FO process for simultaneous wastewater pre-concentration and seawater concentrate disposal: Impacts of seawater concentrate on membrane fouling in long-term operation

Using seawater desalination concentrate (SC) as DS of FO for simultaneous wastewater pre-concentration and disposal of waste SC has presented good application potential. Nevertheless, knowledge of membrane fouling mechanisms in FO process using SC as DS, especially the influence of SC on membrane fouling, is still inadequate, mostly due to the limited operation time in previous studies. Hence, present study focused on membrane fouling behaviors in FO with SC as DS, with particular focus on impact of SC in long-term operation. The results indicated that using SC as DS induced noticeable aggravation of membrane fouling on both active layer (AL) and support layer (SL) sides of FO membrane in long-term operation, which was not observed in short-term operation. On AL side, accumulation of Ca²⁺ and Mg²⁺ via back-diffusion from SC became significant during long-term operation, thereby substantially altered fouling behavior on AL side, resulting in more refractory membrane fouling. On SL side, calcium-based scaling was formed both in and within the porous matrix of SL during long-term operation. Accordingly, a combined membrane cleaning strategy (NaClO for AL + HCl and EDTA for SL) was developed. The results could facilitate the application of SC as an economical and feasible DS source.