The water page

REVERSE OSMOSIS METHOD

Osmosis Definition:Water flows from a less concentrated solution through a semipermeable membrane to a more conentrated saline solution until contentration on both sides of the membrane are equal.

Fig 1 Reverse osmosis

Reverse osmosis requires external pressure to reverse natural osmotic flow. As the pressure is applied to the saline solution, water flows from a more concentrated saline solution through the semipermeable membrane

Fig 2 Reverse osmosis membrane

A reverse osmosis membrane has a thin microsporous surface that rejects impurities, but allow water to pass through. The membrane rejects bacteria, 85-95% of inorganic solids. Organic solids with a molecular weight greater than 300 are rejected by the membrane, but dissolved gases pass through. REVERSE OSMOSIS IS A PRECENT REJECTION TECHNOLOGY The purity of water depends on the purity of inlet water. The purity of reverse osmosis product water is much bigger than the purity of feeding water.

Fig 3 Types of membranes

There are usually three types of membrane 1.Cellulose acetate 2.Polyamide 3.Thin film composite

Factors affecting membranes performance

1 pH

2 Free chlorine and Bacteria

Cellulose acetate membranes require constant free chlorine to prevent bacterial growth and membrane damage. In contrast, polyamide and thin film composite membrane are damaged by free chlorine. Therefore, activated carbon must used to remove free chlorine when polyamide and thin film membranes are chosen.

3 Temperature

Membrane performance is based on feedwater temperature of 25 degree Celcius. For every 1 below 25 degree Celcius product water qauntity is reduced by 3%. Feed water which is greater than 35 degree Celcius will damage the membrane.

4 Silt Density Index

The SDI is a measurement of submicron particles and their tendency to block membranes. The feeding water should have a SDI smaller than 5%. Therefore, it is better to use prefilter before Reverse osmosis membrane vessel.

5 Turbidity

Fig 4 * Rerverse osmosis method will waste a lot of water(rejected water). It is because 95% of the feed-water does not pass through the membrane but flows across the membrane surface and drain away as waste water. Feed water pressure also affect both the quality and purity of RO produce water. Lower feedwater pressure will cause lower produced water with a lower water purity.*