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Osmosis
To understand reverse osmosis we must first understand osmosis.
During natural osmosis, water flows from a less concentrated solution through
a semipermeable membrane to a more concentrated saline solution until
concentrations on both sides of the membrane are equal (see figure 2).
Reverse Osmosis
Reverse osmosis requires external pressure to reverse natural osmotic flow.
As pressure is applied to the saline solution, water flows through the semipermeable
membrane (see figure 3).
Reverse Osmosis Membrane
A reverse osmosis membrane has a thin microporous surface that rejects impurities,
but allows water to pass through. The membrane rejects bacteria, pyrogens,
and 85%-95% of inorganic solids. Polyvalent ions are rejected easier than monovalent
ions. Organic solids with a molecular weight greater than 300 are rejected by
the membrane, but dissolved gases pass through. Reverse osmosis is a percent
rejection technology. The purity of the product water depends on the purity of
the inlet water. The purity of reverse osmosis product water is much higher
than the purity of the feedwater (see figure 4).
Reject Water
A large
percentage (50-90%) of the feedwater does not pass through the membrane but flows
across the membrane surface, constantly cleaning it and carrying the inorganic
and organic solids to drain. This water is called "reject."
Feedwater Factors
Feedwater factors affecting membrane performance and life include the following:
- Pressure
Feedwater pressure affects both the quantity and the purity of reverse osmosis product water. Lower feedwater pressure causes lower product flow
rate and lower product purity.
- pH
Feedwater pH range is important. It is recommended that you use wider pH range membranes when feedwater is basic, acidic or unstable.
- Langlier Saturation Index (LSI)
The LSI indicates the tendency for scale to form on a membrane surface. It requires feedwater testing for temperature, total inorganic solids, calcium
hardness alkalinity, and pH. If the LSI index calculation is positive, it is recommended that you install a water softener prior to the reverse osmosis system.
- Free Chlorine and Bacteria
Cellulose acetate membranes require constant free chlorine to prevent bacterial growth and membrane damage. In
contrast, polyamide and thin film membranes are damaged by free chlorine. Activated carbon is used to remove free chlorine when polyamide and thin film
membranes are chosen.
- Temperature
Membrane performance is based on feedwater temperature of 25°C. For every 1°C below 25°C product water quantity is reduced by 3%.
When feedwater temperature is regularly below 25°C, it is recommended that hot and cold water are mixed to increase the temperature to 25°C.
Feedwater which is greater than 35° will damage most membranes.
- Silt Density Index
The SDI is a measurement of submicron particles and their tendency to block membranes. Flowing water at specific pressure is filtered through a
membrane disc and collected for a fixed period of time. The speed of water flow and total volume collected determines the index value.
- Turbidity
Turbidity is a measurement of suspended submicron particles that obscure light rays.
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