Osmosis: It is a natural phenomenon in which pure water moves from low concentration solution to high concentration solution through a semi permeable membrane. The membrane is permeable to water and some ions. The process continues until it attains chemical potential equilibrium on both the side of the membranes.
Reverse Osmosis: Reverse Osmosis is a technology in which pure water moves from high concentration solution to low concentration solution through semi-permeable membrane and pressure is applied on the high concentration solution side. This technology removes large quantity of contaminants from the water.
Greater the ionic charge on the contaminants are more likely to be trapped in the RO membrane. RO water is used in pharmaceutical industry, food and water industry, boiler feed water.
This process involves cross filtration method. In this water is made to pass through the filter and goes out through two outlets. In one outlet pure water or permeate passes and in another outlet contaminated water flows. They both flows in different direction. In Cross filtration method all the contaminants trapped in the filter are swept away. Accumulation of contaminants are avoided.
Stages in the Reverse Osmosis plant:
- MGF (Multi Grain Filter): It used to remove suspended particles in the water. It consists of three layers of media such as anthracite coal, garnet and sand. A layer of gravel is present at the bottom. Less dense media is at the top and higher dense media is present at the bottom. This type of arrangement allows larger contaminated particles gets trapped at the top and smaller contaminants particles trapped at bottom. It has ability to remove 15 – 20 microns size particles.
- ACF (Activated Carbon Filter): It is used to remove organic material, chlorine and chloramines from water. ACF is generally made up of coal, nutshells, wood. It is generally using adsorption process for the removal of contaminants. The residue of chlorine and chloramines left on the carbon filter is removed by transfer of electrons from the carbon filter to the chlorine and chloramines.
- Micron cartridge filter: These filters are used to remove debris or contaminants from the water. It is placed after the MGF and ACF. Reason behind placing of Micron cartridge filter after the MGF and ACF is that in case if MGF or ACF fails it may cause damage to the downstream pumps. The size of cartridge filter is 5 microns. If the cartridge filter used of lesser microns size, then the filter will fill up faster and this will lead to clogging of the filter.
- Antiscalants Dosing: these are the chemicals which are added in the water to prevent RO system from scale formation. Antiscalants prevents scale formation by getting absorbed to the scale, and forms salt crystal thus preventing attraction between saturated contaminants and crystal surface.
- RO Membrane: Type of the membrane used is Spiral wound TFC- polyamide. Its main components are feed spacer, permeate tube, permeate spacers and membranes. The feed spacer is placed between the membranes. The main function of the feed spacer is to provide space for the water to flow between the membranes and to maintain uniform flow. The permeate spacer is attached with the permeate tube and membrane is attached to the permeate spacer.
The feed water travels tangentially over the membranes across the length. The feed water enters the membranes. It removes almost all the ions and total dissolved solids present in the water. Then the water enters the permeate space where it is carried to the permeate tube.
The pore size of RO membrane is 0.0005 microns. The size of sodium chloride molecule is 0.0007 microns. So the membrane filters the sodium chloride as its size is greater than the size of RO membrane. Also it does not allow the bacteria, virus and other organic molecule to pass through it.