Enhanced methane production from anaerobic co-digestion of rice straw and hydrilla verticillata and its kinetic analysis

Abstract

Rice straw is a widely existing lignocellulosic waste with high potential for methane production. However, high carbon/nitrogen (C/N) ratio of rice straw causes the poor process stability for mono-digestion. This study investigated the co-digestion of rice straw and hydrilla verticillata (nitrogen rich co-substrate) at different C/N ratios to enhance biomass degradation. Different C/N ratios revealed notably enhanced process stability and higher volumetric methane yields than mono-digestion (control). C/N-25 (Co-digestion) showed the highest methane productivity with an increase of 40% than the control. This increase was caused by the improvement of the metabolic pathway due to proper nutritional structure (C/N). C/N-15 obtained the lowest methane yield due to ammonia inhibition (315 mg/L). Biodegradation reaction kinetics parameters were also calculated during the digestion process and it showed reasonably good agreement between the experimental and predicted value (R2 > 0.98) for modified gompertz model. The average Rmax of C/N-25 was 2.78% less than Rmax observed in control. The favourable reducing environment established during the degradation stage was represented by low oxidation-reduction potential (−312.8 mV) values. Since no added energy inputs are required for this process, it is highly practicable on farm-scale in contrast to pretreatment.