Bulking and foaming caused by microthrix parvicella at three large sewage treatment plants in the greater stockholm area

Microbiological studies of the activated sludge have been performed continuously at three of the largest sewage treatment plants in Stockholm (Bromma, Henriksdal and Himmerfjärdsverket). All three plants are operated with nitrogen removal. The filamentous organism Microthrix parvicella has been observed at all plants during the study. Large bulking and foaming problems have only been observed at Himmerfjärdsverket although Himmerfjärdsverket is operated at a higher F/M ratio and a lower aerated sludge age than Henriksdal. Bulking has mainly occured at filament abundances above 3 (relative scale 0-6, 0; none, 6; excessive). Large variations in the settleability of the sludge have been observed in the case of sludge collected from different periods but with the same filamentous abundance. The structure of the flocs and the location of the filaments within the flocs have a major impact on the final settleability 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 sludge age above 3 days and F/M ratios below 0.3 kg BOD7/kg MLSS, day. Operation with a low F/M ratio can be obtained without excessive growth of Microthrix parvicella.