Influence of ferrous iron on the granularity of a UASB reactor

The effect of ferrous ion addition on the granularity of an upflow anaerobic sludge blanket (UASB) reactor was investigated. Two UASB reactors (R1 and R2) (35 °C; pH 7) were operated for 3 months at a 20-h hydraulic retention time (HRT) at organic loads from 1.4 to 10.0 g COD L−1 d−1. Ferrous iron was fed only to reactor R1 in a range of load from 0.014 to 0.100 g Fe2+ L−1 d−1. The addition of ferrous iron induced a stable and excellent COD conversion rate. The enhancement of the performance of reactor R1 may be accredited to the mean granule diameter increase as well as to the sludge bed porosity decrease. The addition of iron to reactor R1 had a pronounced effect on the quality of the granular sludge, increasing the characteristic settling velocities. The formation of ferrous sulphide was mainly responsible for the promotion of sludge granulation.