Subsurface Sewage Treatment Systems



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II. C. Flow determination criteria

II. C. 1.Introduction


The determination of sewage design flow is one of the most important items in the planning of a new or expanded SSTS. Sewage is defined in Minn. R. 7080.1100 subp. 73:

"Sewage" means waste produced by toilets, bathing, laundry, or culinary operations or the floor drains associated with these sources, and includes household cleaners, medications, and other constituents in sewage restricted to amounts normally used for domestic purposes.”

The following methods and tables can be used to determine the sewage flow for all systems regulated under chapters 7080 and 7081. To determine the design flow, the correct flow method must be chosen. Helpful explanatory notes, worksheets, tables, and examples are found in Appendices C through G.

These methods provide guidance on estimating volume of sewage flow only. For systems anticipated to receive both sewage and non-sewage, the non-sewage volume must be determined. It should be noted that non-sewage may only be discharged if the waste is suitable for discharge into the soil and in accordance with applicable local, state, and federal regulations. Please refer Section I A.

Choose the appropriate method to determine the flow. The choices are as follows:



  • Flow estimation for SSTS serving multiple dwellings

  • Flow estimation for SSTS serving Other Establishments

  • Flow determination from measured flow for SSTS serving Other Establishments

In addition to use of one of the above flow determination methods, a Final Flow Worksheet
(Appendix G) must be completed to account for any additional flow that may enter the system.

Flow amounts determined by these methods plus final flow calculation are to be used for system classification (ISTS, MSTS or systems needing an SDS permit) and therefore the type of permit needed (local or state). This flow amount may need to be modified when designing specific system components (e.g., collection system design, groundwater mounding, phosphorus mitigation, surge capacity, timed dosing, etc.).



II. C. 2. Definitions


The following definitions are provided to aid in understanding this section:

"Dwelling" means any building with provision for living, sanitary, and sleeping facilities (7080.1100 subp. 25). For more detail on bedroom determination, please refer to the MPCA fact sheet on bedroom determination at http://www.pca.state.mn.us/index.php/view-document.html?gid=5323

"Other establishment" means any public or private structure other than a dwelling that generates sewage that discharges to an SSTS (7081.0020).


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


  1. Introduction

This method is to be used for estimating flow from multiple dwellings. The governing rule is found at Minn. R. 7081.0120 subp. 1 and 2 as follows:

Subpart 1. Sum of design flow for existing dwellings.



The design flow for MSTS serving existing dwellings is determined by the following calculation in conjunction with part 7080.1850:

the total flow from the ten highest flow dwellings + (total flow from the remaining dwellings * 0.45)” (NOTE: I and I will be added during the final calculation)

Subp. 2. New housing developments.

For new housing developments to be served by a common SSTS, the developer must determine and restrict the total number of bedrooms for the development. Proposed dwellings are determined to be Classification I dwellings for flow determination purposes unless different classifications are approved by the local unit of government. The determined classification system must be used in conjunction with the flow calculation method in subpart 1. If the ultimate development of phased or segmented growth meets or exceeds the thresholds in part 7081.0040, subpart 1, item B, the initial system or systems and all subsequent systems require a state disposal system permit.”

For SDS permit determination, this calculation method is to be used for multiple dwellings connected to one large SSTS, multiple dwellings connected to several small SSTS, SSTS found on individual lots, or any combination of the above. This method is allowed to be used as the smaller number of units has a higher peaking (safety) factor. Minn. R. 7081.0040 subp. 1 item D allows the permit flow volume to be reduced if a factor of safety is added to the flow amounts.

If Other Establishments are to be included with multiple dwellings, their flows are to be included with the total flow and included in the ranking to determine the ten highest flow units.

b. Method

1) Determine the number of bedrooms and dwelling classification for each dwelling. For new developments, the developer must limit the total number of bedrooms for the development.

2) Determine the flow based on the dwelling classification and number of bedrooms. For new developments, dwellings must be classified as Classification I, unless strong evidence exists that the dwellings will not be Classification I.

3) Add the flow together for the ten dwellings/other establishments with the highest flow.

4) Multiply the flow for the remaining dwellings/other establishments by 0.45 and add together.

5) Add the values in Step 3 and Step 4.

6) Go to final flow determination.

An example is provided in Appendix D.

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


a. Introduction

This method in this section is to be used to estimate the sewage flow if the SSTS will serve Other Establishments. The governing rule is found at Minn. R. 7081.0120 subp. 2.

If non-domestic/non-hazardous wastes will also be discharged with the sewage, that waste must be suitable for soil discharge. Please see Section II A. The non-sewage flow, if found suitable, must be added to the values determined by this worksheet.

b. Method

Determine the flow based on Table I in the Appendix E (e.g., Table I in 7081.0130). For some establishments, more than one way to determine the flow is provided (e.g., either number of meals served or square foot of the restaurant). All methods provided should be calculated and the highest volume chosen. Be sure to include 15 gallons per employee per eight-hour shift unless the table indicates that employees are already included in the flow. If average daily flow amounts are to be used, consideration should be given if a peak hourly or other sub-daily flow value should be used to design some SSTS components. Please refer to Section IV A for flow equalization methodology.

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


a. Introduction

This method is to be used to determine the design flow by measurements of flow from Other Establishments. Measured flow data cannot be used if one or more of the following conditions apply:

1) If the flow measuring device has not been calibrated within six months from the start of the measurement

2) If the data is old as compared to the current use of the facility

3) If the facility is planning on a change of use or expansion

4) The peak 90-day flow has not been measured in daily measurements

5) If the daily use (e.g. percent occupancy or percent capacity of use) cannot be reasonably estimated

Measured flow data that cannot be used to determine the permitting flow for the system in question may be useful to use in conjunction with the estimated flow values for design purposes.

b. Method

The method to determine design flow from measured flow is provided as follows:

1) Measure the anticipated peak 90-day flow values along with percent occupancy or capacity of use of the facility each day of the flow reading (e.g., percent of the campgrounds occupied)

2) Divide each day’s flow value by the percent capacity (in decimal form)

3) Determine the peak consecutive seven-day flow from values calculated in Step 2.

4) Average the values in Step 3.

5) Multiply the value in Step 4 by 2.0 if the system was experiencing treatment or hydraulic problems because water use may have been significantly reduced to mitigate the problems.

6) The computed measured flow value must be compared with the estimated flow values provided in rule or other published documents if values are not found in rule (if available). The result of the comparison must be evaluated to assure the measured flow value is reasonable.

7) Follow Final Flow Determination method to complete this process.

It is desirable that the average daily operating flow volume be approximately 70 percent of the daily design flow. This 70 percent value is chosen for average flow values with the intent that maximum daily flows do not exceed the design flow.

c. Cautions with using measured flow

Much care and discernment should be used if measured flow values are to be used for design. It is important to look at the measured values with a holistic mindset. For example, the measured flow values should be viewed against:

1) Did the business just change ownership?

2) Will the business change its business plan?

3) What is the anticipated future growth plans for the business?

4) What is the future growth of the area that could increase business of the facility?

It is thought that underestimation of flow values is one of the main reasons for system failure. Therefore, it is strongly recommended that the chosen design flow value be conservative. Use of a conservative flow volume will not result is a wasteful expenditure, as a more conservative design will decrease operation and maintenance requirements and increase system longevity.

II. C. 6. Final flow determination


Other factors affecting flow amounts must be added to the flow amounts calculated in Methods a. to c. as described below:

a. Flow volumes must be increased if construction of additional dwellings or bedrooms, the installation of water-using devices or other factors likely to affect the operation of the system can be reasonably anticipated (Minn. R. 7081.0120 subp. 3).

b. For clustered development of dwellings, the flow must be determined by the total developed area if the development is to be phased or segmented over time (7081.0120 subp. 2). The developer must place restrictions on the lots so the total number of bedrooms cannot be exceeded.

c. If the system is served by a sewage collection system, the leakage (infiltration) into the collection system must be estimated. The amount is equal to 200 gallons per inch diameter of pipe per mile per day (minimum pipe diameter of two (2) inches) (Minn.R. 7081.0140).

d. If non-sewage is to be discharged into the system, its volume must be added to the flow. CAUTION: 1) the non-sewage flow must be suitable for treatment in the soil, and may require a MPCA permit (see Section II. A. 3 b.); 2) the quantities must be reliably known; and 3) if non-sewage flow will widely fluctuate, the system must be designed based on peak flow or flow equalization must be used.

The final step is to add the flow amounts from Sections II. C. 3 through II. C. 5 with the flow calculated from items a. to c. above.


An assessment should be made regarding the variation in the flow amount calculated. The variation can be a variation within one day’s time (e.g., all the flow in a two-hour period), or a weekly, monthly or seasonal variation or any special events. This variation should also consider the time of year of the variation to reduce the potential for system freezing. Please refer to Section IV A for flow equalization methodology.

Flow amounts determined by the provided methods (plus final flow calculation) are to be used for system classification (ISTS, MSTS or if a State permit is needed) and therefore the type of permit needed (local or State). This flow value may be modified when designing specific system components (e.g., collection system design, groundwater mounding, phosphorus mitigation, surge capacity timed dosing, etc.).


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