Go With the Flow to Scope Out Potential Treatment Issues

Many of today’s onsite systems are complex. Charting water moving through the system is critical to ensuring they continue to work properly over time.

We have spent a lot of time lately addressing questions about estimating or determining design flows. But recently these discussions and questions have been about the importance of measuring flow after a system is installed.

To consider decentralized wastewater treatment systems a viable alternative to the “big pipe” solution for small communities, we have also spent quite a bit of time and space on installing systems, to make management of the systems easier. This includes specific discussions on installing risers to the surface, making pumps in pump tanks more accessible, and installing inspection ports or access in all parts of the system. This allows the service provider the opportunity to access and observe all parts of the system to identify any potential problems early and solve them before they become real problems.

We thought it was time to explain in greater detail methods used to measure flows in the system. One obvious device to measure water flow or use is to install a water meter. A colleague of ours has long advocated installation of water meters whenever there is an onsite sewage treatment system.

We would agree this is a good idea, but the industry has been slow to embrace the concept. The good news is that many proprietary systems have incorporated measuring and tracking flows to determine maintenance schedules and needs. In addition, we see more codes requiring some type of water measurement in conjunction with systems that require pumps or are considered larger systems or systems serving other establishments.

There are several ways to measure flows in systems, including water meters, event counters, running time clocks and electronically controlled dosing. Today there are also a lot of options where the results can be accessed remotely, which means it does not take a system visit to look at the flow values and assess for potential problems. Water use data can be entered into a spreadsheet to provide a continuous picture of system operation. The service provider can easily determine whether a site visit is necessary to correct a problem.

We will discuss methods for determining flows.

Water meters come in a variety of forms. Installation typically costs $200 to $400, so it’s not an expensive item. Water meters are meant to measure clean water flows, so they need to be installed where water enters the house. Installation should include the cost of additional plumbing to place the water meter in a location where it does not measure water delivered to outside sillcocks so only in-house water use is measured. This is the water that will be delivered to the wastewater treatment system. When accessing data from a water meter, check if the flow is measured in gallons or cubic feet and make the proper conversions for the period of time flow is measured.

If pumps are used in the system, an event or cycle counter can be used to determine flow to the next system component. A cycle counter records when the pump starts up and runs. Count the number of times the pump turns on during the day and add how many gallons the pump moves each time it is activated to determine the wastewater flow. Cycle counters can be set up to operate through a panel to be read inside the house or from a remote location.

Counters are most appropriate for use in on-demand systems. Determining the volume delivered includes knowing how much water it takes to fill the pipe and how much will drain back to the system. The total amount delivered minus drainback will tell the volume of wastewater delivered. While this is a relatively cheap and easy method to monitor flow, it is not as accurate as either a water meter or an elapsed time meter. This is because floats are used to turn the pump on and off, and there is some variability in when the floats turn the pump on and off for each pump cycle.

Let’s say in a typical pump tank there is a half-inch variation in the level when the pump turns on and there are 17-gallons/inch volume in the pump tank. The calculations could be off by 8.5 gallons every time the pump runs. If it runs four time a day, that would be a discrepancy of 34 gallons per day. Over a week or month, the volumes begin to add up. Nevertheless, having a counter tells the service provider whether the pump is running and gives a relative value of the effluent delivered.

The other common measuring device is an elapsed-time meter. These days, this is the preferred method to measure flow into the system, and many proprietary products incorporate them into the system or require they be installed. Again, through use of a panel, data can be read remotely. Timers are used to ensure flow is more evenly spaced throughout the day. Times of peak flow are stored and delivered at regular intervals.

To use a timer, the pump must be calibrated to determine the flow rate and the amount of water delivered. An example uses the 17-gallon/inch tank. The pump is run for two minutes and the level in the tank drops 6 inches. This means 6 inches x 17 gallon/inch = 102 gallons delivered in two minutes or 51 gallons per minute, which is the pump rate. If the pump runs 10 minutes a day, it will pump 510 gallons.

Once the pump rate is determined, it is used to determine the volume of effluent delivered by accounting for any drainback from the piping. Service providers should check the pump periodically to make sure it continues to deliver at the same rate. Over time, pumps will begin to wear out, changing the rate.

Measuring flow by any of these means will help the service provider keep the system performing for a long period of time and provide a means of identifying problems before they cause system failure.



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