Due to its advanced technical and economic characteristics, reverse osmosis system has formed a huge user group in various industries in China.

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, on-line 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 off-line.

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 there will be no threat to safe production due to insufficient water supply due to the decrease of water production or desalination rate of reverse osmosis system and the increase of differential pressure of reverse osmosis system in case of emergency, But in fact, it is because of the existence of these margins that sometimes hidden faults can not be displayed in time, which may eventually evolve into severe pollution of reverse osmosis membrane components.

1.1 concept of severe pollution of reverse osmosis membrane elements

It refers to the surface adhesion and Deposition Pollution of RO membrane caused by suspended solids, colloids, organic matter, microorganisms and other particles contained in the influent of reverse osmosis system, or the formation of chemical scale after the ion product is greater than the solubility product due to concentration polarization and other factors on the membrane surface. Severe pollution refers to the situation that the single section pressure difference after pollution is more than 2 times of the single section pressure difference at the initial stage of system operation, the water production of reverse osmosis system decreases by more than 30%, or the weight of single reverse osmosis membrane element exceeds the normal value by more than 3kg.

Severe pollution is often the superposition of severe physical pollution and severe 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 RO pretreatment filter, when the source water is not well water, the equipment filtration rate is greater than 8.5 M / h, while the equipment filtration rate in the surface water design is greater than 8 m / h, resulting in the SDI of RO 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) the COD in the source water is greater 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 above (1), (2) and (3) design defects 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 the second section differential pressure. All design defects on the first section may lead to the decrease of the 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. For example, the type of reverse osmosis scale inhibitor is incompatible with the source water. For example, the source water contains aluminum ions, iron ions or iron, aluminum and other flocculants are used in pretreatment dosing. Try to select organic reverse osmosis scale inhibitor and avoid polycarboxylic acid type as much as possible; the dosage of reverse osmosis scale inhibitor is generally between 2-4ppm (calculated according to the water inflow), it can not exceed 6ppm at most. Improper selection of scale inhibitor type and dosage will lead to rapid decrease of system water production and obvious increase of system differential pressure; (2) the quality of reverse osmosis security filter element shall be reliable in the selection process. The filter element known as 5 microns but the actual accuracy can not reach is very unfavorable to the safe operation of 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 due to 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 inlet water), which worsens the operation condition of reverse osmosis device and forms severe pollution.

1.2.4 system operation management

(1) Failure to operate in accordance with the operating procedures; (2) untimely adjustment of equipment status parameters; (3) improper adjustment of operating parameters; (4) the measurement of added chemicals is not optimized; (5) the system is not cleaned 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 operation 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 severe pollution.

2 off line cleaning requirements for heavily polluted RO membrane

When:, Off line cleaning of heavily polluted RO membrane elements is required: (1) the pollution of reverse osmosis membrane elements meets the "heavy pollution" standard; (2) the reverse osmosis system cannot meet the system rated standard through on-line cleaning; (3) the water treatment system cannot be cleaned on-line due to tight water supply or there is no on-line cleaning equipment; (4) The reverse osmosis pollution type is 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 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 all parameters and indexes of the operating reverse osmosis system and make original records;

3.5 judge the cleaning process according to the user's full raw water 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 pollutant types.

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 a series of formulas provided by the membrane manufacturer and choose one or two more suitable formulas; Or choose a special formula (when RO membrane is contaminated by special pollutants, the ordinary formula is not good or the economic formula is more economical). At present, there are many special agents for cleaning the RO membrane components at home and abroad, such as Kaiyuan Hengye KY series cleaning agent and fungicide. According to the author's experience, the effect is good, simultaneous interpreting the economy with traditional medicine. Not bad.

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 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 reverse osmosis units (bw30-400 membrane elements) for water treatment in a power plant is 2 × 45 t / h, one-stage two-stage treatment and 6:3 arrangement are adopted. 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. After that, due to the scaling problem, the first on-line 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.

4.2 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 problem, 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 membrane element pollution, 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, the charged particles in the circulating water or hydrochloric acid added at the reverse osmosis inlet may cause loss to the water production of the reverse osmosis membrane, and other organic colloidal impurities at the inlet of the system The pollution of particles and impurities on membrane elements can not be ignored.

Finally, in the case of no effect after testing the traditional reagent, 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 module is installed on the system is also satisfactory. See Table 2 for the operating parameters after cleaning.

5. Evaluation of cleaning effect

5.1 system qualification (excluding polysilicon deposits)

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 system water yield 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 test water yield of a single membrane element after cleaning, and QS is the standard water yield of the 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.

6 Conclusion

In the pollution control of reverse osmosis, the most fundamental measure is the control of 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 management level and so on. A good grasp of these aspects plays a vital role in the safe and healthy operation of 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 recommended to consult senior water treatment experts or professional technical companies before cleaning, so as to strive for the completeness of the cleaning scheme.

7. Some misunderstandings in off-line cleaning of reverse osmosis membrane

1. The water yield of membrane element cannot be restored to 100% after cleaning

In fact, after reasonable off-line cleaning of reverse osmosis membrane, basically all membrane elements can return to the state of new membrane, and the water production is no exception. This can be detected by off-line cleaning equipment or verified after membrane element installation;

2. The desalting rate of membrane components can certainly be recovered or decreased

At present, there are two completely different views on and desalination rate, but the fact is that it should be finally determined according to what kind of pollution the membrane element has suffered. If the pollution does not damage the desalination layer, the recovery degree is high, if there is damage, the recovery degree is small, but at least it can be higher than the desalination rate before cleaning; Water treatment equipment_ Demineralized water equipment_ Purified water equipment

3. Cleaning can only be assured on site

In fact, putting the off-line cleaning work on site is only a way designed by a small number of small service companies without spare membrane components. A perfect cleaning workshop environment is not necessary for off-line cleaning, and the cleaning effect in Party B's cleaning workshop may be the best;

In fact, there are still many misunderstandings or misleading claims in the cleaning process of membrane components. In order to avoid such misunderstandings, multiple consultations should be conducted before the formal cleaning work to ensure the cleaning effect and maintain the safety of equipment and membrane components