Effect of organic load on performance and bioavailable metal concentration of straw anaerobic digestion system

Abstract: Although anaerobic digestion is an important way of straw resources utilization, methanogenesis is the rate-limiting step of this process and then metal element could have pivotal importance to methanogens. Rumen fluid and anaerobic sludge were used as inocula, whose content was 25%, and their main components and metal concentration were investigated. Using straw materials, the anaerobic digestion performance and metal concentration change were investigated with 6 organic loading rate (OLR: 1.5, 2, 3, 4, 4, 6 g/(L·d)) and solid retention time (SRT: 100, 100, 100, 100, 66.6, 40 d) in a semi-continuous reactor. Stirring speed was 80 r/min, pH value was controlled at 6.8-7.1. The result indicated that CH4 yield reached 217 mL/g volatile solid (VS) at OLR of 4 g/(L·d) and SRT of 66.7 d, which reached 56%-58% of the theoretical value. The xylanase and cellulase activities in this operating condition were higher than those of other stages, which were 6.45 and 50.06 U/mL respectively. Modified Tessier method was adopted to analyze the distributions of metal speciation in 6 different operational conditions. It was shown that bioavailable metal cofactor was supplied by Zn, Cu, Fe, Ni and Mo in the co-inoculum of rumen fluid and anaerobic sludge during the methanogenesis process, which was indispensable to hydrolytic and methanogenic activities. At the OLR of 1.5-4 g/(L·d) and SRT of 66.6-100 d, bioavailable concentrations of Zn, Cu, Fe, Ni and Mo ranged within 10.20-20.79, 0.53-0.84, 7.77-11.80, 1.15-1.57 and 0.83-1.18 mg/kg respectively. And the anaerobic reactor could run steadily. At the OLR of 6 g/(L·d) and SRT set to 40 d, the concentrations of dissolved organic matter (DOM) increased to 13.67 g/L. And the metal speciation and concentrations changed accordingly. Bioavailable concentrations of Zn and Fe in biogas residues decreased significantly (P<0.05), equal to 4.39 and 5.54 mg/kg respectively. And bioavailable concentration of Cu declined to 0.40 mg/kg. The Ni and Mo in this stage changed slightly because of lower concentrations. As a result, CH4 yield decreased to 110 mL/g and volatile fatty acids (VFAs) accumulated seriously. The concentrations of acetate, propionate and butyrate were 2.02, 6.54 and 0.53 g/L respectively, which indicated the "acidification" in the digestion reactor. This study provides a better understanding for the bioavailable concentrations of metals (especially Zn, Cu and Fe) correlated with digestion performance and methanogenic activities provoked by organic overload in the anaerobic system. Thus, the conclusions are beneficial to enhance biogas production from straw by addition of trace metals.