Membranes in Zero-Liquid-Discharge Systems for Efficient Processes toward Sustainable Environment: A Review

With growing environmental concern over the pollutants being disposed of and the consequent pollution of surface water/ground water, the stringent environmental regulation called zero-liquid-discharge effluent treatment has gained popularity. Zero-liquid-discharge effluent treatment technologies avoid the liquid effluent being released outside the plant and can reuse water, which is an important commodity for any process plant. However, such technologies incur significant capital costs and recurring costs in terms of energy consumption. With the growth in industries in different sectors, such as chemicals, pharmaceuticals, textile, dyes and dye intermediates, dairy, food, and pulp, it is of paramount importance for the government and environmental regulating authorities to impose zero-liquid-discharge as the norm for the certain plants producing a certain amount of effluents. Thus, industries should develop sustainable and cost-effective zero-liquid-discharge technologies. Conventional zero-liquid-discharge technology comprises pretreatment, brine concentrator, and brine crystallizer to solidify all the impurities by thermal evaporation; such plants often have very high-energy consumption per cubic meter of effluent treated. Membrane-based technologies prior to brine concentrators can decrease the load of thermal evaporators by concentrating the effluent to a significant extent. However, the membrane used in zero-liquid-discharge technologies should have certain specific characteristics. They should be able to withstand the high organic load and high total dissolved solids concentration and consequently high pressure. The organic, inorganic, and biofouling on the membrane surface should not be so high so that the membranes can last longer. The present paper reviews different membranes used in zero-liquid-discharge technologies, their characteristics, and their performances and shows the perspectives and scope for future research in the area. Osmotically assisted reverse osmosis and low-salt-rejection reverse osmosis are relatively newer developments in zero and minimal discharge applications. The present paper shows new insight into this domain and provides directions for developing zero-liquid-discharge processes.