The Use of Redox Potential to Control Ferric Sulphate Dosing during Phosphate Removal

Abstract

In recent years, the Norfolk Broads have been enriched with sewage effluent and runoff from heavily fertilized land. Nitrogen and phosphorus compounds from these sources can cause ecological problems by stimulating increase in algal biomass in the water, so part of Anglian Water's programme of research in Broadland has been to remove phosphate from major sewage effluents discharged to the Rivers Ant and Bure. This is achieved by dosing ferric sulphate to secondary effluent and settling the resultant insoluble complex.At three sites on the River Bure, the performance of timeclock-and redox-based dose control systems was evaluated. Redox control, by restricting dosing when phosphate load was low, increased the efficiency of phosphate removal and produced up to 19 per cent savings in chemical costs. Modifications are described which made the system self-regulating and ensured that savings were not eroded by maintenance costs. The system now operates efficiently and routinely at major sites on the Rivers Ant and Bure.

Eutrophication of Freshwaters

“…Precipitation with lime (producing calcium hydroxyapatite) has also been used; this produces larger sludge volumes because of the simultaneous production of calcium carbonate, but the sludge has better thickening and stabilisation properties. Removal of total phosphorus in excess of 90% is achieved (Mekerson, 1973;Kerrison et al, 1989), and treatment may have additional benefits such as the more efficient precipitation of suspended organic solids. Costs are approximately 10-30% in addition to the costs of treatment to the secondary stage.…”