Classifying selectors are used to control the population of foam-causing organisms in activated-sludge plants to prevent the development of nuisance foams. The term, classifying selector, refers to the physical mechanism by which these organisms are selected against; foam-causing organisms are enriched into the solids in the foam and their rapid removal controls their population at low levels in the mixed liquor. Foam-causing organisms are wasted "fi rst" rather than accumulating on the surface of tanks and thereby being wasted "last", which is typical of the process. This concept originated in South Africa, where pilot studies showed that placement of a fl otation tank for foam removal prior to secondary clarifi ers would eliminate foam-causing organisms. It was later simplifi ed in the United States by using the aeration in aeration tanks or aerated channels coupled with simple baffl ing and adjustable weirs to make continuous separation of nuisance organismsfromthemixedliquor. Water Environ. Res., 75, 83(2003).

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