Fractionating various nutrient ions for resource recovery from swine wastewater using simultaneous anionic and...

Different from current nutrient recovery technologies of recovering one or two nutrient components (PO₄^3- or NH₄⁺) from wastewater, this study aimed to fractionate various nutrient anions and cations simultaneously, including PO₄^3-, SO₄^2-, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺, into several streams. The recovered streams could be further paired together to produce high-value products. A novel electrodialysis process was developed by integrating monovalent selective anion and cation exchange membranes into an electrodialysis stack. Results revealed that nutrient recovery was achieved effectively by fractionating PO₄^3- and SO₄^2- into the anionic product stream, whereas bivalent cations (Mg²⁺ and Ca²⁺) were extracted in the cationic product stream and the monovalent cations (K⁺ and NH₄⁺) were concentrated in the brine stream. For the permeation capabilities of anions, SO₄^2- and Cl^- possessed the higher preference, whereas PO₄^3- permeated the membrane more difficult. As to the cations, the permeation sequence was: NH₄⁺≈K⁺ >Ca²⁺>Mg²⁺≈Na⁺. Enhancing voltage values not only promoted ion migration rates, but also led to the increase of energy consumption. Although elevating initial phosphate concentration in the anionic product streams from 60 mg/L to 470 mg/L did not influence phosphate fractionation significantly, the current efficiency decreased from 3.55% to 0.65% and a remarkable increased of energy consumption from 29.42 kWh/kg NaH₂PO₄ to 160.13 kWh/kg NaH₂PO₄ was observed. Further experiments were conducted for phosphorus recovery by pairing two recovered product streams, which revealed that phosphate precipitation could be achieved by using inherent Ca²⁺ and Mg²⁺ in the wastewater without dosing external cation sources.