How much water should a pump be set to deliver per dose? The answer depends on the application, as discussed below. In cold climates, drainback to the pump tank must also be added.

1. Pumping to a gravity distribution soil treatment area

If you are pumping to a distribution box or dropbox, you do not want too large of a dose coming in too fast as it may overwhelm the box and cause inappropriate loading. In residential applications, pumping less than 75 gallons is recommended using a pump under 40 gpm.

2. Pumping to a pressured soil treatment area

In this case, both a maximum and minimum dose are important. The maximum dose should not exceed 25 percent of the design flow. The minimum dose is based on the volume of wastewater needed to fully charge the laterals and deliver a dose under pressure. After the pipes are full, even pressure distribution is achieved. The recommended volume to deliver is four times the volume of the pipes. Therefore, smaller pipe diameters will allow for smaller doses and may be needed with larger piping systems to meet the 25 percent of the design flow maximum. The designer or installer should then choose a dose volume between four times the volume of the distribution piping (at a minimum) and 25 percent of the design flow (at a maximum). The minimum gallons per minute of the pump is set based on the number of orifices, size of orifices and the required pressure head.

3. Pumping to a pretreatment unit

The volume to pump to a pretreatment system such as an ATU or media filter must be confirmed with the product manufacturer. It should be set to maximize the treatment in the unit. 

Once the volume is set, the designer or installer must determine how to ensure that amount will be delivered. When demand dosing is used, the floats alone control the pump. The “ON” float separation distance is calculated by taking the dose volume divided by the gallons per inch of the tank. The alarm depth then needs to be determined. Typically, the alarm is triggered when the effluent gets 2-3 inches above the pump start level. The “OFF” float distance is calculated by taking the pump height + block height + 2 inches (to keep the pump covered).

With time dosing, a timer is being used to control the pump and the actual gallons per minute of the pump being used must be determined. During installation or for an existing system, the gallons per minute must be calculated by performing a drawdown test where the change in depth in a tank in inches is recorded over a period of time and multiplied by the gallons per inch of the tank. This is the only way to have an accurate dose.

Timer on: To calculate the time the pump will run, the dose volume is divided by the gallons per minute of the pump. This will provide the number of minutes the pump will run each time it turns on. The pump will not activate if sufficient sewage is not present to be dosed.

Timer off: To determine the time between doses, take the number of minutes in a day (1,440) divided by the likely or typical doses per day and subtract the “timer on” time. This will establish the amount of time between doses in minutes. This time may need to be adjusted over time if current usage per day is unknown. 

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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 and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.