Effect of iron on phosphate recovery from sewage sludge

Iron is omnipresent in sewage treatment systems. It can unintentionally be present because of, e.g., groundwater seepage into sewers, or it is intentionally added for odor and corrosion control, phosphate removal, or prevention of hydrogen sulfide emissions. The strong affinity of iron for phosphate has advantages for efficient removal of phosphate from sewage, but it is also often considered a disadvantage for phosphate recovery. For instance, the strong affinity between iron and phosphate may reduce recovery efficiencies via struvite precipitation or for some phosphate recovery methods from ash. On the other hand, iron may also have positive effects on phosphate recovery. Acid consumption was reported to be lower when leaching phosphate from sewage sludge ash with higher iron content. Also, phosphate recovery efficiencies may be higher if a Fe-P compound like vivianite (Fe₃(PO₄)₂ 8H₂O) could be harvested from sewage sludge. Developers of phosphate recovery technologies should be aware of the potential and obstacles the iron and phosphate chemistry bears.