Polarity reversal-driven electroprecipitation/electrochlorination coupled system: Enhancing P recovery and N removal of swine wastewater via pH self-regulation

Magnesium electro-precipitation (Mg-EP) can recover equimolar nitrogen (N) and phosphorus (P) from wastewater but is unsuitable for high N/P molar ratio swine wastewater. Although electrochlorination (EC) could theoretically compensate this defect, mismatched reaction conditions and electrode passivation make direct coupling infeasible. This study developed a Mg-EP/EC integrated system via polarity reversal, achieving in-situ and efficient coupling of struvite precipitation with electrochemical NH 4 + -N oxidation through periodic electrode switching. The spontaneous formation of alkaline environment in the Mg-EP stage prevented struvite redissolution and the decrease in the proportion of high oxidative free chlorine caused by continuous H + generation from NH 4 + -N oxidation during subsequent EC. Meanwhile, the localized pH decline and H 2 evolution at the electrode surface during EC contributed to divest the passivation layers formed during Mg-EP, thereby maintaining electrode interfacial activity. Under optimized conditions (concentric tube electrode, 750 mA current, 1000 mg L −1 initial Cl − , 30-min Mg-EP, and 90-min EC), 87.7% of PO 4 3− -P and 13.2% of NH 4 + -N were recovered as struvite, while 59.8% of NH 4 + -N was oxidized to N 2 by free chlorine, thereby achieving compliant discharge of N and P from swine wastewater. Notably, the struvite purity was up to 96.6%, realizing waste-to-resource conversion. Furthermore, the coupled system maintained high repeatability over nine consecutive operating cycles, and the specific energy consumptions for pollutant removal in treating actual swine wastewater (127.2 kWh/kg N and 48.8 kWh/kg P) were significantly lower than related reports. This study provides a promising engineering solution for advanced treatment and resource recovery of high N/P molar ratio wastewater.