Recovering double-metal hydroxides precipitate from desalination process of saline wastewater as conditioner for excess sludge dewatering

The massively generated double-metal hydroxides (DHs) precipitate has restricted the application of the lime-aluminum precipitation method in the zero liquid discharge (ZLD) process of saline wastewater. Recovering DHs precipitate as conditioners for the dewatering of municipal sludge is a promising solution for above issue. In this study, individual use of DHs from the ZLD process, including ettringite (Ett), Friedel's salt (FS) and their mixtures (EF), and combined use of the DHs and FeCl₃ in sludge conditioning were investigated. The EF precipitate was commonly generated from the chloride precipitation process by the lime-aluminum method, and composed of 16.7% of Ett and 83.3% of FS. The dewaterability of excess sludge conditioned with FeCl₃ prior to DHs was much better than opposite dosing sequence and individual use of DHs. Sludge conditioned by FeCl₃ prior to EF possessed the best dewatering performance ascribing to its high fluidity, strong structure strength and uniformly dispersed mesoporous structure. The variations of extracellular polymeric substances (EPS) further confirmed that EPS were largely released after FeCl₃ addition and then efficiently adsorbed by the subsequently added DHs, with the highest adsorption efficiency of 60% by EF. The kinetics for EPS adsorption also suggested that EF had the highest adsorption capacity compared to FS and Ett. Overall, FeCl₃ addition could form interstitial channels in flocs by penetration and release external and intracellular substances, while the latter added EF, Ett could function as uniformly skeleton material and dispersing agent to improve EPS adsorption by FS. Hence, the synergic coupling of FeCl₃ and DHs as chemical conditioner is inspirable and economically beneficial to ZLD and advanced sludge dewatering process.