Water Tec Int’l. Inc. Version 02A
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FACTORS THAT AFFECT THE OPERATION AND THE USEFUL LIFE OF THE MEMBRANE:
There are five main factors that affect a reverse osmosis membrane:
1.
PRESSURE: Excessive pressure tends to deform or compact the membrane. The compaction causes the
membrane to become less porous, thus decreasing the amount of product water.
2.
HYDROLYSIS: Hydrolysis is the effect of chemicals in the feed water on the membrane. In general, this
happens when the water temperature is high and the pH is below 2.5 or above 7. For optimum life of the
membrane, the pH should be between 5 and 6.
3.
BACTERIA: Bacteria, if allowed to grow on the membrane, will digest the top layer of the membrane and
reduce the ability of the membrane to reject salt.
4.
TEMPERATURE: Temperatures above 95ºF are generally avoided because of the problems in the
membrane support structures and the accelerated compaction and hydrolysis rates. Membrane production
rates will go up with higher temperatures and down with lower temperatures.
5.
SURFACE COATING OR FOULING: Surface Coating or Fouling is one of the most common problems with
reverse osmosis. It is the salts (calcium carbonate, etc.) that precipitate on the membrane. These salts plug
the pores and channels, reducing the water production rate.
EFFECTS OF TEMPERATURE ON THE REVERSE OSMOSIS UNITS:
In system design, it is very important to consider incoming water temperature. The units are rated for a product flow
at 77ºF (25ºC). The product flow reduces with lower temperatures. On an average, membranes will lose about 1.8%
production for every degree below 77ºF.
SOME COMMON TERMS AND DEFINITIONS RELATED TO REVERSE OSMOSIS:
FEED WATER
- The raw water introduced into the R.O. modules.
PERMEATE WATER
- The pure water produced by the R.O. membranes.
CONCENTRATE WATER
- The wastewater that will not pass through the membrane and is directed to the drain.
MEMBRANE
- Commonly referred to as the R.O. membrane without the membrane housing.
MODULE
- Referred to as the complete membrane in the membrane housing.
G.P.D
. - (gallons per day) This is the standard at which R.O. systems are sized. Example: a 1,000 GPD R.O.
system will produce 1,000 gallons of pure water in a 24-hour period. Systems are generally sized for
maximum amount of water used in a 24-hour period.
PPM
- (parts per million) The method by which the quality of the R.O. products water is measured.
PERCENT RECOVERY
- The amount of water that is recovered by the R.O. membranes. Example: if you
introduce 100 gallons of feed water into the membrane and you produce 60 gallons of
product water and 40 gallons of reject water, this is known as 60% recovery.
PERCENT REJECTION
-
The amount of salts or chemicals rejected by the R.O. membranes.
