High rate domestic wastewater treatment at 15 °C using anaerobic reactors inoculated with cold-adapted sediments/soils-shaping robust methanogenic communities

To choose the reactor format in which to employ a low temperature adapted seed for wastewater treatment, we compared a UASB and an AnMBRUASB (UF) reactor at low HRT and temperature (15 °C). The reactors were inoculated with biomass from reactors seeded with soils and sediments from low temperature environments, and fed real municipal wastewater. Both systems met the UWWT Directive (91/271/EEC) COD effluent standard (<125.0 mg L−1) with the AnMBR COD removal efficiency being slightly higher (86.2 ± 1.5%) than that of the UASB (79.3 ± 2.0%). Methane production rates were also higher for the AnMBR than for the UASB, resulting in a COD : CH4 of 26.1 ± 3.0 and 18.2 ± 2.1%, respectively. Higher methanogenic cell abundance was observed in the AnMBR (3בUASB'). The low conversion of COD to methane was attributed to (i) the presence of SO4 in the influent (120.4 ± 17.4 mg L−1), which scavenged up to 50% of the COD, and (ii) accumulation of un-hydrolysed lipids in the mixed liquor especially in the case of the AnMBR. The UASB was energy positive (0.041 ± 0.013 kW h m−3) whilst the AnMBR was energy negative (−0.221 ± 0.016 kW h m−3). The reactor design appeared to have a profound effect on the numbers and diversity of the methanogens: the hydrogenotrophic activity being favoured in the UASB. But both reactors had comparatively high cell specific rates of methanogenesis. We concluded that the slightly better performance of the AnMBR was not sufficient to offset its higher running cost and the complexity of its design.