Analysis on scaling phenomenon of reverse osmosis membrane in Hongjie Water Service

1. Fouling analysis

1.1 Treatment of raw water

To avoid clogging the reverse osmosis device, the raw water should be pre-treated to eliminate suspended solids in the water and reduce its turbidity; In addition, sterilization should also be carried out to prevent microbial growth within reverse osmosis

1.2 Suspended solids

The amount of suspended solids is an indicator of water quality. If there is a large amount of suspended matter that is not filtered in a timely manner, it will deposit on the reverse osmosis membrane, hindering its water permeability. As a result, the membrane pores are blocked and cannot function properly. So filter out the suspended solids before the water enters the surface of the membrane.

1.3 Turbidity

Turbidity is the degree of obstruction caused by suspended solids in water to the transmission of light. That is to say, due to the presence of insoluble substances in the water, some of the light passing through the water sample is absorbed or scattered, without being able to penetrate in a straight line. Therefore, the phenomenon of turbidity is an optical property of water samples. If the turbidity of water is too high, it indicates that there are many insoluble pollutants in the water. If it enters the pores of the reverse osmosis membrane, it will also block the pores (pore size<5nm ^ 40m), causing the reverse osmosis membrane to fail.

1. Microorganisms

There are many types of microorganisms, mainly divided into anaerobic and aerobic organisms. Anaerobic microorganisms oxidize ammonia or organic ammonia in water to nitrogen and high valent nitric acid or nitrite compounds, all of which have strong oxidizing properties. The composition of the membrane is that aromatic acylamide is oxidized under the action of oxidants, and the pore size of the membrane is deformed, resulting in long chain fractures of the polymer, So if the membrane fails to penetrate, it will fail and cause economic losses to producers. Currently, membranes are imported from abroad, and there are few domestic manufacturers producing them, which are expensive. Some microorganisms proliferate in large numbers and parasitize on the surface of reverse osmosis membranes, which also physically block the pores of the membrane.

1.5 Pollution index

According to the regulations of SDGJ21-85 issued by the Ministry of Water Resources and Electric Power, the requirements for the inlet water quality of reverse osmosis devices are listed in the table, and the pollution index (fi) is an artificial indicator used to represent the pollution of water quality by suspended impurities. The measurement method is to continuously pass water through a small ultrafiltration filter with a pore size of 0 00045mm), record the time (t0) required to flow out 500ml of water at the beginning of water flow. After 15 minutes of water flow, measure the time (t0) required to flow out 500ml of water again. After 15 minutes of water flow, measure the time (t15) required to flow out 500ml of water again. Based on this, the pollution index (fi) can be calculated according to equation 1:

2. Cleaning

After long-term operation, the reverse osmosis device needs to be cleaned, otherwise scaling will occur and the water production rate will decrease. Chemical agents can be used for cleaning. The agents used for cleaning include dilute HCl solution, complexing agents such as citric acid, sodium bisulfite, and sodium hexametaphosphate, which can be selected according to specific circumstances.

The ionization degree of general dilute HCl solution is very high, and the reaction is also fast. If pH>2, the performance of the membrane will be reduced. These high aromatic amides have internal structures when encountering strong acids and bases

Deformation occurs. This has affected its production efficiency. Citric acid (c6h8o7) is a moderately strong acid with a chiral structure, which has a chelating effect and can effectively remove scale.

A buffer solution of sodium bisulfite (nahso3) and sodium hexametaphosphate complexation can also remove scale, but sodium ions may be residual because the membrane's electrical properties are negative, and sodium ions can adsorb on the surface, which is also detrimental to the membrane.

3. Operating pressure

In order for reverse osmosis to proceed, the applied pressure must be such that the pressure difference (p) on both sides of the membrane is greater than its osmotic pressure difference (). The effective pressure for reverse osmosis is the difference between p and, and the flux of reverse osmosis water can be expressed as equation 2

In the formula, f - -- flux of reverse osmosis water, m3/(m2h)

W ------- Permeability coefficient, m3/(m2hmpa)

P - Pressure difference on both sides of the membrane, i.e. the pressure applied on the saline side, mpa

The osmotic pressure difference on both sides of a membrane, me

The selection of operating pressure determines the concentration of the original solution, as it is related to the osmotic pressure difference. In addition, it also depends on the permeability of the membrane and the water recovery rate. In general, increasing the operating pressure will increase the water production, but excessive pressure will cause a decrease in water permeability due to membrane pressure. After the membrane is compacted, the water flow rate decreases and the efficiency of the membrane decreases, which can cause economic losses in production.

4. Temperature

Membranes are organic substances that also conform to thermal expansion and contraction. When the temperature is high, they will expand. However, the hydrolysis rate of organic membranes is related to temperature, and the higher the temperature, the easier it is to hydrolyze. So the permeability of the membrane increases with the increase of water temperature, but it also accelerates the hydrolysis rate of the membrane and makes the organic membrane soft and easy to compact. So, for organic membranes, it is usually better to control the temperature within the range of about 20 ^ 30 kernels.

5. Concentration polarization

During the reverse osmosis process, due to the continuous passage of water through the membrane, there is a concentration difference between the salt water near the membrane surface and the inlet salt water. The concentration of the solution on the membrane surface is relatively high, which is known as concentration polarization. As a large amount of water passes through the membrane surface, the concentration greatly increases to a supersaturated solution. Some salts with low solubility, such as caso4 and mgso4, will slowly precipitate crystals. At first, these salts were only small single crystals and did not have crystal nuclei, so they could not grow. It can only deposit on the surface of the membrane or form a dissolution equilibrium in the solution. As the concentration of the solution on the surface of the membrane continues to increase, the water flow reaches a constant equilibrium degree, and crystal nuclei appear. The crystal nuclei begin to grow and gradually form a surface or spiral state. If the external temperature is suitable and there are not many corrosive substances, the crystal will gradually grow. The formation of strong scale on the surface of the membrane will block the membrane, greatly reducing the water production efficiency.

6. Prevent scaling

6.1 Pretreatment of raw water

Removing suspended solids, microorganisms, colloids, calcium sulfate, algae, bacteria, oxidants, residual chlorine, oil or lipid substances, organic and iron organic complexes, metal oxides such as iron, copper, and aluminum corrosion products from water will have a huge impact on the lifespan and performance of RO components and ix resins.

6.2 Turbidity and pollution index

Before entering reverse osmosis, the pollution index should be measured.

6.3 Use of scale inhibitors

The classification and principle of scale inhibitors commonly used in water treatment include polyphosphate, organic phosphate, phosphocarboxylic acid, organic phosphonate ester, polycarboxylic acid, etc.

6. Polyphosphate

Commonly used polyphosphates include sodium tripolyphosphate and sodium hexametaphosphate. Long chain anions generated in water are easily adsorbed on small calcium carbonate grains, and these anions are also prone to displacement with carbonate ions, thereby preventing the precipitation of calcium carbonate.

6.5 Organic phosphonic acid

Scientific names such as hedp and edtmp have good effects on inhibiting the precipitation or precipitation of calcium carbonate, hydrated iron oxide, or calcium sulfate.

6.6 Organic phosphonates

Organic phosphonates have a good inhibitory effect on calcium sulfate scale, but their inhibitory effect on calcium carbonate scale is poor. It has low toxicity and is easy to hydrolyze.

6.7 Polycarboxylic acid

Polycarboxylic acid compounds have a good scale inhibition effect on calcium carbonate scale, and their dosage is also very small. Commonly used include polyacrylic acid and hydrolyzed maleic anhydride.

Scale inhibitors have a certain degree of reducibility. Excess oxidants can be removed from the water, which will increase the ksp of some salts such as caso4 and mgso4 several times, causing the dissolution equilibrium to shift towards the dissolution direction. It can also dissolve crystals, making it difficult for them to grow and inhibiting the scaling of crystals on the surface of the membrane. This effectively protects the reverse osmosis membrane and also protects the reverse osmosis device.

7. Shutdown of operation

Do not operate reverse osmosis for a short period of time. If the water temperature is within the range of 20-30C, clean it every 4 hours. If the water temperature is below 20C, clean it every 8 hours. If it is in a shutdown state, it is advisable to continuously add scale inhibitors. To prevent the growth and crystallization of concentrated water or residual salt during shutdown.

7.1 Operating pressure

After debugging the reverse osmosis device, the frequency of the high-pressure pump is fixed. Without the approval of a professional debugging agency, the frequency and inlet pressure cannot be changed arbitrarily. To prevent the membrane from being compacted.

7.2 Temperature

The temperature of water also has an impact on reverse osmosis, and the temperature of raw water is generally 20-30C.

The above is the reason for the high conductivity of pure water equipment introduced by Hongjie Water, hoping to be helpful to you. Before purchasing equipment, we need to have a certain understanding of its composition, then understand its characteristics and defects, and make a comprehensive selection.