Desalination and fouling of NF/low pressure RO membrane for shale gas fracturing flowback water treatment

Shale gas fracturing flowback water (SGFFW) generated during shale gas extraction is of great concern due to its high salinity and complicated components. In this study, the feasibility of nanofiltration (NF) and reverse osmosis (RO) for SGFFW desalination in assisting SGFFW reuse and discharge were systematically evaluated after the pretreatment of coagulation-ultrafiltration (UF) process. Under the same initial water flux of 1.27 × 10⁻⁶ m/s, the water flux decreased tremendously within 800–1000 min for both NF(NF90) and low pressure RO(ESPA1) membranes. The stable water flux was only 38.2% and 15.8% of the initial water flux for NF90 and ESPA1 membrane, respectively. The removal efficiency of the Na⁺ was 81.2% for NF90 membrane and 88.1% for ESPA1 membrane, while the rejection ratios of Cl⁻ were 83.3% and 87.3% for NF90 and ESPA1 membrane, respectively. The rejection ratios for multivalent ions (i.e., Fe³⁺, Ca²⁺ and Ba²⁺) were typically very high (>90%). According to fluorescence regional integration (FRI) under the Excitation—Emission—Matrix Spectra (EEM) within each region volume, the dissolved organics removed by NF in different region decreased in the order of VI (86.0%), I (47.2%), III (31.8%), II (20.8%), V (6.2%), IV(0.2%), while the overall rejection ratio of RO decreased in the order of VI (100.0%), I (88.3%), II (70.7%), III (66.8%), V (58.9%), IV(0.3%). The difference in molecular weight(MW) distribution between the feed and permeate of NF/RO samples showed that the organics with MW of 20 kDa were greatly removed, indicating the NF/RO mainly intercepted this portion of organics such as guar gum, polyacrylamide and various surfactants. However, the effluent of the NF/RO still contained a large portion of biopolymers, which was plausible that small portion of polyacrylamide passed through the membrane. The elemental analysis by scanning electron microscope (SEM)-energy dispersive spectrometer (EDS) demonstrated the fouling layer was mainly composed of organics (i.e., polyacrylamide and aromatic protein). It seemed that developing new-type anti-fouling membrane was a feasible approach to decelerate the membrane fouling.