Reverse Osmosis or RO is used to purifying water containing more than 200 PPM of total dissolved solids. The process can be compared to filtration, except that the particles to be filtered are extremely small, up to molecular level. The solids are no longer in suspension, but are dissolved in the water.
Semi-permeable membranes are used as the filter medium.
In order for the water to pass through the microscopic pores of the membranes, it has to be pressurized to a high pressure by RO pumps. The water has to overcome the osmotic pressure exerted by the different salt concentrations on the 2 sides of the membrane. It has also to force the water through the membrane. The pressure is usually more than 10 bar.
Pure water passes through the membrane and the molecular particles remain at the supply water. The membrane is able to remove suspended particles, organic carbon, microorganism salts, most inorganic salts. These remaining impurities are led away as reject water. As much as 99 percent of the total dissolved solids are removed.
Because the membranes have such microscopic-sized pores, it is important that the water supplied to them is prefiltered. The filters are for filtering suspended particles up to10 micron in size. The membranes do get clogged up. It is important to have some pre-treatment for the water before they are pumped into the membranes to prolong their lifespan.
Chlorine in the incoming water is bad for the RO membranes. The pre-treatment for this is to inject Sodium Sulfite into the water to remove the chlorine. Some systems use activated carbon prefilter for removing chlorine. The RO system can also be designed to perform 10 minutes of low pressure washing every 24 hours by the operation of valves.
A typical RO membrane system will have 80 percent pure water outlet, and 20 percent reject water. Membranes are costly. Various parameters like flow, pressures, and conductivity have to be closely monitored in order to detect abnormalities. Periodic membrane cleaning is also required.