Electrocoagulation–UF/NF membrane separation of residual metalworking-fluid wastewater from induction-assisted dry hard turning for high-grade workshop water reuse

Residual metalworking-fluid (MWF) wastewater from machining workshops contains emulsified oil, surfactants, suspended solids, metallic fines, and persistent dissolved organics. This study evaluates an electrocoagulation–UF/NF membrane-separation route to upgrade source-reduced wastewater generated during induction-assisted dry hard turning of hardened SKD61 steel. Aluminum electrocoagulation (EC) was used for bulk destabilization, followed by ultrafiltration (UF) and nanofiltration (NF), to compare the EC, EC+UF, EC+NF, and EC+UF+NF routes within a common analytical framework that included water quality, rejection, fouling, energy, residual load, storage, and reuse suitability. The EC+UF+NF route reduced COD from 620 to 18.0 mg/L, TOC to 7.60 mg/L, oil and grease to 0.500 mg/L, turbidity to 0.350 NTU, total Fe to 0.050 mg/L, total Cr to 0.0040 mg/L, and residual Al to 0.020 mg/L. UF pretreatment improved NF hydraulic behavior, increasing terminal flux retention from 60.5% to 71.4% and reducing irreversible NF resistance by 29.8%, although substantial short-term fouling remained. The process-only and comprehensive estimated SEC values were 3.06 and 3.24 kWh/m³, equivalent to 0.00055 and 0.00058 kWh/workpiece. The route supports fit-for-purpose workshop reuse, but long-term operation, storage stability, concentrate management, and coolant-makeup compatibility require further validation.