Reverse osmosis is a membrane separation operation that uses the pressure difference as a driving force to separate the solvent from the solution. Because it is opposite to the direction of natural penetration, it is called reverse osmosis. According to the different osmotic pressure of various materials, reverse osmosis pressure greater than osmotic pressure can be used, that is, reverse osmosis method, to achieve the purpose of separation, extraction, purification and concentration.
Preparation principle
Reverse osmosis water treatment equipment is usually composed of three parts: raw water pretreatment system, reverse osmosis purification system and ultra-purification post-treatment system. The purpose of pretreatment is to make the raw water meet the water intake requirements of the reverse osmosis membrane separation component and ensure the stable operation of the reverse osmosis purification system. The reverse osmosis membrane system is the most economical and efficient purification method to remove more than 98% ions, organic matter and 100% microorganisms in raw water at one time (theoretically). The ultrapure post-treatment system further removes the remaining trace ions, organic matter and other impurities in reverse osmosis pure water through a variety of integrated technologies to meet the final water quality index requirements of different uses.
Working principle
Reverse osmosis is the most precise membrane liquid separation technology. The operating pressure is applied to the inlet (concentrated solution) side to overcome the natural osmotic pressure. When the operating pressure higher than the natural osmotic pressure is added to the concentrated solution side, the flow direction of the natural osmosis of water molecules will be reversed, and the water molecule component in the inlet (concentrated solution) will become the purification water on the dilute solution side through the reverse osmosis membrane. Reverse osmosis equipment can block all dissolved salt and molecular weight greater than 100 organic matter, but allow water molecules to pass through, reverse osmosis composite membrane desalination rate is generally greater than 98%, they are widely used in industrial pure water and electronic ultra-pure water preparation, drinking pure water production, boiler water supply and other processes, the use of reverse osmosis equipment before ion exchange can greatly reduce the emissions of the operation of water and wastewater.
Pretreatment
The pretreatment system of reverse osmosis water treatment equipment usually consists of polypropylene fiber (PP) filter and activated carbon (AC) filter. For raw water with high hardness, it is also necessary to install softening resin filters. PP filter cartridge can efficiently remove mechanical particle impurities, rust and large colloids and other pollutants above 5μm in raw water, protect the subsequent filter, which is characterized by a large amount of pollution and low price. Activated carbon filter can efficiently adsorb residual chlorine and part of organic matter and colloids in raw water, and protect polyamide reverse osmosis composite membrane from residual chlorine oxidation. The softening resin can remove most of the calcium and magnesium ions in the raw water, prevent the subsequent scaling and blockage on the surface of RO membrane, and improve the recovery rate of water.
Reverse osmosis
Reverse Osmosis (RO) is a high-tech membrane separation technology driven by pressure difference, which has the characteristics of high separation degree, no phase change, simple and efficient. The "pore size" of the reverse osmosis membrane is as small as nanometers (1nm=10-9m), and no "filter" holes on the surface can be seen under SEM. Under the operating pressure higher than the osmotic pressure of raw water, water molecules can reverse osmosis through the RO semi-permeable membrane to produce pure water, and a large number of inorganic ions, organic matter, colloids, microorganisms, pyrogen, etc. in the raw water are trapped by the RO membrane.
Usually, when the raw water conductivity is less than 200μS/cm, the primary RO pure water conductivity is less than 5μs/cm, which meets the laboratory grade 3 water standard. For areas with high raw water conductivity, in order to save the replacement cost of subsequent mixed bed ion exchange resin and improve the quality of pure water, customers can consider choosing a two-stage reverse osmosis purification system, and the second-stage RO pure water conductivity is about 1 to 5μS/cm, which is related to the raw water quality. The principle of reverse osmosis: the same volume of dilute solution (such as fresh water) and concentrated liquid (such as seawater or salt water) are placed on both sides of a container, and the middle is blocked by a semi-permeable membrane. The solvent in the dilute solution will naturally pass through the semi-permeable membrane and flow to the concentrated solution side. The liquid level on the concentrated solution side will be higher than the liquid level of the dilute solution at a certain height, forming a pressure difference and achieving osmotic equilibrium. This pressure difference is called osmotic pressure. The size of the osmotic pressure depends on the type of concentrated liquid, and the concentration and temperature are independent of the nature of the semi-permeable membrane. If a pressure greater than the osmotic pressure is applied on the concentrated solution side, the solvent in the concentrated solution will flow to the dilute solution, and the flow direction of the solvent is opposite to the direction of the original penetration, this process is called reverse osmosis.
General tap water or groundwater after the first level treatment of reverse osmosis water treatment equipment, water production conductivity <10μS/cm, after the second level of reverse osmosis water treatment equipment, water production conductivity <5μS/cm or even lower, in the reverse osmosis water treatment equipment system supplemented by ion exchange equipment or EDI equipment can be prepared ultra-pure water, To achieve a resistivity of 18 megohm (conductivity =1/ resistivity), reverse osmosis is to use enough pressure to make the solvent in the solution (usually refers to water) through the reverse osmosis membrane (a semi-permeable membrane) and separated from the opposite direction of penetration, can be used greater than the osmotic pressure of the reverse osmosis method for separation, purification and concentration of the solution. The main separation object of reverse osmosis membrane is the ion range in solution.
The standard process flow is as follows:
Well water (tap water) → raw water tank → raw water pump → sand filter → activated carbon filter → automatic softening water equipment → security filter → reverse osmosis host → pure water tank
1 Raw water tank
The device prevents the booster pump from directly pumping the water of the pipe network from damaging the booster pump or affecting the normal operation of the system due to insufficient flow and unstable pressure. The stainless steel floating ball valve and level sensor are built into the original water tank. Large equipment must be equipped with water inlet electric disc valve.
1) The function of the floating ball valve is to control the amount of raw water intake, and it can be replenished in time when the system is running.
2) The liquid level sensor has a middle water level and a low water level, which is used to control the start and stop of the booster pump; When the water level of the water tank is above the middle water level, the booster pump can automatically start; When the water level of the tank is lower than the low water level, the booster pump automatically stops.
2 Raw water pump
In order to ensure the constant flow and pressure of the system water supply, the system raw water booster pump can be selected from imported brands, maintenance-free mechanical seal pumps, high efficiency, low noise, stable and reliable performance. Or choose domestic well-known brands ------ Lingxiao pump, New Territories pump, etc. The raw water pump is automatically started and stopped by the water level of the original water tank.
3 Pretreatment
At present, we often have three kinds of pretreatment filters such as stainless steel filter, glass fiber reinforced plastic filter and carbon steel lined rubber filter. Pretreatment filtration is divided into three parts: multi-media filtration, iron and manganese removal device, activated carbon filtration and softening filtration. The American Atuzu multiway valve can be used to automatically control the backwashing and forward washing of the filter material in the filter tank according to the set time; Or choose manual operation control.
The preliminary treatment of water before refining treatment is carried out in advance so as to achieve good results in the fine treatment of water and improve the water quality. Because the natural water has a large number of impurities, such as sediment, clay, organic matter, microorganisms, mechanical impurities, etc., the existence of these impurities, seriously affect the quality of refined water and treatment effect, so it is necessary to reduce or remove some impurities before fine treatment, which requires pretreatment.
1) Multi-media filter (also known as mechanical filter)
Multi-media filter is mainly used to remove suspended matter, sediment and granular impurities in water.
The main filter materials are: quartz sand, anthracite, fiber balls and so on.
2) Activated carbon filter
Activated carbon filter contains granular pure water type activated carbon, which mainly removes macromolecular organic matter, colloid, odor, residual chlorine and other impurities in water, and its absorbance is strong, and can remove more than 90% on average. The residual chlorine is a strong oxidizing agent and has an oxidation effect on the RO membrane, which must be limited to ≤ 0.1 mg/L.
There are three kinds of activated carbon: round carbon, amorphous fruit shell carbon and coconut shell carbon.
3) Cationic resin softener
The principle of sodium ion exchange softening treatment: the raw water is passed through the sodium cation exchange resin, so that the hardness components of Ca2+ and Mg2+ in the water are exchanged with the Na2+ ions in the resin, so that the Ca2+ and Mg2+ in the water are absorbed and the water is softened.
Function: It has positive significance to prevent scale on the surface of reverse osmosis membrane, improve the service life of reverse osmosis and the treatment effect, so the softener is configured in the system. When the total hardness is less than 200mg/LCaCO3, the softener is not needed. It is best to choose automatic control.
