Effect of sewage sludge ash contents on the performance of thermo-sensitive hydrogel as draw agent for forward osmosis application

In this study, a novel and environmentally friendly technology for sludge incineration ash disposal was developed. We proposed to enhance the performance of thermo-sensitive hydrogel as draw agent for forward osmosis (FO) application by loading sewage sludge ash (SSA). The effect of SSA contents on the swelling and deswelling properties and FO performance of the hydrogels was evaluated. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and diffusion model were applied to characterize a series of hydrogels with various SSA contents (0–4.5 wt%). The results showed that as the SSA increased, the swelling ratio of hydrogel and the water flux of SSA-hydrogel-based FO increased initially and then declined. This was due to the combined effects of hydrogen bonding and high-density crosslinking within the hydrogels. In addition, the water diffusion mechanism of all hydrogels was super Case II diffusion type. Furthermore, the SSA-loaded hydrogels not only had excellent swelling property but also exhibited good deswelling performance (water recovery ratios of them were almost 90% at 60 °C water bath for 5 min). Moreover, all hydrogels in this study showed promising reusability with almost no flux drop in four consecutive FO permeation-regeneration cycles.