Nutrient recovery from animal manure using bipolar membrane electrodialysis: Study on product purity and energy efficiency

Abstract

Bipolar membrane electrodialysis (BMED) is promising in nutrient recovery from wastewater containing nitrogen (N) and phosphorus (P), while studies on its product impurity and energy efficiency are insufficient. In this study, we systematically assessed the application of BMED into nutrient recovery from animal manure, with three membrane configurations investigated, i.e. three-compartment BMED, base-BMED and acid-BMED. The three-compartment BMED presented efficient removal of both NH₄⁺ and phosphate from the feed solution but high impurity in the base and acid product solutions. The base-BMED generated a pure base solution but was insufficient in NH₄⁺ recovery with around 40% of NH₄⁺ unrecovered, while the acid-BMED showed severe NH₄⁺ contamination of the acid solution similar to the three-compartment BMED. The energy in BMED was mainly consumed by membranes, electrodes and the solution resistance in the late operational stage, being around 55%, 15% and 28% for each, respectively; while the water dissociation in bipolar membranes contributed to 80% of the total energy consumption on membranes. Undesired NH₃ diffusion from the base compartment to adjacent compartments containing acidic solution was the main cause of product impurity. Hence, a simultaneous flowing mode integrating the base-BMED with the acid-BMED configurations was proposed. In this mode, NH₃ diffusion was prevented and NH₃ was concentrated up to 16 ​g/L in the base solution. This study provided valuable insights into BMED and proposed a novel BMED flowing mode to increase product purity, which is of high practical significance.