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Standards and Administration of Mid-Sized Individual Sewage Treatment Systems (MSTS)

This document outlines the standards, performance criteria, and administrative regulations for Mid-Sized Individual Sewage Treatment Systems (MSTS), catering to multiple dwellings or establishments with a wastewater flow between 2,500 and 10,000 gallons per day. The framework emphasizes the protection of surface and groundwater, public health, and safety. It includes requirements for sewage collection systems, site evaluation, groundwater investigation, septic tank specifications, and final treatment and disposal. Local permitting authorities oversee compliance with these standards under EPA Class V regulations.

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Standards and Administration of Mid-Sized Individual Sewage Treatment Systems (MSTS)

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  1. Rule Change UpdateMPCA1/13/05

  2. 7081 Mid-Sized ISTS (MSTS)

  3. I. Scope • A. Standards to reasonably protect surface water, ground water, and public health, safety, and general welfare.

  4. I. Scope • B. Mainly performance outcomes provided with some prescriptive design, construction, and operational standards.

  5. I. Scope • C. Systems designed under this part may require additional design requirements pursuant to EPA Class V regulations.

  6. I. Scope • Chapter 7081 - mid-sized ISTS (MSTS) • Multiple dwellings or Other Establishments - 2,500 to 10,000 gpd

  7. I. Scope • Sewage collection system • Piping, lift stations or components that conveys sewage • On-lot sewage tank = MSTS

  8. I. Scope • H. Administration of chapter 7081 standards is done by local permitting authorities.

  9. SDS Permit Threshold • One or more systems • Average daily flow of greater than 10,000 gallons per day • ½ mile radius • Single ownership • ????????????????????

  10. SDS Permit Threshold • Any system the commissioner has determined needs a SDS permit. • Flow amounts to determine if an SDS permit is required must be per 7081.0110

  11. Performance and Compliance Criteria • No contact with sewage. • Safe systems - protection from physical injury and harm. • Accepted no hazardous material.

  12. Performance and Compliance Criteria • Groundwater protection • No fecal organism in groundwater • Systems that comply with any nitrogen or phosphorus standard

  13. Performance and Compliance Criteria • Other conformance • In conformance with the operating permit.

  14. IV. Professional Requirements. • A. MSTS Designer • B. Installer • C. MSTS Inspector • D. Maintainer • E. Service Provider

  15. Sewage Flow Determination • Dwellings • Use Type II flows for 10+ dwellings

  16. V. Sewage Flow Determination • Other • Include 200 gallons of infiltration and inflow per inch of collection pipe diameter per mile per day.

  17. VI. Soil And Site Evaluation • Normal preliminary evaluation plus: • Distance to surface water • Waste strength

  18. Soil And Site Evaluation • Soil pits required - one pit per corner and one the center, plus one additional soil pit per 10,000 ft2.

  19. Soil And Site Evaluation • Must determine perc rate by both water testing and texture, structure and consistence.

  20. Groundwater Investigation • A preliminary groundwater evaluation for all MSTS

  21. Groundwater Investigation • A determination of : • shallow groundwater setting • regional groundwater setting • aquifer(s) used for water supply • wells • groundwater flow direction

  22. Groundwater Investigation • A determination if contaminant plumes may intersect water supply wells capture zones

  23. Groundwater Investigation • An estimation of the extent of groundwater mounding from the operating system. • The accuracy of the approximation shall be determined

  24. Groundwater Investigation • The designer must consult with the permitting authority for the extent of any further investigation or monitoring

  25. Design Standards. • A flow measure device must be employed on all MSTS.

  26. Design Standards. • The system must be designed with sufficient access and ports to monitor the system as applicable.

  27. IX. Sewage Tanks. • Septic tank size = average daily flow by 3.0. • Septic tank size under pressure = the average daily flow by 4.0.

  28. Sewage Tanks. • Septic tank capacity prior to other treatment devices, shall be per manufacturer’s requirements or accepted engineering principals.

  29. Sewage Tanks. • Tanks shall be connected in series. Parallel OK if sufficient justification is provided and adequate monitoring is provided.

  30. Sewage Tanks. • All tanks used for MSTS shall be tested for water tightness. • Food preparation facility, shall install an external grease trap.

  31. Distribution Of Effluent. • Distribution in accordance with 7080.0160 • Designed for a 2 foot head.

  32. DOSING OF EFFLUENT. • Dosing in accordance with 7080.0160

  33. Final Treatment and Disposal. • Systems setback from site boundaries, downgradient of groundwater flow 50’ + ((average design flow – 1500) x .03’)

  34. Final Treatment and Disposal. • Sized with a 50% increased plus 50% reserve. (not for less than 2,500 gpd).

  35. Final Treatment and Disposal. • Sizing • Hydraulic • flow • soil • gw mounding • Nitrogen • Phosphorus • Organics (biomat) • Pathogen removal

  36. Final Treatment and Disposal.

  37. Final Treatment and Disposal. • Separation Distance depends on: • Soil texture • Loading rate • Dosing frequency • System width and depth • Capillary fringe • Groundwater mounding • Concentrations of contaminants • Hydraulic head • Factor of safety

  38. Final Treatment and Disposal. • An unsaturated zone must be maintained. • Measure the height of the seasonally saturated soil during operation. • Systems might need to employ nitrogen and phosphorus reduction methods.

  39. XConstruction • Designer must observe critical periods of system construction

  40. Operation and Maintenance. • Operated under a local operating permit

  41. System Ownership and Responsibility • Apply for and obtain construction and operating permits; • Ensure submittal of required reporting and compliance status to the permitting authority; • Negotiate contracts as necessary; • - recover costs of damages to the system; • - able to obtain financing

  42. System Ownership and Responsibility • Provide annual education to users on suitable discharges; • Monitor compliance with local ordinances requirements • - financial instrument to continue the operation, maintenance, management, repairs, of the system for a period of one year

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