This work evaluates the theoretical availability of energy and the sludge production in a wastewater treatment plant (WWTP) that associates UASB reactors and submerged aerated biofilters (BF). A comparison among the behaviour of a pilot plant operating under constant hydraulic load (trial 1) and under hourly variation of hydraulic load, with sludge recirculation from the BF to the UASB (trial 2), was carried out. The results show that it is possible to suppress complementary units of thickening and digestion, once the sludge in the bottom of the UASB reaches concentrations of 4,5% TS and 50 to 70% (VS/TS). A comparison of the biogas availability in a WWTP with primary settlers and anaerobic digesters is accomplished. The biogas production in a UASB + BF WWTP is larger than double that produced on the conventional WWTP. The UASB reactor acts on the total COD present in the wastewater (soluble COD + suspended COD), which does not happen in a conventional WWTP (only the suspended COD retained in the primary settler and in the biological sludge is converted to methane). The sludge production, the energy requirement for aeration and the necessary volume for the reactors of the two kinds of WWTP are appraised.

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