Optimization on influencing factors of rice chopped straw anaerobic digestion

Abstract: Mechanical size reduction of the straw ruptures the cell walls and makes the organic matter more readily available for the microorganisms to decompose. However, crushing equipment and energy are demanded to change the particle size. Fermentation process of chopped straw with larger length and higher biogas yield can reduce the energy demand and improve the operating profit of biogas plant. In the study, chopped rice straw was investigated on the main factors affecting digestion process, including temperature (30℃, 35℃, 40℃), TS（ total solid）(6%, 8%, 10%), particle size (1cm, 2 cm, 3 cm) and retting period (1 d, 2 d, 3 d). The solid ratio of inoculum to substrate was kept at 3:7 for all the experimental setups. The main response selected for analysis was biogas yield. The study was a two-part test. In test 1, the optimum combination was determined by four-factor and three-level orthogonal test; each reactor had a capacity of 1000 ml and contained 500 ml liquid, and batch experiments were carried in water bath for 40 days. In test 2, after determination of the optimum combination, the verification experiments were carried in a constant incubator, the reactors were replaced by vacuum flasks with a capacity of 2 500 mL and contained total liquid of 1500 ml, and parallel reactor of TS=2% (temperature of 40℃, 2-day retting and particle length of 1 cm) was also run to verify the phenomenon of self-heating during the rice anaerobic digestion. The results showed that temperature was the leading factor to biogas production potential, and the two had a significant relation(P＜0.01). Optimal digestion conditions were defined, that was, temperature of 40℃, 2-day retting, TS of 8% and particle length of 1 cm. Maximum biogas yield of volatile solid was 385.9 mL/g, achieved by test 2. It has reached a high biogas production potential according to the current fermentation research knowledge. During the tests, biogas yields of reactors at 40℃ were higher than those at 30℃ and 35℃, and all of the cumulative biogas productions in early 20 days were above 73% of total. In test 1, liquid temperature in reactors was observed higher than water temperature in water bath in several days of testing; in test 2, liquid temperature in the reactor of TS=8% was higher than those in the reactors of the comparisons throughout the test. It indicated that the phenomenon of self-heating in anaerobic reactors occurred in rice anaerobic digestion. The result is coincident to the thermodynamic knowledge that a little heat can be produced by anaerobic digestion. The self-produced heat is helpful to raise the digestion liquid temperature and reduce energy input to meet the demand of digestion. What's more, it means that the chopped straw biogas production is profitable when the biogas yield is above 299.3 m3/t, the local biogas price is 1.5 RMB/m3, and rice straw price is 226 RMB/t. The collection and treatment of chopped rice straw through anaerobic digestion is a viable option for producing clean, renewable energy with less equipment input and a high biogas yield. Research on feeding and discharging technology still needs to be conducted before this concept becomes a preferred treatment option.