Reverse osmosis (RO)
Ⅰ. Introduction
Films that are selective of the material through are called semi-permeable films. Films that can only penetrate solvent but not solute are generally regarded as ideal semi-permeable films. When the same volume of dilute solution and the sides of the thick liquid were placed in a container in the middle of a semi-permeable membrane, dilute solution of the solvent nature through a semipermeable membrane, flowing the concentrated solution side, the side of the strong solution liquid level than the dilute solution of liquid level above a certain height, and form a pressure difference, achieve osmotic balance state, the pressure difference is the osmotic pressure. The osmotic pressure depends on the kind of concentrated solution, concentration and temperature and the semi-permeable material has the selective film is called the semi-permeable film, generally can only pass through the solvent but not through the solute film is called the ideal semi-permeable film. When a dilute solution (e.g. fresh water) and a concentrated solution (e.g. salt water) of the same volume are placed on either side of the semi-permeable membrane, the solvent in the dilute solution will naturally pass through the semi-permeable membrane and spontaneously flow to the concentrated solution. This phenomenon is called osmosis. When the permeation reaches equilibrium, the liquid level of the concentrated solution side will be higher than the liquid level of the dilute solution to a certain height, namely, the formation of a pressure difference, which is osmotic pressure. The osmotic pressure depends on the intrinsic properties of the solution, that is, it is related to the type, concentration and temperature of the concentrated solution but not to the properties of the semi-permeable membrane. If a pressure greater than osmotic pressure is applied on one side of the concentrated solution, the solvent will flow in the opposite direction from the original osmotic direction and begin to flow from the concentrated solution to the thin solution side. This process is called reverse osmosis. Reverse osmosis is a reverse migration movement, is a pressure driven, with the aid of interception effect on the choice of a semipermeable membrane separate solution of the solute and solvent separation method, it has been widely used in all kinds of liquid purification and concentration, which is the most common examples of application in water treatment process, using reverse osmosis technology of inorganic ions, bacteria, viruses, organic matter in raw water and colloidal impurities removal, in order to obtain high quality of pure water.
Ⅱ.Main indicators
1、Desalination rate and penetration rate
Desalination rate - The percentage of soluble impurity concentration removed from the system water by reverse osmosis membrane.
Salinity - The percentage of soluble impurities in the water that penetrate the membrane.
Desalination rate =(1- salt content of produced water/salt content of inlet water)*100%
Penetration rate =100% - desalination rate
The desalting rate of the membrane element is determined when it is manufactured and formed. The desalting rate depends on the density of the ultra-thin desalting layer on the surface of the membrane element. The denser the desalting layer, the higher the desalting rate and the lower the water yield. The desalination rate of reverse osmosis for different substances is mainly determined by the structure and molecular weight of the substances. The desalination rate for high-priced ions and complex monovalent ions can exceed 99%. The desalination rate for monovalent ions, such as sodium ion, potassium ion and chloride ion, is slightly lower, but also exceeds 98%. The removal rate of organic compounds with molecular weight greater than 100 can also reach 98%, but the removal rate of organic compounds with molecular weight less than 100 is lower.
2、Water yield (water flux)
Water yield (water flux)- refers to the capacity of the reverse osmosis system, that is, the amount of water passing through the membrane per unit time, usually expressed in tons/hour or gallons/day.
Osmotic flow rate - The osmotic flow rate is also an important indicator of the water yield of reverse osmosis membrane element. The rate of fluid flow per unit membrane area, usually expressed in gallons per square foot per day (GFD). Too high permeation rate will lead to the water flow velocity perpendicular to the film surface and aggravate the film pollution.
3、The recovery rate of
Recovery - the percentage of water in the membrane system that is converted to producing or permeating water. The recovery rate of the membrane system is determined at design time and is based on the preset inlet water quality.
Recovery =(water yield/water inflow)×100%
Ⅲ、Influence factor
1、Influence of inlet pressure on reverse osmosis membrane
The inlet pressure itself does not affect the salt penetration, but the increase of the inlet pressure increases the net pressure driving the reverse osmosis, making the water yield increase. Meanwhile, the salt penetration is almost unchanged. The increased water yield dilutes the salt penetration of the membrane, reduces the salt penetration rate, and improves the desalination rate. When the inlet water pressure exceeds a certain value, the high recovery rate will increase the concentration polarization and lead to the increase of salt penetration, which will offset the increased water yield and prevent the desalination rate from increasing.
2、Influence of inlet water temperature on reverse osmosis membrane
The water-producing conductance of reverse osmosis membrane is very sensitive to the change of inlet water temperature. With the increase of water temperature, the water-flux linearly increases. For every 1℃ increase of inlet water temperature, the water yield increases by 2.5-3.0%. (Take 25℃ as standard).
3、Influence of inlet PH value on reverse osmosis membrane
The PH value of influent water has little influence on the water yield, but has great influence on the desalination rate. The PH value is between 7.5 and 8.5, and the desalination rate reaches the highest.
4、Influence of influent salt concentration on reverse osmosis membrane
Osmotic pressure is a function of the concentration of salt or organic matter in the water. The higher the salt content in the water, the greater the difference in concentration, and the higher the permeation rate, thus leading to the decrease of desalination rate.
Ⅳ、Purpose
Food, beverage and purified water production process;
Reverse osmosis principle process pharmaceutical, electronic and other industries water preparation;
Chemical process of concentration, separation, purification and water distribution preparation;
Boiler make-up water desalting soft water;
Desalination of sea and brackish water;
Water and wastewater treatment in papermaking, electroplating, printing and dyeing industries。
