A review of inland nanofiltration and reverse osmosis membrane concentrates management: Treatment, resource recovery and future development

Nanofiltration and reverse osmosis have become prevalent methods for treating inland wastewater because of superior separation efficiency and operational simplicity, yet they produce highly polluting membrane concentrates (e.g., from municipal potable water and municipal and industrial wastewater) containing biological, organic, and salt pollutants. Managing membrane concentrates poses significant challenges, often resulting in underutilization of their valuable components. This paper discusses strategies for reducing, rendering harmless, and utilizing resources from membrane concentrates, along with their future development trends. Methods for reducing membrane concentrate formation, such as optimizing operational modes and membrane modification, are examined. Generally, the preprocessing methods can be used as an oxidation pretreatment to improve the removal of macromolecular pollutants in the membrane concentrates, and provide conditions for the targeted removal of non-biodegradability emerging contaminants by free radicals in the oxidation process. The biological treatment further treats biodegradable pollutants. Finally, biosafety and water quality can be guaranteed through the membrane process, so as to achieve near-zero emissions. Resource recovery options include the production of salt crystals (e.g., potassium and magnesium phosphate, hydroxyapatite), microalgae resources, and others. Additionally, non-chemical pretreatment before membrane processing is proposed to simplify the treatment of membrane concentrates. A new analytic hierarchy process aids in selecting appropriate treatment and resource utilization processes for membrane concentrates. Furthermore, there is a call for the establishment and reinforcement of water quality legislation to address emerging contaminants in membrane concentrates, as well as regulations governing the purity of resources such as salts recovered from membrane concentrates.