Data are reported on the role of Acinetobacter in phosphate removal in activated sludge. The fluorescent antibody (FA) technique was applied in an autecological study of Acinetobacter in this habitat.

Results indicated that Acinetobacter was present in numbers exceeding 106 organisms per cm3 throughout all zones of the activated sludge process, with higher numbers occurring in the aeration zone. It was also found that Acinetobacter numbers did not vary regardless of whether phosphate was removed or not. Combined electron microscopy and electron dispersive micro-analysis of X-rays (EDAX) furthermore indicated that bacteria from phosphate removing activated sludge plants contained phosphate granules, whereas those from non-removing plants or where removal had stopped, did not contain phosphate granules at all.

Hence, it was concluded that no major population shifts of Acinetobacter occurred in activated sludge and that the process of phosphate accumulation is induced in Acinetobacter. However, Acinetobacter never occurred in numbers sufficiently large to account for phosphate removal to the extent sometimes observed in activated sludge.

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