Selection of RO membranes for seawater desalination
Before the Selection of RO membranes for seawater desalination, you should understand the classification of common reverse osmosis membranes on the market. Just to know the raw water quality suitable for different types of reverse osmosis membranes.
The choice of membrane type should be made according to the water quality and degree of pollution. And a number of other factors that we explain in this article. In order to better meet customer’s water quality requirements.
According to different reverse osmosis device manufacturers, the common types of reverse osmosis membranes on the market are:
- Low-pressure reverse osmosis membrane, ultra low-pressure reverse osmosis membrane.
- Reverse osmosis membrane for salt water.
- Anti-fouling reverse osmosis membranes.
- Reverse osmosis membrane for ultra-pure water.
There are many types of reverse osmosis membranes for seawater desalination, but in the end, how do you choose?
The selection of RO membrane series should be based on the end user’s requirements for desalination rate, operating pressure, fouling rate, cleaning frequency, and effluent water quality.
Brackish water membrane is mainly used for well water, river water, lake water, and industrial wastewater as a water source. The anti-pollution membrane can reduce the frequency of chemical cleaning and the operating cost of more polluted water.
Ultra-low pressure membranes are more suitable for small and medium-sized pure water systems. This effectively saves the cost of pumps, piping, and pressure vessels, at low operating pressure. It can still have high performance in water desalination rate.
Ultra-pure water membrane is used in high-purity water, and ultra-pure water systems, micronutrient removal rate is more beneficial. Seawater desalination membrane is mainly used in seawater and other polluted water.
Here is a summary of reverse osmosis membrane options:
Desalination rate and salt penetration rate of reverse osmosis membrane
The desalination rate of the RO membrane element is determined when it is manufactured.
The desalination rate depends on the density of the very thin desalination layer on the surface of the RO membrane element.
The higher the layer density, the higher the desalination rate and, at the same time, the lower the yield.
The reverse osmosis membrane desalination rate of various materials is mainly determined by the composition and molecular weight of the materials.
The desalination rate of high valent ions and complex monovalent ions can exceed 99%, and the desalination rate of monovalent ions such as sodium ion, potassium ion, and chloride ion is slightly lower but can exceed 98% (the longer the length of the reverse osmosis membrane) used, the longer the times Chemical cleaning, reverse osmosis membrane desalination rate decreased), it can also exceed 98% for organic substances with a molecular weight of more than 100, but the removal rate of organic substances with a molecular weight of less than 100 is lower.
Method of calculating salt rejection and salt permeability for reverse osmosis membrane:
RO membrane salt permeability = water concentration in RO membrane product / influent water concentration x 100%
RO membrane desalination rate = (1-RO membrane water salt content / influent salt content) x 100%
Salt permeability of reverse osmosis membrane = 100% – desalination rate
Water production and permeation flow rate of reverse osmosis membrane
RO Membrane Water Production – Refers to the water production capacity of the RO system, i.e. the amount of water that permeates the RO membrane per unit time, usually expressed in tons/hour or gallons/day.
The permeate flow rate of the RO membrane is also an important indicator of the water production of the reverse osmosis membrane element.
It refers to the permeate flow rate per unit membrane area and is usually expressed in gallons per square foot per day (GFD).
Too high a permeation flow rate will increase the water velocity perpendicular to the surface of the RO membrane, which will aggravate membrane fouling.
Reverse osmosis membrane recovery rate
RO membrane recovery rate refers to the percentage of feed water converted into product water or permeated into the reverse osmosis membrane system.
It depends on the quality of the pre-treated influent water and the water requirements of the reverse osmosis system.
The recovery rate of the RO membrane system is determined at design time.
- RO membrane recovery rate = (RO membrane production water flow / effluent) x 100%
- The calculation formulas for the recovery rate, salt permeability, and desalination rate of reverse osmosis (nanofiltration) membrane units are as follows:
- Recovery rate of reverse osmosis membrane unit = water production of RO membrane unit/amount of water x 100%
- Salt permeability of RO membrane unit = Produced water concentration of RO membrane unit / Influent water concentration x 100%
Water output capacity of reverse osmosis membrane
Under normal conditions 4040 membrane elements are mostly used for reverse osmosis equipment with water production less than 4T/H; For 0.25T/H reverse osmosis equipment, choose 4040 membrane for one person, 0.5T/H for two people, and 1T/H for reverse osmosis. 4, and so on; 8040 membrane elements are mostly used for reverse osmosis equipment with water production ≥ 4T/H; 8040 membrane elements are about 1t/h, and 4 8040 membrane elements are selected for reverse osmosis equipment with 4t/h.
A decrease in desalination rate and water production is the most common failure in reverse osmosis and nanofiltration systems.
Blockage of the membrane element inlet flow channel and increased pressure difference in the unit are other typical failures.
If the desalination rate and water production are relatively high. A gradual decline indicates the presence of normal fouling in the system, which can be dealt with by proper and regular cleaning.
However, rapid or sudden performance degradation indicates defects in the system Or incorrect operations.
It is necessary to take corrective action as soon as possible. Because any delay in treatment will lose the opportunity to restore system performance. There will also be too low water production and too high water production.
Also, learn about the pros and cons of desalinating seawater using RO technology.
