Addition of ferric chloride in anaerobic digesters to enhance sulphide removal and methanogenesis

Abstract

A feasibility study on the addition of ferric chloride (FeCl3) as selective regulator to reduce the sulphide reduction activities of sulphate reducing bacteria (SRB) in anaerobic digester was investigated. A lab-scale up-flow anaerobic sludge blanket (UASB) fed with synthetic sulphate enriched wastewaters were operated continuously for 56 d. The influent sulphate concentrations with an addition of potassium sulphate (K2SO4) were operated at corresponding COD/SO4 2– ratios of 5.3, 2.5 and 1.3, respectively. An amount of FeCl3 at dosages of 10.2, 22.2 and 44.5 mM were added on days 66, 86 and 108, respectively (i.e. 8–10 d after each COD/SO4 2– ratio was introduced). Results showed that sCOD removal efficiencies averaged at 78%, 80% and 70%, respectively while methane yield averaged at 0.35, 0.32 and 0.27 L CH4. gCOD destroyed −1 when FeCl3 dosage were added at 10.2, 22.2 and 44.5 mM, respectively. Furthermore, the scanning electron microscopy (SEM) examinations demonstrated that UASB operated at COD/ SO4 2– ratio of 1.3 were dominated by several filamentous rod-shaped bacteria attached to the structural matrix of the digesting sludge as compared to COD/SO4 2– ratio of 2.5. However, when UASB was supplemented with FeCl3 at dosages of 22.2 mM and 44.5 mM, the sludge contained no rod-shape bacterium and the morphology of sludge showed the presence of iron sulphide precipitation.