Treatment of wastewater phosphate by reductive dissolution of iron

A new method for the treatment of wastewater phosphate is presented that relies on reductive iron dissolution (RID process). It is analogous to existing treatment methods that use iron salts (e.g., FeCl2) to precipitate P from wastewater, except that in this case ferrous iron is made available by the reductive dissolution of ferric iron solids that are contained in a reactive porous media. The lense solids are minerals such as amorphous Fe(OH)3 that occur naturally in soils and sediments. A laboratory column study and a pilot scale field trial demonstrated that these solids are susceptible to reductive dissolution when mixed with sewage effluent, resulting in increased Fe(II) in solution. This promotes the precipitation of Fe(II)-P solids and then Fe(III)-P solids when the effluent is subsequently oxidized. Both experiments demonstrated the ability of the RID media to passively solubilize consistent, moderate concentrations of Fe (1-9 mg L-1) over extended periods (2-3 yr). In the column test, influent PO4 of 9.0 ± -3.7 mg P L-1 was lowered to 2.1 ± 1.1 mg P L-1. In the field trial, influent PO4 of 10.2 ± 6.0 mg P L-1 was lowered to <0.05 mg P L-1. This technique may be attractive for use with smaller wastewater treatment systems such as septic systems, because it can be maintenance-free for long periods and it avoids excess sludge accumulation.