At Design Flow

During the various training classes held by the Operator Certification Program, I’m sometimes told by an operator about low-flow issues that occur. One such issue has a 40,000-gallon per day (GPD) plant at his mobile home park, but he only gets 12,000 GPD. Shouldn’t the plant run just fine if he leaves it to itself? The answer is no. Low flow to a package wastewater treatment plant or any plant is something that is often discussed but no definitive answers are given due to the complexity of the issues and the differences that exist from system to system. Each facility will have to do its own independent evaluation of how to make adjustments to reach its own best operating procedures.

Treatment plants are designed to run at full capacity, not at a third of their design flow. For example, at a package plant with a design flow of 25,000 gallons per day (GPD), the return sludge pump is a four-inch airlift pump that, set at its lowest, pumps 25 gallons per minute (GPM) of return activated sludge (RAS), which is 144 percent of the influent design flow (RAS flow should be 50 – 150 percent of the influent flow.). This facility should operate correctly if the influent flow is free of toxins, has the needed amount of biochemical oxygen demand (BOD) (about 200 mg/l) and suspended solids (about 200 mg/l) with the proper pH (6.8-7.2) with the right amount of dissolved oxygen (DO)(2-4 mg/l) supplied by aeration.

If the influent flow to this 25,000 GPD plant drops to 8,000 GPD, the RAS flow is still 36,000 GPD ( that four-inch pump at 25 GPM), which is now 450 percent of the influent (You can’t slow the RAS flow any more or the line will have the potential to clog.). The pounds of BOD have been reduced from 41.7 per day to 13.3 per day. The BOD is the food for the microorganisms that degrade the waste, and without adequate food, there will not be an adequate numbers of microorganisms, and the waste will not be treated sufficiently to meet the permit requirements.

In this blog, I mentioned one aspect of the changes that occur with a low flow to a treatment plant. Operators have to know what changes need to be made to optimize treatment, and that knowledge comes from experience at a facility and doing the required process control testing.

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