Electrochemical phosphorus leaching from digested anaerobic sludge and subsequent nutrient recovery.

With the rising concern over the depletion of phosphorus rock, phosphorus recovery from wastewater has become a key step for sustainable economy. Herein, simultaneous phosphorus leaching and nutrient recovery were accomplished in an electrochemical nutrient recovery cell (ENRC) treating digested anaerobic sludge. The anode reaction of water electrolysis lowered the sludge pH from 8.0 to 2.0 at a current density of 25 A m, elevating the PO-P concentration from 27.72 to 253.47 mg L, comparable to that from direct acid leaching. The released PO-P was transferred to the cathode chamber for recovery, where PO-P recovery efficiency was enhanced from 42.0% to 90.3% by 0.26 M HCl catholyte acidification. The ENRC recovered 90-98% of the coexisting NH-N in the sludge. Increasing current density accelerated both phosphorus leaching and PO-P & NH-N recovery, but at the expense of a higher energy consumption. After five consecutive cycles of operation, the PO-P and NH-N concentrations reached 404.56 and 3493.56 mg L, respectively, at a normalized energy consumption of 229.20 ± 30.13 kWh kg P or 25.67 ± 3.07 kWh kg N. At pH 8.5, 99% of the recovered aqueous PO-P in the recovery solution precipitated, mainly as calcium phosphate that can have a good soil phosphorus availability. The results of this study have provided a foundation for further exploration of electrochemically leaching P from waste sludge with simultaneous nutrient recovery.