Impact of zero valent iron on blackwater anaerobic digestion

The source diverted blackwater treatment is receiving growing attention as an alternative to conventional energy intensive wastewater management and treatment systems. Blackwater, containing concentrated organic materials, can be anaerobically digested to recovery bioenergy. However, the methane recovery from blackwater is often inhibited by the presence of high free ammonia (FA) in blackwater. In order to improve the methane production in blackwater, nano-scale zero valent iron (nZVI, 35 nm or 50 nm) or micro-scale zero valent iron (mZVI, 200 μm) at different dosages (i.e., 0.5, 1, and 10 g/L) were applied respectively in the anaerobic digestion (AD) reactor for blackwater treatment. The results demonstrated that low doses (0.5-1 g/L) of nZVI slightly improved methane (CH₄) production, possibly due to a reduced oxidation-reduction potential (ORP) and improved hydrolysis-acidification in the nZVI supplemented systems. However, a lower biochemical methane potential (BMP) of blackwater was observed with high doses (10 g/L) of nZVI which induced a pH increase (>8.5) in AD reactor leading to a higher FA inhibition of CH₄ production. In contrast, the effect of mZVI on blackwater AD system was not significant. The study demonstrated the successful application of nZVI for improving AD of blackwater, however, which requires dosage control.