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

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 m(2)/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 m(2)/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. Graphic abstract

Automated summary

Anaerobic co-digestion is widely used for re-utilizing livestock manure, but the low methane yield remains a key issue. This study produced and characterized sycamore sawdust biochar (SSB) and investigated its promoting effect on a swine manure-rice straw co-digestion system. The results showed that applying SSB could improve the stability of the system and increase methane yield.