Formation of hydroxyapatite during toilet wastewater treatment by electrolysis

Abstract

Electrolysis of toilet wastewater with TiO_2-coated semiconductor anodes and stainless steel cathodes is a potentially viable onsite sanitation soln. in parts of the world without infrastructure for centralized wastewater treatment. In addn. to providing disinfection, COD redn., and removal of ammonium ion, onsite electrolysis can remove phosphate by cathodic pptn. The formation of Mg-contg. hydroxyapatite (Mg_xCa_(5-x)(PO_4)_3OH, 0 ≤ x <1) on stainless steel cathodes during toilet wastewater electrolysis at pilot- and bench-scales was obsd. Phosphate removal levels up to 80% of the influent concns. were achieved concomitant of electrochem. treatment. While initial Ca^(2+), phosphates, and carbonates concns. were crit. for phosphates removal, Mg^(2+) and Cl- concns. had only minor impacts on phosphates removal rates in synthetic wastewaters at concns. typical of toilet wastewater. Expts. suggested that the pptn. of hydroxyapatite at the cathode during toilet wastewater electrolysis may have been initiated by alk. near-cathode pH values and migration of Ca^(2+) to the cathode. Onsite electrochem. reactors may be designed to recover pptd. hydroxyapatite for use as a phosphorus-rich fertilizer.