Abstract
Ammonium permeation to draw solution (DS) is very common in forward osmosis (FO) treatment due to the negative charge nature of the membrane, which restricts the application of FO for wastewater treatment particularly with a purpose of ammonium recovery and water reuse. Membrane modification by reducing the negative charge of membrane surface could improve ammonium rejection. This study investigated ammonium rejection in the FO process by selecting DSs with different physicochemical properties. Results show an increased ammonium rejection rate with divalent cation-based DS (i.e. Mg2+) due to larger hydrated cation radius compared with monovalent cations such as Na+ under the same osmotic pressure. Non-ionic DSs such as glucose, glycine and ethanol are able to minimize ion exchange across the membrane, leading to a 98.5–100% ammonium rejection. This indicates that cation exchange is the critical factor for ammonium permeation. Real wastewater treatment such as municipal wastewater and sludge digestate liquor by FO with NaCl and glucose as DS, respectively, further confirms that minimizing cation exchange by selecting appropriate DS could significantly improve ammonium rejection, which is not negatively affected by the more complex nature of real wastewater. This study developed an alternative method by selecting appropriate DS to increase ammonium rejection. Furthermore, it shed light on the mechanism of enhanced ammonium rejection with different types of DSs, which could provide general guidance on the selection of DS from the perspective of wastewater treatment with ammonium considered.
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