What Is Reverse Osmosis?

RO membrane technology has been widely used in industrial and domestic water purification treatment in Europe and the United States since the 1980s. In the mid to late 1990s, ro membrane technology began to be widely used in China. Ro membrane technology is mainly used in power plant boiler feed water treatment, food and pharmaceutical water treatment, and bottled drinking water treatment. In recent years, emerging industrial wastewater reuse devices and process water in the chemical industry have made ro membrane technology the core process.

Understanding of Reverse Osmosis

When the pressure on the dilute solution side and the concentrated solution side of the semi-permeable membrane is the same, the phenomenon that water in the dilute solution passes through the semi-permeable membrane into the concentrated solution side, resulting in a decrease in the concentration of the concentrated solution, is called osmosis. At this time, the number of water molecules that pass through the semi-permeable membrane from the dilute solution side to the concentrated solution side per unit time is more than the number of water molecules that pass through the semi-permeable membrane from the concentrated solution side to the dilute solution side, resulting in a decrease in the concentration of the concentrated solution. When the number of water molecules that pass through the semi-permeable membrane from both directions is equal per unit time, osmosis reaches equilibrium.

If a certain pressure is applied to the concentrated solution side, which can bring the osmosis on both sides to equilibrium, the external pressure is called osmotic pressure. The magnitude of the osmotic pressure depends not only on the solution system but also on the concentration of the solute and the temperature.

If the external pressure on the concentrated solution side exceeds the osmotic pressure, the number of water molecules passing through the semi-permeable membrane from the concentrated solution side to the dilute solution side per unit time will be more than the number of water molecules passing through the semi-permeable membrane from the dilute solution side to the concentrated solution side, and this process is called reverse osmosis. The separation process of  ro membrane technology uses the property that the semi-permeable membrane only allows water to pass through and traps dissolved solids. The net pressure difference on both sides of the membrane is used as the driving force to make water pass through the semi-permeable membrane from the concentrated solution side to the dilute solution side, thereby realizing the separation of solute and solvent on the concentrated solution side in the membrane process.

Therefore, the separation process of ro membrane technology should have the following conditions: first, the semi-permeable membrane used should have high permeability and selectivity; second, the net pressure difference on both sides of the semi-permeable membrane is greater than zero.

Structural Properties of Reverse Osmosis

The RO membrane sheet generally consists of three layers: non-woven fabric, polysulfone layer, and desalination layer. For some special membrane elements, the surface layer of the membrane sheet will be coated with a functional modification layer.

The main performance indicators of reverse osmosis:

•  Desalination rate: under normal circumstances, the desalination rate is above 98%, and the salt permeability rate is 1 - desalination rate = 1% - 98% = 2%.

  
  

•  Water permeation rate: generally, the first-stage reverse osmosis design flux is 8-14GFD, and the second-stage reverse osmosis is 20-30GFD. 1GFD=1.698LMH. The water production of a single membrane element = membrane area * design flux. For example, if the membrane area is 400ft2, which is 37.2m2, and the design flux is 14GFD, which is 23.8LMH, then the water permeation rate of the membrane is 37.2 * 23.8 / 1000 = 0.89m3/h.

  
  

•  Salt rejection rate increase: considering it every year at a 10% rate and if the salt rejection rate is 2% in the first year, which means the desalination rate is 98%. Then in the second year, the salt rejection rate would be 2% * (1 + 10% ) = 2.2%, and the desalination rate would be 97.8%.

  
  

•  Water decay rate: considering it every year at 7%, which is the same calculation as above.

  
  

•  Recovery rate: that is the conversion rate of the supply water to the permeate, which directly affects the cost of the desalination system. For brackish water, the recovery rate is about 90%; for highly brackish water, it drops to 60%-65%; for industrial seawater systems, the recovery rate is 35%-45%.