Enhanced methane production from pig manure in covered lagoons and digesters: Project 4C-109

Enhanced methane production from pig manure in covered lagoons and digesters: Project 4C-109

Methane production in covered anaerobic ponds or anaerobic digesters is a natural process whereby microorganisms convert manure into methane. Manure methane is increasingly used at Australian piggeries to produce heat and generate electricity. Currently, about 13.5% of total Australian pork production captures manure methane, and the majority use it in engine generators or hot water systems. System design and operation, the methane potential of the waste (e.g. manure or other) and the health of the microorganisms, dictate the performance of a covered anaerobic pond or anaerobic digester in terms of methane production.

A common strategy to produce more methane is anaerobic co-digestion, whereby two or more wastes are simultaneously digested. Abroad, a number of carbon-rich wastes are regularly co-digested with animal manures. Examples include vegetable by-products, industrial organic wastes, agricultural industrial by-products and residues, food wastes, fodder and brewery wastes, organically rich industrial wastewaters, and biofuel and biorefinery by-products. Methane yields vary widely between different wastes. Some wastes are excellent methane boosters, being highly biodegradable and/or are very concentrated. However, co-digestion should be carefully managed to prevent unsafe organic loading rates and/or to prevent chemical inhibition. Some other wastes require much longer treatment times than pig manure to be adequately converted into methane. The availability of co-digestion wastes is an on-going challenge in Australia. Transport costs and the availability of wastes dictate cost feasibility. A major future incentive could be the revenue from gate fees to divert waste away from landfill and instead sustainably converting them into methane for beneficial use. Unfortunately, in Australia, the imposition of landfill levies that drive such gate fees is a relatively recent and uncertain phenomenon, and varies from state to state. Unique waste handling infrastructure may be required for co-digestion (e.g. high solids) and should be considered during the planning and design stages of a biogas project. Systems designed for piggery loadings may have limited capacity to co-digest other wastes.

Desludging of covered ponds has been an on-going concern with biogas use in the Australian pork sector. Settled sludge eventually displaces active pond volume and requires removal. The present research project measured residual methane potential in sludge samples extracted from covered ponds at Pork CRC demonstration piggeries. At these piggeries, sludge is readily extracted by pumping via extraction ports, do not require removal of the pond cover, and the pond can remain in full operation whilst being desludged. The results showed that the sludge samples were reasonably stable, with over 50% of the organic matter already converted into methane. This was also the case for ponds with very short desludging periods of 1 year. It could be beneficial to decrease frequency of desludging to every 2 years. However, some piggeries may elect to desludge more frequently to manage water balances during wet/dry seasons and to sustainably apply sludge nutrients to cropland. Sludge that is extracted less frequently than every 2 years could become excessively consolidated inside the covered pond, and could then become more difficult to pump/extract.

Chemical inhibitors, such as sodium or ammonia nitrogen, can affect anaerobic digestion of pig manure. In this
research, inhibition resilience and adaptation of microbial
communities were assessed using inoculum samples from full-scale and pilot scale digesters, and separately by subjecting microbes in continuous digesters to chronic high levels of inhibitor. The aim was to identify intervention options to promote inhibition resilience. The results showed that inhibition resilience varied moderately between different inoculum sources, with some microbial communities being more resilient than others. Also, subjecting microbial communities to chronic inhibitor stress showed clear acclimation. These results were encouraging, because accordingly, microbial communities could adapt to inhibitors. Unfortunately, no statistical links were found between intervention strategies and inhibition resilience. Instead, acclimation occurred naturally, and could not be greatly promoted by intervention. The only way was to chronically expose microorganisms to high levels of inhibitor, which then promotes adaptation, but the negative impacts on digestion performance would probably negate this option in practice. In general, when an inhibitor is unavoidable, it should be gradually introduced into the covered pond or digester, to allow time for natural microbial acclimation.

Overall, there is considerable opportunity for co-digestion in Australia, including as a way to deal with chemical inhibition. Future research is needed to better understand the effects of temperature and organic loading rate on co-digestion performance, especially because ambient temperature systems are common in Australia because of a temperate climate. It is also worth exploring in future research, the growing of microbial inoculums in smaller dedicated systems under a stress condition such as inhibitor presence, and then using these adapted microbial inoculums to seed a larger digester or covered pond in preparation for exposure to the same stress condition.

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