Why does reverse osmosis water treatment technology need pretreatment and what are the commonly used pretreatment processes?

Why does reverse osmosis water treatment technology need pretreatment?

The desalination rate of the reverse osmosis effluent is greater than 98%, so the reverse osmosis influent index directly determines the desalination effect of the membrane and the service life of the membrane. In order to ensure the influent requirements of the reverse osmosis membrane, the influent water of the reverse osmosis membrane must be nominally treated before the membrane. As the standard treatment before the membrane enters the water, pretreatment is mainly set up to protect the normal working pressure, water production, and membrane life of the membrane. The purpose of pretreatment is to remove various suspended solids and colloids in the water and achieve subsequent reverse osmosis water treatment equipment. Water intake requirements.

What are the commonly used pretreatment processes for reverse osmosis water treatment technology?

At present, the commonly used pretreatment processes mainly include: coagulation sedimentation, multi-media filtration, activated carbon filtration, ion exchange softening or dosing for scale inhibition, temperature and PH value adjustment, security filtration, etc. Special pretreatment processes also need to reduce the content of metal ions in water (such as iron, manganese, copper, etc.), sterilize microorganisms, and remove silica.

Addition of coagulant

It is mainly to remove the suspended colloid of tiny particle size in water. Because these tiny particles will not settle in the water under the action of gravity, and it is difficult to remove them in the subsequent filter, so the raw water needs to be coagulated. It is by adding a coagulant to the raw water to form larger flocs with the suspended solids and colloids in the water, and then filtering and removing them through a multi-media filter. When the content of iron in the raw water is less than 0.3 mg/L and the content of suspended solids is less than 20 mg/L, this device may not be used for pretreatment. When the content of suspended solids and colloidal substances in the water is relatively high, a coagulant addition device is generally required.

Multi-media filter

Multi-media filter is an essential pretreatment equipment, its function is to filter out fine particles, suspended solids, colloids and other impurities in raw water.

It can remove free chlorine and organic matter in water to protect reverse osmosis facilities. Free chlorine can oxidatively damage the reverse osmosis membrane, and reverse osmosis generally requires residual chlorine in the influent <0.1ppm; organic matter is not only the bait for microorganisms, but when it is concentrated to a certain extent, it can dissolve the organic membrane material and deteriorate the membrane performance. There are many kinds of organic matter in water.

Precision filter

also known as security filter, its function is to intercept particles larger than 5 microns in raw water to ensure that the reverse osmosis membrane is not scratched by the suspended matter of large particles. Because the thickness of the reverse osmosis membrane is about 10 microns, the larger particles in the raw water can easily scratch the desalination skin layer on the surface of the reverse osmosis membrane or may break down the reverse osmosis membrane module after being accelerated by the high-pressure pump. A 5-micron precision filter must be installed. The working process of reverse osmosis is that the raw water flows from one end to the other end on one side of the membrane, the water molecules permeate the membrane surface, from the raw water side to the other side, and the inorganic salt ions stay on the original side. With the gradual growth of the raw water process, water molecules are continuously removed from the raw water, and the salt content in the raw water gradually increases, that is, the raw water is gradually concentrated, and finally becomes concentrated water, which is discharged from the device. After the concentrated water is concentrated Various ion concentrations will be multiplied. The ion depth product of Ca2+, Mg2+, Ba2+, Sr2+, SiO2, SO42-, HCO3- and other ions tending to scale in natural water sources is generally less than its equilibrium constant, so scaling will not occur, but after concentration, various The concentration product of ions is likely to greatly exceed the equilibrium constant, resulting in severe fouling.

The water after the above treatment can reach the influent standard of the reverse osmosis membrane. Under the action of the pressure, the water passes through the reverse osmosis membrane to become pure water; the impurities in the water are intercepted by the reverse osmosis membrane and taken out by the concentrated water. Osmosis technology can effectively remove impurities such as inorganic salt ions, colloids, bacteria, viruses, bacterial endotoxins and most organic matter in water. The salt removal rate of the reverse osmosis system is generally 95-99%. The online monitoring conductivity of the effluent of the primary reverse osmosis equipment is generally between 1-10us.

Different organics have different hazards to the reverse osmosis membrane. Therefore, when the reverse osmosis pretreatment system is designed, if the total organic carbon (TOC) content in the water exceeds 2mg/L, it generally needs to be treated. For the purification of tap water as the water source system, activated carbon adsorption is necessary because of the high organic content in surface water.

Due to the advantages of simple equipment, small investment, simple operation and easy control of treatment effect, activated carbon adsorption method is widely used in water treatment and purification systems.

Softening and anti-scaling agent facilities

Prevent insoluble salt from scaling on the surface of reverse osmosis membrane. Softening is to use ion exchange resin to exchange calcium and magnesium ions in water to remove calcium and magnesium ions in water. The advantage of softening is that it can completely remove calcium and magnesium ions in the water and prevent these ions from scaling in the reverse osmosis device. The disadvantage is that the equipment investment is high, and it needs to be regenerated after a certain period of operation. Adding scale inhibitors can delay the growth of salt crystals to delay the precipitation process, and this method can also prevent scale pollution from reverse osmosis. However, since calcium and magnesium ions still exist in the water, the calcium and magnesium ions in the reverse osmosis effluent will be higher than that of softening, and it is easier to achieve automatic control by adding chemicals and scale inhibition, and the equipment is simple and low-cost. In addition, the use of scale inhibitors can also reduce silicon precipitation, but softening cannot, which is very suitable for groundwater source treatment. The cost of the softening facility is proportional to the flow rate, while the cost of the scale inhibitor addition facility has little relationship with the flow rate. Therefore, the two-stage reverse osmosis and device type systems mostly use the scale inhibitor addition system.

Heat exchanger

Adjust the temperature of reverse osmosis feed water, ensure the water yield of reverse osmosis and protect the reverse osmosis membrane. Because within a certain range, the effluent of the reverse osmosis membrane is proportional to the inlet water temperature, and the effluent will drop by 3-5% for every 1°C drop in temperature. At the same time, when the inlet water temperature of the reverse osmosis membrane is less than 5°C, the structure of the reverse osmosis membrane will be destroyed. , and the manufacturer does not guarantee the quality. In some areas, the winter is cold, and the raw water temperature affects the normal water output of reverse osmosis, so it is necessary to add this device to ensure the water output and protect the reverse osmosis membrane.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

Hardness Water Softening System

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