Reverse Osmosis Membranes Explained
This is a simplified explanation of RO membranes and the RO process, meant for the layperson. For an in-depth detailed description from an industrial perspective, please visit our Reverse Osmosis webpage.
There are various methods of water purification, but reverse osmosis is one of the most economical and effective methods used in industrial applications today. Reverse Osmosis (RO) is water purification method that uses the basic osmosis principle. Water molecules are forced through semi-permeable membranes to filter out impurities, and all contaminants are flushed away, leaving only the pure usable water.
This process is accomplished through the following three stages:
- Sediment Filter: the first stage is called pre-filtering to remove suspended solids greater than 1 micron, which can accumulate on the surface of the RO membranes during the main filtration process.
- Chlorine Removal: most reverse osmosis membranes in use today are susceptible to degradation from chlorine. This is removed from water by activated carbon filtration or by metering a reducing agent such as sodium thiosulfate or sodium bisulfite upstream of the reverse osmosis machine
- RO Membranes: semi-permeable membranes used for the RO process are typically made of a thin polyamide layer (<200 nm) deposited on top of a polysulfone porous layer (about 50 microns) on top of a non-woven fabric support sheet. Pore size is about 0.0001 micron, which excludes most dissolved contaminants while allowing water molecules to pass through.
How Do These RO Membranes Work?
The most important part of any RO system are the membranes. In a typical RO system, membranes are made of two- to eight-inch diameter sized, spiral wound sheets of semi-permeable material designed to let pure water flow through, while keeping back all other impurities. Within industrial applications, the standard length of each membrane has been set to forty inches. These membranes, typically six at a time, are loaded into single housings, which are then arranged into several parallel flow streams, or stages. Typically, reverse osmosis systems contain multiple stages in a single series, as explained above. The more stages, the less water is wasted as each stage reclaims some of the waste water from the previous stage.
Using pressure and force (typically from a pumping system), the natural process of osmosis is reversed via feed water being pushed through the semi-permeable membranes, overcoming the natural process of osmosis, and leaving the impurities behind, while the filtered water continues through the system. Once this has been achieved, the highly-purified water can be reused for a plethora of other purposes, while the “rejected” impurities are flushed to drain or are recycled for cooling water or other potential uses.