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

Orthophosphate was mobilized from iron phosphate (FePO(4)) contained in digested sewage sludge by microbial fuel cell power. FePO(4) 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. (31)P NMR showed a singlet at δ(p)=3.72 ppm indicating that orthophosphate (H(3)PO(4), HPO(4)(-), HPO(4)(2-) and PO(4)(3-)) was recovered. The phosphate containing supernatant solution was reacted with stoichiometric amounts of MgCl(2) and NH(4)OH to precipitate struvite (MgNH(4)PO(4)·6H(2)O). 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.