The removal of lead from drinking water by AFM
Lead Pb2+ is converted to lead hydroxide in water with a redox potential above 300mv, and does not react with water.
Under acidic conditions with a low redox potential a soluble form of lead as lead acetate can form. Also in the presence of anaerobic conditions in combination with sulphide S2- or phosphate PO42- a highly soluble form of lead can develop which will enter the water network.
Most drinking water treatment systems use sand either in pressure or gravity flow filters. We know that bio-coagulation of the sand leads to the development of low redox potential acidic zones. At a low redox potential sulphide will be generated which reacts to form soluble lead. Elemental lead my react with dissolved salts to form soluble lead carbonate.
The removal of lead by pressure or RGF filters can be improved by the use of AFM, activated filter media as opposed to sand.
Ferric chloride should be added to the water prior to filtration. The ferric reacts with phosphate, in the presence of oxygen it also increases the redox potential which converts the lead to insoluble lead oxide. The application of ferric chloride should be via a ZPM or high quality aggressive static mixer. A polyamide should also be applied as a flocculent.
The pretreatment of the water prior to AFM filtration is important. The AFM filter bed, will remove virtually all of the ferric phosphate precipitate. If sand were used, it usually ends up as a solid block of mineralized media after a few months. AFM on the other hand will continue to perform for many years and is not affected as long as the backwash water velocity exceeds 45m/hr.
AFM does not bio-coagulate which means unfiltered water will never pass the filter bed and that anaerobic zones do not form. If there are no anaerobic low pH, low redox potential zones, then lead will not form soluble salts with sulphur or carbonates.
Using existing infrastructure, by replacing the sand with AFM and by modification of the chemical pretreatment strategy, existing pressure or RGF filters with AFM become an effective means of lead control in municipal drinking water supplies.