Compared with traditional water treatment methods, membrane water treatment has many advantages, such as low energy consumption, simple process, stable operation and high effluent quality, which has been widely applied in many fields. Membrane water treatment technology is mainly used in sewage treatment (including municipal sewage treatment and industrial sewage treatment), water purification (including municipal water purification and industrial water purification), seawater desalination (brackish water desalination) and pure water preparation.

Membrane water treatment technology 

Composition of membrane water treatment：

1. Pretreatment

2. Membrane treatment unit

3. Post treatment

The role of pretreatment：

◆ Removal of suspended solids, colloids and various organics

◆ Inhibition and control of precipitation of slightly soluble salts

◆ Adjust the inlet water temperature and pH

◆ Killing and inhibiting the growth of microorganisms

◆ Prevent iron, manganese and other metal oxides and silica precipitation

SDI value of silt density index:

The best technology to judge the degree of colloid and particle pollution in reverse osmosis and nanofiltration influent is to measure the influent sedimentation index (SDI), sometimes also known as the pollution index (FI). It is an important index that should be measured before designing Ro / NF pretreatment system. At the same time, it also needs to be detected regularly during ro daily operation (surface water is generally recommended three times a day). The determination method of sedimentation index has been specified in ASTM d4189-82. 

Measuring instrument：

◆ 47mm diameter test capsule

◆ 47mm test diaphragm (aperture 0.45  m)

◆ 1～5bar (10 ～ 70psi) pressure gauge

◆ Pressure regulating needle valve

Measurement steps：

◆ Place the test diaphragm carefully in the test capsule, wet the diaphragm with a little water, tighten the "O" ring, and place the capsule vertically. Pay attention to the difference between the positive and negative sides of the diaphragm

◆ Adjust the inlet water pressure to 2.1bar (30psi) and immediately measure the time to start filtering 500ml water sample t0 (keep the inlet water pressure unchanged through continuous adjustment)

◆ Filter continuously for 15 minutes at inlet pressure of 2.1 bar (30 psi)

◆ After 15 minutes, continue to record the time T15 required for filtering the same 500ml, and keep the diaphragm on the filter for further analysis

The way of pretreatment：

◆ Coagulation and sedimentation of water

◆ Multi media filtration of water

◆ Activated carbon filtration of water

◆ Softening of water

◆ Other pretreatment

Multi media filter：

The multi-media filter uses the filter material to accumulate on the long handle water cap（ As shown in the right figure, the particle size decreases gradually from bottom to top.

The commonly used particle size is & phi; 5~2.5，Φ 2.5~1.6，Φ 1.6~0.6，Φ 6~0. 4, and the grain size is & phi; 0.8~1.8 anthracite.

The main function of all levels of filter material：

◆ Particle size phi; 5 ~ 2.5 quartz sand: it is mainly used to support the upper filter material

◆ Particle size phi; 2.5 ~ 1.6 quartz sand: it is mainly used to support the upper filter material

◆ Particle size phi; 1.6 ~ 0.6 manganese sand: mainly used to remove iron ions in influent water

◆ Particle size phi; 0.6 ~ 0.4 quartz sand: mainly used to remove large particle pollutants and suspended solids in the influent

◆ Particle size phi; 0.8 ~ 1.8 anthracite: further removal of large particle pollutants and suspended solids in influent

Operation of multi media filter：

1、Mechanical filter operation

1.1、Before the mechanical filter is put into operation each time, first exhaust, and start st metering pump at the same time; When exhausting, open the inlet valve and exhaust valve on the filter. When the exhaust pipe is discharged, the exhaust is completed.

1.2、After exhausting, open the lower exhaust valve and close the exhaust valve at the same time to regulate the inflow flow. At this time, the filter enters into the washing state, when the effluent pollution index SDI15 & le; At 4:00, the filter was put into operation, and the flow rate was 5-10 M/h.

2、Backwashing of mechanical filter

With the extension of the operation time, the pressure difference of the mechanical filter increases. When the pressure difference between the inlet and outlet of the filter increases, backwashing is needed.

2.1、Backwashing and loosening: after opening the lower inlet valve, open the upper discharge valve, and open a backwashing pump at the same time. By adjusting the outlet valve of the backwashing pump or the lower inlet valve of the filter, the water inflow is generally controlled to be 2-3 times of the normal water inflow, and the loosening time is 2-5min. After loosening, close all valves and pumps.

2.2 、Water discharge: open the exhaust valve and lower discharge valve, discharge the water to 20-30cm above the filter layer in the mechanical filter, and then close the lower discharge valve.

2.3、Gas scrubbing: keep the exhaust valve and upper exhaust valve open, open the exhaust valve of Roots blower and start roots blower. Then open the inlet valve of sand filter, close the exhaust valve of Roots blower, and the sand filter enters into the air scrubbing state. The air inlet pressure is 0.03 ~ 0.1MPa, the air inlet volume is 18 ~ 20L / s.m2, and the scrubbing time is 3 ~ 5min. Open the exhaust valve of Roots blower, close the inlet valve of filter, then stop roots blower, then close the exhaust valve of Roots blower, and the air scrubbing is finished.

2.4、Large water backwashing: start a backwashing pump, open the outlet valve of the pump, and then open the lower inlet valve, upper discharge valve and exhaust valve of the sand filter. After backwashing for 2-3 minutes, start another backwashing pump. At this time, the sand filter enters the large flow backwashing state, and the backwashing flow is controlled at 15-16l / m2. S. After 30 minutes of backwashing, when the backwashing water is clear, stop one backwashing pump. After 3-5 minutes, stop the other backwashing pump. Then close all valves on the filter, backwashing pump and outlet valve, and the backwashing is finished.

3、Washing of mechanical filter

3.1、Close the lower inlet valve and exhaust valve, open the upper inlet valve and lower discharge valve, and open the st metering pump at the same time, and the filter starts washing.

3.2、When the effluent SDI15 & Le, the washing time is 10-30 min; At 4 o'clock, after washing, the filter can be put into normal operation.

Activated carbon filter：

The main function of the activated carbon filter is to adsorb the organic matter, chroma and oxidizing matter in the influent.

The specific surface area of good activated carbon is generally more than 1000m2 / g, the total pore volume is generally up to 0.6 ~ 1.18ml/g, and the pore size is from 0.001 ~ 10 & mu; m. According to the pore size, it can be divided into macropore, transition pore and micropore.

Operation of activated carbon filter：

1、Operation of activated carbon filter

1.1Before the operation of the activated carbon filter, open the inlet valve and exhaust valve. When the exhaust pipe comes out of the water, the exhaust of the filter is over. Open the outlet valve and close the exhaust valve. The carbon filter is put into operation. The operating flow rate of the carbon filter is usually 10-15m / h.

2、Backwashing of activated carbon filter

2.1、With the increase of the operating pressure difference of the activated carbon filter, when the effluent does not meet the index, when the pressure difference of the carbon filter increases to 0.05Mpa, the carbon filter begins to backwash. When the carbon filter is backwashed, open the upper discharge valve and exhaust valve of the filter, open a backwashing pump and outlet valve of the backwashing pump, and then open the lower inlet valve of the carbon filter. Adjust the backwashing flow rate to 8-10l / m2h, the flow rate is about 200 m2 / h, and the backwashing time is 20-30 minutes. When the water is clear, close all valves on the filter, the outlet valve of the backwashing pump and the backwashing pump, and the backwashing is finished.

2.2、After the backwashing of the activated carbon filter, open the inlet valve and lower discharge valve of the filter, and use the outlet water of the sand filter to wash the activated carbon filter. The forward washing flow is about 70 m2 / h of the activated carbon operation flow. The washing time is about 20-30 minutes. When there is no carbon dust in the effluent, the effluent is clear and fi index is qualified, the washing is finished and the filter is put into operation.

Disinfection and sterilization：

◆ Ultraviolet sterilization

◆ Chlorination sterilization

◆ Ozone sterilization

Ultraviolet sterilization：

◆ Ultraviolet sterilization is a kind of physical sterilization. After the bacteria are irradiated by ultraviolet light, the bacterial nucleic acid changes its activity due to the absorption of energy, which eventually leads to the mutation or death of microorganisms. The ultraviolet with wavelength of 200-295nm has bactericidal ability, and the ultraviolet with wavelength of 253.7nm has the best bactericidal effect.

◆ The advantages of ultraviolet sterilization are short contact time, strong sterilization ability, simple equipment, convenient operation and management, treatment does not change the physical and chemical properties of water, will not bring additives to cause secondary pollution. Its disadvantage is that there is no continuous bactericidal effect, and the water may be secondary polluted in the pipeline, so it is not suitable for the later sterilization of drinking water and purified water.

Chlorination sterilization：

◆ Chlorine, sodium hypochlorite, chlorine dioxide

◆ In recent years, the harm of trihalomethane, a by-product of chlorine and sodium hypochlorite sterilization, has been known by people, and the application of chlorine dioxide has developed rapidly.

◆ Chlorine dioxide has strong adsorption and penetration ability to the cell wall of bacteria, rapid control of microbial protein synthesis, and strong inactivation ability to bacteria and viruses. The dosage of chlorine dioxide for sterilization is 0.4-0.45mg/l, which is about half less than that of chlorine and sodium hypochlorite.

Ozone sterilization：

◆ Ozone is a kind of commonly used bactericide, ozone releases new ecological oxygen in decomposition, which has strong oxidation ability and strong lethality to stubborn microorganisms. Ozone has the strongest oxidation and sterilization ability and the best effect among several fungicides.

◆ The sterilization effect of ozone mainly depends on the residual amount and contact time. In actual production, the dosage of ozone used in pure water sterilization is 1-3mg / L. if it is used in mineral water, the dosage is slightly higher, and the concentration of ozone in barrels or bottles is generally controlled to be greater than 0.5mg/l. The contact time is generally not less than 5 minutes.

Membrane separation classification：

◆ Microfiltration (MF)

◆ Ultrafiltration (UF)

◆ Nanofiltration (NF)

◆ Reverse osmosis (RO)

Microfiltration (MF)：

The particles between 0.1-10 microns are intercepted. The microfiltration membrane allows macromolecular organic matter and dissolved solids to pass through. It can block the transmission of suspended solids, bacteria, some viruses and large-size colloids. The operating pressure difference on both sides of the microfiltration membrane is generally less than 1 bar.

Nanofiltration (NF)：

Nanofiltration is a special kind of separation membrane, which is named for its ability to intercept substances with the size of about 1 nm (0.001 μ m). The operation range of nanofiltration is between ultrafiltration and reverse osmosis. Its molecular weight of organics is about 200-400, and its ability to intercept soluble salts is 20-80%. The removal rate of monovalent anionic salt solution is lower than that of high valent anionic salt solution, Nanofiltration membrane is generally used to remove the organic matter and chroma of surface water, remove the hardness of well water, partially remove the soluble salt, concentrate the useful substances in food and medicine, etc. the operating pressure difference on both sides of nanofiltration membrane is generally 3.5-16bar.

Reverse osmosis (RO)：

The most precise membrane liquid separation technology can block all soluble salts and organic matters with molecular weight greater than 100, but allow water molecules to pass through. The desalination rate of cellulose acetate reverse osmosis membrane is generally greater than 95%, and that of reverse osmosis composite membrane is generally greater than 98%. The operating pressure difference on both sides of reverse osmosis membrane is generally greater than 5 bar.

Selection of membrane module：

◆ Determine the number of components

◆ Selection of pressure vessel

Arrangement and combination of membrane modules：

◆ Segmented

◆ Multistage 

Accessories of reverse osmosis unit：

◆ High pressure pump

◆ Piping

◆ Valve

◆ Meter 

Signs of RO membrane need cleaning：

◆ When the reverse osmosis device runs on the membrane, it will accumulate colloids, metal oxides, bacteria, organic colloids, scale and other substances, resulting in a certain decline in performance. When the performance declines to a certain extent, the water yield is lower than that after the initial or last cleaning.

◆ Desalting rate of produced water decreased

◆ System differential pressure 15%

Chemical cleaning of membrane components：

Acid cleaners are used to remove inorganic pollutants including iron pollution, while alkaline cleaners are used to remove organic pollutants including microorganisms. As the use of sulfuric acid may cause the risk of calcium sulfate precipitation, it should not be used as a cleaning agent. It is better to use the produced water of membrane system to prepare cleaning solution. Of course, in many cases, the qualified pretreated effluent can be used to prepare cleaning solution. The buffer capacity of raw water may be large, and more acid or alkali is needed to reach the specified pH value. The pH value of acid cleaning is about 2, and that of alkaline cleaning is about 12.

Record of pretreatment operation parameters：

Because the performance of Ro / NF system mainly depends on whether the pretreatment operation is reasonable, the operation characteristics of pretreatment instruments and equipment must be recorded. Because the pretreatment process varies from place to place, it is impossible to propose a unified record form. Generally speaking, the following items need to be recorded：

◆ Record the outlet water pressure of any well water or booster pump twice a day

◆ Record the pressure drop of all filters twice a day

◆ Any abnormal operation, such as failure or shutdown, etc

Connotation of nanofiltration membrane：

There was a popular saying in the market: nanofiltration is a kind of loose reverse osmosis. In fact, this is a misleading technical concept. Table 2-2 shows the comparison of rejection rates of different membranes. The real separation concept of nanofiltration is the filtration membrane with selective rejection for the example, a kind of membrane whose sodium chloride permeability is proportional to the concentration of sodium chloride and the ratio is greater than 0.4. It is mainly used for desalination and concentration of various materials.

Comparison of rejection rates of different membranes：

High temperature resistant film：

The continuous operating temperature of common organic film components must be lower than 50 ℃. If it exceeds this temperature, the film will soon lose its normal performance. With its strong R & D capability of membrane technology, GE has developed duratherm series of high temperature resistant membrane components, which are mainly used in food and beverage, pharmaceutical, electronic and other industries.

Duratherm applied watershed：

Duratherm HWS hot water disinfection membrane elements can withstand periodic 90 ℃ hot water cleaning and disinfection, which is used to replace the traditional chemical disinfection of membrane system, and can be used in & le; Continuous operation at 50 ℃. In Europe and America, hot water disinfection water treatment system is more and more used. The following table describes the operating conditions (temperature and time) of the moist heat sterilization method.

In order to increase the effectiveness of thermal disinfection, the temperature and time of thermal disinfection in practical application should be higher than this theoretical parameter.

Suggestions on hot water disinfection：

◆ Wash with antifouling water before disinfection

◆ During disinfection, the inlet pressure should not exceed 40psi (275kpa)

◆ During disinfection, the heating and cooling rate should not be greater than 5 ℃ / min

◆ The highest disinfection temperature is 90 ℃

Source: Industrial Wastewater Circle