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

Authors: Mulbry, Walter; Selmer, Kaitlyn; LANSING, STEPHANIE - University Of Maryland

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2017
Publication Date: 10/4/2017
Publication URL: http://handle.nal.usda.gov/10113/5852186
Citation: Mulbry III, W.W., Selmer, K.J., Lansing, S. 2017. Influence of liquid surface area on hydrogen sulfide oxidation during micro-aeration in dairy manure digesters. PLoS One. 12(10):e0185738. https://doi.org/10.1371/journal.phon.0185738.

Interpretive Summary: Biogas is produced from organic substrates by bacteria during anaerobic digestion. Although the main components in biogas are methane, carbon dioxide and water vapor, significant levels of hydrogen sulfide (H2S), can be found at levels from 200 parts per million by volume (ppmv) to over 3,000 ppmv. H2S in biogas is the main cause of corrosion of the structures and materials of a biogas facility. Microaeration, i.e., the dosing of very small amounts of air (oxygen) into an anaerobic digester, is a simple and economically feasible technique for hydrogen sulfide removal from biogas. During microaeration, sulfide is oxidized to elemental sulfur by the action of sulfide oxidizing bacteria. The first objective of this study was to evaluate microaeration for reducing H2S levels in low cost plug-flow manure digesters. Six field-scale replicate digesters were used to determine the effect of aeration rate on H2S concentrations and methane production under field conditions. The second objective was to determine the relationship between the liquid surface area in digesters and H2S oxidation rates during headspace aeration. We predicted that H2S oxidation rates were limited, in part, by the surface area available for growth of biofilms containing H2S oxidizing bacteria. Results from experiments with replicate field scale digesters showed that H2S levels decreased from 3500 ppmv to <100 ppmv when headspace oxygen levels were 0.5% or higher. Methane production was not affected by aeration rates that resulted in headspace oxygen levels of up to 1%. Pilot scale experiments showed that H2S oxidation rates increased with increases in liquid surface area. Maximum rates corresponded to 40 to 100 g S per square meter of liquid surface area per day. Results also suggest that it is possible to significantly increase H2S oxidation rates by adding short vertical curtains of digestate-saturated fabric in the digester headspace. These results are important because they are the first instance of microaeration results from replicated field scale manure digesters. The operational parameters established by this study will be of interest to farm digester operators and to farmers and agricultural industries considering installation of anaerobic digesters for waste treatment.

Technical Abstract: The specific objectives of this study were to evaluate headspace aeration for reducing hydrogen sulfide levels in low cost plug flow digesters, and to characterize the relationship between the liquid surface area and hydrogen sulfide oxidation rates. Experiments with replicate field scale plug flow digesters show that hydrogen sulfide levels decreased from 3500 to less than 100 parts per million by volume (ppmv) when headspace oxygen levels were 0.5% or higher. Methane production was not affected by aeration rates that resulted in headspace oxygen levels of up to 1%. Pilot scale experiments showed that hydrogen sulfide oxidation rates increased with increases in liquid surface area. Maximum rates corresponded to 40 to 100 g S per square meter per day of liquid surface area at biogas retention times of 30 to 40 min. Results also suggest that it is possible to significantly increase hydrogen sulfide oxidation rates by adding short vertical curtains of digestate-saturated fabric in the digester headspace.