Reverse osmosis refers to a process of water purification that has been used primarily for the desalination of seawater. To understand reverse osmosis, it is first necessary to understand osmosis.
Osmosis is the term for the phenomenon whereby if a semi-permeable membrane separates two salt solutions of different concentration, water will migrate from the weaker solution through the membrane to the stronger solution, until the solutions are of the same salt concentration. Reverse osmosis subverts this process.
It involves applying pressure to reverse the natural flow of water, forcing the water to move from the more concentrated solution to the weaker. The semi-permeable membrane is porous, allowing water to pass through, but blocking the passage of the bulkier salt molecules (Binnie, Kimber, & Smethurst, 2002). The end result is water sans salt on one side of the membrane.
The semi-permeable membranes for reverse osmosis treatment are generally constructed from polyamide-based materials. These materials are resistant to biological degradations, but are subject to chemical attacks from chlorine.
Reverse osmosis has been used as a method of purification for ground and surface fresh water, in addition to its role as a desalinating agent. Working with such water sources creates some problems for the reverse osmosis system. Because of the very small pore sizes involved in the membrane, it is vital that ground and surface water is adequately pre-treated prior to the reverse osmosis process.
Depending upon the hardness of the water involved, scaling of the membrane is likely to occur. If the concentration of the calcium or magnesium in the water (the chemicals that determine water’s hardness) is at a high enough level where the chemicals are insoluble, it will create a hard mineral on the inside of the membrane, rendering it impotent (Vigneswaran & Visvanathan, 1995).
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