Details are presented concerning the operation of a two-stage selector used in conjunction with an activated sludge process. The first-stage of the selector was not aerated, the second-stage had the facility for variable aeration. The prime objective of using the selector was to effect a sufficient reduction in the numbers ofMicrothrix parvicellato minimise the potential for the formation of stable foam in the aeration tank. In an extended trial ( 1 year) it was found that the most effective regime was when the second-stage was subjected to an aeration period of between 9 and 12 hours. It was noted that none of the changes in the aeration times had any effect on the population of the secondary filamentous species,Nostocoida limicola.
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