THE MICROTHRIX-PARVICELLA PUZZLE

Nowadays the excessive growth of many filamentous bacteria in activate d sludge plants can be controlled by the application of so-called sele ctors. In the Netherlands, about 80 selectors have been constructed at full scale plants so far. However, selectors are frequently not very effective for controlling bulking if M. parvicella is dominating the s ludge population. As M. parvicella has also been frequently observed i n nutrient removal plants, the development of control strategies for t his bacterium has become very urgent. From the substantial information on the occurrence of and attempts to control M. parvicella collected during the last decade, several main conclusions can be drawn. The dev elopment of M. parvicella in Dutch treatment plants shows a very disti nctive seasonal rhythm, the population reaching its maximum size in ea rly spring. Foaming in Dutch treatment plants is usually caused by M. parvicella. M. parvicella and Actinomycetes spp. seem to compete for t he same ecological niche in treatment plants. By comparing the populat ion size of M. pavicella as a function of various modifications in the activated sludge process, it is concluded that this bacterium grows b etter in so-called carrousel systems than in other extended aeration p lants. M. pavicella causes severe bulking in carrousels fed with prese ttled sewage. The impact/usefulness of selectors for controlling M. pa rvicella seems to decrease as the overall sludge load of the plant inc reases. This information is presented to start a discussion about poss ibilities to solve the M. parvicella puzzle.