Development of a Concept for Closing the Water Cycle in the Surface Treatment of Ferrous and Non-Ferrous Metals

This study examines the treatment of industrial wastewater generated during vibro-abrasive steel and Zn-Al alloy parts machining in a Polish metal-processing plant. The machining process uses grinding fluids, which are sent for disposal after becoming saturated with contaminants, incurring high costs. A two-stage filtration process was investigated: an initial bag filtration (pore size 5 µm) followed by a low-pressure (4 bar) ultrafiltration with polyacrylonitrile membranes (30 kDa cut-off). The studies were carried out on a laboratory scale in a cross-flow system using a batch configuration. The initial filtrate flux was 0.116 mL min−1 cm−2 and 0.050 mL min−1 cm−2 for Zn-Al alloy and the steel wastewater, respectively. Key physicochemical parameters, including turbidity, COD, and TOC, were analysed for raw wastewater, feed, retentate, and permeate. Significant reductions in contaminant concentrations were achieved, with comparable total efficiencies for both the wastewaters tested. The reductions in turbidity, COD, TOC, anionic surfactants, total phosphorus and non-ionic surfactants ranged from 80% to almost 100%. A complete removal of total suspended solids was achieved. The novelty of this research lies in applying polyacrylonitrile flat-sheet membranes to treat wastewater from vibratory machining of ferrous and non-ferrous materials and recycle reclaimed water, which has not been systematically explored in previous studies. The study demonstrates the potential of low-pressure membrane filtration for wastewater recycling, offering insights into environmentally friendly and energy-efficient management of industrial wastewater.