Electrochemical Solvent Extraction for Nitrogen Recovery from Anaerobically Digested Sludge

Recovering ammonia from anaerobically digested sludge (ADS) by electrochemical treatment is an attractive approach for nutrient circularity and pollution reduction, but it is often limited by slow mass transfer and energy inefficiency. Herein, an electrochemical solvent extraction (ESE) system was developed to achieve selective ammonia recovery from ADS via integration of a two-chamber electrochemical cell with an external solvent extraction reservoir containing di(2-ethylhexyl) phosphoric acid (D2EHPA) and tributyl phosphate (TBP), enabling continuous proton exchange and pH stabilization in the catholyte. The ESE system achieved 80% NH4+-N recovery at A/O = 1 and 50 A m–2 over 2 h operation, maintaining a near-neutral catholyte pH (6–7) and preventing ammonia volatilization. In the anolyte, proton generation at the anode leached phosphorus from the ADS, increasing its concentration from 53.88 ± 4.14 to 204.51 ± 2.82 mg L–1, which is available for subsequent recovery. Increasing the solvent ratio (A/O = 2) or current density (75 A m–2) induced a third-phase formation, which led to incomplete separation and lower ammonia recovery. Extended operation without solvent regeneration successfully enriched NH4+-N by 2.05-fold after 2 cycles. The results of this study have demonstrated the feasibility of the proposed ESE system for ammonia recovery from ADS and warrant further investigation to improve solvent stability and minimize third-phase formation.