BULKING AND FOAMING CAUSED BY MICROTHRIX PARVICELLA AT 3 LARGE SEWAGE-TREATMENT PLANTS IN THE GREATER STOCKHOLM AREA

Microbiological studies of the activated sludge have been performed co ntinuously at three of the largest sewage treatment plants in Stockhol m (Bromma, Henriksdal and Himmerfjardsverket). All three plants are op erated with nitrogen removal. The filamentous organism Microthrix parv icella has been observed at all plants during the study. Large bulking and foaming problems have only been observed at Himmerfjardsverket al though Himmerfjardsverket is operated at a higher F/M ratio and a lowe r aerated sludge age than Henriksdal. Bulking has mainly occured at fi lament abundances above 3 (relative scale 0-6, 0; none, 6; excessive). Large variations in the settleability of the sludge have been observe d in the case of sludge collected from different periods but with the same filamentous abundance. The structure of the flocs and the locatio n of the filaments within the flocs have a major impact on the final s ettleability of the sludge. Foaming in the activated sludge caused by Microthrix parvicella has only been observed at filamentous abundance above 3. During this study, Microthrix parvicella has been observed in the activated sludge when the plant is operated with an aerated sludg e age above 3 days and F/M ratios below 0.3 kg BOD/kg MLSS, day. Opera tion with a low F/M ratio can be obtained without excessive growth of Microthrix parvicella. Copyright (C) 1996 IAWQ.