Due to its advanced technical and economic characteristics, reverse osmosis system has formed a huge user group in various industries in China. According to incomplete statistics, there are more than thousands of domestic reverse osmosis water treatment users.

As a deep filtration means, the surface of reverse osmosis membrane element will inevitably remain colloids, microorganisms, impurity particles and insoluble salts on its surface. Therefore, once put into use, the reverse osmosis devices used in many fields will eventually need to be cleaned, but the length of cleaning cycle is different. However, as a means of reverse osmosis system cleaning and maintenance, impact sterilization and regular protection, online cleaning is powerless in the face of severe pollution of reverse osmosis membrane elements. At this time, it is necessary to clean the reverse osmosis membrane elements offline.

1 causes and characteristics of severe pollution of reverse osmosis membrane elements

Although there will be a certain degree of margin in the design of reverse osmosis system to ensure that in an emergency, there will not be a threat to safe production due to insufficient water supply due to the decline of water production or desalination rate of reverse osmosis system and the rise of differential pressure of reverse osmosis system, in fact, it is the existence of these margins that sometimes hidden faults can not be displayed in time, In this way, it may eventually evolve into heavy pollution of reverse osmosis membrane elements.

1.1 concept of severe pollution of reverse osmosis membrane elements

It refers to the phenomenon that the suspended solids, colloids, organics, microorganisms and other particles contained in the influent of reverse osmosis system adhere to the surface of RO membrane, deposit pollution, or the formation of chemical scale after the ion product is greater than the solubility product due to concentration polarization and other factors. Severe pollution refers to the situation that the pressure difference of a single section after pollution is more than twice the pressure difference of a single section at the initial stage of system operation, the water production of the reverse osmosis system decreases by more than 30%, or the weight of a single reverse osmosis membrane element exceeds the normal value by more than 3kg.

Heavy pollution is often the superposition of heavy physical pollution and heavy chemical pollution. In some cases, they are accompanied at the same time.

1.2 causes and characteristics of severe pollution of reverse osmosis membrane elements

Due to the different water quality of water sources and different treatment methods, the causes of RO membrane elements are also different. The common pollution causes and characteristics are as follows:

1.2.1 design and manufacturing defects of water treatment process system

(1) For the reverse osmosis pretreatment filter, when the source water is not well water, the filtration rate of the equipment is greater than 8.5m/h, while the filtration rate of the equipment in the surface water design is greater than 8m / h, resulting in the SDI of the reverse osmosis inlet water exceeding the standard; (2) The source water contains colloid, suspended solids and other impurities, but there are no removal measures in the design; (3) COD in source water is more than 3 without removal measures; (4) The scaling and differential pressure of the second section of the reverse osmosis device with excessive design flux of reverse osmosis membrane elements increase rapidly, and the water yield decreases rapidly and the desalination rate decreases rapidly

The design defects of (1), (2) and (3) above may lead to the increase of the first section differential pressure of the reverse osmosis system; (3) It may also lead to the increase of differential pressure in the second section, and all design defects in the first section may lead to the reduction of system water production.

1.2.2 improper selection of reverse osmosis additives and other consumables

(1) Improper selection of reverse osmosis scale inhibitor type or dosage, such as: the type of reverse osmosis scale inhibitor is incompatible with the source water, such as aluminum ion and iron ion in the source water or iron, aluminum and other flocculants are used in pretreatment dosing. Try to choose organic reverse osmosis scale inhibitor and avoid polycarboxylic acid type as much as possible; The dosing amount of reverse osmosis scale inhibitor is generally between 2-4ppm (calculated according to the water inflow), and can not exceed 6ppm at most. Improper selection of scale inhibitor type and dosing amount will lead to the rapid decline of system water production and the obvious increase of system differential pressure; (2) The quality of the filter element of the reverse osmosis security filter shall be reliable in the selection process. The filter element known as 5 microns and the actual accuracy can not reach is very unfavorable to the safe operation of the reverse osmosis membrane element; (3) Improper selection of flocculant and coagulant aid in reverse osmosis pretreatment system will cause serious pollution of membrane elements, while improper selection of bactericide will seriously damage the desalination rate of reverse osmosis system; (containing Cl --)

1.2.3 system abnormality caused by sudden change of water treatment operating conditions

Generally, it is due to the increase of suspended matter content in source water caused by climate change, or the change of source water type (for example, the change from groundwater to tap water leads to the increase of residual chlorine content in reverse osmosis influent), which worsens the operation condition of reverse osmosis device and forms severe pollution.

1.2.4 system operation management

(1) Do not operate according to the operating procedures; (2) The adjustment of equipment status parameters is not timely; (3) Improper adjustment of operating parameters; (4) The metering of added chemicals is not optimized; (5) Do not clean the system in time when the system reaches the cleaning conditions; (6) The system meter is not adjusted on time; (7) Lack of operation management summary.

1.2.5 shortage of water supply for water treatment system

In addition to the above, in a considerable number of enterprises, due to the shortage of water supply in the water treatment system, even if the operating parameters of the reverse osmosis device deteriorate and the system is polluted, the system cannot be cleaned in time due to the shortage of water supply, resulting in the gradual severity of system pollution and the formation of severe pollution.

2 off line cleaning requirements for heavily polluted RO membrane

When the following situations occur, it is necessary to clean the heavily polluted RO membrane elements offline: (1) the pollution of reverse osmosis membrane elements meets the "heavily polluted" standard; (2) The RO system cannot reach the rated standard of the system through on-line cleaning; (3) The water treatment system cannot be cleaned online due to the shortage of water supply or there is no online cleaning equipment; (4) The types of reverse osmosis pollution are relatively complex, and it is easy to cause cross pollution through on-line cleaning; (the pollutants in the front section of the reverse osmosis system may be brought into the rear section of the system through on-line cleaning, and the membrane elements in the rear section may be polluted, which is called cross pollution)

3 off line cleaning methods and steps of reverse osmosis membrane elements

3.1 first, replace the membrane elements to be cleaned on the reverse osmosis system with standby membrane elements with excellent performance, so as to ensure that the reverse osmosis system does not stop running and ensure the continuous stability of the whole production process.

3.2 performance test of reverse osmosis membrane element:

① Test the performance indexes of each membrane element separately, including desalination rate, water production, differential pressure, weight, etc., and make records before test

② Test conditions for desalination rate, water yield and differential pressure: meet the standards provided by different types of membrane manufacturers.

3.3 understand the current operation status of the system before system cleaning;

3.4 collect the parameters and indexes of the operating reverse osmosis system and make original records;

3.5 judge the cleaning process according to the user's raw water full analysis report, performance test results and known system information;

3.6 identification of pollutants. First, make a preliminary judgment according to the analysis results in 3.5, and then make further verification through special equipment and appliances to determine the specific types of pollutants.

3.7 determine the required cleaning formula according to the analysis results in 3.5 and 3.6. When the pollutants on the RO membrane are determined, we can choose the series formula provided by the membrane manufacturer and choose one or two more suitable formulas; Or choose special formula (when RO membrane is polluted by special pollutants, the effect of using ordinary formula is not good, or from the perspective of economy, the special formula is more economical). At present, there are many special agents for reverse osmosis membrane element cleaning at home and abroad, such as KY series cleaning agents and bactericides of Kaiyuan Hengye. According to the author's experience, the use effect is good, and the economy is good compared with traditional agents.

3.8 carry out experimental cleaning with the above cleaning agents combined with physical treatment cleaning means on the special cleaning equipment for reverse osmosis, so as to select the appropriate cleaning formula and cleaning procedure;

3.9 determine the cleaning method and treat all the above membrane elements;

3.10 the cleaned membrane elements shall be tested on the test platform and recorded. If they do not meet the requirements, they will be sent back to the cleaning equipment for treatment;

3.11 sort out cleaning data and write cleaning summary report.

4 offline cleaning example

The initial design capacity of two sets of reverse osmosis units (bw30-400 membrane elements) for water treatment in a power plant in Dengfeng is 2 × 45 t / h, with one-stage two-stage treatment and 6:3 arrangement. The system has been put into operation since February 2004. The initial system inlet pressure is 11.6kgf/cm2 and the system water production is 45 t / h. Later, due to the scaling problem, the first online cleaning was carried out. Due to the improper selection of cleaning agent, the desalination rate decreased by 1 percentage point; After the water production drops again, online cleaning again has no effect.

4.1 operating parameters before cleaning

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4.2 offline cleaning process of the above two systems

Since on-line cleaning had no effect on restoring the water production of the system, and the boiler system was blowing pipes at that time, the water supply was very large. Considering the above factors, the off-line cleaning mode is finally selected, and the spare membrane element is used to replace the membrane element in the existing system, which not only solves the urgent need, but also buys time for the cleaning of the contaminated reverse osmosis membrane.

According to the analysis of the process flow of the system and the observation of the pollution of the membrane element, it is preliminarily determined that the falling off of the fiber filament of the filter element of the security filter may enter the interior of the membrane element, and the charged particles in the circulating water or hydrochloric acid added at the reverse osmosis inlet may cause losses to the water production of the reverse osmosis membrane. At the same time, the pollution of other organic colloidal impurities and particle impurities at the system inlet to the membrane element can not be underestimated.

Finally, in the case of no effect after testing the traditional medicine, ky-430 product developed by Zhengzhou Kaiyuan Hengye was used, and after repeated cleaning and testing of all membrane elements combined with physical methods, it was found that the effect was good, and the test parameters of basically all membrane elements were close to the level of the new membrane. Similarly, the effect after the membrane element is installed on the system is also satisfactory. See Table 2 for the operating parameters after cleaning.

5. Evaluation of cleaning effect

Evaluation of silicon deposits (except 1.5)

Q≧0.98 × Q0

T≧T0

P≯ P0+0.5 × a

Where q is the water yield of the system after cleaning, and Q0 is the water yield of the system when the system is just put into operation; T is the desalination rate of the system after cleaning, and t0 is the desalination rate of the system before cleaning; P is the water inlet pressure of the system after cleaning (unit: bar), P0 is the system inlet pressure when the system is just put into operation; A represents the number of years of operation.

5.2 evaluation of single membrane element

Q≧0.99 × Qs

T≧Ts0

Δ P≯15PSI

Where q is the water yield of single membrane element after cleaning, and QS is the standard water yield of new membrane; T is the desalination rate of single membrane element after cleaning, and TS0 is the desalination rate of single membrane element before cleaning; P is the pressure difference of single membrane element after cleaning.

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6 Conclusion

In the pollution control of reverse osmosis, the most fundamental measure is to control the design, manufacturing and installation process of water treatment system with reverse osmosis as the core, the selection of various consumables of reverse osmosis system, operation and management level and so on. A good grasp of these aspects plays a vital role in the safe and healthy operation of the reverse osmosis system.

Of course, when the reverse osmosis system is seriously polluted, the first measure to be taken must be to analyze the causes of pollution, find out the methods to solve the pollution, and clean the reverse osmosis system in the shortest time through appropriate ways, because with the extension of time, it means that the difficulty of cleaning decreases.

At the same time, it is suggested to consult senior water treatment experts or professional technology companies before cleaning, so as to strive for the completeness of the cleaning scheme.