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/m3 d and between 44 and 71 d, respectively. Specific oxygen and air consumptions of 0.88 kg O2/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 O2 was used for processes other than partial H2S oxidation to elemental sulfur (S0). 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 S0 accumulation in the headspace. The results of this study can be used for designing the aeration systems of full-scale thermophilic digesters.