[Effect on Ammonia Inhibition Mitigation in the Anaerobic Digestion Process with Zero-Valent Iron]

Ammonia inhibition is an important factor impacting methane production efficiency during the anaerobic digestion of high-solid organic wastes. This study investigated the effect of micro-sized zero-valent iron (m-ZVI) on the anaerobic digestion of excess sewage sludge and thermal hydrolyzed sludge using batch mode experiments. The effect of m-ZVI on ammonia inhibition mitigation was also studied. Results showed that the kinetic characteristics of the methane production rate, lag phase, and methane production potential of the anaerobic digestion of excess sludge and thermal hydrolyzed sludge were not impacted by the addition of m-ZVI at a dosage of 4 g·L^-1 and 10 g·L^-1. However, during the inhibited anaerobic digestion process with a high ammonia concentration, the addition of 4 g·L^-1 and 10 g·L^-1 of m-ZVI was able to shorten the lag phase from 18.61 d (the control) to 17.22 d and 16.18 d, respectively. Moreover, the maximum methane production rate (based on the VS) increased from 6.34 mL·(d·g)^-1(the control) to 7.84 mL·(d·g)^-1 (4 g·L^-1 m-ZVI) and 7.39 mL·(d·g)^-1 (10 g·L^-1 m-ZVI). The pH buffer system was not influenced by the chemical reaction of m-ZVI in the anaerobic digestion, although the relative abundance of the dominant methanogenic archaea (Methanosarcina) improved greatly from 30.71% (the control) to 53.50% (4 g·L^-1 m-ZVI) and 60.30% (10 g·L^-1 m-ZVI) at 27 d. This study proved that m-ZVI was incapable of improving the methane production potential of sewage sludge, while the mitigation of ammonia inhibition during anaerobic digestion was enhanced by the stimulating effect on methanogenic archaea.