Application of softening-coagulation-flocculation-UF hybrid process for internal reuse of shale gas fracturing flowback water: performance and fouling analysis

Shale gas fracturing flowback water (SGFFW) recycling for another fracking is currently the dominant option but is subject to certain restrictions, including the declined performance of hydraulic fracturing due to divalent metal ions, organics and particles. In this study, we investigated the performance of softening-coagulation-flocculation-ultrafiltration (S-C-F-UF) process in treating Changning SGFFW and membrane foulings. The results indicated that (1) the S-C-F process could achieve better water quality than S-C process, the 99.3% of turbidity, 34% of total organic carbon (TOC) and 85.9% of hardness can be removed under the optimal pretreatment condition (pH = 10, 500 mg/L polyaluminum chloride [PAC], 0.2 mg/L polyacrylamide [PAM]); (2) under the optimal condition, the normalized water flux decline was minimum and the membrane life was the longest; (3) with increase dosage of PAC, the fouling mechanism changed from complete blocking to standard blocking model, while the standard blocking changed to cake blocking model with increase dosage of PAM; (4) the performances of slick water prepared by the effluent treated by the S-C-F-UF hybrid process met the performance requirements of slick water. The results of this paper demonstrated that the hybrid process was an effective technology for internal reuse of SGFFW in shale plays.