Nocardia control in activated sludge by classifying selectors

Laboratory‐ and full‐scale experiments investigated the use of a classifying selector for Nocardia control in activated sludge. A classifying selector is a device that produces surface foam from the bulk mixed liquor and then removes it. The laboratory‐scale classifying selector activated sludge system operating at a 10‐day mean cell residence time (MCRT), pH = 6.6, dissolved oxygen (DO) = 8.0 mg/L, and temperature = 24°C produced mixed liquor Nocardia levels lower than an unfoamed control activated sludge plant. Addition of low doses (0.5 mg/L) of a readily biodegradable nonionic surfactant (NEODOL 25–7) to the classifying selector system just before foaming reduced the mixed liquor Nocardia levels to below those achieved in a classifying selector system without surfactant addition. A classifying selector installed in the RAS channel combined with selective foam wasting from the mixed liquor channels and secondary clarifiers in a full‐scale activated sludge plant reduced Nocardia levels below those achieved during operation without a classifying selector.