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USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 4-1

Chapter 4: Treatment Processes and Systems

4.1 IntroductionThis chapter contains information on individualonsite/decentralized treatment technologies or unitprocesses. Information on typical application,design, construction, operation, maintenance, cost,and pollutant removal effectiveness is provided formost classes of treatment units and their relatedprocesses. This information is intended to be usedin the preliminary selection of a system of treat-ment unit processes that can be assembled toachieve predetermined pollutant discharge concen-trations or other specific performance require-ments. Complete design specifications for unitprocesses and complete systems are not included inthe manual because of the number of processes andprocess combinations and the wide variability intheir application and operation under various siteconditions. Designers and others who require moredetailed technical information are referred to suchsources.

Chapter 4 is presented in two main sections. Thefirst section contains information about conven-tional (soil-based or subsurface wastewater infiltra-tion) systems, referred to as SWISs in this docu-ment. Both gravity-driven and mechanized SWISsare covered in this section of chapter 4. The secondsection contains a general introduction to sandfilters (including other media), and a series of factsheets on treatment technologies, alternativesystems (e.g., fixed-film and suspended growthsystems, evapotranspiration systems, and otherapplications), and special issues pertaining to thedesign, operation, and maintenance of onsitewastewater treatment systems (OWTSs). This

Chapter 4Treatment processes and systems

approach was used because the conventional systemis the most economical and practical system typethat can meet performance requirements in manyapplications.

The first section is further organized to provideinformation about the major components of aconventional system. Given the emphasis in thismanual on the design boundary (performance-based) approach to system design, this section wasstructured to lead the reader through a discussion ofsystem components by working backwards fromthe point of discharge to the receiving environmentto the point of discharge from the home or otherfacility served by the onsite system. Under thisapproach, soil infiltration issues are discussed first,the distribution piping to the infiltration systemincluding graveless sytems is addressed next, andmatters related to the most common preliminarytreatment device, the septic tank, are covered last.

The fact sheets in the second section of this chapterdescribe treatment technologies and discuss specialissues that might affect system design, perfor-mance, operation, and maintenance. These treat-ment technologies are often preceded by a septictank and can include a subsurface wastewaterinfiltration system. Some treatment technologiesmay be substituted for part or all of the conven-tional system, though nearly all alternative ap-proaches include a septic tank for each facilitybeing served. Fact sheets are provided for the morewidely used and successful treatment technologies,such as sand filters and aerobic treatment units.

4.1 Introduction

4.2 Conventional systems and treatment options

4.3 Subsurface wastewater infiltration

4.4 Design considerations

4.5 Construction management and contingency options

4.6 Septic tanks

4.7 Sand/media filters

4.8 Aerobic Treatment Units


4-2 USEPA Onsite Wastewater Treatment Systems Manual

The component descriptions provided in thischapter are intended to assist the reader in screen-ing components and technologies for specificapplications. Chapter 5 presents a strategy andprocedures that can be used to screen and selectappropriate treatment trains and their componentsfor specific receiver sites. The reader should reviewchapter 5 before selecting system components.

4.2 Conventional systems andtreatment options

The three primary components of a conventionalsystem (figure 4-1) are the soil, the subsurfacewastewater infiltration system (SWIS; also called aleach field or infiltration trench), and the septictank. The SWIS is the interface between theengineered system components and the receivingground water environment. It is important to notethat the performance of conventional systems reliesprimarily on treatment of the wastewater effluentin the soil horizon(s) below the dispersal andinfiltration components of the SWIS. Informationon SWIS siting, hydraulic and mass loadings,design and geometry, distribution methods, andconstruction considerations is included in thischapter. The other major component of a conven-tional system, the septic tank, is characterized bydescribing its many functions in an OWTS.

Treatment options include physical, chemical, andbiological processes. Use of these options isdetermined by site-specific needs. Table 4-1 lists

common onsite treatment processes and methodsthat may be used alone or in combination toassemble a treatment train capable of meetingestablished performance requirements. Specialissues that might need to be addressed in OWTSdesign include treatment of high-strength wastes(e.g., biochemical oxygen demand and grease fromschools and restaurants), mitigation of impactsfrom home water softeners and garbage disposals,management of holding tanks, and additives (seerelated fact sheets).

4.3 Subsurface wastewaterinfiltration

Subsurface wastewater infiltration systems (SWISs)are the most commonly used systems for thetreatment and dispersal of onsite wastewater.Infiltrative surfaces are located in permeable,unsaturated natural soil or imported fill material sowastewater can infiltrate and percolate through theunderlying soil to the ground water. As the waste-water infiltrates and percolates through the soil, itis treated through a variety of physical, chemical,and biochemical processes and reactions.

Many different designs and configurations are used,but all incorporate soil infiltrative surfaces that arelocated in buried excavations (figure 4-1). Theprimary infiltrative surface is the bottom of theexcavation, but the sidewalls also may be used forinfiltration. Perforated pipe is installed to distributethe wastewater over the infiltration surface. A porous

Figure 4-1. Conventional subsurface wastewater infiltration system

USEPA Onsite Wastewater Treatment Systems Manual 4-3


Table 4-1. Commonly used treatment processes and optional treatment methods



medium, typically gravel or crushed rock, is placedin the excavation below and around the distributionpiping to support the pipe and spread the localizedflow from the distribution pipes across the excavationcavity. Other gravelless or “aggregate-free” systemcomponents may be substituted. The porousmedium maintains the structure of the excavation,exposes the applied wastewater to more infiltrativesurface, and provides storage space for the waste-water within its void fractions (interstitial spaces,typically 30 to 40 percent of the volume) during peakflows with gravity systems. A permeable geotextilefabric or other suitable material is laid over the porousmedium before the excavation is backfilled to preventthe introduction of backfill material into the porousmedium. Natural soil is typically used for backfilling,and the surface of the backfill is usually slightlymounded and seeded with grass.

Subsurface wastewater infiltration systems provideboth dispersal and treatment of the applied waste-water. Wastewater is transported from the infiltrationsystem through three zones (see chapter 3). Two ofthese zones, the infiltration zone and vadose zone, actas fixed-film bioreactors. The infiltration zone, whichis only a few centimeters thick, is the most biologi-cally active zone and is often referred to as the“biomat.” Carbonaceous material in the wastewater isquickly degraded in this zone, and nitrification occursimmediately below this zone if sufficient oxygen ispresent. Free or combined forms of oxygen in the soilmust satisfy the oxygen demand generated by themicroorganisms degrading the materials. If sufficientoxygen is not present, the metabolic processes of themicroorganisms can be reduced or halted and bothtreatment and infiltration of the wastewater will beadversely affected (Otis, 1985). The vadose (unsatur-ated) zone provides a significant pathway for oxygendiffusion to reaerate the infiltration zone (Otis, 1997,Siegrist et al., 1986). Also, it is the zone where mostsorption reactions occur because the negative moisturepotential in the unsaturated zone causes percolatingwater to flow into the finer pores of the soil, resultingin greater contact with the soil surfaces. Finally, muchof the phosphorus and pathogen removal occurs inthis zone (Robertson and Harman, 1999; Robertson etal., 1998; Rose et al., 1999; Yates and Yates, 1988).

4.3.1 SWIS designs

There are several different designs for SWISs.They include trenches, beds, seepage pits, at-grade

systems, and mounds. SWIS applications differ intheir geometry and location in the soil profile.Trenches have a large length-to-width ratio, whilebeds have a wide, rectangular or square geometry.Seepage pits are deep, circular excavations that relyalmost completely on sidewall infiltration. Seepagepits are no longer permitted in many jurisdictionsbecause their depth and relatively small horizontalprofile create a greater point-source pollutantloading potential to ground water than othergeometries. Because of these shortcomings, seepagepits are not recommended in this manual.

Infiltration surfaces may be created in natural soilor imported fill material. Most traditional systemsare constructed below ground surface in naturalsoil. In some instances, a restrictive horizon abovea more permeable horizon may be removed and theexcavation filled with suitable porous material inwhich to construct the infiltration surface (Hinsonet al., 1994). Infiltration surfaces may be con-structed at the ground surface (“at-grades”) orelevated in imported fill material above the naturalsoil surface (“mounds”). An important differencebetween infiltration surfaces constructed in naturalsoil and those constructed in fill material is that asecondary infiltrative surface (which must beconsidered in design) is created at the fill/naturalsoil interface. Despite the differences between thetypes of SWISs, the mechanisms of treatment anddispersal are similar.

4.3.2 Typical applications

Subsurface wastewater infiltration systems arepassive, effective, and inexpensive treatmentsystems because the assimilative capacity of manysoils can transform and recycle most pollutantsfound in domestic and commercial wastewaters.SWISs are the treatment method of choice in rural,unsewered areas. Where point discharges to surfacewaters are not permitted, SWISs offer an alterna-tive if ground water is not closely interconnectedwith surface water. Soil characteristics, lot size, andthe proximity of sensitive water resources affect theuse of SWISs. Table 4-2 presents characteristics fortypical SWIS applications and suggests applicationsto avoid. Local codes should be consulted forspecial requirements, restrictions, and otherrelevant information.



4.3.3 Typical performance

Results from numerous studies have shown thatSWISs achieve high removal rates for most waste-water pollutants of concern (see chapter 3) with thenotable exception of nitrogen. Biochemical oxygendemand, suspended solids, fecal indicators, andsurfactants are effectively removed within 2 to 5feet of unsaturated, aerobic soil (figure 4-2).Phosphorus and metals are removed throughadsorption, ion exchange, and precipitation reac-tions. However, the retention capacity of the soil isfinite and varies with soil mineralogy, organiccontent, pH, redox potential, and cation exchangecapacity. The fate of viruses and toxic organiccompounds has not been well documented (Tomsonet al., 1984). Field and laboratory studies suggestthat the soil is quite effective in removing viruses,but some types of viruses apparently are able toleach from SWISs to the ground water. Fine-textured soils, low hydraulic loadings, aerobicsubsoils, and high temperatures favor destruction ofviruses and toxic organics. The most significantdocumented threats to ground water quality from

SWISs are nitrates. Wastewater nitrogen is nearlycompletely nitrified below properly operatingSWISs. Because nitrate is highly soluble andenvironments favoring denitrification in subsoil arelimited, little removal occurs (see chapter 3).Chlorides also leach readily to ground waterbecause they, too, are highly soluble and arenonreactive in soil.

Figure 4-2. Lateral view of conventional SWIS-based system

Source: Bouma, 1975.

aAvoid when possible.Source: Adapted from WEF, 1990.

Table 4-2. Characteristics of typical SWIS applications



Dispersion of SWIS percolate in the ground wateris often minimal because most ground water flow islaminar. The percolate can remain for severalhundred feet as a distinct plume in which the soluteconcentrations remain above ambient ground waterconcentrations (Robertson et al., 1989, Shaw andTuryk, 1994). The plume descends in the groundwater as the ground water is recharged from thesurface, but the amount of dispersion of the plumecan be variable. Thus, drinking water wells somedistance from a SWIS can be threatened if they aredirectly in the path of a percolate plume.

4.4 Design considerations

Onsite wastewater treatment system designs varyaccording to the site and wastewater characteristicsencountered. However, all designs should strive toincorporate the following features to achievesatisfactory long-term performance:

• Shallow placement of the infiltration surface(< 2 feet below final grade)

• Organic loading comparable to that of septictank effluent at its recommended hydraulicloading rate

• Trench orientation parallel to surface contours

• Narrow trenches (< 3 feet wide)

• Timed dosing with peak flow storage

• Uniform application of wastewater over theinfiltration surface

• Multiple cells to provide periodic resting,standby capacity, and space for future repairs orreplacement

Based on the site characteristics, compromises toideal system designs are necessary. However, thedesigner should attempt to include as many of theabove features as possible to ensure optimal long-term performance and minimal impact on publichealth and environmental quality.

4.4.1 Placement of the infiltrationsurface

Placement of a SWIS infiltration surface may bebelow, at, or above the existing ground surface (inan in-ground trench, at grade, or elevated in a

mound system). Actual placement relative to theoriginal soil profile at the site is determined bydesired separation from a limiting condition(figure 4-3). Treatment by removal of additionalpollutants during movement through soils and thepotential for excessive ground water mounding willcontrol the minimum separation distance from alimiting condition. The depth below final grade isaffected by subsoil reaeration potential. Maximumdelivery of oxygen to the infiltration zone is mostlikely when soil components are shallow andnarrow and have separated infiltration areas.(Erickson and Tyler, 2001).

4.4.2 Separation distance from alimiting condition

Placement of the infiltration surface in the soilprofile is determined by both treatment and hy-draulic performance requirements. Adequateseparation between the infiltration surface and anysaturated zone or hydraulically restrictive horizonwithin the soil profile (secondary design boundaryas defined in section 5.3.1) must be maintained toachieve acceptable pollutant removals, sustainaerobic conditions in the subsoil, and provide anadequate hydraulic gradient across the infiltrationzone. Treatment needs (performance requirements)establish the minimum separation distance, but thepotential for ground water mounding or theavailability of more permeable soil may make itadvantageous to increase the separation distance byraising the infiltration surface in the soil profile.

Most current onsite wastewater system codesrequire minimum separation distances of at least 18inches from the seasonally high water table orsaturated zone irrespective of soil characteristics.Generally, 2- to 4-foot separation distances haveproven to be adequate in removing most fecalcoliforms in septic tank effluent (Ayres Associates,1993). However, studies have shown that theapplied effluent quality, hydraulic loading rates,and wastewater distribution methods can affect theunsaturated soil depth necessary to achieve accept-able wastewater pollutant removals. A few studieshave shown that separation distances of 12 to 18inches are sufficient to achieve good fecal coliformremoval if the wastewater receives additionalpretreatment prior to soil application (Converse andTyler, 1998a, 1998b; Duncan et al., 1994). How-ever, when effluents with lower organic and



oxygen-demanding content are applied to theinfiltration surface at greater hydraulic loadingrates than those typically used for septic tankeffluents (during extended periods of peak flow),treatment efficiency can be lost (Converse andTyler, 1998b, Siegrist et al., 2000).

Reducing the hydraulic loading rate or providinguniform distribution of the septic tank effluent hasbeen shown to reduce the needed separationdistance (Bomblat et al., 1994; Converse and Tyler,1998a; Otis, 1985; Siegrist et al., 2000; Simon andReneau, 1987). Reducing both the daily andinstantaneous hydraulic loading rates and providinguniform distribution over the infiltration surfacecan help maintain lower soil moisture levels.Lower soil moisture results in longer wastewaterretention times in the soil and causes the wastewa-ter to flow though the smaller soil pores in theunsaturated zone, both of which enhance treatmentand can reduce the necessary separation distance.

Based only on hydraulics, certain soils requiredifferent vertical separation distances from ground

water to avoid hydrologic interference with theinfiltration rate. From a treatment standpoint,required separation distances are affected by dosingpattern, loading rate, temperature, and soil charac-teristics. Uniform, frequent dosing (more than 12times/day) in coarser soils maximizes the effective-ness of biological, chemical, and physical treatmentmechanisms. To offset inadequate vertical separa-tion, a system designer can raise the infiltrationsurface in an at-grade system or incorporate amound in the design. If the restrictive horizon is ahigh water table and the soil is porous, the watertable can be lowered through the use of drainagetile or a curtain drain if the site has sufficient reliefto promote surface discharge from the tile piping.For flat terrain with porous soils, a commercialsystem has been developed and is being field tested.It lowers the water table with air pressure, therebyavoiding any aesthetic concerns associated with araised mound on the site. Another option usedwhere the terrain is flat and wet is pumped drain-age surrounding the OWTS (or throughout thesubdivision) to lower the seasonal high water tableand enhance aerobic conditions beneath the

Figure 4-3. Suggested subsurface infiltration system design versus depth (below the original ground surface) to alimiting condition

Source: Otis, 2001.



drainfield. These systems must be properly oper-ated by certified operators and managed by a publicmanagement entity since maintenance of off-lotportions of the drainage network will influenceperformance of the SWIS.

The hydraulic capacity of the site or the hydraulicconductivity of the soil may increase the minimumacceptable separation distance determined bytreatment needs. The soil below the infiltrationsurface must be capable of accepting and transmit-ting the wastewater to maintain the desired unsatur-ated separation distance at the design hydraulicloading rate to the SWIS. The separation distancenecessary for satisfactory hydraulic performance isa function of the permeability of the underlyingsoil, the depth to the limiting condition, thethickness of the saturated zone, the percentage ofrocks in the soil, and the hydraulic gradient.Ground water mounding analyses may be necessaryto assess the potential for the saturated zone to riseand encroach upon the minimum acceptableseparation distance (see section 5.4). Raising theinfiltration surface can increase the hydrauliccapacity of the site by accommodating moremounding. If the underlying soil is more slowlypermeable than soil horizons higher in the profile,it might be advantageous to raise the infiltrationsurface into the more permeable horizon wherehigher hydraulic loading rates are possible (Hooveret al., 1991; Weymann et al., 1998). A shallowinfiltration system covered with fill or an at-gradesystem can be used if the natural soil has a shallowpermeable soil horizon (Converse et al., 1990;Penninger, and Hoover, 1998). If more permeablehorizons do not exist, a mound system constructedof suitable sand fill (figure 4-4) can provide morepermeable material in which to place the infiltra-tion surface.

4.4.3 Depth of the infiltration surface

The depth of the infiltration surface is an importantconsideration in maintaining adequate subsoilaeration and frost protection in cold climates. Themaximum depth should be limited to no more than3 to 4 feet below final grade to adequately reaeratethe soil and satisfy the daily oxygen demand of theapplied wastewater. The infiltrative surface depthshould be less in slowly permeable soils or soilswith higher ambient moisture. Placement belowthis depth to take advantage of more permeable

soils should be resisted because reaeration of thesoil below the infiltration surface will be limited.In cold climates, a minimum depth of 1 to 2 feetmay be necessary to protect against freezing.Porous fill material can be used to provide thenecessary cover even with an elevated (at-grade ormound) system if it is necessary to place theinfiltration surface higher.

4.4.4 Subsurface drainage

Soils with shallow saturated zones sometimes canbe drained to allow the infiltration surface to beplaced in the natural soil. Curtain drains, verticaldrains, underdrains, and mechanically assistedcommercial systems can be used to drain shallowwater tables or perched saturated zones. Of thethree, curtain drains are most often used in onsitewastewater systems to any great extent. They canbe used effectively to remove water that is perchedover a slowly permeable horizon on a sloping site.However, poorly drained soils often indicate othersoil and site limitations that improved drainagealone will not overcome, so the use of drainageenhancements must be carefully considered. Anysloping site that is subject to frequent inundationduring prolonged rainfall should be considered acandidate for upslope curtain drains to maintainunsaturated conditions in the vadose zone.

Curtain drains are installed upslope of the SWIS tointercept the permanent and perched ground waterflowing through the site over a restrictive horizon.Perforated pipe is laid in the bottom of upslopetrenches excavated into the restrictive horizon. Adurable, porous medium is placed around thepiping and up to a level above the estimatedseasonally high saturated zone. The porous mediumintercepts the ground water and conveys it to thedrainage pipe (figure 4-5). To provide an outfallfor the drain, one or both ends of the pipe areextended downslope to a point where it interceptsthe ground surface. When drainage enhancementsare used, the outlet and boundary conditions mustbe carefully evaluated to protect local waterquality.

The drain should avoid capture of the SWISpercolate plume and ground water infiltrating frombelow the SWIS or near the end of the drain. Aseparation distance between the SWIS and the drainthat is sufficient to prevent percolate from the



SWIS from entering the drain should be main-tained. The vertical distance between the bottom ofthe SWIS and the drain and soil permeabilitycharacteristics should determine this distance. Asthe vertical distance increases and the permeabilitydecreases, the necessary separation distance in-creases. A 10-foot separation is used for mostapplications. Also, if both ends of the drain cannotbe extended to the ground surface, the upslope endshould be extended some distance along the surfacecontour beyond the end of the SWIS. If not done,

ground water that seeps around the end of the draincan render the drain ineffective. Similar cautionsshould be observed when designing and locatingoutlet locations for commercial systems on flatsites.

The design of a curtain drain is based on thepermeability of the soil in the saturated zone, thesize of the area upslope of the SWIS that contrib-utes water to the saturated zone, the gradient of thedrainage pipe, and a suitable outlet configuration.

Figure 4-4. Raising the infiltration surface with a typical mound system.

CurtainDrain

FillMaterial

PerchedWaterTable Gravel Filled

Above HighWater Table

Drainage Pipe

Impermeable Layer

AbsorptionTrenches

Fill

Figure 4-5. Schematic of curtain drain constructionSource: USEPA, 1980

Source: ASAE, Converse and Tyler, 1998b.



If the saturated hydraulic conductivity is low andthe drainable porosity (the percentage of pore spacedrained when the soil is at field capacity) is small,even effectively designed curtain drains might havelimited effect on soil wetness conditions. Penningeret al. (1998) illustrated this at a site with a siltyclay loam soil at field capacity that became com-pletely re-saturated with as little as 1-inch ofprecipitation. Figure 4-6 provides a useful designchart that considers most of these parameters. Forfurther design guidance, refer to the U.S. Depart-ment of Agriculture’s Drainage of AgriculturalLand (USDA, 1973).

4.4.5 Sizing of the infiltration surface

The minimum acceptable infiltration surface area isa function of the maximum anticipated dailywastewater volume to be applied and the maximuminstantaneous and daily mass loading limitations ofthe infiltration surface (see chapter 5). Both thebottom and sidewall area of the SWIS excavationcan be infiltration surfaces; however, if the sidewallis to be an active infiltration surface, the bottomsurface must pond. If continuous ponding of theinfiltration surface persists, the infiltration zonewill become anaerobic, resulting in loss of hydrau-lic capacity. Loss of the bottom surface for infiltra-tion will cause the ponding depth to increase overtime as the sidewall also clogs (Bouma, 1975; Keyset al., 1998; Otis, 1977). If allowed to continue,

hydraulic failure of the system is probable. There-fore, including sidewall area as an active infiltra-tion surface in design should be avoided. Ifsidewall areas are included, provisions should bemade in the design to enable removal of the pondedsystem from service periodically to allow thesystem to drain and the biomat to oxidize naturally.

Design flowAn accurate estimation of the design flow is criticalto infiltration surface sizing. For existing buildingswhere significant changes in use are not expected,water service metering will provide good estimatesfor design. It is best to obtain several weeks ofmetered daily flows to estimate daily average andpeak flows. For new construction, water usemetering is not possible and thus waste flowprojections must be made based on similar estab-lishments. Tables of “typical” water use or waste-water flows for different water use fixtures, usagepatterns, and building uses are available (seesection 3.3.1). Incorporated into these guidelinesare varying factors of safety. As a result, the use ofthese guides typically provides conservatively highestimates of maximum peak flows that may occuronly occasionally. It is critical that the designerrecognizes the conservativeness of these guides andhow they can be appropriately adjusted because oftheir impacts on the design and, ultimately, perfor-mance of the system.

Curtain drain designCurtain drain designCurtain drain designCurtain drain designCurtain drain design

Curtain drain design (see preceding figures) is dependent on the size of the contributing drainage area, theamount of water that must be removed, the soil’s hydraulic properties, and the available slope of the site.

The contributing drainage area is estimated by outlining the capture zone on a topographic map of the site.Drainage boundaries are determined by extending flow lines perpendicular to the topographic contours upslopefrom the drain to natural divides (e.g., ridge tops) or natural or man-made “no-flow” boundaries (e.g., rockoutcrops, major roads). The amount of water that must be removed is an estimate of the volume of precipitationthat would be absorbed by the soil after a rainfall event. This is called the drainage coefficient, which is expressedas the depth of water to be removed over a specified period of time, typically 24 hours. Soil structure, texture,bulk density, slope, and vegetated cover all affect the volume of water to be drained.

The slope of the drain can be determined after the upslope depth of the drain invert and the outfall invert areestablished. These can be estimated from the topographic map of the site. The contributing drainage area, watervolume to be removed, and slope of the drain are estimated. Figure 4-6 can be used to determine the draindiameter. For example, the diameter of a curtain drain that will drain an area upslope of 50 acres with a drainagecoefficient of ¾ inch on a slope of 5 percent would be 8 inches (see figure). At 0.5 percent, the necessary draindiameter would be 12 inches.



DRAINAGE CHART FOR

CORRUGATED PLASTIC DRAINAGE TUBING

GRADE IN CENTIMETERS PER METER

0.05 0.1 0.2 0.3 0.4 0.5 1.0 2.0 3.0 5.04.0 101009080807060

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GRADE IN FEET PER 100 FEET

Space between lines is the range of drain

capacity for the size shown between lines

V= velocity in feet per second

n=0.015

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DRAINAGE

COEFFICIENT

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Figure 4-6. Capacity chart for subsurface drains

Source: USDA, 1973.



Table 4-3. Suggested hydraulic and organic loading rates for sizing infiltration surfaces

Source: Adapted from Tyler, 2000.

Infiltration surface loading limitationsInfiltration surface hydraulic loading design ratesare a function of soil morphology, wastewaterstrength, and SWIS design configuration. Hydrau-lic loadings are traditionally used to size infiltrationsurfaces for domestic septic tank effluent. In thepast, soil percolation tests determined acceptablehydraulic loading rates. Codes provided tables thatcorrelated percolation test results to the necessaryinfiltration surface areas for different classes ofsoils. Most states have supplemented this approachwith soil morphologic descriptions. Morphologicfeatures of the soil, particularly structure, texture,and consistence, are better predictors of the soil’shydraulic capacity than percolation tests (Brown etal., 1994; Gross et al., 1998; Kleiss and Hoover,

1986; Simon and Reneau, 1987; Tyler et al., 1991;Tyler and Converse, 1994). Although soil textureanalysis supplemented the percolation test in moststates by the mid-1990s, soil structure has onlyrecently been included in infiltrative surface sizingtables (table 4-3). Consistence, a measure of howwell soils form shapes and stick to other objects, isan important consideration for many slowlypermeable soil horizons. Expansive clay soils thatbecome extremely firm when moist and very stickyor plastic when wet (exhibiting firm or extremelyfirm consistence) are not well suited for SWISs.

Not all soil conditions are represented in table 4-3,which is a generic guide to the effects of soilproperties on the performance of SWISs. Also



available are many other state and local guides thatinclude loadings for soils specific to local geomor-phology. North Carolina, for example, uses thelong-term acceptance rate (LTAR) for soil load-ings, which is the volume of wastewater that can beapplied to a square foot of soil each day over anindefinite period of time such that the effluentfrom the onsite system is absorbed and properlytreated (North Carolina DEHNR, 1996). In theNorth Carolina rules, LTAR and loading rate valuesare the same.

Increasingly, organic loading is being used to sizeinfiltration surfaces. Based on current understand-ing of the mechanisms of SWIS operation, organicloadings and the reaeration potential of the subsoilto meet the applied oxygen demand are criticalconsiderations in successful SWIS design. Anaero-bic conditions are created when the applied oxygendemand exceeds what the soil is able to supply bydiffusion through the vadose zone (Otis, 1985,1997; Siegrist et al., 1986). The facultative andanaerobic microorganisms that are able to thrive inthis environment are less efficient in degrading thewaste materials. The accumulating waste materialsand the metabolic by-products cause soil cloggingand loss of infiltrative capacity.

Further, higher forms of soil fauna that would helpbreak up the biomat (e.g., worms, insects, non-wetland plants) and would be attracted to thecarbon and nutrient-rich infiltration zone arerepelled by the anoxic or anaerobic environment. Ifwastewater application continues without ampletime to satisfy the oxygen demand, hydraulicfailure due to soil clogging occurs. Numerousstudies have shown that wastewaters with low BODconcentrations (e.g., < 50 mg/L) can be applied tosoils at rates 2 to 16 times the typical hydraulicloading rate for domestic septic tank effluent (Jonesand Taylor, 1965; Laak, 1970, 1986; Louden et al.,1998; Otis, 1985; Siegrist and Boyle, 1987; Tylerand Converse, 1994).

The comparatively higher hydraulic loadings thathighly treated wastewater (highly treated in termsof TSS, ammonium-nitrogen, and BOD) maypermit should be considered carefully because theresulting rapid flow through the soil may allowdeep penetration of pathogens (Converse and Tyler,1998a, 1998b; Siegrist et al., 2000; Siegrist andVan Cuyk, 2001b; Tyler and Converse, 1994). Thetrench length perpendicular to ground water

movement (footprint) should remain the same tominimize system impacts on the aquifer.

Unfortunately, well-tested organic loading rates forvarious classes of soils and SWIS design configura-tions have not been developed. Most organicloading rates have been derived directly from thehydraulic loadings typically used in SWIS designby assuming a BOD5 concentration (see box andtable 4-3). The derived organic loading rates alsoincorporate the implicit factor of safety found inthe hydraulic loading rates. Organic loadings doappear to have less impact on slowly permeablesoils because the resistance of the biomat that formsat the infiltrative surface presents less resistance toinfiltration of the wastewater than the soil itself(Bouma, 1975). For a further discussion of SWISperformance under various environmental condi-tions, see Siegrist and Van Cuyk, 2001b.

Constituent mass loadingsConstituent mass loadings may be a concern withrespect to water quality. For example, to use thesoil’s capacity to adsorb and retain phosphoruswhen systems are located near sensitive surfacewaters, a phosphorus loading rate based on the soiladsorption capacity might be selected as thecontrolling rate of wastewater application to theinfiltration surface to maximize phosphorusremoval. Placement of the effluent distributionpiping high in the soil profile can promote greaterphosphorus removal because the permeability ofmedium- and fine-textured soils tends to decreasewith depth and because the translocation of alumi-num and iron—which react with phosphorus toform insoluble compounds retained in the soilmatrix—occurs in some sandy soils, with themaximum accumulation usually above 45 cm(Mokma et al., 2001). Many lakes are surroundedby sandy soils with a low phosphorus adsorptioncapacity. If effluent distribution systems areinstalled below 45 cm in these sandy soils, lessphosphorus will be removed from the percolatingeffluent. In the case of a soluble constituent ofconcern such as nitrate-nitrogen, a designer mightdecide to reduce the mass of nitrate per unit ofapplication area. This would have the effect ofincreasing the size of the SWIS footprint, therebyreducing the potential concentration of nitrate inthe ground water immediately surrounding theSWIS (Otis, 2001).



4.4.6 Geometry, orientation, andconfiguration of the infiltrationsurface

The geometry, orientation, and configuration of theinfiltration surface are critical design factors thataffect the performance of SWISs. They are impor-tant for promoting subsoil aeration, maintaining anacceptable separation distance from a saturatedzone or restrictive horizon, and facilitating con-struction. Table 4-4 lists the design considerationsdiscussed in this section.

GeometryThe width and length of the infiltration surface areimportant design considerations to improve perfor-mance and limit impacts on the receiving environ-ment. Trenches, beds, and seepage pits (or drywells) are traditionally used geometries. Seepagepits can be effective for wastewater dispersal, butthey provide little treatment because they extenddeep into the soil profile, where oxygen transferand treatment are limited and the separationdistance to ground water is reduced. They are notrecommended for onsite wastewater treatment andare not included as an option in this manual.

Width

Infiltration surface clogging and the resulting lossof infiltrative capacity are less where the infiltra-tion surface is narrow. This appears to occurbecause reaeration of the soil below a narrowinfiltration surface is more rapid. The dominantpathway for oxygen transport to the subsoil appearsto be diffusion through the soil surrounding theinfiltration surface (figure 4-7). The unsaturatedzone below a wide surface quickly becomesanaerobic because the rates of oxygen diffusion aretoo low to meet the oxygen demands of biota andorganics on the infiltration surface. (Otis, 1985;Siegrist et al., 1986). Therefore, trenches performbetter than beds. Typical trench widths range from1 to 4 feet. Narrower trenches are preferred, butsoil conditions and construction techniques mightlimit how narrow a trench can be constructed. Onsloping sites, narrow trenches are a necessitybecause in keeping the infiltration surface level, theuphill side of the trench bottom might be excavatedinto a less suitable soil horizon. Wider trenchinfiltration surfaces have been successful in at-grade systems and mounds probably because theengineered fill material and elevation above thenatural grade promote better reaeration of the fill.

Factors of safety in infiltration surface sizingFactors of safety in infiltration surface sizingFactors of safety in infiltration surface sizingFactors of safety in infiltration surface sizingFactors of safety in infiltration surface sizingSizing of onsite wastewater systems for single-family homes is typically based on the estimated peak daily flowand the “long term acceptance rate” of the soil for septic tank effluent. In most states, the design flow is based onthe number of bedrooms in the house. A daily flow of 150 gallons is commonly assumed for each bedroom. Thisdaily flow per bedroom assumes two people per bedroom that generate 75 gpd each. Bedrooms, rather thancurrent occupancy, are used for the basis of SWIS design because the number of occupants in the house canchange.

Using this typical estimating procedure, a three-bedroom home would have a design flow of 150 gpd/bedroom x 3bedrooms or 450 gpd. However, the actual daily average flow could be much less. Based on the 1990 census, theaverage home is occupied by 2.8 persons. Each person in the United States generates 45 to 70 gpd of domesticwastewater. Assuming these averages, the average daily flow would be 125 to 195 gpd or 28 to 44 percent of thedesign flow, respectively. Therefore, the design flow includes an implicit factor of safety of 2.3 to 3.6. Of course,this factor of safety varies inversely with the home occupancy and water use.

Unfortunately, the factors of safety implicitly built into the flow estimates are seldom recognized. This isparticularly true in the case of the design hydraulic loading rates, which were derived from existing SWISs. Inmost codes, the hydraulic loading rates for sand are about 1.0 to 1.25 gpd/ft2. Because these hydraulic loadingrates assume daily flows of 150 gpd per bedroom, they are overestimated by a factor of 2.3 to 3.6. Fortunately,these two assumptions largely cancel each other out in residential applications, but the suggested hydraulicloading rates often are used to size commercial systems and systems for schools and similar facilities, where theratios between design flows and actual daily flows are closer to 1.0. This situation, combined with a lack of usefulinformation on allowable organic loading rates, has resulted in failures, particularly for larger systems whereactual flow approximates design.



However, infiltration bed surface widths of greaterthan 10 feet are not recommended because oxygentransfer and clogging problems can occur (Con-verse and Tyler, 2000; Converse et al., 1990).

LengthThe trench length is important where downslopelinear loadings are critical, ground water qualityimpacts are a concern, or the potential for ground

water mounding exists. In many jurisdictions,trench lengths have been limited to 100 feet. Thisrestriction appeared in early codes written forgravity distribution systems and exists as an artifactwith little or no practical basis when pressuredistribution is used. Trench lengths longer than 100feet might be necessary to minimize ground waterimpacts and to permit proper wastewater drainagefrom the site. Long trenches can be used to reducethe linear loadings on a site by spreading the

Comparing hydraulic and organic mass loadings for a restaurant wastewaterComparing hydraulic and organic mass loadings for a restaurant wastewaterComparing hydraulic and organic mass loadings for a restaurant wastewaterComparing hydraulic and organic mass loadings for a restaurant wastewaterComparing hydraulic and organic mass loadings for a restaurant wastewater

Infiltration surface sizing traditionally has been based on the daily hydraulic load determined through experienceto be acceptable for the soil characteristics. This approach to sizing fails to account for changes in appliedwastewater strength. Since soil clogging has been shown to be dependent on applied wastewater strength, itmight be more appropriate to size infiltration surfaces based on organic mass loadings.

To illustrate the impact of the different sizing methods, sizing computations for a restaurant are compared. Aseptic tank is used for pretreatment prior to application to the SWIS. The SWIS is to be constructed in a sandyloam with a moderate, subangular blocky structure. The suggested hydraulic loading rate for domestic septic tankeffluent on this soil is 0.6 gpd/ft2 (table 4-3). The restaurant septic tank effluent has the following characteristics:

BOD5

800 mg/L

TSS 200 mg/L

Average daily flow 600 gpd

Infiltration area based on hydraulic loading:Infiltration area based on hydraulic loading:Infiltration area based on hydraulic loading:Infiltration area based on hydraulic loading:Infiltration area based on hydraulic loading:

Area = 600 gpd/0.6 gpd/ft2 = 1,000 ft2

Infiltration area based on organic loading:Infiltration area based on organic loading:Infiltration area based on organic loading:Infiltration area based on organic loading:Infiltration area based on organic loading:

At the design infiltration rate of 0.6 gpd/ft2 recommended for domestic septic tank effluent, the equivalent organicloading is (assuming a septic tank BOD

5 effluent concentration of 150 mg/L)

Organic Loading = 150 mg/L x 0.6 gpd/ft2 x (8.34 lb/mg/L x 10-6 gal)

= 7.5 x 10-4 lb BOD5/ft2-d

Assuming 7.5 x 10-4 lb BOD5/ft2-d as the design organic loading rate,

Area = (800 mg-BOD5/L x 600 gpd x 8.34 lbs/mg/L x 10-6 gal)

(7.5 x 10-4 lb BOD5/ft2-d)

= 4.0 lb BOD5/d = 5337 ft2 (a 540% increase)

(7.5 x 10-4 lb BOD5/ft2-d)

Impact of a 40% water use reduction on infiltration area sizingImpact of a 40% water use reduction on infiltration area sizingImpact of a 40% water use reduction on infiltration area sizingImpact of a 40% water use reduction on infiltration area sizingImpact of a 40% water use reduction on infiltration area sizing

Based on hydraulic loading,

Area = (1 – 0.4) x 600 gpd = 600 ft2

0.6 gpd/ft2

Based on organic loading (note the concentration of BOD5 increases with water conservation but the mass of

BOD5

discharged does not change),

Area = (800 mg-BOD5/L x 600 gpd) x (8.34 lb/mg/L x 10-6 gal)

[(1 – 0.4) x 600 gpd] x (7.5 x 10-4 lb BOD5/ft2-d)

= 4.0 lb BOD5/d = 5337 ft2 (an 890% increase)

(7.5 x 10-4 lb BOD5/ft2-d)



Figure 4-7. Pathway of subsoil reaeration

Source: Ayres Associates, 2000

Table 4-4. Geometry, orientation, and configuration considerations for SWISs



wastewater loading parallel to and farther along thesurface contour. With current distribution/dosingtechnology, materials, and construction methods,trench lengths need be limited only by what ispractical or feasible on a given site. Also, use ofstandard trench lengths, e.g., X feet of trench/BR,is discouraged because it restricts the design optionsto optimize performance for a given site condition.

HeightThe height of the sidewall is determined primarilyby the type of porous medium used in the system,the depth of the medium needed to encase thedistribution piping, and/or storage requirements forpeak flows. Because the sidewall is not included asan active infiltration surface in sizing the infiltra-tion area, the height of the sidewall can be mini-mized to keep the infiltration surface high in thesoil profile. A height of 6 inches is usually suffi-cient for most porous aggregate applications. Useof a gravelless system requires a separate analysisto determine the height based on whether it is anaggregate-free (empty chamber) design or one thatsubstitutes a lightweight aggregate for washedgravel or crushed stone.

OrientationOrientation of the infiltration surface(s) becomesan important consideration on sloping sites, siteswith shallow soils over a restrictive horizon orsaturated zone, and small or irregularly shaped lots.The long axes of trenches should be alignedparallel to the ground surface contours to reducelinear contour hydraulic loadings and ground watermounding potential. In some cases, ground wateror restrictive horizon contours may differ fromsurface contours because of surface grading or thesoil’s morphological history. Where this occurs,consideration should be given to aligning thetrenches with the contours of the limiting conditionrather than those of the surface. Extending thetrenches perpendicular to the ground water gradientreduces the mass loadings per unit area by creatinga “line” source rather than a “point” source alongthe contour. However, the designer must recognizethat the depth of the trenches and the soil horizonin which the infiltration surface is placed will varyacross the system. Any adverse impacts this mighthave on system performance should be mitigatedthrough design adjustments.

ConfigurationThe spacing of multiple trenches constructedparallel to one another is determined by the soilcharacteristics and the method of construction. Thesidewall-to-sidewall spacing must be sufficient toenable construction without damage to the adjacenttrenches. Only in very tight soils will normallyused spacings be inadequate because of high soilwetness and capillary fringe effects, which canlimit oxygen transfer. It is important to note thatthe sum of the hydraulic loadings to one or moretrenches or beds per each unit of contour length(when projected downslope) must not exceed theestimated maximum contour loading for the site.Also, the finer (tighter) the soil, the greater thetrench spacing should be to provide sufficientoxygen transfer. Quantitative data are lacking, butCamp (1985) reported a lateral impact of morethan 2.0 meters in a clay soil.

Given the advantages of lightweight gravellesssystems in terms of potentially reduced damage tothe site’s hydraulic capacity, parallel trenches mayphysically be placed closer together, but thedownslope hydraulic capacity of the site and thenatural oxygen diffusion capacity of the soil cannotbe exceeded.

4.4.7 Wastewater distribution onto theinfiltration surface

The method and pattern of wastewater distributionin a subsurface infiltration system are importantdesign elements. Uniform distribution aids inmaintaining unsaturated flow below the infiltrationsurface, which results in wastewater retention timesin the soil that are sufficiently long to effecttreatment and promote subsoil reaeration. Uniformdistribution design also results in more completeutilization of the infiltration surface.

Gravity flow and dosing are the two most com-monly used distribution methods. For each method,various network designs are used (table 4-5).Gravity flow is the most commonly used methodbecause it is simple and inexpensive. This methoddischarges effluent from the septic tank or otherpretreatment tank directly to the infiltration surfaceas incoming wastewater displaces it from thetank(s). It is characterized by the term “trickleflow” because the effluent is slowly dischargedover much of the day. Typically, tank discharges



are too low to flow throughout the distributionnetwork. Thus, distribution is unequal and local-ized overloading of the infiltration surface occurswith concomitant poor treatment and soil clogging(Bouma, 1975; McGauhey and Winneberger, 1964;Otis, 1985; Robeck et al., 1964).

Dosing, on the other hand, accumulates the waste-water effluent in a dose tank from which the wateris periodically discharged under pressure in “doses”to the infiltration system by a pump or siphon. Thepretreated wastewater is allowed to accumulate inthe dose tank and is discharged when a predeter-mined water level, water volume, or elapsed time isreached. The dose volumes and discharge rates areusually such that much of the distribution networkis filled, resulting in more uniform distributionover the infiltration surface. Dosing outperformsgravity-flow systems because distribution is moreuniform. In addition, the periods between dosesprovide opportunities for the subsoil to drain andreaerate before the next dose (Bouma et al., 1974;Hargett et al., 1982; Otis et al., 1977). However,which method is most appropriate depends on thespecific application.

Gravity flow

Gravity flow can be used where there is a sufficientelevation difference between the outlet of thepretreatment tank and the SWIS to allow flow toand through the SWIS by gravity. Gravity flowsystems are simple and inexpensive to construct but

are the least efficient method of distribution.Distribution is very uneven over the infiltrationsurface, resulting in localized overloading (Con-verse, 1974; McGauhey and Winneberger, 1964;Otis et al., 1978; University of Wisconsin, 1978).Until a biomat forms on the infiltration surface toslow the rate of infiltration, the wastewater resi-dence time in the soil might be too short to effectgood treatment. As the biomat continues to form onthe overloaded areas, the soil surface becomesclogged, forcing wastewater effluent to flowthrough the porous medium of the trench until itreaches an unclogged infiltration surface. Thisphenomenon, known as “progressive clogging,”occurs until the entire infiltration surface is pondedand the sidewalls become the more active infiltra-tion surfaces. Without extended periods of little orno flow to allow the surface to dry, hydraulicfailure becomes imminent. Although inefficient,these systems can work well for seasonal homeswith intermittent use or for households with lowoccupancies. Seasonal use of SWISs allows theinfiltration surface to dry and the biomat to oxi-dize, which rejuvenates the infiltration capacity.Low occupancies result in mass loadings of waste-water constituents that are lower and less likely toexceed the soil’s capacity to completely treat theeffluent.

Perforated pipe

Four-inch-diameter perforated plastic pipe is themost commonly used distribution piping for

Table 4-5. Distribution methods and applications.



gravity flow systems. The piping is generallysmooth-walled rigid polyvinyl chloride (PVC), orflexible corrugated polyethylene (PE) or acryloni-trile-butadiene-styrene (ABS). One or two rows ofholes or slots spaced 12 inches apart are cut into thepipe wall. Typically, the piping is laid level ingravel (figure 4-1) with the holes or slots at thebottom (ASTM, undated). One distribution line isused per trench. In bed systems, multiple lines areinstalled 3 to 6 feet apart.

Distribution box

Distribution boxes are used to divide the wastewa-ter effluent flow among multiple distribution lines.They are shallow, flat bottomed, watertight struc-tures with a single inlet and individual outletsprovided at the same elevation for each distributionline. An above-grade cover allows access to theinside of the box. The “d-box” must be laid levelon a sound, frost-proof footing to divide the flowevenly among the outlets. Uneven settlement orfrost heaving results in unequal flow to the laterallines because the outlet hole elevations cease to belevel. If this occurs, adjustments must be made toreestablish equal division of flow. Several devicescan be used. Adjustable weirs that can level theoutlet inverts and maintain the same length of weirper outlet are one option. Other options includedesigns that allow for leveling of the entire box(figure 4-8). The box can also be used to takeindividual trenches out of service by blocking theoutlet to the distribution lateral or raising the outletweir above the weir elevations for the other outlets.Because of the inevitable movement of d-boxes,their use has been discouraged for many years(USPHS, 1957). However, under a managed caresystem with regular adjustment, the d-box isacceptable.

Serial relief line

Serial relief lines distribute wastewater to a seriesof trenches constructed on a sloping site. Ratherthan dividing the flow equally among all trenchesas with a distribution box, the uppermost trench isloaded until completely flooded before the next(lower) trench receives effluent. Similarly, thattrench is loaded until flooded before dischargeoccurs to the next trench, and so on. This methodof loading is accomplished by installing “relieflines” between successive trenches (figure 4-9).

Figure 4-8. Distribution box with adjustable weir outlets

Figure 4-9. Serial relief line distribution network and installationdetail

Source: USEPA, 1980.

Source: Ayres Associates.



The relief lines are simple overflow lines thatconnect one trench to the adjacent lower trench.They are solid-wall pipes that connect the crown ofthe upper trench distribution pipe with the distribu-tion pipe in the lower trench. Successive relief linesare separated by 5 to 10 feet to avoid short-circuiting. This method of distribution makes fullhydraulic use of all bottom and sidewall infiltrationsurfaces, creates the maximum hydrostatic headover the infiltration surfaces to force the water intothe surrounding soil, and eliminates the problem ofdividing flows evenly among independent trenches.However, because continuous ponding of theinfiltration surfaces is necessary for the system tofunction, the trenches suffer hydraulic failure morerapidly and progressively because the infiltrationsurfaces cannot regenerate their infiltrative capacity.

Drop box

Drop box distribution systems function similarly torelief line systems except that drop boxes are usedin place of the relief lines. Drop boxes are installedfor each trench. They are connected in manifolds totrenches above and below (figure 4-10). The outletinvert can be placed near the top of each trench toforce the trench to fill completely before it dis-charges to the next trench if a serial distributionmode of operation is desired. Solid-wall pipe isused between the boxes.

The advantage of this method over serial relieflines is that individual trenches can be taken out ofservice by attaching 90 degree ells to the outletsthat rise above the invert of the manifold connec-tion to the next trench drop box. It is easier to addadditional trenches to a drop box system than to aserial relief line network. Also, the drop boxsystem may be operated as an alternating trenchsystem by using the 90 degree ells on unused lines.With this and the serial distribution system, thedesigner must carefully evaluate the downslopecapacity of the site to ensure that it will not beoverloaded when the entire system or specifictrench combinations are functioning.

Gravelless wastewater dispersal systemsGravelless systems have been widely used. Theytake many forms, including open-bottomed cham-bers, fabric-wrapped pipe, and synthetic materialssuch as expanded polystyrene foam chips (fig-

ure 4-11). Some gravelless drain field systems uselarge-diameter corrugated plastic tubing coveredwith permeable nylon filter fabric not surroundedby gravel or rock. The area of fabric in contactwith the soil provides the surface for the septic tankeffluent to infiltrate the soil. The pipe is a mini-mum of 10 to 12 inches (25.4 to 30.5 centimeters)in diameter covered with spun bonded nylon filterfabric to distribute water around the pipe. The pipeis placed in a 12- to 24-inch (30.5- to 61-centime-ter)-wide trench. These systems can be installed inareas with steep slopes with small equipment and inhand-dug trenches where conventional gravelsystems would not be possible.

Reduced sizing of the infiltration surface is oftenpromoted as another advantage of the gravellesssystem. This is based primarily on the premise thatgravelless systems do not “mask” the infiltrationsurface as gravel does where the gravel is in directcontact with the soil. Proponents of this theoryclaim that an infiltration surface area reduction of50 percent is warranted. However, these reductionsare not based on scientific evidence though theyhave been codified in some jurisdictions (Amersonet al., 1991; Anderson et al., 1985; Carlile andOsborne, 1982; Effert and Cashell, 1987). Al-though gravel masking might occur in porousmedium applications, reducing the infiltrationsurface area for gravelless systems increases theBOD mass loading to the available infiltrationsurface. Many soils might not be able to supportthe higher organic loading and, as a result, moresevere soil clogging and greater penetration ofpollutants into the vadose zone and ground watercan occur (University of Wisconsin, 1978), negat-ing the benefits of the gravelless surface.

A similar approach must be taken with any con-taminant in the pretreatment system effluent thatmust be removed before it reaches ground water ornearby surface waters. A 50 percent reduction ininfiltrative surface area will likely result in lessremoval of BOD, pathogens, and other contami-nants in the vadose zone and increase the presenceand concentrations of contaminants in effluentplumes. The relatively confined travel path of aplume provides fewer adsorption sites for removalof adsorbable contaminants (e.g., metals, phospho-rus, toxic organics). Because any potential reduc-tions in infiltrative surface area must be analyzed ina similar comprehensive fashion, the use of



Figure 4-10. Drop box distribution network

Source: National Small Flows Clearinghouse.

Figure 4-11. Various gravelless systems

Source: USEPA, 1980



gravelless medium should be treated similarly topotential reductions from increased pretreatmentand better distribution and dosing concepts.

Despite the cautions stated above, the overallinherent value of lightweight gravelless systemsshould not be ignored, especially in areas wheregravel is expensive and at sites that have soils thatare susceptible to smearing or other structuraldamage during construction due to the impacts ofheavy machinery on the site. In all applicationswhere gravel is used (see SWIS Media in thefollowing section), it must be properly graded andwashed. Improperly washed gravel can contributefines and other material that can plug voids in theinfiltrative surface and reduce hydraulic capability.Gravel that is embedded into clay or fine soilsduring placement can have the same effect.

Leaching chambers

A leaching chamber is a wastewater treatmentsystem that consists of trenches or beds and one ormore distribution pipes or open-bottomed plasticchambers. Leaching chambers have two keyfunctions: to disperse the effluent from septic tanksand to distribute this effluent throughout thetrenches. A typical leaching chamber consists ofseveral high-density polyethylene injection-moldedarch-shaped chamber segments. A typical chamberhas an average inside width of 15 to 40 inches (38to 102 centimeters) and an overall length of 6 to 8feet (1.8 to 2.4 meters). The chamber segments areusually 1-foot high, with wide slotted sidewalls.Depending on the drain field size requirements, oneor more chambers are typically connected to forman underground drain field network.

Typical leaching chambers (figure 4-12) aregravelless systems that have drain field chamberswith no bottoms and plastic chamber sidewalls,available in a variety of shapes and sizes. Use ofthese systems sometimes decreases overall drainfield costs and may reduce the number of trees thatmust be removed from the drain field lot.

Millions of leaching chamber sysems have beeninstalled in the 50 states, Canada, and overseas.About 750,000 chamber systems have been installedover the past 15 years. Currently, a high percentageof new construction applications use lightweightplastic leaching chambers for new wastewatertreatment systems in states like Colorado, Idaho,North Carolina, Georgia, Florida, and Oregon. Thegravel aggregate traditionally used in drain fieldscan have large quantities of mineral fines that alsoclog or block soil pores. Use of leaching chambersavoids this problem. Recent research sponsored bymanufacturers shows promising results to supportreduction in sizing of drain fields through the useof leaching chambers without increased hydraulicand pollutant penetration failures (Colorado Schoolof Mines, 2001; Siegrist and Vancuyk, 2001a, 2001b).These studies should be continued to eventually yieldrational guidelines for proper sizing of these systemsbased on the type of pretreatment effluent to bereceived (septic tank effluent, effluent from filtersor aerobic treatment units, etc.), as well as differentsoil types and hydrogeological conditions. Manystates offer drain field sizing reduction allowanceswhen leaching chambers are used instead ofconventional gravel drain fields.

Because leaching chamber systems can be installedwithout heavy equipment, they are easy to install

Figure 4-12. Placement of leaching chambers in typical application

Source: Hoover et al., 1996.



and repair. These high-capacity, open-bottom drainfield systems can provide greater storage thanconventional gravel systems and can be used inareas appropriate for gravel aggregate drain fields.Leaching systems can operate independently andrequire little day-to-day maintenance. Theirmaintenance requirements are comparable to thoseof aggregate trench systems.

The lightweight chamber segments available on themarket stack together compactly for efficienttransport. Some chambers interlock with ribswithout fasteners, cutting installation time bymore than 50 percent reused and conventionalgravel/pipe systems. Such systems can be reusedand relocated if the site owner decides to buildon another drain field site. A key disadvantage ofleaching chambers compared to gravel drainfields is that they can be more expensive if alow-cost source of gravel is readily available.

Porous media should be placed along the chambersidewall area to a minimum compacted height of8 inches above the trench bottom. Additional backfillis placed to a minimum compacted height of 6 to12inches above the chamber, depending on the chamberstrength. Individual chamber trench bottoms shouldbe leveled in all directions and follow the contour ofthe ground surface elevation without any dams orother water stops. The manufacturer’s installationinstructions should be followed, and systems shouldbe installed by an authorized contractor.

Dosed flow distributionDosed-flow distribution systems are a significantimprovement over gravity-flow distribution systems.The design of dosed-flow systems (figure 4-13)includes both the distribution network and thedosing equipment (see table 4-6). Dosing achievesbetter distribution of the wastewater effluent overthe infiltration surface than gravity flow systems andprovides intervals between doses when no wastewateris applied. As a result, dosed-flow systems reduce therate of soil clogging, more effectively maintainunsaturated conditions in the subsoil (to effect goodtreatment through extended residence times andincreased reaeration potential), and provide a meansto manage wastewater effluent applications to theinfiltration system (Hargett et al., 1982). They can beused in any application and should be the method ofchoice. Unfortunately, they are commonly perceivedto be less desirable because they add a mechanical

Table 4-6. Dosing methods and devices.

Source: National Small Flows Clearinghouse

Figure 4-13. Typical pressurized distribution system layout



component to an otherwise “passive” system andadd cost because of the dosing equipment. Theimproved performance of dosed-flow systems overgravity flow systems should outweigh these perceiveddisadvantages, especially when a managemententity is in place. It must be noted, however, that ifdosed infiltration systems are allowed to pond, theadvantages of dosing are lost because the bottominfiltration surface is continuously inundated andno longer allowed to rest and reaerate. Therefore,there is no value in using dosed-flow distribution inSWISs designed to operate ponded, such as systemsthat include sidewall area as an active infiltrationsurface or those using serial relief lines.

Perforated pipe

Four-inch perforated pipe networks (with orwithout d-boxes or pressure manifolds) that receivedosed-flow applications are designed no differentlythan gravity-flow systems. Many of the advantagesof dosing are lost in such networks, however,because the distribution is only slightly better thanthat of gravity-flow systems (Converse, 1974).

Pressure manifold

A pressure manifold consists of a large-diameterpipe tapped with small outlet pipes that dischargeto gravity laterals (figure 4-14). A pump pressur-izes the manifold, which has a selected diameter toensure that pressure inside the manifold is the sameat each outlet. This method of flow division ismore accurate and consistent than a distributionbox, but it has the same shortcoming since flowafter the manifold is by gravity along each distribu-

tion lateral. Its most common application is todivide flow among multiple trenches constructed atdifferent elevations on a sloping site.

Table 4-7 can be used to size a pressure manifoldfor different applications (see sidebar). This table wasdeveloped by Berkowitz (1985) to size the manifolddiameter based on the spacing between pressure lateraltaps, the lateral tap diameter, and the number oflateral taps. The hydraulic computations made todevelop the table set a maximum flow differentialbetween laterals of 5 percent. The dosing rate isdetermined by calculating the flow in a single lateraltap assuming 1 to 4 feet of head at the manifoldoutlets and multiplying the result by the number oflateral taps. The Hazen-Williams equation for pipeflow can be used to make this calculation.

Pressure distribution is typically constructed ofSchedule 40 PVC pipe (figure 4-15). The lateraltaps are joined by tees. They also can be attachedby tapping (threading) the manifold pipe, but themanifold pipe must be Schedule 80 to provide athicker pipe wall for successful tapping. Valves oneach pressure tap are recommended to enable eachline to be taken out of service as needed by closingthe appropriate valve. This allows an opportunityto manage, rest, or repair individual lines. Toprevent freezing, the manifold can be drained backto the dose tank after each dose. If this is done, thevolume of water that will drain from the manifoldand forcemain must be added to the dose volume toachieve the desired dose.

Rigid pipe pressure network

Rigid pipe pressure distribution networks are usedto provide relatively uniform distribution of

Figure 4-14. Pressure manifold detail



Figure 4-15. Horizontal design for pressure distribution

Source: Washington Department of Health, 1998.

wastewater effluent over the entire infiltrationsurface simultaneously during each dose. They arewell suited for all dosed systems. Because theydeliver the same volume of wastewater effluent perlinear length of lateral, they can be used to dosemultiple trenches of unequal length. Although rigidpipe pressure networks can be designed to deliverequal volumes to trenches at different elevations(Mote, 1984; Mote et al., 1981; Otis, 1982), thesesituations should be avoided. Uniform distributionis achieved only when the network is fully pressur-ized. During filling and draining of the network,

the distribution lateral at the lowest elevationreceives more water. This disparity increases withincreasing dosing frequency. As an alternative onsloping sites, the SWIS could be divided intomultiple cells, with the laterals in each cell at thesame elevation. If this is not possible, otherdistribution designs should be considered.

The networks consist of solid PVC pipe manifoldsthat supply water to a series of smaller perforatedPVC laterals (figure 4-16). The laterals are de-signed to discharge nearly equal volumes of

Table 4-7. Pressure manifold sizing

Source: Adapted from Berkowitz, 1985.



wastewater from each orifice in the network whenfully pressurized. This is accomplished by main-taining a uniform pressure throughout the networkduring dosing. The manifolds and laterals are sizedrelative to the selected orifice size and spacing toachieve uniform pressure. A manual flushingmechanism should be included to enable periodicflushing of slimes and other solids that accumulatein the laterals.

Design of dosed flow systems

A simplified method of network design has beendeveloped (Otis, 1982). Lateral and manifoldsizing is determined using a series of graphs andtables after the designer has selected the desiredorifice size and spacing and the distal pressure inthe network (typically 1 to 2 feet of head). Thesegraphs and tables were derived by calculating thechange in flow and pressure at each orifice betweenthe distal and proximal ends of the network. Themethod is meant to result in discharge rates fromthe first and last orifices that differ by no morethan 10 percent in any lateral and 15 percent acrossthe entire network. However, subsequent testing offield installations indicated that the design modeloverestimates the maximum lateral length by asmuch as 25 percent (Converse and Otis, 1982).Therefore, if the graphs and tables are used, themaximum lateral length for any given orifice sizeand spacing should not exceed 80 percent of themaximum design length suggested by the lateralsizing graphs. In lieu of using the graphs andtables, a spreadsheet could be written using theequations presented and adjusting the orificedischarge coefficient.

Pressure manifold designPressure manifold designPressure manifold designPressure manifold designPressure manifold designA SWIS consisting of 12 trenches of equal length is to be constructed on a slope. To divide the septic tankeffluent equally among the 12 trenches, a pressure manifold is to be used. The lateral taps are to be spaced 6inches apart on one side of the manifold.

Table 4-7 can be used to size the manifold. Looking down the series of columns under the Single-sided manifold,up to sixteen ½-inch taps could be made to a 4-inch manifold. Therefore, a 4-inch manifold would be acceptable. If¾- or 1-inch taps were used, a 6-inch manifold would be necessary.

Using the orifice equation, the flow from each lateral tap can be estimated by assuming an operating pressure inthe manifold:

Q = Ca(2gh)2

where Q is the lateral discharge rate, C is a dimensionless coefficient that varies with the characteristics of theorifice (0.6 for a sharp-edged orifice), a is the area of the orifice, g is the acceleration due to gravity, and h is theoperating pressure within the manifold. In English units using a 0.6 orifice coefficient, this equation becomes

Q = 11.79 d2hd1/2

where Q is the discharge rate in gallons per minute, d is the orifice diameter in inches, and h is the operatingpressure in feet of water.

Assuming ½-inch taps with a operating pressure of 3 feet of water, the discharge rate from each outlet is

Q = 11.79 (½)2 31/2 = 5.1 gpm

Thus, the pump must be capable of delivering 12 x 5.1 gpm or approximately 60 gpm against an operatingpressure of 3 feet of water plus the static lift and friction losses incurred in the forcemain to the pressuremanifold.

Septic TankPumping (Dosing)

Chamber

EffluentPump

Small DiameterPressure Distribution

Cleanout

Figure 4-16. Rigid pipe pressure distribution networks with flushingcleanouts



To achieve uniform distribution, the density oforifices over the infiltration surface should be ashigh as possible. However, the greater the numberof orifices used, the larger the pump must be toprovide the necessary dosing rate. To reduce thedosing rate, the orifice size can be reduced, but thesmaller the orifice diameter, the greater the risk oforifice clogging. Orifice diameters as small as 1/8inch have been used successfully with septic tankeffluent when an effluent screen is used at theseptic tank outlet. Orifice spacings typically are 1.5to 4 feet, but the greater the spacing, the lessuniform the distribution because each orificerepresents a point load. It is up to the designer toachieve the optimum balance between orificedensity and pump size.

The dose volume is determined by the desiredfrequency of dosing and the size of the network.Often, the size of the network will control design.During filling and draining of the network at thestart and end of each dose, the distribution is lessuniform. The first holes in the network dischargemore during initial pressurization of the network,and the holes at the lowest elevation dischargemore as the network drains after each dose. To

Design procedure for rigid pipe pressure distribution networkDesign procedure for rigid pipe pressure distribution networkDesign procedure for rigid pipe pressure distribution networkDesign procedure for rigid pipe pressure distribution networkDesign procedure for rigid pipe pressure distribution network

The simplified design procedure for rigid pipe pressure networks as presented by Otis (1982) includes thefollowing steps:

1. Lay out the proposed network.

2. Select the desired orifice size and spacing. Maximize the density of orifices over the infiltration surface,keeping in mind that the dosing rate increases as the orifice size increases and the orifice spacingdecreases.

3. Determine the appropriate lateral pipe diameter compatible with the selected orifice size and spacing using aspreadsheet or sizing charts from Otis (1982).

4. Calculate the lateral discharge rate using the orifice discharge equation (0.48 discharge coefficient or 80percent of 0.6).

5. Determine the appropriate manifold size based on the number, spacing, and discharge rate of the lateralsusing a spreadsheet or sizing table from Otis (1982).

6. Determine the dose volume required. Use either the minimum dose volume equal to 5 times the networkvolume or the expected daily flow divided by the desired dosing frequency, whichever is larger.

7. Calculate the minimum dosing rate (the lateral discharge times the number of laterals).

8. Select the pump based on the required dosing rate and the total dynamic head (sum of the static lift, frictionlosses in the forcemain to the network, and the network losses, which are equal to 1.3 times the networkoperating pressure).

minimize the relative difference in dischargevolumes, the dose volume should be greater thanfive times the volume of the distribution network(Otis, 1982). A pump or siphon can be used topressurize the network.

Dripline pressure network

Drip distribution, which was derived from dripirrigation technology, was recently introduced as amethod of wastewater distribution. It is a methodof pressure distribution capable of delivering small,precise volumes of wastewater effluent to theinfiltration surface. It is the most efficient of thedistribution methods and is well suited for all typesof SWIS applications. A dripline pressure networkconsists of several components:

• Dose tank

• Pump

• Prefilter

• Supply manifold

• Pressure regulator (when turbulent, flowemitters are used)



• Dripline

• Emitters

• Vacuum release valve

• Return manifold

• Flush valve

• Controller

The pump draws wastewater effluent from the dosetank, preferably on a timed cycle, to dose thedistribution system. Before entering the network,the effluent must be prefiltered through mechanicalor granular medium filters. The former are usedprimarily for large SWIS systems. The backflushwater generated from a self-cleaning filter shouldbe returned to the headworks of the treatmentsystem. The effluent enters the supply manifoldthat feeds each dripline (figure 4-17). If turbulentflow emitters are used, the filtered wastewater mustfirst pass through a pressure regulator to control the

maximum pressure in the dripline. Usually, thedripline is installed in shallow, narrow trenches 1 to 2feet apart and only as wide as necessary to insertthe dripline using a trenching machine or vibratoryplow. The trench is backfilled without any porousmedium so that the emitter orifices are in directcontact with the soil. The distal ends of eachdripline are connected to a return manifold. Thereturn manifold is used to regularly flush thedripline. To flush, a valve on the manifold isopened and the effluent is flushed through thedriplines and returned to the treatment systemheadworks.

Because of the unique construction of drip distribu-tion systems, they cause less site disruption duringinstallation, are adaptable to irregularly shaped lotsor other difficult site constraints, and use more ofthe soil mantle for treatment because of the shallowdepth of placement. Also, because the installed costper linear foot of dripline is usually less than thecost of conventional trench construction, driplinecan be added to decrease mass loadings to theinfiltration surface at lower costs than otherdistribution methods. Because of the equipmentrequired, however, drip distribution tends to bemore costly to construct and requires regularoperation and maintenance by knowledgeableindividuals. Therefore, it should be considered foruse only where operation and maintenance supportis ensured.

The dripline is normally a ½-inch-diameter flexiblepolyethylene tube with emitters attached to theinside wall spaced 1 to 2 feet apart along its length.Because the emitter passageways are small, frictionlosses are large and the rate of discharge is low(typically from 0.5 to nearly 2 gallons per hour).

Two types of emitters are used. One is a “turbulent-flow” emitter, which has a very long labyrinth.Flow through the labyrinth reduces the dischargepressure nearly to atmospheric rates. With increas-ing in-line pressure, more wastewater can be forcedthrough the labyrinth. Thus, the discharges fromturbulent flow emitters are greater at higherpressures (figure 4-18). To more accurately controlthe rate of discharge, a pressure regulator isinstalled in the supply manifold upstream of thedripline. Inlet pressures from a minimum of 10 psito a maximum of 45 psi are recommended. Thesecond emitter type is the pressure-compensating

Figure 4-17. Pressure manifold and flexible drip linesprior to trench filling




emitter. This emitter discharges at nearly a constantrate over a wide range of in-line pressures (fig-ure 4-18).

Head losses through driplines are high because ofthe small diameter of the tubing and its in-lineemitters, and therefore dripline lengths must belimited. Manufacturers limit lengths at variousemitter spacings. With turbulent flow emitters, thedischarge from each successive emitter diminishesin response to pressure loss created by friction orby elevation changes along the length of thedripline. With pressure-compensating emitters, thein-line pressure should not drop below 7 to 10 psiat the final emitter. The designer is urged to workwith manufacturers to ensure that the system meetstheir requirements.

Pressure-compensating emitters are somewhat moreexpensive but offer some important advantagesover turbulent-flow emitters for use in onsitewastewater systems. Pressure-compensatingdripline is better suited for sloping sites or siteswith rolling topography where the dripline cannotbe laid on contour. Turbulent-flow emitters dis-charge more liquid at lower elevations than thesame emitters at higher elevations. The designershould limit the difference in discharge ratesbetween emitters to no more than 10 percent. Also,because the discharge rates are equal when underpressure, monitoring flow rates during dosing of apressure-compensating dripline network canprovide an effective way to determine whetherleaks or obstructions are present in the network oremitters. Early detection is important so that simpleand effective corrective actions can be taken.Usually, injection of a mild bleach solution into thedripline is effective in restoring emitter perfor-mance if clogging is due to biofilms. If this actionproves to be unsuccessful, other corrective actionsare more difficult and costly. An additional advan-tage of pressure-compensating emitters is thatpressure regulators are not required. Finally, whenoperating in their normal pressure range, pressure-compensating emitters are not affected by soilwater pressure in structured soils, which can causeturbulent-flow emitters to suffer reduced dosingvolumes.

Controlling clogging in drip systemsWith small orifices, emitters are susceptible toclogging. Particulate materials in the wastewater,soil particulates drawn into an emitter when thedripline drains following a dose, and biologicalslimes that grow within the dripline pose potentialclogging problems. Also, the moisture and nutrientsdischarged from the emitters may invite rootintrusion through the emitter. Solutions to theseproblems lie in both the design of the dripline andthe design of the distribution network. Emitterhydrodynamic design and biocide impregnation ofthe dripline and emitters help to minimize some ofthese problems. Careful network design is alsonecessary to provide adequate safeguards. Monitor-ing allows the operator to identify other problemssuch as destruction from burrowing animals.

To control emitter clogging, appropriate engineer-ing controls must be provided. These includeprefiltration of the wastewater, regular driplineflushing, and vacuum release valves on the net-work. Prefiltration of the effluent through granularor mechanical filters is necessary. These filtersshould be capable of removing all particulates thatcould plug the emitter orifices. Dripline manufactur-ers recommend that self-cleaning filters be designedto remove particles larger than 100 to 115 microns.Despite this disparate experience, pretreatment withfilters is recommended in light of the potential costof replacing plugged emitters. Regular cleaning ofthe filters is necessary to maintain satisfactoryperformance. The backflush water should bereturned to the head of the treatment works.

Figure 4-18. Turbulent-flow and pressure-compensating emitterdischarge rates versus in-line pressure



The dripline must be flushed on a regular scheduleto keep it scoured of solids. Flushing is accom-plished by opening the flush valve on the returnmanifold and increasing the pumping rate toachieve scouring velocity. Each supplier recom-mends a velocity and procedure for this process.The flushing rate and volume must include waterlosses (discharge) through the emitters during theflushing event. Both continuous flushing and timedflushing are used. However, flushing can add asignificant hydraulic load to the treatment systemand must be considered in the design. If intermit-tent flushing is practiced, flushing should beperformed at least monthly.

Aspiration of soil particles is another potentialemitter clogging hazard. Draining of the networkfollowing a dosing cycle can create a vacuum in thenetwork. The vacuum can cause soil particles to beaspirated into the emitter orifices. To prevent thisfrom occurring, vacuum relief valves are used. It isbest to install these at the high points of both thesupply and return manifolds.

Placement and layout of drip systems

When drip distribution was introduced, the ap-proach to sizing SWISs using this distributionmethod was substantially different from that forSWISs using other distribution methods. Manufac-turer-recommended hydraulic loading rates wereexpressed in terms of gallons per day per squarefoot of drip distribution footprint area. Typically,the recommended rates were based on 2-footemitter and dripline spacing. Therefore, eachemitter would serve 4 square feet of footprint area.Because the dripline is commonly plowed into thesoil without surrounding it with porous medium,the soil around the dripline becomes the actualinfiltration surface. The amount of infiltrationsurface provided is approximately 2/3 to 1 squarefoot per 5 linear feet of dripline. As a result, thewastewater loading rate is considerably greater thanthe hydraulic loadings recommended for traditionalSWISs. Experience has shown however, that thehydraulic loading on this surface can be as much asseven times higher than that of traditional SWISdesigns (Ayres Associates, 1994). This is probablydue to the very narrow geometry, higher levels ofpretreatment, shallow placement, and intermittentloadings of the trenches, all of which help toenhance reaeration of the infiltration surface.

The designer must be aware of the differencesbetween the recommended hydraulic loadings fordrip distribution and those customarily used fortraditional SWISs. The recommended drip distribu-tion loadings are a function of the soil, driplinespacing, and applied effluent quality. It is necessaryto express the hydraulic loading in terms of thefootprint area because the individual dripline trenchesare not isolated infiltration surfaces. If the emitterand/or dripline spacing is reduced, the wettingfronts emanating from each emitter could overlapand significantly reduce hydraulic performance. There-fore, reducing the emitter and/or dripline spacing shouldnot reduce the overall required system footprint.Reducing the spacing might be beneficial for irrigat-ing small areas of turf grass, but the maximum dailyemitter discharge must be reduced proportionately byadding more dripline to maintain the same footprintsize. Using higher hydraulic loading rates must becarefully considered in light of secondary boundaryloadings, which could result in excessive groundwater mounding (see chapter 5). Further, the instanta-neous hydraulic loading during a dose must becontrolled because storage is not provided in thedripline trench. If the dose volume is too high, thewastewater can erupt at the ground surface.

Layout of the drip distribution network must beconsidered carefully. Two important consequencesof the network layout are the impacts on dosepump sizing necessary to achieve adequate flushingflows and the extent of localized overloading dueto internal dripline drainage. Flushing flow ratesare a function of the number of manifold/driplineconnections: More connections create a need forgreater flushing flows, which require a largerpump. To minimize the flushing flow rate, thelength of each dripline should be made as long aspossible in accordance with the manufacturer’srecommendations. To fit the landscape, the driplinecan be looped between the supply and returnmanifolds (figure 4-19). Consideration should alsobe given to dividing the network into more thanone cell to reduce the number of connections in anindividual network. A computer program has beendeveloped to evaluate and optimize the hydraulicdesign for adequate flushing flows of driplinenetworks that use pressure-compensating emitters(Berkowitz and Harman, 1994).

Internal drainage that occurs following each doseor when the soils around the dripline are saturated



can cause significant hydraulic overloading tolower portions of the SWIS. Following a dosecycle, the dripline drains through the emitters. Onsloping sites, the upper driplines drain to the lowerdriplines, where hydraulic overloading can occur.Any free water around the dripline can enterthrough an emitter and drain to the lowest eleva-tion. Each of these events needs to be avoided asmuch as possible through design. The designer canminimize internal drainage problems by isolatingthe driplines from each other in a cell, by aligningthe supply and return manifolds with the site’scontours. A further safeguard is to limit the numberof doses per day while keeping the instantaneoushydraulic loadings to a minimum so the driplinetrench is not flooded following a dose. This trade-off is best addressed by determining the maximumhydraulic loading and adjusting the number ofdoses to fit this dosing volume.

Freezing of dripline networks has occurred insevere winter climates. Limited experience indicatesthat shallow burial depths together with a lack ofuncompacted snow cover or other insulatingmaterials might lead to freezing. In severe winter

climates, the burial depth of dripline should beincreased appropriately and a good turf grassestablished over the network. Mulching the area thewinter after construction or every winter should beconsidered. Also, it is good practice to install thevacuum release valves below grade and insulate theair space around them. Although experience withdrip distribution in cold climates is limited, thesesafeguards should provide adequate protection.

Dosing methods

Two methods of dosing have been used (table 4-6).With on-demand dosing, the wastewater effluentrises to a preset level in the dose tank and the pumpor siphon is activated by a float switch or othermechanism to initiate discharge (figure 4-20).During peak-flow periods, dosing is frequent withlittle time between doses for the infiltration systemto drain and the subsoil to reaerate. During low-flow periods, dosing intervals are long, which canbe beneficial in controlling biomat developmentbut is inefficient in using the hydraulic capacity ofthe system.

Figure 4-19. Dripline layout on a site with trees

Source: Adapted from American Manufacturing, 2001.



Timed dosing overcomes some of the shortcomingsof on-demand dosing. Timers are used to turn thepump on and off at specified intervals so that onlya predetermined volume of wastewater is dischargedwith each dose. Timed dosing has two distinctadvantages over on-demand dosing. First, the dosescan be spaced evenly over the entire 24-hour day tooptimize the use of the soil’s treatment capacity.Second, the infiltration system receives no morethan its design flow each day. Clear water infiltra-tion, leaking plumbing fixtures, or excessive wateruse are detected before the excess flow is dischargedto the infiltration system because the dose tank willeventually fill to its high water alarm level. At thatpoint, the owner has the option of calling a septagepumper to empty the tanks or activating the pump todose the system until the problem is diagnosed andcorrected. Unlike on-demand dosing, timed dosingrequires that the dose tank be sized to store peakflows until they can be pumped (see sidebar).

Dosing frequency and volume are two importantdesign considerations. Frequent, small doses arepreferred over large doses one or two times perday. However, doses should not be so frequent thatdistribution is poor. This is particularly true witheither of the pressure distribution networks. Withpressure networks, uniform distribution does notoccur until the entire network is pressurized. Toensure pressurization and to minimize unequaldischarges from the orifices during filling anddraining, a dose volume equal to five times the

network volume is a good rule of thumb. Thus,doses can be smaller and more frequent with driplinenetworks than with rigid pipe networks because thevolume of drip distribution networks is smaller.

4.4.8 SWIS media

A porous medium is placed below and around SWISdistribution piping to expand the infiltration surfacearea of the excavation exposed to the applied waste-water. This approach is similar in most SWIS designs,except when drip distribution or aggregate-freedesigns are used. In addition, the medium alsosupports the excavation sidewalls, provides storage ofpeak wastewater flows, minimizes erosion of theinfiltration surface by dissipating the energy of theinfluent flow, and provides some protection for thepiping from freezing and root penetration.

Traditionally, washed gravel or crushed rock,typically ranging from ¾ to 2½ inches in diam-eter, has been used as the porous medium. Therock should be durable, resistant to slaking anddissolution, and free of fine particles. A hardnessof at least 3 on the Moh’s scale of hardness issuggested. Rock that can scratch a copper pennywithout leaving any residual meets this criterion.It is important that the medium be washed toremove fine particles. Fines from insufficientlywashed rock have been shown to result in signifi-cant reductions in infiltration rates (Amerson etal., 1991). In all applications where gravel isused, it must be properly graded and washed.Improperly washed gravel can contribute fines andother material that can plug voids in the infiltra-tive surface and reduce hydraulic capability.Gravel that is embedded into clay or fine soilsduring placement can have the same effect.

In addition to natural aggregates, gravelless systemshave been widely used as alternative SWIS medium(see preceding section). These systems take manyforms, including open-bottomed chambers, fabric-wrapped pipe, and synthetic materials such asexpanded polystyrene foam chips, as described inthe preceding section. Systems that provide an openchamber are sometimes referred to as “aggregate-free” systems, to distinguish them from others thatsubstitute lightweight medium for gravel or stone.These systems provide a suitable substitute inlocales where gravel is not available or affordable.Some systems (polyethylene chambers and light-

Figure 4-20. Pumping tank (generic)

Source: Purdue University, 1990



Dose tank sizing for timed dosingDose tank sizing for timed dosingDose tank sizing for timed dosingDose tank sizing for timed dosingDose tank sizing for timed dosing

Timed dosing to a SWIS is to be used in an onsite system serving a restaurant in a summer resort area. Timeddosing will equalize the flows, enhancing treatment in the soil and reducing the required size of the SWIS.

The restaurant serves meals from 11 a.m. to 12 midnight Tuesday through Saturday and from9 a.m. to 2 p.m. Sundays. The largest number of meals is served during the summer weekends. The restaurant isclosed on Mondays. The metered water use is as follows:

Average weekly water use (summer) 17,500 gal

Peak weekend water use (4 p.m. Friday to 2 p.m. Sunday) 9,500 gal

The dose tank will be sized to equalize flows over a 7-day period. The dosing frequency is to be six times daily orone dose every 4 hours. Therefore, the dose volume will be

Dose volume = 17,500 gal/wk ¸ (7 d/wk x 6 doses/day) = 417 gal/dose

The necessary volume of the dose tank to store the peak flows and equalize the flow to the SWIS over the 7-dayweek can be determined graphically.

The accumulated water use over the week and the daily dosing rate (6 doses/day x 417 gal/dose = 2,500 gpd) isplotted on the graph. Lines parallel to the dosing rate are drawn tangent to points 1 and 2 representing themaximum deviations of the water use line above and below the dosing rate line. The volume represented by thedifference between the two parallel lines is the tank volume needed to achieve flow equalization. A 4,500-gallontank would be required.

Both siphons and pumps can be used for dosing distribution networks. Only drip distribution networks cannot bedosed by siphons because of the higher required operating pressures and the need to control instantaneoushydraulic loadings (dose volume). Siphons can be used where power is not available and elevation is adequate toinstall the siphon sufficiently above the distribution network to overcome friction losses in the forcemain andnetwork. Care must be taken in their selection and installation to ensure proper performance. Also, owners mustbe aware that siphon systems require routine monitoring and occasional maintenance. “Dribbling” can occur whenthe siphon bell becomes saturated, suspending dosing and allowing the wastewater effluent to trickle out underthe bell. Dribbling can occur because of leaks in the bell or a siphon out of adjustment. Today, pumps are favoredover siphons because of the greater flexibility in site selection and dosing regime.




weight aggregate systems) can also offer substantialadvantages in terms of reduced site disruption overthe traditional gravel because their light weightmakes them easy to handle without the use ofheavy equipment. These advantages reduce laborcosts, limit damage to the property by machinery,and allow construction on difficult sites whereconventional medium could not reasonably be used.

4.5 Construction management andcontingency options

Onsite wastewater systems can and do fail toperform at times. To avoid threats to public healthand the environment during periods when a systemmalfunctions hydraulically, contingency plansshould be made to permit continued use of thesystem until appropriate remedial actions can betaken. Contingency options should be consideredduring design so that the appropriate measures aredesigned into the original system. Table 4-8 listscommon contingency options.

4.5.1 Construction considerationsConstruction practices are critical to the perfor-mance of SWISs. Satisfactory SWIS performancedepends on maintaining soil porosity. Construc-tion activities can significantly reduce the porosityand cause SWISs to hydraulically fail soon afterbeing brought into service. Good constructionpractices should carefully consider site protectionbefore and during construction, site preparation,and construction equipment selection and use.Good construction practices for at-grade andmound systems can be found elsewhere (Converseand Tyler, 2000; Converse et al., 1990). Many ofthem, however, are similar to those described inthe following subsections.

Site protection

Construction of the onsite wastewater system isoften only one of many construction activities thatoccur on a property. If not protected againstintrusion, the site designated for the onsite systemcan be damaged by other, unrelated construction

Table 4-8. Contingency options for SWIS malfunctions



activities. Therefore, the site should be staked androped off before any construction activities beginto make others aware of the site and to keep trafficand materials stockpiles off the site.

The designer should anticipate what activities willbe necessary during construction and designateacceptable areas for them to occur. Site accesspoints and areas for traffic lanes, material stockpil-ing, and equipment parking should be designatedon the drawings for the contractor.

Site preparationSite preparation activities include clearing andsurface preparation for filling. Before these activi-ties are begun, the soil moisture should be deter-mined. In nongranular soils, compaction will occurif the soil is near its plastic limit. This can be testedby removing a sample of soil and rolling it betweenthe palms of the hands. If the soil fails to form a“rope” the soil is sufficiently dry to proceed.However, constant care should be taken to avoidsoil disturbance as much as possible.

Clearing

Clearing should be limited to mowing and rakingbecause the surface should be only minimallydisturbed. If trees must be removed, they should becut at the base of the trunk and removed withoutheavy machinery. If it is necessary to remove thestumps, they should be ground out. Grubbing ofthe site (mechanically raking away roots) should beavoided. If the site is to be filled, the surfaceshould be moldboard- or chisel-plowed parallel tothe contour (usually to a depth of 7 to 10 inches)when the soil is sufficiently dry to ensure maxi-mum vertical permeability. The organic layershould not be removed. Scarifying the surface withthe teeth of a backhoe bucket is not sufficient.

Excavation

Excavation activities can cause significant reduc-tions in soil porosity and permeability (Tyler et al.,1985). Compaction and smearing of the soilinfiltrative surface occur from equipment trafficand vibration, scraping actions of the equipment, andplacement of the SWIS medium on the infiltrationsurface. Lightweight backhoes are most commonlyused. Front-end loaders and blades should not be used

because of their scraping action. All efforts shouldbe made to avoid any disturbance to the exposedinfiltration surface. Equipment should be kept offthe infiltration field. Before the SWIS medium isinstalled, any smeared areas should be scarified andthe surface gently raked. If gravel or crushed rockis to be used for SWIS medium, the rock should beplaced in the trench by using the backhoe bucketrather than dumping it directly from the truck. Ifdamage occurs, it might be possible to restore thearea, but only by removing the compacted layer. Itmight be necessary to remove as much as 4 inchesof soil to regain the natural soil porosity andpermeability (Tyler et al., 1985). Consequences ofthe removal of this amount of soil over the entireinfiltration surface can be significant. It will reducethe separation distance to the restrictive horizonand could place the infiltration surface in anunacceptable soil horizon.

To avoid potential soil damage during construction,the soil below the proposed infiltration surfaceelevation must be below its plastic limit. Thisshould be tested before excavation begins. Also,excavation should be scheduled only when theinfiltration surface can be covered the same day toavoid loss of permeability from wind-blown silt orraindrop impact. Another solution is to use light-weight gravelless systems, which reduce thedamage and speed the construction process.

Before leaving the site, the area around the siteshould be graded to divert surface runoff from theSWIS area. The backfill over the infiltrationsurface should be mounded slightly to account forsettling and eliminate depressions over the systemthat can pond water. Finally, the area should beseeded and mulched.

4.5.2 Operation, maintenance, andmonitoring

Subsurface wastewater infiltration systems requirelittle operator intervention. Table 4-9 lists typicaloperation, maintenance, and monitoring activitiesthat should be performed. However, more complexpretreatment, larger and more variable flows, andhigher-risk installations increase the need formaintenance and monitoring. More information isprovided in the USEPA draft Guidelines for Onsite/Decentralized Wastewater Systems (2000) and in thechapter 4 fact sheets.



4.5.3 Considerations for large andcommercial systems

Designs for systems treating larger flows follow thesame guidelines used for residential systems, but theymust address characteristics of the wastewater to betreated, site characteristics, infiltration surface sizing,and contingency planning more comprehensively.

Wastewater characteristics

Wastewaters from cluster systems serving multiplehomes or commercial establishments can differsubstantially in flow pattern and waste strength fromwastewaters generated by single family residences.The ratio of peak to average daily flow from residen-tial clusters is typically much lower than what istypical from single residences. This is because themoderating effect associated with combining multiplewater use patterns reduces the daily variation in flow.Commercial systems, on the other hand, can varysignificantly in wastewater strength. Typically,restaurants have high concentrations of grease andBOD, laundromats have high sodium and suspendedsolids concentrations, and toilet facilities at parksand rest areas have higher concentrations of BOD,TSS, and nitrogen. These differences in daily flowpatterns and waste strengths must be dealt with inthe design of SWISs. Therefore, it is important to

characterize the wastewater fully before initiatingdesign (see chapter 3).

Site characteristics

The proposed site for a SWIS that will treat waste-water from a cluster of homes or a commercialestablishment must be evaluated more rigorouslythan a single-residence site because of the largervolume of water that is to be applied and thegreater need to determine hydraulic gradients anddirection. SWIS discharges can be from 10 to morethan 100 times the amount of water that the soilinfiltration surface typically receives from precipi-tation. For example, assume that an area receives anaverage of 40 inches of rainfall per year. Of that, lessthan 25 percent (about 10 inches annually) infiltratesand even less percolates to the water table. A waste-water infiltration system is designed to infiltrate0.4 to 1.6 inches per day, or 146 to 584 inches peryear. Assuming actual system flows are 30 percentof design flows, this is reduced to 44 to 175 inchesper year even under this conservative approach.

The soils associated with small systems can usuallyaccommodate these additional flows. However,systems that treat larger flows load wastewaters tothe soil over a greater area and might exceed thesite’s capacity to accept the wastewater. Restrictivehorizons that may inhibit deep percolation need to

Table 4-9. Operation, maintenance, and monitoring activities



be identified before design. Ground water moundinganalysis should be performed to determine whetherthe hydraulic loading to the saturated zone (second-ary design boundary), rather than the loading to theinfiltration surface, controls system sizing (see Chap-ter 5). If the secondary boundary controls design, thesize of the infiltration surface, its geometry, and evenhow wastewater is applied will be affected.

Infiltration surface sizing

Selection of the design flow is a very importantconsideration in infiltration surface sizing. Statecodified design flows for residential systemstypically are 2 to 5 times greater than the averagedaily flow actually generated in the home. Thisoccurs because the design flow is usually based onthe number of bedrooms rather than the number ofoccupants. As a result, the actual daily flow is oftena small fraction of the design flow.

This is not the case when the per capita flows forthe population served or metered flows are used asthe design flow. In such instances, the ratio ofdesign flow to actual daily flow can approachunity. This is because the same factors of safety aretypically not used to determine the design flow. Initself, this is not a problem. The problem ariseswhen the metered or averaged hydraulic loadingrates are used to size the infiltration surface. Theserates can be more than two times what the soilbelow the undersized system is actually able toaccept. As a result, SWISs would be significantlyundersized. This problem is exacerbated where thewaste strength is high.

To avoid the problem of undersizing the infiltrationsurface, designs must compensate in some way.Factors of safety of up to 2 or more could beapplied to accurate flow estimates, but the morecommon practice is to design multiple cells thatprovide 150 to 200 percent of the total estimatedinfiltration surface needed. Multiple cells are agood approach because the cells can be rotated intoservice on a regular schedule that allows the cellstaken out of service to rest and rejuvenate theirhydraulic capacity. Further, the system providesstandby capacity that can be used when malfunc-tions occur, and distribution networks are smallerto permit smaller and more frequent dosing,thereby maximizing oxygen transfer and thehydraulic capacity of the site. For high-strengthwastewaters, advanced pretreatment can be speci-

fied or the infiltration surface loadings can beadjusted (see Special Issue Fact Sheet 4).

Contingency planning

Malfunctions of systems that treat larger flows cancreate significant public health and environmentalhazards. Therefore, adequate contingency planningis more critical for these systems than for residen-tial systems. Standby infiltration cells, timeddosing, and flow monitoring are key designelements that should be included. Also, professionalmanagement should be required.

4.6 Septic tanks

The septic tank is the most commonly used waste-water pretreatment unit for onsite wastewater systems.Tanks may be used alone or in combination withother processes to treat raw wastewater before it isdischarged to a subsurface infiltration system. Thetank provides primary treatment by creating quiescentconditions inside a covered, watertight rectangular,oval, or cylindrical vessel, which is typically buried.In addition to primary treatment, the septic tank storesand partially digests settled and floating organic solidsin sludge and scum layers. This can reduce the sludgeand scum volumes by as much as 40 percent, and itconditions the wastewater by hydrolyzing organicmolecules for subsequent treatment in the soil or byother unit processes (Baumann et al., 1978). Gasesgenerated from digestion of the organics are ventedback through the building sewer and out of the houseplumbing stack vent. Inlet structures are designed tolimit short circuiting of incoming wastewater acrossthe tank to the outlet, while outlet structures (e.g., asanitary “tee” fitting) retain the sludge and scumlayers in the tank and draw effluent only from theclarified zone between the sludge and scum layers.The outlet should be fitted with an effluent screen(commonly called a septic tank filter) to retain largersolids that might be carried in the effluent to theSWIS, where it could contribute to clogging andeventual system failure. Inspection ports and manwaysare provided in the tank cover to allow access forperiodically removing the tank contents, including theaccumulated scum and sludge (figure 4-21). Adiagram of a two-compartment tank is shown laterin this section.

Septic tanks are used as the first or only pretreat-ment step in nearly all onsite systems regardless of



daily wastewater flow rate or strength. Othermechanical pretreatment units may be substituted forseptic tanks, but even when these are used septictanks often precede them. The tanks passivelyprovide suspended solids removal, solids storageand digestion, and some peak flow attenuation.

4.6.1 TreatmentA septic tank removes many of the settleable solids,oils, greases, and floating debris in the raw waste-water, achieving 60 to 80 percent removal(Baumann et al., 1978; Boyer and Rock, 1992;University of Wisconsin, 1978). The solids removedare stored in sludge and scum layers, where theyundergo liquefaction. During liquefaction, the firststep in the digestion process, acid-forming bacteria

partially digest the solids by hydrolyzing theproteins and converting them to volatile fatty acids,most of which are dissolved in the water phase. Thevolatile fatty acids still exert much of the biochemicaloxygen demand that was originally in the organicsuspended solids. Because these acids are in thedissolved form, they are able to pass from the tank inthe effluent stream, reducing the BOD removalefficiency of septic tanks compared to primary sedi-mentation. Typical septic tank BOD removal efficien-cies are 30 to 50 percent (Boyer and Rock, 1992;University of Wisconsin, 1978; see table 4-10). Com-plete digestion, in which the volatile fatty acids areconverted to methane, could reduce the amount of BODreleased by the tank, but it usually does not occur to asignificant extent because wastewater temperatures inseptic tanks are typically well below the optimumtemperature for methane-producing bacteria.

Gases that form from the microbial action in thetank rise in the wastewater column. The rising gasbubbles disturb the quiescent wastewater column,which can reduce the settling efficiency of the tank.They also dislodge colloidal particles in the sludgeblanket so they can escape in the water column. Atthe same time, however, they can carry active anaero-bic and facultative microorganisms that might helpto treat colloidal and dissolved solids present in thewastewater column (Baumann and Babbit, 1953).

Septic tank effluent varies naturally in qualitydepending on the characteristics of the wastewaterand condition of the tank. Documented effluentquality from single-family homes, small communi-ties and cluster systems, and various commercialseptic tanks is presented in tables 4-10 through 4-12.

Table 4-10. Characteristics of domestic septic tank effluent

Figure 4-21. Profile of a single-compartment septictank with outlet screen



4.6.2 Design considerations

The primary purpose of a septic tank is to providesuspended solids and oil/grease removal throughsedimentation and flotation. The important factorto achieving good sedimentation is maintainingquiescent conditions. This is accomplished byproviding a long wastewater residence time in theseptic tank. Tank volume, geometry, and compart-mentalization affect the residence time.

Volume

Septic tanks must have sufficient volume to providean adequate hydraulic residence time for sedimenta-tion. Hydraulic residence times of 6 to 24 hours havebeen recommended (Baumann and Babbitt, 1953:Kinnicutt et al., 1910). However, actual hydraulicresidence times can vary significantly from tank totank because of differences in geometry, depth, andinlet and outlet configurations (Baumann and Babbitt,1953). Sludge and scum also affect the residencetime, reducing it as the solids accumulate.

Table 4-12. Average septic tank effluent concentrations of selected parameters from various commercial establishmentsa

Table 4-11. Average septic tank effluent concentrations for selected parameters from small community and cluster systems



Most state and national plumbing codes specify thetank volume to be used based on the building sizeor estimated peak daily flow of wastewater. Table4-13 presents the tank volumes recommended inthe International Private Sewage Disposal Codespecified for one- and two-family residences (ICC,1995). The volumes specified are typical of mostlocal codes, but in many jurisdictions the minimumtank volume has been increased to 1,000 gallons ormore. For buildings other than one- or two-familyresidential homes, the rule of thumb often used forsizing tanks is to use two to three times the esti-

mated design flow. This conservative rule of thumbis based on maintaining a 24-hour minimumhydraulic retention time when the tank is ready forpumping, for example, when the tank is one-half totwo-thirds full of sludge and scum.

GeometryTank geometry affects the hydraulic residence timein the tank. The length-to-width ratio and liquiddepth are important considerations. Elongated tankswith length-to-width ratios of 3:1 and greater havebeen shown to reduce short-circuiting of the rawwastewater across the tank and improve suspendedsolids removal (Ludwig, 1950). Prefabricated tanksgenerally are available in rectangular, oval, andcylindrical (horizontal or vertical) shapes. Verticalcylindrical tanks can be the least effective becauseof the shorter distance between the inlets andoutlets. Baffles are recommended.

Among tanks of equal liquid volumes, the tankwith shallower liquid depths better reduces peakoutflow rates and velocities, so solids are less likelyto remain in suspension and be carried out of thetank in the effluent. This is because the shallowtank has a larger surface area. Inflows to the tankcause less of a liquid rise because of the largersurface area. The rate of flow exiting the tank(over a weir or through a pipe invert) is propor-

Figure 4-22. Two-compartment tank with effluent screen and surface risers

Source: Washington Department of Health, 1998.

Table 4-13. Septic tank capacities for one- and two-family dwellings (ICC, 1995).



tional to the height of the water surface over theinvert (Baumann et al., 1978; Jones, 1975). Also,the depth of excavation necessary is reduced withshallow tanks, which helps to avoid saturatedhorizons and lessens the potential for ground waterinfiltration or tank flotation. A typically specifiedminimum liquid depth below the outlet invert is 36inches. Shallower depths can disturb the sludgeblanket and, therefore, require more frequentpumping.

CompartmentalizationCompartmentalized tanks (figure 4-23) or tanksplaced in series provide better suspended solidsremoval than single-compartment tanks alone,although results from different studies vary(Baumann and Babbitt, 1953; Boyer and Rock,1992; Weibel et al., 1949, 1954; University ofWisconsin, 1978). If two compartments are used,better suspended solids removal rates are achievedif the first compartment is equal to one-half to two-thirds the total tank volume (Weibel et al., 1949,1954). An air vent between compartments must beprovided to allow both compartments to vent. Theprimary advantage of these configurations is whengas generated from organic solids digestion in thefirst compartment is separated from subsequentcompartments.

Inlets and outletsThe inlet and outlet of a septic tank are designed toenhance tank performance. Their respective invertelevations should provide at least a 2- to 3-inchdrop across the tank to ensure that the buildingsewer does not become flooded and obstructedduring high wastewater flows (figure 4-24). A clearspace of at least 9 inches should be provided abovethe liquid depth (outlet invert) to allow for scumstorage and ventilation. Both the inlet and outletare commonly baffled. Plastic sanitary tees are themost commonly used baffles. Curtain baffles(concrete baffles cast to the tank wall and fiberglassor plastic baffles bolted to the tank wall) have alsobeen used. The use of gasket materials that achievea watertight joint with the tank wall makes plasticsanitary tees easy to adjust, repair, or equip witheffluent screens or filters. The use of a removable,cleanable effluent screen connected to the outlet isstrongly recommended.

The inlet baffle is designed to prevent short-circuiting of the flow to the outlet by dissipatingthe energy of the influent flow and deflecting itdownward into the tank. The rising leg of the teeshould extend at least 6 inches above the liquidlevel to prevent the scum layer from plugging theinlet. It should be open at the top to allow ventingof the tank through the building sewer and out theplumbing stack vent. The descending leg shouldextend well into the clear space between the sludgeand scum layers, but not more than about 30 to 40percent of the liquid depth. The volume of thedescending leg should not be larger than 2 to 3gallons so that it is completely flushed to expelfloating materials that could cake the inlet. For thisreason, curtain baffles should be avoided.

Figure 4-23. Examples of septic tank effluent screens/filters

Source: Adapted from various manufacturers’ drawings.



The outlet baffle is designed to draw effluent fromthe clear zone between the sludge and scum layers.The rising leg of the tee should extend 6 inchesabove the liquid level to prevent the scum layerfrom escaping the tank. The descending leg shouldextend to 30 or 40 percent of the liquid depth.Effluent screens (commonly called septic tankfilters), which can be fitted to septic tank outlets,are commercially available. Screens prevent solidsthat either are buoyant or are resuspended from thescum or sludge layers from passing out of the tank(figures 4-22 and 4-23). Mesh, slotted screens, andstacked plates with openings from 1/32 to 1/8 inchare available. Usually, the screens can be fitted intothe existing outlet tee or retrofitted directly into theoutlet. An access port directly above the outlet isrequired so the screen can be removed for inspec-tion and cleaning.

Quality-assured, reliable test results have not shownconclusively that effluent screens result in effluentswith significantly lower suspended solids and BODconcentrations. However, they provide an excellent,low-cost safeguard against neutral-buoyancy solidsand high suspended solids in the tank effluentresulting from solids digestion or other upsets.Also, as the effluent screens clog over time, slowerdraining and flushing of home fixtures may alerthomeowners of the need for maintenance beforecomplete blockage occurs.

Tank access

Access to the septic tank is necessary for pumpingseptage, observing the inlet and outlet baffles, andservicing the effluent screen. Both manways andinspection ports are used. Manways are largeopenings, 18 to 24 inches in diameter or square. Atleast one that can provide access to the entire tankfor septage removal is needed. If the system iscompartmentalized, each compartment requires amanway. They are located over the inlet, the outlet,or the center of the tank. Typically, in the pastmanway covers were required to be buried understate and local codes. However, they should beabove grade and fitted with an airtight, lockablecover so they can be accessed quickly and easily.Inspection ports are 8 inches or larger in diameterand located over both the inlet and the outlet unlessa manway is used. They should be extended abovegrade and securely capped.

(CAUTION: The screen should not be removed forinspection or cleaning without first plugging theoutlet or pumping the tank to lower the liquid levelbelow the outlet invert. Solids retained on the screencan slough off as the screen is removed. Thesesolids will pass through the outlet and into theSWIS unless precautions are taken. This cautionshould be made clear in homeowner instructionsand on notices posted at the access port.)

Septic tank designs for large wastewater flows donot differ from designs for small systems. How-ever, it is suggested that multiple compartments ortanks in series be used and that effluent screens beattached to the tank outlet. Access ports andmanways should be brought to grade and providedwith locking covers for all large systems.

Construction materials

Septic tanks smaller than 6,000 gallons are typi-cally premanufactured; larger tanks are constructedin place. The materials used in premanufacturedtanks include concrete, fiberglass, polyethylene,and coated steel. Precast concrete tanks are by farthe most common, but fiberglass and plastic tanksare gaining popularity. The lighter weight fiber-glass and plastic tanks can be shipped longerdistances and set in place without cranes. Concretetanks, on the other hand, are less susceptible tocollapse and flotation. Coated steel tanks are nolonger widely used because they corrode easily.Tanks constructed in place are typically made ofconcrete.

Tanks constructed of fiberglass-reinforced polyester(FRP) usually have a wall thickness of about 1/4inch (6 millimeters). Most are gel- or resin-coatedto provide a smooth finish and prevent glass fibersfrom becoming exposed, which can cause wicking.Polyethylene tanks are more flexible than FRPtanks and can deform to a shape of structuralweakness if not properly designed. Concrete tankwalls are usually about 4 inches thick and rein-forced with no. 5 rods on 8-inch (20-centimeter)centers. Sulfuric acid and hydrogen sulfide, both ofwhich are present in varying concentrations inseptic tank effluent, can corrode exposed rods andthe concrete itself over time. Some plastics (e.g.,polyvinyl chloride, polyethylene, but not nylon)are virtually unaffected by acids and hydrogensulfide (USEPA, 1991).



Quality construction is critical to proper perfor-mance. Tanks must be properly designed, rein-forced, and constructed of the proper mix ofmaterials so they can meet anticipated loadswithout cracking or collapsing. All joints must bewatertight and flexible to accommodate soilconditions. For concrete tank manufacturing, a“best practices manual” can be purchased from theNational Pre-Cast Concrete Association (NPCA,1998). Also, a Standard Specification for PrecastConcrete Septic Tanks (C 1227) has been publishedby the American Society for Testing and Materials(ASTM, 1998).

Watertightness

Watertightness of the septic tank is critical to theperformance of the entire onsite wastewater system.Leaks, whether exfiltrating or infiltrating, areserious. Infiltration of clear water to the tank fromthe building storm sewer or ground water adds tothe hydraulic load of the system and can upsetsubsequent treatment processes. Exfiltration canthreaten ground water quality with partially treatedwastewater and can lower the liquid level below theoutlet baffle so it and subsequent processes canbecome fouled with scum. Also, leaks can cause thetank to collapse.

Tank joints should be designed for watertightness.Two-piece tanks and tanks with separate coversshould be designed with tongue and groove or lapjoints (figure 4-24). Manway covers should havesimilar joints. High-quality, preformed joint sealersshould be used to achieve a watertight seal. Theyshould be workable over a wide temperature rangeand should adhere to clean, dry surfaces; they mustnot shrink, harden, or oxidize. Seals should meetthe minimum compression and other requirementsprescribed by the seal manufacturer. Pipe and

inspection port joints should have cast-in rubberboots or compression seals.

Septic tanks should be tested for watertightnessusing hydrostatic or vacuum tests, and manwayrisers and inspection ports should be included in thetest. The professional association representing thematerials industry of the type of tank construction(e.g., the National Pre-cast Concrete Association)should be contacted to establish the appropriatetesting criteria and procedures. Test criteria forprecast concrete are presented in table 4-14.

4.6.3 Construction considerations

Important construction considerations include tanklocation, bedding and backfilling, watertightness,and flotation prevention, especially with non-concrete tanks. Roof drains, surface water runoff,and other clear water sources must not be routed tothe septic tank. Attention to these considerations

Table 4-14. Watertightness testing procedure/criteria for precast concrete tanks

Figure 4-24. Tongue and groove joint and sealer

Source: Ayres Associates



will help to ensure that the tank performs asintended.

Location

The tank should be located where it can be accessedeasily for septage removal and sited away fromdrainage swales or depressions where water cancollect. Local codes must be consulted regardingminimum horizontal setback distances frombuildings, property boundaries, wells, water lines,and the like.

Bedding and backfilling

The tank should rest on a uniform bearing surface.It is good practice to provide a level, granular basefor the tank. The underlying soils must be capableof bearing the weight of the tank and its contents.Soils with a high organic content or containinglarge boulders or massive rock edges are notsuitable.

After setting the tank, leveling, and joining thebuilding sewer and effluent line, the tank can bebackfilled. The backfill material should be free-flowing and free of stones larger than 3 inches indiameter, debris, ice, or snow. It should be added inlifts and each lift compacted. In fine-textured soilssuch as silts, silt loams, clay loams, and clay,imported granular material should be used. This isa must where freeze and thaw cycles are commonbecause the soil movement during such cycles canwork tank joints open. This is a significant concernwhen using plastic and fiberglass tanks.

The specific bedding and backfilling requirementsvary with the shape and material of the tank. Themanufacturer should be consulted for acceptablematerials and procedures.

Watertightness

All joints must be sealed properly, including tankjoints (sections and covers if not a monolithictank), inlets, outlets, manways, and risers (ASTM,1993; NPCA, 1998). The joints should be cleanand dry before applying the joint sealer. Only high-quality joint sealers should be used (see previoussection). Backfilling should not proceed until thesealant setup period is completed. After all jointshave been made and have cured, a watertightness

test should be performed (see table 4-14 for precastconcrete tanks). Risers should be tested.

Flotation prevention

If the tank is set where the soil can be saturated,tank flotation may occur, particularly when thetank is empty (e.g., recently pumped dose tanks orseptic tank after septage removal). Tank manufac-turers should be consulted for appropriateantiflotation devices.

4.6.4 Operation and maintenance

The septic tank is a passive treatment unit thattypically requires little operator intervention.Regular inspections, septage pumping, and periodiccleaning of the effluent filter or screen are the onlyoperation and maintenance requirements. Commer-cially available microbiological and enzymeadditives are promoted to reduce sludge and scumaccumulations in septic tanks. They are not neces-sary for the septic tank to function properly whentreating domestic wastewaters. Results from studiesto evaluate their effectiveness have failed to provetheir cost-effectiveness for residential application.For most products, concentrations of suspendedsolids and BOD in the septic tank effluent increaseupon their use, posing a threat to SWIS perfor-mance. No additive made up of organic solvents orstrong alkali chemicals should be used because theypose a potential threat to soil structure and groundwater.

Inspections

Inspections are performed to observe sludge andscum accumulations, structural soundness, water-tightness, and condition of the inlet and outletbaffles and screens. (Warning: In performinginspections or other maintenance, the tank shouldnot be entered. The septic tank is a confined spaceand entering can be extremely hazardous because oftoxic gases and/or insufficient oxygen.)

Sludge and scum accumulations

As wastewater passes through and is partiallytreated in the septic tank over the years, the layersof floatable material (scum) and settleable material(sludge) increase in thickness and gradually reducethe amount of space available for clarified waste-



water. If the sludge layer rises to the bottom of theeffluent T-pipe, solids can be drawn through theeffluent port and transported into the infiltrationfield, increasing the risk of clogging. Likewise, ifthe bottom of the thickening scum layer moveslower than the bottom of the effluent T-pipe, oilsand other scum material can be drawn into thepiping that discharges to the infiltration field.Various devices are commercially available tomeasure sludge and scum depths. The scum layershould not extend above the top or below thebottom of either the inlet or outlet tees. The top ofthe sludge layer should be at least 1 foot below thebottom of either tee or baffle. Usually, the sludgedepth is greatest below the inlet baffle. The scumlayer bottom must not be less than 3 inches abovethe bottom of the outlet tee or baffle. If any ofthese conditions are present, there is a risk thatwastewater solids will plug the tank inlet or becarried out in the tank effluent and begin to clogthe SWIS.

Structural soundness and watertightness

Structural soundness and watertightness are bestobserved after the septage has been pumped fromthe tank. The interior tank surfaces should beinspected for deterioration, such as pitting,spalling, delamination, and so forth and for cracksand holes. The presence of roots, for example,indicates tank cracks or open joints. These observa-tions should be made with a mirror and brightlight. Watertightness can be checked by observingthe liquid level (before pumping), observing alljoints for seeping water or roots, and listening forrunning or dripping water. Before pumping, theliquid level of the tank should be at the outletinvert level. If the liquid level is below the outletinvert, exfiltration is occurring. If it is above, theoutlet is obstructed or the SWIS is flooded. Aconstant trickle from the inlet is an indication thatplumbing fixtures in the building are leaking andneed to be inspected.

Baffles and screens

The baffles should be observed to confirm that theyare in the proper position, secured well to thepiping or tank wall, clear of debris, and notcracked or broken. If an effluent screen is fitted tothe outlet baffle, it should be removed, cleaned,inspected for irregularities, and replaced. Note that

effluent screens should not be removed until thetank has been pumped or the outlet is first plugged.

Septic tank pumping

Tanks should be pumped when sludge and scumaccumulations exceed 30 percent of the tankvolume or are encroaching on the inlet and outletbaffle entrances. Periodic pumping of septic tanksis recommended to ensure proper system perfor-mance and reduce the risk of hydraulic failure. Ifsystems are not inspected, septic tanks should bepumped every 3 to 5 years depending on the size ofthe tank, the number of building occupants, andhousehold appliances and habits (see Special IssuesFact Sheets). Commercial systems should beinspected and/or pumped more frequently, typicallyannually. There is a system available that providescontinuous monitoring and data storage of changesin the sludge depth, scum or grease layer thickness,liquid level, and temperature in the tank. Long-term verification studies of this system are underway. Accumulated sludge and scum material storedin the tank should be removed by a certified,licensed, or trained service provider and reused ordisposed of in accordance with applicable federal,state, and local codes. (Also see section 4.5.5.)

4.6.5 Septage

Septage is an odoriferous slurry (solids content ofonly 3 to 10 percent) of organic and inorganicmaterial that typically contains high levels of grit,hair, nutrients, pathogenic microorganisms, oil, andgrease (table 4-15). Septage is defined as the entirecontents of the septic tank—the scum, the sludge,and the partially clarified liquid that lies betweenthem—and also includes pumpings from aerobictreatment unit tanks, holding tanks, biological(“composting”) toilets, chemical or vault toilets,and other systems that receive domestic wastewa-ters. Septage is controlled under the federal regula-tions at 40 CFR Part 503. Publications and otherinformation on compliance with these regulationscan be found at http://www.epa.gov/oia/tips/scws.htm.

Septage also may harbor potentially toxic levels ofmetals and organic and inorganic chemicals. Theexact composition of septage from a particulartreatment system is highly dependent upon the typeof facility and the activities and habits of its users.



For example, oil and grease levels in septage fromfood service or processing facilities might be manytimes higher than oil and grease concentrations inseptage from residences (see Special Issues FactSheets). Campgrounds that have separate graywatertreatment systems for showers will likely havemuch higher levels of solids in the septage from theblackwater (i.e., toilet waste) treatment system.Septage from portable toilets might have beentreated with disinfectants, deodorizers, or otherchemicals.

Septage management programs

The primary objective of a septage managementprogram is to establish procedures and rules forhandling and disposing of septage in an affordablemanner that protects public health and ecologicalresources. When planning a program it is importantto have a thorough knowledge of legal and regula-tory requirements regarding handling and disposal.USEPA (1994) has issued regulations and guidancethat contain the type of information required fordeveloping, implementing, and maintaining aseptage management program. Detailed guidancefor identifying, selecting, developing, and operat-ing reuse or disposal sites for septage is provided inProcess Design Manual: Surface Disposal ofSewage Sludge and Domestic Septage (USEPA,

1995b), which is on the Internet at http://www.epa.gov/ORD/WebPubs/sludge.pdf. Addi-tional information can be found in DomesticSeptage Regulatory Guidance (USEPA, 1993), athttp://www.epa.gov/oia/tips/scws.htm.

States and municipalities typically establish publichealth and environmental protection regulations forseptage management (pumping, handling, trans-port, treatment, and reuse/disposal). Key compo-nents of septage management programs includetracking or manifest systems that identify accept-able septage sources, pumpers, transport equip-ment, final destination, and treatment, as well asprocedures for controlling human exposure toseptage, including vector control, wet weatherrunoff, and access to disposal sites.

Septage treatment/disposal: landapplication

The ultimate fate of septage generally falls intothree basic categories—land application, treatmentat a wastewater treatment plant, or treatment at aspecial septage treatment plant. Land application isthe most commonly used method for disposing ofseptage in the United States. Simple and cost-effective, land application approaches use minimalenergy and recycle organic material and nutrientsback to the land. Topography, soils, drainagepatterns, and agricultural crops determine whichtype of land disposal practice works best for agiven situation. Some common alternatives aresurface application, subsurface incorporation, andburial. Disposal of portable toilet wastes mixedwith disinfectants, deodorizers, or other chemicalsat land application sites is not recommended. Ifpossible, these wastes should be delivered to thecollection system of a wastewater treatment plant toavoid potential chemical contamination risks atseptage land application sites. Treatment plantoperators should be consulted so they can deter-mine when and where the septage should be addedto the collection system.

When disposing of septage by land application,appropriate buffers and setbacks should be pro-vided between application areas and water re-sources (e.g., streams, lakes, sinkholes). Otherconsiderations include vegetation type and density,slopes, soils, sensitivity of water resources, climate,

Table 4-15. Chemical and physical characteristics of domesticseptage



and application rates. Agricultural products fromthe site must not be directly consumed by humans.Land application practices include the following:

Spreading by hauler truck or farm equipment

In the simplest method, the truck that pumps theseptage takes it to a field and spreads it on the soil.Alternatively, the hauler truck can transfer itsseptage load into a wagon spreader or other special-ized spreading equipment or into a holding facilityat the site for spreading later.

Spray irrigation

Spray irrigation is an alternative that eliminates theproblem of soil compaction by tires. Pretreatedseptage is pumped at 80 to 100 psi through nozzlesand sprayed directly onto the land. This methodallows for septage disposal on fields with roughterrain.

Ridge and furrow irrigation

Pretreated septage can be transferred directly intofurrows or row crops. The land should be relativelylevel.

Subsurface incorporation of septage

This alternative to surface application involvesplacing untreated septage just below the surface.This approach reduces odors and health risks whilestill fertilizing and conditioning the soil. Themethod can be applied only on relatively flat land(less than 8 percent slope) in areas where theseasonally high water table is at least 20 inches.Because soil compaction is a concern, no vehiclesshould be allowed to drive on the field for 1 to 2weeks after application. Subsurface applicationpractices include the following:

• Plow and furrow irrigation: In this simplemethod, a plow creates a narrow furrow 6 to 8inches (15 to 20 centimeters) deep. Liquidseptage is discharged from a tank into thefurrow, and a second plow covers the furrow.

• Subsurface injection: A tillage tool is used tocreate a narrow cavity 4 to 6 inches (10 to 15centimeters) deep. Liquid septage is injectedinto the cavity, and the hole is covered.

Codisposal of septage in sanitary landfills

Because of the pollution risks associated withrunoff and effluent leaching into ground water,landfill disposal of septage is not usually a viableoption. However, some jurisdictions may allowdisposal of septage/soil mixtures or permit otherspecial disposal options for dewatered septage(sludge with at least 20 percent solids). Septage orsludge deposited in a landfill should be coveredimmediately with at least 6 inches of soil to controlodors and vector access (USEPA, 1995b). (Note:Codisposal of sewage sludge or domestic septage ata municipal landfill is considered surface disposaland is regulated under 40 CFR Part 258.)

Septage treatment/disposal: treatmentplantsDisposal of septage at a wastewater treatment plantis often a convenient and cost-effective option.Addition of septage requires special care andhandling because by nature septage is more concen-trated than the influent wastewater stream at thetreatment plant. Therefore, there must be adequatecapacity at the plant to handle and perhaps tempo-rarily store delivered septage until it can be fed intothe treatment process units. Sites that typicallyserve as the input point for septage to be treated ata wastewater treatment plant include the following:

Upstream sewer manhole

This alternative is viable for larger sewer systemsand treatment plants. Septage is added to thenormal influent wastewater flow at a receivingstation fitted with an access manhole.

Treatment plant headworks

The septage is added at the treatment plant up-stream of the inlet screens and grit chambers. Theprimary concern associated with this option is theimpact of the introduced wastes on treatment unitprocesses in the plant. A thorough analysis shouldbe conducted to ensure that plant processes canaccept and treat the wastes while maintainingappropriate effluent pollutant concentrations andmeeting other treatment requirements. In anyevent, the treatment plant operator should beconsulted before disposal.



Sludge-handling process

To reduce loading to the liquid stream, the septagecan be sent directly to the sludge-handling process.Like the headworks option, the impact on thesludge treatment processes must be carefullyanalyzed to ensure that the final product meetstreatment and other requirements.

Treatment at a special septage treatment plant

This method of septage disposal is usually em-ployed in areas where land disposal or treatment ata wastewater treatment plant is not a feasibleoption. There are few of these facilities, whichvary from simple lagoons to sophisticated plantsthat mechanically and/or chemically treat septage.Treatment processes used include lime stabilization,chlorine oxidation, aerobic and anaerobic digestion,composting, and dewatering using pressure orvacuum filtration or centrifugation. This is themost expensive option for septage management andshould be considered only as a last resort.

Public outreach and involvement

Developing septage treatment units or land applica-tion sites requires an effective public outreachprogram. Opposition to locating these facilities inthe service area is sometimes based about incom-plete or inaccurate information, fear of the un-known, and a lack of knowledge on potentialimpacts. Without an effective community-basedprogram of involvement, even the most reasonableplan can be difficult to implement. Traditionalguidance on obtaining public input in the develop-ment of disposal or reuse facilities can be found inProcess Design Manual: Surface Disposal ofSewage Sludge and Domestic Septage (USEPA,1995b), which is on the Internet at http://www.epa.gov/ORD/WebPubs/sludge.pdf.

Additional information can be found in DomesticSeptage Regulatory Guidance (USEPA, 1993),posted at http://www.epa.gov/oia/tips/scws.htm.General guidance on developing and implementinga public outreach strategy is available in Getting InStep: A Guide to Effective Outreach in YourWatershed, published by the Council of StateGovernments (see chapter 2) and available at http://www.epa.gov/owow/watershed/outreach/documents/.

4.7 Sand/media filters

Sand (or other media) filters are used to provideadvanced treatment of settled wastewater or septictank effluent. They consist of a lined (lined withimpervious PVC liner on sand bedding) excavationor watertight structure filled with uniformly sizedwashed sand (the medium) that is normally placedover an underdrain system (figure 4-25). Thesecontained media filters are also known as packedbed filters. The wastewater is dosed onto thesurface of the sand through a distribution networkand is allowed to percolate through the sand to theunderdrain system. The underdrain collects thefiltrate for further processing, recycling, or dis-charging to a SWIS. Some “bottomless” designsdirectly infiltrate the filtered effluent into the soilbelow.

4.7.1 Treatment mechanisms and filterdesign

Sand filters are essentially aerobic, fixed-filmbioreactors used to treat septic tank effluent. Othervery important treatment mechanisms that occur insand filters include physical processes such asstraining and sedimentation, which remove sus-pended solids within the pores of the media, andchemical adsorption of dissolved pollutants (e.g.,phosphorus) to media surfaces. The latter phenom-enon tends to be finite because adsorption sitesbecome saturated with the adsorbed compound, andit is specific to the medium chosen. Bioslimes fromthe growth of microorganisms develop as attachedfilms on the sand particle surfaces. The microorgan-isms in the slimes absorb soluble and colloidal wastematerials in the wastewater as it percolates aroundthe sand surfaces. The absorbed materials areincorporated into new cell mass or degraded underaerobic conditions to carbon dioxide and water.

Figure 4-25. Underdrain system detail for sand filters

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Most of the biochemical treatment occurs withinapproximately 6 inches (15 centimeters) of thefilter surface. As the wastewater percolates throughthis active layer, carbonaceous BOD and ammo-nium-nitrogen are removed. Most of the suspendedsolids are strained out at the filter surface. TheBOD is nearly completely removed if the wastewa-ter retention time in the sand media is sufficientlylong for the microorganisms to absorb and reactwith waste constituents. With depleting carbon-aceous BOD in the percolating wastewater, nitrify-ing microorganisms are able to thrive deeper in thisactive surface layer, where nitrification will readilyoccur.

To achieve acceptable treatment, the wastewaterretention time in the filter must be sufficiently longand reaeration of the media must occur to meet theoxygen demand of the applied wastewater. Thepore size distribution and continuity of the filtermedium, the dose volume, and the dosing fre-quency are key design and operating considerationsfor achieving these conditions. As the effective sizeand uniformity of the media increases, thereaeration rate increases, but the retention timedecreases. Treatment performance might decline ifthe retention time is too short. If so, it may benecessary to recirculate the wastewater through thefilter several times to achieve the desired retentiontime and concomitant treatment performance.Multiple small dose volumes that do not create asaturated wetting front on the medium can be usedto extend residence times. If saturated conditionsare avoided, moisture tensions within the mediumwill remain high, which will redistribute theapplied wastewater throughout the medium,enhancing its contact with the bioslimes on themedium. The interval between doses provides timefor reaeration of the medium to replenish theoxygen depleted during the previous dose.

Filter surface clogging can occur with finer mediain response to excessive organic loadings. Biomassincreases can partially fill the pores in the surfacelayer of the sand. If the organic loadings are toogreat, the biomass will increase to a point wherethe surface layer becomes clogged and is unable toaccept further wastewater applications. However, ifthe applied food supply is less than that required byresident microorganisms, the microorganisms areforced into endogenous respiration; that is, theybegin to draw on their stored metabolites or

surrounding dead cells for food. If the microorgan-isms are maintained in this growth phase, netincreases of biomass do not occur and clogging canbe minimized.

Chemical adsorption can occur throughout themedium bed, but adsorption sites in the mediumare usually limited. The capacity of the medium toretain ions depends on the target constituent, thepH, and the mineralogy of the medium. Phospho-rus is one element of concern in wastewater thatcan be removed in this manner, but the number ofavailable adsorption sites is limited by the charac-teristics of the medium. Higher aluminum, iron, orcalcium concentrations can be used to increase theeffectiveness of the medium in removing phospho-rus. Typical packed bed sand filters are not effi-cient units for chemical adsorption over an ex-tended period of time. However, use of specialmedia can lengthen the service (phosphorus re-moval) life of such filters beyond the normal, finiteperiod of effective removal.

Filter designs

Sand filters are simple in design and relativelypassive to operate because the fixed-film process isvery stable and few mechanical components areused. Two types of filter designs are common,“single-pass” and “recirculating” (figure 4-26).They are similar in treatment mechanisms andperformance, but they operate differently. Single-pass filters, historically called “intermittent” filters,discharge treated septic tank effluent after one passthrough the filter medium (see Fact Sheet 10).Recirculating filters collect and recirculate thefiltrate through the filter medium several timesbefore discharging it (see Fact Sheet 11). Each hasadvantages for different applications.

Single-pass filters

The basic components of single-pass filters (seeFact Sheet 10) include a dose tank, pump andcontrols (or siphon), distribution network, and thefilter bed with an underdrain system (figure 4-25).The wastewater is intermittently dosed from thedose tank onto the filter through the distributionnetwork. From there, it percolates through the sandmedium to the underdrain and is discharged. On-demand dosing has often been used, but timeddosing is becoming common.



To create the wastewater retention times necessaryfor achieving desired treatment results, single-passfilters must use finer media than that typically usedin recirculating filters. Finely sized media results inlonger residence times and greater contact betweenthe wastewater and the media surfaces and theirattached bioslimes. BOD removals of greater than90 percent and nearly complete ammonia removalare typical (Darby et al., 1996; Emerick et al., 1997;

University of Wisconsin, 1978). Single-pass filterstypically achieve greater fecal coliform removalsthan recirculating filters because of the finer mediaand the lower hydraulic loading. Daily hydraulicloadings are typically limited to 1 to 2 gpd/ft2, de-pending on sand size, organic loading, and espe-cially the number of doses per day (Darby et al.,1996).

Figure 4-26. Schematics of the two most common types of sand media filters



Recirculating filters

The basic components of recirculating filters (seeFact Sheet 11) are a recirculation/dosing tank,pump and controls, a distribution network, a filterbed with an underdrain system, and a return linefitted with a flow-splitting device to return aportion of the filtrate to the recirculation/dosingtank (figure 4-26). The wastewater is dosed to thefilter surface on a timed cycle 1 to 3 times perhour. The returned filtrate mixes with fresh septictank effluent before being returned to the filter.

Media types

Many types of media are used in packed bed filters.Washed, graded sand is the most common medium.Other granular media used include gravel, anthra-cite, crushed glass, expanded shale, and bottom ashfrom coal-fired power plants. Bottom ash has beenstudied successfully by Swanson and Dix (1987).Crushed glass has been studied (Darby et al., 1996;and Emerick et al., 1997), and it was found toperform similarly to sand of similar size anduniformity. Expanded shale appears to have beensuccessful in some field trials in Maryland, but thedata are currently incomplete in relation to long-term durability of the medium.

Foam chips, peat, and nonwoven coarse-fibersynthetic textile materials have also been used.These are generally restricted to proprietary units.Probably the most studied of these is the peat filter,which has become fairly common in recent years.Depending on the type of peat used, the early perfor-mance of these systems will produce an effluent with

a low pH and a yellowish color. This is accompa-nied by some excellent removal of organics andmicrobes, but would generally not be acceptable asa surface discharge (because of low pH and visiblecolor). However, as a pretreatment for a SWIS,low pH and color are not a problem. Peat mustmeet the same hydraulic requirements as sand (seeFact Sheets 10 and 11). The primary advantage ofthe proprietary materials, the expanded shale, and tosome degree the peat is their light weight, whichmakes them easy to transport and use at any site.Some short-term studies of nonwoven fabric filtershave shown promise (Roy and Dube, 1994).System manufacturers should be contacted forapplication and design using these materials.

4.7.2 Applications

Sand media filters may be used for a broad rangeof applications, including single-family residences,large commercial establishments, and small com-munities. They are frequently used to pretreatwastewater prior to subsurface infiltration on siteswhere the soil has insufficient unsaturated depthabove ground water or bedrock to achieve adequatetreatment. They are also used to meet water qualityrequirements before direct discharge to a surfacewater. They are used primarily to treat domesticwastewater, but they have been used successfully intreatment trains to treat wastewaters high in organicmaterials such as those from restaurants andsupermarkets. Single pass filters are most fre-quently used for smaller applications and siteswhere nitrogen removal is not required. Recirculat-ing filters are used for both large and small flows

Performance of sand and other filtersPerformance of sand and other filtersPerformance of sand and other filtersPerformance of sand and other filtersPerformance of sand and other filters

Twelve innovative treatment technologies were installed to replace failed septic systems in the Narragansett Baywatershed, which is both pathogen- and nitrogen-sensitive. The technologies installed consisted of an at-graderecirculating sand filter, single pass sand filters, Maryland-style recirculating sand filters, foam biofilters, and arecirculating textile filter. The treatment performance of these systems was monitored over an 18-month period. Inthe field study, TSS and BOD

5 concentrations were typically less than 5 mg/L for all sand filter effluent and less

than 20 mg/L for both the foam biofilter and textile filter effluents. Single pass sand filters achieved substantialfecal coliform reductions, reaching mean discharge levels ranging from 200 to 520 colonies per 100 mL for all 31observations. The at-grade recirculating sand filter achieved the highest total nitrogen reductions of anytechnology investigated and consistently met the Rhode Island state nitrogen removal standard (a TN reduction of50 percent or more and a TN concentration of 19 mg/L or less) throughout the study.

Source: Loomis et al., 2001.



and are frequently used where nitrogen removal isnecessary. Nitrogen removal of up to 70 to 80percent can be achieved if an anoxic reactor is usedahead of the recirculation tank, where the nitrifiedreturn filtrate can be mixed with the carbon-richseptic tank effluent (Anderson et al., 1998; Boyleet al., 1994; Piluk and Peters, 1994).

4.7.3 Performance

The treatment performance of single-pass andrecirculating filters is presented in table 4-16. Themedium used was sand or gravel as noted. Recircu-lating sand filters generally match or outperformsingle-pass filters in removal of BOD, TSS, andnitrogen. Typical effluent concentrations fordomestic wastewater treatment are less than 10 mg/L for both BOD and TSS, and nitrogen removal isapproximately 50 percent. Single-pass sand filterscan also typically produce an effluent of less than10 mg/L for both BOD and TSS. Effluent is nearlycompletely nitrified, but some variability can beexpected in nitrogen removal capability. Pell andNyberg (1989) found typical nitrogen removals of18 to 33 percent with their intermittent sand filter.Fecal coliform removal is somewhat better insingle pass filters. Removals range from 2 to 4 logsin both types of filters. Intermittent sand filter fecalcoliform removal is a function of hydraulic load-ing; removals decrease as the loading rate increasesabove 1 gpm/ft2 (Emerick et al., 1997).

Effluent suspended solids from sand filters aretypically low. The medium retains the solids. Mostof the organic solids are ultimately digested. Gravelfilters, on the other hand, do not retain solids aswell.

excessive solids buildup due to the lack of periodicsludge pumping and removal. In such cases, thesolids storage capacity of the final settling compart-ment might be exceeded, which results in thedischarge of solids into the effluent. ATU perfor-mance and effluent quality can also be negativelyaffected by the excessive use of toxic householdchemicals. ATUs must be properly operated andmaintained to ensure acceptable performance.

4.8 Aerobic treatment units

Aerobic treatment units (ATUs) refer to a broadcategory of pre-engineered wastewater treatment

devices for residential and commercial use. ATUsare designed to oxidize both organic material andammonium-nitrogen (to nitrate nitrogen), decreasesuspended solids concentrations and reduce patho-gen concentrations.

A properly designed treatment train that incorpo-rates an ATU and a disinfection process can providea level of treatment that is equivalent to that levelprovided by a conventional municipal biologicaltreatment facility. The AUT, however, must beproperly designed, installed, operated and main-tained.

Although most ATUs are suspended growth de-vices, some units are designed to include bothsuspended growth mechanisms combined withfixed-growth elements. A third category of ATU isdesigned to provide treatment entirely through theuse of fixed-growth elements such as tricklingfilters or rotating biological contactors (refer tosheets 1 through 3). Typical ATU’s are designedusing the principles developed for municipal-scalewastewater treatment and scaled down for residen-tial or commercial use.

Most ATUs are designed with compressors oraerators to oxygenate and mix the wastewater.Partial pathogen reduction is achieved. Additionaldisinfection can be achieved through chlorination,UV treatment, ozonation or soil filtration. In-creased nutrient removal (denitrification) can beachieved by modifying the treatment process toprovide an anaerobic/anoxic step or by addingtreatment processes to the treatment train.

4.8.1 Treatment mechanisms

ATUs may be designed as continuous or batch flowsystems (refer to fact sheets 1 through 3). Thesimplest continuous flow units are designed with noflow equalization and depend upon aeration tankvolume and/or baffles to reduce the impact ofhydraulic surges. Some units are designed withflow-dampening devices, including air lift or float-controlled mechanical pumps to transfer thewastewater from the aeration tank to a clarifier.Other units are designed with multiple-chamberedtanks to attenuate flow. The batch (fill and draw)flow system design eliminates the problem ofhydraulic variation. Batch systems are designed tocollect and treat wastewater over a period of time.


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Pumps are used to discharge the settled effluent atthe end of the cycle (usually one day). Fixed filmtreatment plants typically are operated as continu-ous flow systems.

Oxygen is transferred by diffused air, spargedturbine, or surface entrainment devices. Whendiffused air systems are used, blowers or compres-sors are used to force the air through diffusers nearthe bottom of the tank. The sparged turbine istypically designed with a diffused air source and anexternal mixer, e.g., a submerged flat-bladedturbine. The sparged turbine is more complex thanthe simple diffused air system. A variety of surfaceentrainment devices aerate and mix the wastewater.Air is entrained and circulated in the mixed liquorthrough violent agitation from mixing or pumping.

The separation of process-generated solids byclarification or filtration is a critical design factorfor successful ATU performance. Most ATUs aredesigned to rely on the process of simple gravityseparation to remove most of the solids. Somesystems include effluent filters within the clarifierto further screen and retain solids in the treatmentplant. Gas deflection barriers and scum baffles area part of some designs and are a simple way tokeep floating solids away from the weir area.Properly managed uplow clarifiers can improveseparation.

4.8.2 Design Considerations

ATU’s are typically rated by hydraulic capacity andorganic and solids loadings. ATU daily treatmentvolumes may range from 400 gpd to a maximumof 1,500 gpd. ATUs typically can be used to treatresidential wastewaters with influent concentrationswhich have 100 mg/L to 300 mg/L total organiccompounds and 100 mg/L to 350 mg/L totalsuspended solids. Design flows are generally set bylocal sanitary codes for residential and commercialdwellings using methods described in Section 3.3.

ATU’s should be equipped with audio and visualalarms to warn of compressor/aerator failure andhigh water. These alarms alert the owner and/orservice provider of service issues that requireimmediate attention.

ATU’s should be constructed of noncorrosivematerials, including reinforced plastics and

fiberglass, coated steel, and reinforced concrete.Buried ATU’s must be designed to provide easyaccess to mechanical parts, electrical controlsystems, and appurtenances requiring maintenancesuch as weirs, air lift pump lines, etc. ATU’sinstalled above ground should be properly housedto protect against severe climatic conditions.Installation should be in accordance with manufac-turers’ specifications.

Appurtenances should be constructed of corrosion-free materials including polyethylene plastics. Airdiffusers are usually constructed of PVC or ceramicstone. Mechanical components must be eitherwaterproofed and/or protected from the elements.Because blowers, pumps, and other prime moverscan be subject to harsh environments and continu-ous operation, they should be designed for heavyduty use. Proper housing can reduce blower noise.

4.8.3 Applications

ATUs are typically integrated in a treatment train toprovide additional treatment before the effluent isdischarged to a SWIS. ATU-treatment trains canalso be designed to discharge to land and surfacewaters; ATU discharge is suitable for drip irrigationif high quality effluent is consistently maintainedthrough proper management. Although somejurisdictions allow reductions in vertical separationdistances and/or higher soil infiltration rates whenATUs are used, consideration must be given to thepotential impacts of higher hydraulic and pollutantloadings. Increased flow through the soil mayallow deeper penetration of pathogens anddecreased treatment efficiency of other pollutants(see sections 4.4.2 and 4.4.5).

4.8.4 Performance

Managed ATU effluent quality is typicallycharacterized as 25 mg/L or less CBOD5 and 30mg/L or less TSS. Fecal coliform counts aretypically 3-4 log # / 100 ml (Table 3-19) when theATUs are operated at or below their design flowsand the influent is typical domestic sewage.Effluent nutrient levels are dependent on influentconcentrations, climate, and operating conditions.

Other wastewater characteristics may influenceperformance. Cleaning agents, bleach, caustic



agents, floating matter, and other detritus can plugor damage equipment. Temperature will affectprocess efficiency, i.e., treatment efficiencygenerally will improve as the temperatureincreases.

Owners should be required by local sanitary codesor management program requirements to maintainongoing service agreements for the life of thesystem. ATU’s should be inspected every threemonths to help ensure proper operation andtreatment effectiveness. Many ATU manufacturersoffer a two-year warranty with an optional serviceagreement after the warranty expires. Inspectionsgenerally include visual checks of hoses, wires,leads and contacts, testing of alarms, examinationof the mixed liquor, cleaning of filters, removal ofdetritus, and inspection of the effluent. ATU’sshould be pumped when the mixed-liquor (aerator)solids are above 6,000 mg/L or the final settler ismore than 1/3 full of settled solids.

4.8.5 Risk management

ATU’s should be designed to protect the treatmentcapability of the soil dispersal system and also tosound alarms or send signals to the managemententity (owners and/or service providers) wheninspection or maintenance is needed. All biologicalsystems are sensitive to temperature, powerinterruptions, influent variability, and shockloadings of toxic chemicals. Successful operationof ATUs depends on adherence to manufacturers’design and installation requirements and goodmanagement that employs meaningful measure-ments of system performance at sufficientlyfrequent intervals to ascertain changes in systemfunction. Consistent performance depends on astable power supply, an intact system as designed,and routine maintenance to ensure that componentsand appurtenances are in good order. ATU’s, likeall other onsite wastewater treatment technologies,will fail if they are not designed, installed, oroperated properly. Vigilance on the part of ownersand service providers is essential to ensure ATUsare operated and maintained to function asdesigned.

4.8.6 Costs

Installed ATU costs range from $2500 to $9000installed. Pumping may be necessary at any timedue to process upsets, or every eight to twelvemonths, depending on influent quality, temperatureand type of process. Pumping could cost from$100-to-$300, depending on local requirements.Aerators/compressors last about three to five yearsand cost from $300 to $500 to replace.

Many communities require service contracts.These contracts typically range in cost between$100 and $400 per year, depending on the optionsand features the owners choose. The high endincludes pumping costs. Power requirements aregenerally quoted at around $200/year.

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