Interest in the use of biogas from anaerobic digestion has been increasing within the Australian pork industry in recent years, driven by a significant potential for biogas use to buffer rising energy costs and to reduce carbon emissions from individual piggeries and across the whole pork industry. A recent life cycle assessment study suggested that a 64% reduction in piggery GHG emissions could be achieved by installing biogas capture and use systems. Further Government incentives have also contributed to the growing interest in on-farm biogas.
One of the major obstacles to adoption of on-farm biogas technology in the Australian pork sector is the presence of relatively high concentrations of hydrogen sulphide (H2S) in raw piggery biogas, commonly in the range of 500 to 3000 ppm. Smelling like rotten eggs, H2S is highly toxic and corrosive. Exposure to H2S, even at relatively low concentrations, can result in severe human health impacts, while corrosion and increased maintenance of biogas use equipment necessitates some form of biogas treatment to remove H2S to suitable levels.
Many of the existing biogas treatment technologies used in other industries are not ideally suited for on-farm application in the Australian pork industry, because on-farm systems must be relatively simple, low-cost, safe, robust and scalable, producing minimal hazardous waste products. However, a literature review, which examined various existing biogas purification technologies, identified biological oxidation of H2S and chemisorption with iron-based solid media as potential options for piggeries, provided that some key research and development gaps could be addressed. A particular issue with regard to chemisorption was the high cost of replacing commercial chemisorption medium (accounting for up to 5% of the savings derived from biogas use). Of further interest was the observation that the active components in commercial chemisorption media were also relatively common in natural materials such as soils, and even in some agricultural and industrial waste and by-products. However, such alternative chemisorption media required targeted laboratory and on-farm testing. With regard to biological oxidation, there was a need to determine whether the treated effluent outflow from a covered anaerobic lagoon could be used as a viable nutrient source in an external packed column system. This concept required testing on-farm.
To assess chemisorption options, a detailed and carefully designed laboratory study tested and compared the H2S removal capacity of a commercial iron-based medium (cg5) with that of a range of low-cost alternative media. The results of these trials indicated a far superior performance of a commercial cg5 medium, probably due to its engineered high porosity and high iron content (the active ingredient). However, a locally sourced red soil was a potentially feasible alternative medium, with reasonable chemisorption capacity and likely low cost and ready availability, depending on the piggery locality. While the pressure drop through the red soil bed was ten times that of the commercial cg5 pellets, this was effectively reduced by mixing the red soil with a bulking agent (sugar cane mulch, SCM), albeit with a dilution of the active ingredient. An unexpected but highly favourable increase in H2S chemisorption capacity was observed following repeated regeneration of used red soil media by exposure to air, perhaps due to mechanical disruption or chemical reaction.
The laboratory data was extended to on-farm trials which confirmed the performance of the red soil and cg5 media. Interestingly, the on-farm chemisorption performance of both media was superior to the laboratory results, possibly due to the ingress of traces of oxygen with the biogas, causing continuous regeneration. The on-farm performance of a low-cost, biological H2S treatment system was also tested, using treated CAL outflow as the liquid nutrient source sprayed over the column packing. The results were very promising, showing removal of over 90% of the H2S from the raw biogas, reducing H2S concentrations from 4,000 ppm to less than 400 ppm. The recycled CAL effluent proved to be an effective liquid nutrient source, without needing excessive air addition to meet O2 scavenging requirements of residual carbon in the liquid, and with no notable changes to the prevailing pH of the liquid in the biological vessel. These results suggested that a simple biological oxidation system has considerable potential as a low-cost option for removing bulk H2S from raw piggery biogas.
Overall, the relatively low chemisorption capacity of the red soil+SCM medium suggested that it would not be suitable as a primary treatment medium, but with the highly effective biological treatment step first removing the bulk of the H2S, perhaps red soil mixtures could be feasible for polishing of biogas to a consistent quality. The potential economic viability of this scenario was assessed in a feasibility analysis given in the thesis. Further testing of red soil in tandem with biological oxidation is recommended, as are regeneration studies to establish the actual chemisorption capacity of red soil mixtures and viable means to regenerate these media.
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