Measuring Flow for Onsite System Design and Management

Determining the actual flow for a system, rather than an estimated amount, is one of the most important aspects of troubleshooting a problem

Measuring Flow for Onsite System Design and Management

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Flow measurement describes any method used to accurately quantify the flow of liquid. There are several ways to accurately measure water or sewage flow including water meters, event counters, running time clocks or electronically controlled dosing.

It’s very easy to add flow measurement such as a running time clock in combination with an event counter in septic systems with a pump. Determining what the flow for a system has been is one of the most important aspects of troubleshooting a problem.

Water meters

Septic systems are becoming more expensive both to install and to repair, so one goal is to design them to treat the actual amount of flow rather than an estimated amount, which may be high or low. Another goal is to get optimum use over the longest possible time from existing systems.

In order to achieve these goals, it is helpful to know actual flow rates, which a water meter can provide. While it is often necessary to use estimated values from a table/code to estimate sewage flows, more accurate data should be obtained if possible. For example, if a chain restaurant is to be located beyond the reach of municipal sewer, then data should be obtained from the parent company on water use rates of comparable facilities. Accurate measurement of water use can help ensure successful septic system design and operation.

To track the amount of water entering the septic system, include a water meter in the design of the system or add one to an existing system. Water meters come in many different shapes and sizes. Most water meters are designed to deal with clean water, which means that they may not function properly if they are used to measure the flow of sewage. For example, many water meters have small paddles or wheels that move to measure flow. These moving parts can be easily plugged by solids in sewage. One way to avoid this problem is to measure the flow of clean water before it is used.

These meters should measure the water used inside the building, but not the water used outside for watering lawns and gardens, filling swimming pools, or washing cars since this water does not enter the septic system. A filter to catch small particles should be installed to protect the water meter. Placing the meter after the water softener is common. If it’s difficult to install a water meter so that it does not include the water to be used outdoors, try to estimate outside use, or use only data from December to March, when there is typically no outdoor use of water. 


Water meters measure flow in either gallons or cubic feet. Before doing any calculations using data from the meter, check to be sure of the units of measurement. Designs for septic systems typically use gallons per day. If you take two meter readings seven days apart and determine the number of gallons used during that period, you would divide the gallons used by seven to get the number of gallons per day. If the meter records in cubic feet, you would multiply the cubic feet used by 7.48 to convert to gallons.

Although a meter is installed directly into the waterline, it will not affect water pressure. Meters are better installed on the incoming line from the well because some meters are not as accurate when the flow is small (for instance, trickle flow from a leaky fixture).

Another type of clean water meter often found in houses is an on-demand water softener. These water softeners measure flow and recycle at certain set flow amounts. The water softener may also have a metering device that can be periodically read to record clean water use. 

Event counter (cycle counter)

Another way to track flow is to use a pump with an event counter. An event or cycle counter is a device used to record the number of times a component has been activated (e.g., activation of a pump followed by deactivation is one cycle). An event counter is a meter that records every instance of the pump turning on. By knowing the number of times the pump turns on during a day, you may have a record of the flow of wastewater going out to the system, provided that you know (from the system design) how many gallons are to be pumped each time the pump turns on.

This method is not as accurate as a running time clock because the floats that turn the pump on have some variability. That is, the pump may turn on at 6 inches the first time and then 6 l/2 inches the second time. Also, with displacement dosing, the pump may run for longer periods of time during periods of larger water use (for instance, on laundry day), resulting in inaccuracies.  That can be a 15 to 20 gallon discrepancy each dose. If the event counter is triggered five times a day, at a 20-gallon per time discrepancy, your calculations could be off by as much as 100 gallons of water that day. A leaky toilet could actually cause a pump to run longer pump cycles for very long periods until the problem is corrected. If the pump tank is set up for drainback of the pump line into the dose tank, this value must be subtracted from the daily totals.

Top: running time clock; Bottom: event counter
Top: running time clock; Bottom: event counter

Elapsed time meter

When reading an elapsed time meter, or ETM, be sure to note the units of measurement on the dial or screen (for instance: seconds, minutes or hours). All pumps run at a certain designed pumping rate, so effluent flow can be calculated from the measured pump runtime. This calibration can be done in the following steps:

1. The interior dimensions of the pump tank are determined and used to calculate gallons per inch contained in the tank.

2. The water level in the tank is measured and recorded. When doing this calibration, the water level in the tank should be near normal operation levels.

3. The pump is run for a known amount of time (recommended to be two minutes or even longer).

4. The water level in the tank is measured again, and the volume pumped is calculated.

5.  The volume pumped is divided by the amount of time the pump was running in decimal minutes.

6.  A pumping rate in gallons per minute is the result.

Using this rate, the amount of water pumped over a period of time can be calculated based on how long the pump has been running.

  • For example, assume you measured and calculated that a tank contains 10 gallons per inch of water depth, and the depth of wastewater is 3 feet deep. A pump is run for two minutes, and now the wastewater is measured as 2 feet deep.
  • 12 inches x 10 gallons = 120 gallons have been pumped in two minutes; so the rate is 120 gallons ÷ 2 minutes = 60 gallons per minute.
  • If the same pump ran for a total of 10 minutes over a two-day period, then during that time it pumped 10 x 60 = 600 gallons. Divided by two days equals 300 gallons per day as the measured water use.

By measuring and monitoring system flows with these tools, a person has valuable information with which to troubleshoot problems that occur. This is a quick way to use a pump and a clock to calculate how much water is being used. Once you know the pump’s rate, check it regularly (annually at a minimum). The rate may slow to the point where it is not evenly distributing wastewater to the soil treatment system, or it may be failing. It is good to know before the pump stops working that there is a problem.

However, if it is measured, rate of flow is critical data that will allow the best design and operation of the septic system. Flow estimation is a great design tool. It allows for a safety factor and peace of mind. Measured flow is used both to design systems and to verify performance. By using both flow figures appropriately, you give the system the best chance of good long-term performance.

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


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