Try to avoid recirculation of ...

Try to avoid recirculation of the scum. Try to discharge the scum separately. If scum is due to Microthrix Parvicella polyaluminum chloride dosage (few mg/l) may help. Alternatively sprinkle a diluted solution of sodium hypochlorite on the surface of the settling tank. In this case pay attention to clogging of sprinkling nozzles due to carbonate precipitation. To avoid it, provide dropping NaHClO on a jet of tap water at the sprinkling point. This technique proved effective at a large plant in Milan.