Ferrous Salt Demand for Sulfide Control in Rising Main Sewers: Tests on a Laboratory-Scale Sewer System

The addition of ferrous salts is a commonly used strategy for sulfide control in sewer networks. The Fe2+ dosing requirement in rising main sewers which takes into account of the effect of anaerobic sewer biofilms on the dosing demand is investigated. A laboratory-scale rising main sewer, consisting of four biofilm reactors in series and fed with real sewage, was operated for over 12 months, during which FeCl2 was dosed at several locations and at various dosing rates. The experimental results consistently revealed that approximately 0.7 mol of Fe2+ was required to precipitate sulfide formed from the reduction of 1 mol of sulfate by anaerobic sewer biofilms. This ratio is significantly lower than the ratio expected from reaction stoichiometry (molar ratio of 1:1), and also the Fe2+ to sulfide ratio (1.07-1.10 mol:1 mol) observed in batch tests conducted with real wastewater in the absence of sewer biofilms. Biofilms adapted to Fe2+ addition were found to contain a substantially higher amount of elemental sulfur than biofilms not receiving Fe2+ dosage. This suggests Fe2+ addition might have altered the final product of sulfate reduction by anaerobic sewer biofilms. The study also showed that the addition of ferrous salts at the inlet of a rising main sewer can effectively control sulfide throughout the whole system despite of the presence of competing anions in wastewater. Phosphate precipitation with ferrous iron in anaerobic rising main sewers is negligible.