General Knowledge - Reverse Osmosis Principle

In 1748 French scientists first discovered the phenomenon that water can permeate a pig’s gallbladder but, based on the production technology of the time, this knowledge could not be used in a practical way. This phenomenon was used over the next two hundred years to explain other various natural phenomena, such as the theory on how trees absorb moisture. The principle is as follows:

If the permeate membrane separates brine on two sides with different concentrations, the water molecules will move from the low concentration side to the high concentration side in order to balance the potential energy on both sides, until the water level and water pressure difference between the two sides are equal whereby water molecules cannot be relocated and material exchange is balanced. This is the osmotic effect (Osmosis) phenomenon. Such as 4.11.

FIGURE 4.11 The phenomena of thin membrane permeation
Data source:

Reverse osmosis, as its name implies, is able to stop the above mentioned infiltration effect, such as pressure moving from the high concentration side and reverse it, so that the water from the high concentration side is forced to the low concentration side for water purification. As shown in Figure 4.12.

FIGURE 4.12 The phenomena of permeation
Data source:

In simple terms, in the raw water side of the application the pressure is greater than in reverse osmosis. In reverse osmosis, contaminated components concentrated in the raw water such as: dissolved and insoluble inorganic salts, heavy metals, organic matter, bacteria, and particles cannot pass through the semipermeable membrane. Only water molecules and smaller molecules of salt can penetrate the film, and flow to the water on the other side.

To concentrate raw water the pressure has to be gradually and infinitely increased. In actual operation, if you could continue to supply raw water to maintain a constant pressure you could drain the concentrate and achieve the purpose of reverse osmosis.

When it comes to the reverse osmosis membrane, it is included in a small part of the separation membrane. The separation membrane (CROSS-FLOW) can be subdivided into: Refer to Table 4.16.

-  Reverse Osmosis (RO)
-  Ultra Filtration (UF)
-  Microfiltration (MF)
-  GAS Separation (GS)

In all separation membrane projects reverse osmosis membrane permeability holes are the smallest, between 0.0001 ~ 0.002 micron. For comparison the human hair has a diameter of about 10micron. You can therefore see the precision in the reverse osmosis membrane pore. In addition it effectively filters bacterial viruses and also removes toxic heavy metals in water, as shown in Figure 4.13 separation membrane application and Table 4.16 separation membrane type.

Table 4.16 Types of separation membrane​
NO. Abbreviation Separation membrane English name Chinese name Water permeation porous diameter (MICRON)
1 RO REVERSE OSMOSIS 逆滲透 0.0001 ~ 0.002
2 UF ULTRAFILTRATION 超過濾 0.006 ~ 0.11
3 MF MICROFILTRATION 微孔過濾 0.08 ~ 1.2
4 ED ELECTRODIALYSIS 電滲析(或稱電透析) ----
9 NF NANOFILTRATION 極濾膜 0.0009 ~ 0.009

Data Source: consolidated by EASY WELL

FIGURE 4.13 Application of separation membrane
(Source: Lin Dongpo Master's thesis)
R.O. (reverse osmosis method) is the great invention of human beings in the 21 centuries.  It is the most advanced and safest water filtration method.  The main reasons are that R.O. reverse osmosis method is the alternates of the following products.
1. Ion exchanger resin.
2. Distill water machine.
3. Water softening device.
4. UV disinfection machine.
5. Activated carbon water filter.
6. Water filtration canister.