Integrating solvent extraction with electrochemical separation for nutrient recovery from anaerobically digested sludge

• Electrochemical method enables P leaching in anolyte and N enrichment in catholyte. • NH 4 + is selectively separated by solvent extraction using D2EHPA. • NH 4 + purity can be improved by a two-stage acid stripping process. • Residual Ca 2+ after P precipitation is recovered by D2EHPA with 94.3 % efficiency. Anaerobic digested sludge (ADS) from municipal wastewater treatment plants contains large amount of inorganic nutrients such as phosphorus and ammonia, and recovery of those nutrients will not only reduce pollutants but also contribute to a circular economy. Herein, an integrated nutrient recovery system was developed by coupling electrochemical treatment with solvent extraction to enable efficient and selective nutrient recovery. Ammonium was enriched in the catholyte through electromigration, accompanied by an electrolysis-induced pH increasement. Subsequent extraction using di(2-ethylhexyl) phosphoric acid (D2EHPA) achieved more than 70 % efficiency with evident selectivity over sodium and potassium. Consistence performance was maintained over ten consecutive extraction – stripping cycles. To further improve ammonium purity, a two-stage acid stripping process, scrubbing with 0.1 M HCl followed by 0.5 M HCl, yielded 94.4 % ammonium in the total cations with an ammonia concentration of 1448.4 ± 129.7 mg L −1 . Meanwhile, phosphate leached into the anolyte was recovered via calcium precipitation with a 7 mM calcium dosage, achieving a recovery efficiency of 92.0 %. The unreacted calcium was subsequently recovered using D2EHPA with a 94.3 % of extraction efficiency. Those results encourage further exploration of integrating solvent extraction with electrochemical processes for nutrient recovery.