Osmosis is the natural passage of water from a weaker solution to a stronger solution to equalize the chemical potentials of the water in the membrane-separated solution.
Osmotic pressure is the driving force for osmosis to occur.
In the given figure in slider1, the water (moisture) from the soil rises up in the roots to equalize the concentration by diluting the nutrients (salts) present in the roots.
Reverse osmosis is a membrane separation process in which a semi permeable membrane is used to permeate high quality water while rejecting the passage of dissolved solids.
In reverse osmosis an external pressure greater than the osmotic pressure is applied to the solution, causing water to flow against the natural direction through the membrane, thus producing high –quality dematerialized water. The membrane rejects most of the ions and molecules while permitting acceptable rates of water passage.
Many types of membranes have been developed, but cellulose acetate and polyamide (nylon) are currently the most widely used membrane materials. Reverse Osmosis modules suitable for water treatment involve arrangement of membranes and their supporting structures so that feed water under high pressure up to 10,000 kN/m2 can pass through the membrane surface while product water is collected from the opposite face without brine contamination.
Four different types of module designs have been developed: plate and frame, large tube, spiral wound, and hollow fine fiber as shown in slider. In MBR process, we use Plate type and Hollow fiber membranes. Typical membranes are approximately 100 micrometer thick, having a surface skin of about 0.2 micrometer thick that serves the rejecting surface. The remaining layer is porous and spongy and serves as backing material. The hollow fibers have outer and inner diameters of 50 & 25 micrometers.
