Enhanced methane production with anaerobic co-digestion of rice straw with ternary mixtures

Abstract

A high carbon-to-nitrogen ratio makes rice straw unsuitable for mono-digesting. This study investigates the effect of various proportions of total solids (15%, 20%, 25%, 30%) on the biodegradability of rice straw during co-digestion with nitrogen-rich substrates, such as food waste, cow manure, sewage sludge, and chicken manure. As a result of co-digestion, the process stability and volumetric yields of biogas were significantly increased. The mixture of RSCMChM (rice straw + cow manure + chicken manure) with a solids ratio of 20% reported higher biogas yield of 442 mL/gVS with a higher methane yeld of 55.88%, followed by the mixture of RSSSChM (rice straw + sewage sludge + chicken manure) with 408 mL/g-VS with a solid’s ratio of 20%. In mixtures of rice straw, cow manure, and food waste biogas production is lowest due to the pile up of volatile fatty acids from the co-substrates that can be easily biodegraded. The rice straw mixed with sewage sludge and chicken manure consistently performs better than all other mixtures at all four total solids contents. The Modified Gompertz model fitted the experimental values well and noticed rise in lag phase times ranging from 1.28 to 10.04 days in case of RSSSCM mixture. In accordance with the results of the experiment, the maximum production (P_max) of biogas obtained from RSSSChM (rice straw + sewage sludge + chicken manure) at 20% TS was 470.01 mL/g-VS and RSCMChM (rice straw + cow manure + chicken manure) at 20% TS was 447.47 mL/g-VS. Thus, ternary mixtures with rice straw enhances the digestion and biogas production.