Frothing which is both a hazard and a nuisance, has frequently been associated with increased detergent use. Froth control may be accomplished by plant operational methods, sprays, defoamers, or a combination of these devices. Foam control of any kind costs money and determination of the best type of control depends on the individual plant. Evidence has been presented, in spite of some disagreement in the field, to indicate that detergents do interfere with sedimentation, coagulation, trickling filters, the activated sludge process, and sludge digestion. It does appear that different detergents have different effects on the treatment processes, and that the activated sludge process is most susceptible to their interference.