A simple, effective technique for controlling solids retention time in activated sludge plants

A technique is presented whereby the sludge wastage rate and recycle ratio for the activated sludge process may be adjusted to ensure efficient operation. The technique requires the use of three types of graphs. The operational diagram allows the operator to see immediately whether a proposed solids retention time will be feasible for a given loading on a plant and to choose a recycle ratio which will minimize pumping costs while maintaining a margin of safety against settler failure.