Why nanofiltration membranes can achieve high desalination rates at lower operating pressures

Nanofiltration membrane technology is a membrane separation technology between ultrafiltration and reverse osmosis, with a molecular weight cut-off in the range of 80-1000 and a pore size of a few nanometers, so it is called nanofiltration.

The nanofiltration membrane is between ultrafiltration and reverse osmosis, and has a certain rejection rate for inorganic salts, and the molecular weight of organic matter is optional from 200 to 1000 dalton. Toray low-pressure nanofiltration membranes are often used to remove organic matter and color from surface water, remove hardness of groundwater, partially remove dissolved salts, concentrate fruit juices, and separate useful substances from pharmaceuticals.

The working principle of Dran Meier's low-pressure nanofiltration membrane is the same as that of ultrafiltration and reverse osmosis membrane separation, and nanofiltration is also an irreversible process driven by pressure difference. The separation mechanism can be described by using charge models, fine pore models, electrostatic repulsion and stereo resistance models, etc. Compared with other membrane separation processes, nanofiltration has the advantage of being able to retain the small molecular weight of organic matter through the ultrafiltration membrane, and can dialyze part of the inorganic salts trapped by the reverse osmosis membrane, that is, it can make the concentration and desalination go at the same time.

When the nanofiltration membrane is used to separate the solute in the solution, its rejection rate will be affected by some factors, so as to show different change rules, and a detailed understanding of this law is conducive to the better application of the separation performance of the nanofiltration membrane.

Here we will mainly introduce some changes of nanofiltration membranes according to the different solutes treated in the process of separating the solution

1. If the pressure of the system is kept constant, the rejection rate of the nanofiltration membrane will decrease with the increase of solution concentration.

Second, the rejection rate of this membrane is proportional to the change of molar mass of the solute, and when the molar mass decreases, then the rejection rate will also decrease.

3. If the concentration of the solution remains constant, the rejection rate of the membrane will be proportional to the change of pressure difference on both sides, and the decrease of pressure difference will lead to a decrease in the rejection rate.

Fourth, for some common anions in the solution, the rejection rate of the membrane will increase in the order of nitrate ions, chloride ions, hydroxide ions, and sulfuric acid ions.

5. For some common cations in solution, the rejection rate of the membrane will increase in the order of hydrogen ions, sodium ions, potassium ions, calcium ions, magnesium ions, and copper ions.

Summary. Not the same. The permeability of a nanofiltration membrane to pure water is much greater than that of salt water, because the pore size of a nanofiltration membrane is smaller than that of a molecule in brine, so pure water can pass through the nanofiltration membrane, while brine cannot pass through the nanofiltration membrane.

The solution is: firstly, select the appropriate nanofiltration membrane according to the actual situation to ensure that the pore size of the nanofiltration membrane is smaller than the size of the brine molecule; Secondly, multi-layer nanofiltration membranes should be used to increase the filtration efficiency of nanofiltration membranes; Finally, the nanofiltration membrane should be cleaned regularly to maintain the filtration efficiency of the nanofiltration membrane.

Not the same. The permeability of a nanofiltration membrane to pure water is much greater than that of salt water, because the pore size of a nanofiltration membrane is smaller than that of a molecule in brine, so pure water can pass through the nanofiltration membrane, while brine cannot pass through the nanofiltration membrane. The workaround is:

First of all, it is necessary to select the appropriate nanofiltration membrane according to the actual balance of the calendar to ensure that the pore size of the nanofiltration membrane is smaller than the size of brine molecules; Secondly, multi-layer nanofiltration membranes should be used to increase the filtration efficiency of nanofiltration membranes; Finally, the nanofiltration membrane should be cleaned regularly to maintain the efficiency of the nanofiltration membrane.

Can you add, I don't quite understand it.

Different. The ability of membrane permeability refers to the ability of the membrane to permeate substances, and the permeability of nanofiltration membranes to pure water and salt water is different. Pure water has a smaller molecular weight and can be permeated by nanofiltration membranes, while brine has a larger molecular weight and cannot be permeable by nanofiltration membranes.

As a result, the permeability of nanofiltration membranes to pure water and brine is different. The permeability of nanofiltration membranes is affected by factors such as membrane structure, membrane material, membrane thickness, temperature, and pressure. The thicker the film thickness, the lower the membrane's permeability capacity, the higher the temperature, the lower the membrane's permeability and the greater the pressure, the lower the membrane's permeability ability.

In addition, the permeability of nanofiltration membranes is also affected by factors such as the concentration of the solution, the pH value, and the ionic strength of the solution. The permeability of nanofiltration membranes is a complex process, which is affected by a variety of factors, therefore, the permeability of nanofiltration membranes to pure water and salt water is different.

1. The influence of temperature on water production: the activity of water molecules increases and the viscosity decreases when the temperature increases, so the water yield increases. Conversely, the water yield decreases, so the difference in water yield between winter and summer is significant, even for the same nanofiltration system.

2. The influence of operating pressure on water production: the water yield of the nanofiltration membrane is proportional to the pressure in the low-pressure section, that is, the water production increases with the increase of pressure, but when the pressure value is exceeded, even if the pressure rises again, the increase of water production is very small, mainly due to the increase of permeability resistance caused by the compression of the nanofiltration membrane under high pressure.

3. The influence of influent turbidity on water production: the larger the turbidity of the inlet water, the less water yield of the nanofiltration membrane, and the greater turbidity of the influent water, the more likely it is to cause the blockage of the nanofiltration membrane.

4. The influence of flow rate on water production: the influence of flow rate change on water production is not as obvious as temperature and pressure, and the flow rate is too slow to cause the nanofiltration membrane to be blocked, and too fast to affect water production.

Summary. In contrast, the permeability of nanofiltration membranes to salt water is lower than that of pure water, because salt molecules are larger than water molecules, making it difficult to pass through the pores of nanofiltration membranes.

In contrast, the permeability of nanofiltration membranes to salt water is lower than that of pure water, because salt molecules are larger than water molecules, making it difficult to pass through the pores of nanofiltration membranes.

You've done a great job! Can you elaborate on that?

Nanofiltration membrane is a filter material that can separate small particles, organic matter, microorganisms, ions, etc. from water, so that smaller molecules or ions are retained on the membrane. Because the pore size of nanofiltration membranes is small, typically less than 1 micron, nanofiltration membranes are effective at filtering out most solute particles and ions, including salts. As a result, the permeability of nanofiltration membranes to pure water and brine is different.

The permeability of pure water is higher, while the permeability of brine is lower, because brine contains salt plasma, and it is necessary to overcome factors such as potential difference to pass through the nanofiltration membrane with a small pore size, so the nanofiltration membrane has a higher barrier effect on the brine.