Cleaning mechanism of gypsum scaling in hydrophobic porous membranes

Mineral scaling is one of the major challenges for membrane distillation in concentrating high-salinity brine. This work first investigated the characteristic of CaSO 4 scaling, then explored different cleaning reagents to evaluate their efficiencies and mechanisms. By analyzing the SEM and EDX mapping images, the CaSO 4 scaling process could be divided into surface and pore scaling stages. Three cleaning solutions, i.e., commercial detergent (20 %), EDTA-4Na (0.05 M), and MgCl 2 (0.8 M), were chosen for their high CaSO 4 solubility (0.7 g CaSO 4 per 100 g H 2 O). At the surface scaling stage, three cleaning solutions exhibited excellent removal of CaSO 4 scaling with approximately similar 96 % cleaning efficiencies. However, at the pore scaling stage, only the commercial detergent showed high efficiency (89 %). Such high cleaning efficiency was mainly attributed to the detergent's high total dissolved solids (salinity), low pH, and high diffusion rate (5.46 μm cm −1  min −1 ) across the membrane. Moreover, periodic cleaning at the surface scaling stage with the commercial detergent followed by drying throughout a 43.5 h continuous concentration test on an actual RO brine could restore the PTFE membrane performance completely, giving 90 % of the total water recovery . This work provides new insights into the cleaning mechanism for gypsum scaling in a hydrophobic porous membrane.