Climate - Adaptive anaerobic digestion of food waste in household digesters: Insights from extreme temperature conditions

Abstract

Anaerobic digestion (AD) is a widely used method for organic waste treatment, but the energy requirement for temperature control poses challenges, especially for household digesters. This study focuses on the semi-continuous AD process of FW in household digesters at an ambient temperature in Pilani, adjusting organic loading rates (OLR) based on daily temperature fluctuations. Additionally, it assesses ammonium removal efficiency during the degradation of FW using a low-cost magnesite-derived stabilizing agent. The findings highlight the feasibility of generating inoculum from cow dung in anaerobic conditions within a few weeks. The average specific biogas production was 0.591 ± 0.20 m³/kgVS, with a 58 ± 3 % methane concentration. The addition of the stabilizing agent resulted in a 30.3 % increase in biogas production by precipitating struvite, which led to a 22.6 % reduction in total ammonia nitrogen, thereby preventing the inhibition of methanogenic bacteria. Characterization studies, including FTIR, XRF, XRD, and SEM analyses, validate the stabilizing agent's formation and struvite precipitation. However, during a sudden and significant drop in winter temperatures, biogas production decreased to 0.210 ± 0.052 m³/kg VS, with methane content falling to 49 %, highlighting the need for microbial acclimatization. The study indicates that anaerobic digesters can operate effectively at low temperatures with a reduced OLR when the microbial community is adequately acclimatized. Furthermore, the effluent characteristics post-digestion exhibits favorable nitrogen and potassium values, and phosphate recovery through struvite precipitation is evident. Economically, the study demonstrated that replacing non-subsidized LPG with biogas yielded a pay-back period of 6 years and an internal rate of return of 15.6 %. Additionally, the challenges of household biogas production and corresponding potential recommendations are thoroughly addressed. The study highlights the potential of the investigated AD system at ambient conditions, incorporating cost-effective innovations to enhance the efficiency of the process. It provides valuable insights for decision-makers and waste management planners, extending its relevance beyond Pilani to similar settings.