Pilot-scale tests of poly ferric sulfate synthesized using SO2 at Des Moines Water Works

Poly ferric sulfate (PFS) was synthesized by absorbing a dilute sulfur dioxide gas stream into a bench-scale reactor containing a stirred solution of ferrous sulfate, with sodium chlorate added as an oxidant. The reaction product was a solution containing approximately 50 wt.% PFS, a highly effective coagulant used in water treatment. The reaction took place near atmospheric pressure and at temperatures of 30-80 °C. SO2 removal efficiencies greater than 90% were achieved with ferrous iron concentrations of less than 0.1% in the product. The product solution was evaluated by wet chemistry methods to verify that the process was capable of consistently producing high quality PFS. Quality parameters examined were total iron concentration, ferrous iron concentration, basicity (mass ratio of OH/Fe(III)), density, and pH. The PFS product was used in pilot-scale tests at a municipal water treatment facility, and gave good results in the removal of turbidity and superior results in the removal of organic materials when compared with equal doses of ferric chloride.