The results obtained from the operation of a treatment system composed of an anaerobic (up-flow sludge blanket - UASB) reactor followed by an aerobic (sequencing batch - SBR) reactor treating domestic sewage are presented and discussed. The pilot plant was monitored during 6 months, aiming to obtain performance data on organic matter, nitrogen and phosphate removal under different operating conditions.

The UASB reactor was operated at a constant hydraulic detention time (θ) of 6 h while the SBR performance was monitored in four different duration cycles (24, 12, 6 and 4 h) corresponding to aeration times (AT) of 22, 10, 4 and 2 h, respectively.

COD and TSS overall removal efficiencies (Eo) up to 91% and 84%, respectively, were achieved independently on the aeration time applied to the SBR. In respect to nitrification and phosphate removal, AT was found to be a determinating operating parameter. TKN removal of approximately 90% was achieved for AT equal to or higher than 10 h; complete nitrification occurred for AT higher than 4 h; significant phosphate removal (72%) occurred only at the AT of 2 h. It was not possible to achieve simultaneous efficient removal of nitrogen and phosphate, under the operating conditions imposed on the treatment system.

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