Anaerobic deflocculation and aerobic reflocculation of activated sludge

Activated sludge deflocculated when it was exposed to anaerobic conditions and shear. The extent of deflocculation, measured as turbidity, varied with length of anaerobic period and sludge type (age). In order to restore the deflocculated sludge, oxygen was added, and subsequently, in less than 1 h, most of the deflocculated matter was reflocculated. A certain part of the deflocculated hoc components remained, however, deflocculated even after se veral hours of aeration. As the sludge was exposed to repeated deflocculati on-reflocculation conditions, the flocs were progressively damaged, and an accumulation of non-flocculated matter was observed. Measurements of the nu mber of bacteria, the concentration of protein, humic substances and carboh ydrate in the sludge and the supernatant indicated that the deflocculated m aterial was mainly composed of bacteria and extracellular polymeric substan ces (EPS) attached to cells or hoc fragments. The non-flocculated particles after reflocculation were enriched in bacteria and protein, compared to th e average sludge composition. A reduced deflocculation was observed when ni trate acted as an electron acceptor (anoxic conditions), compared to anaero bic conditions. Increased shear and decreased temperature reinforced the de flocculation. Field experiments showed a temporal variation in flee strengt hs with the weakest flocs present in the winter period. (C) 2000 Elsevier S cience Ltd. All rights reserved.