Spiral-wound membrane reverse osmosis and the treatment of industrial effluents

Reverse osmosis (RO) is increasingly used as a separation technique in chemical and environmental engineering where desalination, selective separation and wastewater treatment are well established examples. Treatment by RO reduces high levels of dissolved salts but has certain limitations when used for the removal of organic compounds from effluents of the chemical industry. The spiral-wound membrane element is the most widely used membrane device because it has a high membrane surface area to volume ratio, it is easy to replace and can be manufactured from a wide variety of materials. This work forms part of a wider research project aimed at recovering products and reducing the concentration of pollutants in wastewater by using membrane process. The synthetic effluent stream that was treated contained an organic product (acrylnitrile) and four inorganic species (sulphate, ammonium, cyanide and sodium). It was found that the pH value ofthe solution plays an important role in the ionization of the different species and, subsequently, in their retention. The object of this study was to reduce the concentration of pollutants to the levels of mentioned regulations concerning discharges, using a pilot plant with a spiral-wound membrane element (0.56 m² surface area) and a polyamide membrane, which was previously selected. The RO measurements were carried out in a closed-loop controlled-pressure system, with the solution being constantly fed through the spiral-wound membrane. The rejection percentage of sulphate ion was high in the two treatments assayed, regardless ofthe operational pH and sequence of steps followed. The degree to which NH₄⁺ and CN⁻ were eliminated was strongly dependent on the pH of the feed stream. Ammonium ion, too, was strongly eliminated regardless ofthe sequence ofthe steps, while the best results with CN⁻ were obtained when the first step had a nearly neutral pH and the second a pH of 11.0. Acrylnitrile showed low rejection percentages in all the steps carried out.