Activated sludge bulking often occurs in French wastewater treatment plants. Precise identification of filaments and better knowledge of their ecophysiology helped to clarify their re1ations with the operational conditions of plants. Effective techniques were therefore developed for control. The “mixing zone” is suitable for many cases of bulking ocurring in extended aeration plants (the most common type in France). A test method was developed to provide the elements for dimensioning the system. Unreserved cooperation between the owner of the plant, the manager, the SATESE (Service of technical aid to wastewater treatment plant managers) and specialized laboratories is required to implement such measures. Some cases, about which CEMAGREF was consulted, were examined; three of them are detailed and reasons for success or fai1ure are analysed. Seeking funds for the works too often leads to excessive delays.

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