Continuous microaeration of mesophilic anaerobic digestion for in situ biogas desulfurization

Abstract

Anaerobic digestion can be vital in helping water resource recovery facilities improve their renewable energy portfolio through the generation of methane-rich biogas. The biogas must be stripped of various trace components such as hydrogen sulfide (H2S) before reuse to maintaining the operation life of the machinery. Microaeration, which is the addition of trace amounts of oxygen or air, has been demonstrated to be an effective low-cost method for in situ biogas desulfurization in single stage anaerobic digesters. There is further evidence that suggests that microaeration aids hydrolysis, thereby improving solids destruction. This study aimed to understand the efficacy of continuous microaeration, using bubble-free supply of oxygen into the sludge recirculation line, in the methanogenic stage of a mesophilic two-stage anaerobic digestion process for biogas desulfurization and for improving solids destruction. The doses tested in this investigation were 0.0, 0.67, 5.35, 7.50, and 15.0 mLO2/Lreactor–day. Microaeration led to a decrease in the H2S from 750 ± 120 ppm at a microaeration dose of 0.0 mLO2/Lreactor–day, i.e., when completely anaerobic, to 220 ± 80 ppm at 15.0 mLO2/Lreactor–day in the Experimental bioreactor. There was no significant change in the volatile solids destruction efficiency across the different doses of microaeration, suggesting that, up to a dose 15.0 mLO2/Lreactor–day, microaeration does not impact solids destruction. There was an increase in the methane fraction in the biogas, from 64.4 ± 2.0% at 7.50 mLO2/Lreactor–day to 65.7 ± 1.6% at 15.0 mLO2/Lreactor–day, while CO2 reduced from 34.3 ± 1.3% during the same period to 29.7 ± 0.9%. Methane yield was consistent almost throughout the operation period, at 0.75 ± 0.03 LCH4/gVS in the Experimental

bioreactor compared to 0.67 ± 0.04 LCH4/gVS in the Control bioreactor. This study validated that continuous microaeration can be used for in situ biogas desulfurization in the methanogenic stage of a mesophilic two-stage anaerobic digestion process, without any significant adverse effect to the anaerobic community.