Synchronously recovering different nutrient ions from wastewater by using selective electrodialysis

Digestive slurry normally contains various nutrient ions with high concentrations, including NH4+, PO43-, K+, Mg2+, Ca2+ and SO42-, which is a resource pool for nutrient recovery. In this study, a synchronously cationic and anionic selective electrodialysis (SCAE) was developed to recover anionic and cationic nutrient ions. Results showed that SCAE could synchronously recover more than 85.0%, 90.2% and 97.8% of PO43-, SO42- and other cations (including NH4+, K+, Ca2+, Mg2+) from the simulated digestive slurry, respectively. The ionic permeation sequence, NH4+ > K+ > Ca2+ > Mg2+ for cations, and SO42- > PO43- for anions, was affected by hydrated radius and hydration numbers, and did not alter despite the variation in electric field. High electrolyte concentration in the product streams would promote the recovery efficiency of both divalent cations and anions due to the ionic replacement effect and the demand for charge neutrality. Under continuous operation, the maximum concentrations of PO43-, SO42-, Mg2+, Ca2+, NH4+ and K+ in product streams reached 231.9, 496.6, 180.7, 604.3, 9,648.4 and 4,571.4 mg·L-1, respectively. By directly mixing different streams, the feasibility of producing mineral fertilizers without dosing externally precipitating chemicals was proved. Struvite, NH4HSO4 and potassium chloride minerals were produced successfully. The outcome provided an optional method for nutrient recovery from wastewater.