Subsurface Sewage Treatment Systems



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Subsurface Sewage Treatment Systems

Prescriptive Designs and Design Guidance for Advanced Designers



October 2013
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Authors and Contributors


Brett Ballavance P.E., Sara Heger, Brian Fitzpatrick P.E, David Gustafson P.E., Corey Hower, David Kortan P.E., Corey Mathisen P.E., Barbara McCarthy PSS, Bill Priebe P.E., Amy Douville, Gretchen Sabel, David Sahli P.E., Randy Thorson P.E., and Mark Wespetal PSS.

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Disclaimer


This is a guidance document produced by the Minnesota Pollution Control Agency for the benefit of the general public and wastewater treatment professionals alike. This document is not intended as a substitute for professional engineering consulting advice and/or a substitute for the assistance from wastewater treatment professionals who should address the unique, onsite conditions (soils, topography, etc.) and decide on the appropriate treatment products to be used in that particular situation. While the scenarios described in this document may appear to provide solutions for some situations, users of this guide are strongly recommended to seek the advice of their own consultants and wastewater treatment professionals who can assess the unique circumstances and address the specific concerns and needs of an individual project. The State of Minnesota Pollution Control Agency and/or its employees accept no responsibility or liability for the contents of this guidance document, the designs, or advice contained herein. This guidance document is not intended, and cannot be relied upon, to create any rights, substantive or procedural, that can be enforced in litigation or any administrative proceeding with the State of Minnesota or the Minnesota Pollution Control Agency.


Contents

Authors and Contributors 2

Disclaimer 3

I. Introduction 5

I. A. Purpose of this document 5

I. B. Design guidance for systems 6

I. C. Limitations of advanced design 8

I. D. Desired goals and outcomes 9

I. E. How to use this document 11

I. F. Contacts 14

II. Assessment methods 15

II. A. Suitable waste for subsurface sewage treatment systems 15

II. A. 1. Introduction 15

II. A. 2. Definitions 15

II. A. 3. Regulations 17

II. A. 4. Assessment methodology 21

II. A. 5. Instructional help for methodology 21

I. A. 6. References 24

II. B. Waste strength assessment 24

II. B. 1. Introduction 24

II. B. 2. Waste strength assessment methods 27

II. B. 3. At-risk design recommendations 33

II. B. 4. High strength waste design standards 34

II. B. 5. Who can do the work 35

II. B. 6. Additional assistance 35

II. B. 7. Literature reviewed 35



II. C. Flow determination criteria 36

II. C. 1.Introduction 36

II. C. 2. Definitions 38

II. C. 3. Flow estimation for SSTS serving multiple dwellings 38

II. C. 4. Flow estimation for SSTS serving Other Establishments 39

II. C. 5. Flow determination from measured flow for SSTS serving Other Establishments 39

II. C. 6. Final flow determination 41



II. D. Permit determination 42

II. D. 1. Introduction 42

II. D. 2. Examples 43

59

II. E. Site suitability assessment 60

II. E. 1. Introduction 60

II. E. 2. Preliminary site evaluation 60

II. E. 3. Field site evaluation 62

II. E. 4. Site evaluation interpretation 63

II. E. 5. Site evaluation reporting 63



II. F. Assessing nitrogen impacts to aquifers 63

II. F. 1. Introduction 63

II. F. 2. Aquifer impact assessment for ISTS with a flow between 2,501 gpd and 5,000 gpd (LISTS) 65

II. F. 3. Aquifer impact assessment for MSTS (flow between 5,001 gpd and 10,000 gpd) 69

II. F. 4. Aquifer impact assessment conducted by a AELSLAGID Board Professional 71

II. F. 5. Literature reviewed 74



II. G. Groundwater mounding assessment 74

II. G. 1. Introduction 74

II. G. 2. Definitions 77

II. G. 3. Site conditions that affect mounding 79

II. G. 4. Two mounding determinations must be considered - MSTS 80

II. G. 5. Effluent mounding in an unsaturated restrictive layer – MSTS 80

II. G. 6. Groundwater mounding on a saturated layer - MSTS 83

II. H. Surface water impact from phosphorus 86

II. H. 1. Introduction 86

II. H. 2. Individual sewage treatment systems 86

II. H. 3. Mid-sized sewage treatment systems 87



III. Design selection 87

III. A. Introduction 87

III. B. Design options for non-domestic waste sources 87

III. C. Design options for high strength wastes 89

III. D. Design options for variation in flow 92

III. E. Soil dispersal design options for site conditions 92

III. F. Design options for soil conditions 94

III. G. Design options for groundwater mounding 97

III. H. Design options for nitrogen sensitive aquifers 97

III. I. Design options for surface water protection 98

III. J. Advanced treatment devices considerations 99

IV. Component design 101

IV. A. Flow equalization 101

IV. A. 1. Introduction 101

IV. A. 2. When to employ flow equalization 101

IV. A. 3. Methodology to determine flow equalization 101

IV. A. 4. Methods of flow equalization 114

IV. A. 5. Maintenance concern 114



IV. B. Tank requirements and tank sizing 114

IV. B. 1. - General 114

IV. B. 2. – Definitions 115

IV. B. 3. - Requirements/specifications 116

IV. B. 4. – Septic tank sizing 117

IV. B. 5. – Stilling tank sizing 118

IV. B. 6. – Combined septic tank and stilling tank sizing (one compartment tank) 118

IV. B. 7. – STEP pump tank sizing 118

IV. B. 8. – Common pump tank sizing 118

IV. B. 9. – Combination stilling/ pump tank sizing (one compartment tank, all septic tank capacity is at the dwellings) 119

IV. B. 10. – Flow equalizing tank sizing 119

IV. B. 11. – Combination stilling/flow equalizing tank sizing (one compartment tank) 119

IV. B. 12. - Recirculation tank sizing 119

IV. B. 13. – Combination septic/recirculation tank (one compartment tank) 120



IV. C. Recirculating Media Filter (RMF) Design Guidance 120

IV. D. Single Pass Sand Filter (SPSF) Design Guidance 120

IV. E. Registered proprietary treatment and distribution products 120

IV. E. 1. Introduction 120

IV. E. 2. How to use the list of registered products 120

IV. E. 3. Proprietary treatment products and treatment levels 121

IV. E. 4. Nutrient listing 122

IV. E. 5. Distribution media products 122

IV. E. 6. Product information 123

IV. E. 7. Product performance and renewal 124

IV. E. 8. Upscaling of registered treatment products; splitting flow to treatment and disinfection devices 124

IV. F. Soil dispersal system design 125

IV. F. 1. Introduction 125

IV. F. 2. Performance of soil dispersal systems 125

IV. F. 3. Major design parameters for all soil dispersal systems types 126

IV. F. 4. Design for nitrogen reduction components 129

IV. F. 5. Dispersal component selection 131

IV. F. 6. Determine hydraulic soil loading rates based on organic loading rates 136

IV. F. 7. Design considerations for groundwater mounding 137

IV. F. 8. Design of trenches 138

IV. F. 9. Design of seepage beds 141

IV. F. 10. Design of at-grade systems 143

IV. F. 11. Design of mound system 145



IV. G. Design worksheets 146

V. Collection system design 147

V.A. Overview 147

V. B. Gravity collection system design 148

V. B. 1. Introduction 148

V. B. 2. Definitions 149

V. B. 3. Design flow 149

V. B. 4. Design pipe diameter, materials and slope 149

V. B. 5. Design sewer setback, alignment, and depth 151

V. B. 6. Design sewer main trench, trench bedding and backfill material 155

V. B. 7. Air, hydrostatic and deflection testing 156

V. B. 8. Air testing 156

V. B. 9. Hydrostatic testing 156

V. B. 10. Deflection testing 158

V. B. 11. References 158



V. C. Pressure sewer collection system design 159

V. C. 1. Introduction 159

V. C. 2. Definitions 159

V. C. 3. Design flows and minimum velocity 160

V. C. 4. Pump selection 160

V. C. 5. Other construction requirements 162

V. C. 6. Material specifications 168

V. C. 7. Wastewater lift station requirements when installed in the main collection system 168

V. C. 8. References 170

VI. Operation and maintenance 171

VI. A. Management plans 171

VI A. 1. Introduction 171

VI A. 2. General system considerations 172

VI A. 3. Collection system 172

VI A. 4. Grease interceptors 173

VI A. 5. Septic tanks 173

VI A. 6. Advanced pretreatment units 173

VI A. 7. Soil treatment systems 177

VI. B. Operating permits 177

VI. C. Groundwater mounding monitoring 179

VI. C. 1. Introduction 179

VI. C. 2. Applicable rules – Minn. R. ch. 4725 – Wells and Borings 180

VI. C. 3. Suitable devices 181

VI. C. 4. Monitoring location 181

VI. C. 5. Device depth and screen depth 182

VI. C. 6. Testing and maintenance 188

VI. C. 7. Taking water-level measurements 188

VI. C. 8. Timing, frequency, and duration of readings 188

VI. C. 9. Interpreting results 189

VI. C. 10. Reporting results 189

VI. C. 11. Device removal 190

VI. C. 12. References 190

VII. Appendices 190

Appendix A 190

State system classification and permit worksheet 190



Appendix A-2 193

Federal system classification and permit worksheet suitable waste for subsurface sewage treatment systems worksheet 193



Appendix B 195

High Strength Waste Worksheet 195



Appendix C 198

Flow determination for Individual Dwellings Worksheet 198



Appendix D 200

Appendix E 202

Flow estimation for other establishments 202

Notes for flow estimates for other establishments 207

Definitions for other establishments 207

Commercial/industrial 208

Eating and drinking establishments 209

Entertainment establishments 210

Outdoor recreation and related lodging facilities 210

Transportation 211

Institutional 213

Other public institutes 213

Miscellaneous 213



Appendix F 214

Appendix G 215

Appendix H 216

Appendix I 217

Nitrogen impacts to aquifers assessment for advanced designers 217



Appendix I (cont.) 218

Appendix I (cont.) 219

Nitrogen impacts to aquifers from SSTS 221

Worksheet 221

Appendix J 224

Setbacks 224



Appendix K 225

Acronyms and abbreviations 225



Appendix L 226

Interpretation of site evaluation 226



Appendix M 228

Appendix N – Check of Hantush spreadsheet 232

Instructions for Hantush spreadsheet 235

2. Fraction of drainfield subunit that is trench area (aka “absorption area”): 236

Appendix O 237

Instruction sheet for aquifer impacts determination 243

I. Determine well impacts 243

II. Calculate treatment 243



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