Sewage sludge ash-based thermo-responsive hydrogel as a novel draw agent towards high performance of water flux and recovery for forward-osmosis

This work made use of the sewage sludge ash (SSA) to enhance thermo-responsive hydrogel for driving forward-osmosis (FO) process. Results showed that owing to the porous structure and intermolecular interactions between the ionized groups of monomers and the chemical surface of silica, the SSA-based thermo-responsive hydrogel (SSA2.5H) had increased swelling capacity and osmotic pressure, resulting in an average water flux (2.33 LMH) twice as high as the hydrogel without adding SSA (1.18 LMH) during 24 h FO with DI water or different concentrations (2000–8000 ppm) of NaCl as feed solution. More importantly, SSA2.5H can maintain good thermo-responsive dehydration performance in spite of an enhancement to its swelling ability. 93.8% of the adsorbed water can be retrieved from SSA2.5H in water bath of moderate temperature (60 °C) for 10 min and water flux can be recovered to 94.4% of the initial value after 4 times of hydrogel regeneration during FO as draw agent. Therefore, this work showed that SSA was an excellent modifier to enhance the thermo-responsive hydrogel for improving permeation performance in FO and lowering down the actual energy requirement for water recovery and draw agent regeneration. [Display omitted] •Hydrogel modified with SSA as draw agent for FO was proposed.•SSA increased swelling capability of hydrogel.•High water flux was achieved due to high osmosis pressure of hydrogel.•Hydrogel maintained good thermo-responsive dehydration performance.•Low energy was consumed for water recovery in FO with SSA hydrogel.