New insights into inhibition of high Fe(III) content on anaerobic digestion of waste-activated sludge.

The impacts of the increased iron in the waste-activated sludge (WAS) on its anaerobic digestion were investigated. It was found that low Fe(III) content (< 750 mg/L) promoted WAS anaerobic digestion, while the continual increase of Fe(III) inhibited CH production and total chemical oxygen demand (TCOD) removal. As the Fe(III) content increased to 1470 mg/L, methane production has been slightly inhibited about 5 % compared with the group containing 35 mg/L Fe(III). Particularly, as Fe(III) concentration was up to 2900 mg/L, CH production, and TCOD removal decreased by 43.6 % and 37.5 %, respectively, compared with the group with 35 mg/L Fe(III). Furthermore, the percentage of CO of the group with 2900 mg/L Fe(III) decreased by 52.8 % compared with the group containing 35 mg/L Fe(III). It indicated that Fe(II) generated by the dissimilatory iron reduction might cause CO consumption, which was confirmed by X-ray diffraction that siderite (FeCO) was generated in the group with 2900 mg/L Fe(III). Further study revealed that Fe(III) promoted the WAS solubilization and hydrolysis, but inhibited acidification and methane production. The methanogenesis test with H/CO as a substrate showed that CO consumption weakened hydrogenotrophic methanogenesis and then increased H partial pressure, further causing VFA accumulation. Microbial community analysis indicated that the abundance of hydrogen-utilizing methanogens decreased with the high Fe(III) content. Our study suggested that the increase of Fe(III) in sludge might inhibit methanogenesis by consuming or precipitating CO. To achieve maximum bioenergy conversion, the iron content should be controlled to lower than 750 mg/L. The study may provide new insights into the mechanistic understanding of the inhibition of high Fe(III) content on the anaerobic digestion of WAS.