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 floc components remained, however, deflocculated even after several hours of aeration. As the sludge was exposed to repeated deflocculation–reflocculation conditions, the flocs were progressively damaged, and an accumulation of non-flocculated matter was observed. Measurements of the number of bacteria, the concentration of protein, humic substances and carbohydrate in the sludge and the supernatant indicated that the deflocculated material was mainly composed of bacteria and extracellular polymeric substances (EPS) attached to cells or floc fragments. The non-flocculated particles after reflocculation were enriched in bacteria and protein, compared to the average sludge composition. A reduced deflocculation was observed when nitrate acted as an electron acceptor (anoxic conditions), compared to anaerobic conditions. Increased shear and decreased temperature reinforced the deflocculation. Field experiments showed a temporal variation in floc strengths with the weakest flocs present in the winter period.