Water purification information

Improvement and innovation of domestic RO water treatment technology issues

Update time : 2021-12-21 11:36:27

1. Problems in the application of reverse osmosis equipment

Compared with other desalination devices, such as evaporator, electrodialysis, multiple bed, etc., reverse osmosis desalination has unique characteristics and advantages. The localization of reverse osmosis has also received increasing attention. With the increase in the application of reverse osmosis technology, the problems that have arisen have become increasingly serious. In recent years, the author has conducted extensive research on the application of reverse osmosis water treatment equipment. A total of 99 sets of RO water treatment equipment in various industries across the country have been collected, including a full set of 76 sets imported from abroad, part of the domestic and part of the imported equipment. There are 13 sets, and 10 sets are all domestically produced. After finishing research, it is found that the normal utilization rate of a full set of imported equipment is 30%; the normal utilization rate of some domestic and partially imported equipment is 60%; the normal utilization rate of a full set of domestically produced equipment is 10%. The above-mentioned problems are mainly due to the following reasons:

① Due to the different quality of the raw water, the whole set of imported equipment lacks technical demonstration and process modification, so it is copied and copied, which is not suitable for the actual situation of our country. Therefore, the reverse osmosis influent must be pretreated according to the quality of the raw water to meet the requirements of the equipment for the influent water quality.

②Some companies with poor technical capabilities do not understand the rational selection of membrane elements and their quantity in reverse osmosis devices, and the rational arrangement of membrane elements, which causes some membrane elements to operate under abnormal conditions.

③The quality of some brand-name films is not good enough. The quality of the membrane directly affects the removal rate of salt and other impurities.

④ The operation management is not strict. When the system is running, the pressure must be within the working pressure range that the membrane can withstand to prevent excessive strength and overload operation, which may cause mechanical damage to the membrane and cause leakage. When the reverse osmosis system has been running for a period of time, the water production volume drops sharply, the quality of the water production deteriorates, or the pressure difference increases, indicating that the membrane needs to be cleaned. At this time, the machine should be switched to the cleaning state and the system can be cleaned by itself to restore the membrane. Function.

2. Technical improvement

2.1 Design of mechanical filter

The main reason for the low normal use rate of imported equipment is that the pretreatment equipment does not incorporate the characteristics of poor quality of the raw water in our country, the mechanical filter is not completely backwashed, the upper filter sand is agglomerated, and the SDI (pollution index) rises, resulting in membrane fouling. , Affect system operation. RO devices generally require SDI <4 (each membrane element manufacturer has different requirements for SDI). To meet the above requirements, the author puts forward the following suggestions through research and practice:

2.1.1 Selection of mechanical filter

Considering the quality of raw water in my country, equipment materials and fillers, it is recommended to use a double-layer filter material filter. From the point of view of the filtering mechanism, it should be large and small. In fact, mechanical filters are intercepted by the finest sand layer in the upper layer, so the sand in the upper layer is easy to block and agglomerate, and the head loss increases rapidly. If the granular anthracite is added to the upper layer of sand, the pollution holding capacity will be increased, the operation period will be long, the head loss will increase slowly, and the application effect will be good in practice.

2.1.2 Backwashing of mechanical filter

The mechanical filter is not easy to backflush due to the high proportion of quartz sand filled inside. Many systems are unstable because they ignore the process of thorough and clean backflushing. The backflushing devices set on the system do not meet the requirements of backflushing strength. It is a problem existing in many water treatment equipment manufacturers and engineering companies. After the author and a certain treatment equipment co., Ltd. jointly research and practice, using the method of repeated air and water flushing, the backwashing effect of the mechanical filter after being blocked is very obvious, the sand layer is cleaned very cleanly, and the performance is restored. The specific measures are:

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 Use compressed air to scrub the filter material to make the sludge 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 the internal packing, quartz sand of different particle sizes can be selected according to the different drainage structure, but the particle size of the uppermost layer of quartz sand should be 0.3mm. The uppermost layer is filled with 0.5～1.0mm particles of anthracite, and the height is not less than 200mm.

2.2 Application of activated carbon adsorption

The activated carbon adsorber has two main functions: ① It absorbs part of the organic matter in the water, and the adsorption rate is about 60%; ② It absorbs the residual chlorine in the water. For users who directly extract groundwater, activated carbon can be eliminated. If the hardness is greater, a water softener must be used. For surface water, activated carbon must be used. Because the active residual chlorine of the fungicide in water has strong oxidizing properties, it will damage the RO membrane. The system inlet water requires residual chlorine to be less than 0.1mg/L, so activated carbon is used to adsorb residual chlorine. In addition, the removal of residual chlorine by activated carbon is not pure adsorption, but catalysis on its surface, so activated carbon does not have the problem of adsorption saturation, but only loses carbon.

2.3 The choice of coagulation agent

Add various coagulants and polymer flocculants in front of the mechanical filter to remove suspended solids, colloids and other impurities in the water. However, if you blindly add them according to the actual conditions of the water source, not only will the water quality not be improved, but on the contrary, it will be caused by the agent itself or in the agent. The impurities contained in the water bring substances that are harmful to the RO membrane. There are many domestic pharmaceutical factory water treatment systems that have the above problems. So the choice of medicine is very particular. According to the characteristics of RO membrane:

①Aluminum salt should be avoided for coagulant. The aluminum salt coagulant makes it easy to produce aluminum glue during the coagulation process, and it is not easy to clean after entering the RO surface;

② Cationic polymer flocculants should not be used. The RO membrane is anionic, and the cationic polymer flocculant is easy to combine with the membrane to form a polymer membrane that is difficult to clean. If you do not pay attention to the above situation, the life of the membrane will be shortened in the slightest, and part of the membrane element will be scrapped. At the same time, the compatibility between medicaments cannot be ignored. If ST polymer flocculant is selected, it should be used together with ArgoAF150ul.

2.4 Discussion on RO system

2.4.1 The importance of security filters

The main purpose of the security filter is to ensure that the RO water inflow does not damage the membrane components. Generally, the filter pore size is 5μm, and the filter element is determined to be exchanged according to the pressure difference between the front and rear, and the pressure difference is controlled within 58.8kPa.

At present, the domestic system uses two kinds of disposable filter elements: wire wound or folded. Even if the filter element has a small pressure difference between the front and rear, the use time should not be too long. 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 solids pollution, chemical pollution, colloidal pollution, bacterial pollution and so on. At present, the most used scale inhibitor in reverse osmosis systems is sodium hexametaphosphate, but sodium hexametaphosphate is easily decomposed into phosphate, and phosphate is the source of nutrients for bacteria, so improper use can easily cause biological pollution. In addition, sodium hexametaphosphate is not easy to dissolve, and its own scaling also affects the operation of the system. The author investigated several manufacturers with good system operation and found that they all use FLOCON260 products made in the UK. As a scale inhibitor, this product also has:

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

②It can prevent the dissolved and insoluble iron in the raw water from forming iron glue, which will affect the operation of the system and cause irreversible membrane pollution;

③The saturation critical value is increased (LSI value can reach 2.5), and acid can be omitted for most of the raw water, and the addition of acid can be greatly reduced for a small part of the raw water, thereby reducing the CO2 in the reverse osmosis effluent;

④The composition of the medicine is stable, and it can be stored and used for a long time. In order to ensure the normal operation of the RO system, in addition to the selection of suitable scale inhibitors, the dosage should be calculated according to the quality of the raw water. The British Argo has developed a complete set of software, which only needs to input the quality of the raw water into the computer and the dosage of various chemicals Determined by the program.

2.4.3 Configuration of large flow flushing

In the process of water separation by reverse osmosis, the membrane surface contains many contaminants. Since the direction of water separation is 90° to the direction of water flow, the contaminants on the membrane surface can be removed by a large amount of washing. In fact, the original domestic assembly equipment ignores this Cleaning device, and the imported equipment is equipped with cleaning device. At present, some water treatment equipment engineering companies that the author has contacted have all developed PLC automatic control large-flow flushing systems. This set of devices is conducive to prolonging the service life of RO membranes. [FS:PAGE]

2.4.4 Choice of chemical cleaning fluid

Under normal operating conditions, the RO system only needs to be cleaned 3 or 4 times a year, and different agents should be used for different pollution. In China, citric acid and EDTA are generally used as the main ingredients, but the cleaning effect is often poor, while the cleaning effect of imported cleaning fluid is obvious. For example, the Bengbu No. 1 Pharmaceutical Factory, due to serious water pollution in the Huai River and some problems in its operation and management, resulted in serious fouling of the RO membrane. 3 or 4 pickling and alkali washings had no obvious effect. In response to the above situation, the author and the equipment manufacturer analyzed the cause of the fouling and basically judged it as biological pollution. After boldly using the targeted imported chemical cleaning agent FIOCLEANMC11, the original performance was basically restored, and the performance has been stable after strengthened management.

2.4.5 Design of reverse osmosis device

There is a set of complex calculation methods for RO device design and calculation. At present, all foreign membrane element manufacturers have developed special software. As long as the designer initially determines the plan based on the raw water quality report and the performance of each membrane element, then the raw water quality is input into the computer, and the program software To verify the feasibility of the preliminary plan, if not, issue a warning and inform which part of the design is unreasonable. In addition, the designer should pay attention to the various protection measures that the design should be equipped with in accordance with the requirements in the membrane element user manual.

2.5 Selection of mixed bed ultraviolet sterilizer and membrane filter

Mixed bed is the ultimate means to improve water quality. Ultraviolet sterilizer can kill bacteria in the water, and then filter through membrane to remove bacterial corpses. These designs can use conventional products, so I won’t describe them here.

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