SURVEY OF FILAMENTOUS POPULATIONS IN NUTRIENT REMOVAL PLANTS IN 4 EUROPEAN COUNTRIES

A joint EU research project aimed at solving activated sludge bulking in nutrient removal plants was initiated in 1993. The project started with a survey of the size and composition of the filamentous populatio n in nutrient removal plants in Denmark, Germany, Greece and the Nethe rlands. The results show that biological nutrient removal process cond itions indeed favour filamentous microorganisms in their competition w ith flee forming organisms. An increase in the size of the filamentous population resulted in a deterioration of the settling properties of the biomass, except for plants with Bio-P removal conditions. It is as sumed that in the latter case the dense clusters of Bio-P bacteria inc rease the weight of the flees, and compensate for the effect of the la rger number of filaments. Although exceptions frequently occur, the fo llowing sequence in decreasing filamentous organism population size wa s observed for the process conditions indicated: completely mixed + si multaneous denitrification; completely mixed + intermittent aeration/d enitrification; alternating anoxic/oxic process conditions, with an an aerobic tank for biological phosphate removal (Bio-Denipho); alternati ng anoxic/oxic process conditions (Bio-Denitro); predenitrification Th e surveys provided little information about the effect of nutrient rem oval in plants with plug flow aeration basins. Simultaneous precipitat ion with aluminium salts nearly always resulted in a low number of fil aments and a good settling sludge. The size of the filamentous organis m population showed a seasonal pattern with a maximum in winter/early spring and a minimum during summer (in Greece: during autumn). This se asonal variation is primarily caused by the effect of the season on th e population sizes of M. parvicella, N. limicola and Type 0092. M. par vicella is by far the most important filamentous species in nutrient r emoval plants. In Denmark only, Type 0041 also frequently dominates th e filamentous population, but seldom causes severe bulking. Considerin g their frequency of occurrence, approx. 10 other filamentous micro-or ganisms are of minor importance. Growth of some of these species, viz. those which use soluble substrate, can be prevented by the introducti on of Bio-P process conditions. M. parvicella and Type 0041 (and proba bly also Actinomycetes and the Types 1851 and 0092) seem to compete fo r the same substrates i.e. the influent particulate fraction. Most of the differences in composition of the filamentous microorganism popula tion can be explained by whether or not premixing of influent and recy cled sludge is used. In general, premixing for a short period of time followed by anoxic conditions favours Type 0041. M. parvicella seems t o proliferate if the particulate fraction is first hydrolysed or if it enters the plant via an oxic zone. It is concluded that bulking in nu trient removal plants is mainly caused by filamentous species requirin g the particulate fraction for their growth. (C) 1998 IAWQ. Published by Elsevier Science Ltd.