Zwitterions as alternative draw solutions in forward osmosis for application in wastewater reclamation

The draw solution is the driving force in forward osmosis (FO) processes. The reverse solute leakage of the draw solution is however a major constraint due to cost and energy requirements when reconcentrating the solutes subsequent to the FO process. Several zwitterions as draw solutions (π≈24 bar and 7 bar) were systematically investigated to enhance the FO performance and minimise the solute loss. The highly soluble zwitterions: glycine, l-proline and glycine betaine demonstrated comparable water fluxes to NaCl (~5 L/m2 h), but with significantly lower solute loss (Js: 2.13±0.54 g/m2 h; 1.37±0.09 g/m2 h, 0.96±0.4 g/m2 h respectively and JsNaCl: 3.26±0.53 g/m2 h), which is advantageous for cost reduction. The physico-chemical properties, charge and size played a dominant role in the flux efficiencies. The Js/Jv ratios decreased with (i) a decrease in hydrophobicity and (ii) an increase in size. The FO mass transfer model verified the experimental investigations of the solute transport through the membrane.