Microaeration reduces hydrogen sulfide in biogas

Authors: Mulbry, Walter; LANSING, STEPHANIE - University Of Maryland; COKER, CRAIG - Jg Press, Inc

Submitted to: Biocycle
Publication Type: Trade Journal
Publication Acceptance Date: 12/22/2016
Publication Date: 1/20/2017
Publication URL: https://handle.nal.usda.gov/10113/5695418
Citation: Mulbry III, W.W., Lansing, S., Coker, C. 2017. Microaeration reduces hydrogen sulfide in biogas. Biocycle. 58:57-59.

Interpretive Summary: Biogas is produced from organic substrates by bacteria in anaerobic digestion (AD) from the conversion of organic carbon into methane, with (predominantly) carbon dioxide and (secondarily) water vapor forming as byproducts. However, in many biogases, significant levels of another byproduct, hydrogen sulfide (H2S), can be found in the raw biogas at levels varying from 200 parts per million (ppm) to over 3,000 ppm. H2S in biogas is the main cause of corrosion of the structures and materials of a biogas facility. The hydrogen sulfide component is oxidized during combustion, leading to the formation of the corrosive gas sulfur dioxide. Sulfur dioxide causes overacidification of the engine oils during combustion in the gas engine. This results in increased wear, frequent bearing damage, higher consumable costs and longer plant downtimes. Adding a small amount of air (or oxygen) to biogas promotes the growth of sulfur-reducing (oxidizing) bacteria, who convert the sulfide compounds to elemental sulfur. Microaeration, i.e., the dosing of very small amounts of air (oxygen) into an anaerobic digester, is a very efficient, simple and economically feasible technique for hydrogen sulfide removal from biogas. Due to microaeration, sulfide is oxidized to elemental sulfur by the action of sulfide oxidizing bacteria. This process takes place directly in the digester. Researchers at the U.S. Department of Agriculture‘s Beltsville Agricultural Research Center (BARC) in Beltsville, Maryland, want to understand how the oxidation rates for H2S change as a function of oxygen levels and retention times, and also to understand how biologically-active surfaces are involved in the oxidation of sulfide compounds. Using pilot-scale digesters constructed at the BARC dairy, the team found that H2S levels in the biogas were routinely below about 100 ppmv as long as the oxygen (O2) levels were above 0.4% (v/v). 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: Although there are a variety of biological and chemical treatments for removal of hydrogen sulfide (H2S) from biogas, all require some level of chemical or water inputs and maintenance. In practice, managing biogas H2S remains a significant challenge for agricultural digesters where labor and operational funds are very limited compared to municipal and industrial digesters. As an alternative treatment for H2S, relatively low volumes of oxygen or air can be injected into the digester headspace. Under the resulting micro-aerobic conditions, sulfide-oxidizing bacteria in the manure feedstock will convert dissolved sulfide into elemental sulfur. Although the feasibility of sulfide oxidation under micro-aerobic conditions has been demonstrated in small scale experiments and sludge digesters, further research is needed to improve this approach for use on small agricultural digesters. the objective of this study was to determine the effect of different aeration rates on hydrogen sulfide emissions during anaerobic digestion of dairy manure. Duplicate plug flow field-scale digesters were operated at different levels of aeration or without aeration. Results showed that hydrogen sulfide levels decreased rapidly from 3500 ppmv to less than 100 ppmv when oxygen levels in the digesters were 0.4% or higher. Control of aeration rates using simple pumps coupled to inexpensive oxygen sensors may lead to improved use of microaeration in both large and small digester operations.