Effect of liquid surface area on hydrogen sulfide oxidation during micro-aeration in dairy manure digesters

Although there are a variety of commercially available biological and chemical treatments for removal of hydrogen sulfide (H(2)S) from biogas, managing biogas H(2)S remains a significant challenge for agricultural digesters where labor and operational funds are very limited compared to municipal and industrial digesters. The objectives of this study were to evaluate headspace aeration for reducing H(2)S levels in low cost plug flow digesters and to characterize the relationship between the liquid surface area and H(2)S oxidation rates. Experiments with replicate field scale plug flow digesters showed that H(2)S levels decreased from 3500 ppmv to <100 ppmv when headspace oxygen levels were 0.5 to 1%. Methane production was not affected by aeration rates that resulted in headspace oxygen levels of up to 1%. Pilot scale experiments using 65 to 104 L desulfurization units showed that H(2)S oxidation rates increased with increases in liquid surface area. These results support the hypothesis that H(2)S oxidation rates are limited, in part, by the surface area available for oxygen transfer, and can be increased by growth of biofilms containing H(2)S oxidizing bacteria. Maximum removal rates corresponded to 40 to 100 g S m(-2) d(-1) of liquid surface area at biogas retention times of 30 to 40 min.