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).
Since 1981 the activated‐sludge plants at Halifax sewage‐treatment works have been operated in the ‘Halifax’mode, which has successfully prevented previous bulking of the activated sludge. The paper describes an investigation of the ‘Halifax’mode; also the ‘Halifax’mode plus a sludge bypass arrangement. Plant performance, sewage flows, and electricity and oxygen usages were monitored. Practical measurements showed that an increase of 65% in peak final settling tank flow rates could be achieved, and up to twice the BOD load could be treated with the assistance of extra oxygen, using the existing Vitox plant. The improved settling tank performance can be explained in terms of the Water Research Centre's mass loading theory.
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