(94d) Bio-Electrochemical Treatment of Dairy Manure for in-Situ Sulfide Remediation

As a highly toxic and corrosive gas with a characteristic rotten-egg odor, hydrogen sulfide (H₂S) emission spikes are often detected during dairy manure storage because of high levels of organic content (e.g., sulfur-containing proteins) and sulfate-containing minerals in feed and bedding materials. Most large sized farms collect liquid manure and slurry in a reception pit (or transition pit) before manure is pumped to the long-term storage or processed via anaerobic digestion, and these reception pits are often close in proximity to the barn animals and farm workers. Thus, this can be an emission point of H₂S and offensive odor, especially when manure agitation and pumping events occur. In addition to the toxicity to farm workers and animals, H₂S in the air of pits as well as in the subsequent biogas can be very corrosive for the equipment and infrastructure. All these clearly show that mitigating the H₂S generation during dairy manure storage and treatment is important and urgent. Conventional H₂S mitigation strategies typically include biogas cleaning technologies using biofilters or aqueous solutions (e.g., sodium hydroxide, ferrous chloride, or ferric hydroxide), which always require a separate unit housing the facilities and are chemical- and energy-intensive. Therefore, an in-situ H₂S mitigation method is critical to the sustainable manure management with less energy and chemical input as well as an easier operation. In the present study, a bio-electrochemical unit was integrated into the dairy manure storage and anaerobic digestion treatment for in-situ H₂S remediation. Through the lab-scale tests, a series of materials, including sacrificial metals (i.e., low carbon steel, stainless steel, aluminum, and copper) and non-sacrificial materials (i.e., graphite and titanium), were examined for the efficiency of H₂S removal, among which low carbon steel anodes showed the most effective H₂S removals (over 95%). This implied that the precipitation of insoluble sulfide minerals seems to be the major contributor to the sulfide remediation, rather than anodic sulfide oxidation. Subsequently, an experiment to determine the optimum applied voltage to the electrode pairs (anode: low carbon steel; cathode: stainless steel with a longer lifespan) for sulfide removal were conducted. It was recorded that an applied voltage of 0.6 V was high enough to release ferrous ions from the low carbon steel anode into dairy manure for ferrous sulfide precipitation as well as lead to higher methane production. In contrast, higher applied voltages over 0.8 V resulted in faster anode material consumption and triggered significant electrochemical hydrogen production with the inhibition of methane generation. Then, at the optimum voltage of 0.6 V applied to the low carbon steel-stainless steel electrode pair, strategies of intermittent electrochemical treatment were examined aiming to reduce the electricity and anode material consumption. It was observed that the electrochemical treatment of 6h/6h (on/off) and 12h/12h (on/off) were efficient in H₂S remediation, whereas that of 3h/3h (on/off) did not reduce the H₂S release effectively, indicating that a continuous electrochemical treatment period over 3 h was definitely required to properly function. With all the selections and optimizations above, a pilot-scale electrochemical unit was accordingly designed and then installed in the dairy manure pit in a local dairy farm in Minnesota, and its effects in in-situ H₂S remediation in a real application scenario were documented. In summary, this proposed bio-electrochemical system can successfully reduce the odor and improve the safety of a dairy farm during manure storage and treatment.