Long-term biogas desulfurization under different microaerobic conditions in full-scale thermophilic digesters co-digesting high-solid sewage sludge

Long-term (81 weeks) microaerobic biogas desulfurization was investigated in three full-scale thermophilic digesters in series co-digesting sewage sludge with co-products of source segregated domestic food waste. Residual oxygen (RO) concentrations in the digester headspaces were maintained at fixed values from 2.0% to 0.2% (v/v). Complete H2S removal was obtained at all tested RO concentrations under organic loading rate (OLR) and hydraulic retention time (HRT) varying between 1.5 and 2.3 kg VS/m(3) d and between 44 and 71 d, respectively. Specific oxygen and air consumptions of 0.88 kg O-2/ton VSin and 0.73 %ton air/ton biogas, respectively, were obtained by linear regressions with the RO values maintained in the biogas. Mass balance and stoichiometric evaluations revealed that from 13% to 59% of the consumed O-2 was used for processes other than partial H2S oxidation to elemental sulfur (S-0). The relatively high ammonium concentrations (3.5-3.8 g N-NH4+/kg digestate) in the liquid phase of the three digesters presumably limited the biological H2S production and S-0 accumulation in the headspace. The results of this study can be used for designing the aeration systems of full-scale thermophilic digesters.