The Examination and Comparison between Reverse and Forward Osmosis: Which Is the Solution for the Future?
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Water shortages are becoming a global problem which is further aggravated by rapid population and economic growth. Owing to the scarce amount of fresh water available, various water desalination and purification methods have been developed over the past decades to mitigate the scarcity of fresh water. Reverse osmosis (RO) emerged as one of the most viable techniques due to its reduced energy cost compared to distillation, which requires massive amount of heat. RO uses hydraulic pressure to push the flow of water across a semi-permeable membrane to separate water from the salt it dissolves. The past few decades have seen the development of RO membranes, through which optimized materials and compositions have been synthesized for excellent water flux and salt rejection. Recently, forward osmosis (FO) has been gaining attention because no hydraulic pressure needs to be applied. By using a draw solution, a spontaneous flow of water occurs in a FO system, allowing for much lower energy consumption compared to RO. In addition, RO suffers from severe membrane fouling that is intensified by the pressure applied. In contrast, FO membranes show less fouling. However, internal polarization of FO is a major obstacle that causes a severe flux decline. In addition, FO lacks truly optimized membranes since the majority of FO membranes are modified RO membranes. Furthermore, FO needs an optimized draw agent that is easily removable and non-toxic. Hence, a literature review addressing the advantages and limitations of FO with regard to membranes, energy cost, draw agents and fouling problems has been conducted. A comparison between FO and RO in each aspect has been drawn, and the challenges as well as future research directions for FO have been highlighted.