Hydrogen sulphide removal in the anaerobic digestion of sludge by micro-aerobic processes: pilot plant experience

H₂S removal from biogas produced in anaerobic digestion of sludge through the introduction of oxygen under micro-aerobic conditions is studied. Research was carried out in two pilot plant reactors (working volume, 200 L each) treating sludge from WWTP with HRT of 20 days. Mixing was provided via sludge or biogas recirculation. Introduction of very low oxygen flow (0.013–0.024 L/L_reactor d) successfully removed H₂S content in biogas with an efficiency above 99%. Reactor performance during micro-aerobic operation in terms of biogas production, methane yield and COD removal were not affected by the amount of oxygen supplied, remaining stable and similar to the anaerobic behaviour. Sludge recirculation (∼50 L/h) and biogas recirculation (∼3.5 L/min) as mixing methods were found not significant in H₂S removal from biogas while biogas recirculation reduced by 10 times dissolved sulphide concentration compared to sludge recirculation.