There are numerous tanks and varying configurations to choose from when designing commercial systems. Below are common tanks with design considerations related to managing commercial flows.
Holding tanks are useful in a few circumstances. The first may be for source segregation for toxic or hard-to-treat waste streams. Examples could include the drain for mop buckets, an RV dump station at a campground, or a sink where hair coloring is rinsed out at a beauty salon. These streams have the potential to kill off the beneficial bacteria and fungi in a system. Holding tanks can also be used in commercial design where there is not enough available soil treatment area or funding to deal with peak flows. In these situations, a timer can set up the system to only dose what the dispersal system can handle and during peak usage the holding tank can be used to handle the overflow.
Grease traps/interceptors are useful for wastewater from food preparation. These facilities should first have and maintain an internal grease interceptor. Secondly, the external grease traps should collect only the kitchen wastewater and discharge to the downstream septic tanks, whereas the toilet and other wastewater discharges directly into the septic tank. A clean-out outside the structure should be designed on the building sewer line for cleaning. It is recommended to keep the building sewer short to limit the wastewater cooling and grease solidifying in the pipe. If fat and grease are excessive, more and smaller tanks are better for cooling, as there is more surface area contact with the soil. A minimum of 24 hours of retention should be provided, with more being beneficial.
An effluent filter on the grease trap should be commercially sized and have an access riser to the surface for regular servicing. The presence of a grease trap does not guarantee success if deep fryers are emptied into the drains, or if excessive disinfectants are used in the facility. The grease from fryers should be handled through a grease-recycling program. Degreasers for cleaning fume hoods should not be disposed of in the grease trap or septic tank, as they can cause emulsification of the tanks, moving the grease downstream into the treatment unit.
Septic tanks for commercial systems are recommended to have at least three days of design hydraulic retention time with four-plus days for challenging streams. More retention time is likely needed for systems with grinder pumps or with large amounts of fat, oils and grease; dairy; alcohol; or blood. Tank sizing should also account for the frequency of solids removal and the size of available pump trucks. Effluent filters should be used on the final tank in series unless the system is going into a secondary treatment unit that does not want an effluent screen prior to treatment. Septic tanks set in series are preferable to double-compartment tanks if temperature regulation is an issue, again with secured risers to the surface in order to service the tanks and effluent filter.
Flow equalization/pump tanks utilize a larger dosing tank and include an adjustable timer that controls pump rest intervals and runtime for specific dosing regimens. In commercial systems, they should be considered when effluent is pumped from one system component to another. Flow equalization is a management concept that can help reduce stress on system performance due to high peak flows. In flow equalization, the peak flows are stored for a period of time to be delivered over a longer period. Usually the flow for one day is equalized over a 24-hour period, but it can be done for longer periods of time, especially if peak flows last for longer than one full day. For this to be accomplished, the tank must be large enough to handle these flows, and the pump operation must be controlled by a timer as opposed to a float.
When designing commercial systems you should also consider having multiple pumps to provide continued use when one pump fails. For commercial design, consider the use of hatches and pumps on rails as appropriate. A control panel with remote access is another design item to consider, to allow for remote monitoring and adjustment.
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 firstname.lastname@example.org.
This article is part of a series on the design and maintenance of commercial septic systems.