Co-production of biogas and humic acid using rice straw and pig manure as substrates through solid-state anaerobic fermentation and subsequent aerobic composting

Compared with wet anaerobic digestion, solid-state fermentation possesses many merits such as low water consumption, high biogas yield and low processing cost. In this work, co-producing biogas and humic acid (HA) by two-step solid-state fermentation was innovatively investigated using rice straw and pig manure as materials. The result indicates that C/N ratio, straw particle size, and total solid content (TS%) caused significant effects on the solid-state fermentation process. At the first step for anaerobic biogas fermentation, the optimal fermentation conditions included C/N ratio of 27.5, straw particle size of 0.85 mm and TS% of 25%. The maximal biogas productivity and methane content were up to 0.43 m3/(m3·d) and 64.88%, respectively. This means that biogas production was significantly improved by adjusting C/N ratio during the co-fermentation of rice straw and pig manure. Following, the digested residue was aerobically composted for HA biosynthesis to improve the fertilizer efficiency of the fermented substrate. The optimal aeration rate of 0.75 L/min was obtained, and the volatile solid (VS) degradation rate, HA content, and the germination index (GI) value were up to 19.16%, 100.89 mg/g, and 103.07%, respectively, which indicates that HA biosynthesis and compost maturity were significantly enhanced. Therefore, the co-production of biogas and HA using rice straw and pig manure as fermentation materials was achieved by adopting the two-step solid-state fermentation, and the bioconversion efficiencies of livestock manure and straw were significantly improved.