Recovery of nutrients and volatile fatty acids from pig manure hydrolysate using two-stage bipolar membrane electrodialysis

Animal manure should be regarded as a resource rather than a waste as it contains abundant nutrients (nitrogen and phosphorus) and organic matter. In this study, a laboratory-scale bipolar membrane electrodialysis (BMED) system was set up to assess the recovery of ammonium (NH₄⁺), phosphate (PO₄³⁻) and volatile fatty acids (VFAs) from both synthetic and real pig manure hydrolysate for the first time. Synthetic hydrolysate was used as feed first to investigate the ionic migrations in the BMED system. After 5.5 h of operation, 52% of NH₄⁺ migrated to the base compartment, and 98% of PO₄³⁻ and 95% of VFAs migrated to the acid compartment. A BMED model was established to quantify the ion flux balance in the membrane stack. It substantiated that the low recovery efficiencies of NH₄⁺ and the impurity of acid solution were primarily caused by the undesired diffusion of ions through bipolar membranes. Subsequently, a novel two-stage BMED operation based on the “inflection point” of voltage was developed to minimize the NH₄⁺ loss and separate PO₄³⁻ and VFAs from the acid compartment. Through this operation, the recovery efficiency of NH₄⁺ increased to 78%, and 75% of PO₄³⁻ and 87% of VFAs were separated from Cl⁻ and SO₄²⁻ in the acid compartment. Finally, real pig manure hydrolysate was tested and the variations of ions in the BMED were consistent as those using synthetic wastewater. This study demonstrates that it is feasible to recover valuable nutrients and VFAs from pig manure hydrolysate using two-stage BMED technology.