Can micro-scale anaerobic digestion be viable? An investigation of the techno-economics and flexibility of micro-scale anaerobic digestion for future routes to the market.

In the developed world, anaerobic digestion (AD) is commonly installed on a large scale (over 500 kWe) and far less at micro scale (less than 50 kW). One reason for this discrepancy is that there is an economy of scale. However, micro-scale AD has potential advantages in social, technical and environmental terms. This thesis aimed to evaluate and quantify these advantages, through a combined experimental and techno-economic approach. Flexible biogas production can enhance AD profitability and was investigated experimentally. Two experimental streams were run with different loading patterns and the operation and stability were studied under the different conditions. Under a variable load pattern, the test digester showed better volatile solids degradation, a more pronounced immediate response to feeding events, and a higher methane production rate than a digester fed at a continuous load. An operational 2 m3 micro-scale AD plant in London, UK was monitored for a year whilst running on local food waste. The plant averaged a processing rate of 12.6 kg day-1 and achieved a specific methane production of 132.4 m3 CH4 tonne-1 wet waste with an average biogas methane content of 60.6%. Signs of ammonia toxicity were successfully addressed by the addition of a trace element solution. The plant had a simple payback period of 148 years due to low revenues. A technoeconomic analysis was performed for a theoretical micro-scale AD plant with a yearly input of 119 tonnes of food waste and 6 tonnes of vegetable oil. The simple payback time of the scenarios ranged from 5.4 to 11.9 years. The best solution included a biogas boiler and a composting system, adding cardboard and green waste to the digestate output. A sensitivity analysis showed that the simple payback time was most affected by the value of compost, the value of electricity and by government initiatives such as the Renewable Heat Incentive.