It was found that heavy scum production occurred when cell numbers of actinomycetes were higher than 106 ml−1 in mixed liquor. The reduction of sludge retention time (SRT) was one methodology for controlling actinomycete scum production. This method was successfully applied in the Yokosuka treatment plant when sludge age was maintained at 1.5-2.0 days. The number of actinomycetes cells in mixed liquor decreased to the order of 104 or 105 ml−1. However, in order to reduce SRT, mixed liquor suspended solids (MLSS) had to be low, resulting in a high BOD load. Therefore, it was very difficult to control the scum formation by SRT reduction, while still meeting the water quality standard. The number of actinomycetes in the influent in the Okinawa treatment plant was very high, about 106 ml−1. Since the reduction of SRT was not effective in the Okinawa treatment plant, the anaerobic-oxic process was introduced. Using this process with low SRT, the abnormal foaming and scum production were reduced to 1/2 to 1/3 from the conventional activated sludge process. The number of actinomycete cells in anaerobic-oxic process decreased compared to that of the conventional process.

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