Estimated Return Rate

Many different methods are available for estimating the proper return sludge rate. A simple method described in the Operation of Wastewater Treatment Plants: A Field Study Training Program (Kerri et al., 2007) uses the 60-minute percent settled sludge volume (%SSV60), which can provide an approximation of the appropriate return activated sludge rate. The results of this calculation can then be adjusted based on sampling and visual observations to develop the optimum return sludge rate.

Note: The %SSV60 must be converted to a decimal percent and total flow rate, and wastewater flow and current return rate in million gallons per day (MGD) must be used.

^Influent Flow (MGD) + Current Return Flow (MGD)

x %SSV60

where we assume that %SSV60 is representative and the return rate, in percent, equals %SSV60. The actual return rate is normally set slightly higher to ensure that organisms are returned to the aeration tank as quickly as possible. The rate of return must be adequately controlled to prevent the following:

• Aeration and settling hydraulic overloads

• Low MLSS levels in the aerator

• Organic overloading of aeration

• Solids loss due to excessive sludge blanket depth

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