Abstract
A UCT-type pilot plant was operated for two and a half year. The phosphate removal activity tended to be unstable in low temperature terms although the nitrogen removal activity was stable through the operation period. As it was considered that the point at issue is low MLSS concentration in the anaerobic tank, high circulation ratio of denitrified mixed liquor from the anoxic to anaerobic tank was adopted. MLSS concentration in the anaerobic tank became higher a little, but phosphate removal was still unstable. In order to hold the MLSS concentration in the anaerobic tank much higher, the return sludge sent back to not only the anoxic tank but also the anaerobic tank. Since high MLSS concentration made high amount of organic matter consumption in the anaerobic tank, carry-overs of the organic matter to the anoxic tank decreased. Consequently, a population ratio of the PAOs rose, and the higher phosphate removal activity was achieved even if in low-temperature terms.
