Optimization of methane production in a swine manure–rice straw anaerobic co-digestion process with sycamore sawdust biochar application

Abstract

Anaerobic co-digestion has been widely applied to re-utilize livestock manure; however, the low methane yield is still a key issue. Biochar could play an important role in anaerobic digestion. This study aims to produce and characterize sycamore sawdust biochar (SSB), determine the promoting effect on the swine manure–rice straw co-digestion system by applying the SSB, and obtain the optimal digesting conditions by using a Box–Behnken design of response surface methodology (RSM). The results showed that SSB had a porous structure composed of micropores and mesopores. The micropore surface area reached 162.87 m2/g, and the mesopore volume accounted for 43.67% of the total pore volume. SSB was also rich in C=O surface functional groups, and the BET specific surface area was up to 276.67 m2/g. Application of SSB could improve the pH value and the stability of the co-digestion system, and eventually improve the cumulative methane yield. The maximum cumulative methane yield in this test was 217.99 mL/(g VS) when 4% SSB was added. According to the response surface optimization analysis, the cumulative methane yield could be 247.20 mL/(g VS) under the optimal conditions: 94.54% swine manure, 8.78% TS and 5.49% SSB.