Simultaneous effect of mixing and substrate to microbial ratio on anaerobic digestion of food waste

Abstract

Anaerobic digestion (AD) is an environmentally-friendly way to treat food waste (FW). FW is regarded as a potential substrate of AD due to the high organic content in FW. However, there are still challenges in the digestion of FW since it has daily and seasonal fluctuations, complex compositions, and low pH values. Therefore, it is required to find appropriate operating parameters in the anaerobic digestion of FW. In this study, mixing and food to microorganism ratio (F/M) were chosen as independent variables. Response surface analysis (RSA) with central composite face-centered (CCF) design was conducted to study the effect of mixing intensity and F/M ratio on biogas production rate and methane yield. In the first study, FW (non-filtrated) was used as substrate and in the second study, FW was filtrated by 0.45 μm filters to reduce the hydrolysis step and focus on biogas production. The mixing intensity had a positive effect on both biogas production rate and methane yield in the digestion of FW. The F/M ratio increased the biogas production rate but not methane yield in the first study. It may be because higher concentration of VFA accumulation during the process inhibited methane yield. The results of filtrated FW were quite different from FW. The maximum biogas production rate and methane yield were obtained at 108.7 rpm and 115.7 rpm each and then the response values decreased above that. The different results between the first and second studies may be because higher mixing intensity was required to disintegrate FW that was not filtrated, and vigorous mixing had a negative impact on methanogenesis in the digestion of filtrated FW. It could be concluded that the effect of mixing intensity can be different depending on the complexity of the substrate, while the F/M ratio had a similar effect.