Subsurface Sewage Treatment Systems

II. B. Waste strength assessment

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II. B. Waste strength assessment

II. B. 1. Introduction

Hydraulic performance, treatment performance, and longevity of an SSTS can drastically be affected by the concentration of organic substances and solids in the waste. The concentration of organic substances and solids in the sewage is referred to as the “waste strength”. The compounds causing high strength waste are generally derived from food wastes and are not to be confused with other organic compounds such as petroleum products, solvents, volatile cleaners, etc., which were dealt with previously in Section A.

The concern with waste strength is not with environmental protection, but with soil system longevity. If excessive organic material or solids reach the soil system, the biological clogging mat can become restrictive to the point of hydraulic failure. In addition, excessive oil and grease can clog pipes.

Soil system longevity is of great importance, especially if the soil system prematurely clogs and there is limited or no area for a replacement system. Another concern is an economic one, as the life of the soil system may be limited to the point that the gallons treated per system costs are very high. It has been reported to the agency that designs that did not account for high strength waste failed within a matter of months.

  1. Definitions

The terms used in describing waste strength are as follows:

    • Biochemical oxygen demand - "Biochemical oxygen demand" (BOD) means the measure of the amount of oxygen required by bacteria while stabilizing, digesting, or treating biodegradable organic matter under aerobic conditions over a five-day incubation period.

    • Oil and grease - “Oil and grease” (O&G) means a component of sewage typically originating from foodstuffs such as animal fats or vegetable oils or consisting of compounds of alcohol or glycerol with fatty acids such as soaps and lotions (also known as FOG or fats, oil, and grease).

    • Settleable Solids - “Settleable solids” means the solids which are of sufficient size and weight to settle in a given period of time.

    • Total suspended solids or TSS - "Total suspended solids" (TSS) means solids that are in suspension in water and that are removable by laboratory filtering.

  1. Domestic strength waste

The following table lists the maximum waste strength concentrations from a typical dwelling. Waste strength that is less than these concentrations is considered “domestic strength waste”.

Influent (Raw Sewage)

Final Sewage Tank Effluent

Biochemical Oxygen Demand (BOD5)

<300 mg/l

< 170 mg/l (125 mg/l CBOD5)

Total Suspended Soils (TSS)

<200 mg/l

<60 mg/l

Fats, Oils and Grease (FOG)

<50 mg/l

<25 mg/l

These concentrations can be achieved if the raw waste is domestic in nature, the flow amounts are typical, septic tanks are sized in accordance with applicable rules, and the tanks are maintained (pumped, baffled and watertight). Domestic strength waste is a combination of high strength waste from plumbing fixtures such as toilets, which is diluted with low strength waste from plumbing fixtures such as showers or laundry.

For typical residences with systems meeting Type I septic tank criteria, no assessment of high strength waste is necessary. However, it may be prudent to determine if atypical household practices and uses may be present or anticipated. Atypical use could be a home business, extensive hobbies, ethnic practices, or use of water conservation devices.

  1. High strength waste

High strength waste is defined as concentrations that exceed those set for domestic waste (see table above).

Systemic high strength waste can be cause by the following:

  • High inputs of BOD, TSS and Oil & Grease

  • Lack of dilution from low waste strength inputs (shower, laundry, etc.)

  • Chemical upset of the septic tank

  • Operational/maintenance issues (lack of tank pumping or missing baffles)

It should be understood that waste concentration thresholds were based on Type 1 flow estimates, and Type I sizing of septic tanks and soil dispersal systems. In other words, the size and treatment abilities of a Type 1 system drive the acceptable concentrations in the waste.

To clarify the previous paragraph, the actual concentration is not the issue; it is the concentration along with the volume of flow. The concentration multiplied by the flow equals the amount (or mass) of the contaminant. If water conservation is used, the mass remains the same. For example, if the flow is reduced by 15 percent, then the concentration will increase by 15 percent but the mass would remain the same. However, it is assumed in this guidance that the Type 1 flow values will be generated, so an increase in concentration will result in an increase in mass.

Therefore, if the concentrations are in excess of Type 1 values, chapter 7080 stipulates that the system must be designed with additional treatment (7080.1550 subp. 2 item A).

  1. At-risk waste

At-risk waste is a waste that is difficult to ascertain if the waste strength will cause operational, hydraulic, or longevity problems.

II. B. 2. Waste strength assessment methods

  1. Existing establishments

The designer needs to conduct a general assessment if the waste strength is clearly domestic, clearly high strength, or is uncertain (at-risk waste). The estimation methods in Section b. below can be used to help make this determination. If the waste is at-risk or clearly high strength, then measuring the waste strength is highly recommended. Please see Sections 1) and 2) below.

  1. Measuring waste strength

Due to variation in flow and waste strength, knowing when to sample to collect representative samples is very important. Incorrect sampling can result in over or under design of the system. Over-design may result in unnecessary costs and under-design can result in shorter system life.

Good sampling methods should be:





Things to consider when wastewater strength measurements are to be taken:

  • The suitability of the collection point (static water sample in a tank versus flowing water from a monitoring port)

  • Number and timing of samples to get representative concentration (a single grab sample is likely not to be representative)

  • Sampling method (sample the cleanest water first)

  • Sample preservation and shipping

  • The type of analysis to be conducted (BOD5, CBOD5, COD, O & G, O&G, etc.). It is recommended to analyze for BOD5, TSS and O&G.

  • The expected concentration so the lab can use the appropriate dilution

  • Interpretation of sample results

  1. Other things to consider along with the measured value

  • If working with a manufacturer of a registered product that treats high strength waste, the measured value can be used in conjunction with the knowledge and experience of the manufacturer.

  • Determine the life of the current soil system as compared with the level of treatment before the soil.

  • Ask the system’s maintainer of issues or estimation of the concentration of the waste.

  • Take a survey of number and use the plumbing fixtures (those that generate high strength waste and those that dilute the waste) – do a mass balance estimate.

  1. New establishments

  1. The first step is to make the determination if the proposed system will initially be designed for domestic, at-risk, or for high strength waste. The methods provided in this section can be used for guidance to make this determination.

  • If it is decided to initially design for high strength waste (such as for a restaurant), then a concentration or mass must be determined for design purposes. This may best be done by contacting a manufacturer of a registered high strength waste treatment device.

  • If initially designed for at-risk waste, the newly operating system (after a period of start-up), should be sampled to see the adequacy of the at-risk design standards.

The recommended sampling procedure for a newly operating at-risk system is as follows. This can be included in the Operating Permit.

  • After three months, but before six months of facility operation, the owner(s) will test the effluent prior to soil system discharge on three separate occasions during peak use.

  • The parameters tested will be BOD, TSS, and O&G, if oil and grease are identified as a possible problem.

  • If the concentrations of these parameters exceed those of domestic wastes, the at-risk design and system operation must be assessed for adequately. The system must be retrofitted within the current or next construction season if system performance is found to be inadequate.  

  • The owner(s) will take full responsibility for the system’s performance and operation. If failure occurs, the owner(s) will discontinue use of the system and it will be repaired or replaced.

  1. Estimation methods if waste may be at-risk or high strength.

The following methods are provided to assist in the determination if the waste is high strength or at-risk.
Method #1 - Facility chart

The following chart is offered as guidance of which wastes are likely candidates as at-risk or high strength wastes:


At-Risk Waste

High Strength Waste

Dwelling Units

Dwelling, groups of Dwellings, Apartment Buildings > 2500


Hotel (no restaurant or bar)


Motel (no restaurant or bar)


Rooming House


Day Care (in-home, no meals)


Day Care (in-home, with meals)


Day Care (commercial)


Group homes




Labor Camp


Labor (semi- permanent)



Retail Store


Shopping Center




Medical Office


Industrial Building




Barber Shop


Beauty Shop (sewage only)


Flea Market


Eating and Drinking Establishments

Eating and Drinking Establishment with commercial kitchen(all types)


Entertainment Establishments

Drive-in Theater




Bowling Alley


Country Club


Fairground and similar gatherings




Dance Hall


Health Club


Outdoor Recreation Facilities



Day Camp (with meals)


Day Camp (w/o meals)


Camp - day and night (w/meals)




Resort Cabin




Swimming Pool


Visitor Center



At-Risk Waste

High Strength Waste


Gas Station/Convenience Store (sewage only)


Service Station (sewage only)


Car Wash – (sewage only)


Airport, bus and train station




Call the MPCA

Mental health hospital

Call the MPCA

Prison or Jail


Nursing home or similar facility

Call the MPCA

Other Public Institution


School – no cafeteria, no showers


School –cafeteria, no showers


School – no cafeteria, showers


School –cafeteria and showers


School - Boarding




Assembly Hall



Public Lavatory


Public Shower


Method #2 - Measuring similar establishments

One problem with estimation from data received from similar establishments is that seemingly similar establishments are affected by subtle and often intangible influences that can cause significant variation in wastewater characteristics. For example, popularity, price, cuisine, management, and location can produce substantial variations in wastewater flow and waste strength among seemingly similar restaurants.

Method #3 - Published waste strength data

Waste strength data is limited for most types of establishments. Therefore it is difficult to apply this data with any degree of confidence. Limited estimated waste strength values are available from:

  • The University of Minnesota Onsite Sewage Treatment Program Basic Design Form – Waste Strength tab (, or in Appendix B-2)

  • Research literature

  • Engineering textbooks or manufacturers of treatment systems engineered specifically for high strength wastes

If a specific establishment is not listed in a publication, average concentrations of pollutants from specific plumbing fixtures are typically provided in the University of Minnesota Onsite Design Manual – Section 5 (, or in Appendix B. which may provide assistance to the designer in determining potential influent characteristics.

A recommended estimated waste strength value for design of a new restaurant would be a BOD5 concentration of 1500 mg/l (Leskiar et. al, 2005).

Method #4 – Survey and query

The designer can ask questions about the waste type from the owner of the establishment. Helpful areas to explore would be:

  • If establishment will have high toilet paper use

  • If the establishment will require frequent sanitation

  • If the establishment will have significant surge/peak flows

  • If the establishment will have flow is from equipment or building  wash-down

  • Any establishment that gives a designer “pause”

  • Look at the plan sheet of the proposed establishment for the number and use of the plumbing fixtures (those that generate high strength waste and those that dilute the waste) – do a mass balance estimate

II. B. 3. At-risk design recommendations

The design recommendations for at-risk waste or for high strength waste brought down to Treatment Level C concentrations are as follows:

Based on the assessment methods provided and professional judgment, increase the Type I system components with the intention that the soil dispersal system does not exceed Type l waste strength loading rates (see Section IV. F. 6.). Note: Septic tanks do not remove soluble BOD

Provide the required cleanouts

Install sampling ports

Consider pressure distribution to minimize biomat formation

If pressure distribution is used, use larger perforations and greater distal head

Employ an effluent filter on the outlet of the last septic tank

Include in Management Plan that solids levels in the tank be measured more frequently

Soil dispersal system inspection pipes can be finished above the ground surface and include in Management Plan that ponding in the soil dispersal system must be measured yearly

Provide space for a future advanced treatment unit and additional soil dispersal capacity

Recommend that the system be operated under an operating permit (already required for Type IV Treatment Level C systems)

Annual visit with the system owner about waste strength concerns

II. B. 4. High strength waste design standards

    1. Existing establishment

  • An existing establishment will be designed on the measured waste strength in combination with any other relevant information gathered about the establishment.

    1. New establishment

  • If a new establishment will initially be designed for high strength waste an estimated concentration needs to be determined. Since an advanced treatment device will likely be necessary, it is best to contact the manufacture for a design recommendation for their product.

Factors of safety should be incorporated in the design concentration.

II. B. 5. Who can do the work

  1. Basic or intermediate designers

Basic or intermediate designers can:

  • Sample existing establishments for waste strength

  • Determine if the initial design should be an at-risk or high strength waste design

  • Assess if an at-risk design is adequately performing

  • Design an at-risk system

  1. Advanced designers

Advanced designers can:

  • Do all the functions of a basic/intermediate designer

  • Design a system to treat and disperse high strength waste

II. B. 6. Additional assistance

Additional helpful information is found in the publication: Analyzing Wastewater Treatment Systems for High Strength and Hydraulic Loading developed by the Consortium of Institutes for Decentralized Wastewater Treatment. Please refer to the following websites for downloadable analysis forms and additional information:



Additional helpful information can be found in paragraphs 11.251 to 11.253 in the document: Recommended Standards for Wastewater Facilities issued by the Great Lakes Upper Mississippi Board of State and Provincial Public Health and Environmental Managers (aka – “10 State Standards”) at the following website:

II. B. 7. Literature reviewed

Metcalf and Eddy, Wastewater Engineering Treatment Disposal and Reuse – 1972.

Minnesota Pollution Control Agency, Minnesota Rules Chapter 7081 – Mid-Sized Subsurface Sewage Treatment Systems Office of the Revisor of Statutes – 2011

R. L. Siegrist, D. L. Anderson, and J.C. Converse – 1984.Commercial Wastewater On-site Treatment and Disposal pp. 210 to 219.In Proceedings of the Fourth National Symposium on Individual and Small Community Sewage Systems, American Society of Agricultural Engineers, St. Joseph Missouri.

United States Environmental Protection Agency, Onsite Wastewater Treatment Systems Manual, February 2002 pages 3-11.

Recommended Standards for Wastewater Facilities issued by the Great Lakes Upper Mississippi Board of State and Provincial Public Health and Environmental Managers (10 states), 2004 Edition.

State of Maine – Septic System Code 2008

University of Minnesota – Onsite Sewage Treatment Manual – Appendix A-5, 2009

B.J. Lesikar, O.A.Garza, R.A.Persyn, A.L.Kenimer, and M.T.Anderson, Food-Service Establishment Wastewater Characterization, 2005

Consortium of Institutes for Decentralized Wastewater Treatment. Analyzing Wastewater Treatment Systems Serving Residential and Commercial Facilities for High Strength and Hydraulic Loading, 2009.

Policy for Processing High-Strength Wastewater POWTS Plans, April 2009. Wisconsin Department of Safety and Professional Services.

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