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Performance and feasibility of small-scale anaerobic digesters of food waste

Lou, Xian Fang (2013) Performance and feasibility of small-scale anaerobic digesters of food waste. PhD thesis, Murdoch University.

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The majority of putrescible wastes currently generated in Australia are disposed in landfills, placing increasing stress on current infrastructure and the environment. Anaerobic digestion offers a technological option that cannot only reduce Australia's dependency on landfills, generates a source of renewable energy but also helps to mitigate greenhouse gas (GHG) emissions. One benefit of anaerobic digestion is its ability to be undertaken at various scales. Unfortunately, the study of small-scale digesters utilizing food waste remains limited, and feasibility studies are currently unavailable in Australia. This study aimed to evaluate the performance and determine the feasibility of energy generation from small-scale anaerobic digestion of food waste in Australia.

These aims were achieved through the development of a decision support tool which can estimate the digester's technical performance together with the cost-benefits analysis. The technical performance used in the tool was based upon laboratory studies conducted to yield the kinetic coefficients of food waste digestion at 38°C and 28°C. Field studies of small-scale digesters treating food waste were also conducted to, firstly contribute quantitative data pertaining to the technical performance of small-scale digesters treating food waste, which is currently lacking in the literature and secondly to validate the decision support tool. Sensitivity analysis using the model was then performed to determine the minimum digester size required to enable the technical feasibility of electricity generation and the minimum digester size to ensure financial feasibility in terms of payback period and the ability to achieve grid parity. The tool was also used to determine whether the benefits from the installation of temperature regulators would be a worth while investment for a small-scale digester in Australia.

Results revealed that although CH4 can be generated from digesters of any capacity, a minimum of 10.9 m3 is required to ensure financial feasibility. This implies, at current energy tariffs and cost of digester, payback and grid parity is not achievable for household applications. In addition, the minimum size required for electricity generation varies between 3.5-6.3 m3; however only digesters larger than 30 m3 may be financially worthwhile in terms of achieving grid parity, and larger than 17 m3 to achieve a reasonable payback period. Therefore, unless there are strong demands for electricity generation, digesters smaller than 30 m3 are strongly discouraged from utilizing the gas for electricity generation purposes and should be used for heat generating purposes instead. Installing temperature regulators will allow for a higher rate of waste degradation, higher gas yield, require a smaller digester capacity and is more cost effective (in terms of payback period) as compared to a digester without temperature regulators treating the small amount of waste. It is thus recommended that for small-scale digesters in Australia, temperature-regulated digesters should be installed to ensure a functional temperature all year round especially during the colder months.

Finally, the potential of energy generation and GHG mitigation from food waste anaerobic digesters in Australia was estimated and case study scenarios were created evaluated and analyzed. It was found that the maximum energy generation potential from anaerobic digestion of food waste was approximately 9.82 PJ/year which would divert up to 2 million ton of putrescible waste from landfills mitigating up to 39% of total GHG emissions. While it is difficult to assess and determine the demand and likelihood of anaerobic technology in Australia, there is a strong indication of a large potential of small-scale anaerobic digestion for the Northern Territory due to the large percentage of remote communities in the State, the high potential energy yield per capita and the prevailing tropical climate throughout the year.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Nair, Jaya and Ho, Goen
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