Comparative evaluation of high solids anaerobic digestion of rice husk and rice straw: Impact of substrate characteristics and solids concentration on biogas yield and digestate quality

Open burning of crop residues in agricultural fields causes soil infertility and contributes to global warming. Solid-state or dry anaerobic digestion (AD) is an emerging process for converting high solids (90 ̶ 95%) biomass such as rice husk (RH) and rice straw (RS) into energy and resources without the need for large volmes of water. This paper describes laboratory experiments investigating the dry AD of RH and RS across a range of total solids (TS) concentrations from 20 to 60%. Conducted in 500 mL bioreactors under batch mode at mesophilic temperatures, the experiments had a batch retention time of 45 days. An increase in the F/I ratio from 8.37 to 25.01 correlated with a rise in biogas generation from 2.9 to 4.6 L in RH reactors. The highest biogas yield from RS (0.49 L/g VS) and RH (0.17 L/g VS) was obtained at 20 and 50% TS, respectively. From the mass balance, 68.7 g of biomanure and 4.6 g of biogas were produced from the 50H reactor, while the 20S reactor yielded 20.9 g of biomanure and 8.4 g of biogas. From the modified Gompertz model a shorter lag phase of 1.9 days was observed in reactor 50H compared to the 4.6 days observed in reactor 20S.