Effect of combined addition amount of nano zero-valent iron and biochar on methane production by anaerobic digestion of corn straw

Abstract: Anaerobic digestion (AD) is an attractive straw resource treatment technology as it can improve the utilization efficiency of straw resource. Raw straw materials contain complex polymers, so some enhancements are needed to achieve better biodegradability. The addition of various additives has become an effective method to improve the AD efficiency, among which the effect of nano zero-valent iron (NZVI) and biochar (BC) on AD has become a research hot spot. In this paper, the powder of NZVI and BC (mixing ratio 1:1) was used as additives to study the effect of different addition amount (3%, 6%, 9%, 12% and 15%) on the AD of corn straw for methane production. The cycle of AD was 28 days, the fermentation temperature was 35 ℃, and the total solid (TS) concentration was 4%. The combined addition of NZVI and BC enhanced the pH stability of the digestion process and the degradation of organic acids. The greatest enhancement of methane production was obtained when the combined addition amount of NZVI and BC was 9%, and the cumulative methane production was 151.06 mL/g VS, which is 20.73% higher than the control group. The combined addition of NZVI and BC could increase the methane content within a certain range, but an inhibitory effect was observed when exceeded 9%. When the addition amount reached 12% and 15%, the cumulative gas production and cumulative methane production of corn straw AD were inhibited to varying degrees. The VS removal efficiency was the highest in the group with the addition amount of 9%, which was 20.41% higher than the control. The modified Gompertz equation fitted well with the maximum methane production rate (Rm) and lag time (λ) when the addition amount was 9%, with high correlation coefficients. Considering that NZVI could be recovered by magnetic separation to further reduce the cost of additives, while the cost of biochar was relatively low, it was believed that the crop straw AD technology had certain commercial application value. Graphic abstract: [Figure not available: see fulltext.]