Flow Equalization Revisited

Sharpen your pencil and do a little math to overcome lingering concerns about optimal tank capacities for every residential or commercial system design.
Flow Equalization Revisited
A series of tanks are buried at a resort where flow equalization is critical to proper wastewater treatment. (Photo courtesy of Jim Anderson)

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In the past we’ve talked about the advantages of flow equalization strategies in residential or commercial systems to even out daily or weekly flows to a final soil dispersal unit or to pretreatment devices such as ATUs or media filters. This has led to some questions about tank capacities and necessary storage for flow equalization.

Consider three volume levels in flow equalization tanks: the minimum operating volume, the storage volume and the alarm volume. The alarm and storage volume will be determined based on the flow pattern from the residence or facility. The alarm volume is often designated by local regulatory requirements as a particular amount of reserve volume left in the tank after the alarm has been triggered. The minimum operating volume is dependent on tank characteristics, pump, pump discharge assembly and special site characteristics. The minimum volume level in the tank is heavily influenced by the pump intake. The pump intake must remain underwater to prevent air from being drawn into the pump and to provide for sufficient cooling of the pump’s housing.


For a residential system in Minnesota, for example, code requires a 1,000-gallon minimum tank or two times the estimated daily average flow rate. So we would estimate a three-bedroom house at 450 gallons per day x 2 = 900 gallons per day, or the 1,000-gallon minimum. It’s straightforward as long as the tank allows the pump to be covered with water that is the minimum volume and any requirements for storage after a high-water alarm is triggered. Bottom line, regardless of the situation, we recommend designing on the basis of the peak measured flow and then build in safety capacity.

Storage and flow equalization are especially critical, for example, during peak summer usage periods at family summer lakefront cabins in our backyard in Minnesota. These properties have small lots and limited drainfield capacity, and onsite systems face severe overuse when everyone comes to the cabin to escape the heat in the cities.

In the absence of actual flow data, a design for capacity would be to look at the estimated daily flow handled by the drainfield and then look at the maximum weekend or four-day holiday stretch during a family reunion. Sum those estimated rates over the time period and add the daily flow estimated drainfield capacity, and you get the storage volume needed to handle the peak flow.

An example we use in workshops says we have 200 gallons per day acceptance in the drainfield, and over the four-day Fourth of July holiday we have 1,350 gallons over and above the daily average, which means we need 1,350 gallons plus 800 gallons – 2,150 gallons of storage capacity – to handle the peak weekends. If we had designed based on minimum usage, the family would be paying a premium for a pumper to visit at least twice on that weekend to remove the excess. Probably not a realistic expectation!


Total capacity of a flow equalization tank used in a commercial treatment system can be calculated two different ways. The first method for calculating total capacity is to multiply the surge day-loading by 1 1/5. The other method is to add the surge day-loading volume to the average daily flow.

Again, depending on the type of establishment being considered, this can be straightforward or create some other interesting questions. For example, a fast-food restaurant or other franchise operation usually has solid information on peak days and has consistent flows, making establishing tank capacity a simple process.

The other end of the spectrum is the tourist-area Wisconsin or Minnesota supper club. Often these restaurants are closed one or two days a week and the peak flows occur over the weekend from Friday night (fish fry night) to Sunday afternoon (everyone goes home). Finding information can be difficult, but owners of these businesses can tell you how many meals they serve and when their income is the highest. Multiplying the meals per day times 5 gives an estimated flow per day, which can be averaged and indicate the peak flow periods and estimates to base the tank size either by multiplying by 1 1/2 or taking the peak and adding the surge or peak loading volume to the daily average.

A fast-food restaurant or other franchise operation usually has solid information on peak days and has consistent flows, making establishing tank capacity a simple process.


The result of doing the math is often the need for a large tank capacity. Since the total operating volume of a flow equalization tank is calculated relative to the inlet pipe, the operating volume can be increased by elevating the tank’s inlet. This allows for the entire tank volume to function as the operating volume. With this configuration, smaller tanks can be used because overall storage volume is increased and costs can be minimized. However, be aware code requirements in some states may prohibit this approach. If the inlet is raised, the installer must pay careful attention to the watertightness to avoid the risk of infiltration.

Another way to provide tank capacity is through linking multiple tanks. This allows for utilizing smaller tanks, which may be more readily available in some areas. The tanks are usually plumbed together at both the top and bottom. The top connection allows for airflow and the bottom connection is used to accommodate water flow.

When using tanks in series, it is extremely important that all of the tanks sit on a stable base. If the tanks are allowed to shift on their base, the connecting pipes will be strained and can fracture. Another installation consideration is to drop the elevation of the last tank in the series because its elevation sets the operating volume in all of the preceding tanks. By lowering the last tank, optimal operating volumes in all of the tanks can be preserved.


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