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

摘要

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 600mg/l. Optical emission spectroscopy was used for phosphate dosage. ~(31)P NMR showed a singlet at δ_p=3.72ppm indicating that orthophosphate (H3PO4, HPO4-, HPO42- and PO43-) was recovered. The phosphate containing supernatant solution was reacted with stoichiometric amounts of MgCl_2 and NH_4OH to precipitate struvite (MgNH_4PO_4·6H_2O). 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.