Zero liquid discharge treatment of brackish water by membrane distillation system: Influencing mechanism of antiscalants on scaling mitigation and biofilm formation

Abstract Membrane distillation (MD) is increasingly regarded as a promising technique for brine management and zero liquid discharge (ZLD) disposal of seawater and brackish water owing to its excellent effluent quality and higher salt tolerance. However, membrane fouling issues of MD, especially scaling, have been confirmed as a crucial challenge impeding its large-scale utilization. Presently, the scaling issue of conventional membrane-based desalination techniques including reverse osmosis (RO) and ultrafiltration (UF), has been effectively controlled via the usage of antiscalants. Hence, this work applied three representative antiscalants of 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), diethylene triamine penta methylene phosphonic acid (DTPMPA), and polymaleic acid (HPMA) in MD devices to restrain the scaling of polytetrafluoroethylene (PTFE) membrane during the ZLD treatment of synthetic brackish water. Furthermore, influences of antiscalant presence on variations of feed water qualities and the corresponding biofilm formation were also comprehensively studied via the high throughput sequencing method of 16S rDNA and 16S rRNA as well as a confocal laser scanning microscopy (CLSM). Compared to DTPMPA (77.4%-97.8%) and HPMA (42.1%-97.8%), a higher anti-scaling efficiency was obtained for PBTCA (90.6%-98.1%) in MD system, while the membrane biofilm formed in PBTCA group exhibited a remarkable appearance of live cells (14.046 μm3/μm2). The secondary fouling of phosphate scale and pore wetting might be induced by the utilization of DTPMPA in MD plants, sharply increasing the effluent conductivity to 176.4 μS/cm (12 d). Comparatively, HPMA could be largely combined with salt ions to form loose slags with good fluidity, promoting the reuse of MD membranes and the subsequent disposal of MD brine.