The case of both energetic uncoupling and metabolic selection of microorganisms in the OSA activated sludge system

Abstract

A modified activated sludge system with anaerobic treatment of returned sludge, named OSA (Oxic‐Settling‐Anaerobic) was studied. The main objective of the research was to create conditions of a physiological stress on microbial growth, resulting in uncoupling of catabolic processes from anabolic ones. This phenomenon, largely described in the literature, is accompanied by energy wastage resulting in biomass yield reduction. The experimental results obtained with a mixed culture (activated sludge) fed on synthetic substrate showed that the anaerobic conditions exerted a kind of selection pressure on microbial metabolism of the activated sludge microorganisms, resulting in metabolic selection of the species being able to favour catabolic pathways. These bacteria, capable of storing polyphosphates under oxic conditions and using them as energy reserves under anaerobic conditions are called poly‐P bacteria. The OSA mixed culture contained about 50 to 60% of these poly‐P bacteria. It has been concluded that the insertion of an anaerobic zone into the returned sludge circuit changed both composition and behaviour of activated sludge microorganisms. The typical phenomena observed in the OSA system were enhanced phosphate removal, reduction of the excess activated sludge production and the suppression of activated sludge filamentous bulking.