Industrial Wastewater Treatment by Nanofiltration—A Case Study on the Anodizing Industry

The anodizing industry generates several alkaline and acidic wastewater streams often with high concentrations of heavy metals. In this study, nanofiltration (NF) was used to treat wastewater from individual baths, i.e., wastewater from color rinse, alkaline pickling rinse, acidic pickling rinse and anodizing rinse, as well as a mixture of all the wastewater streams. The experiments were carried out by using a commercial membrane (NF99HF) exhibiting pure water permeability of 10 L/(m2·h·bar). For all wastewater streams except one, pH was adjusted to bring it within the recommended pH limits of the membrane, whereby part of the heavy metals precipitated and was removed. The NF of the color rinse offered high-quality permeate (heavy metals below detection limit) and high permeability (9 L/(m2·h·bar)), whereas the nanofiltration of the alkaline pickling rinse exhibited no permeability. The NF of the acidic pickling rinse showed a permeability of 3.1-4.1 L/(m2·h·bar), but low ion rejection (7%-13%). NF of the neutralized mixed wastewater, after the removal of precipitate, produced high-quality permeate with a stable permeability of 1 L/(m2·h·bar). Treatment of the mixed wastewater is therefore the best option if the water has to be discharged. If the water has to be reused, the permeate conductivity in the color rinse and anodizing rinse baths have been reduced significantly, so the treatment of these streams may then be a better option.