Would you drive a car without an odometer? How would you know when to change your oil?
This is how we have to operate most of our onsite wastewater treatment systems — with no access to the volume of wastewater being processed. Whenever it is possible to truly understand what is going on in the home, it’s important to use that information to determine the volume of wastewater that enters or exits the system.
Measured flow is used both to design systems and to verify performance. This data can be gathered during design, when conducting routine maintenance or when evaluating system performance. Anytime the data is available, the service provider should assess the flow the system is processing. Depending upon the tools available there may be daily or average daily flow which can be compared to the design flow.
Methods to obtain data
A water meter on the potable waterline to the house is one option to collect flow data. If it measures all the water entering the property it will include activities that do not generate wastewater such as filling swimming pools, watering lawns or washing cars. If it is 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 months with reduced outside use which is winter in colder climates.
Those on public water supplies will often have access to monthly average flows, but the service provider or property owner should determine if hourly or daily flow data is collected by the meter.
A water meter within the home only measures interior water usage is a preferred option to track wastewater flows. Placing the meter after the water softener or other water treatment is common as that will typically eliminate outside use of water.
Water meter readings can be compared on subsequent visits to determine the flow through the system. The average gallons per day through the system can be determined by dividing the difference between readings by the number of days since the last reading. New smart water meters can include data logging ability to capture hourly and daily water usage.
If the system includes a dosing tank and controls, use the information recorded from a cycle counter or elapsed time meter to calculate the flow. If the pump does not currently have a cycle counter or elapsed time meter one can be added. These are more common with systems employing advanced pretreatment along with commercial systems but should be considered for all systems with a pump.
Event counter (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 counting the number of times the pump turns on, you can measure the flow of wastewater going out to the system provided that you know how many gallons are pumped each time it turns on. To use a cycle counter, follow these steps:
- Calculate the dose volume
- Calculate the drainback (if applicable)
- Calculate the net dose volume (if applicable)
- Calculate the number of doses/cycles
- Calculate the total flow by taking the number of cycles times net dose volume
- Calculate the number of days between measurements
- Calculate the average flow by taking the total flow divided by the number of days
This method is not as accurate as an elapsed time meter 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 1/2 inches the second time. That can be a 15 to 20 gallon discrepancy for each dose. If the even t counter is turning 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. The number of cycles is critical for the drainback calculation.
An elapsed time meter is a device that tracks the minutes or hours a pump has run and since all pumps run at a certain rate or gallons per minute the effluent flow can be calculated and calibrated from the pump system. To use an elapsed time meter the pump must be calibrated to determine its delivery rate. This can be done in the following steps:
- Calculate the gallons per inch or obtain this information from the tank manufacture
- Measure the wastewater level in the tank in inches (from top of rise down is common)
- Manually turn the pump on for a known amount of time (such as two or three minutes)
- Remeasure the wastewater level
- Calculate the drop in liquid in inches
- Calculate the total gallons pumped by taking the drop in liquid and multiply by the gallons per inch
- Calculate the gallons per minute by taking the volume pumped and dividing it by the time the pump ran the gallons
The rate should be confirmed throughout the life of the pump as the delivery rate will decrease over time. Using this rate, the amount of water pumped can be calculated based on how long the pump has run between visits and divided by the number of days to determine the average daily flow. Drainback will need to be factored in if it is part of the system design.
- Telemetry allows for remote communication of system status via a phone line or increasingly over Wi-Fi. Data can be shared with the owner, service provider, responsible management entity or manufacturer. In these instances, the pump operational data can be analyzed to determine hourly, daily and annual usage. New options are hitting the market that will allow direct access of this data to the system owner.
- As a last resort, flow can be estimated based on 50 to 70 gallons generated per day per person living in the residence
The service provider can use this data in reference to the design hydraulic capacity of the various treatment components. Periodic data recording provides an estimate of average hydraulic loading (annually, daily and hourly) while continuous recording of data can facilitate the identification of peak flows. The longer apart these measurements the more averaged out the data will be.
Actual flow data is critical for evaluating performance and evaluating maintenance needs. This data is increasingly required by regulatory programs critical to verifying design assumptions.
