Fundamentals of reverse osmosis (nanofiltration)

Release Time:

2022-04-11 10:05

The principle of reverse osmosis (nanofiltration)

The principle of osmosis and reverse osmosis
The phenomenon that pure water flows from the low-concentration brine side through the membrane to the higher-concentration brine side is called osmosis. The phenomenon of osmosis shows that when a semi-permeable membrane is placed between two compartments, the semi-permeable membrane will allow certain substances to pass through, while refusing other substances to pass through. Assuming that this permeable membrane is permeable to water but not salt, when a salt solution is placed in one zone and pure water is placed in another zone, the water can permeate the membrane to the other side, but the salt cannot. This system will tend to reach equilibrium, that is, return to the same concentration on both sides, and a possible way to reach equilibrium is for water to flow from the pure water zone through the membrane to the salt-containing zone. It was also shown that osmosis can cause the water level of the salt solution to increase until the pressure of the water column in the salt solution region is high enough to resist the flow. When the balance is reached, the water pressure represented by the height of the water column is called the osmotic pressure. If pressure is applied to this water column, the direction of the water flow can be changed, which is the principle of reverse osmosis.


Because the salt cannot penetrate the membrane, the result of the countercurrent is pure water from the salt-containing solution. That is, the operating pressure is applied on the water inlet (concentrated solution) side to overcome the natural osmotic pressure. When the operating pressure higher than the natural osmotic pressure is applied to the concentrated solution side, the flow direction of the natural infiltration of water molecules will be reversed, and the influent (concentrated solution) flow direction will be reversed. ) part of the water molecules pass through the membrane to become purified water on the dilute solution side.

The principle of nanofiltration
There is no clear boundary between nanofiltration and reverse osmosis. Named for its ability to exclude anything larger than 1 nm (1 nanometer). The nanofiltration membrane is not a perfect barrier for soluble salts or solutes. The permeability of these solutes through the nanofiltration membrane depends on the composition of the salt and the type of the nanofiltration membrane. The higher the osmotic pressure, the closer it is to the reverse osmosis process. On the contrary, if the permeability is higher, the osmotic pressure on both sides of the nanofiltration membrane will be lower, and the osmotic pressure will have less influence on the nanofiltration process. The performance of nanofiltration is between ultrafiltration and reverse osmosis. Organic substances with a relative molecular mass greater than 200 ~ 1000 can be excluded. In addition, the desalination rate of dissolved salts is 20% to 90%. Transmembrane pressure is generally 3.5 ~ 16bar (50 ~ 225psi).


Reverse Osmosis (Nanofiltration) Process
The process of reverse osmosis (nanofiltration) is to continuously apply a certain pressure on the water inlet side (concentrated salt solution side) to overcome the osmotic pressure of the solution, so that the permeate (product water) is continuously produced on the water production side (low salt solution side) the process of.


The raw water is continuously pressurized by a high-pressure pump and sent to the device. The device includes a pressure vessel and a membrane element. The influent water is divided into a permeate containing trace salt and a concentrate containing high salt. The percentage of concentrate and permeate produced from the raw water is controlled by the concentrate. In addition to the pressure vessel, the roll-type membrane group has several groups of roll-type membrane elements. Generally, there are 1-6 groups of membrane elements in a pressure vessel, and there can be up to 7 groups of membrane elements. When the water passes through the next element,
The salt concentration will gradually increase, and finally flow out from the last group, enter the concentrate control valve, and depressurize there. The permeate produced by each membrane element is collected by a common permeate pipe in the center of the group of membrane elements and flows into a permeate collection pipe located outside the pressure vessel.

Related nouns

Reverse osmosis membrane:A functional semi-permeable membrane that allows solvent molecules to pass through but not solute molecules is called reverse osmosis membrane;

Nanofiltration membrane:A functional semipermeable membrane that allows solvent molecules or some low molecular weight solutes or low valent ions to pass through is called a nanofiltration membrane. filter membrane;

Membrane element:The reverse osmosis or nanofiltration membrane membrane is combined with the inlet flow channel grid, the permeate flow channel material, the permeate flow center tube and the anti-stress The smallest unit of the reverse osmosis or nanofiltration process that separates the influent and the produced water is called a membrane element;

Membrane module:The membrane element is installed in the pressure vessel shell to form the membrane module;

Membrane device:A self-contained and self-contained system consisting of membrane modules, instruments, pipes, valves, high pressure pumps, security filters, local control panels and racks The operating complete unit membrane equipment is called membrane device, through which reverse osmosis and nanofiltration processes are realized;

Membrane system:Designed for specific water source conditions and production water requirements, it consists of pretreatment, dosing device, booster pump, water tank, membrane device and The complete membrane water treatment process controlled by electrical instrument chain is called membrane system.

Flow rate:Flow rate refers to the flow rate of water entering the membrane element, usually m3/h or gpm.

Concentrated water flow: refers to the "influent" flow that leaves the portion of the membrane element system that does not permeate the membrane. This part of the concentrated water contains soluble components brought in from the raw water source, usually m3/h or gpm.

Flux:The flow rate of permeate per unit membrane area, usually in l/m2h or gfd.

Dilute solution:The purified aqueous solution is the product water of reverse osmosis or nanofiltration system.

Concentrated solution:The part of the solution that does not pass through the membrane, such as concentrated water from reverse osmosis or nanofiltration systems.

Salt content and total solids:Salt content refers to solids that can be dissociated into ionic states in water, and the sum of their anions and cations is called salt content quantity. The conductivity and resistance of water are related to the salt content. In general, the higher the salt content, the higher the conductivity and the lower the resistance.

Acidity:Acidity refers to the content of acidic substances in water that can react with strong bases (such as NaOH, KOH).

Alkalinity:Alkalinity refers to the content of alkaline substances in water that can react with strong acids (usually 0.1mol/L HCI standard solution). The alkalinity index is often used to evaluate the buffer capacity of water and the solubility and toxicity of metals in it.

Hardness:The hardness of water is divided into two categories: carbonate hardness and non-carbonate hardness, the sum of which is called total hardness.

Carbonate hardness:Refers to the salt composed of calcium, magnesium and other ions and bicarbonate in water. After the water is heated, the bicarbonate is decomposed into carbonate, and the solubility is reduced to form a precipitate and precipitate. Therefore, it is also called temporary hardness.

Non-carbonate hardness:Mainly refers to the hardness formed by calcium and magnesium sulfate, nitrate and chloride. Because water is heated to boiling under normal pressure, they do not form precipitation, so it is also called permanent hardness.

Conductivity:Conductivity is a numerical representation of the ability of a solution to conduct electrical current. The conductivity of pure water is very small. When the water contains inorganic acids, alkalis or salts, the conductivity of the solution increases. Conductivity is often used to indirectly infer the total concentration of ions in water. The conductivity of an aqueous solution depends on the nature and concentration of the ions, the temperature and viscosity of the solution. For every 10°C increase in temperature, the conductivity increases by about 2% to 2.5%, and 25°C is usually specified as the standard temperature for measuring conductivity.

The standard unit of conductivity is S/m (ie Siemens 1 meter). Generally, the actual unit used is μS/cm.

Turbidity: The ISO International Standard defines turbidity as the reduction in the clarity of a liquid due to the presence of insoluble substances. According to the different turbidity standard solutions used in the test, the obtained turbidity values ​​and units are different.

SDI value: SDI ( Silt Density Index ), also known as fouling index, is an important indicator to characterize the influent quality of reverse osmosis system . Compared to turbidity, it represents water quality from a different perspective, but SDI values ​​are more accurate and reliable than turbidity. The measurement of turbidity is to use spectrophotometry or visual turbidimetry to determine the content of particulate impurities in water, but it cannot accurately measure some colloidal particles that are not sensitive to light in water.

Oxidation-reduction potential ORP:Oxidation-reduction potential ORP is a parameter that characterizes the amount of oxidizing and reducing substances in water. The redox potential is generally measured in millivolts (mV). When the redox potential is positive, it means that the water contains oxidizing substances, and when the redox potential is negative, it means that the water contains reducing substances.

Organic matter:There are many kinds of organic matter, and the composition in natural water is ever-changing. There is no accurate direct measurement method, and several indicators related to organic matter , such as oxygen consumption, total solid residue burning weight loss, total organic carbon, etc., can not accurately represent the content and composition of organic matter.


Common units and their conversions:

1in2(1 square inch)≈6.45cm2
1ft2(1 square foot)≈0.0929m2
1gallom(gallon)≈3.785L(US) 1GPM(gallons per minute)≈0.227m3/h
1GPD (gallons per day)≈3.785L/D
14.5PSI≈0.1Mpa≈1 bar(kg/cm2)


The main index to measure the performance of reverse osmosis membrane

Salt rejection and salt permeability

Salt permeability = permeate concentration 1 influent concentration x 100%

Salt removal rate = (1-salt content of produced water/salt content of influent water) x 100%

Salt permeability=100%-salt rejection

The desalination rate of the membrane element is determined when it is manufactured and formed. The level of the desalination rate depends on the density of the ultra-thin desalination layer on the surface of the membrane element. The denser the desalination layer, the higher the desalination rate and the lower the water production. The desalination rate of different substances by reverse osmosis is mainly determined by the structure and molecular weight of the substance. The desalination rate of high-valent ions and complex monovalent ions can exceed 99%, and the desalination rate of monovalent ions such as sodium ion, potassium ion and chloride ion is slightly lower. , but also exceeded 98%; the removal rate of organic matter with molecular weight greater than 100 can also reach 98%, but the removal rate of organic matter with molecular weight less than 100 is lower.


Water production

Water production refers to the water production capacity of the reverse osmosis system, that is, the amount of water permeating the membrane per unit time, usually expressed in ton per hour or gallon per day.

The permeate flow rate is also an important indicator to express the water output of the reverse osmosis membrane element. Refers to the flow rate of permeate per unit membrane area, usually expressed in gallons per square foot per day (GFD). Excessive permeate flow rate will lead to faster water flow rate perpendicular to the membrane surface, aggravating membrane fouling.



Recovery refers to the percentage of the feed water in the membrane system converted into product water or permeate. It depends on the water quality and water requirements of the pretreated water. The recovery rate of the membrane system has been determined in the original design. The calculation formulas of the recovery rate, salt transmission rate and salt rejection rate of the reverse osmosis (nanofiltration) membrane module are as follows;

Recovery rate = water production/influent x 100%

Salt transmission rate = product water concentration 1 influent concentration x 100%

Salt Rejection Rate = (1-Salt Pass Rate) x 100%

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