Experiments were performed in parallel using six identical UASB (upflow anaerobic sludge blanket) reactors to examine the effects of iron (Fe2+) on sludge granulation during start-up. Introduction of Fe2+ at concentrations of 300 and 450 mg/L enhanced the granulation process in UASB reactors, while dosage of Fe2+ (150 mg/L) had little effect on the sludge granulation. The Fe2+concentration in granules was nearly proportional to the influent Fe2+concentration. The specific activity of granules decreased with increasing Fe2+/concentration in the feed. For high Fe2+ dosages, a large amount of minerals was deposited within the granules, and FeS and the compounds formed by iron and exopolysaccharide polymers were the main precipitates. This, along with the significant decrease in the water content in granules and the possible toxicity of high-level Fe2+accumulated inside granules, reduced bacterial specific activity.

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