The effluent of a potato-starch factory is separately treated in a two-stage activated sludge plant. The occurrence of bulking sludge in the second biological stage due to excessive growth of type 021N was repeatedly observed. The installation of an aerobic selector, consisting of three cascades, prior to the aeration tank of the second stage, where the return sludge and all the influents flow together, has been effective in improving the SVI to values below 150 ml/g. The design procedure was carried out in accordance with Prendl's work, with a simple calculation method based on findings derived from aerobic treatment of sugar mill wastewater.

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