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The solution for a filamentous bulking episode (associated with foaming) at a municipal wastewater plant in La Spezia (North-West Italy) is reported. Two main bacterial Types were involved: one belonging to the group of the so-called Nocardia Amarae Like Organisms (NALO); the other was identified as Type 0092. The identification was made according to Jenkins (1986) by the observation of Mixed Liquor fresh samples at direct microscopy before and after Gram and Neisser Stains. To evaluate treatment plant performance, the Sludge Biotic Index (SBI) was also calculated. As a result of different environmental conditions existing between the two treatment lines currently in use, the population dynamics of both protozoan ciliates and filamentous bacteria in the oxidation tanks were also different. The sludge rehabilitation procedure included the use of a cationic polyelectrolite instead of NaCIO thus allowing the bulking problem to be solved within 20 hours, without significant negative alterations on treatment performance which improved in the following days. Hence, the proposed method would represent an alternative to the disinfection of Mixed Liquor in bulking control. [ABSTRACT FROM AUTHOR]