The behaviour of As(III), As(V), MMAv and DMAv in batch activated sludge process were investigated. Experiments were carried out by using aerobic and anoxic reactors with an initial As concentration of 100 μg l−1. Under aerobic condition, As(III) was oxidized to As(V) within 9 hours, some part of MMAv was methylated to DMAv and some other part was demethylated to As(III), which in turn was immediately oxidized to As(V). Under anoxic condition, As(V) was reduced to As(III) within the same time-course. No significant transformation occurred during experiments conducted with DMAv. It was found that all reactions were biologically mediated. The overall As removal was low (<20%) during the experiments. Although a relationship seems to exist between the sludge concentration and As removal, it is concluded, under the conditions of our study, that the activated sludge process cannot remove arsenicals efficiently. However, it can control their transformations well. Thus, if associated with an appropriate technology, the activated sludge can be used for As pre-oxidation to treat As contaminated wastewaters. Finally, care must be taken on possible presence of MMAv in the influent of any wastewater treatment plant as it can be easily oxidized by the activated sludge.

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