Before Using Reverse Osmosis Membranes, We must Pay Attention to these Six Points.

Apr 2019
Before Using Reverse Osmosis Membranes, We must Pay Attention to these Six Points.

1.  Influent pressure

Influent pressure will affect the flux and desalination rate of reverse osmosis membranes. The permeability increases with the increase of influent pressure. Because reverse osmosis membranes can not absolutely intercept dissolved salts in the water, there is always a certain amount of permeation with the increase of pressure, because the permeation rate of the membranes is faster than that of salts, and the increase of salt permeability is quickly restrained. Therefore, the influent pressure of reverse osmosis should be considered according to the performance of various membranes.

2. Scope of PH Value Use

The reverse osmosis membrane enjoys the highest desalination rate around PH7, and the desalination rate of the membrane system increases with the change of PH value. The change of PH has a great influence on the electrical conductivity. Reverse osmosis membranes with a wide range of PH values allow us to use stronger, faster and more effective chemical cleaning methods, but too high or too low PH values are likely to cause membrane damage.

3. Influent temperature

Some membranes can increase water permeability by about 2.7% when the water temperature is increased by 1 C. However, when the temperature is too high, the hydrolysis rate of the membrane will be accelerated. Ordinary organic membranes become soft as the temperature rises, and the compaction of the membranes also increases. But too low temperature also affects the normal water production of reverse osmosis membranes. Therefore, the raw water temperature of organic membranes should be controlled at about 20 - 30 C.

4. Flushing Membrane Elements

During the reverse osmosis process, the concentration of solution on the membrane surface increases due to the continuous water permeation through the membrane, and a concentration gradient is formed between the membrane surface and the feed liquid. If the concentration of the solution on the membrane surface is increasing, the osmotic pressure of the solution on the membrane surface increases greatly due to this concentration polarization phenomenon, which leads to an increase in the resistance of water passing through the membrane, and consequently the permeability and desalination rate of the membrane decrease, and some insoluble salts will precipitate on the membrane surface. In order to avoid concentration polarization, the reverse osmosis membrane is washed after the equipment runs to a certain time to prevent the increase of membrane surface concentration.

5. Pretreatment

Because the treated water usually contains inorganic, organic, microbial, granular and colloidal impurities, it must be pretreated before reverse osmosis. Organic membrane filters usually use quartz sand filters, activated carbon filters and ion exchangers. The pretreatment scheme depends on the composition and application conditions of raw water source, and the treatment of well water, surface water and municipal wastewater is different. The water quality of well water is stable, the possibility of pollution is low, and the pretreatment method is simple. However, some sediments are not treated thoroughly, which will cause damage to the membrane. Surface water is a water source directly affected by the season. It has the possibility of high pollution of microorganisms and colloids. So it is necessary to use organic membrane filters for pretreatment, and other pretreatment steps include chlorine disinfection, flocculation/coagulation aids, clarification, multi-media filtration, dichlorination, adding acid or adding scale inhibitors, etc.

6. Security filters

Before reverse osmosis membranes, security filters must be installed to prevent impurities from entering the membranes and to avoid damage to the membranes. The filter element in the security filter is replaced once every 3-6 months.
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