Role of Membranes in Wastewater Treatment

Abstract

Demand for water is on the rise as population and human activities increase including industries and agriculture. Freshwater resources have a skewed distribution besides being inadequate to meet the demands. Even though actual water consumed by humans and their activities is much less, large quantum of water is used for peripheral activities and discharged into the environment as wastewater. Hence, to meet the demand for water and to protect the environment, wastewater treatment is necessary. Conventional methods of treatment are cumbersome requiring large footprints, use of chemicals, and subsequent management of sludge generated.

Membrane is a barrier which helps in the preferential transport of some species under a potential gradient, be it mechanical, chemical, or electrical. Membranes can be made from different materials, in different forms, and with different morphologies. Membranes can be porous or nonporous, charged or neutral, and solid or liquid. Because of its flexibility, a variety of membrane processes has been developed and is being used to mitigate many industrial challenges. Membrane processes used in wastewater treatment are ambient temperature processes with no phase change and are rate-governed. The chemical requirements are significantly less compared to conventional processes leading to less sludge production.

An overview of different membrane processes motivated by pressure, concentration, and thermal and electrical gradients is discussed in the context of mitigating water stress situations. The technologies discussed include desalination, water recovery, and recycle and removal of toxic contaminants from wastewater streams including the latest developments in application areas. Utility of membrane contactors in improving the performance of the conventional separation processes is highlighted through membrane solvent extraction, supported liquid membranes, and membrane bioreactors. The potential applications of forward osmosis in water treatment are also indicated. The roles of electrically driven membrane processes such as electrodialysis, bipolar membrane-based electrodialysis, electrodialysis reversal, and electro-deionization in water treatment are explained along with its limitations and challenges. The role of membranes in providing safe drinking water at the point of use has also been highlighted.

The prospects of combining two or more membrane processes like nanofiltration, reverse osmosis, and electrodialysis in water and wastewater treatment are highlighted. With increasing environmental consciousness and the need to recover value from waste, the concept of decentralization of wastewater treatment is proposed wherein the source of waste is isolated, as membrane processes can operate on any scale.

In the future, environmental protection is going to become a critical concern, and the best strategy is to recover everything in the wastewater stream as value toward realizing the concept of “Waste is unutilized Wealth.” The best way to achieve this is by isolating the individual wastewater streams as produced and treating them at the source without mixing with other waste streams. In this context, membrane processes have varieties and are economically viable for different capacities. Since the various streams are isolated, both the product and retentate streams can be recycled, thus leading not only to recovering value but also zero discharge to the environment. This chapter aims at providing necessary background knowledge to select a suitable scheme for the treatment of the specific wastewater including point-of-use devices and value recovery.