Tweak Septic System Designs For Optimal Flow Equalization

You’ve decided to incorporate flow equalization principles in an onsite system. The next step is knowing how to tweak the design and operation for optimal results.

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Last month we discussed the reasons for employing flow equalization as a way to manage systems. The purpose is to moderate peak flows and spread use of the system out more evenly over time. This can be done on a daily or weekly basis depending on how the system is used. There are two primary benefits: improving the treatment efficiency of any downstream units and helping prevent system overloads that can ultimately damage the final soil treatment component.

There are two design approaches, depending on the situation, for using flow equalization to improve system operation and efficiency.


The first is cases where you can look at the actual flow and then design each of the components – including the soil treatment area – according to flow. This is the situation that we would all prefer so the most efficient and cost-effective components can be chosen in terms of the operation and sizing. In this case, the larger peak flows can be accounted for in the design.

The second situation is where there is limited area for final treatment. This reduces the choices and means the system will require additional storage or buffer capacity to remove some of the effluent generated during peak flows. It is important to make sure flow numbers for design purposes should be the actual flows, showing the peaks and variations.

So how do you measure or capture the flow numbers? Using the estimated daily flows based on the number of bedrooms, water-using fixtures, number of people or general estimates for various establishments is the least desirable method. These figures are based on averages and while it may be the “best” guess, it is still that: a guess. Water meters can be installed and monitored, but that raises questions about whether all of the water actually gets delivered to the system.

The ideal solution is measuring where the pump delivers the effluent to the next downstream component. This gives the most accurate reading of actual flows and offers the most information for system management going forward. This requires installing a pump-cycle counter and an elapsed-time meter. The counter counts the times the pump turns on and off, and the time meter tells how long the pump runs during the time interval selected.

Using these methods requires the pump delivery be calculated, which is simply done by running the pump to the system, pumping out a known quantity of effluent over a set time period and then dividing the quantity pumped by the time to obtain the rate.


Flow equalization requires use of a timer. In a time-dosed system, the timer is set to run a pump for a certain amount of time on and off and logs the cycles. This gives a record of daily flows that can be tracked over time and adjusted to manage the flows. A programmable logic unit or computer is used to control and track the times. Computers give more management options. The system requires on and off floats or sensors, as well as floats or sensors for high- and low-water alarms and usually a redundant off feature.

Effective flow management requires a control panel to track the number of doses or times the pump runs. In addition, pump-off events should be tracked to inform the service provider if the predetermined flow was set higher than the actual amount of flow. Alarms should be tracked to indicate if initial flow estimates were too low.

Please note: If you are working with a demand system, you can add an hour meter to provide much more information about how the system works. This will give an accurate picture of flow, allowing you to make better-informed decisions about the cost-effectiveness of changing system components to achieve better equalization.

An hour meter can be added to systems where the typical piggyback method is used to plug into the floats and pump. Adding an hour meter is a simple, inexpensive way to determine flow and peak flow. If you want to know the peak flow, you need to read it every day. It’s not uncommon to see an average flow fall far below design maybe one day a month or one day a week. But our peak one-day flow might be twice the design, so you really need to look closely for that peak flow number.


Another point to ponder when considering flow equalization: Having a timed-dose system protects the system as the pump wears out and the gallons delivered per minute go down. The timer will operate the pump for the same amount of time but it won’t deliver the same number of gallons. Eventually this will trigger the high-water alarm and the service provider will know the pump needs attention to prevent a system overload.

Then there is a problem of human error. Timed-dose systems often incorporate a manual override feature. This is a bad idea, leading to homeowners consistently tripping the override rather than having the problem fixed, which then leads to problematic system overloads.


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