Commercial System Design: Wastewater Characterization

Each commercial onsite system has its own unique set of requirements that need to be taken into account when considering design and maintenance

Commercial System Design: Wastewater Characterization
The first tank in a commercial septic system. A one-size-fits-all approach does not work with commercial design, as the wastewater treatment system starts in the facility — and each one has different demands.

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A one-size-fits-all approach does not work with commercial design, as the wastewater treatment system starts in the facility. These facilities include restaurants, golf clubs, schools, resorts, churches, office buildings, supermarkets, mini-marts, day care centers, campgrounds, hotels — the list could go on and on.

When designing any septic system, a site visit is essential, but with commercial systems, doing a survey of use within the facility is critical in order to identify what kind of wastewater will be generated. This survey also provides an opportunity to educate the owner about how various practices impact the wastewater and system. The checklist developed by CIDWT is a great resource  for many of the common commercial facilities.

For facilities preparing food, pay particular attention to how they are using sanitizers and the amount being used, along with degreasers. During the survey you should determine if food is being prepared, what kind, how they clean the facility, graywater versus blackwater usage. (Is there laundry, showering, etc., to balance out toilet/kitchen usage?) Characteristics vary even based on cuisine type served at restaurants and the use of soda fountains, deep fryers, salad bars and ice cream machines. Discussion with the owner should include utilizing solid waste and reducing cleaners.

The hydraulic load of a commercial system is a key design parameter. For commercial facilities the amount of water used typically relates to the meals served, cash flow or number of users. An evaluation of inputs from water treatment devices and clean water should be performed. With existing facilities, it may be wise to update older high water-using fixtures such as toilets and washing machines, and identify and fix interior leaks. For any existing commercial property flows should be measured during peak flow time periods, as this will eliminate the need to use estimates.

Care should be taken and a safety factor added when there is the possibility of flow increasing over time or a change in use, which is common in strip-mall applications. Many state rules have requirements for flow estimation. The 2002 EPA design manual also has a chart for various commercial establishments, but take care when using estimates, as they typically overestimate the average flow, but occasionally underestimate the peak flows. Manufactures are another good resource for flow estimates. They have often collected data from similar facilities that may be of use.

The organic load relates to the amount of BOD, TSS and FOG in the wastewater. With an existing system the wastewater should be sampled, pulling the best representational sample available in the existing system (often from outlet of final septic tank or pump/dosing tank). Multiple samples should be taken over a period of time, as grab samples and flow fluctuations tend to provide results with a wide variation. CIDWT has assembled a table of waste strengths from various references for commercial properties shown in the table for BOD. Designers should consider isolating streams that are very concentrated or toxic to microorganisms. 

The results of the survey along with the hydraulic and organic loading rates will then serve as the basis for the design.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to

This article is part of a series on the design and maintenance of commercial septic systems:


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