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I I I I I I I I


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EPA 625/1-80-012

DESIGN MANUAL

ONSITE WASTEWATER TREATMENTAND DISPOSAL SYSTEMS

U.S. ENVIRONMENTAL PROTECTION AGENCY

O f fi i c e o f Water Program Operations

Of f i ce o f Research and DevelopmentMuni cipa l Envi ronmental Research Labor ato ry

October 1980


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NOTICE

The mention of trade names or commercial products i n t h i s p u b l i ca t i on i sfor i l lust rat ion purposes and does not const i tu te endorsement orrecommendation for use by the U. S. Environmental Protection Agency.

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FOREWORD

Rural and suburban communit ies a re con fro nte d w i t h problems th a t areunique to t he i r s i ze and popu la t i on densi t y, and a re o f ten unab le t o

super impose so lu t ion s t yp ic a l l y app l i ca b le t o la r ge r u rban areas . A

good example o f such problems i s th e p ro v i s i on of wastewater serv ic es.

I n t h e p as t, p r i o r i t i e s f o r wa te r p o l l u t i o n c o n t r o l f oc us ed on t h ec i t i e s , s ince waste generat ion f rom these areas was most ev ident . I nsuch high-densi ty development, the t radit ional sani tary engi neer i ngapproach was t o co ns tr uc t a network o f sewers t o convey wastewater t o ac e n t r a l l o c a t i o n f o r t reatment and d isposal t o sur face waters. Since al a rg e number o f users ex is ted per un i t leng th o f sewer l in e , the cos tso f c on st ru ct io n and operat ion co ul d be d iv id ed among many people, thuskeepi ng the f inancia l burden on each user re lat ive ly low.

Wi th in the pas t severa l decades, mig ra t i on o f the popu la t ion f rom c i t i e s

t o suburban and ru ra l areas has been s i gn i f ic an t . Wi th t h i s s h i f t camethe problems o f p r o v i d i n g u t i l i t y s e r v i c e s t o t h e r e s i d e n t s .Unfortunately, in many cases, solutions to wastewater problems in urbanareas have been ap pl ie d t o r ur a l communities. With the advent o ffedera l p rograms th a t p rov i de g rants f o r cons t r uc t ion o f was tewaterf a c i l ities, sewers and cen tra l ized t reat ment p la nts were construct ed inthese low-dens i ty ru ra l se t t ings . I n many cases the cos t o f opera t ingand ma in ta ini ng such f a c i l i t i e s impose severe economic burdens on th ecommuni t i e s .

Although wastewater tre atm ent and di sposal systems se rv i ng s in g l e homeshave been used fo r many year s, they have of t e n been con sid ere d aninadequate or temporary sol ut ion unt i l sewers could be constructed.However, re se ar ch has demonstrated th a t such systems, i f constructed andmain ta ined proper ly , can prov ide a r e l i a b l e and e f f i c i e n t means o fwastewater treatment and disposal a t r e l a t i ve ly low co st .

Th is document pr ov i des tec hni cal in fo rm at io n on onsi t e wastewatertreatment and disposal systems. It does n ot cont a in s tandards fo r thosesystems, no r does i t c o n t a i n r u l e s o r r e g u l a t i o n s p e r t a i n i n g t o o n s i t esy stems.

The in tended audience fo r t h i s manual inc lud es those inv o lv ed i n th edesign, const ruct io n, operat ion, maintenance, and re gu lat io n of onsi t ewastewater systems.

D i r e c t o rm i n i st r a t o r

f o r Water Program Opera tions Munic i pal EnvironmentalResearch Laboratory

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ACKNOWLEDGMENTS

There were th ree g roups o f pa r t i c ipa n ts invo lved i n the p repara t io n o fthis manual : (1) the con t rac to r - au thors , (2) the con t rac t superv iso rs ,and (3 ) th e tec hni ca l review ers. The manual was w r i t t e n by personnelfrom SCS Engi nee rs and Rural Systems Engi ne er i ng (RSE). Cont rac tsupervi si on was pro vi ded by U.S. Environmental P r ot ec t io n Agency (EPA)personnel f rom the Munic ipa l Const r uct ion D iv is io n i n Washington, D.C. ,and from t he Municipal Environmental Research Laborat ory i n Cin ci nna t i ,Ohio. The tec hni ca l re view ers were exp ert s in ce r t a i n a re a s o f o n s i t ewaste treatment and disposal , and included professors, h e a l t h o f f i c i a l s ,consu ltan ts , and government o f f i c i a l s . Each prov ided tech nica l reviewo f a se ct io n or sect ions of the rep ort . The membership o f each group i slisted bel ow.

CONTRACTOR-AUTHORS:Di re ct io n: Cu rt is J. Schmidt, SCS

W i l l i a m C. Boyle, RSE

Senior Authors: Ern est V. Clements, P ro j e c t Manager, SCSRichard J . Ot is , RSE

Con tr ibu t in g Authors: David H. Bauer, SCSRobert L. S i e g r i s t , E. Je r r y Ty l e r ,Davi d E. Stewart, James C. Converse, RSE

CONTRACT SUPERVISORS:P r o j e c t O f f i c e r s : R o b e r t M. Southworth, OWPO, EPA, Washington, D.C.

Robert P. G. Bowker, MERL, EPA, Cincinnat i , Ohio

Revi ewers: James K re i ssl, MERL , EPA, Cinc i nna t i , OhioDenis Lussier , CERI , EPA, Cincinnat i , Ohio

Sherwood Reed, CRREL, COE, Hanover, N.H.

TECHNICAL REVIEWERS:1. Michael Hansel - Minnesota P ol lu t i o n Cont rol Agency2. Roger Machmeier - U n i v e r s i t y of Minnesota3. Jack Abney - P a r r o t t , Ely & Hurt, Inc., Lexing ton, Kentucky4. Wi l l i am Me l len - Lake County Heal th Depar tment, I l l i n o i s5. Rei n Laak - Un i ve rs i t y o f Co nn e ct i cu t6. Gary Plews - Washington Department o f Social & Health Services7. B. L . C a r l i l e - Nor th Caro l ina S ta te Un ive rs i t y8. John Clayton - Fai r fax County Heal th Department, V ir g in ia9. W i l l iam Sharpe - Pennsyl vani a State Uni vers i ty

10. Elmer Jones - U.S. Department o f Agr icu l ture11. Edwin Bennett - Unive rs i t y o f Co lo rado12. Harry Pence - V i r g i n i a P o l y t e c h n i c I n s t i t u t e13. B r i a r Cook - U.S. Department o f Ag r ic u l t ure , Fore st Service14. Marek Brandes - Onta r io Mi n i s t ry o f the Env i ronment (Re t i red )15. Michael Hines - I l l i n o i s S ta te Department o f Pu b l i c He al t h16. John Fancy - John Fancy, In c. , Waldoboro, Maine

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CONTENTS

PageChapter

FORWORD i i i

ACKNOWLEDGEMENTS VCONTENTS v i i

LIST OF FIGURES i x LIST OF TABLES xv

1 INTRODUCTION

1.1 Background 1 1.2 Purpose 2

1.3 Scope 3

2 STRATEGY FOR ONSITE SYSTEM DESIGN

2.1 In t roduc t ion 42.2 Ons ite System Design St rat egy 4

3 SITE EVALUATION PROCEDURES

3.1 In t ro duc t io n 13

3.2 Disposal

Al ter na t i ves 13

3.3 S i t e Eval uat i on Strateg y 17 3.4 References 48

4 WASTEWATER CHARACTERISTICS

4.1 In t r od uc t i on 50

4.2 Resident i a l Wastewater Cha rac te r is t ic s 50

4.3 Nonresi de nt ia l Wastewater Characteri s t i cs 574.4 Predi c t i ng Wastewater Characteristi cs 65

4.5 References

68

5 WASTEWATER MODIFICATION

5.1 In t r od uc t i on 705.2 Water Con ser va tio n and Wastewater

Flow Redu ct io n 71

5.3 P ol lu t an t Mass Reduction 84 5.4 Onsite Containment - Ho ld in g Tanks 88 5.5 R e l i a b i l i t y 88

5.6 Impacts on Onsite Treatment and DisposalPract ices 92

5.7 References 95

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CONTENTS (c on t i nued)

Chapter

6 ONSITE TREATMENT METHODS

6.1 In t r od uc t i on 6.2 Se pt ic Tanks 6.3 In te rm i t t en t Sand F i l t e r s 6.4 Aerobic Treatment Un it s 6 .5 D i s i n fec t i on 6.6 Nu t r i e n t Removal

9798

113140161184

6.7 Waste Segr egat ion and Recycl e Systems 1976.8 References 199

Page

7 DISPOSAL METHODS

7.1 In t r od uc t i on 206

7.2 Subsurfac e Soil Abs orp tio n 2077.3 Ev ap or at io n Systems 3007.4 Ou tf al l t o Surface Waters 3167.5 References 316

8 APPURTENANCES

8.1 I n t r o d u c t i o n 321 8.2 Grease Traps 3218.3 Dosing Chambers 3278.4 Flow Div ers ion Methods f o r Al t er na ti ng Beds 3358.5 References 337

9 RESIDUALS DI SPOSAL

9.1 In t r od uc t i on 3389.2 Resi dual s C ha r ac te r i s t i cs 3389.3 Residuals Handl ing Opti on 3439.4 Ul ti ma te Disposal o f Septage 3439.5 Refe rences 351

10 MANAGEMENT OF ONSITE SYSTEMS

10.1 In t roduc t ion 35310.2 Theory o f Management 354

10.3 Types o f Management E n t i t i e s 35510.4 Management Program Fu nct i ons 35810.5 References 366

APPENDIX - Soi l Properties and Soil -Water Relationships 367

GLOSSARY 382

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FIGURES

Number Page

2-1 Onsi t e Wastewater Management Opt ions 5

2-2 Onsi t e System Design St ra te gy 7

3-1 Po te nt ia l Eva por atio n Versus Mean AnnualP r ec i p i t a t i on 16

3-2 Example of a Port ion of a Soi l Map as Publ ishedi n a Det a i l ed So i l Survey (Ac tual S ize ) 20

3-3 Tr ans lat ion o f Typical So i l Mapping U n i t Symbol 20

3-4 P l o t Pl an Showing So i l Serie s Boundaries fromS o i l Survey Report 23

3-5 Plo t Pl an Showi ng Surfac e F eatures 25

3-6 Landscape Posi t ions 27

3-7 Methods o f Expressing Land Slopes 27

3-8 Prepara t ion o f So i l Sample f o r F i e l d De te rm ina ti ono f So i l Tex tu re 30

3-9 Soi l Tex tur e Determi n a t ion by Hand: Physical

Appearance o f Var i ous Soi l Textur es 32

3-10 Comparison o f Ribbons an d Cas ts o f Sandy Loam andClay (Ribbons Above, Cas ts Below) 33

3-11 Exampl e Procedure f o r C o l l e c t i ng Soi l P i t Observat ion In format ion 34

3-12 Types o f So i l Str uctu re 36

3-13 Ty pic al Observation Well f o r Determining Soi lSaturat ion 38

3-14 Con stru ct io n o f a Percometer 42

3-15 Pe rc ola t io n Te st Data Form 43

3-16 Compi lation o f Soi ls and S i t e Information( In fo rma t ion Includes Topographic, Soi l Survey,Onsi te Slope and Soi l P i t Observations) 45

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FIGURES ( c o n t i nued)

Number Page

4-1 Frequency D i s t r i b u t i o n f o r Ave rage Da i l yResi de nt ia l Water Use/Waste Flows 53

4-2 Peak Discharge Versus F ix tu re Un i t s Present 64

4-3 S t ra t eg y f o r P re di c t i ng Wastewater Ch ar ac te r i s t ic s 67

5-1 Exampl e St ra te gi es f o r Management o f SegregatedHuman Wastes 89

5-2 Exampl e St ra te gi es f o r Management o f Resi de nt ia lGraywa t e r 89

5-3 Flow Reduct ion Ef f ec t s on Pol l u t a n t Concent rat i ons 93

6-1 Typ ica l Sept i c Tank Outlet S t ruc tu res t o Min im ize Suspended S o l i ds i n Discharge 105

6-2 Se pt ic Tank Scum and Sludge C lea r Spaces 107

6-3 Typical Two-Compartment Se pt ic Tank 108

6-4 Four Precast Reinforced Concrete Septic TanksCombined i n t o One U n i t f o r Large FlowApplication 114

6-5 T yp ic al B u r ie d I n t e r m i t t e n t F i l t e r I n s t a l l a t i o n 129

6-6 Typi cal Free Access Int erm i t t e n t Fi l ter 131

6-7 Typ ica l Rec i rc u l a t i ng In te rm i t t e n t F i lt e r System 134

6-8 Reci r c u la tion Tank 134

6-9 By-Pass Al ternatives f o r Reci rc u l a ti ng Fi l ters 135

6-10 Aer obi c and Anae robi c Decomposi tion Products 142

6-11 Examples of Extended Aeration Package PlantConfi gu ra t i ons 144

6-12 Examples o f Fi xe d F i l m Package Pl a nt Con fig ura t io ns 156

6-13 Stack Feed Chl o r i nat or 171

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FIGURES (continued)

Number Page

6-14 Io di ne Sa tu rat or 172

6-15 Sample Co nt ac t Chamber 174

6-16 Typical UV

D i s i n f e c t i on

U n i t 177

6-17 Typical UV Ster i lizing Chamber 178

6-18 Onsi t e Deni t r i f icat ion Systems 190

7-1 Ty pi ca l Trench System 209

7-

2 Ty pi ca l Bed System 210

7-3 Al te r n a t i n g Trench System with D i vers ion Valve 218

7-4 Pr ov is io n o f a Reserve Area Between Trenches o fth e I n i t i a l System on a S lop ing Si te 220

7-5 Trench System In s t a l l e d t o Overcome a Shallow WaterTable or Re s t r i c t i v e Layer 222

7-6 Typical Inspect ion P i pe 228

7-7 Backhoe Bucket w i t h Removable Raker Tee th 228

7-8 Methods o f Soi l Ab so rp ti on Fiel d R ehab i l i t a t io n 232

7-9 Seepage P i t Cross Se ct ion 236

7-10 Typ ic a l Mound Systems 240

7-11 De ta i l ed Schematic o f a Mound System 241

7-12 Proper Or ie n ta t ion o f a Mound System on a ComplexSlope 246

7-13 Mound Dimensions 247

7-14 T i e r e d Mound System 257

7-15 Cur ta i n Dra in t o I n t e r c e p t L a t e r a l l y Movi ng PerchedWater Table Caused by a Shallow, Impermeable La ye r 261

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FIGURES (continued)

Page Number

7-16 Vertical Drain t o Intercept Laterally Moving

Thin, Impermeable Layer 261

7.-17 Underdrains Used t o Lower Water Table 262

7-18 Typical Electro-Osmosis System 270

7-19 Single Line Dis t r i bu t ion Network 273

7-20 Drop Box Distr i but ion Network 274

7- 21 Closed Loop D i s tr i buti on Network 276

7-22 Distri bution Box Netwo rk 277

7-23 Relief Line Distri bution Network 279

7-24 Central Manifold Distri bu t i on Network 280

7-25 End Manifold Dist ri bution Network 281

7-26 Lateral Detail - Tee t o Tee Construction 282

7-27 Lateral Detail - Staggered Tees or CrossConstructi on 283

7-28 Required Lateral Pipe Diameters for Various HoleDiameters, Hole Spaci ngs , and Lateral Lengths(for Plastic Pipe Only) 285

7-29 Recommended Manifold Diameters for Various ManifoldLengths, Number of Laterals, and LateralDischarge Rates (for Plastic Pipe Only) 286

7-30 Nomograph for Determining the Minimum Dose Volumefor a Given Lateral Diameter, Lateral Length,a n d Number o f Laterals 287

7-31 Distri b u t i on Network fo r Example 7-2 289

7-32 Dis t r i b u t i on Network fo r Exampl e 7-3 294

Perched Water Table Caused by a Shallow,

7-33 Schematic o f a Leaching Chamber 298

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FIGURES (continued)

PageNumber

A-6 Hydraul ic Conducti v i ty (K) Versus Soi l

Mois tu re Retent ion 37 9

A-7 Schematic Representat ion o f Water Movement Through a S o i l w i t h C r u st s o f Di f f e r en t Resi stances 381

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TABLES

Number Page

2-1 Sel ec t i on o f D i sposal Methods Under Var i ousSi te Cons t ra i n t s 9

3-1 Suggested S i t e E v a l u a t ion Procedure 18

3-2 Soi l L imi ta t ions Rat ings Used by SCS f o r S e p t i c Tank/Soil Absorp t ion Fie l ds 22

3-3 Soil Survey Repor t I n fo rma t i on f o r Pa rce l i n F i g u r e 3-4 24

3-4 Tex tura l Proper t i es o f M i neral Soi ls 3 1

3-5 Grades o f Soi l St ruc t ure 36

3-6 Desc r i p t i on o f S o i l M o t t l e s 37

3-7 Es timated Hyd rau l i c Charac te r i s t i c s o f So i l 39

3-8 Fa l l i ng Head Per co la t ion Tes t Procedure 41

4-1 Summary o f Average D a i l y Resi de nt ia l Wastewater Flows 52

4-2 Resi de nt ial Water Use by A c t i v i t y 54

4-3 Character is t ics o f Typ ical Resi de nt ia l Wastewater 56

4-4 Po l lu ta n t Cont r ibu t ions o f Major Res ident ia l Wastewater Fr ac t i ons (gm/ cap/ day) 58

4-5 Pol lu t an t Concent rat i ons o f Ma jo r Res iden t i a l Wastewater Fract ions (mg/ l ) 58

4-6 Ty pi cal Wastewater Flows fr om Commercial Sources 60

4-7 Typical Wastewater Flows f rom Ins t i tu t iona l Sources 6 1

4-8 Ty pic al Wastewater Flows fr om R ec re at i onal Sources 62

4-9 Fixture-Uni ts per F ix tu re 63

5-1 Example Wastewater Flow Reduct i on Methods 72

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TABLES ( co n t i nued)

PageNumber

5-

2 Wastewater

Flow Reduction - Water Carriage

T o i l e t s and Systems 74

5-3 Wastewater Flow Reducti on - Non-Water Carriage Toi let s 77

5-4 Wastewater Flow Reduction - Bathing Devices and Systems 79

5-5 Wastewater Flow Reduction - Miscel laneous Devices and Systems 82

5-6 Wastewater Flow Reduction - Wastewater Recycle and Reuse Systems 85

5-7 Exampl e Pol lut a n t Mass Reduct i on Methods 87

5-8 Addi t ional Conside ra t i ons in the Design, I n s t a l l a t i on and Opera t ion o f Hol di ng Tanks 90

5-9 Po ten t ia l Impacts of Wastewater Mo di f i ca t i on on Onsi t e Disposal Pract i ces 94

6-1 Summary o f E f f l u e n t Data from Vario us Sep tic Tank Studies 100

6-2 Typical Septic Tank Liqu i d Volume Requirements 103

6-3 Locat io n of Top and Bottom of O ut le t Tee or B a f f l e 107

6-4 Performance o f B ur ie d I n t e r m i t t e n t F i l t e r s -Sept ic Tank Ef f l ue nt 121

6-5 Performance o f Free Access In te rm i t te n t F i l t e rs 122

6-6 Performance of Reci r c u l a t i ng I n te r mi t t e n t Fil ters 123

6-7 Desig n C r i t e r i a f o r B u ri ed I n t e r m i t t e n t F i l t e r s 124

6-8 Des ign C r i t e r i a f o r Free Access In te rm i t te n t Fi l ters 126

6-9 Des ign C r i t e r i a f o r Reci r c u l a t i ng In t e r m i t t en t Fil ters 128

6-10 Operati on and Maintenance Requirements f o r Bur ied I n t e r m i t t e n t

Fil ters 138

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TABLES (continued)

Number Page

6-11 Operat i on and Mai ntenance Requirements f o r Fr eeAccess In termi t ten t Fi l ters 138

6-12 Ope rati on and Mai ntenance Requirements f o rReci r c u l a t i ng I n t e rm i t t e n t

Fi l ters 139

6-13 Summary o f E f f l u en t Data f rom Var ious Aerobic U n i tFi e l d Studi es 146

6-14 Typi cal Operat i ng Parameters f o r Onsi t e ExtendedAera tion Systems 151

6-15 Suggested Maintenance f o r Onsi t e Extended Ae ra t i on

Package Pl an ts 153

6-16 Operat i ona l Pr obl ems - Extended Aeration PackagePlants 154

6-17 Typical Operat i ng Parameters f o r Onsi t e F i xed F i lmSystems 158

6-18 Suggested Maintenance f o r Onsi t e Fi xe d F i l m PackageP l a n t s 160

6-19 Operational Problems - Fixe d F i l m Package Plan ts 1 62

6-20 Selected Po te nt ia l D i s i n f e c ta n t s f o r Onsi t eApplica tion 163

6-21 Halogen Pr ope rt i es 164

6-22 Ch lo ri ne Demand o f Sel ecte d Domest ic Wastewaters 165

6-23 Performance o f Halogens an d Ozone a t 25°C 167

6-24 Halogen Dosage Desi gn

Guide l i nes 168

6-25 UV Dosage f o r Selecte d Organi sms 180

6-26 Potent ia l Onsi te N i t rogen Con tro l Op ti ons 186

6-27 Potent ia l Onsi t e Phosphorus Removal Opti ons 194

6-28 Phosphorus Adso rpt i on Estimates f o r SelectedNatural Mater ia ls 198

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TABLES (conti nu ed )

Number

7-

1 Site Criteria for Trench and Bed

Systems

2127-2 Recommended Rates of Wastewater Application for

Trench a n d Bed Bottom Areas . 214

7-3 Typical Dimensions f o r Trenches and Beds 221

7-4 Dosi ng Frequencies for Various Soi1 Textures 224

7-5 Methods o f Wastewater Application for VariousSystem Designs and Soil Permea bi 1i t i es 225

7 -6 Sidewall Areas of Circular Seepage Pits ( f t2) 237

7-

7 Site Criteria for Mound Systems 242

7-8 Commonly Used Fill Materials and their DesignInf i1 t ra t ion Rates 245

7-9 Dimensions for Mound Systems 248

7-10 Infi 1tration Rates for Determining Mound BasalArea 249

7-11 Drainage Methods for Various S i te Ch arac te ri st ic s 266

7-

12 Distribution Networks for Various System Designsan d Application Methods 271

7-13 Discharge Rates for Various Sized Holes at VariousPressures (gpm) 284

7-14 Friction Loss in Schedule 40 Pla sti c Pipe, C=150(ft/100 ft)

291

7-15 Pipe Materials for Nonpressurized Dis t r i bution Networks 297

7-16 Sample Water Balance for Evaporation Lagoon Design 314

8-1 Recommended Ratings for Commercial Grease Traps 324

9-1 Residuals Generated from Onsit e Wastewater Systems 339

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TABLES (continued)

Page Number

9-

2 Characteristics of Domestic Septage 340

9-3 Indicator Organism and Pathogen Concentrationsin Domestic Septage 342

9-4 Land Disposal Alternati ves for Septage 345

9-5 Independent Septage Treatment Faci l i t ies 340

9-6 Septage Treatment at Wastewater Treatment P l an t s 350

10-1 Si t e Evaluat i on a nd System Design Functi ons 359

10-

2 Instal 1ation Functi ons 362

10-3 Operation a n d Maintenance Functions 364

10-4 Rehabilitation Functi ons 365

A -1 U.S. Department of Agriculture Size Limits forSoil Separates 367

A - 2 Types and Classes of Soil Structure 372

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CHAPTER 1

INTRODUCTION

1.1 Background

Approximately 18 m i l l i o n hous ing un i t s , o r 25% o f a l l housing u n i t s i nthe Uni ted States, d ispose o f t h e i r wastewater using onsi te wastewatertre atm ent and dispos al systems. These systems in cl ud e a v ar ie t y o f components and co nf igu ra ti ons , th e most common being the se pt ic t ank /so i lab sor pt io n system. The number o f ons i te systems i s increas ing , w i t habout one-hal f m i l l i o n new systems being i ns t a l le d each year.

The f i r s t o ns it e treatm ent and disposal systems were construc ted byhomeowners themselves o r by l oc al entre preneu rs i n accordance wit h des ign c r i t e r i a fu rn ished by federa l o r s ta te hea l th depar tments . Usua l l y , a sep t ic tank fo l lowed by a so i l absorpt ion f i e l d was ins ta l led .Trenches i n the s o i l abs orpt io n system were dug wide enough t o accommodate open- jo inted d ra i n t i l e l a i d d i r e c t l y on the exposed trench bottom.Some he al th departments suggested t h a t deeper and wid er trenc hes be usedi n "dense" soi ls and that the bottom of those trenches be covered wi thcoarse aggregate befor e the dr ai n t i l e was l a i d . The purposes o f theaggregate were to provide a porous media through which the septic tanke f f l u e n t c o u l d f l o w and t o p ro v id e s to ra ge o f t h e l i q u i d u n t i l i t cou ldi n f i l t r a t e i n t o t h e su rr ou nd in g s o i l .

It has been estimated that only 32% of the total land area in the UnitedStates has so i l s su i tab le fo r ons i te systems wh ich u t i l i z e the so i l f o rf i n a l t reatment and d isposal o f wastewater. I n areas where there i spressure f o r development, onsi t e systems have of te n been i ns t al 1ed onl and t ha t i s no t su i ta b le f o r convent ional so i l absorp tion systems.Cases o f con tamina ted we l ls a t t r ibu ted to inadequate ly t rea ted sep t ictank e f f l uen t , and nu t r ie n t enr ichment o f lakes from near -shore development are examples o f what may occur when a s o i l abs orpt io n system isinstalled in an area with unsuitable soil or geological conditions.

Alarmed by the

potential

health hazards of improperly functioning sys-

tems, publ ic he al t h o f f i c i a l s have con t i nua l l y sought methodst o

improvethe design and performance of onsite systems.

Unfortunate ly, the great increases i n populat ion have exacerbated theproblems associated wi th on s i t e systems. The luxu ry o f va st amounts o fland for homesites is gone; instead, denser housing in rural areas ismore common.

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I n many areas, onsite systems have been plagued by poor publ ic accep-tance; feelings t h a t those systems were second rate, temporary, or f a i l -ure prone. Th i s perspect ive cont ributed to poorly designed, poorly con-structed, and inadequately maintained onsite systems.

Recently, the si tu at io n has begun to change. Federal, s ta t e , and localgovernments have refocused their attention on rural wastewater disposaland, more particularly, on wastewater systems affordable by the rural population. Onsite systems are now gaining desired recognition as aviable wastewater management alternative t h a t can provide excellent,rel iable service at a reasonable cost, while s t i l l preserving environ-mental qu al it y. Federal and many state and local governments haveinitiated public education programs dealing with the technical and administrative aspects of onsite systems and other less costly waste-water handli n g a l ternatives for rural areas.

In this time of population movements t o rural and semirural areas, h i g h

costs of centralized sewage collection and treatment,

and new f u n d i n gincentives for cost and energy saving technologies, those involved w i t hrural wastewater management need more information on the planning, de-s i g n , construction, and management o f onsite systems. Th i s process de-sign manual provides primarily technical guidance on the design , con-struction, and maintenance of such systems.

1.2 Purpose

This document provides information on generic types of onsi t e wastewatertreatment and disposal systems. It contains neither standards for thosesystems nor ru les and regulations pertaining to onsi te systems. The de

-s i g n information presented herein i s intended as technical guidance re-f lec t ive of sound, professional pract ice. The intended audience fo r themanual includes those involved in the design, construction, operation,maintenance, and regulation of onsite systems.

Techno1 ogies discussed i n t h i s manual were selected because of pastoperating experience and/or because of the avai l abi l ty o f informationand performance da ta on those processes. Because a particular waste-water handling option is not discussed in this manual does not mean that it is not acceptable. All available technogies should be consideredwhen p l anni ng wastewater management systems for rural and suburban com-mun i t i e s .

Groundwater and surface water p o l l u t i o n are major environmental consid -erations when onsite systems are used. All wastewater treatment anddisposal systems must be designed, const ructed, operated, and maintained

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CHAPTER 2

STRATEGY FOR ONSITE SYSTEM DESIGN

2.1 Introduction

A wide variety of onsite system designs exist from which to select themost appropriate for a given si t e . The primary cr it er io n fo r select ionof one design over another i s protec tion of the public health while preventing environmental degradation. Secondary c r i t e r i a are co st and easeof operating and maintaining the system. The fat e of any re sidual s resulting from the treatment and disposal system must be considered in the

selection process.

Figure 2-1 summarizes wastewater management options for onsite systems.Because of the wide variety, selection of the system t h a t prevents publ ic heal th hazards and ma in t a i n s environmental quality a t the least costi s a d i f f i c u l t task. The purpose of this chapter i s to present a s t rategy for selecting the optimum onsite system for a particular environment. A t each step, the reader i s referred to the appropriate chaptersi n the manual for site evaluation, and subsequent system design, construction, operation and maintenance, and residuals disposal .

2.2 Onsite System Design Strategy

Traditionally , subsurface soil absorption has been used almost exclu-sively for onsite disposal of wastewater because of i t s ability to meetthe publ ic health and environmental criteria without the necessity forcomplex design or h i g h cost. A properly designed, cons truc ted , andmaintained subsurface abso rp t i on system performs reliably over a long period of time w i t h l i t t l e a t t en t ion . This i s because of the largena t u r a l capacity of the soil t o assimilate the wastewater pollutants.

Unfortunately, much of the l a n d area i n the United States does n o t havesoil s suited for conventional subsurface soil absorption fields. I fsoil absorption cannot be ut i l ized , wastewater also may be saf ely di s

posed of i n t o surface waters or evaporated into the atmosphere. However, more complex systems may be required t o reliably meet the publichealth and environmental criteria where these disposal methods are used. Not only are complex systems of ten more costly t o construct, b u t theyare also more difficult and costly to main ta in . Therefore, the onsitesystem select ion str ategy described here i s based on the assumption t h a t

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FIGURE 2-1

ONSITE WASTEWATER MANAGEMENT OPTIONS

SURFACE

DISCHARGE-

Disinfection

I

I

II

I

I

I

I

I

I

- Evaporation

Lagoon

FURTHER TREATMENT

- Aerobic Un it

- Granular Filter

- Nutrient Removal

L ---

SUBSURFACE SOIL

ABSORPTION

- Trenches

- Beds- Pits

- Mounds

- Fill Systems

- Ar tif ic ial ly Dra ined

Systems


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subsurface soil absorption i s the preferred onsite disposal option be-cause of its greater reliability with a minimum of attention. Where thes i t e cha ra cte ri st ic s are unsuitable for conventional subsurface soilabsorption systems, other subsurface soil absorption systems may be poss ib le. Though these o ther systems may be more cost ly to const ru ctt h a n systems employing surface water discharge or evaporation, theirreliable performance under a minimum of supervision may make them the preferred ternative. Figure 2-2 i ll u st r a t es the onsi t e system designstrategy discussed i n th is chapter.

2.2.1 Preliminary System Screening

The f i r s t step i n the design of an onsit e system i s the select ion of themost appropriate components t o make up the system. Since the s i t e char acteristics constrain the method of disposal more t h a n other components-,the disposal component must be selected f i rs t . Selection of wastewater

modification and treatment components f ol l ow. To select the disposalmethod properly, a detai 1ed s i t e eval uati on i s required. However, thes i te charac ter ist ics t h a t must be evaluated may vary with the disposalmethod. Since i t i s n o t economical nor practical to evalua te a s i t e f o revery conceivable system design, the purpose of this f i r s t s tep i s t oeliminate the disposal options w i t h the least potential so t h a t the detailed site evaluation can concentrate on the most promising options.

To effectively screen the disposal options, the wastewater t o be treatedand disposed must be characterized , and an ini t ia l s i te invest igationmade.

2.2.1.1 Wastewater Characterization

The est imated daily wastewater volume and any short- or long-termvariations i n flow affect the size of many of the system components. Inaddition, the concentrations of va r i ous constituents can affect thetreatment a n d disposal options chosen. Characteri s t i cs are presented i nChapter 4 for wastewater from residential dwellings as well as from

opera tions.commercial

2.2.1.2 Initial Site Evaluation

All useful information a b o u t the site should be collected. T h i s may beaccomplished by c l i en t cont act , a review of available published resourceinformation and records, and an i n i t i a l si t e vi sit . Client contact anda review of published maps and reports should provide i n f o rma t ion reg a r d i n g the soil s , geology, topography, cl imate, and other physical

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F I G U R E 2-2

ONSITE SYSTEM DESIGN STRATEGY

(Ch.

Characterization

Residuals (Ch.

Evaluation

3.3.1,

Selection of

Disposal Options Disposal Option

I

7

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features of the site (See 3.3.1 and 3.3.2). An initial site visitshould al so be made, and should i nc lu de a vis ual survey o f the area andpreliminary field testing, if required, with a hand auger (See 3.3.3).From this site visit, general site features such as relative soil perme-a b i l i t y , depth and nature o f bedrock, depth t o water tab le , s lope, l o ts ize, and landscape po si t i on should be id en t i f i ed . Sources o f informa-

t i o n and eva luat i on procedures f o r s i t e eva lua t i on are de ta i l ed i nChapter 3.

2.2.1.3 Pr el im in ar y Screening o f Disposal Option s

From the wastewater cha ra c t er i s t i cs and s i t e in fo rmat ion gathered i nt h i s s tep, a pre l im inar y screening o f the d isposal opt ions can be madeus ing Table 2-1. Th i s t ab le i nd i ca tes the ons i t e d i sposa l op t i ons tha tpo te n t i a l l y may work fo r t he g i ven s i t e cons t ra i n t s . The po te n t i a l l yfeas ib le d isposa l op t ions are i d e n t i f i e d by no t ing which ones perfo rme f f e c t i v e l y u n d e r a l l t h e g i ve n s i t e c o n st r a i nt s . Note t h a t w i t h s u f f i -c i e n t t reatment and presence o f rec eiv ing waters, sur face water d i s -charge i s always a po te nt i a l d isposa l op t ion .

As an example, suppose a s i t e f o r a si ng le - family home has t h e f o l l o w i n ggenera l cha rac te r i s t i c s :

1. Very rapid ly permeable soi l2. Deep bedrock3. Shal low water table4 . Five to 15 percent s lope

5. Large l o t6. Low evaporat ion potent ia l

From Table 2-1, the d isposa l op t ions most app l i cab le t o the example s i t econs t ra i n t s a re :

1. Mounds2. F i l l s3. Surface water discharge

The design sect ions i n Chapter 7

wou ld be consu lted a t t h i s po i n t t odetermine the spec i f i c cha rac te r i s t i c s t o be eva lua ted a t t h e s i t e i norder to se lec t the most feas ib le d isposa l op t ions ,

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TABLE 2-1

SELECTION OF DISPOSAL METHODS UNDER VARIOUS SITE CONSTRAINTS

Construct only during dry soil conditions. Use trench configuration only.

Trenches only.

Flow reduction suggested. X means system can function effectively

High Evaporation potential required.with that constraint.

Recommended for south-facing slopes only.


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2.2.2 System Select ion

Wi th the po te n t i a l l y f eas ib l e d i sposa l op t i ons i n mind, a d e t a i l e d s i t eeva lua t i on i s per formed. The i n fo rma t i on co l l ec ted i s used t o i d e n t i f yth e system opt ions t h a t meet the p ubl ic hea l th and envi ronmental c r i

teria. If

more than one system is feasible, final selection is based onresults of a cost effective analysis. Local codes should be consultedt o determine which ons i t e t reat ment and d isposal methods are perm i t te di n t h e area.

2.2.2.1 Deta i l ed S i te Eva lua t i on

A c a r e f u l , d e t a i l e d s i t e e v a l u a ti o n i s needed t o p r ov i de s u f f i c i e n tin f o rm at i on t o se l ec t the most appropr ia te t rea tment and d isposa l systemfrom the po te nt ia l l y fe as ib le system opt ions. The eval uat i on should beperformed i n a systematic manner so as to i nsu re tha t t he i n fo rma t i onc o l l e c te d i s u se fu l and i n s u f f i c i e n t d e t a i l . A s i t e e v a l u a t i on p ro ce

dure i s suggested i n Chapter 3, i n c l u d i n g d e s c r i p t i o n s o f t h e t e s t s andobse rvat ions t o be made. This procedure i s based on th e assumption t h a tsubsurface soil absorption is the preferred method of disposal. If subsurface absorpt io n cannot be used, techniques are expla ined f o r eval uati n g t h e s u i t a b i l i t y o f a s i t e f o r s ur fa ce wa te r di sc ha rg e o r evaporat i o n .

2.2.2.2 Sel ect ion o f Most Appropriate System

The di sposal op t i on se lec ted a f t e r t he de ta i l e d s i t e eva lua t i on d i c ta te sthe qua l i t y o f the was tewater requ i red p r i o r t o d isposa l . I f s u i t a b l e

so i l s e x i s t on s i t e t o employ one o f the subsurface s o i l absor pt io n meth-ods o f d isposa l , the qual i t y o f the wastewater appl i e d need no t be highdue t o t he ass im i l a t i ve capac i t y o f the so i l . Where su i t ab le so i l s don o t e x i s t ons i t e , o the r methods o f d i sposa l t h a t requ i re a h i ghe r qua l -ity of wastewater may be necessary. These wastewater quality requirements are es tab l i shed dur ing the s i t e eva lua t ion (Chapter 3). Wastewater reduct ion and t reatment opt ions are selected t o meet the requi redwastewater qual i ty .

Altering the characteristics of the wastewater generated can have a

major im pact on th e design of th e treatme nt and disp osal system.Alteration can be beneficial in reducing the size or complexity of thesystem. Chapter 5 describes a var ie t y o f was tewater reduc t ion opt ions .


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Chapter 6 provides detailed information regarding the design,construction and operation of vari ous treatment opti ons. Selection of the most appropriate treatment option is based on performance and cost.Various o ns it e systems may be synthesized from the data presented i nChapters 5 and 6. As an example of the synthesis of treatment anddisposal systems followi ng the detai 1ed site evaluation, assume that all

thr ee d isposal opt ions se lected i n 2.2.1.3 proved t o be feasi b le .

Examina ti on o f the f i r s t two d i sposal op t i ons i nd i c a tes th a t on l yminimal pretreatment may be re qui re d. Thus, two systems might be:

1. Sept ic tank - mounds2. Septi c tank - f i l l

If groundwater quality is a constraint, however, it may be necessary todevelop other systems. Thus, if nitrogen di scharges from the di sposalsystem t o th e groundwater must be con tr ol 1ed, t he two trea tment

-

di sposalsystems may be r e v i sed t o i n c l u d e t h e f o l lowi ng:

1. Sept ic tank - mound - d e n i t r i f i c a t i o n2. In-house toilet segregation/graywater - septic tank - fill

Note t ha t a v ar ie ty o f o ther systems may be developed as w e l l . Theother disposal option listed in 2.2.1.3 is surface water discharge.Several treatment options exist if the wastewater is disposed of bydischarge to surface waters. Fi ltration and disinfection may be

req uir ed as pa r t o f those treatment opt ions, depending on the waterqual i ty requi rements of the approp r ia t e regula to ry agency.

Residuals produced from the treatment processes also require safedisposal . Th is must be cons idered i n the se l ec t ion o f the t rea tment anddisposal system. Chapter 9 provides information regarding thecharacter, required treatment, and methods of ultimate di sposal of various residuals produced.

2.2.3 System Design

Once a l l the components are selected, design of the system fol lo ws .Chapters 5 , 6, 7, 8, and 9 should be consul ted for design in format ion.

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2.2.4 Onsite System Management

Pa st exper ience has shown t h a t on si te management d i s t r i c t s have manybene f i ts , inc lud ing improved s i t e se lec t ion , sys tem design, cons t ruction, and operation and maintenance. Management districts also facili-

t a t e t he use o f more complex systems or l ar ge r systems se rv ic in g a c lus -t e r o f several homes. These d i s t r i c t s can take many forms w i t h var yin gpowers. Chapter 10 prov ides an overview o f management opt ion s f o r ons i t e systems.

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determined largely by the physical properties of the soil. Descriptionsof some of the more important physical proper ti es appear i n Appendix A.

The soil i s capable of treating organic materials, inorganic substances,and pathogens i n wastewater by acting as a filter, exchanger, adsorber,

and a surface on which many chemical and biochemical processes mayoccur. The combination of these processes acting on the wastewater asi t passes through the soil produces a water of acceptable qua l i ty fordischarge into the groundwater under the proper conditions.

Physical entrapment of par ti cu la te matter i n the wastewater may beresponsible for much of the treatment provided by so i l . Th i s process performs best when the soi l i s unsaturated. I f saturated soil conditions prevail, the wastewater flows through the larger pores and receives minimal treatment. However, i f the soil i s kept unsaturated byrestricting the wastewater flow into the so i l , f i l t ra t i on i s enhanced

because the wastewater i s forced t o flow through the smaller pores ofthe soi l .

Because most soil particles and organic matter are negatively charged,they at t ract and hold positively charged wastewater components and repelthose of l ike charge. The to ta l charge on the surfaces of the soil system i s called the cation exchange capacity, and i s a good measure of thesoi l ' s ab i l i t y to re ta in wastewater components. The charged s i t e s i nthe soil are able to sorb bacteria, viruses, ammonium, nitrogen, and phosphorus, the principa l wastewater constituents o f concern. Theretention of bacteria and viruses allows time for thei r die-off ordestruction by other processes, such as predation by other soil micro

organisms

(1)(2).

Ammonium ions can be adsorbed onto clay particles.Where anaerobic conditions prevail, the ammonium ions may be retained onthe pa rt ic le s. If oxygen i s present, bacteria can quickly n i t r i f y theammonium to nitrate which i s soluble and i s easily leached t o thegroundwater. Phosphorus, on the other hand, i s quickly chemisorbed ontomineral surfaces of the so i l , and as the concentration of phosphorusincreases with time, precipitates may form with the iron, aluminum, or calcium naturally present i n most so i l s . Therefore, the movement o f phosphorus through most s o i l s i s very slow (1)(2).

Numerous studies have shown that 2 ft to 4 ft (0.6 to 1. 2 m) of unsaturated soil i s suff ic ient t o remove bacteria and viruses t o

acceptable levels and nearly all phosphorus (1)(2). The needed depth isdetermined by the permeability of the soi l. Soils w i t h r a p i d

permeabi l i t i es may require greater unsaturated depths bel ow theinfi1 t ra t ive surface than soi1s with slow permeabi1itiers.

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3.2.2 Wastewater Treatment and Disposal by Evaporation

Wastewater can be returned directly to the hydrologic cycle by evaporat ion. This has appeal i n onsite wastewater disposal because i t can beused i n some areas where site conditions preclude soil absorption or i n

areas where surface water or groundwater contamination i s a concern.The wastewater can be confined and the water removed t o concentrate the

pollutants w i t h i n the system. L i t t l e or no treatment i s required prior t o evaporation. However, cl imatic conditions r e s t r i c t the appl ica tio no f this method.

Evaporation can take place from a free water surface, bare soil , or plant canopies. Evaporation from plants i s call ed tran spir atio n. Sincei t i s often difficult to separate these two processes on par t i a l ly baresoil surfaces, they are considered as a single process called evapot ranspirat ion (ET).

I f evaporation i s t o occur continuously, three conditions must be met(3). First, there must be a continuous supply of heat to meet the latent heat requirements of water (approximately 590

cal/gm of water evap-orated at 15o C). Second, the vapor pressure in the atmosphere over theevapora tive surf ace must remain lower than the vapor pressu re a t thesurface. Th i s vapor pressure g radi ent i s necessary to remove the moisture e i ther by diffusion, convection, or both. Th i r d , there must be acontinuous supply of water to the evaporative surface. The f i r s t twoconditions are strongly influenced by meteorological fa ct or s such as airtemperature, hum id i t y , wind velocity, and solar radiation, while t h et h i r d can be controlled by design.

Successful use of evaporation fo r wastewater disposal requ ires t ha tevaporation exceed the total water i n p u t t o the system. Rates of evaporation decrease dramatically du r i ng the cold winter months. In thecase of evaporative lagoons or evapotranspiration beds, input from pre-cipitation must also be included. Therefore, appl ication of evaporationfor wastewater disposal i s largely res t r ic ted t o areas where evaporationra te s exceed pre cip ita tio n rat es. These areas occur primari ly i n thesouthwestern United States (see Figure 3-1) . In other areas , evaporat i o n can be used t o augment percolation i n t o the soil.

Transpi rat ion by pl an ts can be used t o augment evaporation in soi l-cov-ered systems (5)(6). Plants can transpire at high rates, but only dur i n g d a y l i g h t hours of the growing season. During such periods, evapotranspiration rates may exceed ten times the rates measured in Class Aevaporation pans ( 7 ) (8)( 9 ) . However, overall monthly evaporation ratesexceed measured evapotranspi rati on ra te s. Ratios of evapotranspirationto evaporation (as measured from Class A pans) are estimated t o be 0.75

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FIGURE 3-1

POT ENTI AL EVAPORATION VERSUS MEAN ANNUAL PRE CIP IT ATI ON (4)

( inches)

Potentia l Evapotranspirat ion mo re tha n

+ mean annual precipi tat ionPotential Evapotranspirat ion less than

m ean annual precipitat ion


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t o 0.8 ( 6 )( 7 ). Therefore, i f covered disp osal systems ar e t o be used,they must be larger than systems with a free water surface.

3.2.3 Wastewater Treatment and Disposal i n Surface Waters

Surface waters may be used f o r the disposal o f t rea ted wastewaters i f permi t ted by the lo ca l re gula tory agency. The capaci ty o f surfacewaters t o ass i mi la te wastewater p o l l u ta nts v ar ie s wi t h the s iz e and typeo f the body o f water. In some cases, because o f th e po te nt i al f o r humancontac t as wel l as the concern f o r mainta in ing the qu al i t y o f lakes,streams, and wetlands, the use o f such waters f o r disposal are l i mi te d.Where they can be used, th e minimum q u a l i t y o f th e wastewater e f f l u e n tt o be discharged i s sp ec i f ie d by the appropr ia te water qu al i t y agency.

3.3 S i t e Evaluat ion St ra tegy

The object ive o f a s i t e i n v e s t i ga t i o n i s t o e va lu at e t he c h a r a c t e r i st i c so f t h e a rea fo r t h e i r po ten t ia l t o t r ea t and dispose o f wastewate r. Agood s i t e eva lua t ion i s one th a t p rovides s u f f i c i en t i n fo rmat ion t o sel e c t the most appropr iate treatment and disposal system from a broadrange of feas ib le options . Th is requi res th at the s i t e eva luat ion beginw i th a l l op t ions i n mind, e l im ina t ing in f eas ib l e op tions on ly ascollected site data indicate (see Chapter 2). At the completion of thei nv es t i ga ti on , f i n a l s e l ec t i on o f a system from those feasible opt ionsi s based on costs, aest het i cs, and personal preference.

A s i t e eva luat ion should be done i n a systematic manner t o ensure thein fo rmat ion co l lec t ed i s use ful and i s s u f f i c i e n t i n de t a i l . A sugges ted procedure i s ou t l i ned i n Table 3- 1 and discussed i n the fo l low ingsect ion. This procedure, which can be used t o evaluate the f e a s i b i l i t yo f s i t es f o r s i ng l e dwel l ings o r small c l us t e r s o f dwe l li ngs ( up t o 10to 12) is based on the assumption that subsurface soil disposal is themost appr opri ate method o f wastewater disposal. Therefore, the su i t a b i l i t y o f t he s o i l s and ot he r s i t e c ha r ac t e r i s t i c s f o r s ubsu rfacedisposal are evaluated first. If found to be unsuitable, then thes i t e ' s s ui t a b i l i ty for o ther d isposal opt ions i s eval uated.

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TABLE 3-1

SUGGESTED SI T E EVALUATION PROCEDURE

Step

Cl ient Contact

Prel imi nary Eva lua t ion

F i e l d Testing

Other Si teCharacter i s t ic s

Organi z a t i on o f F ie l dInf orma ti on

3.3.1 C l i e n t Contact

Data Co ll ected

Loca ti on and des c ri p t i on o f l o t Type of use Vol ume and cha rac ter i s t i cs o f

wastewater

Avai lab l e resource informa t ion( s o i l maps, geology, etc).

Records of onsite systems insurrounding area

Topography and landscape features

Soi l p r o f i le charac ter i s t ic sHydraulic condu cti v i t y

If needed, s i t e s u i t ab i l i t y f o revaporat ion or di scharge t osurface waters should beeval uate d

Compi lat ion o f a l l da ta in touseable form

Before performing any onsi te test ing, i t i s impor tan t t o ga ther informat i o n a bout t he s i t e t h a t w i l l be useful i n e va lu a ti n g i t s p o t e nt i a l f o rt r ea t i ng and disposing of wastewater. This begins wi th the party developing the l o t . The lo ca t i on o f the l o t and the intended developmentshoul d be est abl ish ed. The volume and cha rac ter o f th e generated wastewater should be estimated. Any wastewater constituents that may posepo te nt ia l problems i n treatment and disposal, such as stron g organicwastewaters, la rg e qu an t i t i es o f greases, f a t s o r o i l s , hazardous and

toxic substances, etc., should be identified. This information helps tofocus t he s i t e eva lua tion on the impor tan t s i t e charac te r i s t i cs .

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3.3.2 Preliminary Evaluat ion

The next step i s t o gather any available resource information about thesite. This includes soil s, geol ogy, topography, e tc . , t h a t may be pub

lished on maps or i n reports. Local records of soil tests, system de

signs, and reported problems w i t h onsite systems installed i n the sur rounding area shou ld also be reviewed. This information may lack accuracy, b u t i t can be useful i n identifying potent ial problems or particular features t o investigate. A p l o t p l a n of the l o t and the l a n d immediately adjacent t o i t should be drawn t o a scale large enough so t h a tthe information gathered i n this and later steps can be displayed on thedrawing. The proposed layout of all buildings and other manmade features should also be sketched i n .

3.3.2.1 Soil Surveys

Soil surveys are usually found a t the local USDA Soi l Conservation Ser vice office. Also, some areas of t h e country have been mapped bya state agency and these maps may be located a t the appropriate stateoffice. I n counties now being mapped, advance field sheets w i t h inter pretive t ab les often can be obtained from the SCS.

Modern soil survey reports are a collection of aerial photographs of themapping area, usually a county, on which the distribution and k i n d ofsoils are indicated. Interpretations about the potential uses of eachsoil for farming, woodland, recreation, engineeering uses, and othernonfarm uses are provid ed. Detailed descriptions of each soil seriesfound i n the area are also given . The maps are usually drawn t o a scaleof 4 i n . t o 1 mile. An example of a por t ion of a soil map i s shown i nFigure 3-2.

The map symbols for each mapping u n i t give the name of the soil series,slope, and degree of erosion The soil series name i s given a two-letter symbol, the first i n upper case, the second i n lower case. Slopei s indicated by an upper case letter from A t o F. A slopes are f l a t o rnearly f l a t and F slopes are steep. The specific slope range t h a t eachletter represents differs from survey t o survey. The degree of erosion,i f indicated, is given a number representing an erosion class. The

classes usually range from 1 t o 3, representing slightly eroded t o severely eroded phases. The legend for the map symbols i s found immedia t e ly preceding and following the map sheets i n the modern publishedsurveys. An example translation of a map symbol from Figure 3-2 i sgiven i n Figure 3-3.

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FIGURE 3-2

EXAMPLE OF A PORTION OF A SOIL MAP AS PUBLISHEDI N A DETAILE D SOIL SURVEY (ACTUAL SI ZE )

FIGURE 3-3

TRANSLATION OF TYPICAL SOIL MAPPING UNIT SYMBOL

Soil Series Erosion Class(Mode rately Eroded)

Slope Class(In This Survey 2-6%)

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Interpretations about potential uses of each soil series are listed i ntables w i t h i n the text of the report.

The

soil's suitabiliy for subsur face soil absorption systems and lagoons are specifically indica ted.Engineering properties are

also 1isted, often incl uding depth to bed rock, seasonal h i g h water table, percolation rate , shrink -swell poten

t i a l , drainage potential, etc. Flooding hazard and other important fac

tors are discussed for each mapping unit with the profile descriptions.

While the soil surveys offer good preliminary information about an area,i t i s not complete nor a substitute for a field study. Because of thescale used, the mapping units cannot represent areas smaller than 2 to 3acres (8,100 t o 12,100 m2 Thus, there may be inclusions of soils withsignificantly different character within mapping units t h a t cannot beind ica ted . For typical b u i l d i n g lots, the map loses accuracy. Therefore, these maps cannot be substituted for onsite testing i n most cases.

Limitations ratings used by SCS for septic tank -soil absorption systems ,are based on conventional trench or bed designs, and t h u s do no t i n d i cate the soil's suitability for other designs. Table 3-2 lists thecriteria used i n making the l imi ta t ion ratings. They are based on asoil absorption system w i t h the bo ttom surface located 2 f t (0.6

below the soil surface. In many cases, the 1imitations can be overcomethrough proper design. Therefore, the interpretations shoul d be usedonly as a guide.

The information provided by the soil survey should be transferred t o thesite drawing along with other important information. An example for a pa rcel i s shown i n Figure 3-4. Information for each o f the soil sites

shown on Figure 3-4 is presented i n Table 3-

3.

3.3.2.2 U.S. Geological Survey Quadrangles

Quadrangles published by the U.S.

Geological Survey may be useful inestimating sl ope, topography, 1ocal

depressions or wet areas, rock outcrops, and regional drainage patterns and water table elevations. Thesemaps are usually drawn to a scale of 1:24,000 (7.5 minute series) or 1:65,000 (15 minute series). However, because of their scale, they areof limited value for evaluating small parcels.

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--- -

---

--- ---

TABLE 3-2

SOIL LIMITATIONS RATINGS USED BY SCSFOR SEPTIC TANK/SOIL ABSORPTION FIELDS

Proper ty

USDA Texture

Floodi ng

Depth t o Bedrock, >72in.

Depth t o Cemented

>72Pan, i n .

Depth t o H i gh

>6Water Table, f tbe l ow ground

Permeab i l ity ,in. /hr

24-60 in . l ayer layers 3 in.,percent by w t

[Modi f ied a f ter (10) ]

L im i t sSlight Moderate Severe

I c e

None, Rare CommonProtected

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FIGURE 3-4

PLOT PLAN SHOWING SOIL SERIES BOUNDARIESFROM SOIL SURVEY REPORT

Soi IDrainage

oundary

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TABLE 3-3

SOIL SURVEY REPORT INFORMATIONFOR PARCEL IN FIGUR E 3-4

Soi l

MapSymbol

Soil Series

L i m i t a t i o n

Rating

Flood

Hazard

High Water

Bedrock Dept h Perm.in.

Dodge 2-6 M o d e ra te No 5-10 0-40 0.63-2.040-60

Tro x e l 2-6 Severe Yes 3-5 0-60 0.63-20

2-6 Modera te No 3-5 0-41 0.63-2.041-60 2.0-6.3

Ab sor pt io n Depth t o

3.3.3 Field Testing

Field testing begins w i t h a visual survey of the parcel t o locate potential sites for subsurface soil absorption. Soil borings are made in the potential sites t o observe the soil characteristics. Percolation te st smay be conducted in those soils t h a t appear t o be well suited. If no potential sites can be found from either the visual survey, soil bor-i ng s , or percolation tests, then other means of disposal should beinvesti gated.

3.3.3.1 Visual Survey

A visual survey is made t o locate the areas on the l o t w i t h the greatest potential for subsurface soil absorption. The location of any depressions gullies, 'steep slopes, rocks or rock outcrops, or other obviousland and surface features are noted and marked on the plot plan. Vege

t a t i o n types are also noted t h a t may indicate wetness or shallow soils.Locations and distances from a permanent benchmark t o l o t lines, wells,surface waters, buildings, and other features or structures are alsomarked on the plot plan (see Figure 3.5). If a suitable area cannot be

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F IGURE 3-5

PLOT PLAN SHOWING SURFACE FEATURES

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found for a subsurface soil absorption system based on this informationother disposal options must be considered (see Chapter 2 ) . The remainder of the field testing can be altered accordingly.

3.3.3.2Landscape Posi t ion

The landscape position and landform for each suitable area should benoted. Figure 3-6 can be used as a guide for identifying landscape positions. This information i s useful i n estimating surface and subsur face drainage pa tte rns . For example, h i l l t o p s and sideslopes can beexpected t o have good surface and subsurface drainage, while depressionsand footslopes are more likely t o be poorly drained.

3.3 .3 .3 Slope

The type and degree of s lope of the area should be determined. The typeof slope indicates what surface drainage problems may be expected. Forexample, concave slopes cause surface runoff t o converge, while convexslopes disperse the runoff (see Figure 3-6).

Some treatment and disposal systems are limited by slopes. Therefore,slope measurement i s important. Land slopes can be expressed i n severalways (see Figure 3-7):

1. PERCENT OF GRADE - The fee t of ver tica l ri se or fa ll i n 100 f thorizontal distance.

2. SLOPE - The ratio of ver t ical r i se or f a l l to horizontaldistance.

ANGLE - The degrees and minutes from horizontal. 3 .

4. TOPOGRAPHIC ARC - The feet of ver tical r i se or fa l l in 66 f t(20 m) hori zontal distance.

Land slopes are usual ly determined by measuring the slope of a l ine

parallel t o the ground w i t h an Abney Level either a t eye height or a tsome other f i x e d height above the ground. I f an ordinary hand level i sused, then slopes are determined by horizontal l i n e of si gh t which givechanges in elevation for specific horizontal distances. A hand level islimited in use because it is best suited for slope determinations upgrade only , b u t has the advantage t h a t only one person i s needed f o rmapping slopes. Three methods of slope determinations are discussed bel ow.

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FIGURE 3-6

LANDSCAPE POSITIONS

Slope

ConcaveSlope

FIGURE 3-7

METHODS OF EXPRESSING LAND SLOPES

,-

o r

0' Horizonta I

Percent of Grade - 2 0 Slope-1:5Angle - 11O 19' Topographic Arc - 13.2

66' 100'

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Instrument Supported - Abney Level : For accurate slope determinations,no tch two s t i cks o r cu t fo r ked s t i cks so they w i l l h o l d t h e l e v e l 5 f t(1.5 m). above the ground. Rest the level in the notch or fork and sightto the notch or fork of the other stick held by another person at apo in t on the s lope. The land s lope i s read d i r ec t l y i n percent on theAbney Level.

Abney Level: On l e v el ground, s i g h t the person working with yo u t od et er mi ne t h e p o i n t o f i n t e r se c t i o n o f you r l i n e o f sight on him whenthe i ns tr um en t i s i n po si t i on f o r use as a hand le ve l ( ze ro l ev e l posit i on ) . When he i s on the slope, s i g h t th e same p o i n t on th e personas s i s t in g you and read the s lope d i re ct ly .

Hand Level: Height o f eye must be determined. Then s ig ht th e p o i n t o fi n t e r c e p t i o n w i t h th e ground surfac e and determine, by tape measuremento r pacing, the ground surface distance between the s ig ht in g po i nt andthe po in t o f i n te r cep t . To ca l cu la te l and s lope i n percent, d i v i de yourheig ht o f eye by the ground sur face d istanc e and mul t ip ly by 100.

Using one o f th e above procedures or ot her surveying methods, slopes a tselected sites can be determined so t h a t topography can be mapped. Thenumber o f sites needed wil l depend on th e com ple xit y o f slopes. Slopedeterm inati ons should be made a t each apparent change i n slope a t knownl o ca t i o n s so steep slope areas can be ac cu ra te ly drawn. Experience w i l lbe req ui red fo r pr ofi cie ncy and accuracy i n mapping. Steep slope areasi n nat ural topography have i r re g ul ar form and curved boundaries.Un i f o rm boundar ies havi ng st ra ig ht lines and angular corners in d i c at eman-al tered condi t ions. For la rge areas i t may be necessary t o drawcon tou r l i nes so t h a t s lopes a t d i f f e r e n t p o i n ts i n t he p l o t can bedetermined.

3.3.3.4 Soi l Bor ings

Observat ion and eva lua t ion o f s o i l cha rac ter is t i cs can best be de ter mined from a p i t dug by a backhoe o r ot he r excavati ng equipment. However , an experienced so i l te st er can do a sa t i sf act ory j ob by using ahand auger o r probe. Both methods ar e suggested. Hand t o o l s can beused to determine s oi l var iab i l i ty over the area and p i t s used to descr ibe the var ious soi l types found.

So i l p i t s should be prepared a t the perimeter o f the expected so i labsorpt ion area . P i ts p repared w i th in the absorp t ion area o f ten s e t t l eafter the system has been instal1ed and may di sr up t the d i s t r ib u t i onnetwork. I f hand augers are used, th e holes may be made within the

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absorption area. Sufficient borings or pits should be made t o adequately describe the soils i n the area, and should be deep enough t oassure t h a t a sufficient depth of unsaturated soil exists below the proposed bo tt om elevation of the absorption area. Variable soilconditions may require many pits.

Since i n some cases subtle differences i n color need t o be recognized,i t i s often advantageous t o prepare the soil p i t so the sun will beshining on the face du r ing the observation period. Natural l i g h t willgive true color interpretations. Artificial 1i g h t i n g should no t beused.

3 . 3 . 3 . 5 Soil Texture

Texture i s one of the most important physical properties of soil because

of i t s close re la t ionsh ip t o pore size , pore size distr ibution, and porec o n t i n u i t y . I t refers t o the relative proportion of the various sitesof s o l i d particles i n the soil t h a t are smaller than 2 mm in diameter.The soil texture i s determined i n the field by r ubb ing a moist sample

between the thumb and forefinger. A water bot tle i s useful for moistur i z i n g the sample. The grittiness, "silkiness," or stickiness can beinterpreted as being caused by the soil separates of sand, s i l t , andclay . I t i s extremely helpful t o work w i t h some known samples t o ga inexperience w i t h f i el d texturing.

While laboratory analysis of soil texture is done routinely by many l aboratories, field texturing can give as good information as laboratory

da t a and therefore expenditures of time and money fo r laboratory analyses are not necessary. To determine the soil texture, moisten a sampleof soil about one-half t o one i n c h i n diameter. There should be justenough moisture so t ha t the consistency i s l ike put ty. Too much moisture results i n a sticky material, which i s hard t o work. Press andsqueeze the sample between the thumb and forefinger. Gradually pressthe thumb forward t o try to form a ribbon from the soil (see Figure3-8) . By using this procedure, the texture of the soil can be easilydescribed.

Table 3-4 and Figures 3-9 and 3-10 describe the feeling and appearanceo f the various soil textures for a general soil classification.

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FIGURE 3-8

PREPARATION OF SOIL SAMPLE FOR FIELDDETERMINATION OF SOIL TEXTURE

(A) Moistening Sample

(B) Forming Cast

Ribboning

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TABLE 3-4

TEXTURAL PROPERTIES OF MINERAL SOILS

Feeling and AppearanceSoi l

Class

Sand

Sandy Loam

Loam

S i l t Loam

Clay Loam

Clay

Dry So i l

Loose, s in gl e gra ins whichfee l g r i t t y . Squeezed i nth e hand, the so i l mass f a l l s apa r t when thepressure is released.

Aggregates easily crushed;very f a i n t ve lvet y f e e l i n gi ni t i a l l y but w i th con t inuedr u b b i n g t h e g r i t t y f e e l i n go f sand soon dominates.

Aggregates are crushed undermoderate pressure; clods canbe qu i te f i r m . When pulver ized, loam has velvety fe eltha t becomes gr i t ty w i thco nt i nued rubbing. Castsbear carefu l handl ing.

Aggregates are f i r m b u t maybe crushed under moderatepressure. Clods ar e f i r m t o

hard. Smooth, f lou r -l ikefeel dominates when soil is

pul v er i zed.

Very f i r m aggregates andha rd c lods tha t s t r ong lyr e s i s t crus hi ng by hand.When pulve r ized , the s o i ltakes on a somewhat g r i t t yfe el in g due t o the harshnessof the very small aggregateswhich persist .

Aggregates ar e hard; cl od sare extremely hard ands t rong ly re s i s t c rush ing byhand. When pu l ve r i zed, i thas a gr i t - l i k e tex tu r e duet o t he harshness o f numerousvery small aggregates whichpe rs i s t.

Moist So i l

Squeezed i n th e hand, i tforms a ca st which crumbleswhen touched. Does no t fo rma ribbon between thumb andfo re f i nge r .

Forms a cast which bearscare f u l hand l ing w i tho utbre aki ng. Does not form aribbon between thumb andfo re f i nge r .

Cast can be handled quitef ree ly w i thout b reak ing .Very s l i g h t tendency t oribbon between thumb andf o r e f i nger. Rubbed surf acei s rough.

Cast can be freely handledwit hou t breaki ng. Sl ighttendency t o r ib bo n between

thumb and f or ef in ge r. Rubbedsurface has a broken o rr i p pl e d appearance.

Cast can bear much handlingwi th ou t breaki ng. P i nchedbetween the thumb andfo re f i nge r , i t forms a r ibbonwhose sur fac e tends t o feels l i g h t l y g r i t t y when dampenedand rubbed. So i l i s p l as t i c ,st icky and puddles easi ly.

Casts can bear consi derabl ehandling wi th ou t break i ng.Forms a f lexib le r ibbonbetween thumb and forefingerand r et a in s i t s p l a s t i c i t ywhen elongated. Rubbedsurface has a very smooth,sa t i n fee l i ng . St icky whenwet and easily puddled.

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FIGURE 3-9

SOIL TEXTURE DETERMINATION BY HAND: PHYSICAL

APPEARANCE OF VARIOUS SOIL TEXTURES

SandyLoam

Weak Aggregates

Si l tLoam

Fir m Aggregates

Clay

Hard Aggregates

No Ribbon; Non-Plastic Cast

Very Slight RibboningTendency; ModerateIy

Plastic Cast

Ribbons Easily; Plastic Cast

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FIGURE 3-10

COMPARISON OF RIBBONS AND CASTS OF SANDY LOAMAND CLAY (RIBBONS ABOVE, CASTS BELOW)

I f the so i l sample r ibbo ns (loam, cl ay loam, or cl ay ), i t may be desir

able to determine i f sand o r s i l t predominate. I f t here i s a g r i t t yf ee l and a lack o f smooth ta lc - 1 ike fee l , then sand very 1i k e l y predominates. I f t h er e i s a l a ck o f a g r i t t y f e e l b u t a smooth t a l c - l i k e f e e l ,then silt predominates. If there is not a predominance of either thesmooth or g r i t t y fe e l , then the sample should not be ca l l ed anyth ingother than a clay, clay loam, or loam. I f a sample feels quite smoothw i t h l i t t l e o r no g r i t i n it, and w i l l n o t form a ribbo n, th e samplewou ld be ca l l ed s i l t loam.

Beginn ing a t the top o r bo t tom o f the p i t s idewal l , obv ious changes i nt ex tu re wi th depth are noted. Boundaries t h a t can be seen ar e marked.The texture o f each la ye r o r h or izon i s determined and the demarcationso f boundaries changed as app rop ria te. When th e te xt ur es have beendetermined fo r each lay er, the depth, thickness, and te xt ur e o f eachlay e r i s r eco rded ( see Figure 3-11).

3.3.3.6 So il St ru ct ur e

S o i l s t r uc t u re has a s i gn i f i ca n t i n f l uence on the soi l 's acceptance andtransmission of water . So i l s t r uc tu re re fe r s to the agg rega t ion o f s o i lparticles into clusters of particles, called peds, that are separated bysurfaces o f weakness. These su rf aces o f weakness open pl an ar pores

between the peds th a t are of te n seen as cracks i n the so i l . These p la -nar pores can grea t ly mod i fy the in f luence o f so i l tex tu r e on watermovement. Well- s t r u c t u r e d so i l s wi th large voids between peds w i l lt ransmi t water more rap id ly than s t ructu re less so i ls o f the same text u r e , p a r t i c u l a r l y i f the s oi l has become dry before the water i sadded. Fine-textured, massive soil s (soils' with little structure) havevery slow percolat ion rates.

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FIGURE 3-11

EXAMPLE PROCEDURE FOR COLLECTINGSOI L P I T OBSERVATION INFORMATION

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I f a detailed analysis of the soil structure i s necessary, the sidewallof the soil p i t s hou l d be carefully examined, us i ng a pick or similardevice to expose th e natural cleavages and planes of weakness. Cracksi n the face of the soil profile are indications of breaks between soil

peds. The shapes created by the cracks should be compared to the shapes

shown i n Figure 3-12. If cracks are no t visible, a sample of soi lshould be careful ly picked out and, by hand, carefully separated i n t o

the structural units u n t i l any further breakdown can only be achieved byfracturing.

Since the structure can significantly al ter the hydraulic characterist i c s of s o i l s , more detailed descriptions of soil structure are sometimes desi rabl e. Size and grade of dur ab ili ty o f the structural units provide useful information t o estimate hydraulic cond ucti viti es. Descriptions of types and classes of soil structure used by SCS are giveni n Appendix A. Grade descriptions are given i n Table 3-5. The type,size, and grade o f each horizon or zone i s recorded i n Figure 3-11.

3.3.3.7 Soil Color

The color and color patterns i n soil are good indicators o f the drainagecharacteristics of the so il . Soil propertie s, location in the land-scape, and climate all influence water movement i n the soil. Thesefactors cause s ome s o i l s t o be saturated or seasonally saturated,affect ing their abil i t y t o absorb and t r e a t wastewater. Int erp ret at ionof soil color aids i n identifying these conditions.

Color may be described by estimating the true color for each horizon or by comparing the soi l w i t h the colors i n a soil color book. In eithercase, i t i s particularly important t o note the colors or color patterns.

Pick up some soil and , without crushing, observe the color. I t i simportant t o have good s u n l i g h t and know the moisture status of thesample. If ped faces are dry , some water applied from a mist bot t l ewill allow observation of moist colors.

Though i t i s often adequate t o speak of soil colors i n general terms,

there i s a standard method o f describing colors us ing Munsell colornotation. Th i s notation i s used i n soil survey reports and soil description. Hue is the dominant spectral color and refers to the lightness or darkness of the color between black and white. Chroma i s there la t ive purity of strength of the color, and ranges from gray t o a b r i g h t color of that hue.

Numbers

are given to each of the variablesand a verbal description is also given. For example, 10YR 3/2 corresponds t o a color hue of 10YR value o f 3 and chroma 2. Th i s i s a verydark grayi sh brown.

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FIGURE 3-12

TYPES OF SOIL STRUCTURE

BlockyPrismatic o l u m n a r

TABLE 3-5

GRADES OF SOIL STRUCTURE

Grade

Structu re1 ess

Weak

Moderate

Strong

Character i s t ic s

No observable aggregation.

Poor ly formed and d i f f icu l t to see.W i l l

not retain shape on handling.E vident bu t no t d i s t i nc t i n und is tu r bed

s o i l .Moderately durable on handling.

Visual ly d ist inct in undisturbed soi l . Durable on ha ndl ing.

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If a soil color book i s used to determine soil colors, ho l d the soil and book so the sun shines over your shoulder. Match the soil colo r w i t hthe color c h i p i n the book. Record the hue, chroma and va lue, and thecolor name.

Mottling i n s o i l s i s described by the col or of the soil matrix and thecolor or colors, size, and number of the mottles. Each color may begiven a Munsell des ignat ion and name. However, i t i s often suff icientt o say the soil i s mottled. A class i f ica t ion of mottles used by theUSDA is shown i n Table 3-6. Some examples of soil mottling are shown onthe inside back cover of this manual.

DESCRIPTION

Character Class

Abundance FewCommonMany

Size Fi neMediumCoarse

Contrast Fai n tDi stinctProminent

TABLE 3-6

OF

SOIL

MOTTLES (10)

L i m i t

20% of exposed face

15mm 1ongest d imensi on

Recognized only by cl ose observat ionReadily seen b u t no t str ikingObvious and striking

3.3.3.8 Seasonally Saturated So il s

Seasonally saturated soils can usually be detected by soi l borings madeduring the wet season or by the presence of mottled soils (see 3.3.3.7).For large cluster systems or for developments where each dwelling i sserved by an onsite system, the use of observat ion well s may be ju s t ifi ed . They ar e constructed as shown i n Figure 3-13. The well should be

placed i n , b u t not extended through, the horizon t h a t i s t o be monito re d. More than one well i n each horizon that may become seasonallysaturated i s desi rable. The wells ar e monitored over a normal wet season by observing the presence and duration o f water i n the well. Ifwater remains i n the well for several days, the water level elevation ismeasured and assumed t o be the elevation of the seasonally saturatedsoil hor izon.

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3.3.3.10 Hydraulic Conductivity

Several methods of measuring the hydraul i c conducti vity of soi ls have been developed (1)(11). The most commonly used tes t i s the percolationtest. When r u n proper ly, the test can give an approximate measure ofthe soi l ' s saturated hydraulic conduct ivity. However, the per col ati onof wastewater through soil bel ow soil disposal systems usual l y occursth rough unsaturated so il s. Therefore, empirical fac tor s must be used toestimate unsaturated conductivities. The unsaturated hydraulic conduc-tivi ties can vary dramatically from the saturated hydraulic conductivityw i t h changes i n soi l characteristics and moisture content (see Appendix

The percolation tes t i s often criticized because of i t s variability andfailure to measure the hydraulic conductivity accuratel y. Percolati ontests conducted in the same soils can vary by 90% or more (1)(11)(12)(13)(14). Reasons for the large variability are attributed to the procedure used, the soil moisture conditions at the time of the test, and the individual performing the test. Despite these shortcomings, the percolation test can be useful i f used together w i t h the soil boringsdata. The test can be used to rank the relative hydraulic conductivityo f the soil. Estimated percolation rates for various soil textures aregiven i n Table 3-7.

TABLE 3-7

CHARACTERISTICS OF SOIL (15)ESTIMATED HYDRAULIC

Soil Texture

Sand >6.0


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I f t e s t r e su l t s a g r e e wi th t h i s table , the te s t and bo r ing da ta areprobably co rr ec t and can be used i n design. If not , e i th er the t es t wasrun improper ly o r so i l s t r uct u re o r c la y minera logy have a s ig n i f i ca n te f f ec t on the hyd rau l i c conduc t i v i t y . For example, i f t h e t e x t u r e o f as o i l i s de te rm ined t o be a c lay l oam, the est imated pe rco la t i on ra t e i sslower than 45 min/in. (18 min/cm). If the measured percolation rate is15 min/in.

(6

min/cm), however, e i th e r the tex t u re i s i nc o r re c t o r thes o i l has strong str uc tu re wi t h la rg e cracks between peds. The te st ershould be caut ious i n such s oi l s because the unsaturated hydrau l icc on d uc t i v i t y may be many time s less. Expandable cl ay s may be pr es en t .t h at could clo se many o f the pores.

Several percolation test procedures are used (11)(16). The most commonprocedure is the falling head test (11). Though less reproducible thanother procedures, it is simple to perform in the field (11)(12). Thef a l l i n g head procedure i s o u t l i ne d i n Table 3-8. A diagram o f a“percometer” designed to simplify the testing is illustrated in Figure

3-14. For a discussi on of ot her methods see the Nat ion al EnvironmentalHeal th Associat ion ’s “On-Site Wastewater Management” (16).

Data co l lec ted f rom the perco l a t ion t e s t can be tabu la ted us ing a fo rms i m i l a r t o t he one i l l u s t r a t e d i n Figure 3-15.

3.3.4 Other Si te Cha rac te r i st ic s

If subsurface disposal does not appear to be a viable option or cost-effective, other methods of disposal are evaluated (see Chapter 2).

Eva por ati on and discharge t o sur face waters are other opt ions t oinves t i gate. Each requ i res fu r t her s i t e eva luat ion .

3.3.4.1 S i t e Eva lua tio n o f Evaporat ion Potent ia l

Evaporation and eva pot rans pir ati on can be used as th e sol e means o f d is posal or as a supplement t o s o i l absorption. To be ef fect ive, evaporat i on shou ld exceed p rec ip i ta t i on i n the area. The di f fe re nc e betweenevapo ra t i on and p re c i p i t a t i o n ra tes p rov ides es t ima tes o f quan t i t i e s o fwater t h a t can be evaporated from a fr ee water surf ace.

Weather data can be obtained from local weather stations and the Nat i o n a l Oceanic and Atmosphere Ad mi ni st ra ti on (NOAA). Ra i n f a l l and snowf a l l measurements are av ai la bl e from NOAA f o r thousands o f weather stat i o ns throughout the country. Many lo ca l agencies al so mai nta in records. A c r i t i c a l we t ye ar i s t yp i c a l l y u sed f o r d es ig n based on a tle as t 10 years o f r ecords (18 ).

40


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FIGURE 3-14

CONSTRUCTION OF PERCOMETER

Yard Stick

Wh en making percolat iontests, m ark l ine s here atregular time intervals,

Measur ing St ick

Guide L Other FixedBatter Board or

Reference

2" Layer of Gravel

(a) Float ing Indicator Fixed Indicator


61/408

FIGURE 3-15

PERCOLATION TEST DATA FORM (17)

Test hole number

Depth to b o t t o m of hole inches. Diameter of inches.

Depth, inches Soil texture

fat

4-

S C

Percolation test by

Date of test

I I I I I

Percolation rate minu tes per inch.

43


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Estab l ish ing evapora t ion da ta a t a speci f ic lo ca t i on can be a mored i f f i c u l t problem. Measurements of Class A pan evaporat ion rates arer e p or t e d f o r a l l o f t h e s t at e s by NOAA i n t h e p u bl i ca t io n ,"Climato logica l Data," U.S. Department o f Commerce, av ai la bl e i ndeposi tory l i b r a r ies f o r government documents a t major uni ve rsi ties i neach st at e. Pan ev ap or at io n measurements a r e made a t a few ( 5 t o 30)

weather s ta t i ons i n each s

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