Microbial fuel cell enables phosphate recovery from digested sewage sludge as struvite

Orthophosphate was mobilized from iron phosphate (FePO4) contained in digested sewage sludge by microbial fuel cell power. FePO4 was reduced through electrons and protons obtained from metabolic activity of Escherichia coli. The process yielded up to 82% or 600 mg/l. Optical emission spectroscopy was used for phosphate dosage. 31P NMR showed a singlet at δp = 3.72 ppm indicating that orthophosphate (H3PO4H3PO4, HPO4-, HPO42- and PO43-) was recovered. The phosphate containing supernatant solution was reacted with stoichiometric amounts of MgCl2 and NH4OH to precipitate struvite (MgNH4PO4·6H2O). The crystalline fertilizer was analyzed by scanning electron microscopy comprising elemental analysis, revealing a composition accuracy of ∼90% and the absence of any toxic metals such as As, Cd, Pb, or Cr. The phosphate extraction is also a means to reduce the volume of digested sewage sludge while increasing the heat of combustion. This study represents a concept for sustainable decentralized phosphate recycling.