Optimization of methane production during anaerobic co-digestion of rice straw and hydrilla verticillata using response surface methodology

Anaerobic co-digestion is a realistic approach to concurrently manage agriculture residue and harness renewable energy. To achieve the anaerobic degradation requirements and to recompense the nitrogen deficiency of rice straw, it should be digested with another nitrogen-rich co-substrate to advance its characteristics. This study exhibits the necessity of co-digestion to improve physicochemical and biochemical progression compared to mono-digestion. The individual effect of carbon/nitrogen (C/N) ratio, food/microorganisms (F/M) ratio and pH, in addition to their interaction effects on methane yield (mL CH4/g-VSadded) were explored in this study. A central composite design - response surface methodology was used for defining the experimental design for anaerobic co-digestion of rice straw and hydrilla verticillata. Results of this study showed significant interaction of C/N ratio and F/M ratio, and individual response parameters on methane yields. The optimum condition for anaerobic co-digestion (C/N ratio 29.7, F/M ratio 2.15 and pH 7.34) showed methane yield of 287.6 mL CH4/g-VSadded, 1.84 fold (156.32 mL CH4/g-VSadded) higher than mono-digestion (control). Model validation proved the high adequacy of the model and methane yield is good output response variable for co-digestion study and it is necessary to optimize the transient variation in C/N ratio, F/M ratio and pH.