(153d) Electrochemical Approach for Removal of H2S and Inorganic Nutrients in Anaerobic Digestion

Anaerobic digestion (AD) is a commercial waste treatment process that can also generates biogas for energy and heating use. Sulfate (SO₄^2-) in organic waste feedstock is reduced by sulfate-reducing bacteria (SBR), e.g., Desulfovibrio desulfuricans, to either stay in liquid effluent or emit into biogas as hydrogen sulfide (H₂S). Hydrogen sulfide decreases the value of the biogas and poses environmental challenges because it is odorous, toxic, and corrosive. Another issue regarding AD is that the effluent still contains high levels of total nitrogen and phosphorus.

Considering that electrochemical reactors are generally easy and cost-effective to incorporate and operate and can be precisely controlled, this study coupled AD systems with electrochemical reactors to reduce concentrations of sulfide, total nitrogen, and phosphorus. Different anode materials, including carbon cloth, stainless steel mesh, nickel foam, and mixed metal oxide (MMO) coated titanium mesh, were evaluated for their electro-activity for sulfide removal via cyclic voltammetry in various types of media, including phosphate buffer solution, Na₂S solution, and AD influent. Independent variables, such as position of electrode (immersed, half-immersed, and in headspace), applied voltage (0, 0.5 V, 1 V, 2 V, 3 V), and temperature were optimized for hydrogen sulfide removal. Under the similar condition, electro-activity for phosphorus accumulation on cathode was optimized. Batch reactors of anaerobic digestion were performed with the selected anode materials, and biogas production rate, oxidation-reduction potential of medium, and hydrogen sulfide level in biogas were periodically monitored. Total nitrogen and total phosphorus levels were analyzed at the end of the batch reaction.

Two mechanisms of anodic sulfide removal were elucidated and quantified: first, at an anode potential less than the required level for oxygen generation, the sulfide generated by SRB can be reduced to elemental sulfur which deposits on the surface. Second, higher anode potential created micro-aerobic conditions due to oxygen gas formation, and it reduced SBR activity and therefore decreased the rate of sulfide buildup. This oxygen generation also transformed sulfide to more oxidized forms. Meanwhile, the micro-aeration condition boosted the activity of ammonia- and nitrite-oxidizing bacteria, generated more nitrate as feedstock for denitrification and resulted in a reduction in total nitrogen level in AD effluent. Microbial and archael analysis for regular and electrochemical reactors were carried out by 16SrRNA gene amplification, cloning, and sequencing. The sequences were compared with the GenBank databases to determine the approximate phylogenetic affiliations, and the phylogenic tree was constructed using the neighbor-joining method. The shift in microbial community between regular and electrochemical reactors, especially the sulfate-reducing bacteria, ammonia- and nitrite-oxidizing bacteria, as well as denitrifying bacteria was elucidated. This study demonstrated that electrochemically assisted AD reactors successfully reduced hydrogen sulfide, total nitrogen, and total phosphorus concentrations in biogas and AD effluent.