LAUSR

Abstract World energy demand is growing and the negative effects of greenhouse gases (GHGs) and climate change are being felt more acutely. Accordingly, technologies that reduce GHG releases and produce renewable energy, such as anaerobic digestion (AD) with combined heat and power (CHP) systems, are potentially attractive for agricultural wastes, including rice straw (RS). Asia produces over 500 Mt of RS per year that is usually burned, wasting potential energy, causing air pollution and GHGs, and having negative health impacts. Therefore, making RS AD options more attractive is urgently needed. This paper shows biomethane (CH 4 ) yields from infrequently fed RS AD units, which match better with RS harvest production cycles, can be very efficient at specific CH 4 production without the need for co-digestion. Using Biomethane Potential (BMP) data to guide AD reactor conditions, five feeding frequencies (FFs) were operated for over 250 days in bench-scale units, ranging from five feeds per seven days (5/7; frequent) to one feed per 21 days (1/21; infrequent), using OLRs of 1 g VS/L/d and 2 g VS/L/d. Highest specific methane yields (148 ± 6.3 mL CH 4 /g VS/d) were observed at 1/21 FF and the lower OLR. In contrast, highest volumetric yields were seen for a FF of 1/7 at 2 g VS/L/d (276 ± 10.6 mL CH 4 /L/d), although AD units failed at this OLR for FFs of 1/14 and 1/21 due to volatile fatty acids accumulation. This study shows RS AD is feasible without co-digestion, producing biogas that can be coupled with CHP technology to provide renewable energy. However, less frequent feeding regimes performed better than more frequent feeding regimes, suggesting infrequently-fed batch AD units may be a better option for biomethane production, especially for rural locations.