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Do we need Flow Equalization?

The answer by USEPA is "Flow equalization should be considered as an alternative to additional treatment capacity wherever influent peak-to-average (P/A) ratios exceeds levels of 1.3 to 1.5 : 1 (Evaluation of flow equalization in municipal wastewater treatment, EPA-600/2-79-096, May 1979) .

What is this P/A ratio in actual plants? The graph on the left shows the flow fluctuation from a mobile home park in 24 hours. There are three peaks which is occurring at, 8:30am, 1pm and 10:30 pm. The third peak is the largest one. In this case, P/A = 2.31. According to USEPA, this mobile home park requires flow equalization.

Sludge Discharges from Clarifier with Peak Flow

A major problem in this flow fluctuation is that sludge blanket is swollen at night with the largest flow peak and sludge sometimes overflows from the clarifier. Operators find some signs of sludge discharge in the morning. This kind of sludge discharge from peak flows happens, if the sewer line connected to the wastewater treatment plant has the I/I problem. When the operator comes back to the plant after heavy rain, he finds no sludge in the aeration tank.

The owner and the operator were suffering from this sludge discharge problems. One day a flow equalization tank was equipped as shown in the picture. The owner and the operator was happy. But the problem was not solved quite well, after the owner spent a big amount of money for this flow equalization tank. Why didn't this flow equalization tank work well?

This graph shows the flow fluctuation with and without the flow equalization tank. P/A was reduced from 2.3 to 1.9 but it was not good enough, according to USEPA's suggestion.

Not recommended Practice of FEQ

The problem comes from too small " effective depth". The effective depth of the flow equalization tank is usually 1 feet. In this case, when peak flow occurs, the submersible pump with a big capacity is operated for 15 minutes or so and it is stopped. The pump was operated 16 times a day in the particular mobile home park. This is, what is called, "Pump Tank". Many people are not spending a big amount of money for flow equalization tank but "Pump Tank".

Better way, but waste of money

When the upper float switch is moved upward, the effective depth of the flow equalization tank is increased. In this case, the fluctuated flow in sewage is almost equalized and the occasion of sludge discharge from the clarifier will be reduced. However, there is another thing to consider, that is, electricity cost. Typically the submersible pump for the flow equalization tank has a big capacity to avoid clogging, therefore, it is 1 HP to sometimes 4 HP or bigger. In this case, the pumping rate is 200,000 GPD to more than 500,000 GPD, but the required flow is 40,000 GPD. That means, most of the electricity is used for just recirculating water in the flow equalization tank and it costs $50 to $200 per month.