BULKING-SLUDGE CONTROL - PROGRESS, PRACTICE AND PROBLEMS

Since bulking sludges are the primary cause of failure of activated sludge systems, the first obligation of the designer is to minimize the possibility of such an occurrence. This paper traces the evolution of bulking control concepts from the earliest date when batch and semi-continuous selectors were employed, to more recent experiences with anaerobic, anoxic, oxic and high F/M biological selectors. This paper also summarizes recent results obtained from 12 USA facilities employing various selectors. Preliminary conclusions support high to low F/M gradient as a dominant selector factor. Whether the system is aerated or not, the available oxygen in the initial contact zones (ICZs) should be substantially less than the oxygen demand in the zone to ensure that anaerobic functions occur within the cell mass. In addition, F/M gradients should be provided in each environment.