Assessment of the use of mainstream iron addition for phosphorous control on H2 S content of biogas from anaerobic digestion of sludges.

A material flux analysis on sulfur (S), phosphorus (P), aluminum (Al), and iron (Fe) was conducted for two WWTPs (Galt and Kitchener) to evaluate the potential of coagulants that are employed for phosphorus control to reduce hydrogen sulfide (H2 S) emissions in the biogas from anaerobic digestion. It was found that while the Galt WWTP receives higher concentrations of S in the raw wastewater than the Kitchener WWTP, this had only a modest impact on the speciation of S entering anaerobic digestion. At both plants, only 2%-4% of influent S entered the digesters. The presence of Fe in the sludge stream was found to cause S, that is released by volatile solid destruction and sulfate ( SO 4 2 - SO 4 2 - ) reduction, to become particulate-bound. A dosage of 1.1 mg/L of Fe into the raw wastewater (11% of the Fe dosed for P control) was sufficient for sulfide (S2- ) control. Transitioning the Galt WWTP from Al to Fe dosing for P control had no significant impact on effluent P concentrations and resulted in a substantial reduction in the biogas H2 S concentration. An additional secondary benefit was an increase in the solid content of the dewatered cake. PRACTITIONER POINTS: Material flux analyses can be employed to gain insight into the fate of key elements contributing to biogas quality. The use of iron for phosphorous control can effectively control H2 S in anaerobic biogas. Conversion from Al2 (SO4 )3 to FeSO4 dosing for P control resulted in increased solid content of centrifuged biosolids.