The internal reuse of flowback and produced water (FPW) for another hydraulic fracking is currently the most dominant and economical option but is subject to certain restrictions, including the declined performance of hydraulic fracturing due to residual divalent metal ions. In this study, we investigated the performance of coagulation-ultrafiltration (UF)-nanofiltration (NF) in treating Weiyuan shale gas FPW. Different coagulants (aluminum, iron), dosages (0-1200 mg/L), types of NF membranes (VNF1, NF90, NF270), water recoveries (50-85%) and working pressures (100-400 psi) have been systematically studied and analyzed. The results indicated that (1) aluminum and iron coagulation at optimal dosage decreased OF membrane fouling resistance by 64% and 84%, respectively; coagulation followed by OF was suitable as NF pretreatment; (2) Membrane type significantly influenced permeate flux and contaminant rejection of NF membranes; An decrease in operating pressure (100-200 psi) resulted in a slight fouling suggesting the presence of a limiting flux; (3) Coagulation (iron, 900 mg/L)-UF-NF (200 psi) process removed 99.9% of turbidity, 94.2% of COD and most divalent ions (72.8% of Ca2+, 86.3% of Mg2+, 82.8% of Ba2+, 80.1% of Sr2+ and 91.7% of SO42-). The integrated coagulation-UF-NF process was an effective technology for internal reuse of FPW in shale plays.
Weiyuan shale gas;Flowback and produced water;Internal reuse;Coagulation-ultrafiltration;Nanofiltration;