Organic fouling of membrane distillation for shale gas fracturing flowback water desalination: a special interest in the feed properties by pretreatment

Shale gas fracturing flowback water (SGFFW) contains a high concentration of colloids and organics which can cause severe water flux decline for membrane distillation (MD). It is desirable to identify the key foulants for MD fouling/wetting for real SGFFW treatment. In this study, coagulation and membrane filtrations with different molecular weight cut-off (MWCO) were applied to try to separate the different fractions of pollutants and evaluate their contributions to MD flux decline. The organics with a molecular weight of 20 kDa, which also belonged to humic acid-like components, protein-like components and fulvic acid-like components removed by coagulation could effectively mitigate MD fouling. However, the rest of the fractions of high molecular weight components of 20 kDa and low molecular weight components (i.e., 200 Da) removed by the UF membrane, had a less significant effect on the water flux of MD. Despite the further removal of small molecular weight compounds and Ca2+/Mg2+ by NF, negligible change for the water flux could be observed, indicating that the rest of the aromatic protein (21.2%) could still cause severe wetting for the MD membrane. In addition, SEM-EDS demonstrated that the combination of organic fouling and crystallization of Ca and Ba contributed to the fouling of the MD membrane. These studies demonstrated that the removal of high molecular weight organics by coagulation and removal of aromatic protein with the molecular weight of 200 Da might be vital for the alleviation of MD fouling or wetting.