Dynamic Response of Sulfate-Reducing and Methanogenic Activities of Anaerobic Sewer Biofilms to Ferric Dosing

A recent study showed that, in addition to precipitating sulfides, Fe3+ addition to sewers reduces the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms. The dynamic response of these activities to the commencement and termination of ferric dosage is investigated in this study. Both the sulfate-reduction and methane-production rates of anaerobic sewer biofilms decreased upon the addition of ferric ions, with a maximum inhibition of approximately 50% achieved after 3 and 7 days, respectively. In comparison, the sulfate-reducing activity of suspended biomass in the Fe3+ receiving reactor was completely inhibited. The volatile solids-to-total solids (VS/TS) ratio of sewer biofilms decreased from 91±3% before Fe3+ addition to 54±6% after a full adaptation of the biofilms to Fe3+ addition. Energy-dispersive spectroscopy profiles showed higher iron and sulfur contents in biofilms adapted to Fe3+ addition, suggesting FeS deposition in biofilms. We hypothesize that FeS precipitation primarily occurred in the upper layer of sewer biofilms causing inhibition to sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) in this layer (and in the suspended phase). However, SRB and MA were likely to be active in the deep layer. The sulfate-reducing activity recovered to approximately 90% that of the reference reactor in 5 weeks after Fe3+ injection was stopped, whereas there was no recovery of the methanogenic activity in the same period. The results suggest that a temporary suspension of ferric dosage would not lead to immediate recovery of the sulfate-reducing and methanogenic activities. The recovery of sulfate-reducing activity was found to be linearly correlated with the recovery of biofilm VS/TS ratio, following the termination of Fe3+ addition.