The filamentous bacterium Microthrix parvicella often causes poor settling of activated sludge in oxidation ditches supplied with domestic sewage. This organism utilizes preferably fatty acids as a carbon and energy source and it requires reduced nitrogen and reduced sulphur compounds for growth. In activated sludge fed with a synthetic wastewater containing oleic acid, abundant growth of M. parvicella was obtained in a bench scale plant operated at a low oxygen concentration. On the other hand, in control plants operated at a high oxygen concentration or fed with a synthetic wastewater containing glucose, this organism disappeared. These results may explain the excessive growth of M. parvicella in oxidation ditches supplied with raw domestic sewage as this type of wastewater contains a considerable amount of fatty matter and the process is usually operated at a low oxygen concentration in the mixed liquor. Bulking of sludge caused by M. parvicella can be cured by an enhanced oxygen supply. © 1988.
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