1. Overview

During the operation of the reverse osmosis system, the surface of the reverse osmosis membrane will be dedeposited due to the presence of mud, colloids, organic matter, microorganisms and other pollutants in the raw water and the concentration of insoluble substances in the process of membrane separation, and then the pollution of the reverse osmosis membrane will be formed. We all know that the pretreatment device of the reverse osmosis system is specially designed to remove as many substances that cause membrane pollution as possible, but even if the system has a fairly complete pretreatment equipment, it cannot completely avoid the pollution of the membrane during use, so it is necessary to periodically remove the pollutants in the membrane system during the operation of the equipment, which is called CIP (Cleaning In Place) of the reverse osmosis system.

When the reverse osmosis membrane is contaminated, there will be a decrease in the water yield of the system, an increase in salt permeability, and other degradation of membrane performance. However, due to the changes in other major factors (pressure, temperature, etc.) that affect the performance of the membrane during the use of reverse osmosis equipment, the phenomenon of membrane fouling may be masked by other factors, so attention should be paid to it.

At present, most of the aromatic polyamide reverse osmosis composite membranes on the market have considerable stability and certain temperature resistance in a wide pH range, so users can clean the reverse osmosis system very effectively. Years of engineering practice have shown that it is very difficult to completely remove the contaminants on the surface of the membrane that have been adhered to for a long time without timely cleaning of the reverse osmosis system that has produced a certain degree of pollution.

In general, when considering the membrane system cleaning scheme, the following points should be noted:

■ The environmental impact (EDTA, biocide, etc.) of the cleaning and discharge liquid should be minimized.

■ Contaminants removed during the cleaning process should be maximized as much as possible.

■ The damage to the membrane should be minimized during cleaning (the selection of a chemical with little impact on the membrane performance should be considered first).

■ In the actual cleaning operation, under the premise of ensuring the cleaning effect, the cleaning cost is minimized as much as possible

2. The cause of the contamination of the reverse osmosis membrane

■ Improper pretreatment

8226; The system is equipped with a pretreatment device that is not suitable for the quality and flow rate of the raw water, or the necessary process equipment and process links are not equipped in the system.

8226; The pretreatment device is not operating normally, that is, the original pretreatment equipment of the system has a low removal ability of raw water SDI components, turbidity, colloids, etc., and the pretreatment effect is not ideal.

■ Improper selection of equipment or material of equipment (pumps, piping, etc.) is selected by the system.

■ Failure of the system chemical injection unit (acids, flocculants/coagulants, scale inhibitors/dispersants, reducing agents, etc.).

■ Failure to take appropriate protective measures after the equipment runs intermittently or the system is out of use.

■ Unreasonable operation and operation of equipment (recovery rate, water production, concentrated water, differential pressure, cleaning and others) by operation management personnel.

■ Long-term accumulation of insoluble precipitates in the membrane system.

■ The composition of the raw water has changed greatly or the characteristics of the water source have changed fundamentally.

■ The reverse osmosis membrane system has undergone a considerable degree of microbial contamination.

3. Analysis of membrane pollutants

■ First of all, the recent equipment operation records recorded before this time and can reflect the operation status of the equipment should be carefully analyzed.

■ Analyze the quality of raw water.

■ Check the results of the previous cleaning.

■ Analyze foreign substances left on the filter membrane during the SDI value test when the system is running.

■ Analyze the build-up on the filter element of the security filter configured with the reverse osmosis system.

■ Check for foreign substances in the piping of the raw water inflow system and at the inlet end of the reverse osmosis membrane.

* Performance at the time of fouling of various contaminants

(1) Carbonate scale

Post-fouling performance: Decreased flow of standard permeate or decreased desalination rate.

Reason: Increased polarization of the film surface concentration

(2) Iron/manganese

Post-contamination manifestations: An increase in standard differential pressure (mainly in the membrane element at the front of the device) may also cause a decrease in water permeability. Usually manganese and iron are present at the same time.

(3) Sulfate scale

If deposition occurs, the membrane element at the back of the system with the highest salt concentration is first affected, and the pressure difference between the two stages is significantly increased. A special cleaning agent is required.

(4) Silicon

Granular silicon: Fouls the water flow path of the membrane element, resulting in an increase in the differential pressure of the system. The use of 0.4% dichloramine is effective in dissolving heavily contaminated silica scale.

Glue: Similar to granular silicon.

Dissolved silica: silicate precipitation is formed, which should be cleaned with dichloramine.

(5) Suspended solids/organic matter

Fouling performance: the permeability decreases, and the pressure difference in the first stage increases significantly. If the feed water SDI is greater than 4 or the turbidity is greater than 1, there is a greater possibility of organic contamination.

(6) Microorganisms

Fouling performance: the standard pressure difference increases or the standard permeability decreases. It can be cleaned with a non-oxidizing biocide and alkali.

(7) Iron bacteria

Fouling manifestations: elevated standard differential pressure. EDTA sodium salt and alkali can be used for cleaning.

4. Judgment and selection of cleaning timing of reverse osmosis system

When the following situations occur, the reverse osmosis membrane system should be cleaned

■ The water yield of the standardized equipment is reduced by 10~15%;

■ The operating pressure of the standardized membrane system has increased by 15%;

■ The salt permeability of the standardized membrane system increased by 10~15% compared with the initial normal value;

■ The operating differential pressure has increased by 15% compared to the initial operation

(It is recommended to use the operation record obtained from the initial operation of the equipment for 25~48 hours as the basis for comparison after standardization)

The performance parameters of reverse osmosis equipment are related to the changes in pressure, temperature, pH, system water recovery rate and salt concentration of raw water. Therefore, it is important to base on the normal technical parameters (product water flow, pressure, differential pressure and system desalination rate) obtained during the initial commissioning and to compare them with the current system data after standardization. In addition, the choice of cleaning time varies depending on the raw water quality conditions and environmental characteristics of the area where the reverse osmosis plant is used, so it is necessary to implement appropriate management measures according to the conditions at the plant site. However, in any case, for any well-designed and well-managed reverse osmosis system, the shortest cycle of chemical cleaning should be guaranteed to be in cumulative continuous operation for more than 3 months, and the operation time is generally about 6-12 months, otherwise it is necessary to consider improving the pretreatment equipment of the original system or its operation management.

5. Determine the volume of the cleaning box and calculate the amount of cleaning liquid

The volume of the cleaning chamber and the amount of cleaning solution can be calculated in the following ways:

1) Estimate the empty volume of the pressure vessel and the empty volume of the pipeline:

The empty volume of the pressure vessel is:

V1 = NπR2L

Where: N = number of pressure vessels per cleaning

R = radius of the pressure vessel

L = effective length of the pressure vessel

The empty volume volume of the pipeline is:

V2=L1πd2/4

Where: L1 = is the total length of the cleaning pipe

d = is the diameter of the cleaning pipe

Total volume of cleaning chamber (i.e. cleaning solution preparation volume):

V= 1.2(V1+ V2)

2) Calculate the volume of the cleaning chamber and the preparation amount of cleaning solution according to the model specification and pollution degree of the membrane element:

For normal contamination, the volume of the reverse osmosis cleaning chamber is generally calculated by preparing 8.5 liters of cleaning solution for each 4-inch membrane element and 34 liters of cleaning solution for each 8-inch membrane element.

For more contaminated cases: 16 litres of cleaning solution per 4-inch element; Each 8-inch membrane element is prepared with 55 litres of cleaning solution, from which the volume of the cleaning chamber and the amount of cleaning solution are obtained.

6. Membrane cleaning process

1) First of all, rinse the reverse osmosis membrane module and system pipeline with reverse osmosis product water (it is best to use reverse osmosis product water, or softened water or filtered water that meets the reverse osmosis inlet water standard).

2) Prepare the cleaning solution with reverse osmosis product water that should at least be qualified softened water, and ensure that it is evenly mixed; Before cleaning, the pH value and temperature of the cleaning solution should be repeatedly confirmed.

3) First, use 1/2 of the normal cleaning flow rate and 40~60PSI operating pressure to inject cleaning solution into the reverse osmosis equipment, and remove the water trapped inside the membrane container. And drain the part of the cleaning solution that has just been circulated back to prevent the cleaning solution from being diluted.

During normal cleaning, the pressure control criterion of the cleaning system is to use the pressure that almost does not produce pure water (i.e., the pressure supplied to the cleaning system is equal to the pressure difference between the raw water and the concentrated water). This is because the right cleaning run pressure minimizes the possibility of re-accumulation of foreign substances on the reverse osmosis membrane surface.

4) When cleaning, drain the water that was previously stored inside the pressure vessel. Then the concentrated water and produced water generated by the cleaning process are circulated to the cleaning tank, and attention is paid to keeping the temperature of the cleaning solution stable. Before starting the cycle cleaning, it is necessary to confirm that the temperature and pH value of the cleaning solution meet the standard. And confirm the turbidity of the reflux cleaning solution: if the reflux cleaning solution has obviously changed color or turbidity, the cleaning solution should be re-prepared; If the pH of the reflux solution changes more than 0.5, it is best to readjust the pH value or replace the cleaning solution.

5) When the system is chemically cleaned, the general operation method is as follows: first, the pressure vessel that needs to be cleaned is cleaned with a low flow rate (1/2 standard cleaning flow) for 5~15 minutes, and then a medium flow rate (2/3 standard cleaning flow) is used for 10~15 minutes.

6) Then stop the pump and close the valve, so that the membrane element is soaked in the cleaning solution, the soaking time is about 1 hour. If the membrane is more contaminated or the contaminants are difficult to remove by cleaning, the soaking time of the process can be appropriately extended. In order to ensure the temperature of the cleaning solution during long-term soaking, a combination of repeated circulation and soaking can also be adopted. Generally speaking, the temperature of the cleaning solution should be kept at least above 20°C and below 40°C, and the appropriate cleaning solution temperature can enhance the cleaning effect; Please note: Cleaning fluids that are too cold may cause chemical precipitation during the cleaning process. When the temperature of the cleaning solution is too low, the cleaning should be arranged after the temperature of the cleaning solution is raised to a more suitable temperature.

Flow control of each reverse osmosis pressure vessel during cleaning

Diameter of the pressure vessel

(inches) Standard wash flow rate for each reverse osmosis pressure vessel to pass

GPM m3/hr

2.5 ～5 ～1.1

4 ～10 ～2.3

8 ～40 ～9

7) During normal cleaning, after the soaking of the cleaning solution is finished, the cleaning can be generally completed by circulating the cleaning again for 20~60 minutes with the standard cleaning flow. The reverse osmosis membrane module is then rinsed with the same volume of reverse osmosis product water, and the flushing water is discharged into the sewer. Once you have confirmed that the flush is clean, you can re-run the reverse osmosis plant. We recommend that the product water produced should be drained at least 15 minutes after the system is re-run after chemical cleaning. After the results of careful chemical analysis of the product water quality of the on-site system are confirmed, the system produced water obtained from the operation of the system is injected into the product water tank. In addition, in order to prevent chemical reactions between chemicals when cleaning with multiple chemicals, it is best to drain the water from the water side of the product before each cleaning.

※ If it is a multi-stage equipment, it is recommended to clean it in stages to avoid the situation that the flow rate is uncontrollable - that is, the flow rate of the first stage is too small or the flow rate of the last stage is too much, which can also prevent the pollution sediment that was washed out in the first stage from flowing back into the next stage and forming secondary pollution.

8) If you want to prevent the re-contamination of microorganisms, after cleaning the system, you can use the sterilization solution allowed by the membrane manufacturer to sterilize and clean the membrane system, and the operation method is the same as before. Please note: It is important to rinse thoroughly after sterilization and cleaning to avoid introducing the disinfectant into the product water.

7. Appendix General cleaning solution for polyamide composite membrane elements

Cleaning fluid

Contaminants 0.1% (W) NaOH or 1.% (W) Na4EDTA

[pH12/30°C (maximum)] 0.1% (W) NaOH or 0.025% (W) Na-SDS

[pH12/30°C (maximum)] 0.2% (W) HCl hydrochloric acid 1.0% (W) Na2S2O4 0.5% (W) H3PO4 phosphoric acid 1.0% (W) NH2SO3H 2.0% (W) citric acid

Inorganic salt scale

(e.g. CaCO3) preferably yes yes can

Sulfate scale

(CaSO4 BaSO4) preferably Yes

Metal oxides

(such as iron) preferably can can can

Inorganic colloids (sludge) are best

Silicon can be best

Microbial membranes can be best

Organics can be cleaned in the first step, and it is best to do the first step to clean, and it is best to do the second step to clean

1. (W) indicates the weight percentage of the active ingredient;

2. The chemical symbol of the pollutant in order is: CaCO3 represents calcium carbonate; CaSO4 denotes calcium sulfate; BaSO4 stands for barium sulfate.

3. The symbol of the sequential cleaning chemicals is: NaOH represents sodium hydroxide; Na4EDTA stands for ethylenediaminetetraacetic acid; Na-SDS stands for sodium dodecylbenzene sulfonate, also known as sodium monthly silicate; HCl stands for hydrochloric acid; Na2S2O4 denotes sodium bisulfite; H3PO4 denotes phosphoric acid; NH2SO3H stands for bisulfite amine.

4. In order to effectively clean the sulfate scale, it must be found and treated as soon as possible, because the solubility of the sulfate scale increases with the increase of the salt content of the cleaning solution, NaCl can be added to the cleaning solution of NaOH and Na4EDTA, when the scale is more than one week, the cleaning success of the sulfate scale is questionable.

5. Citric acid is an optional cleaning agent for inorganic salt scale.

When cleaning the RO membrane, it is best to look at the pH value, the pH value of acid is about 2, the concentration is 2%, the pH value of alkali is about 12, and the concentration is 0. 5%.