Simulation of Hydrogen Sulfide Emission from Deep-Pit Manure Storage During Agitation
Abstract. Human and animal exposure to hydrogen sulfide (H2S) in animal barns has long been a serious issue due to the acute and chronic toxicity of H2S. The H2S concentration in the room air of deep-pit swine barns is usually within hundreds of parts per billion by volume; however, it can sharply increase to hundreds and even thousands of parts per million (ppm) during manure agitation and pump-out. To explore the sudden release and concentration distribution of H2S, this study collected and analyzed samples from varying depths of a normal non-foaming barn and a foaming barn and then mathematically simulated the H2S concentrations and emissions in the pit headspace and room air for both barns during pit agitation. Simulations were conducted for six ventilation scenarios, or six different combinations of pit fan and wall fan ventilation rates. The simulation results suggested that pit ventilation was more effective than wall ventilation in decreasing H2S concentration in room air where pigs may be housed during agitation. A minimal pit ventilation rate of 40 cfm per pig was necessary to lower the peak concentration in room air to less than the permissible exposure limit of 20 ppm. The simulation results also indicated that gas bubble release during agitation accounted for the main part (81%) of H2S emission in the foaming barn, and expedited molecular diffusion contributed the main part (70.2%) of H2S emission in the non-foaming barn. The disturbed air-manure interface during agitation induced a pH decrease and therefore increased the apparent overall mass transfer coefficient of H2S, resulting in a substantially increased mass transfer rate and concentration. The immediately dangerous to life or health (IDLH) concentration of 100 ppm may be reached during pit agitation if pit fan ventilation is not fully provided, and the duration of the exceedance could be more than 30 min. The results provide empirical data for future simulation of spatial and temporal H2S distribution and are beneficial for developing methods to control H2S below hazardous levels so that the health and safety of workers can be better secured.