The liquid has tendency to flow from low concentration to high concentration through a semi permeable membrane, in the Osmosis Process. In reverse osmosis the direction of flow is reversed by increasing the pressure at high concentration liquid side than the low concentration side, which results in the reversed flow of the liquid from high concentration to low concentration, through a semi permeable membrane, leaving behind dissolved minerals.
The above semi permeable membrane for reverse osmosis application consists of a polymeric material, which is several thousand Angstroms thick, thin film.
There are two major groups of polymeric materials, which can be used to produce satisfactory reverse osmosis membranes: cellulose acetate (CA) and polyamide (PA). The performance of membrane elements in reverse osmosis systems is affected by the feed water composition, temperature, pressure and permeate recovery ratio.
The feed water, depending on its source, may contain suspended solids and dissolved matter, in varying concentrations. Suspended solids may consist of inorganic particles, colloids and biological debris such as microorganisms and algae. Dissolved matter may consist of highly soluble salts, such as chlorides, and sparingly soluble salts, such as carbonates, sulfates, sulfates, and silica. During the RO process, suspended particales may settle on the membrane surface, thus blocking feed channels and increasing friction losses (pressure drop across the system Sparingly soluble salts may also precipitate from the concentrate stream, create scale on the membrane surface, and result in lower water permeability trough the RO membranes. Threrefore, the feed water pretreatment, to improve its quality to the level, which would result in reliable operation of the RO membranes is must. The common indicators of suspended particles used in the RO industry are turbidity and Silt Density Index (SDI) The maximum limits for turbidity are 1 NTU and SDI of 4. In the continuous operation of an RO system with feed water, the average values of turbidly and SDI in the feed water should not exceed 0.5 NTU and 2.5 SDI units, respectively. The indicators of saturation levels of sparingly soluble salts in the concentrate stream are the Langley Saturation Index (LSI) and saturation rations. Negative values of LSI indicate the possibility of calcium carbonate precipitation. The saturation ratio is the ratio of the product of the actual concentration of the ions in the concentrate stream to the theoretical solubilitys of the salts at a give conditions of temperature and ionic strength. These ratios are applicable mainly to sparingly soluble sulfates of calcium, Barium and Strontium. Silica could also be a potential scale forming constituent. Depending on the raw water quality, the pretreatment process mainly consists of removal of the followings:-
- SUSPENDED SOLIDS OR TURBIDITY
- REDUCTION OF ALKALINITY, BY PH ADJUSTMENT.
- ADDITION OF SCALE INHIBITOR OR SOFTENIG.
- RESIDUEL FREE CHLORINE REMOVAL.
Seldom Chlorination may be used to oxidize iron and manganese in the well water before filtration of biological removal.
- RO system consists of the following basic components:-
- Feed water supply unit.
- Pretreatment systems.
- High pressure pumping units
- Membrane element assembly unit.
- Instrumentation and Control System.
- Permeate treatment and storage unit.
- Cleaning unit.
CIEPL has the ability to provide all the basic components, as illustrated, as above in accordance, with the individual end users requirement.