Co-digestion of swine manure and corn stalks with biochar as an effective promoter: An optimization study using response surface methodology

Anaerobic digestion (AD) of high solid swine manure has been facing the problem of inhibition and accumulation of ammonia. In order to improve the waste treatment efficiency as well as to obtain a higher methane yield, adding carbon-rich raw materials seems to be a good choice to optimize the carbon to nitrogen ratio (C/N) of the substrate in AD system. Recently, biochar has become an important exogenous additive to relieve the ammonia inhibition of AD due to its excellent mesoporous structure and adsorption performance. Therefore, in order to investigate the influence of three factors including total solid concentration (TS), amount of added biochar and C/N on biogas production rate, a regression equation was obtained using quadratic orthogonal rotation method by a five-level, three-factor experiment. The results of variance analysis showed that the p value of the model was smaller than 0.0001, which was quite significant. The out of fitness of the model was 0.3507 > 0.1, so the fitting effect is good. The optimum theoretical methane yield of 269 mL/g VS was obtained under the parameter combination values of 6.18% (TS), 8.73% (amount of added biochar) and 21.69 (C/N), respectively. The order of the importance of selected parameters from highest to lowest was TS > C/N > biochar addition.