USE OF IRON SALTS TO CONTROL DISSOLVED SULFIDE IN TRUNK SEWERS

Sewer headspace H2S reduction by precipitating dissolved sulfide in wastewater was investigated using iron salt (FeCl3 and FeCl3). Full-scale experiments were conducted in a 40-km (25 mi) sewer with an average flow of 8.7 m(3)/s (200 mgd). Results were sensitive to total Fe dosages and Fe(III)/Fe(II) blend ratios injected. A concentration of 16 mg/L total Fe and a blend ratio of 1.9:1 [Fe(III):Fe(II)] reduced dissolved sulfide levels by 97%. Total sulfide and headspace H2S were reduced by 63% and 79%, respectively. Liquid and gas-phase sulfide reductions were largely due to the effective precipitation of sulfide with Fe(III) and Fe(II) and the limited volatilization of H2S, respectively. Oxidation of sulfide in the presence of Fe(II) and minute amounts of O-2 may have occurred. A combination of Fe(III) and Fe(II) proved more effective than either salt alone. By using excess Fe(III), dissolved sulfide can be reduced to undetectable levels. No specific relation between the concentration of Fe or Fe(III)/Fe(II) blend ratio and sewer crown pH was inferred. Iron salts may retard crown corrosion rates by precipitating free sulfide and reducing its release to the sewer headspace as H2S. A mechanism to inhibit certain responsible bacteria was not established in the 40-km (25 mi) sewer.