Revealing the combined fouling by gypsum and organics in membrane distillation integrated with in-line granular filtration

Membrane distillation (MD) coupled with in-line granular filtration successfully mitigated gypsum scaling-induced wetting, but it encountered great challenges when treating the real wastewater. The impacts of NaCl and humic acid (HA) on gypsum-induced wetting were systematically investigated in this study. Results demonstrated that NaCl exacerbated gypsum scaling while HA mitigated gypsum scaling at identical saturation index of 0.1. HA adsorbed onto filter media enhanced scale capture efficiency via carboxyl group-Ca 2+ binding and reduced membrane scaling, whereas scaling distribution was not significantly impacted by NaCl. HA reduced the size of gypsum crystals on the membrane surface. At high HA concentrations, free HA formed a protective layer on the membrane, inhibiting crystal invasion, while HA adsorbed Ca 2+ increased scaling under high ions concentration. Thermodynamic and kinetic studies confirmed that homogeneous scaling was inhibited by HA but not by NaCl. X-ray diffraction analysis revealed that HA reduced the crystallinity of gypsum to form amorphous lumpy particles with less sharpness. Membrane characterization further demonstrated that HA mitigated structural degradation by altering crystal properties rather than merely by reducing the scaling mass. These findings provide a reference for mitigating MD scaling-induced wetting by utilizing the advantageous interrelationships between inorganics and organics. • Mechanisms of combined fouling by gypsum and organics in MD were revealed. • HA increased final MD flux by 62–75 % by enhancing filtration column capture. • A certain NaCl increased the CaSO 4 reducing the MD end flux by up to 95 %. • HA increased the nuclear energy barrier for gypsum higher than NaCl. • EDTA-4Na was the optimal cleaning reagent, MD flux recovery of 98.9 %.