1. Problems existing in the application of reverse osmosis equipment

Compared with other desalination devices, such as evaporator, electrodialysis and double bed, reverse osmosis desalination has unique characteristics and advantages, and the localization of reverse osmosis has been paid more and more attention. With the increasing application of reverse osmosis technology, the problems are becoming more and more serious. In recent years, the author has conducted extensive research on the application of reverse osmosis water treatment devices, and collected 99 sets of RO water treatment devices in various industries all over the country, including 76 sets imported from abroad, 13 sets composed of some domestic and some imported equipment, and 10 sets of domestic equipment. It is found that the normal utilization rate of a full set of imported products is 30%; The normal utilization rate of some domestic and imported equipment is 60%; The normal utilization rate of the full set of domestic products is 10% The above problems are mainly caused by the following reasons:

① Due to the different raw water quality, the complete set of imported equipment lacks technical demonstration and process modification, which is not suitable for the actual situation of our country. Therefore, RO inlet water must be pretreated according to different raw water quality to meet the requirements of equipment for inlet water quality.

② Some enterprises with poor technical ability do not understand the reasonable selection of membrane elements and their quantity of reverse osmosis device, and the reasonable arrangement of membrane elements, resulting in some membrane elements operating under abnormal conditions.

③ Some miscellaneous films are of poor quality. The quality of the membrane directly affects the removal rate of salt and other impurities.

④ Lax operation management. During system operation, the pressure shall be within the allowable working pressure range of the membrane to prevent over strength and overload operation, resulting in mechanical damage to the membrane and leakage. When the reverse osmosis system operates for a period of time, the water production volume decreases sharply, the water quality deteriorates or the differential pressure increases, it indicates that the membrane needs to be cleaned. At this time, the machine should be switched to the cleaning state to make the system clean by itself, and the function of the membrane can be restored.

2. Technical improvement

2.1 design of mechanical filter

The main reason for the low normal utilization rate of imported equipment is that the pretreatment equipment does not combine the characteristics of poor raw water quality in China, the backwashing of mechanical filter is not complete, and the upper filter sand is caked, The increase of SDI (pollution index) results in membrane fouling and affects the system operation. Generally, SDI of RO device is less than 4 (each membrane element manufacturer has different requirements for SDI). To meet the above requirements, the author puts forward the following suggestions through investigation and practice:

2.1. 1 Selection of mechanical filter

Combined with the raw water quality, equipment material and filler in China, it is recommended to use double-layer filter material filter. In terms of filtration mechanism, it should be large and small. In fact, mechanical filters are intercepted through the finest sand layer on the upper layer, so the sand on the upper layer is easy to be blocked and caked, and the head loss increases rapidly. If granular anthracite is added to the upper layer of sand, the pollution capacity will be increased, the operation cycle will be long, the head loss will increase slowly, and the application effect is good in practice.

2.1. 2 backwashing of mechanical filter

Due to the large proportion of quartz sand filled inside the mechanical filter, it is difficult to recoil. Many systems operate unstable because the process of thorough and clean backwashing is ignored. The backwashing devices set on the system can not meet the requirements of backwashing intensity, which is a problem existing in many water treatment equipment manufacturers and Engineering companies. Through the joint research and practice of the author and a treatment equipment Co., Ltd., using the method of repeated flushing with gas and water, the backwashing effect of the mechanical filter after sewage blockage is very obvious, the sand layer is cleaned very clean and the performance is restored well. The specific measures are as follows:

① When designing the backwashing device, the backwashing pump and pipeline must meet the requirements of backwashing volume, and the backwashing intensity is 12 ~ 15L / (s · m2);

② The filter material is scrubbed with compressed air to make the sludge and other substances on the surface of the filter material fall off, and its strength is 18 ~ 25L / (s · m2)

2.1. 3 selection of internal packing

For internal filler, quartz sand with different particle sizes can be selected according to different drainage structures, but the particle size of the uppermost quartz sand shall be 0.3mm The top part shall be filled with 0.5 ~ 1.0mm granular anthracite, and its height shall not be less than 200mm

2.2 application of activated carbon adsorption

Activated carbon adsorber has two main functions: ① adsorb some organic matter in water, and the adsorption rate is about 60%; ② Adsorb residual chlorine in water. For users who directly draw groundwater, activated carbon can be cancelled. If the hardness is large, water softener shall be selected, and activated carbon must be used for surface water. Because the active residual chlorine of bactericide in water has strong oxidizability and will damage RO membrane, the residual chlorine shall be adsorbed with activated carbon according to the water inlet requirements of RO system. The residual chlorine shall be less than 0.1mg/l. In addition, the removal of residual chlorine by activated carbon is not a simple adsorption, but a catalytic effect on its surface. Therefore, there is no problem of adsorption saturation of activated carbon, but only the loss of carbon.

2.3 selection of coagulant

Various coagulants and polymer flocculants are added in front of the mechanical filter to remove suspended solids, colloids and other impurities in the water. However, if they are not added blindly according to the actual situation of the water source, it will not improve the water quality. On the contrary, substances harmful to RO membrane will be brought into the water due to the reagent itself or the impurities contained in the reagent. The above problems exist in the water treatment system of many pharmaceutical factories in China. So the choice of medicine is very particular. According to the characteristics of RO membrane:

① Aluminum salts shall be avoided for coagulants. Aluminum salt coagulant makes it easy to produce aluminum glue in the coagulation process, which is difficult to clean after entering the RO surface;

② Cationic polymer flocculants shall not be used. RO membrane is anionic and cationic polymer flocculant is easy to combine with the membrane to form a polymer membrane that is difficult to clean. If we do not pay attention to the above situation, the membrane life will be shortened if it is light, and some membrane components will be scrapped if it is heavy. At the same time, the compatibility between agents can not be ignored. For example, St polymer flocculant should be used together with argoaf150ul.

2.4 discussion on RO system

2.4. 1 importance of security filter

The main purpose of the security filter is to ensure that the RO inlet water does not damage the membrane module. Generally, the filter aperture is 5 μ m. The filter element shall be replaced according to the front and rear pressure difference, and the pressure difference shall be controlled within 58.8kpa.

At present, two disposable filter elements, wound or folded, are used in domestic systems. Even the filter element with small front and rear pressure difference should not be used for a long time, because the filter element is easy to grow bacteria. It is recommended to use 14 ~ 15t / (H · m2) (M2 is the filter area of the filter element.)

2.4. 2 use of scale inhibitor

Reverse osmosis membrane pollution can be divided into biological pollution, suspended matter pollution, chemical pollution, colloidal pollution, bacterial pollution, etc. At present, sodium hexametaphosphate is the most used scale inhibitor in reverse osmosis system, but sodium hexametaphosphate is easy to decompose into phosphate, which is the nutrient source of bacteria, so improper use is easy to cause biological pollution. In addition, sodium hexametaphosphate is not easy to dissolve, and its scaling also affects the operation of the system. The author investigated several manufacturers with good system operation and found that they all use flocon260 product made in Britain. As a scale inhibitor, this product also has:

① It can inhibit the growth of bacteria and prolong the cleaning cycle;

② It can prevent dissolved and insoluble iron in raw water from forming iron glue, affecting system operation and causing irreversible membrane pollution;

③ The saturation critical value (LSI value can reach 2.5) is increased, acid can not be added to most raw water, and acid can be greatly reduced to a small part of raw water, so as to reduce CO2 in reverse osmosis effluent;

④ The composition of the medicine is stable and can be stored and opened for a long time. In order to ensure the normal operation of RO system, in addition to selecting appropriate scale inhibitor varieties, the dosage should also be calculated according to the raw water quality. Argo in the UK has developed a complete set of software, which only needs to input the raw water quality into the computer, and the dosage of various agents is determined by the program.

2.4. 3 configuration of large flow flushing

In the process of reverse osmosis water quality separation, the membrane surface contains many pollutants. Because the water separation direction is 90 ° with the water flow direction, the pollutants on the membrane surface can be removed by a large number of flushing. In fact, the cleaning device is ignored by the original domestic assembly equipment, and the inlet equipment is equipped with a cleaning device. At present, some water treatment equipment engineering companies contacted by the author have developed PLC automatic control large flow flushing system, which is conducive to prolonging the service life of RO membrane. [FS：PAGE]

2.4. 4 selection of chemical cleaning solution

Under normal operation, the RO system only needs to be cleaned 3 or 4 times a year, and different agents should be selected for different pollution. Citric acid and EDTA are generally used as the main components in China, but the cleaning effect is often poor, while the cleaning effect of imported cleaning solution is obvious. For example, Bengbu No. 1 pharmaceutical factory, due to the serious water pollution of Huaihe River and some problems in operation and management, the RO membrane was seriously polluted and blocked. After 3 or 4 acid and alkali washing, there was no obvious effect. In view of the above situation, the author and the equipment manufacturer analyzed the causes of pollution and blockage, and basically determined that it was biological pollution. After boldly using the targeted imported chemical cleaning agent fiocleanmc11, the original performance was basically restored, and the performance has been stable since the operation was strengthened.

2.4. 5 design of reverse osmosis device

There is a complex calculation method for the design and calculation of RO device. At present, foreign membrane component manufacturers have developed special software. As long as the designer preliminarily determines the scheme according to the raw water quality report and the performance of each membrane component, and then inputs the raw water quality into the computer, the program software will verify the feasibility of the preliminary scheme. If not, a warning will be issued and which part of the design is unreasonable. In addition, the designer shall pay attention to various protective measures that should be provided in the design according to the requirements in the module operation manual.

2.5 selection of mixed bed ultraviolet sterilizer and membrane filter

Mixed bed is the final means to improve water quality. Ultraviolet sterilizer can kill bacteria in water, and then remove bacterial bodies through membrane filtration. These sets of designs can adopt conventional products, which are not described here