SLUDGE RETENTION TIME IN THE SECONDARY CLARIFIER EFFECTS ON THE GROWTH OF MICROTHRIX-PARVICELLA

Microbiological examinations have been performed over the last 3 years at 3 large sewage treatment plants in the greater Stockholm area. All 3 plants have periodically suffered from excessive growth of the fila mentous bacterium Microthrix parvicella. The growth of Microthrix parv icella was favored by low sludge loading. Results from various plants and from the various parallel systems within the individual plants wer e compared. The bacterial composition of the activated sludges differe d in ways that could not be explained by the sludge loading or the sew age composition. Biological processes in the secondary clarifiers seem to influence the competition between the different organisms in the a ctivated sludge. A clear relationship was shown between high sludge re tention time in the secondary clarifiers, VFA production in the return activated sludge (RAS) and a high abundance of Microthrix parvicella. (C) 1998 IAWQ. Published by Elsevier Science Ltd.