Effects of anaerobic co-digestion of different proportions between rice straw and chicken manure on biogas yield rate

Abstract: For anaerobic digestion of straws, current researches mainly focus on pretreatment of materials, improvement of reactor or process and regulation of nutrition. Traditionally, adding chemicals such as urea or ammonium bicarbonate was employed in nutritional regulation. Co-digestion of straws with other nitrogen-rich materials is a feasible method for nutritional regulation. Chicken manure is a kind of nitrogen-rich material with C/N below 15:1. In order to obtain the optimal proportion of raw materials for anaerobic co-digestion of rice straws and chicken manure, anaerobic batch co-digestions of rice straws (C/N 47.5) and chicken manures (C/N 11.2) were carried out in anaerobic bottles of 2.5 L at initial volatile solid (VS) concentration of 6% at 37℃. The effects of rice straws and chicken manures VS ratio (0:1, 1:2, 1:1, 2:1, 1:0) on anaerobic digestion were investigated. The pH value, volatile fatty acids and ammonia concentrations were monitored to estimate the stability of process. The actual methane yield and the ratio of actual methane yield to theoretical methane yield were both used to evaluate the efficiency of anaerobic co-digestion. The C/N of five kinds of mixed substrates was 11.2, 14.8, 17.8, 22.4 and 47.5. Theoretical methane yields based on volatile solid were 381.88, 365.61, 357.47, 349.83 and 333.06 mL/g respectively. The results showed that there were no inhibition of volatile fatty acids (VFAs) and ammonia for all digestions with pH of 6.8~8.2. The maximum concentrations of acetic acid, propionic acid, butyric acid and ammonia nitrogen were 1958, 2480, 2576 and 2600 mg/L. Compared with mono-digestion of rice straws or chicken manures, co-digestion can significantly increase biogas and methane yields. Methane yields based on volatile solid were 212.43, 240.45, 250.28, 206.09 and 178.03 mL/g for VS ratio of 0:1, 1:2, 1:1, 2:1, 1:0. Methane yields based on total solid were 343.98, 411.85, 446.19, 371.09 and 293.58 mL/g. Average methane volume fractions were 58.8%, 50.5%, 50.8%, 49.2% and 50.1% respectively. When C/N of mixed substrates were between 11.2-47.5, the ratio of actual value and theoretical value (Ya/Ym) was increased and then decreased for biogas and methane yields with maximum Ya/Ym at C/N of 17.8. For engineering application, the rice straws and chicken manures VS ration of 1:1 or C/N of 17.8 for mixed materials was recommended. And designed retention time and biogas yield based on volatile solid were 23 d and 446 mL/g.