Chapter 5: Treatment System Selectiononsite.tennessee.edu/EPA Decentralized CD/Decentralized...

USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 5-1

Chapter 5: Treatment System Selection

5.1 IntroductionSelecting the appropriate system type, size, andlocation at the site depends on the wastewater flowand composition information discussed in chap-ter 3, site- and landscape-level assessments out-lined in chapter 3 and in this chapter, performancerequirements as noted in chapter 3, and the array ofavailable technology options reviewed in chapter 4.Key to selecting, sizing, and siting the system areidentifying the desired level of performance andensuring that the effluent quality at the perfor-mance boundaries meets the expected performancerequirements.

5.2 Design conditions and systemselection

An appropriate onsite wastewater treatment systemconcept for a given receiver site—proposedlocation of the system, regional geologic andhydrologic features, and downgradient soils usedfor treatment—depends on the prevailing designconditions. Designers must consider and evaluatethe design conditions carefully before selecting asystem concept. Design conditions include thecharacteristics of the wastewater to be treated,regulatory requirements, and the characteristics ofthe receiver site (figure 5-1). With sufficientknowledge of these factors, the designer candevelop an effective preliminary design concept.This chapter focuses on general guidance forevaluation of the receiver site, identification of thesite’s design boundaries and requirements, andselection of suitable designs to meet the perfor-

Chapter 5:Treatment System Selection

mance requirements. This chapter also providesguidance for evaluating and rehabilitating systemsthat are not meeting their performance requirements.

5.3 Matching design conditions tosystem performance

Design conditions include wastewater characteristics;system owner preferences for siting, operation andmaintenance, and cost; regulatory requirementsprescribed by the permitting agency’s rules; and thereceiver site’s capability to treat or otherwiseassimilate the waste discharge. Each of these mustbe evaluated in light of the others before an appro-priate system design concept can be developed.

5.3.1 Wastewater source considerations

Wastewater source considerations include projec-tions of wastewater flow, wastewater composition,and owner requirements. Chapter 3 providesguidance for estimating flow and waste strengthcharacteristics. The owner’s needs, capabilities, andexpectations might be explicit or implied. The firstconsideration is the owner’s use of the property(present and projected), which informs analyses ofthe character and volume of the wastewater gener-ated. The footprint and location of existing orplanned buildings, paved areas, swimming pools,and other structures or uses will limit the areaavailable for the onsite system. Second, the owner’sconcern for the system’s visual impact or odor

5.1 Introduction

5.2 Design conditions and system selection

5.3 Matching design conditions to system performance

5.4 Design boundaries and boundary loadings

5.5 Evaluating the receiving environment

5.6 Mapping the site

5.7 Developing the initial system design

5.8 Rehabilitating and upgrading existing systems


5-2 USEPA Onsite Wastewater Treatment Systems Manual

Figure 5-1. Preliminary design steps and considerations.

USEPA Onsite Wastewater Treatment Systems Manual 5-3


potential might restrict the range of alternativesavailable to the designer. Third, the owner’s abilityand willingness to perform operation and mainte-nance tasks could limit the range of treatmentalternatives. Finally, costs are a critical concern forthe owner. Capital (construction) costs and recur-ring (operation and maintenance) costs should beestimated, and total costs over time should becalculated if cost comparisons between alternativesystems are necessary. The owner should have boththe ability and willingness to pay construction andoperation and maintenance costs if the system is toperform satisfactorily.

5.3.2 Regulatory requirements

Designs must comply with the rules and regulationsof the permitting entity. Onsite wastewater systemsare regulated by a variety of agencies in the UnitedStates. At the state level, rules may be enacted aspublic health codes, nuisance codes, environmentalprotection codes, or building codes. In most (butnot all) states, the regulatory authority for onsitesingle-family residential or small cluster systems isdelegated to counties or other local jurisdictions.The state might enact a uniform code requirementthat all local jurisdictions must enforce equally, orthe state might have a minimum code that localjurisdictions may adopt directly or revise to bestricter. In a few states, general guidance ratherthan prescriptive requirements is provided to localjurisdictions. In such cases, the local jurisdictionsmay enact more or less strict regulations or choosenot to adopt any specific onsite system ordinance.

Traditionally, state and local rules have beenprescriptive codes that require specific systemdesigns for a set of specific site criteria. Such rulestypically require that treated wastewater dischargedto the soil be maintained below the surface of theground, though a few states and local jurisdictionsdo allow discharges to surface waters under theirNational Pollutant Discharge Elimination System(NPDES) permitting programs, as authorized by thefederal Clean Water Act. If applications are pro-posed outside the prescriptive rules, the agencyusually requires special approvals or variancesbefore a permit can be issued. Circumstances thatrequire special action (approvals, variances) andadministrative processes for approving thoseactions are usually specified in state or local codes.

5.3.3 Receiver site suitability

The physical characteristics of the site (the locationof the proposed system, regional geologic andhydrologic features, and the soils to be used in thetreatment process) determine the performancerequirements and treatment needs. A careful andthorough site evaluation is necessary to assess thecapacity of the site to treat and assimilate effluentdischarges. Treatment requirements for a proposedsystem are based on the performance boundaryrequirements established by rule and the naturaldesign boundaries identified through the siteevaluation.

5.4 Design boundaries andboundary loadings

Wastewater system design must focus on thecritical design boundaries: between system compo-nents, system/soil interfaces, soil layer and prop-erty boundaries, or other places where designconditions abruptly change (see figures 5-2 and5-3). System failures occur at design boundariesbecause they are sensitive to hydraulic and masspollutant loadings. Exceeding the mass loadinglimit of a sensitive design boundary usually resultsin system failure. Therefore, all critical designboundaries must be identified and the mass load-ings to each carefully considered to properly selectthe upstream performance and design requirementsneeded to prevent system failure (Otis, 1999).

The approach discussed in this chapter is based oncharacterizing the assimilative capacity of thereceiving environment (ground water, surfacewater) and establishing onsite system performancerequirements that protect human health and eco-logical resources. Desired system performance, asmeasured at the final discharge point (after treat-ment in the soil matrix or other treatment traincomponents), provides a starting point for consid-ering performance requirements for each precedingsystem component at each design boundary (e.g.,septic tank-SWIS interface, biomat at the infiltra-tive surface, surface of the saturated zone).Through this approach, system designers candetermine treatment or performance requirementsfor each component of the treatment train byassessing whether each proposed component canmeet performance requirements (acceptable massloading limits) at each subsequent design boundary.



Determining the critical design boundaries of thephysical environment is the primary objective ofthe site evaluation (see section 5.5). Design bound-aries are physical planes or points, or they may bedefined by rule. More than one design boundarycan be expected in every system, but not all of theidentified boundaries are likely to control design.The most obvious design boundaries are those towhich performance requirements are applied(figure 5-2). These are defined boundaries thatmight or might not coincide with a physicalboundary. For a ground water discharge, the designboundary might be the water table surface, theproperty line, or a drinking water well. For surfacewater discharges the performance boundary istypically designated at the outfall to the receivingwaters, where permit limits on effluent contami-nants are applied. Physical boundaries are particu-larly significant for conventional wastewatertreatment systems that discharge to ground water orto the atmosphere. Soil infiltrative surfaces,hydraulically restrictive soil horizons, or zones ofsaturation are often the critical design boundariesfor ground water discharging systems.

The site evaluation must be sufficiently thorough toidentify all potential design boundaries that mightaffect system design. Usually, the critical designboundaries are obvious for surface water discharg-ing and evaporation systems. Design boundaries for

subsurface wastewater infiltration systems, how-ever, are more difficult to identify because theyoccur in the soil profile and there might be morethan one critical design boundary.

5.4.1 Subsurface infiltration systemdesign boundaries and loadings

Subsurface wastewater infiltration systems (SWISs)have traditionally been used to treat and dischargeeffluent from residences, commercial buildings,and other facilities not connected to centralizedsewage treatment plants. These systems accept andtreat wastewater discharged from one or moreseptic tanks in below-grade perforated piping,which is usually installed in moderately shallowtrenches 1.5 to 3.0 feet deep on a bed of crushedrock 0.5 to 1.5 inches in diameter. Leachingchambers, leach beds, and other SWIS technologieshave also been approved for use in some states.Both the trench bottoms and sidewalls provideinfiltrative surfaces for development of the biomat(see chapter 3) and percolation of treated wastewa-ter to the surrounding soil matrix.

The soil functions as a biological, physical, andchemical treatment medium for the wastewater, aswell as a porous medium to disperse the wastewater inthe receiving environment as it percolates to theground water. Therefore, the site evaluation mustdetermine the capacity of the soil to hydraulicallyaccept and treat the expected daily mass loadings ofwastewater. Site and soil characteristics must provideadequate drainage of the saturated zone to maintainthe necessary unsaturated depth below the infiltrativesurface, allow oxygenation of aerobic biota in thebiomat and reaeration of the subsoil, and preventeffluent surfacing at downgradient locations.

Traditional site evaluation and design proceduresconsider only the infiltrative surface of the SWIS as adesign boundary (figure 5-3). Hydraulic loadingrates to this boundary are usually estimated frompercolation tests and/or soil profile analyses. Therecommended daily hydraulic loading rates typicallyassume septic tank effluent is to be applied to the soil(through the SWIS biomat, across the trench bottom/sidewall soil interface, and into the surrounding soil).The estimated daily wastewater volume is divided bythe applicable hydraulic loading rate to calculate theneeded infiltration surface area. This method ofdesign has endured since Henry Ryon first proposed

Design Boundary

Source: Ayres Associates, 2000.

Figure 5-2. Performance (design) boundaries associated withonsite treatment systems



the percolation test and its empirical relationship toinfiltration system size nearly 100 years ago(Fredrick, 1948). Although this method of designhas been reasonably successful, hydraulic andtreatment failures still occur because focusing onthe infiltrative surface overlooks other importantdesign boundaries. Identifying those criticalboundaries and assessing their impacts on SWISdesign will substantially reduce the number andfrequency of failures.

Usually there is more than one critical designboundary for a SWIS. Zones where free water orsaturated soil conditions are expected to occurabove or below unsaturated zones identify perfor-

mance boundary layers (Otis, 2001). In SWISs,these include

• The infiltrative surfaces where the wastewaterfirst contacts the soil.

• Secondary infiltration surfaces that causepercolating wastewater to perch above anunsaturated zone created by changes in soiltexture, structure, consistency, or bulk density.

• The ground water table surface, which thepercolating wastewater must enter withoutexcessive ground water mounding or degrada-tion of ground water quality.

Figure 5-3. Subsurface wastewater infiltration system design/performance boundaries.

Source: Adapted from Ayres Associates, 1993.

BackfillNatural

Geotextile

Distribution pipe

Design boundary

Infiltration zone and biomat

Vadose zone (unsaturated)

"Perched" saturated zone

Design boundary*

Restrictive horizon*

Capillary

Design boundary

Groundwater surface

Saturated

* If present



advantages. For example, denitrification is aidedwhen saturation results in anaerobic conditions ininterstices in the normally unsaturated zones.Perched or otherwise layered boundaries requirecareful characterization, analysis, and assessment ofsystem operation to determine how they will affectthe movement of effluent plumes from the SWIS.

The water table surface is where treatment is usuallyexpected to be complete, that is, where pollutantloadings, with proper mixing and dispersion, shouldnot create concentrations in excess of water qualitystandards. System designers should seek to ensurethat hydraulic loadings from the system(s) to theground water will not exceed the aquifer’s capacityto drain water from the site. If a SWIS is to performproperly, the mass loadings to the critical designboundaries must be carefully considered and incorpo-rated into the design of the system. The types ofmass loadings that should be considered in SWISdesign are presented in table 5-1.

The various design boundaries are affected differ-ently by different types of mass loadings (table 5-2).

The infiltrative surface is a critical design andperformance boundary in all SWISs since freewater enters the soil and changes to water undertension (at pressures less than atmospheric) in theunsaturated zone. Many wastewater quality trans-formations occur at this boundary. For example,biochemical activity usually causes a hydraulicallyrestrictive biomat to form at the infiltrative surface.Failure to consider the infiltrative surface in systemdesign and to accommodate the changes that occurthere can lead to hydraulic or treatment failure.

Other surfaces that are often critical design bound-aries include those associated with hydraulicallyrestrictive zones below the infiltrative surface thatcan cause water to perch. If hydraulic loadings aretoo great for these boundaries, surface seepagemight occur at downslope locations as effluentslides along the perched boundary. Also, thesaturated zone could mound to encroach on theunsaturated zone to the extent that sufficientreaeration of the soil does not occur, which canresult in severe soil clogging. If hydraulic problemsdo not occur, these conditions offer some treatment

Table 5-1. Types of mass loadings to subsurface wastewater infiltration systems.

Source: Otis, 2001.



Table 5-2. Potential impacts of mass loadings on soil design boundaries

The infiltrative surface is the primary designboundary. At this boundary, the partially treatedwastewater must pass through the biomat, enter thesoil pores, and percolate into unsaturated soil. Thewastewater cannot be applied at rates faster thanthe soil can accept it, nor can the soil be overloadedwith solids or organic matter to the point wheresoil pores become clogged with solids or an overlythick development of the biomass. Because solidsare usually removed through settling processes inthe septic tank, the critical design loadings at thisboundary are the daily and instantaneous hydraulicloading rates and the organic loading rate. Systemdesign requires that daily hydraulic and instanta-neous/peak loadings be estimated carefully so thatthe total hydraulic load can be applied as uniformlyas feasible over the entire day to maximize theinfiltration capacity of the soil. Uniform dosingand resting maximizes the reaeration potential ofthe soil and meets the oxygen demand of theapplied wastewater loading more efficiently. Theorganic loading rate is an important considerationif the available area for the SWIS is small. Inmoderately permeable or more permeable soils,lower organic loading rates can increase infiltrationrates into the soil and may allow reductions in thesize of the infiltrative surface. Organic loadings to

slowly permeable, fine-textured soils are of lesserconcern because percolation rates through thebiomat created by the organic loading are usuallygreater than the infiltration rate into the soil.Preventing effluent backup (hydraulic failure) byincreasing the size of the SWIS and implementingwater conservation measures are important consid-erations in these situations.

Secondary design boundaries are usually hydrauli-cally restrictive horizons that inhibit verticalpercolation through the soil (figure 5-2). Water canperch above these boundaries, and the perching canaffect performance in two significant ways. If theperched water encroaches into the unsaturated zone,treatment capacity of the soil is reduced andreaeration of the soil below the infiltrative surfacemight be impeded. Depending on the degree ofimpedance, anoxic or anaerobic conditions candevelop, resulting in excessive clogging of theinfiltrative surface. Also, water will move laterallyon top of the boundary, and partially treatedwastewater might seep from the exposed bound-aries of the restrictive soil strata downslope and outonto the ground surface. Therefore, the contour(linear) loading along the boundary surface contourmust be low enough to prevent water from mounding



above the boundary to the point that inadequatelytreated wastewater seeps to the surface and createsa nuisance and possible risk to human health.Organic loadings at these secondary boundaries areseldom an issue because most organic matter istypically removed as the wastewater passes throughthe infiltrative surface boundary layer.

Hydraulic and wastewater constituent loadings arethe critical design loadings at the water tableboundary. Low aquifer transmissivity createsground water mounding (figure 5-4), which canencroach on the infiltrative surface if the dailyhydraulic loading is too high. Mounding can affecttreatment and percolation adversely by inhibitingsoil reaeration and reducing moisture potential. Afurther potential consequence is undesirable surfaceseepage that can occur downslope. Constituentloadings must be considered where protection ofpotable water supply wells is a concern. Typicalwastewater constituents of human health concerninclude pathogenic microbes and nitrates (seechapter 3). Water resource pollutants of concerninclude nitrogen in coastal areas, phosphorus nearinland waters, and toxic organics and certain metalsin all areas. If the wastewater constituent loadingsare too high at the water table boundary, pretreat-ment before application to the infiltrative surfacemight be necessary.

5.4.2 Surface water dischargingsystem design boundaries andloadings

Surface water discharging systems typically consistof a treatment plant (aeration/activated sludge/sandfilter “package” system with disinfection) discharg-ing to an outfall (pipe discharge) to a surface water.The important design boundaries for these systemsare the inlet to the treatment plant and the outfall tothe surface water. The discharge permit and theperformance history of the treatment processtypically establish the limits of mass loading thatcan be handled at both the inlet to and the outletfrom the treatment process. The loadings are oftenexpressed in terms of daily maximum flow andpollutant concentrations (table 5-3). The effluentlimits and wastewater characteristics establish theextent of treatment (performance requirements)needed before final discharge.

5.4.3 Atmospheric discharging systemdesign boundaries and loadings

Evapotranspiration systems are the most commonlyused atmospheric discharging systems. They cantake several forms, but the primary design bound-

Figure 5-4. Effluent mounding effect above the saturated zone

Source: Adapted from NSFC diagram.



ary is the evaporative surface. Water (effluent)flowing through the treatment system and sitehydrology must be considered in the design. Waterbalance calculations in the system control design(table 5-4). These loadings are determined by theambient climatic conditions expected. Proceduresfor estimating these loadings are provided inchapter 4 (Evapotranspiration Fact Sheet).

5.5 Evaluating the receivingenvironment

Evaluation of the wastewater receiver site is acritical step in system selection and design. Theobjective of the evaluation is to determine thecapacity of the site to accept, disperse, and safelyand effectively assimilate the wastewater discharge.The evaluation should• Determine feasible receiving environments

(ground water, surface water, or atmosphere)• Identify suitable receiver sites• Identify significant design boundaries associ-

ated with the receiver sites• Estimate design boundary mass loading limitations

Considering the importance of site evaluation withrespect to system design, it is imperative that siteevaluators have appropriate training to assess

receiver sites and select the proper treatment train,size, and physical placement at the site. Thissection does not provide basic information on soilscience but rather suggests methods and proceduresthat are standardized or otherwise proven for thepractice of site evaluation. It also identifies specificsteps or information that is crucial in the decision-making process for the site evaluator.

5.5.1 Role and qualifications of the siteevaluator

The role of the site evaluator is to identify, interpret,and document site conditions for use in subsurfacewastewater treatment system selection, design, andinstallation. The information collected should bepresented in a manner that is scientifically accurateand spatially correct. Documentation should usestandardized nomenclature to provide geophysicalinformation so that the information can be used byother site evaluators, designers, regulators, andcontractors.

The site evaluator needs considerable knowledgeand a variety of skills. A substantial knowledge ofsoils, soil morphology, and geology is essentialbecause most onsite systems use the soil as the finaltreatment and dispersal medium. Many states nolonger accept the percolation test as the primary

Table 5-3. Types of mass loadings for point discharges to surface waters

Table 5-4. Types of mass loadings for evapotranspiration systems



NorNorNorNorNorth Carolina guidelines fth Carolina guidelines fth Carolina guidelines fth Carolina guidelines fth Carolina guidelines for Oor Oor Oor Oor OWTS site eWTS site eWTS site eWTS site eWTS site evvvvvaluationsaluationsaluationsaluationsaluations

The Division of Environmental Health of the North Carolina Department of Environment, Health, and NaturalResources uses a 10-point guide for conducting site evaluations. The ten guidelines can be grouped into thefollowing components:

Collecting information before the site visit

Assessing the site and soil at the location

Recording site evaluation data for system design

Relaying the information to the system designer and the applicant.

1. Know the rules and know how to collect the needed information. Applicable codes for sewage treatment anddispersal systems are usually established by the local agency.

2. Determine the wastewater flow rate and characteristics. Information on wastewater quantity and quality is usedto determine the initial size and type of the onsite system to be installed at a particular site.

3. Review preliminary site information. Existing published information will help the evaluator understand the typesof soils and their properties and distribution on the landscape.

4. Understand the septic system design options. Site evaluators must understand how onsite systems function inorder to assess trade-offs in design options.

5. View the onsite system as part of the soil system and the hydrologic cycle. Typically, onsite systems servingsingle-family homes do not add enough water to the site to substantially change the site’s hydrology, except inareas of high densities of onsite systems.

6. Predict wastewater flow through the soil and the underlying materials. The soil morphological evaluation andlandscape evaluation are important in predicting flow paths and rates of wastewater movement through the soiland underlying materials.

7. Determine if additional information is needed from the site. Site and soil conditions and the type of onsitesystem being considered determine whether additional evaluation is required. Some additional evaluations thatmay be required are ground water mounding analysis, drainage analysis, hydrogeologic testing, contour(linear) loading rate evaluation, and hydraulic conductivity measurements.

8. Assess the treatment potential of the site. The treatment potential of the site depends on the degree of soilaeration and the rate of flow of the wastewater through the soil.

9. Evaluate the site’s environmental and public health sensitivity. Installing onsite systems in close proximity tocommunity wells, near shellfish waters, in sole-source aquifer areas, or other sensitive areas may raiseconcerns regarding environmental and public health issues.

10. Provide the system designer with soil/site descriptions and your recommendations. Based on the informationgathered about the facility and the actual site and soil evaluation, the evaluator can suggest loading rates,highlight site and design considerations, and point out special concerns in designing the onsite system.

Source: North Carolina DEHNR, 1996.

suitability criterion. A significant number ofpermitting agencies now require a detailed soilprofile description and evaluation performed byprofessional soil scientists or certified site evaluators.

In addition to a thorough knowledge of soilscience, the site evaluator should have a basicunderstanding of chemistry, wastewater treatment,and water movement in the soil environment, aswell as knowledge of onsite system operation andconstruction. The evaluator should also have basicskills in surveying to create site contour maps and

site plans that include temporary benchmarks,horizontal and vertical locations of site features,and investigation, sample, or test locations. Ageneral knowledge of hydrology, biology, andbotany is helpful. Finally, good oral and writtencommunication skills are necessary to convey siteinformation to others who will make importantdecisions regarding the best use of the site.



5.5.2 Phases of a site evaluation

Site evaluations typically proceed in three phases: apreliminary review of documented site information, areconnaissance of potential sites, and a detailedevaluation of the most promising site or sites. Thescale and detail of the evaluation depend on thequantity and strength of the wastewater to be treated,the nature of local soils and the hydrogeologic setting,the sensitivity of the local environment, and theavailability of suitable sites. Using a phased approach(table 5-5) helps to focus the site evaluation effort ononly the most promising sites for subsurface systems.

5.5.3 Preliminary review

The preliminary review is performed before anyfieldwork. It is based on information availablefrom the owner or local agencies or on generalresource information. The objectives of the pre-liminary review are to identify potential receiversites, determine the most feasible receiving environ-ments, identify potential design boundaries, anddevelop a relative suitability ranking. Preliminaryscreening of sites is an important aspect of the siteevaluator’s role. More than one receiving environ-ment might be feasible and available for use.Focusing the effort on the most promising receivingenvironments and receiver sites allows the evalua-tor to reasonably and methodically eliminate theleast suitable sites early in the site evaluationprocess. For example, basic knowledge of the localclimate might eliminate evaporation or evapotranspi-ration as a potential receiving environment immedi-ately. Also, the applicable local codes often prohibitpoint discharges to surface waters from small systems.Knowledge of local conditions and regulations isessential during the screening process. Resourcematerials and information to be reviewed mayinclude, but are not limited to, the following:

• Property information. This information shouldinclude owner contact information, site legaldescription or address, plat map or boundarysurvey, description of existing site improve-ments (e.g., existing onsite wastewater systems,underground tanks, utility lines), previous andproposed uses, surrounding land use andzoning, and other available and relevant data.

• Detailed soil survey. Detailed soil surveys arepublished by the U.S. Department ofAgriculture’s Natural Resources Conservation

Service (NRCS), formerly the Soil Conserva-tion Service (SCS). Detailed soil surveysprovide soil profile descriptions, identify soillimitations, estimate saturated soil conductivitiesand permeability values, describe typicallandscape position and soil formation factors,and provide various other soil-related informa-tion. Soil surveys are typically based on deduc-tive projections of soil units based on topo-graphical or landscape position and should beregarded as general in nature. Because theaccuracy of soil survey maps decreases asassessments move from the landscape scale tothe site scale, soil survey data should be supple-mented with detailed soil sampling at the site(table 5-5). Individual surveys are performed ona county basis and are available for mostcounties in the continental United States,Alaska, Hawaii, and the U.S. territories. Theyare available from county extension offices or

Table 5-5. Site characterization and assessment activities forSWIS applications



the local NRCS office. Information on availabledetailed soil surveys and mapping status can beobtained from the National Soil Survey Centerthrough its web site at http://www.statlab.iastate.edu/soils/nssc/. The NRCSpublication Fieldbook for Describing andSampling Soils is an excellent manual for use insite evaluation. It is available at http://www.statlab.iastate.edu/soils/nssc/field_gd/field_gd.pdf.

• Quadrangle maps. Quadrangle maps providegeneral topographic information about a siteand surrounding landscape. These maps aredeveloped and maintained by the U.S. GeologicalSurvey (USGS) and provide nationwide coveragetypically at a scale of 1 inch = 2000 feet, witheither a 10- or 20-foot contour interval. At thisscale, the maps provide information related toland use, public improvements (e.g., roadways),USGS benchmarks, landscape position and slope,vegetated areas, wetlands, surface drainagepatterns, and watersheds. More informationabout USGS mapping resources can be found athttp://mapping.usgs.gov/mac/findmaps.html.Quadrangle maps also are available throughproprietary software packages.

• Wetland maps. Specialized maps that identifyexisting, farmed, and former wetlands areavailable in many states from natural resourceor environmental agencies. These maps identifywetland and fringe areas to be avoided forwastewater infiltration areas. On-line andpublished wetland maps for many parts of theUnited States are available from the U.S. Fishand Wildlife Service’s National WetlandsInventory Center at http://www.nwi.fws.gov/.

• Aerial photographs. If available, aerial photo-graphs can provide information regarding pastand existing land use, drainage and vegetationpatterns, surface water resources, and approxi-mate location of property boundaries. They areespecially useful for remote sites or those withlimited or difficult access. Aerial photographsmay be available from a variety of sources, suchas county or regional planning, propertyvaluation, and agricultural agencies.

• Geology and basin maps. Geology and basin mapsare especially useful for providing general inform-ation regarding bedrock formations and depths,

ground water aquifers and depths, flow direc-tion and velocities, ambient water quality,surface water quality, stream flow, and seasonalfluctuations. If available, these maps can beobtained from USGS at http://www.nationalatlas.gov/or Terra Server at http://www.terraserver.microsoft.com.

• Water resource and health agency information.Permit and other files, state/regional wateragency staff, and local health departmentsanitarians or inspectors can provide valuableinformation regarding local onsite systemdesigns, applications, and performance. Regula-tory agencies are beginning to establish TotalMaximum Daily Loads (TMDLs) for criticalwastewater constituents within regional drain-age basins under federal and state clean waterlaws. TMDLs establish pollutant “budgets” toensure that receiving waters can safely assimi-late loads of incoming contaminants, includingthose associated with an onsite system (e.g.,bacteria, nutrients). If the site lies in the re-charge area of a water resource listed as im-paired (not meeting its designated use) becauseof bacteria or nutrient contamination, siteevaluators need to be aware of all applicableloading limits to ground water or surface waterin the vicinity of the site under review.

• Local installer/maintenance firms. Helpfulinformation often can be obtained from inter-views with system installation and maintenanceservice providers. Their experience with othersites in the vicinity, existing technology perfor-mance, and general knowledge of soils andother factors can inform both the site evaluationand the selection of appropriate treatmentsystem components.

• Climate. Temperature, precipitation, and panevaporation data can be obtained from theNational Oceanic and Atmospheric Administra-tion (NOAA) at http://www.nic.noaa.gov. Thisinformation is necessary if evapotranspirationsystems are being considered. The evaluatormust realize, however, that the data from thenearest weather station might not accuratelyrepresent the climate at the site being evaluated.



5.5.4 Reconnaissance survey

The objectives of the reconnaissance survey are toobtain preliminary site data that can be used todetermine the appropriate receiving environment,screen potential receiver sites, and further focus thedetailed survey to follow. A reconnaissance surveytypically includes visual surveys of each potentialsite, preliminary soils investigation using handborings, and potential system layouts. Informationgathered from the preliminary review, soil samplingtools, and other materials should be on hand duringthe reconnaissance survey.

The site reconnaissance begins with a site walkoverto observe and identify existing conditions, selectareas to perform soil borings, or view potentialroutes for piping or outfall structures. The siteevaluator should have an estimate of the total areaneeded for the receiver site based on the projecteddesign flow and anticipated soil characteristics. It isadvisable to complete the site walkover with theowner and local regulatory staff if possible,particularly with larger projects. Selection of anarea for soil investigation is based on the owner’srequirements (desired location, vegetation preserva-tion, and general site aesthetics), regulatoryrequirements (setbacks, slope, and prior land use),and the site evaluator’s knowledge and experience(landscape position, local soil formation factors,and geologic conditions). Visual inspections areused to note general features that might affect sitesuitability or system layout and design. Generalfeatures that should be noted include the following:

• Landscape position. Landscape position andlandform determine surface and subsurfacedrainage patterns that can affect treatment andinfiltration system location. Landscape featuresthat retain or concentrate subsurface flows, suchas swales, depressions, or floodplains, should beavoided. Preferred landscape positions areconvex slopes, flat areas with deep, permeablesoils, and other sites that promote wastewaterinfiltration and dispersion through unsaturatedsoils (figures 5-5 and 5-6).

• Topography. Long, planar slopes or plateausprovide greater flexibility in design than ridges,knolls, or other mounded or steeply slopingsites. This is an important consideration ingravity-flow treatment systems, collection

piping for cluster systems, treatment unit sites,and potential routes for point discharge outfalls.

• Vegetation. Existing vegetation type and sizeprovide information regarding soil depth andinternal soil drainage, which are importantconsiderations in the subsurface wastewaterinfiltration system layout.

• Natural and cultural features. Surface waters,wetlands, areas of potential flooding, rockoutcrops, wells, roads, buildings, buried utilities,underground storage tanks, property lines, andother features should be noted because they willaffect the suitability of the receiver site.

A good approach to selecting locations for soilinvestigations is to focus on landscape position. Theunderlying bedrock often controls landscapes,which are modified by a variety of natural forces.The site evaluator should investigate landscapepositions during the reconnaissance phase toidentify potential receiver sites (figures 5-5, 5-6and 5-7; table 5-6). Ridgelines are narrow areasthat typically have limited soil depth but often agood potential for surface/subsurface drainage.Shoulderslopes and backslopes are convex slopeswhere erosion is common. These areas often havegood drainage, but the soil mantle is typically thinand exposed bedrock outcrops are common.Sideslopes are often steep and erosion is active.Footslopes and depressions are concave areas ofsoil accumulation; however, depressions usuallyhave poor drainage. The deeper, better-drained

Figure 5-5. General considerations for locating a SWIS on asloping site

Source: Purdue University, 1990.



soils are found on ridgelines, lower sideslopes, andfootslopes. Bottomlands might have deeper soilsbut might also have poor subsurface drainage.

The visual survey might eliminate candidatereceiver sites from further consideration. Prelimi-nary soil borings should be examined on theremaining potential sites unless subsurface waste-water infiltration as a treatment or dispersal optionhas been ruled out for other reasons. Shallowborings, typically to a depth of at least 5 feet (1.7meters), should be made with a soil probe or handauger to observe the texture, structure, horizonthickness, moisture content, color, bulk density, andspatial variability of the soil. Excavated test pits arenot typically required during this phase because ofthe expense and damage to noncommitted sites.Enough borings must be made to adequatelycharacterize site conditions and identify designboundaries. To account for grade variations,separation distances, piping routes, managementconsiderations, and contingencies, an area sufficientto provide approximately 200 percent of theestimated treatment area needed should be investi-gated. A boring density of one hole per half-acremay be adequate to accomplish the objectives ofthis phase. On sites where no reasonable number ofsoil borings is adequate to characterize the continu-ity of the soils, consideration should be given toabandoning the site as a potential receiver site.

Onsite treatment with a point discharge (permittedunder the National Pollutant Discharge EliminationSystem) requires evaluation of the potentialreceiving water and an outfall location. Thefeasibility of a point discharge is determined byfederal and state rules and local codes, if enactedby the local jurisdiction. Where the impacts andlocation of the discharge are considered acceptableby the regulating agency, effluent concentrationlimits will be stipulated and an NPDES permit willbe required.

The final step of the reconnaissance survey is tomake a preliminary layout of the proposed systemon each remaining candidate site based on assessedsite characteristics and projected wastewater flows.This step is necessary to determine whether the sitehas sufficient area and to identify where detailedsoils investigations should be concentrated. Inpractice, this step becomes integrated into the fieldreconnaissance process so the conceptual design

Figure 5-6. Landscape position features(see table 5-6 for siting potential)

Source: NRCS, 1998.

Table 5-6. SWIS siting potential vs. landscape position features



unfolds progressively as it is adapted to the grow-ing body of site and soil information.

5.5.5 Detailed evaluation

The objective of the detailed evaluation is toevaluate and document site conditions and charac-teristics in sufficient detail to allow interpretationand use by others in designing, siting, and install-ing the system. Because detailed investigations canbe costly, they should not be performed unless thepreliminary and reconnaissance evaluationsindicate a high probability that the site is suitable.Detailed site evaluations should attempt to identifycritical site characteristics and design boundariesthat affect site suitability and system design. At aminimum, the detailed investigation should includesoil profile descriptions and topographic mapping.(See figure 5-8, Site Evaluation/Site Plan Check-list.) Several backhoe pits, deep soil borings, soilpermeability measurements, ground water charac-terizations, and pilot infiltration testing processesmay be necessary for large subsurface infiltrationsystems. For evapotranspiration systems, fieldmeasurements of pan evaporation rates or otherparameters, as appropriate, might be necessary.This information should be presented with anaccurate site plan.

The detailed evaluation should address surfacefeatures such as topography, drainage, vegetation,site improvements, property boundaries, and othersignificant features identified during the reconnais-sance survey. Subsurface features to be addressedinclude soil characteristics, depth to bedrock andground water, subsurface drainage, presence ofrock in the subsoil, and identification of hydraulicand treatment boundaries. Information must beconveyed using standardized nomenclature for soildescriptions and hydrological conditions. Testingprocedures must follow accepted protocol andstandards. Forms or formats and evaluation pro-cesses specified by regulatory or managementagencies must also be used (for a state example seehttp://www.deh.enr.state.nc.us/oww/LOSWW/soil_form.pdf).

5.5.6 Describing the soil profile

Descriptions and documentation of soil profilesprovide invaluable information for designing onsitesystems that use soil as the final wastewater

treatment and dispersal medium. Detailed soilcharacterizations are provided through observation,description, and documentation of exposed soilprofiles within backhoe-excavated test pits.Profiles can be described using a hand auger ordrill probe for any single-home SWISs site inknown soil and hydrogeology. However, backhoe-excavated test pits should be used wherever largeSWISs or difficult single-home sites are proposedbecause of the quality of information gained. Thegrinding action or compression forces from soilborings taken with a hand auger or drill probe limitthe information obtained for some soil characteris-tics, especially structure, consistency, and soilhorizon relationships. Depending on project size, itmight be necessary to supplement soil evaluationtest pits with deep borings to provide more detailregarding soil substratum, ground water, andbedrock conditions. Table 5-7 summarizes theprocesses and procedures discussed below.

It might not be possible to identify all designboundaries, such as the permanent water table

Figure 5-7. Conventional system layout with SWIS replacementarea



Figure 5-8. Site evaluation/site plan checklist



Figure 5-8. Site evaluation/site plan checklist (cont.)



Table 5-7. Practices to characterize subsurface conditions through test pit inspection

surface or bedrock, if they are beyond shallowexploration depths (5 to 8 feet). However, it isimperative to identify and characterize secondarydesign boundaries that occur within the range ofsubsurface investigation. Soil characteristicsshould be described using USDA NRCS nomencla-ture and assessed by using standardized field soilevaluation procedures as identified in the FieldBook for Describing and Sampling Soils(Shoeneberger et al., 1998), which is available on

the Internet at http://www.statlab.iastate.edu/soils/nssc/field_gd/field_gd.pdf.

Another source for the description of soils in the fieldis American Society for Testing and Materials (ASTM)Standard D 5921-96, Standard Practice for Subsur-face Site Characterization of Test Pits for On-SiteSeptic Systems (ASTM, 1996), which is summarized intable 5-7. The primary ASTM soil characterizationreference is Standard Practice for Classification ofSoils for Engineering Purposes (Unified Soil Classifi-cation System), ASTM D 2487-00. The ASTM and



NRCS soil classification systems have many similari-ties; both describe and categorize soils according tosilt, clay, and sand composition and relative plasticity.However, the NRCS guide cited above is a field guideand is based on soil characterization procedures thatcan be conducted through tactile and visual tech-niques in the field (e.g., the feel of a soil sample,visual identification of the presence and color ofconcretions and mottles) with minimal equipment.The ASTM approach requires laboratory analysis ofsoil particle size (with a series of sieves), plasticity,and organic content (ASTM , 2000) and is morecommonly used in the engineering profession. Bothapproaches meet the technical requirements forconducting the site evaluation process described inthis section.

Based on the proposed design flow, an area equal toapproximately 200 percent of the estimated re-quired treatment area should be investigated. Testpits should be spaced in a manner that provides areasonable degree of confidence that conditions aresimilar between pits. For small cluster systems,three to five test pits may be sufficient if locatedaround the periphery and in the center of theproposed infiltration area. Large projects requiremore test pits. Test pit spacing can be adjustedbased on landscape position and observed condi-tions. Hand auger borings or soil probes may beused to confirm conditions between or at peripheraltest pit locations. Soil profiles should be observedand documented under similar conditions of lightand moisture content. Features that should be notedinclude the following:

• Soil depth. Test pits should be excavated to asafe depth to describe soil conditions, typically4 feet below the proposed infiltrative surface. Avertical wall exposed to the sunlight is best forexamination. The wall should be picked with ashovel or knife to provide an undisturbedprofile for evaluation and description. Horizonthickness should be measured and the soilproperties described for each horizon.

Restrictive horizons that may be significantsecondary design boundaries must be noted. Thedepths of each horizon should be measured todevelop a relationship with conditions in other testpits. Soil below the floor of the backhoe pit can beinvestigated by using hand augers in the excavatedpit bottom or by using deep boring equipment.

Key soil properties that describe a soil profileare horizons, texture, structure, color, andredoximorphic features (soft masses, nodules, orconcretions of iron or manganese oxides oftenlinked to saturated conditions). Other propertiesinclude moisture content, porosity, ruptureresistance (resistance to applied stress), penetra-tion resistance, roots, clay mineralogy, bound-aries, and coatings. Attention to the listed keysoil properties will provide the most value indetermining water movement in soil.

• Horizons. A soil horizon is a layer of soil thatexhibits similar properties and is generallydenoted based on texture and color. Soil horizonsresult from natural soil-forming processes andhuman practices. Horizons are designated asmaster horizons and layers with subordinatedistinctions. All key soil properties and associ-ated properties that are relevant to watermovement and wastewater treatment should bedescribed. Particular attention should be givento horizons with strong textural contrast, stratifiedmaterials, and redoximorphic indicators thatsuggest a restriction to vertical water movement.Certain soil conditions that create a designboundary can occur within a soil horizon orlayer. These include horizons with low perme-

Figure 5-9. Soil textural triangle

Source: USDA, 1951.



ability that perch water, indurated or massivehorizons, or substrata of dense glacial till.

• Texture. Soil texture is defined as the percentageby weight of separates (sand, silt, and clay) thatmake up the physical composition of a givensample. It is one indicator of a soil’s ability totransmit water. The textural triangle (figure 5-9)is used to identify soil textures based on percent-age of separates (Schoeneberger et al., 1998).

The texture of soil profiles is typically identifiedin the field through hand texturing. Theevaluator’s skill and experience play an impor-tant role in the accuracy of field texturing.Several field guides, typically in the form offlow charts, are available to assist the evaluatorin learning this skill and to assist with identify-ing the texture of soils that occur at or neartexture boundaries. (ASTM, 1997)

• Structure. Structure is more important thantexture for determining water movement in soils.Soil structure is the aggregation of soil particlesinto larger units called peds. The more commontypes of structure are granular, angular blocky,subangular blocky, and platy (figure 5-10).Structureless soils include single-grain soils(e.g., sand) and massive soils (e.g., hardpan).The grade, size, shape, and orientation of soilpeds influence water movement in the soilprofile. This is especially true in fine-textured

soils. Smaller peds create more inter-pedalfractures, which provide more flow paths forpercolating water. Grade, which defines thedistinctness of peds, is important for establish-ing a soil loading rate for wastewater dispersal.A soil with a “strong” grade of structure hasclearly defined fractures or voids between thepeds for the transmittance of water. The inter-pedal fractures and voids in a soil with a “weak”grade are less distinct and offer more resistanceto water flow. Soils with a strong grade canaccept higher hydraulic loadings than soils witha weak grade. Platy and massive soils restrictthe vertical movement of water.

• Color. Color is an obvious property of soil thatis easily discernible. It is an excellent indicatorof the soil’s aeration status and moisture regime.Soil colors are described using the Munsellcolor system, which divides colors into threeelements—hue, value, and chroma (Munsell,1994). Hue relates to the quality of color, valueindicates the degree of lightness or darkness,and chroma is the purity of the spectral color.Munsell soil color books are commerciallyavailable and are universally accepted as thestandard for identifying soil color. The dominantor matrix color is determined for each horizon,and secondary colors are determined forredoximorphic features, ped coatings, mineralconcretions, and other distinctive soil features.Dark colors generally indicate higher organiccontent, high-chroma colors usually suggesthighly oxygenated soils or high iron content,and low-chroma soils imply reduced conditionsoften associated with saturation. The siteevaluator must be aware that colors can bemodified by temperature, mineralogy, vegetation,ped coatings, and position in the soil profile.

• Redoximorphic features. Redoximorphicfeatures are used to identify aquic moistureregimes in soils. An aquic moisture regimeoccurs when the soil is saturated with waterduring long periods, an indicator of possiblerestrictive horizons, seasonal high water tables,or perched water tables. The presence ofredoximorphic features suggests that thesurrounding soil is periodically or continuouslysaturated. This condition is important to identifybecause saturated soils prevent reaeration of thevadose zone below infiltration systems andreduce the hydraulic gradients necessary for

Figure 5-10. Types of soil structure

Source: USDA, 1951.



adequate drainage. Saturated conditions can leadto surfacing of wastewater or failure due tosignificant decreases in soil percolation rates.Redoximorphic features include iron nodulesand mottles that form in seasonally saturatedsoils by the reduction, translocation, andoxidation of iron and manganese oxides(Vespaskas, 1996). Redoximorphic featureshave replaced mottles and low-chroma colors inthe USDA NRCS soil taxonomy because mottlesinclude carbonate accumulations and organicstains that are not related to saturation andreduction. It is important to note thatredoximorphic features are largely the result ofbiochemical activity and therefore do not occurin soils with low amounts of organic carbon,high pH (more than 7 standard pH units), lowsoil temperatures, or low amounts of iron, orwhere the ground water is aerated. Vespraskas(1996) provides an excellent guide to theidentification of redoximorphic features andtheir interpretation. As noted, the NRCS onlineguide to redoximorphic and other soil propertiesat http://www.statlab.iastate.edu/soils/nssc/field_gd/field_gd.pdf addresses key identifica-tion and characterization procedures forredoximorphic and other soil features.

• Soil consistence. Soil consistence in the generalsense refers to attributes of soil as expressed indegree of cohesion and adhesion, or in resis-tance to deformation or rupture. Consistenceincludes the resistance of soil material torupture; the resistance to penetration; theplasticity, toughness, or stickiness of puddledsoil material; and the manner in which the soilmaterial behaves when subjected to compres-sion. Consistence is highly dependent on thesoil-water state. The general classifications ofsoil consistence are loose, friable, firm, andextremely firm. Soils classified as firm andextremely firm tend to block subsurface waste-water flows. These soils can become cementedwhen dry and can exhibit considerable plasticitywhen wet. Soils that exhibit extremely firmconsistence are not recommended for conven-tional infiltration systems.

• Restrictive horizons. Soil properties like pen-etration resistance, rooting depth, and claymineralogy are important indicators of soilporosity and hydraulic conductivity. Penetrationresistance is often correlated with the soil’s bulk

density. The greater the penetration resistance,the more compacted and less permeable the soilis likely to be. Rooting depth is another measureof bulk density and also soil wetness. Claymineralogies such as montmorillonite, whichexpand when wetted, reduce soil permeabilityand hydraulic conductivity significantly. Adiscussion of these properties and their descrip-tion can also be found in the USDA Soil SurveyManual (USDA, 1993) and the USDA NRCSField Book for Describing and Sampling Soils(Schoeneberger et al., 1998).

• Other soil properties. Other soil properties thataffect nutrient removal are organic content andphosphorus adsorption potential. Organiccontent can provide a carbon source (fromdecaying organic matter in the uppermost soilhorizons) that will aid denitrification of nitrifiedeffluent (nitrate) in anoxic regions of the SWIS.Phosphorus can be effectively removed fromwastewater effluent by soil through adsorptionand precipitation reactions (see chapter 3). Soilmineralogy and pH affect the soil’s capacity toretain phosphorus. Adsorption isotherm testsprovide a conservative measure of the potentialphosphorus retention capacity.

• Characterization of unconsolidated material.Geologists define unconsolidated material as thematerial occurring between the earth’s surfaceand the underlying bedrock. Soil forms in thisparent material from the actions of wind, water,or alluvial or glacial deposition. Soil scientistsrefer to the soil portion of unconsolidatedmaterial as the solum and the parent material asthe substratum. Typically, site evaluators exposethe solum and the upper portion of the substra-tum. Knowledge of the type of parent materialand noted restrictions or boundary conditions isimportant to the designer, particularly for largewastewater infiltration systems. Often, if thesubstratum is deep, normal test pit depth will beinsufficient and deep borings may be necessary.

5.5.7 Estimating infiltration rate andhydraulic conductivity

Knowledge of the soil’s capacity to accept andtransmit water is critical for design. The infiltrationrate is the rate at which water is accepted by thesoil. Hydraulic conductivity is the rate at which



water is transmitted through the soil. As wastewateris applied to the soil, the infiltration rate typicallydeclines well below the saturated hydraulic con-ductivity of the soil. This occurs because thebiodegradable materials and nutrients in thewastewater stimulate microbiological activity thatproduces new biomass (see chapter 3). The biomassproduced and the suspended solids in the wastewa-ter create a biomat that can fill many of the soilpores and close their entrances to water flow. Theflow resistance created by the biomat can reducethe infiltration rate to several orders of magnitudeless than the soil’s saturated hydraulic conductiv-ity. The magnitude of the resistance created by thebiomat is a function of the BOD and suspendedsolids in the applied wastewater and the initialhydraulic conductivity of the soil.

Estimating the design infiltration rate is difficult.Historically, the percolation test has been used toestimate the infiltration rate. The percolation testwas developed to provide an estimate of the soil’ssaturated hydraulic conductivity. Based on experi-ence with operating subsurface infiltration systems,an empirical factor was applied to the percolationtest result to provide a design infiltration rate. Thismethod of estimating the design infiltration rate hasmany flaws, and many programs that regulate onsitesystems have abandoned it in favor of detailed soilprofile descriptions. Soil texture and structure havebeen found to correlate better with the infiltrationrate of domestic septic tank effluent (Converse andTyler, 1994). For other applied effluent qualitiessuch as secondary effluent, the correlation withtexture and structure is less well known.

Information on the hydraulic conductivity of thesoil below the infiltrative surface is necessary forground water mounding analysis and estimation ofthe maximum hydraulic loading rate for the infiltra-tion area. There are both field and laboratorymethods for estimating saturated hydraulic conduc-tivity. Field tests include flooding basin, single- ordouble-ring infiltrometer, and air entry permea-meter. These and other field test procedures aredescribed elsewhere (ASTM, 1997; Black, 1965;USEPA, 1981; 1984). Laboratory methods are lessaccurate because they are performed on small soilsamples that are disturbed from their natural statewhen they are taken. Of the laboratory tests, theconcentric ring permeameter (Hill and King, 1982)and the cube method (Bouma and Dekker, 1981) are

the most useful techniques. The American Societyfor Testing and Materials posts permeameterinformation on its Internet site at http://www.astm.org (see ASTM Store, ASTM Standards).

5.5.8 Characterizing the ground watertable

Where ground water is present within 5 feet belowsmall infiltration systems and 10 to 15 feet belowlarge systems, the hydraulic response of the watertable to prolonged loading should be evaluated.The ground water can be adversely affected bytreated wastewater and under certain conditions caninfluence system performance. This information isvaluable for understanding potential systemimpacts on ground water and how the systemdesign can mitigate these impacts.

The depth, seasonal fluctuation, direction of flow,transmissivity, and, where possible, thickness of thewater table should be estimated. With shallow, thinwater tables, depth, thickness, and seasonal fluctua-tions can be determined through soil test pitexamination. However, deeper water tables requirethe use of deep borings and possible installation ofpiezometers or monitoring wells. At least threepiezometers, installed in a triangular pattern, arenecessary to determine ground water gradient anddirection of flow, which might be different fromsurface water flow direction. Estimating thesaturated hydraulic conductivity of the aquifermaterials is necessary to determine ground watertravel velocity. Slug tests or pumping tests can beperformed in one or more existing or new wellsscreened in the shallow water table to estimate thehydraulic conductivity of the aquifer (Bouwer,1978; Bouwer and Rice, 1976; Cherry and Freeze,1979). In some cases, it may be possible to estimatethe saturated hydraulic conductivity from a particlesize analysis of aquifer materials collected from thetest pit, if the material is accessible (Bouwer, 1978;Cherry and Freeze, 1979). Pumping tests may alsobe used to determine the effective porosity orspecific yield of the saturated zone.

Ground water mounding beneath an infiltrationsystem can reduce both treatment and the hydraulicefficiency of the system. Ground water moundingoccurs when the rate of water percolating verticallyinto the saturated zone exceeds the rate of groundwater drainage from the site (figure 5-4). Mounding



is more likely to occur where the receiver site isrelatively flat, the hydraulic conductivity of thesaturated zone is low, or the saturated zone is thin.With continuous application, the water moundsbeneath the infiltrative surface and reduces thevertical depth of the vadose zone. Reaeration of thesoil, treatment efficiency, and the infiltrationsystem’s hydraulic capacity are all reduced whensignificant mounding occurs. A mounding analysisshould be completed to determine site limits andacceptable design boundary loadings (linearhydraulic loading) for sites where the water table isshallow or the soil mantle is thin, or for any largeinfiltration system.

Both analytical and numerical ground watermounding models are available. Because of thelarge number of data points necessary for numeri-cal modeling, analytical models are the mostcommonly used. Analytical models have beendeveloped for various hydrogeologic conditions(Brock, 1976; Finnemore and Hantzshe, 1983;Hantush, 1967; Kahn et al., 1976). Also, commer-cial computer software is available to estimatemounding potential. The assumptions used in eachmodel must be compared to the specific siteconditions found to select the most appropriatemodel. For examples of model selection and modelcomputations, see EPA’s process design manual(USEPA, 1981, 1984). A USEPA Office of GroundWater and Drinking Water annotated bibliographyof ground water and well field characterizationmodeling studies can be found on the Internet athttp://www.epa.gov/ogwdw000/swp/wellhead/dewell.html#analytical. USGS has available anumber of software packages, which are posted athttp://water.usgs.gov/software/ground_water.html. For links to software suppliersor general information, visit the National GroundWater Association web site at http://www.ngwa.org/.

5.5.9 Assessments for point source andevapotranspiration discharges

Sites proposed for point discharges to surfacewaters require a permit from the National PollutantDischarge Elimination System (see http://www.epa.gov/owm/npdes.htm) and a suitablelocation for an outfall to a receiving water body.Considerations for locating an outfall structureinclude NPDES regulatory requirements, outfall

structure siting, routing from the treatment facility,construction logistics and expense, and aesthetics.Regulatory requirements generally address accept-able entry points to receiving waters and hydraulicand pollutant loadings. The state regulatory agencytypically sets effluent limits based on the waterresource classification, stream flow, and assimila-tive capacity of the receiving water. Assimilativecapacities take into account the entire drainagebasin or watershed of nearby receiving waters toensure that pollutant levels do not exceed waterquality criteria. (See table 3-21 for applicableDrinking Water Standards; USEPA Drinking WaterStandards are posted at http://www.epa.gov/ogwdw000/creg.html.) In the case of state-listedimpaired streams (those listed under section 303(d)of the Clean Water Act), discharges must considerpollutant loads established or proposed under theTotal Maximum Daily Load provisions of the CleanWater Act. Piping from the treatment facility needsto consider gravity or forcemain, route, existingutilities, and other obstacles to be avoided.

Evapotranspiration (ET) systems treat and dis-charge wastewater by evaporation from the soil orwater surface or by plant transpiration. Thesesystems are climate-sensitive and require large landareas. ET systems function best in arid climateswhere there is large annual net evaporation andactive vegetative growth year-round. In the UnitedStates this generally means only the southwesternstates, where humidity is low, rainfall is minimal,and temperatures are warm enough to permit activeplant growth during the winter season (figure 5-11).Although the macroclimate of an area might beacceptable for the use of ET systems, evaluation ofthe microclimate is often required because it cansignificantly influence system performance. Inaddition to temperature, precipitation, and panevaporation data, exposure position and prevalentwind direction should be considered as part of theevaluation process. Southern exposures in thenorthern hemisphere provides greater solar radia-tion. Exposure to wind provides greater drying ofthe soil and plant surfaces. Surface drainagepatterns should also be assessed. Well-drained siteshave a lower ambient humidity to enhance evapora-tion than poorly drained sites.



5.6 Mapping the siteAt the completion of the site evaluation, a site mapor sketch should be prepared to show physicalfeatures, locations of soil pits and borings, topogra-phy or slopes, and suitable receiver sites. If a mapor aerial photograph was used, field measurementsand locations can be noted directly on it. Otherwiseit will be necessary to take measurements andsketch the site. The level of effort for developing agood site map should be commensurate with theresults of the site evaluation and whether the sitemap is being completed for a preliminary ordetailed site evaluation.

In addition to the features of the site under consid-eration, the site map should show adjacent landsand land uses that could affect treatment systemlayout, construction, and system performance.Maps with a 1- or 2-foot contour interval arepreferred.

5.7 Developing the initial systemdesign

Developing a concept for the initial system designis based on integration of projected wastewatervolume, flow, and composition information; thecontrolling design boundaries of the selectedreceiving environment; the performance require-ments for the chosen receiving environment; andthe needs and desires of the owner (figure 5-12).The site evaluation identifies the critical designboundaries and the maximum mass loadings theycan accept. This knowledge, together with theperformance requirements promulgated by theregulating authority for the receiving environment,establishes the design boundary loadings. Once theboundary loadings are established, treatment trainsthat will meet the performance requirements can beassembled.

Figure 5-11. Potential evaporation versus mean annual precipitation



Figure 5-12. Development of the onsite wastewater system design concept



AssembAssembAssembAssembAssembling SWIS treatment trling SWIS treatment trling SWIS treatment trling SWIS treatment trling SWIS treatment trains fains fains fains fains for a site with shalloor a site with shalloor a site with shalloor a site with shalloor a site with shallowwwww,,,,,slowly permeable soils over bedrockslowly permeable soils over bedrockslowly permeable soils over bedrockslowly permeable soils over bedrockslowly permeable soils over bedrock

Site descriptionSite descriptionSite descriptionSite descriptionSite description

A single-family residence is proposed for a lot with shallow, finely textured, slowly permeable soil overcreviced bedrock. The depth of soil is 2 feet. The slope of the lot is moderate and is controlled by bedrock.Ground water is more than 5 feet below the bedrock surface.

Design boundariesDesign boundariesDesign boundariesDesign boundariesDesign boundaries

Three obvious design boundaries that will affect the SWIS design are present on this site: the infiltrativesurface, the bedrock surface, and the water table. The site evaluation determined that no hydraulicallyrestrictive horizon is present in the soil profile above the bedrock.

PPPPPerferferferferfororororormance requirementsmance requirementsmance requirementsmance requirementsmance requirements

The regulatory agency requires that the wastewater discharge remain below ground surface at all times, thatthe ground water contain no detectable fecal coliforms, and that the nitrate concentrations of the ground waterbe less than 10 mg-N/L at the property boundary. In this case study, wastewater modification (reducing masspollutant loads or implementing water conservation measures; seechapter 3) was not considered.

Design boundary mass loadingsDesign boundary mass loadingsDesign boundary mass loadingsDesign boundary mass loadingsDesign boundary mass loadings

InfiltrInfiltrInfiltrInfiltrInfiltrativativativativative surfe surfe surfe surfe surface:ace:ace:ace:ace: Referring to table 5-2, the mass loadings that might affect the infiltrative surface are thedaily, instantaneous, and organic mass loadings. The selected hydraulic and instantaneous (dose volume persquare foot) loading rates must be appropriate for the characteristics of the soil to prevent surface seepage.Assuming domestic septic tank effluent is discharged to the infiltrative surface and that the surface is placedin the natural soil, the organic mass loading is accounted for in the commonly used daily hydraulic loadingrates. Typical hydraulic loading rates for domestic septic tank effluent control design. Reducing the organicconcentration through pretreatment will have little impact because the resistance of the biomat created by theorganic content is typically less than the resistance to flow through the fine-textured soil.

BedrocBedrocBedrocBedrocBedrock boundark boundark boundark boundark boundary:y:y:y:y: The bedrock boundary is a secondary design boundary where a zone of saturation willform as the wastewater percolates through the soil. This boundary is affected by the daily and linear hydraulicloadings (table 5-2). If these hydraulic loadings exceed the rate at which the water is able to drain laterallyfrom the site or percolate to the water table through the bedrock crevices, the saturated zone thickness willincrease and could encroach on the infiltrative surface, reducing its treatment and hydraulic capacity. Becausethe site is sloping, the linear, rather than the daily, hydraulic loading will control design.

WWWWWater tabater tabater tabater tabater table boundarle boundarle boundarle boundarle boundary:y:y:y:y: The wastewater percolate will enter the ground water through the bedrock crevices.The daily and linear hydraulic loading and constituent loadings are the mass loadings that can affect thisboundary (table 5-2). Because of the depth of the water table below the bedrock surface and the porous natureof the creviced bedrock, the daily and linear hydraulic loadings are not of concern. However, nitrate-nitrogenand fecal coliforms are critical design loadings because of the water quality requirements. Table 5-2summarizes the critical design boundary mass loadings that will affect design.

Assembling feasible treatment train alternativesAssembling feasible treatment train alternativesAssembling feasible treatment train alternativesAssembling feasible treatment train alternativesAssembling feasible treatment train alternatives

Because control of the wastewater is lost after it is applied to the soil, the bedrock and water table boundaryloading requirements must be satisfied through appropriate design considerations at or before the infiltrativeboundary. Therefore, the secondary and water table boundary loadings must be considered first.

Constituent loading limits at the ground water boundary will control treatment requirements. Although theperformance boundary (the point at which performance requirements are measured) may be at the propertyboundary, mixing and dilution in the ground water cannot be certain because the bedrock crevices can act asdirect conduits for transporting undiluted wastewater percolate. Therefore, it would be prudent to ensure thesepollutants are removed before they can leach to the ground water. Research has demonstrated that soilssimilar to those present at the site (fine-textured, slowly permeable soils) can effectively remove the fecal



coliforms if the wastewater percolates through an unsaturated zone of 2 to 3 feet (Florida HRS, 1993).Because the soil at the site extends to only a 2-foot depth, the infiltrative surface would need to be elevated 1foot above the ground surface in a mound or at-grade system. Alternatively, disinfection prior to soil applicationcould be used. Nitrate is not effectively removed by unsaturated, aerated soil; therefore, pretreatment fornitrogen removal is required.

Maintaining the linear loading at the bedrock surface below the maximum acceptable rate determines theorientation and geometry of the infiltrative surface. The infiltrative surface will need to be oriented parallel tothe bedrock surface contour. Its geometry needs to be long and narrow, with a width no greater than themaximum acceptable linear loading (gpd/ft) divided by the design hydraulic loading on the infiltrative surface(gpd/ft2). Note: If a mound is used on this site, an additional design boundary is created at the mound fill/natural soil interface. The daily hydraulic loading will affect this secondary design boundary.

If the perched saturated zone above the bedrock is expected to rise and fall with infiltrative surface loadings,the instantaneous loading to the infiltrative surface should be controlled through timed dosing to maximize thesite’s hydraulic capacity. Failure to control instantaneous loads could lead to transmission of partially treatedwastewater through bedrock crevices, driven by the higher hydraulic head created during periods of peaksystem use. Applying the wastewater through a dosing regime will maximize retention time in the soil whileensuring cyclical flooding of the infiltration trenches, creating optimum conditions for denitrifying bacteria toaccomplish nitrogen removal. The daily and instantaneous hydraulic loadings to the infiltrative surface aredependent on the characteristics of the soil or fill material in which the SWIS is placed.

Alternative Pretreatment Dosing Infiltration

1 Nitrogen removal Timed dosing Mound with pressure distribution

2 Nitrogen removal with disinfection Timed dosing In-ground trenches with pressure distribution

From this boundary loading analysis, potential treatment train alternatives can be assembled. Table 4-1 andthe fact sheets in chapter 4 should be used to select appropriate system components.

AlterAlterAlterAlterAlternativnativnativnativnative 1e 1e 1e 1e 1 elevates the infiltrative surface in a mound of suitable sand fill. With at least a foot of fill and theunsaturated 2 feet of natural soil below, fecal coliform removal will be nearly complete. The mound would bedesigned as long and narrow, oriented parallel to the bedrock surface contours (equivalent to the land surfacecontours since the slope is bedrock-controlled) to control the linear loading on the interface between the sandfill and natural soil or at the bedrock surface. The infiltrative surface would be time-dosed through a pressureor drip distribution network to distribute the wastewater onto the surface uniformly in time and space.

AlterAlterAlterAlterAlternativnativnativnativnative 2e 2e 2e 2e 2 places the infiltrative surface in the natural soil. With this design, there would be an insufficientdepth of unsaturated soil to remove the fecal coliforms. Therefore, disinfection of the treated wastewater priorto application to the soil would be necessary. The trenches would be oriented parallel to the bedrock surfacecontours (equivalent to the land surface contours since the slope is bedrock-controlled) to control the linearloading on the bedrock surface. If multiple trenches are used, the total daily volume of treated wastewaterapplied per linear foot of trench parallel to the slope of the bedrock surface would be no greater than thedesign linear loading for the site. Loadings to the infiltrative surface would be time-dosed through a pressure ordrip distribution network to distribute the wastewater uniformly in time and space.

Note that for the alternatives listed, multiple options exist for each of the system’s components (see table 4-1).

Source: Otis, 2001.



Subsurface wastewater infiltration system design in a restricted areaSubsurface wastewater infiltration system design in a restricted areaSubsurface wastewater infiltration system design in a restricted areaSubsurface wastewater infiltration system design in a restricted areaSubsurface wastewater infiltration system design in a restricted area

Often, the available area with soils suitable for subsurface infiltration of wastewater is limited. Because localauthorities usually do not permit point discharges to surface waters, subsurface infiltration usually is the onlyoption for wastewater treatment. However, a SWIS can perform as required only if the daily wastewater flow isless than the site’s hydraulic capacity.

The hydraulic capacity of the site is determined by the subsurface drainage capacity of the site. The drainagecapacity is defined by the soil profile and the daily hydraulic or linear mass loading to secondary or groundwater boundary surfaces. In some cases, however, the infiltration rate of the wastewater into the soil at theinfiltrative boundary is more limiting. Therefore, it is important to distinguish between the two boundaries if useof the site is to be maximized. Where hydraulic loadings to secondary boundaries are the principal controlfeature, the only option is to limit the amount of water applied to the secondary boundaries. This can beaccomplished through the following:

OrOrOrOrOrientation, geometrientation, geometrientation, geometrientation, geometrientation, geometryyyyy, and controlled dosing of the infiltr, and controlled dosing of the infiltr, and controlled dosing of the infiltr, and controlled dosing of the infiltr, and controlled dosing of the infiltrativativativativative surfe surfe surfe surfe surfaceaceaceaceace

The infiltrative surface should be oriented parallel to and extended as much as possible along the surfacecontour of the secondary boundary. Southern, eastern, and western exposures may provide betterevaporation than north-facing slopes. The daily hydraulic loading rate onto the total downslope projection of“stacked” infiltration surfaces (multiple, evenly spaced SWIS trenches placed on the contour on slopingterrain) should be limited to the maximum linear loading of the secondary boundary. Timed dosing to theinfiltrative surfaces should be used to apply wastewater uniformly over the full length of the infiltrativesurfaces to minimize the depth of soil saturation over the secondary boundary. Note that the presence ofother SWIS-based treatment systems above or below the site should be considered in load calculationsand design concept development.

Installation of water-conserving plumbing fixtures in the building servedInstallation of water-conserving plumbing fixtures in the building servedInstallation of water-conserving plumbing fixtures in the building servedInstallation of water-conserving plumbing fixtures in the building servedInstallation of water-conserving plumbing fixtures in the building served

The total daily volume of wastewater generated can be significantly reduced by installation of water-conserving fixtures such as low-volume flush toilets and low-flow showerheads (see chapter 3). Also,wastewater inputs from tub spas and automatic regenerating water softeners should be eliminated.

Maximizing the evapotranspiration potential of the infiltration systemMaximizing the evapotranspiration potential of the infiltration systemMaximizing the evapotranspiration potential of the infiltration systemMaximizing the evapotranspiration potential of the infiltration systemMaximizing the evapotranspiration potential of the infiltration system

Where the growing season is long or use of the property is limited to the summer months,evapotranspiration can help to reduce the total hydraulic loading to the secondary boundary. The infiltrativesurfaces should be shallow and located in open, grassed areas with southern exposures (in the NorthernHemisphere).

If the infiltrIf the infiltrIf the infiltrIf the infiltrIf the infiltration capacity at the soil’ation capacity at the soil’ation capacity at the soil’ation capacity at the soil’ation capacity at the soil’s infiltrs infiltrs infiltrs infiltrs infiltrativativativativative surfe surfe surfe surfe surface is the limiting condition, measures toace is the limiting condition, measures toace is the limiting condition, measures toace is the limiting condition, measures toace is the limiting condition, measures toincrease infiltrincrease infiltrincrease infiltrincrease infiltrincrease infiltration can be takation can be takation can be takation can be takation can be taken.en.en.en.en. These measures include the fThese measures include the fThese measures include the fThese measures include the fThese measures include the folloolloolloolloollowing:wing:wing:wing:wing:

Reducing the mass loadings of soil clogging constituents on the infiltrative surface

The mass loadings to the infiltrative surface can be reduced either by increasing the infiltrative surfacearea to reduce the mass constituent loading per unit of area or by removing the soil-clogging constituentsbefore soil application. Where the suitable area for the SWIS is limited, increasing the infiltrative surfacearea might not be possible.

Controlled dosing of the infiltrative surfaceControlled dosing of the infiltrative surfaceControlled dosing of the infiltrative surfaceControlled dosing of the infiltrative surfaceControlled dosing of the infiltrative surface

Timed dosing and alternate “resting” of infiltrative surfaces allow organic materials that might clog the soilsurface to oxidize, helping to rejuvenate infiltrative capacity. Using multiple timed doses throughout the daywith intervals between doses to allow air diffusion maximizes the reaeration potential of the subsoil (Otis,1997). Dual infiltration systems that can be alternately loaded allow for annual resting of the infiltrativesurfaces to oxidize the biomat. On small lots dual systems are often not feasible because of spacelimitations.

Source: Otis, 2001.



5.7.1 Identifying appropriate treatmenttrains

Multiple treatment trains (system designs) are oftenfeasible for a particular receiver site and expectedwastewater flow. More than one receiving environ-ment may be suitable for a treated discharge. Forexample, subsurface infiltration or a point dis-charge to surface water might be feasible. Multiplesites on a property might be suitable as a receiversite. In addition, more than one treatment trainmight meet established or proposed performancerequirements. Each of these alternatives must beconsidered to select the most appropriate system fora given application.

Evaluation of the feasible alternatives is a continu-ous activity throughout the preliminary designprocess. It is beneficial to eliminate as manypotential options as possible early in the prelimi-nary design process so that time can be spent on themost probable alternatives. Typically, receivingenvironments are the first to be eliminated. Forexample, in temperate climates atmosphericdischarges are rarely feasible because there isinsufficient net evaporation to evaporate thewastewater. Surface water discharges usually canbe eliminated as well because often they are notpermitted by the local regulatory agency. Wheresuch discharges are permitted, subsurface infiltra-tion is usually less costly if the site meets theregulatory agency’s requirements because monitor-ing costs for compliance with point dischargepermit requirements can be substantial.

At the completion of the site evaluation, thereceiving environment has been tentatively selected(see section 5.5). For each potential receiver site,the design boundaries have been identified.Integrating information on physical limitations andestablished or proposed performance requirementshelps to define the maximum mass loadings to thedesign boundaries (see section 5.3). Defining andcharacterizing the controlling design boundariesand their maximum acceptable mass loadings,estimating the characteristics of the wastewater tobe treated, and evaluating the site conditionsinform the development of a feasible set of poten-tial treatment trains. Treatment train assembly isusually straightforward for surface water dis-charges because the effluent concentration limits atthe outfall control design. With soil-based systems

such as SWISs, however, treatment train selectionis more complex because multiple design bound-aries can be involved.

Because direct control of SWIS performance is lostonce the partially treated wastewater enters the soilat the infiltrative surface, management of theloadings to any secondary design boundaries andwater table boundaries must be accomplishedindirectly through appropriate adaptations at theprimary infiltrative surface. For hydraulic loadings,control can be achieved by changing the geometryor size of the infiltrative surface or the dosingvolume, frequency, and pattern. For organic orconstituent loadings, control is achieved either bypretreating the wastewater before it is applied tothe infiltrative surface or by increasing the size ofthe infiltrative surface.

5.7.2 Treatment train selection

Where multiple treatment trains are feasible andtechnically equivalent, each must be evaluated withrespect to aesthetics, operation and maintenancerequirements, cost, and reliability before selectionof the final design concept.

5.7.3 Aesthetic considerations

Aesthetics are an intangible factor that must beaddressed with the owner, users, adjacent propertyowners, and regulators. They include consider-ations such as system location preferences, appear-ance, disruption during construction, equipmentand alarm noise, and odor potential. It is importantthat these and possibly other aesthetics issues bediscussed with the appropriate parties beforeselecting the design concept to be used. If theexpectations of the concerned parties are not met,their dissatisfaction with the system could affect itsuse and care.

5.7.4 Operation and maintenancerequirements

Specific and appropriate operation and mainte-nance tasks and schedules are essential if a waste-water system is to perform properly over itsintended service life. Important considerationsinclude



• Types of maintenance functions that must beperformed

• Frequency of routine maintenance

• Time and skills required to perform routinemaintenance

• Availability of operation and maintenanceservice providers with appropriate skills

• Availability of factory service and replacementparts

Traditional onsite systems are passive in design,requiring little operator attention or skill. Unskilledowners can usually access maintenance services orbe trained to perform basic maintenance tasks.Septage removal usually requires professionalservices, but these are readily available in mostareas. More complex wastewater systems, however,require elevated levels of operator attention andskill. The designer must weigh the availability ofoperator services in the locale of the proposedsystem against the consequences of inadequateoperation and maintenance before recommending amore complex system. The availability of factoryservice is also an important consideration. Whereoperation and maintenance services are not locallyavailable and the use of alternative systems thathave fewer operation and maintenance require-ments is not an option, the prospective systemowner should be advised fully before proceeding.

5.7.5 Costs

Costs of the feasible alternatives should be arrayedbased on the total cost of each alternative. Totalcosts include both the capital costs incurred inplanning, designing, and constructing the systemand the long-term costs associated with maintainingthe system over its design life (20 to 30 years inmost cases; see table 5-8). This method of costanalysis is an equitable method of comparingalternatives with higher capital costs but lowerannual operating costs to other alternatives withlower capital costs but higher annual operatingcosts. Often, owners are deceived by systems withlower capital costs. These systems might have muchhigher annual operating costs, a shorter design life,and possibly higher replacement costs, resulting inmuch higher total costs. Systems with higher capitalcosts might have lower total costs because therecurring operation and maintenance costs are less.

Choosing between alternatives with varying totalcost options is a financing decision. In some cases,capital budgets are tighter than operating budgets.Therefore, this is a decision the prospective ownermust make based on available financing options.Table 5-8 is an example of such a comparativeanalysis.

The USEPA Office of Wastewater Managementposts financing information for onsite wastewatertreatment systems or other decentralized systems(cluster systems not connected to a wastewatertreatment plant) on the Internet at http://www.epa.gov/owm/decent/funding.htm. Links areavailable at that site to financing programs sup-ported by a variety of federal, state, and otherpublic and private organizations.

5.7.6 Reliability

The reliability of the proposed system and the risksto the owner, the public, and the environment ifmalfunctions or failures occur must be considered.Potential risks include public health and environ-mental risks, property damage, personal injury,medical expenses, fines, and penalties. Where theseor other potential risks are significant, contingencyplans should be developed to manage the risks.Contingencies include storage, pump and haul(holding tank), redundant components, reservecapacity, and designation of areas for repair orreplacement components (e.g., replacement leachfield). These come at additional cost, so theirbenefit must be weighed against the potential risks.

5.7.7 Conceptual design

After evaluating the feasible options, the prelimi-nary treatment train components can be selected. Atthis point in the development of the design, the unitprocesses to be used and their sequence are de-fined. A preliminary layout should be prepared toconfirm that the system will fit on the available site.Sufficient detail should be available to prepare apreliminary cost estimate if needed. It is recom-mended that the conceptual system design andpreliminary layout be submitted to the regulatoryagency for conditional acceptance of the chosensystem. Final design can proceed upon acceptanceby the owner and regulatory agency.


Chapter 5: Treatment System SelectionTa

ble

5-8.

Exa

mpl

e of

a to

tal c

ost s

umm

ary

wor

kshe

et to

com

pare

alte

rnat

ives

a .

a Cos

ts d

ispl

ayed

are

not

typi

cal f

or a

ll sta

tes.

Cos

ts in

oth

er s

tate

s ar

e si

gnifi

cant

ly h

ighe

r.



5.8 Rehabilitating and upgradingexisting systems

Onsite wastewater treatment systems can fail tomeet the established performance requirements.When this occurs, corrective actions are necessary.Successful rehabilitation requires knowledge of theperformance requirements, a sound diagnosticprocedure, and appropriate selection of correctiveactions.

5.8.1 Defining system failure

Failure occurs when performance requirements arenot met (see table 5-9). Under traditional prescrip-tive rules, onsite wastewater systems must complywith specific siting and design requirements,maintain the discharged wastewater below groundsurface, and not cause backup in fixtures. Typi-cally, failures are declared when wastewater isobserved on the ground surface or is backing up inthe household plumbing. However, systems alsomay be declared as failed if they do not complywith the prescriptive design rules. Thus, except forhydraulic failures, systems can be declared failedbased on their design, but rarely based on treatmentperformance to date.

When failure is strictly a code compliance issuerather than a performance issue, enforcing correc-tive actions can be problematic because correctiveactions for code-based compliance might notreduce (and might even elevate) the potential riskto human health or the environment. Also, codecompliance failures can be much more difficult tocorrect because site or wastewater characteristicsmight prevent compliance with the prescriptive

requirements. In such instances, variances to therule requirements are needed to remove thenoncompliant condition. Performance codes, on theother hand, define failures based on performancerequirements consisting of specific and measurablecriteria. Usually, treatment options are feasible toachieve compliance, though costs can be a signifi-cant impediment.

5.8.2 Failure diagnosis

Wastewater system failures occur at the designboundaries when the acceptable boundary loadingsare exceeded. Prescribing an effective correctivemeasure requires that the failure boundary and theunsuitable boundary loading be correctly identified.

The manifestations of boundary failures can besimilar in appearance despite different locations orcauses of failure. For example, the primary infiltra-tive surface might fail to accept the daily wastewa-ter load, causing the discharged wastewater to seeponto the ground surface. The cause of failure mightbe that the daily hydraulic capacity of the infiltra-tive surface was exceeded, the instantaneoushydraulic loading (dose volume) was too great, orthe organic load was too high. In other instances,the linear loading on a site might be exceeded,causing a saturated zone above a secondary restric-tive horizon to rise and encroach on the infiltrativesurface (effluent mounding). The potential gradientacross this surface is reduced in this situation, andthe reaeration of the subsoil is inhibited. As a resultof the reduced gradient and increased clogging, theinfiltrative surface can no longer accept the daily

Table 5-9. Common onsite wastewater treatment system failures



loading and allows wastewater to back up in thetrenches and possibly to surface. Though the causesof failure in these two instances are different, thesymptoms are similar. Thus, it is important that asystematic approach to failure diagnosis be used.Failures occur for a reason. The reason for failureshould be determined before corrective actions areimplemented; if not, failures can recur. Thediagnostic procedure should be comprehensive, butbased on deductive reasoning to avoid excessivetesting and data gathering (figure 5-13). Anotherexample of a failure diagnosis, Failure AnalysisChart for Troubleshooting Septic Systems (FACTS)is provided in Adams et al., 1998.

In addition to specific design boundary failures,failures can be caused by system age. Tanks andpipes buried in the ground begin to deteriorate after20 or more years of use and may require repair orreplacement. In addition, the treatment capabilitiesof soils below infiltration fields that have been inuse for several decades might not be adequate forcontinued use. Years of treatment use can cause theinterstitial spaces between soil particles to becomefilled with contaminants (e.g., TSS, precipitates,biomass). Soil structure can also be affected aftermany years of use. Finally, changes in design andconstruction practices in the past 25 years have ledto marked improvements in system performanceand treatment capacity. These issues make consider-ation of system age a vital component of theoverall failure investigation.

5.8.3 Initial data gathering

When a failure is reported, relevant informationregarding the system should be gathered.

• Visual observation. A visual observation of thefailure should be made to confirm the informa-tion provided. Also, the owner should beinterviewed regarding the owner’s observations,use of the building, and other relevant informa-tion. Each of the system components should beinspected and mechanical components (e.g.,float switches, flow diverters) tested.

• Past operation and maintenance practices.Assessing operation and maintenance actionstaken over the past 3 to 5 years can often aid indetecting relatively simple problems. Perhapsthe tank has not been pumped, the tank filter (ifused) has not been cleaned, the electrical supply

Figure 5-13. Onsite wastewater failure diagnosis andcorrection procedure



to the pumps has not been checked, or theswitches have not been examined.

• System layout and boundary design loadings.The system layout can be obtained from thedesign drawings or from a site survey. From thelayout, the design boundary loadings should bedetermined or estimated based on the originaldesign flow.

• Soil test reports. Soil test reports should beobtained. If none are available, soil augertesting between the trenches or just outside theSWIS perimeter might be necessary to provide asimple description of the soil profile to deter-mine whether any significant secondary designboundaries might be present.

• Age of system. If the system age is less than 2years, it is likely the design boundary loadingswere in error or improper construction tech-niques (e.g., operation of heavy equipment onSWIS area, installation during wet conditions)that significantly altered the soil characteristicswere used. If the age of the system is greaterthan 2 years, it is likely that the design condi-tions changed. Changed conditions couldinclude changes in the building’s use, increasedwastewater flows, infiltration and inflow into thesystem, surface runoff over the system, im-proper maintenance, compaction of SWIS soilsby vehicle traffic, and others.

• Description of failure symptoms. The symptomsof failure are important. Historically, reportedfailures have usually been hydraulic in natureand tended to be manifested by surface seepage.Information on the location and frequency ofthe surface seepage helps to determine thespecific design boundary at which the failureoccurred and possible causes of the failure. Forexample, surface seepage above the infiltrationsystem suggests that the infiltrative surface isoverloaded, either hydraulically or organically.Seepage downslope from the system suggeststhat a secondary design boundary exists and isoverloaded hydraulically. If the failure isseasonal, wet weather conditions are likely to bethe cause; that is, clear water is infiltrating intothe system or causing inadequate subsurfacedrainage.

• Daily flow estimates. Estimates of daily waste-water flows derived from water meter data or

other sources are needed to compare the designloadings with actual loadings. In the absence ofdata, water use should be estimated (see chapter3) with the caveat that such estimates are seldomaccurate. Where practical, water meters shouldbe read or installed as soon as the failure isreported so that metered data can be collected.Initially, daily flow estimates might need tosuffice for the purposes of failure analysis.Leaking plumbing fixtures, such as improperlyseated toilet tank flapper valves, should beinvestigated.

5.8.4 Determining the cause of failure

From the gathered data, hypotheses of potentialcauses of failure should be formulated. Formulat-ing hypotheses is an important step in diagnosingthe problem because the hypotheses can be tested toprovide a systematic and efficient analysis ofpossible causes of failure (see case study). Testingcan take many forms (see table 5-10 as an exampleof a local approach) depending on the hypothesesto be tested. It may include soil profile descrip-tions, soil hydraulic conductivity testing, wastewa-ter characterization, equipment testing and monitor-ing, and other tests.

5.8.5 Designing corrective actions

If the design boundary failure can be identified andits cause identified, selecting an appropriatecorrective action is straightforward. Table 5-11 canbe used to select the appropriate corrective actionfor a given boundary failure. This table presentsclasses of corrective actions and the impacts theycan be expected to have on boundary mass load-ings. Several options typically exist for each classof corrective action. Specific actions will bedetermined by the particular needs of the systemand site.

The failure diagnosis and correction procedureoutlined in figure 5-13 provides a summary ofactivities required to identify and characterize thecause of failure. As noted in the previous discus-sion, data collection, failure cause determination,and testing of hypotheses (e.g., as in the case studyabove) provides key information needed to developcorrective actions. Failures at design boundaries(e.g., exceeding mass pollutant or hydraulic loadlimits) can be rectified by changing boundary



Table 5-10. General OWTS inspection and failure detection processa

Table 5-11. Response of corrective actions on SWIS boundary mass loadings.

Source: Otis, 2001.



Failure hypothesis testing at a system serving a highway rest areaFailure hypothesis testing at a system serving a highway rest areaFailure hypothesis testing at a system serving a highway rest areaFailure hypothesis testing at a system serving a highway rest areaFailure hypothesis testing at a system serving a highway rest area

A wastewater system serving a highway rest area used a drip distribution system for final treatment anddispersal of the wastewater. After the first summer of use, water was observed above the dispersal system.The original soil test results indicated that the soils were deep, loamy sands with no apparent secondaryboundaries. The system design appeared to use appropriate loadings on the infiltrative surfaces.

A visual inspection and interviews with the maintenance staff at the rest area provided important clues:

✔ The site of the dispersal system had been significantly regraded after the soil testing had beencompleted. Up to 5 feet of material had been removed from the site.

✔ The system was a replacement for another system that had also failed. The existing septic tanks were usedin the new system.

✔ Water use was metered and recorded daily.✔ The rest area had a sanitary dump station that discharged into the wastewater system. The dump station

received very heavy use on weekends during the summer. This load was not accounted for in the meteringdata.

From these clues, several hypotheses were formulated for testing.

a. Water discharges to the system exceed the hydraulic and constituent design loadings.

This hypothesis can be tested by estimating daily wastewater discharges. The recorded water meter dataprovide an accurate estimate of water use at the rest area. The metered data would need to be corrected forturf irrigation at the rest area. Turf irrigation can be estimated from staff interviews of irrigation schedules.Unaccounted water from the sanitary dump station must be estimated. Counting the number of vehiclesusing the dump station and assuming an average volume of wastewater discharged per vehicle wouldprovide a reasonable estimate. Because of the strength of the dump station wastewater, wastewatersamples at the septic tank outlets should be taken to determine organic loadings.

Another issue that might need to be considered is load inputs from disinfectants or other chemicals used inholding tanks that are discharged into the dump station. Significant concentrations of these chemicals couldaffect biological processes in the tank and infiltrative zone.

b. Infiltration/inflow of clear water into the system or into the SWIS is excessive.

Only the septic tanks were left in place during the reconstruction of the existing system. All newcomponents were leak tested during construction. It can be assumed that the new portion of the systemdoes not leak if inspection records exist and can be verified. The existing septic tanks could be expected tobe the source of any inflow or infiltration. Infiltration of surface runoff from the area over the septic tanks,revealed by the existence of saturated soils around the tanks, could result in significant infloinfiltrationcontributions. If there is evidence that such conditions exist, the septic tanks should be pumped and testedfor leakage. Runoff of storm water onto the SWIS surface could also cause ponding and might requireregrading of the surrounding site or a diversion to route runoff elsewhere

c. The actual soil characteristics at the receiver site are different from the soil test results.

The characteristics of the soils after regrading might be different from those reported by the original soiltests because of the depth of soil removed. Also, the regrading operations might have compacted thesubsoil, creating a secondary design boundary that was not anticipated. Soil tests could be performed todetermine if the existing profile below the dispersal system is different in texture, structure, and bulk densityfrom that reported earlier. Also, the source of the surface seepage should be investigated. If the seepageoccurs immediately above a dripperline but the soil is not saturated between the lines, the infiltrative surfacesurrounding the dripperline is hydraulically or organically overloaded. If the soil between the lines issaturated, a secondary boundary that is hydraulically overloaded probably exists. If such a boundary ispresent, the soil below the boundary would be unsaturated.

By developing these hypotheses, determination of the failure can be systematic and efficient. The most probablehypothesis can be tested first, or appropriate tests for all the hypotheses formulated can be performed at one timefor later evaluation.

Source: Otis, 2001.



loadings to accommodate the hydraulic or masspollutant assimilative capacities at the designboundary. Loading adjustments may requirelowering water usage through water conservationmeasures, eliminating clear water inputs, orseparating graywater; increasing the area of theinfiltrative surface; or diverting precipitation and/or shallow ground water from the SWIS with bermsor curtain drains.

Approaches for lowering mass pollutant loadsinclude improving pretreatment by upgrading theexisting system and/or adding treatment units,improving user habits (e.g., removing food,kitchen, or dishwashing wastes from the wastewaterstream), reducing or eliminating inputs of cleanersor other strong chemical products, and reducingsolid waste in the wastewater stream (e.g., groundgarbage from garbage disposals). If measures tocorrect failures within the existing receiver site arenot possible, corrective actions may involvechanging the receiver site or changing the receiversite conditions. These options include adoption ofdifferent treatment technologies, physical alterationof the receiver site, and installation of a newinfiltration system, thereby resting the existingsystem for future alternate dosing.

Attention to established performance requirementsand the design boundaries where they are measuredhelps to ensure that corrective actions meet theoverall goals of the management entity and protecthuman health and the environment. Implementationof corrective actions should follow the sameprocesses and procedures outlined in the precedingsections for new or replacement OWTSs.

ReferencesAdams, A., M.T. Hoover, W. Arrington, and G.

Young. 1998. FACTS: Failure Analysis Chart forTroubleshooting Septic Systems. In Onsitewastewater Treatment: Proceedings of the EighthNational Symposium on Individual and SmallCommunity Sewage Systems. American Societyof Agricultural Engineers, St. Joseph, MI.

American Society for Testing and Materials(ASTM). 1996a. Standard Practice for SurfaceSite Characterization for Onsite Septic Systems.ASTM Practice D5879-95 el. American

Society for Testing and Materials, WestConshohocken, PA.

American Society for Testing and Materials(ASTM). 1996b Standard Practice forSubsurface Site Characterization of Test Pitsfor Onsite Septic Systems. ASTM PracticeD5921-96 el. American Society for Testing andMaterials, West Conshohocken, PA.

American Society for Testing and Materials(ASTM). 1997. Standards Related to On-SiteSeptic Systems. ASTM publication code 03-418197-38. American Society for Testing andMaterials, West Conshohocken, PA.

American Society for Testing and Materials(ASTM). 2000. Standard Practice forClassification of Soils for EngineeringPurposes (Unified Soil Classification System).ASTM D 2487-00. American Society forTesting and Materials, West Conshohocken,PA.

Ayres Associates. 1993. Onsite Sewage DisposalSystem Research in Florida—An Evaluation ofCurrent OSDS Practices in Florida. Report tothe Department of Health and RehabilitativeServices. Ayres and Associates, Tallahassee,FL.

Black, C.A., ed. 1965. Methods of Soil Analysis.Part 1: Physical and MicrobiologicalProperties, Including Statistical Measurementand Sampling. American Society of Agronomy,Madison, WI.

Bouma, J., and L.W. Dekker. 1981. A method ofmeasuring the vertical and horizontal hydraulicsaturated conductivity of clay soils withmacropores. Soil Science Society of AmericaJournal 45:662.

Bouwer, H. 1978. Groundwater Hydrology.McGraw-Hill Book Company, New York, NY.

Bouwer, H., and R.C. Rice. 1976. A slug test fordetermining hydraulic conductivity ofunconfined aquifers with completely orpartially penetrating wells. Water ResourcesResearch 12:423-428.

Brock, R.P. 1976. Dupuit-Forcheimer and potentialtheories for recharge from basins. WaterResources Research 12:909.



Finnemore, E.J., and N.N. Hantzshe. 1983.Ground-water mounding due to onsite sewagedisposal. Journal of the American Society ofCivil Engineers Irrigation and DrainageDivision 109:999.

Florida Department of Health and RehabilitativeServices (Florida HRS). 1993. Onsite SewageDisposal System Reseach in Florida.Tallahassee, FL.

Freeze, R.A., and J.A. Cherry. 1979. Groundwater.Prentice-Hall, Englewoods Cliffs, NJ.

Fredrick, J.C. 1948. Solving disposal problems inunsewered areas. Sewage Works Engineering19(6):292-293, 320.

Glegg, G.L. 1971. The Design of Design.Cambridge University Press, London, England.

Hantush, M.S. 1967. Growth and decay of groundwater mounds in response to uniformpercolation. Water Resources Research 3:227.

Hantzsche, N.N. 1995. Data Management Systemfor On-Site Wastewater Inspection Program atThe Sea Ranch, California. In Proceedings ofthe Sixth International Symposium onIndividual and Small Community SewageSystems. American Society of AgriculturalEngineers, St. Joseph, MI.

Hill, R.L., and L.D. King. 1982. A permeameterwhich eliminates boundary flow errors insaturated hydraulic conductivity measurements.Soil Science Society of America Journal46:877.

Hoover, M.T. 1997. A Framework for SiteEvaluation, Design, and Engineering of On-site Technologies Within a ManagementContext. Marine Studies Consortium, WaquoitBay National Esturarine Research Reserve, andad hoc Task Force for DecentralizedWastewater Management. Marine StudiesConsortium, Chestnut Hill, MA.

Kahn, M.Y., D. Kirkham, and R.L. Handy. 1976.Shapes of steady state perched groundwatermounds. Water Resources Research 12:429.

Munsell. 1994. Munsell Soil Color Charts.GretagMacbeth LLC, New Windsor, NY.

North Carolina Department of Environment,Health, and Natural Resources (North Carolina

DEHNR). 1996. On-Site WastewaterManagement: Guidance Manual. NorthCarolina Department of Environment, Health,and Natural Resources, Division ofEnvironmental Health, On-Site WastewaterSection, Raleigh, NC.

National Small Flows Clearinghouse. 2000.National Environmental Service Center. WestVirginia University, Morgantown, WV.

Natural Resources Conservation Service (NRCS).1998. Field Book for Describing and SamplingSoils. Version 1.1. U.S. Department ofAgriculture, Natural Resources ConservationService, National Soil Survey Center, Lincoln,NE. <http://www.statlab.iastate.edu/soils/nssc/field gd/fieldgd.pdf>.

Otis, R.J. 1997. Considering Reaeration. In NinthNorthwest On-Site Wastewater Treatment ShortCourse and Equipment Short Course,University of Washington, Seattle.

Otis, R.J. 1999. Designing on the Boundaries: AStrategy for Design of Onsite TreatmentSystems. In Proceedings of the Eighth AnnualNOWRA Conference and Exhibit. NationalOnsite Wastewater Recycling Association,Northbrook, IL.

Otis, R.J. 2001. Boundary Design: A Strategy forSWIS Designed and Rehabilitation. In OnsiteWastewater Treatment Proceedings of the NinthNational Symposium on Individual and SmallCommunity Sewage Systems. ASAE, St. Joseph,MI.

Powell, G.M. 1990. Why Do Septic Systems Fail?Kansas State University Cooperative ExtensionService, Manhattan, KS. <http://hermes.ecn.purdue.edu:8001/cgi/convertwq?5862>. Accessed August 6, 2000.

Purdue University. 1990. Steps in Constructing aMound (Bed-Type) Septic System. CooperativeExtension Service, Purdue University, WestLafayette, IN. <http://www.agcom.purdue.edu/AgCom/Pubs/ID/ID-163.html>.

Schoeneberger, P.J., D.A. Wysocki, E.C. Benham,and W.D. Broderson. 1998. Field Book forDescribing and Sampling Soils. U.S.Department of Agriculture, Natural Resources



Conservation Service, National Soil SurveyCenter, Lincoln, NE.

Sprehe, T.G. 1997. Onsite Wastewater ManagementPractices in the Upper Patuxent Watershed.Submitted to the Washington SuburbanSanitary Commission, Laurel, MD, by George,Miles & Buhr, Hunt Valley, MD.

Tyler, E.J., and J.C. Converse. 1994. SoilAcceptance of Onsite Wastewater as Affectedby Soil Morphology and Wastewater Quality.In On-Site Wastewater Treatment: Proceedingsof the Seventh International Symposium onIndividual and Small Community SewageSystems, ed. E. Collins. American Society ofAgricultural Engineers, St. Josephs, MI.

U.S. Department of Agriculture (USDA), SoilSurvey Staff. 1993. Soil Survey Manual.USDA handbook no. 18. U.S. GovernmentPrinting Office, Washington, DC.

U.S. Department of Agriculture (USDA), SoilSurvey Staff. 1951. Soil Survey Manual.USDA handbook no. 18. U.S. GovernmentPrinting Office, Washington, DC.

U.S. Environmental Protection Agency (USEPA).1984. Land Treatment of MunicipalWastewaters Process Design Manual—Supplement on Rapid Infiltration and OverlandFlow. EPA/625/1-81-013a. U.S. EnvironmentalProtection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA).1981. Land Treatment of MunicipalWastewaters Process Design Manual. EPA/625/1-81-013. U.S. Environmental ProtectionAgency, Cincinnati, OH.

Vespaskas, M.J. 1996. Redoximorphic Features forIdentifying Aquic Conditions. Technicalbulletin 301. North Carolina AgriculturalResearch Service, North Carolina StateUniversity, Raleigh, NC.


5-40 (Click Here to Return to Bookmarks Page) USEPA Onsite Wastewater Treatment Systems Manual

Chapter 5: Treatment System Selectiononsite.tennessee.edu/EPA Decentralized CD/Decentralized...

Documents

Transcript of Chapter 5: Treatment System Selectiononsite.tennessee.edu/EPA Decentralized CD/Decentralized...