(614e) In-Situ Bio-Electrochemical Sulfide Remediation during the Storage and Anaerobic Digestion of Dairy Manure

Background: Hydrogen sulfide (H₂S) is a highly toxic and corrosive gas with a characteristic rotten-egg odor, and it is always released in agricultural systems such as livestock manure storage. Most large sized farms collect liquid manure and slurry in a reception pit before manure is pumped to the long-term storage or processed via anaerobic digestion, during which constant manure agitation and pumping events occur and lead to the release and accumulation of H₂S. It is not only an environmental concern but also a safety hazard to the agricultural workers and animals that causes casualties every year and corrodes the facilities. Conventional H₂S mitigation strategies include ventilation and some ex-situ gas cleaning technologies applied to the biogas. However, even in well ventilated swine and dairy barns, high H₂S concentrations were reported, while the ex-situ biogas cleaning technologies using biofilters or aqueous solutions (e.g., sodium hydroxide, ferrous chloride, or ferric hydroxide) are always chemical- and energy-intensive and require a separate unit housing the facilities. Therefore, an in-situ H₂S mitigation method with less energy and chemical input and easier operation is critical to the sustainable manure management.

Methods: In this study, we proposed to install a microbial electrochemical system to mitigate the sulfide generation and facilitate the aqueous sulfide oxidation and precipitation, therefore proactively preventing H₂S release from the dairy manure storage and anaerobic digestion. Different metal materials (e.g., low carbon steel, stainless steel, aluminum, copper, etc.) and non-metal materials (e.g., graphite rod, graphite sheet, etc.) were coupled to form electrode pairs, which were then inserted into bottle reactors filled with dairy manure collected from a farm. An external voltage ranging from 0.4 V to 3.0 V supplied by a direct current (DC) power supply was applied to each electrode pair. The current across the electrodes was monitored and the power consumption was then calculated. A group without electrochemical treatment was also established as the control. The biogas released from manure was collected by gas bags connected to the bottle reactors, and the biogas composition (i.e., H₂S, CH₄, CO₂, H₂, N₂, O₂) was then determined using a micro-gas chromatograph. The nutrient values (e.g., phosphorus and nitrogen) of the effluents after treatment will be compared later. The material consumption will also be compared between the sacrificial metal electrodes, and the precipitates on the electrode surface will be characterized.

Results: With low carbon steel as the sacrificial anode material at a relatively low applied voltage (≤1.0 V), the gaseous H₂S release from the dairy manure storage was reduced to almost zero, whilst the biogas generation was very close to the control without electrochemical treatment. Similarly, the copper anode also contributed to a thorough removal of gaseous H₂S release. However, the release of copper ions into the manure liquid significantly increased the toxicity to the microbes, hence leading to a significant decrease in the biogas generation. Stainless steel, which can also release ferrous ions into the liquid, was more resistant to the electrochemical treatment and required a much higher voltage to induce the anodic reactions for sulfide remediation. Similarly, for the non-metal material of graphite rods and sheets, due to the relatively low conductivity, their requirement on the external voltage was much higher, while the efficiency of sulfide removal was still limited due to the lack of sulfide precipitation, regardless of its sustainability and reusability. By comparison, aluminum was inefficient in H₂S remediation, though it exhibited a great conductivity that induced a high current initially with a low voltage and facilitated the carbon dioxide (CO₂) reduction and conversion to methane (CH₄) through electro-methanogenesis and enhanced hydrogenotrophic methanogenesis.

Implications: Different from the mainstream research on mitigation of H₂S in the gas phase, this process removes aqueous sulfide species from the liquid dairy manure, making it an innovative approach to remove H₂S in a more efficient and effective way. By treating the manure with a microbial electrochemical system, the spatial and temporal distribution of mild anodic reactions was the reason for sulfide mitigation, which created the conditions that inhibited the biogenic sulfide production, oxidized part of existing sulfide species to elemental sulfur and sulfur oxoanions, and converted another part of sulfide to insoluble minerals (in the presence of metal ions released from anodes). Among the three routes, the precipitation of insoluble sulfide minerals seems to be the major contributor to the sulfide remediation. Further experiments on the characterization of electrodes consumption are required to obtain more insights into the fundamental mechanisms. In conclusion, this proposed technology will reduce the odor, improve the safety of the farm, and retain further nutrient level of the manure for better land biofertilizer application.