. . . .

Naturai .Systems for Waste ManagemeflJ

and Treatment-

. Sherwood C. Reed Ronald W. Crites

~:~ Joe iVIiddtebrooks

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Second Edition . , .

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McGraw-Hill, lnc. New York San Francisco Washington, D.C. Auckland :'Eicig~

Caracas Usbon london Madrid Mexico CiLY. ~Milan Montreal New Delhi San Juari satgapore

Sydney Tokyo Toronto

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Library of Congress CataloJ,!ing--in-Puhlication Data

Heed. Sherwood C. Z'\r;tural :-ystems for wc:tstt man' ~Iiddlebrook;;. - p. em. Inc.lude~ bibliographical -rc-fer.ences and index. I Si3;-; 0-0 7 -060~)82-9 i acid-free paper' L Sewage-Purification-Biological treatmeJ ,. 2. Sewage sludgc-~lanagemenL L Crites. Ronald W. II. Midd lebrooks. E .. Joe. TIL Title. TD755.R43 1995 628.3'5- dc20 94-33399

CIP

Copyright 1995'. 1988 by McGraw-HilL Inc. All rights reserved. Printed in the United State~ vf America. Except a~ permitted under .the United States Cop}-right Act of1976. no pan of this publication may be reproduced or' dis:tr.hilled in any form or by any means. ur stored in n data base or retri"\:il. sysr~m. v.ithout the prior written per-mi::sion of the publisher.

2 3 4 5 6 7 8 9 10 it 12 ~5 14 Bt:ME!KM 9 9 8 7 6

.ISB:\ 0-iH -06098:?-l?

Th.: . .;;p;n:~oring editor .t;:.r thi.- '-look u:a:-; Larry S. Hager. the t~diting supenisor uas 0/ite B . Coilln. and the production !:w.peni.o.:or was Pamela A Pdton. The nooh u:o::;; set iJ?. Cer:tury Schoolbook by )fcGra.:c-Hrl!'.-; Pro;f:.c;.

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Contents

Preface xi

Chapter 1. Natural Waste Treatment Systems: An Overview 1

1-1 Natural treatment Processes 1 Background 1 Wastewater treatment concepts and expectations 2

1.2 Project Development 9 References 9

Chapter 2.. Pfanning, Feasibility Assessment, and Site Selection 11

2.1 Concept Evaluation 11 Resources required 13 Preliminary estimates of land area 13

2.2 Site Identification 21 Screening procedure 21 Climate 24 Flood hazard 25 Water rights 27

2.3 Site Evaluation 28 Soils investigation 29 tnmtration and permeability 33 Bufferzones 40

2.4 Site and Process selection 40 References 41

Chapter 3. Basic Process Responses and ln.teracfions

3.1 Water Management Fund2mental relationships Movement af pollutants Groundwater mounding Underdrairuige

3.2 Biodegradable Organics Removal of BOD Removal of suspended solids

' 43

43 44 47 51 59 60 61 62

v

vi Contents

3.3 Organic Priority Pollutants Removal methods Removal performance Travel time In sons

3.4 : Pathogens Aquatic systems Wetland systems .Land treatment systems Sludge systems Aerosols

3.5 Metals Aquatic systems Wetland systems Land treatment systems

3.6 Nutrfents Nitrogen Phosphorus Pot:assrum and other micro nutrients

References

Chapter 4. Wastewater Stabilization Ponds 4.1 Preliminary Treatment 4.2 Facultative Ponds

Areat loading rate method Gtoyna equation .,.

Co~plete-mix model Plug 11ow model Wehner-Wilhelm equation Comparison of facultative pond design models

4.3 Partial-Mix Aerated Ponds Partial-mix design model Pond configuration Mixing and aeration

4;4 Controlled-Discharge Ponds 4.5 Complete-Retention Ponds 4.6 Combined Systems 4.7 Anaerobic Ponds 4.8 Pathogen Removal 4.9 Removal of Suspended Solids

Intermittent sand fiJtrcrtion MicroStrainers. Rock filters Other solids-removal techniQues

4 .10 Removal of Nitrogen Design models

4.11 Removal of Phosphorus Batch chemicaf treatment Continuous-overflow chemical treatment

4.1? Physical Design and Construction Dilce construction

Pond sealing Pond hydraulics

4.13 Storage Ponds for Land Treatment Systems References

Chapter 5. Aquatic Treatment Systems 5.1 Aoatlng Plants

Water hyacinth Duckweed

5.2 Submerged Plants Performance expectations Design considerations

5.3 Aquatic Animals Daphnia and brine shrimp rash Marine polyculture

References

>('Chapter 6. Wetland Systems 6.1 Introduction

Naluraf wetlands Mitigation and enhancement Constructed wetlands Design concepts

6.2 Wetland Components Plants Emergent species Submerged specfes Floating species Evapotranspiration losses Oxygen transfer Plant diversity Plant functions Soils Of!PD1isms

6.3 Perf~mrcnce Expectations Removal of BOD Removal of suspended solids ~erf!oval of nitrogen Removal-of phosphorus Remove! of metals Organic priority pollutants Removal of pathogens

6.4 General Design Procedures 6.5 HydrauUc Design Procedures

Free-water-surface wetlands Subsurface-flow wetlands

6.6 Thermal Aspects Subsurface-flow wetlands Free-water-surface wetlands Summary

...

Contents vii

127 128 129 130

133

134 134 158 165 166 167 167 167 168 170 170

173

173 173 174 175 177 178 178 179 180 181 181 182 183 184 185 186 186 187 . 189 191 196 197 198 199 200 202 203 205 210 211 216 221

viii Contents

6.7 Design Models for BOO Removal 221 Free-water-surface :wettands 223 Subsurface-flow weUa.nds 226 Preliminary treatment 231

6.8 Design Models for TSS Removal 232 6.9 Design Models for Nilragen Ramoval 234

Free-water-surface wetlands 235 Subsurface-flow wetlands 239 Nitrification filter bed 2tUi Summary 250

6.10 Design Models for Phosphorus Removal 250 6.11 Design of On-site Sys.tems 252 6.12 Vertical-Flow Wetland Beds 256 6.13 Wetland Applications 258

Domestic wastewaters 258 Municipal wastewaters 259 Commercial and industrial wastewaters 260 Stormwater runoff 261 Combined sewer o.verffaws 263 Agricultural runoff 265 Livestock wastewaters 268 Landfif! leachates 269 Mine drainage 272

6.14 Construction Requirements 275 Subgrade construction and liners 275 Vegetation 2n Inlet and outlet structures 2n Costs 2:79

5.15 Operation and Maintenance 280 Vegetation . 280 Mosquito control 281

References 281 ~ Chapter 7. Land Treatment Systems 285 7.1 System Types 285

Slow-rate systems 285 Overland-flow systems 286. Rapid-infittratio n systems 287

7.2 Slow-Rate Systems 289 Design objectiv~s 289 Preapplication treatment 2..-Cl{J Crop selection 291 Loading rates 2-03 Land requirements .302 Storage requirements 303 Application scheduling 303 Distribution techniques 304 Control of surface runoff 304 Underdrainage 30S System management '307 System monitoring 30S

7.3 Overland-Flow Systems Design objectives Site selection Preapplication treatment Climate and storage Design procedure Land requirements Vegetation selection Distribution system Slope design and construction Runoff collection Recycling System management and monitoring

7.4 Rapid.Jnfiltration Systems Design objectives Design procedure Treatment performance Nitrification Nitrogen removal Phosphorus removal Preappfication treatment Hydraufic loading rates Organic loading rates Land requirements Basin construction Winter operation in cold climates System management System monitoring

References

Chapter 8. Sludge Management and Treatment

a 1 Sludge Quantity and Characteristics . Sludges from oatural treatment systems Sludges from drfnking-wa1er treatme'nt

.8.2 Stabilization and Dewatering Methods for pathogen reduction

8.3 Sludge Freezing Effects of freezlng Process requirements Design procedures Sludge freezing facilities and procedures

8.4 Reed Beds Functi.on of vegetation Design requirements.

Performance Benefits Sludge q~afity

8.5 Vermistabilization Worm species Loading eriteria Procedures and performance Sludge .quality

Contents ix

309 309 31C 310 3i1 311 315 318 319 320 320 320 320 321 321 32'1 322 322 323 325 325 326 32S 326 330 331 331 332 332

335

~.s 337 338 339 339 340 340' ~ 342 345 347 348 349 350 352 353 353 353 '354 354 355

x Contents

8.6 Comparison of Bed-Type Operations 8.7 Composting 8.8 Land Application and Surface Disposal of Sludge

concept and site selection Process d~sign, land application Desrgn of surface disposal systems

References.

Chapter 9_ Oa-site Wastewater Management Systems

9.1 Types E>f On-site Systems 9.2 Site Assessment

Preliminary site evaluation Detailed site assessment

9.3 On-site Treabnent Alternatives Septic tanks Imhoff tanks Oil and grease removal lntennittent sand ft.lters

~ecirculaling fine gravel filters Alternative nitrogen-removal processes Package aeration systems

9.4 On-site Disposal Alternatives Gravity leachfields Pressure-dosed distribution . Fill systems : At::grade systems Mound systems Artificially drained systems Evapotranspiration .systems

9.5 Management of On-site Systems References

Appe.ndix

Table A.1 Metric Conversion Factors (Sf to U.S. Customary Units) Table A.2 Conversion Factors for Commonty Used Design Parameters

Table A.3 Physical Properties of Wafer Table A.4 Dissolved Oxygen Solubility in Fresh. Water

Index 423

. . '

Preface

This book is intended for the practicing engineer involved in the .Plan.:. ning, design, construction, or operation of waste management facili-ties (both wastewater and sludges) for on-site service, municipalities, and industries.

The focus in this book is on waste management processes which depend to a maximum degree on natural components and to a minima] degree on. mechanical elements. This utilization of natural systems can re4uce costs, process energy, and complexity of operation. These natur-al processes should be given priority consideration for planning new systems and for.upgrading or retrofitting existing systems.

Some of the processes included in this book, such as pond systems, may be familiar to many engineers, but the text presents simplified, easy-to-use design procedures. The other, less familiar concepts can provide very effective treatment for significantly less cost than mechanical treatment alternatives. :Design criteria for some of the emerging tec-hnologies, particularly wetland systems, cannot be fmmd in any other text.

Each design chapter provides a complete descriptian of the subject technology, data on performance expectations, and detailed design pro-cedures with supporting examples. Chapter 2 presents the basic responses and interactions common to these natural biological systems. The treatment responses for toxic and hazardous materials are cover.ed in this chapter and discussed as appropriate in ~e de:,-ign ch~s. Chapter 3 provides a rational procedure for planning and for process and site selection for the natural treatment systems. Combined metric and U.S. customary units are used tbrnughout the text.

Sherwood C. Reed

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Chapter

Natural Waste Treatment Systems: .An Overview

The waste treatment systems described jn this hook are designed specifically to utilize natural response$ to the maximum possible degree while obtaining the intended waste treatment or management goal. In most cases this will result in a system that costs less to build and to operate and requires less energy than mecba11ical treatment alternatives.

1.1 Natural Treatment Processes All waste management processes depend on natural responses snch as gravity forces for sedimentation, or on natural components such as biological organisms. However, in the typical ca._~ these natural com-ponents are supported by an often-complex array of energy-intenSive mechanical equipment. The term natural system as used in this text is intended to describe those processes that depend primarily on their natural components to achieve the intended purpose. A natural s-ys-tem might typically include pumps and piping for waste conveyance but would not depend exclusively on e.nernal erlergy sources to main-tain the major treatment responses.

Background Serious interest in natural methods for waste treatment .reemerged in the United states following passage of the Clean Water Act of 1972 (PL 92-500). The major initial response was to assume that the "'zero discharge" mandate of the law could be obtaine4 via a combination of mechanical treatment units capable of aduanced u,.:astewatcr treat-

'.

6 Chapfer One

Chap. 6. Another variation of the concept, used for sludge drying, is described in Chap. 8.

Terrestria~ treatment methods. Table 1.3 pr;sents the typical design f~atures and' performance expectations for the three basic terrestrial con-cepts. AU three depend on the physical, chemical, and biological reac-tions on and within the soil matrix. In addition, the slow-rate (SR) and the overland-flow (OF) methods require the presence of vegetation as a major treatment component. The slow-rate process can utilize a wide range of vegetation? from trees to pastures to row crop vegetables. As described in Chap. 7, the overland-flow process depends on perennial grasses m en~ure a continu~us vegetated cover. The hydraulic loading rates, with some exceptions, on rapid-infiltration systems are typically too high to support ~neficial vegetation. All three concepts can pro-duce high-quality efilnent. In the ty-pical case the slow-rate. process can be designed to produce drinking water quality in the percolate.

Reuse of the treated water is possible with all three concepts. Recovery is easiest with overland flow. since it is a surface system that discharges to ditches at the toe of the treatment slopes. Most slow-rate and rapid-infiltration systems require underdrains or wells for water recovery.

Another type of terrestrial concept is on-site systems which serve single-fannly dwellings, schools, public facilities, and commercial operations. These typically include a preliminary treatment step fol-lowed by in-ground disposal Chapter 9 describes these on-site con-cepts;. small-scale constructed wetlands used for the preli.nllnary treatmen~ s~p are described in Chap. 6.

Sludge management concepts. The freezing, composting:t and reed bed concepts listed in Table L4 are intended to prepare the sludge for final disposal or reuse. The freeze/thaw approach described in Chap. 8

~

-.- ~ ~. , , ... ,. _,.._ - --' -"' . , ,. . ..., ... ._.., ' ' r ... , ,,., , ,..._ '''~ .,. .. _ ""' - - ... -.. .... - - .. -...:"...., ,.,. ... .._~ .-. .... ;.">:;- .. __ , ...... ,,.,,.-,...,:6-..-(-- oloOf_._ - --'"'---

TABLE 1.3 Terrestrial Treatment Units, Design Features, and Performance1113

Typicnl criteria

Concepts

Slow i'nte

H~ pid infil trnt.ion

Overland flow

Onaite

'l'ren tmett t go~l!3

Secondnty, at AW'r

Socnndnry,m AW'l', Ill' ground water toohrugo

Secondary, nltl'ogon tomoval

Secondary Lo tfJr~lnry

*'Fot de::tign flow of :178li m11/d, .

Climate needs

Watmer aonsons

NHnc

Wntmer seasons

None

tNitrogen t'omovul dopohdB Otl type or crop nnd mnnngmnoni. tFC = fccnl coliform, #/100 mL. .

Vegotntlon

Yos

No

Yos

No

Me(lsured In immodint.o vicinity ofbuHin; lncrunH

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8 Chapter One

TABLE 1.4 Sludge Management with Natural Method_s 1

Concept

Freezing .

Compost

Reed beds

Land applicatirm

Description

A method for condi-t ior:.i!'lg and dewater-ing s ludges in the win-ter months in cold cli-mates. More effccti\e and reliable than any ofthe available mechanical devices. Can use eJdsting sand beds. A procedure to further stabilize and dewater sludges, mth signifi-cant pathogen kill, so fewer restrictions on end use of final product. :Narrow i renche:; or be-ds. \\i th sand bottom and underd1ained, planted v.ith reeds. Veg"e tation assists

water remo\'al.

Application of liquid or partially dried sludge on agricultural, forest-c.-d., or re clamation land.

Limitations

Must have freezing weather for long enough to freeze the sludge layer completely.

Requires a bulking agent and mechanical equipment for mixing

and sorting; ~nter operations can be diffi-cult in cold climates.

Best suited for warm to m oderate climates. Annual harvest and disposal of \egetation is required.

State and federal regu-lations limit arinual and cumulative loading of metals. e tc.

Costs and energy. Intere::;t in natural concepts was originally based on the environmental ethic of recycle and reuse of resources -,.vherever possible . .Many of the concepts described in the pr~vious sections do incorporate such potentiaL However~ as more and i:nore systems were . built and operational experience accumulate

Natural Waste Treatm~nt Systems: An Overview 9

al and commercial systems also exist. These process selection deci-sions have been and will continue to be made on the basis of costs and energy requirements.

1.2 Project Development The development of a waste treatment project, either municipal or industrial, involves consideration or'institutional and social issues in addition to technical factors. These issues influence and can often control decisions during the planning and preHminary design stages. Current regulatory reqUirements at the federal, state, and local levels ar e p~rticularly i:r:qportant. The engineer must determine these requirements at the earliest possible stage of project development to ensure that the concepts under consideration are institutionally feasi-ble. References 3, 4, and 11 proVide useful gUidance on the institu-tional and social aspects of project development.

Table 1.5 provides summary guidance on the technical require-ments for project development, and indicates the chapter( s) in this book which provide the needed criteria. Detailed infunnation on waste characterization and on the civil and mechanical engineering details of design are not unique to natural systems and are therefore not included in this text. References 5 and 6 are recommended for that purpose.

References 1. Banks, L . and~ Davis: Wastewater and Sludge Treatment by Rooted Aquatic

Plants in San.d a:ncl Gravel Ba..~, in Proceedings of a Workshop on LGu: Cost Wast~water Treatment. Clemson Uni"-ersity, Clemson, SC. Apr. 1983, pp. 205-218.

2_. Bastian~ R. ~and S. c_ Reed

1 0 Chapter One

.

TABLE 1.5 . Guide to Project Development Task

Characterize waste

Concept feasibility'

Process design

Civil and m echanical details

. Description

Define the volume and the composition of the waste to be treated

Determine which if any of the natural sys-tems are compatible for the particular waste' and the site con-ditions and require-ments

Determine the waste constituent that con-trols design

Pond systems Aquatic systems Wetland systems Terrestrial systems Sludge management On-site systems

Collection network in the community, pump stations, transmission piping, etc.

'*Not covered in this text; see Refs. 5 and 6.

... ~-. -: .

See Chapter

*

2,3

3

4 5 6 7 8 9

*

9. Heed. S.C., R. Bastian. S. Black,. a.'ld R- K Khettr)r: Wetlands for Wastewater Treatment in Cold CliiD:ates, in Proceedings Water Reuse III Symposium, American Water Works Association, Denver, CO, Angnst 1984._ 10~ Ree~ S. C., R W. Crites, R. E_ Thomas, and A. B. Hais: Cost of Land Treatment

Systems. EPA 430/9-75-003, U.S. Environmental Protection Agency, Washington, DC,l979_ .

IL U.S. Environmental Protection .-\gency: Process Design Manua~-Land Treatment of Municipal Wastewater,.. EPA 625/1-81-013, Center for Environmental Research Information. Cincinnati, OH. Oct. 1981.

12. U.S. Environmental Protection Agency: Process Design Manual for Municipal Wastewater Stabilization Ponds, EPA 625/1-83.-015, Center for Environmental

Research Information, Cincinnati, Ohio, 1983. . 13. U.S. Environmental Protection Agency: Process ,Design Man]ml Supplement on

Rapid Infiltration and Overland Flow, EPA 625/l-81-013a? Cente~ for Environmental Research Information, Cincinnati, 0~ Oct. 1984.

Chapter

Planning, Feasi~ility

Assessment; and Site Selection

It is important during the early planning stages of a waste manage-. ment project to include as many alternatives as possible to ensure that

the most cost-effective process is selected. The feasibility of the natur:il treatment processes described in this book depends significantly on. site conditions, climate, and related factors. It is not ~ctical or economi-cal, however, to condnct e~ve field investigations for every process, at ever.y potential .site, dming planning and preliminary design. This chapter provides a sequential approach which first determines poten-tial feasibility and the land area required for treatment, and identifies possible sites. The second step evaluates these sites, based on technical and economic factors, and selects one or more for detailed investiga-tion.. The final step involves detailed field investigations, identification of the most ~st-effective alternative, and development of the criteria needed for final desiga

2.1 Coneept Evaluation A convenient starting point is to divide the many possible processes into discharging and nondischarging systems. ~e former group, .which typically have an ontf~ or other direct discharge to surface

. waters, would usually include treatment ponds, a~tics,. wetlands, and the overland-flow (OF) land treatment concept. The second, nond.ischarging, group includes the other land treatment concepts,

11

.. """~- .

12 Chapter Two

on-.site methods, a nd the sludge treatment methods. Site topography, soils, geology, and groundwater conditions are important factors for the construction of discharging systems, but are often critical campo .. nents in the treatment process itself for the second group. Design fea-tures and performance expectations for both types of systems are

gi~en in Tables L 1, 1.2, and 1.3; other special characteristics and requirements are listed in Table 2.1 and Table 2.2.

TABLE 2 .1 Discharge Systems: Special Site Requirements

Concept

Treatment ponds

Aquaculture

Constructed wetlands

0.-edund tlow 'OF i

Requirement

Proximity to a surface water for discharge, impermeable soils or liner, no steep slopes, out of flood plain or diked, no bedrock or groundwater within excavation depth

Same physical features as ponds, also must have suitable climate to support aquatic plants or other biological compo-nents

Prorimity to surface waters for discharge, impermeable soils or a liner. siope~ 0-39

Planning, Feasibility Assessment, Site Selection 13

It is presumed that any percolate from the nondischarging systems mingles with any groundwater that may be present, and may eventu-

. ally emerge as subflow in adjacent surface waters: These systems are typically designed to satisfy regulatory water-quality requirements in the percolate/groundwater as it reaches tl1:e project boimdary. Some of . these concepts can also be designed as a direct discharge if under- drains, recovery wells, or cutoff ditches are included as system compo-nents. The underdrained slow-rate (SR) land treatment system at Muskegon, Michigan, 4 is an example of this type, while the forested SR system in Clayton County, Georgia,' depends on natural subsur-face flow. This subflow does emerge in surface streams which are part of the community's drinking water sUpplies, but the land. treatment system is not considered to be a discharging system as defined by the U.S. Environmental Protection. Agency (EPA) and by the State of Georgia.

Resources required A preliminary determination of process feasibility and the identifi-cation of potential sites is based on the analysis of maps and other existing information. The requirements in Tables 2.1 and 2.2, along V\rith an estimate of the land area needed for each of th~ .con-cepts, are used in this procedure. The community maps sh~uld

. show: topography, water bodies and streams, flood hazard zones, community layout and land use (e.g., residential, commercial, industrial, agricultural, forest), existing water supply and sewer:-age systems. anticipated areas of growth and expansion, soil types within the community and adjacent areas. Sources for these maps include the U.S. Geological Survey (USGS), the Soil Conservation Service {SCS), state agencies~ as well as local planni.D.g and zoning agencies .

. Preliminary estimates of land area The land area estimates derived in this section are used with the information in Tables 2.1 and 2..2 to determine, with the map study~ if suitable sites exist for the process under consideration. These pre-

.liminary area estimates -are very conservative and are intended only for this preliminary evaiuation: They should not be used. for final design.

Treatment ponds. The area estimate for pond systems will depend on . ,; the effluent quality required [defined in terms of biochemical oxygen demand (BOD) and suspended solidS (SS)], on the type of po~ system

14 Chapter Two

proposed, a nd on the geographic location. A fa~ultative pond in the southern United States \"~.till require less area than the same process in Canada. TP.e .equations ~ven below are for total project area and include an allowance for dikes, roads, and unusable portions of the site.

Oxidation ponds. F_'or a 1-m (3-ft)-deep oxidation pond assumed to be in a wann climate, with 30 days' detention, and organic loading of 90 kg/ha/d (80 lb/add). the expected effiuent quality is BOD = 30 mg/L, 88>30, mg/L. Then

A = (k)(Q) -op .

. whereA0p = total project area, ha (ac) Q = design flow, m3/d (g/d) .k = (3.2)(10- 3 ).,. metric (3.0 x 10-5 , U.S. units).

(2.1)

Facultative ponds in cold crrmates. Assuming more than 80 days" deten-tion in a 1.5-m (5-ftJ.-deep pond and organic loading of 16.8 kg/bald ( 15 lb/add), the expected. effiuent quality is BOD = 30 mg/L, 88>30 mg/L. Then

(2.2)

where A fc = facultative pond site ~ ha (ac) k ~ (1.68)(10 -2)~ metric (1.6o x .I0- 4, U.S.)

(other terms as defined previously)_ Fa.~ultative ponds in warm climates. Assuming more than 60 days, deten-

. tion, in a 1.5-m {5-ft}-deep pond and organic loading of 56 kglha/d (50 . 1b/add), the expected efiluent quality is BOD = 30 ing/L, 88>30 mg/L .

. Then

.4. = (k "J(Q ) fw whe!e A fw = facultafu,.e pond, warm climate, site area, ha (ac)

k = (5.1){10- 3 ), metric (4..8 X 10-5; U.S .. units) {other terms a.S dEfined preciously).

(2.3)

. . .

Conlt'oled-dtscharge ponds. Controlled-discharge ponds are used in northern climates to avoid winter discharges and in warm areas to match effluent. quality to acceptable stream.fiow conditions. The typi-cal depth is 1.5 m (5 ft), maximum detention time is 180 days~ and expected effluent quality is BOD

I .. .. ,

Planning, Feasibility Assessment, Site Selection 15

k = (i.63)( 1o~ ), metric n.32 ;.- io-1, U.S. units) (other terms as defined previously) . .

Partial-mix aerated ponds. The size of partial-mix aerat~d ponds will vary with climate. For this purpose assume more than 50 days deten-tion, 2.5 m (8ft) depth, and organic loading of 100 kg/hald (89lb/add). Expected effiuent quality is BOD = 30 mg/L, 88>30 mg/L. Then

Aap = (k)(Q)

where Aap = aerated pond, site are~ ha (a c) k = (2.9)(10- 3), metric (2.7 x 10-3, U.S. units)

(other terms as defined previously).

(2.5)

Hyacinth systems. Hyacinth systems can be designed for treatment of raw sewage or for any other level up to tertiary polishing of secondary effiuent. As with other types of pond systems7 the critical design para-meter is organic loading. The degree of nutrient removal ac~eved with hyacinth systems is related directly to the frequency of 4,arvest. . . Hyacinth systems are practical only in locations where the plant can survive naturally; see Fig. 2.1 for this range and Chap. 5 for detailed design criteria~

Secondary hyacinth ponds. Hyacinth ponds for secondary treat;ment are designed for a raw sewage input,.. detention tinle more than 50

fJI Year- round 6 mont~s per year

..

Figure 2.1 Suitable areas for hyacinth syst~.

16 Chapter Two

days. depth of 1.5 m c 5 ftJ or less, organic loading rate of 30 kg/ha/d ( 27 lb/ac/dJ, and water temperature above l0C. Expected effiuent quality is BOD

Planning, Feasibility Assessment, Site Selection 17

include the area required for a preliminary treatment system before the wetland.

(2.9) where AC'\\. = cofu-tructed wetland, site area, ha ( ac)

k = (4~31Jf10- 3 ), metric (4.03 x Io-5, U.S. units> (other terms as defined previously)_

Overland flow. The size of an overland-flow (OF J project site will depend on the 'length of the operating season for this process. Figure 2.2 can be used to estimate the number of nonoperating days during which wastewater storage will be required. The design flow to the

overland-~ow system is then calculated using Eq. 2.10:

_ + (ts)(qc) Qm - qt' (f )

a

(2.10)

where Ql~ = average monthly design flow to land treatment site, m 3/mo \gal/n1o)

q:: = average monthly flc;w incommunitym3/mo (gal/mo) t . = number of months storag-e is required " ta = nmnber of months in operating season

f'"::' "1 2 t~ 5 days star-nne for L::.:J. . .....,. operoti.ar.aJ fleJtibi I it-y

Ftgure ~ Recommended stor~ue days for overland-flow systems.

18 Chapter Two

The detention time on the OF slope is about 1-2 h , and depth of water on the slope is a few centimeters or less. The process design is not typ.ically based on organic loading rates. Expected effluent quality is BOD = 10' mg/L, SS = 10 mg/L, tota l N

r

Planning, Feasibility Assessment, Site Selection 19

system is then calculated using Eq. 2.12. Organic loading is not usu-ally the critical design param~ter. Either nitrogen or the hydraulic capacity of the soil will control for most municipal effluents (see Chap. 7); responses to industrial pollutants are considered in Chap. 3. The area estimate given by Eq. 2.12 includes an allowance for preap-plication treatment in an aerated cell as well as a winter storage allowance and the actual land treatment area; a hydraulic loading of 5 c~wk (2 in/wk) is assumed. Expected effluent qua~ity is BOD

20 Chapter Two

TABLE 2.3 Land Area Estimates for 4000-m3/d Systems

Treatment system

Pond systems Oxidation Facultative Cont rolled discharge Partia l mix, aerated Hyacinth , secondary Hyacinth. adva nced secondary Hyacinth , tertiary

Constructed wetland Slow rate Overland flow Rapid infiltration

'-'.NA = not applicable. -i'Includes allowance for primary t reatment. +Includes a 20-ha facultative pond.

North

NA* 67.2 65.2 20.4 NA NA NA 24.0 134.0 92.0 6.0

Area Cha )

Mid-Atlantic

NA . 43.6 65.2

1~.3 NA NA NA 20.2 102.0

. 69.0 6.0

:.

South

12.8 20.4 65.2 ll.6 38.0 4.0t 22.8t 17.2 72.0 47.0 6.0

any preliminary ~reatrnent that might be required and for unused portions of the general site area.

Hyacinth systems ar_e not considered outside the range shown in Fig. 2 .1. Wetland systeni.s are. considered to 'be year-round operations for th?-s ptupose. Treatment responses in a wetland would proceed at a higher rate in warm climates, as descnoed in Chap. 6, so a smaller area would be required as comp~d to a northern site. That differ-ence is not critical at this stage of planning and process selection and is not included in Table 2.3.

Sludge systems. The land area required for sludge systems for com-posting~ sludge freezing~ vermistabilization, or reed bed dewatering is dependent on sludge quantity, moisture content7 and local climate. It is necessary to nse the procedures in Chap. 8 to determine the area required for each situation_ A special site investigation may not be necessary, since thes~ sludge treatment concepts are usually loC?ted in the vicinity of a wastewater treatment facility and require a minor portion of the total site area. The exception may be composting sites for large quantities of sludge, where a remote site may be desirable to avoid residential co~p1aints and to take advantage of lower land costs.

The area required for land application of sludges is also dependent on sludge quantity and characteristics as well as the type of operation intended. The loading rates in Table 2.4 can be ti.sed to calculate an estimate of the land area required for each of the major land applica-tion options.

Planning, Feasibility Assessment, Site Selection 21

TABLE 2.4 Sludge Loadings for Preliminary Site Area Determination

Option*

Agricultural Forest

Reclamation TypeB

Application schedule

Annual, for 10 years One time, or at 5-year intervals for 20 years Onetime Annual

*See Chap. 8 for detailed description of options.

2 .. 2 Site Identification

Typical rate (mtJhaJ

10

45 100 340

The information presented or develop~d in the previous sections is combined with maps of the community area to determine if feasible sites for wastewater treatment or sludge disposal exist within a rea-sonable distance.

It is unlikely that a community or industry will have site conditions, within reasonable proximity, for all of the y;rastewater treatment or sludge concepts listed in Tables 2.1 and 2.2, and several will usually be dropped from consideration at an early stage .. All of the technically suitable sites should be located on the maps. In the next evaluation step, local knowledge regarding. land use commitments, costs, and the

. technical ranking procedure (described in the next section) are consid-ered to determine which process(es) and site(s) are technically feasible. A complex screening procedure is not usually required for the pond, aquatic, and wetland concepts, since the number of potential sites is usually. limited. The critical factors in these cases are close proximity to the. wastewater source and access to surface water for final discharge. The opposite is true for the concepts that involve land application of wastewater or sludge, since a significant number of potential sites may exist. It will not be economjcal to -conduct detailed site investigations on all potential sites: so a preliminary screening is justified.

Screening procedure The screening procedure J:.ecom.mended by the EPA13 utilizes rating factors to evaluate each potential site_ Those sites with moderate to high scores are candidates for ~erions consideration .and site investiga-tion and testing. The conditions included in the general procedure include site grades, depth to groundwater, depth of soil, land use (pre-sent or f.irl;ure),. and the pumpjng distance and elevation for the waste-water treatment concepts. The economical haul distance for sludge disposal/utilization concepts will. depend on solids concentration and. other local factors and must be detei-:mi.ned on a case-by-case basis. Tables 2.5 and 2.6 are applicable for land application uf wastewater.

22 Chapter Two

TABLE 2.5 Physical Rating Factors for Land Application o~ Wastewater16

Concept

Slow Overland Rapid Condition rate flow infiltration

Site grade ( ~ 1 Q..-..5 8 8 8 5-10 6 5 4 10-15 4 2 1 15-20 Forest only, 5 NS* NS 20-39 Forest only, 4 NS NS 30-35 Forest only, 2 NS NS >35 Forest only, 0 NS NS

Soil depth(m)t 0.3--0.6 NS 0 NS 0.6-L5 3 4 NS 1.5-3.0 8 7 4 >3.0 9 7 8

Depth to groundwater (m > 3 6 6 6

Soil permeability. most restrictive layer (em/h)

5.10 8 NS 9 *NS = not suitable. tsoil depth to bedrock or impeoneab1e barrier.

Table 2. 7 may be used for sludge concepts, and Tables 2.a and 2.9 are for the special case of forested sites for either sludge or wastewater. ~e soil eype is not included as a factor in Tables 2.5 to 2.7; it was included in Tables 2.1 and 2.2 .a::ild was part of the basis for preliminary site idEmtifica:tion, so it is not included again as a rating fu.:ctor.

The relative importance of the various conditions in Tables 2.5 and 2.6 is reflected in the magnitndeof the valne assign~ so the: largest value indicates the most important 'characteristic. The final category in Table 2.6 relates to the ~cipated management of the land appli-cation site. It is possible, under favorable conditions, to find farms or forestry operators in rural areas who may be willing to accept waste-water or sludges for their nutrient value and who would prefer to con-tinue to manage the site. .

The ranking for a specific site is obtained by snmmTng the individ-ual values from Tables 2.5 and. 2.6_; The highest-ranking site will be the mosi suitable. The suitability ranlring can be determined accord-ing to the following ranges:

' . :~; , .

Planning, Feasibility Assessment, Site Selection 23 .. .. . .

TABLE 2.6 Land Use and Economic Factors for land Application of Wast~~~er16

Condition

Distance from wastewater source (kmJ 0-3 3-8 S-16 >16

Ele\'ation difference (m) 200

Land use, existing or planned Industrial High density, residential or urban Low density, residential or urban Agricultural, or open space, for agricultural SR or OF Forested.

for forested sites for agricultural SR or OF

Land cost and management No land cost, farmer or forest company mana..:,uement Land pttrehase~ farmer or forest company management Land purchased, operated by industry or city

Low snitability ndoderatesuitabLU~

. High suitability

24 Chapter Two

TABLE 2.7 Physical Rating Factors for Land Application of Sludge17

S ite ~,.rrade 1 ' i( 1 0-3 3--6

Coudit i>:!l

6-12rno liquid s ludge on ground :;urfacE-1 12- 15 i no liquid s ludge 1 >15 rno liquid s ludge .l

Soil depth tml~ 1.2

Soil permeabiljty. most restrictive layer lcm'hl 2.4

Dept h tn 5easuna1 groUi-idw~ter 1m1 1.2

Concept

Agricultural Reclamation

:) 8 6 7 4 6 3 5 ~s 4

NS 2 3 5 8 8

1 3 3 4 5 5 3 4 1 0

0 0 4 4 6 6

KSc~ Chap. S for details on surface treatment ofindustri~l wastes. ~NS -= not suitable. :j:Soil depth to hedrock ot impermeable barrier.

Type B*

8 4

NSt NS NS

NS 2 8 ..

5 5 5 0

NS .

NS 2 6

can also be used fm agricultural sludge operations. In general, it is economical to transport liquid sludges (

Planning, Feasibility Assessment. Site Selection 25

TABLE 2.8 Rating Factors for Sludge or Wastewater in Forests, Surface C(>nditlons 13

Condition Rating value*

Dorrunant vegetation Pine Hardwood or mixed

Vegetatio!l. age I)Tl Pine

>30 20-30 50 3'0-50 40 25-40 35 {}-I .2-6 7-:35

Di;t:ance to surface waters t m .: 15-30 30--60 >'60

Adjacent land use High-denSity residential Low-density residential Industrial Undeveloped

"'Total ratin~ 3-4 = not .suitable, 5-6 = poor, 9-14 = good. >15 = e-xcellent.

2 3

3 3 4

1 2 3

1 2 3

0 2 4 6

1 2 3

1 2 2 3

.straints on operations are already included as a factor in the land area determinations; Seasonal constraints and the local climate are important factors in determining the design hydraulic loading rates and cycles for wastewatr systems~ as well as the length of the oper-ating season and stormwater runoff conditions for all concepts_ Table 2..11lists the pertinent climatic data required for finai design of both sludge and wastewater systems. At least a 10-year return period is recommended_ Refer~nces 87 97 and ~0 are useful sources for this in:fa-rmation..

Flood- hazard Th~ location of sludge or wastewater systems within a flood plain can he either an asset or a liability, depending on the approach used for

26 Chapter Two

TABLE 2:9 Rating Factors for Sludge or Wastewater in Forests, Subsurface Condifrons 13

Condition

Depth to seasonal groundwater (m) 10

Depth to bedrock (ml 3

Type ofbedrock Shale

Sand...~ne Granite-gneiss

Rock oui:c::rops (fk oft33 10-33 1-10 None

SCS eroion. clas3.fication Severely eroded Eroded Not eroded

SCS shri.ck.:...:.-well potential for the soil High Low Moderate

Soil cation-e:n:hange capacity fmEq/100 g> 15 Hydrcn1ic: condw:titity of soil (em/h)

>15

Planning, Feasibility Assessment, Site Selection 27

.

TABLE 2.11 Climatic Data Required for Land Application Oesigns'3

Condition Required data Type of analysis

Precipitatio~ As rain, as snow, annual Frequency averages, maxima, minima

Storm events Intensity, duration Frequency Temperature Length of frost-free Frequency

period Wind Direction, velocity . Assess aerosol risk Evapotranspiration Annual and monthly Annual distribution

averages

p]anning and design. Flood-prone areas may be undesirabl~ ~ecause of variable drainage characteristics and potential flood damage to the structural components of the system.. On the other hand, flood plains and siniilar terrain may be the only deep soils in the area.. If permit-ted by the regul~tory authorities, utilization of such sites for waste-water or sludge can be an integral part of a flood-plain man~ement plan. Off-site storage of wastewater or sludge can be a design feature to allow the site to flood as needed.

!

Maps of flood-prone areas have been produced by the U.S. Geological Survey (USGS) in many areas of the United States as part of the uniform National Program for Managing Flood Losses. The maps are based on the standard 7.5' US~S topographic sheets_ These identify areas with a potential of a 1-in-100 chance of flooding in a given year by means of a black-and-white overprint. Other detailed flood information is usually available from local offices of the U.S. Army Corps of Engineers and flood-control districts. If the screening process identifies potential sites in flood-prone~ local authorities should be consulted to identifY regnlatory requirements before begin-ning any detru1ed site investigation.

Water rights Riparian water laws, primarily in states east of the Mississippi River, proteCt the rights of landowners along a watercolll"se to use the water. Appropriative laws in the Western States protect the rights of prior users ofthe water. Adaption of any of the natu:ral concepts for waste-water treatment can have a direct impact on water right concerns:

Site drainage, both quantity and quality, may be affected. .. A non discharging system, or a new discharge location, will affect

the quaptity .of flow in a body of water where the discharge previ-. ously existed.

28 Chapter Two

a Operational considerations for land treatment systems may alter the pattern and the quality of discharges to a water body.

In addition to surface waters in well-defined channels or basins, many states also regulate. or control other superficial waters and the groundwater beneath the surface. State . and local discharge require-ments for the appropriate case should be determined prior to initia-tion of design. If the project has any potential for legal entanglement, a water rights attorney sho~d be consulted.

2.3 Site Evaluation The ne?tt phase of the site and system selection process involves field surveys to coiLfirm map data and then field testing for verification and to provide the data needed for design. 'This preliminary procedure includes an estimate of capital and operation and maintenance costs so that the sites identified in. previous steps can be evaluated for cost effectiveness. A concept and a site are. then selected for final design based on these results. Each site evaluation must include the following ~ormation:

Property o\V-nership, physical dimensions cf the site, current and future land us~

Surfaee and gr.oundwat~r .conditions: location and depth of wells, surface waters, flooding and drainage problems,. fluctuations in groundwater levels: quality and users of groundwater

Characterization of the soil profile to 1.5 m (5 ft) for SR. and most sludge systems,. to at least 3m (10ft) for RI and pond-type sys-tems, both physical and chemical properties

Agricultural crops: cropping pattern.s .Yields, fertilizers used, tillage a nd irrigation methods, end use of crop, vehicular access within site

Forest site: age and species of trees, commercial or recreational site, irrigation and fertilizer methods, vehicle access to and within site

Reclamation site: existing vegetation, historical causes for distur-bance: previous reclamation efforts, need for regrading or ten~ain modification

. Investigation of RI sites requires special consideration of the topog-raphy, and of soil type and uniformity. Extensiye cut-and-fill or relat-:ed earthmoving operations a re not only expensive but c~ alter the necessary soil characteri.::.-tics through compaction. Sites with signifi-cant and numerous changes in relief over a small area -are not the best choice for RL Any soil \'\-ith a significant clay fractiun (>10 per-

Planning, Feasibility Assessment, Site Selection 29

centJ \\'ould generally exclude RI construction if fill were required by the design. Extremely nonuniform soils over the site do not absolutely p1eclude development of an RI system, but the significantly increase the cost and complexity of site investigation.

Soils investigation Table 2.12 presents a sequential approach to field testing to define the physical and chemical characteristics of the on-site soils. In addi-tion to the on-site test pits and borings, examination of exposed soil profiles in road cuts, borrow pits, and plowed fields on or near the site should be part of the routine investigation. .

Backhoe test pits to a 3-m {10-ft) depth are recommended where soil conditions permit, in each of the major soil types on the site. Soil samples should be obtained from critical layers, particularly from the layer being considered as the infJtration surface for wastewater, or the application layer for sludge. These samples should be reserved for fut-1.1re testing. The walls of the test pit should be carefully examined to define the characteristics listed in Table 2.13; Refs. 11, 15, and 18 are useful sources for this purpose. The test pit should be left open long enough to determine if groundwater seepage occurs, and then the highest level attained should be recorded. Equally important is any indication of seasonally high groundwater, most typically demon-strated by mottling of the soils (see Re[ 15)_

Soil borings should pene~ate to below the groundwater table if it is within 10 to 15m (30 to 50ft) of the surface. At least one boring should be located in every major soil type on the site. If generally uniform con-ditions prevail, there might be one boring for every 1 to 2 ha (2 to 5 ac) for large--scale systems. Small .systems (

., ;)

TAB I.E 2.12 saquonoo of Flold Testing, Typical Order from Left to RlgJlt1

CnnlliHlntA

'ry pc u f lea t:

DntLl noodod;

'!'hen estimate:

Moro teats for:

Also csUmnte:

Number of tests:

'!'oat pit 'fot~t. hmln~-IB lni1ILntJ1m tof"tfl"' Bnckhoe pit, also inspect DJ'lllcq or nugcrod, also But~ln rnolhud if poaHiblc tond cut.H, otc, Jogs of local W!.!lls for

Dopth of' ptofila, t.oxturo, litl'ucturu, l'oat.J'IoUng lnyors Ne(ld t'or hytlruuliu conductivity taata Hydrt\ulic conductivity, If needed

. Loading rntos

3-5 minimum/site, more for lnrgc sites, poor aoll uniformity

aolla dntn and wntcr level Depth to groundwater, depth t~ bnrtiur

Ground water flow dir~;~ction Hotizontal conductivity, if needed Groundwater mounding, need for drainage 3/site minimum, more for RJ than SR, more for poor soil uniformity

lnfiiLtuLion rut.c

Hydraulic capacity

2/site minimum, more for Jorge sites or poor soil uniformity

S!JII t!IH!IliiH!.!'.Y't

Al::m ruvit~w SCS 1-mils BU J'V~YH

N, P, melniH, o.tc., l'elenLiun, Hoi! oncl crop management Soil nmendmonis, crop limitntions

Quality of any percolnte

Depends on type of site, soil uniformity, waste character

*Required only for larid application of wnatowntorj some definition of subsurface po!'tnenbili~y ncoded for pond and sludge systems. t Typic!llly needed only for lnnd Jl.pplichtiun nf sludges ot wn~tewntora.

' .. -~t I ' '- ~j., :AI - .',1,.,.:,. ,:JrJ.'J ..,Hff,hHU:Ot':; .... :,;..v~l!:-"t;. ,j~:'j..~~ :':. .:_~!J!!..,\i(l~:j,~l.s.s.:.ti!~....:.:tr...l4~.;,!t,~~ .....

Planning, Feasibility Assessment, Site Selection 31

TABLE 2.13 Soil Characteristics in Field lnvestigations1

Characteristic

Estim~te pel"Cl'lll gravel, sand an~ fines ~il textural class Soil color

Plasticity of fines

Stratigraphy and structure

Wetness and consistency

Significance

Influences permeabi)ity Influences permeability Indication of seasonal groundwater, soil minerals

0 0

Permeability, and influence on cut or fill earthwork Ability to move water vertically and laterally

Drainage characteristics

TABLE 2.14 Soil Textural Classes and General Terminology Used in Soil Descriptions15

COIIllDDn name Textul:e Sand soils Coaise

Loamy soils Moderately coarse

Clayey soils Medium

Moderately fine

Fme

Class name

Sand Loamy sand

Sandy loam Fine sandy lo0 m Very fine sandy

loam..~silt loam.. silt Clay loam. sandy clay loam., silty clay loam Sandy clay, silty clay, clay

*USCS = Unified Soil Classification System:_

uses symbol* GW,GP.,GM-d sw SP,SM-d

MH.ML

.sc

ClLCL

the in-situ soil structure and significantly change the natmal perme-ability_ Soil strncture can be observed in the side walls of a . test pit; Refs_ 10 and 1:2 are suggested fur additional detaiL

Soil chemistry. The chemical properties of a soil affect plant 'growth, C

32 Chapter Two

application or containment of toxic and hazardous materials. Chapter 8 contains information on land application of toxic sludg~s.

If the proposed concept involves land application of sludges or wastewater and that in tum depends on surface vegetation as a treat-ment component, then soil chemistry is a verj important factor in the development and future maintenance of that vegetatioi1. The follow-ing tests are suggested for each of the major soil types on the site: a pH, cation-exchange capacity (CEC}, exchangeable sodium percent-

age (ESP) (in arid climates)7 background metals (Pb, Zn, Cu, Ni, Cd), electrical conductivity (EC> of soil solution

Plant available nitrogen (N), phosphorus (P), potassi~ (K), lime requirements for pH adjustment and maintenance

. There are few standard test procedures for chemical analysis of soils. References 2, 67 and 12 are suggested for this purpose. Table 2.15 can be used to interpret results of these chemical tests.

The cation-exchange capacity of a soil is a measure of the capacity of negatively charged soi1 colloids ta adsorb cations from the soil solu-tion. This adsorption i:3 not neceSsarily penn.anent, since the cations

TABLE 2.15 interpretation ot Soil Chemical Tests16

Parameter and test result

pH of saturated SGil paste 8.4

CEC CmEq/100 gl 1-10 12-20 >20

Exchangeable cations Sodium Calcium Potassium

ESP tas w- of CEC i 10 >20

EC fmmho3/cm@ 25~C of saturation e..'\.i:ract)

16

Interpretation

Too acid for most crops Suitable for acid-tolerant crops Snitahle for mo:rt crops Too alkaline for most crops

Limited adsorption Desirab"ler.ange fas '1 ofCEC'I

Planning, Feasibility Ass~ssment, Site Selection 33

can be replaced by others in the soil solution. These exchanges do not significantly alter the structure of the soil colloids. The percentage of the CEC that is occupied by a particular cation is termed the percent saturation for th~t cation. The sum of the exchangeable H, Na, K, Ca, and Mg expressed as a percentage of the CEC is called percent base saturation. There are optimum ranges for percent base saturation for various crop and soil combinations. It is important for Ca and Mg to be the dominant cations rather than Na or K. The cation distribution in the natural soil can be easily changed by the use of soil amend-ments such as lime or gypsum.

The nutrient status of the soil is important if vegetation is to become a component in the treatment system or if the soil system is otherwise to remove nitrogen and phosphorus. Potassium is also mea-sured to ensure maintenance of a proper balance with the other nutri-ents. The N:P:K ratios for wastewaters and sludges are not always suitable for optimum crop growth and there have been cases where the addition of supplemental potassium was necessary. See Chap. 3 for a detailed discussion of nutrients.

Infiltration and permeability The ability of water to infiltrate the soil surface and then percolate ver-tically or laterally is a critical factor for most of the treatment concepts

discussed in this book. On the one hand, excessive permeability can negate the design intentions for most ponds, wetlands, and OF systems. Insufficient permeability will limit the usefulness of SR and RI sys'-t.ems and result in undesirable waterlogged conditions for land application of sludges. The hydraulic properties of major concern are the ability of the soil surface to infiltrate water and the flow or ret.ention of water v,itlrin the soil profile. These factors are defined by the saturated penneahility or hydraulic conductivity, the infiltration capacity, ~d the porosity,. specific retention, and specific yield of the soil matrix.

Saturated permeabifrty. A material is considered permeable if it con-tains interconnected pores, cracks, or .other passageways through. which water or gas can flow. H._ydraulic conductivity (synonymous with permeability as used in this te..'rt) is a measure of the ability of liquids and gases to pass through soiL A preliminary estimate of per-meability can be fo~d in most SCS soil surveys. The final site and process selection and design should be based on appropriate field and laboratory tests to confirm the initial estimates. Table 2.16 lists the permeability classes as defined by the SCS.

Natural soils at the low end of the range are best . suited for ponds. wetlands, OF, and treatment of indus~al sludges which might have

.'~ .. , . .

34 Chapter Two

TABLE 2.16 ~CS Permeability Classes for Sat~rated Soif1

Soil penneability Ccm/h) 50.0

Class

Very slow Slow Moderately slow Moderate Moderately rapid Rapid Very rapid

toxic components. Soils in the mid-range are well suited. for SR and for land application of sludges. These soils can be rendered suitable for the former uses via amendments or special treatment. The soils at the upper end of the range are suited only for RI systems in their nat-ural state, but can also be suitable for ponds, wetlands, or OF with construction of a proper liner.

The movement of water through soils can be defined using Darcy's equation:

(2.14)

where q = flux of water, the flow per unit cross-sectional area, cmlh . (in/h)

Q = volmne of flow per unit time, cm3/h (in3/h). A = unit cross-sectional area, cm2 (in2) K = permeability {hydraulic conductivity}, cm/h (in/h) H = total head, m (ft) L = hydraulic path length, m (ft)

!lli/ IlL = hydraulic gradient

The total head can be assumed to be the sum of the soil water pres-sure head (h) and the head due to gravitY (Z), or H = h+Z. When the flow path is essentially vertical, the hydraulic gradient is eq:nal to I and the vertical permeability Kt. is used in Eq. 2.14. When the flow path. is essentially horizontal, then the horizontal permeability" Kh should be used. The perm.eability coefficient K is not a true constant, bnt a rapidly changing function of water content.. Even under saturat-ed conditions, the K value may change dne to swelling of clay parti-cles, and other factors, but for general engineering design purposes it can be consid, red a constant. The Kt. will not necessanly be equal to the Kh for most soils. In general, the lateral K. will be higher, since

. n the interbedding of fine- and coarse-grained layers tends to restrict vertical flow. Typical values are given in Table 2.17_

Pl~nning, Feasibility Assessment, Site Selection 35

TABLE2.17 Measured Ratio of~ to Kv16

42 75 56

100 72 72

2.0 2.0 4.4 7.0

20.Q 10.0

Comments

Silty soil

Gravelly Near terminal morain Irregular sUccession of sand and gra:.tel layers, from field mea.sure-ment.sofK

Infiltration capacity. The infiltration rate of a soil is defined as the rate at which water enters the soil from the surface. When the soil profile is saturated and there is negligible ponding at the surface7 the infiltration rate is equal to the effective saturated permeability or co.nductivity of the immediate soil profile.

Although the measured infiltration rate on a particular site may decrease with time due to surface clogging, the snbsurface vertical permeability at saturation will generally remain constant. As a result, the short-term measurement of infiltration serves reasonably well as an estimate of the long-term saturated vertical permeability within the zone of influence for the test procedure being used.

Porosity~ The ratio of voids to the total volume of the soil is referred to as the soil porosity. It is expressed either as a decimal fraction or as a percentage as defined by Eq. 2..15.

v - v v n = t s=_t'

vt ~ {2.15)

where n = porosity,. % as a decimal ~ = total unit volume of soil~ m3 (ft3} V = tmit volume of soil fractio~ m s (~)

s

V = unit volume of voids, m 3 (ft3} v

Specific yield and specific retention.. The porosity

36 Chapter Two

50

45

40

... 35 E ::I 30 ~ >-..0 25 ....

c -

.Q "0 'l:l

"0 c: c u c:

.c 0 0 ....

f. I "' > 41 c.> . c: c: u ~ i,:;: i.L t/16 118 1/4

'l:l c: CJ "' E CD ::J

"' . :0 a Cl 0 ~ L; 112 t 2'

/Porosity

0

Specific retention

""0 'ii ~ "'0 c: > > c ~ 0 0 g .. ... .... > 00 C' ~ e E E ~ C' c:l ::J -~ >

:0 'l:l o ~ Cl> ~ ....... z 4 8 16 32

Mmirr.urn 101.. grain size, mm

"'ii ~ > > 0 0 ... ~ CJ' CJ'

"' ... ... II> ...

t "' ... "C ,_ ~ :; 0 0 0

u . U III

64 12B 256

Figure 2.4 PurO:>-it}-, specific retention. and s p?cific )ie1d Yarir.tions with grain size. in-s itu consolidated soils. Cl)a.stal bGt.sin, California.

' ~ . . . . . .

'The specific yield is used in defining aquifer properties, particularly in calculating groundwater mounding beneath ponds and wastewater application sites_ For relatively coarse-textured soils. and deep water tables, it is acceptable to assume a constant value for the specific yield. Since the calculations are not especially sensitive to small changes in specific yield. it is usually satisfactory to estimate it from other properties, as shotVn in Fig. 2.4 or 2.5. Neither Fig. 2-4 .nor 2.5 shonld be used to indicate the hydraulic properties of the medium in subsurface-flow constructed wetlands. Groundwater mound analysis can be. more complicated for finer-textured soils because of capillarity effects in tbe soil as the water table moves higher; see Refs. 3 and 5 for details.

Test procedures and evaiuation. In some cases It may be acceptable to utilize SCS estimates of soil permeability after confirnringtbe aetual presence of 'U!,e spec#ic soil on the site during a field investigation. This should be sufficient for pond and OF systems. on soils with natu-rally Inw permeabilities. Concepts where water flow in the soil is .a major design consideration \\

40 30

.,2Q E ::s 0 > >-~ - 10 c

~ 8 8. 6 ~ 5 >. 4 u ~ ... 3 .. Q.

Ul 2

1

I I I

~ /

.; V I

v /

v i I I

Planning, Feasibility Assessment, Site Selection 37

! .... II 1.------1---1--

I i I v~ ~ ~ l I i v ~ .

/,! I I !

. ! I ; I .. I I I

t I I I I I I I I

inlh 0.1 . 02 Q.3 0.4 ()..6 ()..6 1 2 3 4 6 8 10 20 30 40 ED 8l 100 50 80 100 150200250 cmlh Q25 O.S OB t 1.5 2 2.5 5 8 10 15 2025

Hydraulic CX)ndlJCfivity Agure 2.5 General relationship between "specific yield and hydraulic conductiwit}- for fine-textured -~oils.

on the soil sm-face7 and with smaJler-5eale devices in test pits. It is necessary t0 lli.~ize laboratory permeability tests on undisturbed soil samples from test borings if deeper subsurface flow is a project con: cern. A "ariety of methods are available to measure infiltration rate or vertical saturated permeability (K ) in the field; some of the most

C"

common methods are compared in Table 2.18 .. The reliability of test resultS is a function of the test area and the zone of subsurface mater-ial :influenced_ This relationship . is indicated indirectly in Table 2.18 by the ~rolume of water required to conduct a single test. As indicat.ed in Chap. 7~ the increased confidence resulting from larger-scale field tes:ts allows a reduction in th safety factor for the design of some land treatment systems.

FJoocfmgba:si~ test.. A basin te:;t area of at least 7m2 (75 ft;2) is sug-gested for. .all projects where infiltration and percolation of water are design expe.ctations. The area can be surrounded by a low earthen berm with an impermeable plastic cover, or aluminum flashing can be partially set into the soil in a circular con.f4:,auration to define the test area.. The use of a bent.onite seal around the aluminum flas~ng

perime~.r is recommended to prevent le~e of water. Tensiometers at. a -depth of 15. ern: ( 6 ini and .30 em ( 12 in)7 -can be installed near the center {)[ the circle tc? define saturated conditions at these depths as the test progresses .. The test basin should be flooded several times to ensure saturated conditions and to calibrate any instrUmentation_ The actual test nm should he completed within 24 h of the prelimi-

.: ... :~

. :: t; . " '

:' I

38 . Chapter: Two

TABLE 2.18 Comparison of Field Infiltration Testing Melhods1

Water needs Time Equipment Technique per test (L) . :)(}r test Ch) needed Comments

Flooding _ha::,-in 1900-7600 4-12 Backhoe or See _this chap-blade ter for details

Air ent.ry pern:Ie- 10 0.5-1 AEPdevice See this chap-ameter. (AEPI ter for details Cylinder infil- 400-700 1-6 Standard device See Ref. 16 for trometer details

Sprinkler infil- 1000-1200 1.5-3 Pump, pressure See Ref. 16 for trometer ta.nk, sprinkler, details

collection cans

nary trials~ This final test nm may require 3 to 8 h for coarse-tex-tured soils.. The water level in the ba..~ is observed and recorded with time. These valnes are plotted as intake rate (em/h) versus time. This intake rate will be relatively high initially and then drop off with time_ The- test must continue until the intake rate approaches "steadv-::state" condition. This "steady-state" rate can be taken as the limi~g irifiltration rate for the soil within the z~ne of influence of the test. A safety factor is then applied to that rate for system design as descnoedin Chap. 7.

Sine~ it .is the basic purpose of the test to define the hydraulic con-. ductivity afthe near surfu.ce soil layers, the use of clean water (with

. about' tlie Sam~ ionic composition as the expected wastewater) is acceptable in most cases. If~ however, the wastewater is expected to have a high .solids rontent~ ...-vbich might clog the surface, a similar liq-uid should be used for the field test.

This basin test is most critical for the RI land treatment concept . becatise large volumes of wastewater are applied to a relatively small

surfa...~ area. Most RI systems,. as described in Chap. 7, are operated on a cyclic pattern of flooding and dcying to restore the infiltration capacity of the basin.surface. If a particular project design calls for. a. continuously flooded seepage pond mode, then the initial field tests should be rontinued for a long enough period to simUlate this condi-tion_ If site conditioru; require the construction of the full-scale RI basins on backfilled mata-r.:ial (not recommended), a test fill should he constructed on the site with the equipment intended for full-scale use and then the .basin test described above run in that materiaL The test fill should be as deep as required by the site design or L5 m (5 ft\ whichever is less. The top of the fill area should be at least 5 m (15ft)

Planning, Feasibility Assessment, Site Selection 39

wide and 5 m ( 15 ft) long to permit the installation of a. flooding basin test near the center. .

One flooding basin infiltration test should be run on each of the major soil types on the site. For large continuous areas, one test for up to 10 ha (25 ac) is typically sufficient. The test should be per-formed on the soil layer that will become the final infiltration surface i.J;l the constructed system.

Air entry permeameter. The air entry permeameter was developed by the U.S. Department of Agriculture (USDA) to meas.ure point hydraulic conductivity in the absence of a water table. The device is not available commercially, but specifications and fabrication details can be obtained from the U.S. Department of Agriculture, Water Conservation Laboratory, 4332 East Broadway, Phoenix, AZ 85040 . . . The unit defines conditio~ for a very small soil zone, but the small volmne of water required and short time for a single test make it use-ful to verify site conditions between. the larger-scale flooding basin t-ests. It can also be used in a test pit to define the in-situ permeab~ty with depth. The pit is dug with one end inclined to the surface, benches are cut about 1 m (3 ft) wide by band, and the AEP device is used on that surface.

Subsurface per.:Oeability and groundwater tiow. The permeability of deeper soils is usually measured via laboratory tests on undisturbed soil samples obtained during the field boring program. Such data are usn-ally required only for design of RI systems or to ensure that su~soils are adequate to contain undesirahleIeachates. In many situations it

. is desi:I:able for the design of RI systems to determine the horizontal permeability of the subsn:rf:ace layers_ This can be accomplished with .a field test called the auger hale tests- which in essence requires pump-:mg a slug of water out of a bore hole and then observing the time for . the water level to recover via lateral flow. The u_s_ Bureau of Reclamation has developed a standard procedure for this test, and details can. be fonnd in Refs_ i1 and 14.

Definition of the groundwater position and flow direction is essential for most of the treatment concepts discussed in. this book. Overland-flow and wetland syStems have little concern with deep groundwater tables but might. still be affected by near-surface seaso~ally high groun.dwater. Evidence of seasonal groundwater may be observed in the test pits; water levels should be observed in any borings and in any existing wells on site or on adjacent properties. These data can provide

information on the general hydraulic gradient and flow direction for the area. Th~e data are also necessary if groundwater mounding or underdrainage, as described in Chap. 3, are project C?ncerns.

40 Chapter Two

Buffer zones Prior w the site investigation, state and local requirements for buffer zones or setbac~ : listances should be determined, to be su~e - that ade-quate area exisb on site or can be obtained. rvrost requirements for buffer zones or separation distances are 'based on.esthetics. and the avoidance of odor complaints. The potential for aerosol transmission of pathogens is a concern to some for the opetation of land appllcation of wastewater and some types of sludge composting operations (see Chap. 8 for a discussion of the iatter L A number of aerosol studies have been conducted at both conventional and land treatment facili-tie~ with no evidence of significant risk to adjacent populations. E"-'tensive buffer zones for aerosol containment are not recommended. If the system vvill utilize sprinklers, a buffer zone to catch the sprin-kler droplets on windy days should be considered~ A strip 10-15 m

~ 30-50 ft) v;ride planted with conifers should suffice_ Odor potential is the major concern for pond systems of the facultative type~ since the seasonal O\.'erturn may bring anaerobic maten.als to the liquid surface for a :=:hort period each spring and fall A typical requirement in these cases is to locate such ponds at least 0.4 km. (0.25 mi) from habita-tions. Mosquito control for wetland systems may require similar sepa-ration distances unless positive control measures a!e planned for the system_ Recommended. setback distances for sludge systems are listed in Table 2.19.

2A Site and Process setection The evaluation procedure to this poin~ has identified potential sites for a particular treatment alternative and then condu~ field investiga-tioru: to obtai.Tl data fur the feasibility determination. The eYaluation of the field data 1-~.J.t indicate whether the site requirements listed in Tables 2.1 and 2.2 .exist or not. If site conditions are favorable. it can be .concluded that the'siteis apparehtly feasible for the intE:mded eoncept.

TABLE 2.13 Setback freccrnmendationsforSiudgeSystems"

Setback dis:an.ce- l!!ll

15-SO

90-460

Suitable activities

Sludge injection. and only near: .rernote single dwellings. sma."l ponds, 10-yca:r h:igb.:-water mark for streams. roads_ No surface applications. Injection or surface application near: all the above, plus springs and wat~r supply wells; injeci.iou on!y, neEr high-d'-nlrl t:y residendai

de~c:loprne:tt~. Inj;ction tn :surfa ce application at an above.

Planning, Feasibility Assessment, Site ~election 41

If only one site and related treatment concept result from this screening process, then the focus can shift to final design and possibly additional detailed field tests to support that design. If m.ore than one si.te for a particular concept, and/or more than one concept remain tech-nically viable after the screening process, it will be necessary to do a preliminary cost analysis to identify the most cost-effective alternative.

The criteria in Chaps. 4-9 should be used for a preliminary design of the concept in ques:tJon. Equations 2.1~2.13 in this chapter should not be used for this purpose. These equations are intended only as a very preliminary estimate of the total amount of land which \vould be required for a particular concept. The preliminary design s!:i9uld then be used as the basis for a preliminary cost estimate (capital and oper-ating/maintenance) which should include land costs as well as pump-ing or transport costs to move the wastes from their source to the site. A con1parison of these cost data will indicate the most cost-effective altemative. In many cases the final selection will also be influenced by the social and institutional acceptability. of the proposed site and concept to be developed on it_

Referenc.es 1. Asano, T., and G. S. Pettygrove 1 eds. !: Irrigation with Reclaimed 1Hc111;cipal

nastewater-_4. Guidance Manual, Water Re~o'urces Board, State of California, Sacramento, July 1984.

2. Black, A C.ed.): 1.\tethods oj Soil Analysis, Part 2: Chemical and li!icrobiologicd Properties, Agronomy 9, American Society of A,.oronomy, Madison, WI, 1965.

3. Childs; E. C.: An. Introduction to the Physical Basis of Soil Water Phenomena, John Wiley, London~ 1909.

4. Demirjiian, Y. 1L. J . Wilso!l., W. ClarkBon, and L Estes: Mw;kego:z Count)' Wastewater Jfanagemer.l S_v.'item-Progress Report, 19.68-1975, EPA. 905/2-80-004, U.S. Environme..-:ttal Proteetion Agency. Region V. Crucago, Feb. 1980.

5. Duke, H. R.: Capillary Properties of Soils-Influence upon Specific Yields. Transcript.~ P.m. Scl{: __ 4gr. Eng_ 15:688-69~ 1972.

6. Jackson, 1\.L L.: Soil Chemical Prope-rtif.s. Prentice-Hall, E;nglev.-ood Cliffs. NJ. 1958_

7. Mcrci.ID, H. L.~ R. Cole, \V. Sopp.er, and W. Nuttr: Was.tewater App#ca:tions in. Forest Eco-s_ .... stems. CRREL Report S2-19. U.S. Cold Regions .Research and En.gine.cring Laboratory" Hano....-ei', l\1--p-419&2.

8. National Oeeank and ;.-\:tm~-phe..-ic Adm.iP..istration: The Climatic Summary of the United State.s.(a: 10-:)1-ear sumn1ary1. NO.AlL Roekville, !\.ID.

9. National Oceanic and Atmospheric Administration: Local Cli'Tnatologicci Data (ar1noci! summari-es for ::eiecttd locations), NOAA, Rocbille-, ~ID.

10. National Oceanic and Atmospheric Administration: The ~Uonthlv Summary of Climatic. Data:, NOA.-!\. Rockvilfe. MD. . -

11. Reerl, S.C . and. R. W. Crites: l:IandJ;.ook of Land Treatment Systems for Industrial and ~\furricipal Wastes .. Noye~ Public-d.tions. Park Ridge" NJ. 1984. .

12. Richards... L. A. (ed. l: Diagnosis and Improvement o{ Saline and Alkali SoilsJ A::,aricwtnral Handbook Nn. 60, U.S. Department of Agrkulture. Wasf:rington., DC, 1954.

13. Tayl-or. G. L.: A Preliminary Site ..-~nation I-.1ethod for Treatu:ent of .Municipal Wastewater by Spray Img--a.Iion ofForeEt Land. in Proceedings of the Ccmferen~.:e of Applied Resi~arch a.nd Practice on Jfwzic:ipal and industrial W(!.';f.e~ Madi3on. \VI, Sept... 1980.

42 Chapter Two

14. U.S. Dep.artment of the Interior, Bureau of Reclamation: Drainage Manual, U.S. Government Printing Office, Washington, DC, 1978.

15. U.S: Etwironmental Protection Agency: DesigrCManual-Onsite Wastewater Treatment- and Disposal SystemS, EPA 625/1-80-012, Water Engineerlng Research LaboratorY., Cincinnati, OH, Oct. 1980.

16. U.S. Emironrnental Protection Agency: Process Design ltfanual~ Treatment of 1'\tfunoc.ipal Wastewater, EPA 625/1-81-013, Center for Environmental Research Information, Cincinnati, OH, Oct. 1981.

17. U.S. En.,ironmental Protection Agency: Process Design Manual Land Application of Municipal Sludge, EPA 625/1-83-016, Center for Environmental Research Information, Cincinnati, OH, Oct. 1983.

18. U.S. Environmental Protection Agency: Process Design Manual Supplem..ent on Rapid l'nfiltration and Overland Flow, EPA 625/l-81-013a, Center for Environmental Research Information, Cincinnati, OH, Oct. 1984.

~.

..

.. ,;

.

;:

. - :,

.._ ....

.,:

.

. - ~

. ! . .

Chapter

Basi~ Process Responses and Interactions

This chapter describes the basic responses and interactions amqng the waste constituents and the process components of natural tr~atment systems. Many of these responses are common to ~ore t han one uf the treatment concepts, and are therefore discussed in this chapter. If a waste constituent is the limiting factor for. design, it .is also dis-cussed in detail in the appropriate process design chapter.

\Vater is the major constituent of all of the wastes of concern in this book, since even a "dried" sludge can contain more than 50 percent '.\ater. The presence of water is a volumetric concern for all treatment methods~ but it has even greater significance for many of the natural treatment concepts since the flow patb and the flow rate control the -:-:uccessful performance of the system.

. - r'

Other waste constituents of major conern include the simple ear-b.Qnaceous organics (dissolved and suspended), toxic and hazardous._ orgamcs, pai:hogeB:S;-trace ~etals, nutrients (nitrogen~ phosphorus~ potassium), and other micronutrients. The natural s.,3-stem compo-nents which provide the critical reactions and responses inclnde bac-teria, protozoa (algae, etc..), vegetation (aquatic and terrestrial),.~:and the soil_ The responses involved include a range of physical, chemi~ and biological reactions.

3.1 Water Management :\lajor c()ncems of water management include the potential for. t~:v:el of contaminants with groundwater; the risk of leakage from pogp.s and other aquatic systems, the potential for gro:undwater mm.mding beneath a land treatment system, the need for drainage~ and the

43-

. :! . . .

44 Chapter Three

maintenance of design flow conditions in pond~, wetlands, and othet aquatic systems.

Fundame!ltal relationships . . .

Chapter 2 introduced some of the hydraulic parameters (permeabili-ty . etc.) that are important to natural systems and discussed methods for their determi~ation fn the field OT laboratory~ It is necessary to provide furthei details and definition before undertaking any flow analysis.

Permeability. The results from the field and laboratory test progran1 described in the previous chapter n1ay vary with respect to both depth Rnd areal extent, even if the same basic soil type is known to eXist over much of the site. The soii layer with the most restrictive perme-ability is taken as the design basis for those systems which depend on infilt!atitm and percolation of water as a process requirement. In ether cases: where there is considerable scatter to the data it is neces-

. .

sa'!:y to detennine a. '-!mean'' permeability for design. If the 5oil is u.."liform. then the vertical penneabi1itv K should be

~ t

constant \c,!ith depth and area, and any differences in test results :;hould be rlue to variations in the test proced.ure. In this case Kc~ e;an be considered to be the arith~etic mean a~~defined by Eq. 3.1:

K, + K., ~ K3 + K K = .a:. - n am

'3 1) ~. . .

where- K :i~ the arithmetic mean v.ertical permeability and K 1-K am II are indhidual test results.

\Vher~ th"E soil profile consists of a layered series of uniform soils~ each with a distinct K

1_, generally decrea:,-ing \vith depth, the average

value can be represented as the harmonic mean:

~;~:her-e .D = soU profile depth d r. = depth afnth layer

D

K. = harmonif.: mean permeabilitv 1m '.J

(3.2)'

If nr: pattern or preference is indicated by a statistical analysis, then a random distribution of the K .. values fO'r a. layer must be

;.

assumed , and the geometric mean provides the most conseF,at.ive estim.ate of the true K :

c

(3.3)

Basic Process Responses and Interactions 45

where Kgm = geometric mean permeability

(other terms as defined previously). . Equation 3.1 or 3.3 can also be used with appropriate data to deter-~ine. the later~l permeability Kh. Table 2.17 presents typical .values for the ratio Kh I Kv.

Groundwater flow velocity. The .actual flow velocity in a groundwater systeni can be obtained by combining Darcy's iaw ~ the basic velocity equation from hydraulics, and the soil porosity, because flow can occur only in the pore spaces in the soil.

V= (Kh)(~J (n)(!lL)

where V = groundwater flow velocity,. m/d (ft/d) Kh = horizontal saturated penneability, mid (ft/d)

D.H M. = hydraulic gradient~ m./n1 (ft/ft)

(3.4)'

n = porosity {as a decimal fraction; see Fig. 2 .4 for typical val-ues for in-situ soils) .

. Equation 3.4 can also be used to determine vertical flow velocity. In this ease the hycl:rauJic gradient is equal to 1 and KL. should be used in the -equation.

Aquifer transmissivity. The transmissivity of an aquifer is the .product of the permeability of the material and the saturated thickness of the aquifer. In effect7 it represents the ability of a unit width of the aquifer to- transmit water. The volume of water moving through this urlit width~ be calculated using Eq. 3.5:

I !1H) q = (Ki:)(b)(w)\L\L

where q = volume of water moving throngh aquifer, m 3/d (ft}/d) b = depth of saturated thickness of aquifer .. m (ft.')

w = \vidth of aquifer, for unit vtidth w = 1 m (1 ft> !{H

~ = hydraulic gradient, m/m (ft./ft)

(3.5.1

L""l many situations7 well pumpi-11g tests are used to ~efin.e aquifer properties.. The transmissivity of L~e aquifer can be ,estimated using pumping rate and_ iz'aw-down data. from wcll tests; Refs. 6 and 32 provide details.

46 Chapter Three

Dispersion. The dispe-rsion of contaminants in the groundwater is due to a combination of molecular diffusion and hydrodynamic mix-ing. The net result is that the concentration of the material i$less but the zone of contact is greater at down-gradient locations. Dispersion occurs in a longitudinal direction (D) and transverse to the f19w pat~ fD). Dye studies in homogeneous and isotropic granular media indi-cated that dis.persion occurs in the shape of a cone about 6 from the application poi:at.10 Stratification and other areal differences in the field will typically result in much greater lateral and longitudinal dis-persion. For example, the divergence of the co~e could be 20 or more in fractured rock. 6 The dispersion coefficient is related to the seepage velocity as described by Eq. 3.6:

D = (a)(v) (3.6)

where D = dispersion coefficient, D x longitudinal, D . transverse~ m2/d (ft~d) . )

a = dispersivity, a::: longitudinal, a')' transverse, ~ (ft) u = seepage velocity of groundwater system, mid (ftld)

v n

V = Darcys velocity fro~ Eq. 3.5. n = porosity; see F:tg. 2.4 for typical values fQr in-situ .sOil~

The dispersivity is difficult to measure in the field or to determine in the laboratory. Dispersivity is usually measured in the field by adding a tracer at the source and then observing the concentration in surrounding monitoring wells. An average value of 10 m 2/d resulted from field experiments at the Fort Devens, Massachusetts, rapid-infiltration system, 3 bnt predicted levels of contaminant transport changd very little after increasing the assumed dispersivity by' 100 percent or more. Many uf the values reported in the literature are site-specific, "'fittecf' values and cannot be used reliably for projects elsewhere.

Retardation~ The hydrodynamic dispersion discussed in the previous section affects all the contaminant concentrations e'qtially. However, ad~orption, precipitation, and chemical reactions with other ground-water constitn.ents retard the rate of advance of the affected contami-

nants~ This effect is described by the retardation factor R d' which can rang-e from a value of 1 to 50 for organics often encountered at field sites. The lowest values are for conservative substances such as chlo-rides; which are not removed in the groundwater system. Chlorides

Basic Process Responses and Interactions 47

move with the same velocity as the adjacent water in the system, and any change in obse'0'ed chloride concentration is due to dispersion only, not retardation. Retardation is a function of soil and groundwa-ter characteristics and is not necessarily constant fo~ all locations. The Rd for some metals might be close to 1 if th1. aquifer -is flowing through clean sandy soils with a low pH, but close to 50 for clayey soils. The lid for organic compounds depends on . sorption of the com-P9unds to soil organic matter plus volatilization and _biodegradation. The sorptive reactions depend on the quantity of organic matter in the soil and on the solubility of the organic material in the groundwa-ter. Insoluble compounds such as DDT, benzo[a]pyrenes, and some PCBs are effectively removed by most soils: Highly soluble com-pounds such as chloroform, benzene, and toluene are removed less efficiently by even highly organic soils. Because volatilizatio~ and biodegradation are not necessarily dependent on soil type, the removal of organic compoundS via these methods tends to . be more . .. uniform from site to site. Table 3.1 presents retardation factors for a number of organic compounds, as estimated from several literature sources. 310.27

Movement of pollutants The movement or migration of po-llutants. with the groundwater is controlled by the factors discussed in the previous section.. This might be a concern for ponds and other a:qu.atie systems as well as when uti-lizing the slow-rate and re:pid-infil"t!ation land treatment concepts. Figure 3.1 illustrates the suqsnrface zone of influence for a rapid-infiltJ;ation basin system, or a treatment pond where significant seep-age is allowed.

It is frequently necessmy to determine the concentration of a polln- . tant in the groundwater :plnme at a selected -distance down-gradient of the source. Alternatively? it may be desired to determine the dis-tance at which a given concentration will occur at a given time, or the time at which a given concentration will reach a particular point.

TABLE 3.1 Retardation Factots fcrSefected Organic Compounds

Material

Chloride Chloroform Tetrachloroethylene Toluene Dichlorobenzene Styrene Chlorobenzene

1 3 9 3

14 31 35

52 Chapter Three

d = lateral distafice from. the source to the point of concern, m ( ft)

K, = effective horizontal permeability of the soil system, m/d (ft/d)

. .

Q; = lateral discharge f-r-om the unconfined aquifer system. per unit width of the &w system, m 3/d/m ( ff>d ft)

= K, lh 2 - h 2 > 2d. 0 i

I

(3.11)

where d, = distance to the seepage face or outlet point (m) (ft) hi= saturated thickness of the unconfined aquifer at the outlet

point, m (ft)

The travel time for lateral flow is a function of the hydraulic gradi-ent, the distance traveled, the K;,, and the porosity of the soil as defined by Eq. 3.12_:

(n)(d.)2 t = I n

Basic Process Responses and Interactions 53

ei-J;; 0::

w

Figure 3.3 Groundwater mounding curve for center of. a square r-echarge basin.

er-.r_O::

w

.Figure 3.4 Groundwater mounding curves for -center of .a rectangular recharge area "With different ratios oflength !'L \to width (Wl.

w ----- = dimensionless scale factor [( 4)(a)(t-)]ll2

where W = width of the recharge basin~ m (~l (Kh){ho)

o:= . y

"

(3.13)

(3.14)

where Kh = effective horizontal-penneability ofthe .aquifer, m/d (ftld)

54 Chapter Three

1.0

"' 3.0

0.9

0.8

Ej+-.c:. a: 0.5

. 0,4

0.2

0 .1

0.5 ~ w

Edqe of plot

f..O

Figure- 3.5 Rise and horizontaJ spread of .a groundwater mound below a square recharge area.

.,

..,

. ',

Basic Process Responses and Interactions 55

1.0 --3.0

0.9

0.8

~ 1.2

0.7

0.6

--0.8

0.5 w J4a.t

-o.G 0.4 .

0 .3

-:.o.4

0.2

0.1 -0.2-----

0.5 l(

.w

Edge of plot

0.6 0.4

. 1.2

1.0

Figure 3.6 Rise and horizontal spread of a groundwater mound below a rectangular recharge area with a length equal to twice its width. .

'.

56 Chapter Three

h '-' origtnal saturated thickn e::; ~ of the a q uifer. beneath the il center of the recharge area . m < ft i

Ys = specific yield of the soil: use Fig. 2.5 or 2.6 to determine, m:1/m;1 f ff3/ft=>.)

(R>

Basic Process Responses and Interactions 57

:l6.5 W/r 4ct! J1 :! :: = 1.:!8 {!4Ji35.7 lC3JI 1 :!

0.3 R = - - = 2 m/d 0.14

(RJIO-=

58 Chapter