Biological foams in activated sludge plants: Characterization and situation

Abstract

Stable biological foams are a worrying problem for the operation and control of activated sludge plants. The flotation of a fraction of the sludge is related to the metabolism of certain filamentous micro-organisms which synthetize and/or excrete hydrophobic compounds. In France, a survey of 6000 activated sludge plants has shown that 20% of them (87% of these plants which operate by extended aeration) are affected by this problem. Microscopic observations performed on samples collected in 58 selected plants have shown that, among the filamentous germs identified, the most frequent is Microthrix parvicella (45% of cases), followed by type 0675 (26% of cases) and the Nocardioform actinomycetes (14% of cases). Identification criteria for all these germs are documented. The investigation reveals the difficulties encountered by plant operators to control foaming phenomenon. So far, promising results have been obtained either by using the contact zone technique or resorting to chlorination.