The osmotic pressure of reverse osmosis equipment is a function of the concentration of salt or organic matter in the water. The higher the salt content, the higher the osmotic pressure will increase. When the inlet pressure remains unchanged, the net pressure will decrease and the water yield will decrease. The salt permeability is directly proportional to the salt concentration difference between the positive and negative sides of the reverse osmosis membrane. The higher the salt content of the influent, the greater the concentration difference, and the salt permeability increases, resulting in the decrease of the desalination rate. For the same system, if the salt content of feed water is different, the operating pressure and product water conductivity are also different. For each increase of feed water salt content of l00ppm, the inlet pressure needs to be increased by about 0.007mpa. At the same time, due to the increase of concentration, the conductivity of product water also increases accordingly. Suspended matter in water refers to the material left on the surface of the filter material while the water is filtered, with particle composition as the main body. High suspended solids content will lead to serious blockage of reverse osmosis and nanofiltration system soon, affecting the water production and water quality of the system.

The membrane modules of various reverse osmosis equipment have an allowable pH value range, and the pH value of influent has little effect on the water yield; However, even within the allowable range, the pH value also has a great impact on the desalination rate. On the one hand, the pH value also has a certain impact on the conductivity of product water. This is because the reverse osmosis membrane itself mostly contains some active groups. The pH value can affect the electric field on the membrane surface of the reverse osmosis equipment, and then affect the migration of ions. The pH value has a direct impact on the form of impurities in the influent, For example, the rejection rate of dissociatable organic matter decreases with the decrease of pH value.

The recovery rate has a great impact on the pressure drop of each section. Under the condition of maintaining a certain total inflow flow, the recovery rate increases. Due to the reduction of concentrated water flow through the high-pressure side of reverse osmosis equipment, the total pressure drop decreases, the recovery rate decreases and the total pressure drop increases. The actual operation shows that even if the recovery rate changes very little, such as 1%, it will also change the total differential pressure by about 0.02MPa. The dissolved CO2 in water is greatly affected by pH value. When the pH value is low, it exists in the form of gaseous CO2 and is easy to pass through the reverse osmosis membrane. Therefore, when the pH value is low, the desalination rate is also low. With the increase of pH, the gaseous CO2 is transformed into HCO3 - and CO2 - ions, and the desalination rate also increases gradually. When the pH value is between 7.5 and 8.5, the desalination rate reaches the highest. The effect of recovery rate on the conductivity of product water depends on the excess salt penetration and product water. Generally speaking, the increase of system recovery rate will increase the salt content in concentrated water and the conductivity of product water accordingly.

[characteristics of pretreatment technology of reverse osmosis equipment]

In order to improve the operation performance of the reverse osmosis system, the following agents can be added to the influent: acid, alkali, bactericide, scale inhibitor and dispersant.

1 add acid - prevent scaling

Hydrochloric acid (HCl) and sulfuric acid (H2SO4) can be added to the influent to reduce the pH. Sulfuric acid is cheap, will not smoke and corrode the surrounding metal components, and the removal rate of sulfate ion by the membrane is higher than that of chloride ion, so sulfuric acid is more commonly used than hydrochloric acid. Industrial grade sulfuric acid without other additives is suitable for reverse osmosis. Commercial sulfuric acid has two concentration specifications of 20% and 93%. 93% sulfuric acid is also known as 66 Baume degree sulfuric acid. Be careful when diluting 93% sulfuric acid. Heating when diluting to 66% can raise the temperature of the solution to 138 ℃. Be sure to slowly add the acid into the water under stirring to avoid local heating and boiling of the aqueous solution. Hydrochloric acid is mainly used when calcium sulfate or strontium sulfate scaling may occur. The use of sulfuric acid will increase the sulfate ion concentration in the reverse osmosis influent, which will directly increase the scaling tendency of calcium sulfate. The purchase of industrial grade hydrochloric acid (without additives) is very convenient. The general content of commercial hydrochloric acid is 30-37%.

The primary purpose of reducing pH is to reduce the tendency of calcium carbonate scaling in RO concentrated water, that is, to reduce the Langley index (LSI). LSI is the saturation of calcium carbonate in low salinity brackish water, indicating the possibility of scaling or corrosion of calcium carbonate. In reverse osmosis water chemistry, LSI is an important index to determine whether calcium carbonate scaling will occur. When LSI is negative, water will corrode metal pipes, but will not form calcium carbonate scaling. If the LSI is positive, the water is not corrosive, but calcium carbonate scaling will occur.

LSI subtracts the actual pH of water from the pH saturated with calcium carbonate. The solubility of calcium carbonate decreases with the increase of temperature (this is how the scale in the kettle is formed), and decreases with the increase of pH and calcium ion concentration, i.e. alkalinity. The LSI value can be lowered by injecting acid (generally sulfuric acid or hydrochloric acid) into the reverse osmosis inlet water, that is, reducing the pH. The recommended LSI value of reverse osmosis concentrated water is 0.2 (indicating that the concentration is 0.2 pH units lower than the saturated concentration of calcium carbonate). Polymer scale inhibitors can also be used to prevent calcium carbonate precipitation. Some scale inhibitor suppliers claim that their products can make the LSI of reverse osmosis concentrated water up to + 2.5 (the conservative design is that the LSI is + 1.8).

2 add alkali - improve the removal rate

Alkali addition is rarely used in primary reverse osmosis. Alkali liquor is injected into the reverse osmosis inlet water to improve the pH. The commonly used alkali agent is only sodium hydroxide (NaOH), which is easy to buy and soluble in water. Generally, industrial grade sodium hydroxide without other additives can meet the needs. Commercial sodium hydroxide has 100% flake alkali, 20% and 50% liquid alkali. When adding alkali to increase pH, it must be noted that the increase of pH will increase LSI and reduce the solubility of calcium carbonate, iron and manganese. The most common alkali adding application is the secondary RO system. In the secondary reverse osmosis system, the primary ro produced water is supplied to the secondary ro as raw water. Secondary reverse osmosis "polishes" the primary reverse osmosis produced water, and the water quality of secondary ro produced water can reach 4 megohm. There are four reasons for adding alkali to the inlet water of secondary ro:

a. At pH 8 Above 2, all carbon dioxide is converted into carbonate ions, which can be removed by reverse osmosis. Carbon dioxide itself is a kind of gas, which will freely enter ro produced water with the permeate, resulting in improper load on the polishing treatment of the downstream ion exchange bed.

b. Some TOC components are easier to remove at high pH.

c. The solubility and removal rate of silica are higher at high pH (especially above 9).

d. The removal rate of boron is also high at high pH (especially above 9).

There is a special case of alkali adding application, which is usually called Hero (high efficiency reverse osmosis) process, which adjusts the influent pH to 9 or 10. Primary reverse osmosis is used to treat brackish water, which will have pollution problems under high pH (such as hardness, alkalinity, iron, manganese, etc.). Pretreatment usually uses weak acid cationic resin system and degassing device to remove these pollutants.

3 dechlorination agent - eliminate residual chlorine

The free chlorine in RO and NF feed water should be reduced below 0.05ppm to meet the requirements of polyamide composite membrane. There are two pretreatment methods for chlorine removal, granular activated carbon adsorption and the use of reducing agents such as sodium sulfite. In small systems (50-100gpm), activated carbon filters are generally used, and the investment cost is relatively reasonable. It is recommended to use high-quality activated carbon after pickling treatment to remove hardness and metal ions. The content of fine powder should be very low, otherwise it will pollute the membrane. The newly installed carbon filter material must be fully washed until the carbon powder is completely removed, which usually takes several hours or even days. We can't rely on 5 μ M security filter to protect the reverse osmosis membrane from carbon powder pollution. The advantage of carbon filter is that it can remove the organic matter that will cause membrane pollution, and the treatment of all influent is more reliable than adding chemicals. However, its disadvantage is that carbon will become the feed of microorganisms and breed bacteria in the carbon filter, resulting in biological pollution of reverse osmosis membrane.

Sodium bisulfite (SBS) is a typical reducing agent selected for larger ro units. The solid sodium metabisulfite is dissolved in water to prepare a solution. The purity of commercial sodium metabisulfite is 97.5-99%, and the dry storage period is 6 months. SBS solution is unstable in the air and will react with oxygen. Therefore, it is recommended that the service life of 2% solution is 3-7 days and that of less than 10% solution is 7-14 days. Theoretically, 1.47 ppm SBS (or 0.70 ppm sodium metabisulfite) can reduce 1.0 ppm chlorine. Considering the safety factor of industrial brackish water system, the addition amount of SBS is set as 1.8-3.0ppm chlorine per 1.0ppm. The injection port of SBS shall be upstream of the membrane element, and the distance shall be set to ensure that there is a reaction time of 29 seconds before entering the membrane element. A suitable on-line mixing device (static mixer) is recommended.

Multi media filter

The common method of removing suspended solids from water is multi-media filtration. The multi-media filter takes layered anthracite, quartz sand, fine garnet or other materials as the bed. The top layer of the bed consists of light and coarse materials, while the heaviest and finest materials are placed at the bottom of the bed. The principle is to filter according to the depth - the larger particles in the water are removed at the top layer, and the smaller particles are removed at the deeper depth of the filter medium.

Matters needing attention during the operation of RO reverse osmosis membrane:

1. The hydrolysis of cellulose acetate membrane is easy to deteriorate the performance of reverse osmosis device. Therefore, the pH value of water must be strictly controlled, the pH value of feed water must be maintained at 5-6, and the composite membrane can operate in the range of feed water ph3-ph11.

2. When the injected sodium hypochlorite is insufficient so that the free chlorine in the feed water cannot be measured, there will be slime on the membrane module of the reverse osmosis device, and the differential pressure of the reverse osmosis device will increase. However, for composite membrane and polyamide membrane, the amount of free chlorine entering the membrane module must be strictly controlled. Exceeding the specified value will lead to the oxidative decomposition of the membrane.

3. If the water with excessive fi value is supplied to the reverse osmosis device as water supply, dirt will be attached to the surface of the membrane module, so the dirt must be removed by cleaning.

4. Excessive feedwater flow will degrade the membrane module in advance, so the feedwater flow shall not exceed the design standard value. In addition, the flow of concentrated water shall not be less than the design standard value as far as possible. If the concentrated water flow is too small, uneven flow will occur in the pressure vessel of the reverse osmosis device and dirt will be precipitated on the membrane module due to excessive concentration.

5. Even if the high-pressure pump of the reverse osmosis device is interrupted for a very short time, the device may fail.

6. The inlet pressure of reverse osmosis shall be kept with appropriate margin, otherwise the desalination rate will be reduced due to lack of proper compaction.

7. When the reverse osmosis device stops, the low-pressure water supply shall be used to replace the water in the reverse osmosis device. This is to prevent the precipitation of silica during shutdown (because the water temperature drops in winter).

8. Pay attention to the validity period of precision filter. The reason for the sharp rise of differential pressure is mainly the leakage of turbidity of precision filter. On the contrary, the sharp drop of differential pressure is due to the damage of precision filter elements and the loosening of fastening screws of precision filter elements.

9. When the pressure difference between the inlet and outlet of the reverse osmosis device exceeds the standard, it indicates that the membrane surface has been polluted or the feed water flow is above the design value. If the pressure difference problem cannot be solved by flow adjustment, the membrane surface shall be cleaned.

10. In summer, when the water supply temperature is high, the produced water flow is too much, and sometimes the operating pressure has to be reduced, which will lead to the decline of produced water quality. To prevent this, the number of membrane modules can be reduced while the operating pressure remains high.