Integrated physicochemical treatment train for flowback and produced water valorization into chlor-alkali brine

Flowback and produced water (FPW) can serve as sustainable feedstock for the industrial chlor-alkali (CA) process, but elevated concentrations of multivalent cations and dissolved organic matter cause severe diaphragm and membrane fouling, scaling, and efficiency loss. Conventional standalone FPW treatment technologies, including membrane separation, chemical treatment, and advanced oxidation processes, are constrained by performance degradation under hypersaline conditions, high energy and chemical demands, incomplete removal of dissolved organics and divalent cations, and poor selectivity in preserving the NaCl-rich brine required for the CA applications. This study hypothesizes that an optimized integrated treatment train combining coagulation, chemical softening, microfiltration, and granular activated carbon (GAC) filtration may reduce divalent cations and organic matter to levels compatible with diaphragm cells. Treatment is quantified by total organic carbon (TOC) and divalent cations (Ca 2+ , Mg 2+ , Sr 2+ , and Fe 2+ ) concentrations in raw and treated FPW. Batch coagulation-flocculation experiments employ ferric chloride hexahydrate (FeCl₃·6H₂O) followed by dynamic GAC filtration for TOC removal, while divalent cations are precipitated using varied soda ash (Na₂CO₃) and caustic soda (NaOH) dosages. Overall, divalent cations removal exceeds 99.9%, reducing combined Ca 2+ , Mg 2+ , Sr 2+ , and Fe 2+ concentrations below 5 mg.L −1 at 0.42 mol.L −1 of Na 2 CO 3 dosage. GAC adsorption achieves a TOC removal of 90.70%, yielding a treated brine TOC of 38.80 mg.L −1 . The treated brine generally meets diaphragm cell feed quality specifications except for TOC, which remains above the recommended maximum limit, indicating that additional polishing may be required to fully satisfy CA brine standards.