Recent research has indicated the advantages of combining anaerobic and aerobic processes for the treatment of municipal wastewater, especially for warm-climate countries. Although this configuration is seen as an economical alternative, is has not been investigated in sufficient detail on a worldwide basis. This work presents the results of the monitoring of a pilot-scale plant comprising of an UASB reactor followed by an activated sludge system, treating actual municipal wastewater from a large city in Brazil. The plant was intensively monitored and operated for 261 days, divided into five different phases, working with constant and variable inflows. The plant showed good COD removal, with efficiencies ranging from 69% to 84% for the UASB reactor, from 43% to 56% for the activated sludge system only and from 85% to 93% for the overall system. The final effluent suspended solids concentration was very low, with averages ranging from 13 to 18 mg/l in the typical phases of the research. Based on the very good overall performance of the system, it is believed that it is a better alternative for warm-climate countries than the conventional activated sludge system, especially considering the total low hydraulic detention time (4.0 h UASB; 2.8 h aerobic reactor; 1.1 h final clarifier), the savings in energy consumption, the absence of primary sludge and the possibility of thickening and digesting the aerobic excess sludge in the UASB reactor itself.

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