Installing Multiple Flow Equalization Tanks

In larger systems multiple tanks may be needed to meet the requirements for the flow equalization

Installing Multiple Flow Equalization Tanks

Installation of two flow equalization tanks. This photo shows the connection at the bottom. (Photos courtesy of SI Precast)

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A flow equalization tank, also known as a surge tank, is a specialized dosing tank that provides storage of effluent and uses timed dosing to allow for uniform delivery to a subsequent component over a period of several days. In flow-equalization configurations, the dosing tank capacity is determined by the minimum volume required to keep the pump submerged, a surge volume equal to the flow generated during the designated storage period, and the reserve volume above the alarm activation level.

In this case, the tank is typically designed to hold at least twice the normal daily flow of the facility and dose it over the course of more than one day, as specified in the design. In duplex pump configurations, the reserve volume may not be included. The flow from a surge or flow equalization tank is again controlled by a timer that controls pump operation according to fixed on (dose) and off (rest) cycles. In this case, effluent delivery may be spread out over several days.

In larger systems multiple tanks may be needed to meet the requirements for the flow equalization. Typically, these tanks are connected in series to create the total volume required. The pump or pumps and controls that are dosing the downstream component is only located in one of the tanks, so it is necessary to connect the tanks in series near the bottom of the tank. 

If there are concerns about inappropriate items in the tank or sludge production, having that connection around 1 foot off the bottom or right above the pumps may be wise. Using a larger diameter pipe (8-10 inches) is another suggestion to limit blockages and assure quick transfer between the two tanks.

This photo shows the connection between the two tanks at the top.
This photo shows the connection between the two tanks at the top.

Keep in mind these tanks should vent — so having an air transfer connection between tank to tank or some form of venting should be included.

Sometimes the flow equalization tanks are still connected at the top in case there is a problem in the system, which would allow all the tanks to fill to capacity; but without a gravity overflow it will not make a significant difference in the time available before the problem will need to be resolved. 

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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