In spite of global efforts to reduce the generation of food waste, overwhelming quantities are still generated annually. In the United Kingdom for example, a third of the food crops produced annually for consumption end up in the bins. Anaerobic digestion (AD) is currently the most suitable technology for treating food waste, providing energy in the form of methane. However, the highly organic nature of food waste enriches the release of nutrients up to levels, which can be toxic or inhibitory to the acting microorganisms. As a result, the biomethane yields are much lower than the theoretical potential. This study investigates the possibility of improving the stability of AD and enhancing biomethane yield from mono-digestion of food waste, by a sequential optimisation of the biomethane production process.
The first level of optimisation was to identify suitable combinations of food waste particle size and microbial availability (inoculum-to-substrate ratio – ISR), to improve the process stability and biomethane yield. This investigation revealed that PS reduction (≤ 3 mm) resulted in a rapid digestion of food waste, and while this is expected to result in higher rates of acidification within the system, the variation in ISR helped to reduce such effects. Hence, an optimum condition of 1 mm PS and 3:1 ISR was determined; resulting in 38% increase in methane, and was used henceforth.
The second level of optimisation explored the potential for incorporating biomethanation into food waste AD. To optimise the conversion of the injected hydrogen to biomethane, three hydrogen injection points were investigated. As a result, 12.1%, 4% and 9.6% increases in biomethane yield were achieved, when hydrogen was added before hydrolysis, at the peak of acidification and during active methanogenesis respectively.
The third level of optimisation adopted the principle of acclimation to further improve the biomethane yield and explore the possibility of using formic acid (FA) as an alternative source of H2. The H2-acclimated systems performed better than the FA-acclimated systems, and yielded up to 81% biomethane against 65% without acclimation. Based on the results obtained in this study, it is possible to obtain up to 98% biomethane content, with continuous hydrogen acclimation. This reveals that the energy and revenue potential of food waste AD can be improved, by opening up multiple end uses beyond combined heat and power, such as gas-to-grid injection and vehicle fuel.
