Membrane-Based Desalination Technology for Energy Efficiency and Cost Reduction

Reverse osmosis (RO) desalination is reported to reach 3.0kWh/m³ by a certain specific case, and an average product water cost close to 0.75US$/m³ was achieved. After the introduction of an energy recovery device, RO desalination has now reached a technical barrier in energy consumption. To allow for further improvement, two major research trends have recently been highlighted: (1) development of high flux RO membranes, and (2) development of new technologies such as membrane distillation (MD) and forward osmosis (FO). The performance of RO membranes has been improved from 6000 gallons per day (GPD) to 12,000 GPD, due to novel membrane fabrication technologies. Also, membrane modifications are being spotlighted utilizing aquaporin, carbon nanotube, graphene, and other materials. The world's first standalone FO desalination process, by Modern Water (Al Khaluf, Oman), with a capacity of 100m³/d, has demonstrated its potential for commercial application. Nonetheless, FO still lacks economic feasibility due to the inefficiency in draw solute recovery. MD has a high potential to replace prevailing thermal desalination technologies, but in order for it to be economically feasible, a reliable (low grade) heat source is required. Such alternative desalination technologies currently have drawbacks in replacing RO desalination, thus hybridization with RO desalination is now consistently addressed for practical application: FO-RO, utilizing FO as pretreatment for RO or for reducing energy consumption in RO, RO-MD hybrid for improving RO recovery, or RO-PRO (pressure retarded osmosis) hybrid for converting chemical potential to electrical energy. Such hybridizations have just started to be considered in research laboratories and pilot plants, but guidelines for design, operation, and maintenance of such processes are yet to be established.