Reverse osmosis is the best known filtration method and it is widely used in pharmaceutical water systems. It is based upon membrane filtration technology which removes many types of ions and molecules. In normal osmosis the solvent is driven naturally from a region of higher concentration to lower concentration whereas in reverse osmosis a substantial pressure difference is created among the semi permeable membrane to force the solvent (water) to the diluted side (lesser concentration) and hence the rejected water is left with the contaminants behind.
In most of the reverse osmosis system it cleans the membrane itself and the process is known as crossflow. Reverse osmosis is capable to remove the salts, sugars, proteins, dyes, bacteria and other particles having the molecular weight greater than 250 daltons even viruses. But reverse osmosis is not able to remove the dissolved gases, example oxygen and carbon dioxide. It is effective in removing trihalomethanes (THM’s), pesticides, volatile organic compounds (VOC).
Generally an RO equipment consist of following main parts:
- Water Pumps
- Pre-treatment unit
- Semi-permeable membrane
- Disinfection unit
- Storage and Distribution
The reverse osmosis process requires a force to pass the water through the membrane known as driving force. This force is applied in the form of pressure with the help of pumps. Contaminates of water are separated and collected in a tank known as concentrate. There are certain factors that effects the performance of the system which are as follows:
- Membrane type
- Flow Control
- Feed Water Quality
- Temperature and Pressure
The membranes are commonly made of cellulose acetate which are intersegmental spaces of the given polymers and they are having very small pore size from ranging from 0.01μ to 0.0001μ. The size is designed in such a way that it easily allow passage of water molecules but entraps chemical ions and microorganisms. This membrane is fitted in a hollow tube where the water is passed from outside to inner side of the tube through the membrane. The filtered water is collected from the hollow tubing and the remaining water containing water contaminants is collected separately.
The flow control and the feed water quality is essential as it will effect the downstream purification process. The permeate recovery rate is important part in RO systems which means the ratio of water passing through the membrane to amount of water rejected. This is obtained by applying sufficient pressure around the membrane. Generally recovery is 75% which is not essential when it comes to purified water hence a second stage of RO is set up to obtain the permeate of required quality.