Bioenergy Support Program - Transition (Research): Project 4C-115

Bioenergy Support Program - Transition (Research): Project 4C-115

Biogas is increasingly used at Australian piggeries to produce heat and generate electricity, with current uptake of biogas by about 13.5% of total Australian pork production. Biogas is produced by anaerobic treatment, a natural microbial process converting pig manure organic matter into valuable methane (plus carbon dioxide as a by-product). By capturing and burning the biogas in a generator, flare or hot water system, greenhouse gas (GHG) emissions are also reduced by up to 64% across an Australian pork supply chain. Income from the sale of carbon credits has been a strong incentive to adoption of biogas in Australia, with income to date from carbon credits amounting to an estimated $4.5M. The present project facilitated further uptake of biogas, by supporting Pork CRC biogas demonstration piggeries; contributing to other Pork CRC biogas related projects (4C-109, 4C-113, 4C-111), and conducting further targeted biogas-related research in support of piggeries that were installing biogas systems during the project period.

A low cost, and simple design and operation have made covered anaerobic ponds (CAPs) popular for biogas production at Australian piggeries. However, accumulation of float layers and/or sludge are on-going concerns with CAPs. Scum/crust can damage pond infrastructure or restrict methane collection and sludge displaces active pond volume over time and eventually has to be extracted. Solids separation upstream of a CAP reduces solids loading and thereby reduces sludge accumulation in a CAP. However, solids separation also removes organic matter (VS) and so decreases methane yield. The present research quantified these losses for selected solids separation techniques commonly used in the Australian pork sector. Manure samples were collected before and after solids separation equipment at commercial piggeries which were installing or commissioning biogas systems. Methane yield and total solids (TS) and volatile solids (VS) were measured. The results showed a 17-31% decrease in methane yield across screw presses and a 22% decrease in methane yield across a static run-down screen. A higher VS removal extent corresponded to a higher methane yield loss. The majority of methane yield was therefore not in the separated solids, but in the screened liquid fraction. Overall, VS reduction extent seemed to be a reasonable baseline indicator of anticipated methane yield losses by solids separation, an outcome that is important for greenhouse gas accounting, such as conducted for the Emissions Reduction Fund.

Safety and maintenance associated with high concentrations of hydrogen sulphide (H2S) is discouraging further use of biogas at piggeries. Specifically, H2S concentrations in piggery biogas is as high as 2-4 times the immediate lethal dose for humans, and 2-4 times the level readily tolerated by biogas use equipment. A number of commercial H2S removal methods are available, but these are not practical or cost-effective for Australian piggeries. To assess options, on-farm trials were conducted at two commercial piggeries. One trial tested biological H2S oxidation, adding small amounts of air to biogas upstream of an enhanced surface treatment vessel, and using CAP effluent as nutrient source for a biofilm of naturally occurring microorganisms. Results showed that treatment was very effective, removing over 90% of the H2S and reducing H2S concentrations from 4,000 ppm to <400 ppm. Another trial tested chemisorption performance of a natural iron-rich soil as a safer and more cost-effective option to commercial chemisorption media. The red soil removed H2S, but substantially less (~2 g S/kg red soil) than cg5 commercial media (~200 g S/kg media). Therefore, red soil would only be feasible for final polishing of biogas after an initial biological oxidation step removed most of the H2S. Having these two steps in sequence (biological oxidation followed by chemisorption) would help reduce H2S load on the chemisorption media, which can then last longer and require less frequent replacement. In this way operating costs are reduced and hazardous exposure to the high reactivity of commercial chemisorption media is minimised.

In the future, food by-products unsuitable as a pig feed will be anaerobically co-digested to boost methane production and meet onsite energy demands. However, based on results from joint research with an Australian Pork Limited project, it was recommended that food by-products should be used as a pig feed wherever nutritionally, legislatively and economically appropriate. This is because the value of pig feed far exceeds the value of biogas methane energy in the current energy pricing climate in Australia. Recommendations for future research includes further fine tuning and development of the H2S removal options described herein.

Final report prepared for the Cooperative Research Centre for High Integrity Australian Pork.