A comprehensive analysis of the causes of fouling and blockage in reverse osmosis equipment and the precautions for cleaning plans by Hongjie Water

1. Overview

During the operation of the reverse osmosis system, the surface of the reverse osmosis membrane may be contaminated due to the presence of pollutants such as mud, gel, organic matter, and microorganisms in the raw water, as well as the concentration of insoluble substances during membrane separation, resulting in the formation of fouling of the reverse osmosis membrane. We all know that the pre-treatment device of the reverse osmosis system is specially designed to remove as many substances as possible that cause membrane fouling. However, even if the system has a relatively complete pre-treatment equipment, it cannot completely avoid membrane fouling during use. Therefore, periodic removal of pollutants from the membrane system is required during equipment operation. This operation process is called CIP (Cleaning In Place) of the reverse osmosis system.

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

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

When considering membrane system cleaning solutions, the following points should be noted:

The impact of cleaning waste liquid on the environment (EDTA, fungicides, etc.) should be minimized.

Efforts should be made to maximize the removal of pollutants during this cleaning process.

The damage to the membrane should be minimized during cleaning (the selection of agents with minimal impact on membrane performance should be considered first).

In actual cleaning operations, while ensuring the cleaning effect, try to minimize the cleaning cost as much as possible

2. Reasons for fouling of reverse osmosis membranes

Inappropriate preprocessing

8226; The system is equipped with pre-treatment devices that are not suitable for the raw water quality and flow rate, or necessary process devices and processes are not equipped in the system.

8226; The pre-treatment device is not operating properly, which means that the original pre-treatment equipment of the system has a low ability to remove SDI components, turbidity, gel like substances, etc. from the raw water, and the pre-treatment effect is not ideal.

The system has selected inappropriate equipment or selected incorrect equipment materials (pumps, piping, and others).

The system chemical injection device has malfunctioned (acid, flocculant/coagulant, scale inhibitor/dispersant, reducing agent, and others).

Failure to take appropriate protective measures after equipment interruption or system shutdown.

Unreasonable equipment operation and use by operation and management personnel (recovery rate, water production, concentrated water volume, pressure difference, cleaning, and others).

Long term accumulation of insoluble precipitates within the membrane system.

Significant changes in the composition of the raw water or fundamental changes in the characteristics of the water source.

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

3. Analysis of membrane fouling substances

Firstly, it is necessary to carefully analyze the recent equipment operation records that have been recorded before and can reflect the equipment operation status.

Analyze the quality of raw water.

Confirm the previous cleaning results.

Analyze the foreign substances left on the filter membrane during SDI value testing during system operation.

Analyze the accumulation on the security filter cartridge of the reverse osmosis system configuration.

Check for foreign substances inside the piping of the raw water flowing into the system and at the inlet end of the reverse osmosis membrane

The behavior of various pollutants scaling

(1) Carbonate scale

Performance after scaling: a decrease in standard permeate water flow rate or a decrease in desalination rate.

Reason: Increased concentration polarization on the membrane surface

(2) Iron/Manganese

Performance after pollution: An increase in standard pressure difference (mainly occurring in the membrane components at the front end of the device) may also cause a decrease in water permeability. Usually, manganese and iron coexist.

(3) Sulfate scale

If sedimentation occurs, it first affects the membrane element at the back of the system with the highest salt concentration, manifested as a significant increase in the second stage pressure difference. Special cleaning agents are required.

(4) Silicon

Particle silicon: fouling the water flow channel of the membrane element, causing an increase in system pressure difference.The use of 0.4% dichloramine is effective in dissolving heavily polluted silica scale.

Silicone: Similar to granular silicon.

Dissolved silicon: forms silicate precipitates, which should be cleaned with dichloramine.

(5) Suspended solids/organic matter

Pollution blockage performance: The permeability decreases and the pressure difference increases significantly in a certain section.If the SDI of the water supply is greater than 4 or the turbidity is greater than 1, there is a higher possibility of organic pollution.

(6) Microorganisms

Pollution blockage performance: the standard pressure difference increases or the standard permeability decreases.Non oxidizing bactericides and alkali can be used for cleaning.

(7) Iron bacteria

Pollution blockage performance: The standard pressure difference increases.EDTA sodium salt and alkali can be used for cleaning.

4. Judgment and selection of cleaning timing for reverse osmosis systems

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

The standardized equipment has reduced water production by 10-15%;

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

The salt transmittance of the standardized membrane system increased by 10-15% compared to the initial normal value;

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

The performance parameters of reverse osmosis equipment are related to various factors such as pressure, temperature, pH value, system water recovery rate, and salt concentration of raw water.Therefore, it is very important to use the normal technical parameters obtained during the initial trial run (product water flow rate, pressure, pressure difference, and system desalination rate) as a basis and compare them with the current standardized system data. In addition, the selection of cleaning time also varies depending on the differences in raw water quality conditions and environmental characteristics in the areas where reverse osmosis equipment is used. Therefore, it is necessary to implement appropriate management measures based on the on-site conditions of the equipment. However, for any well-designed and well managed reverse osmosis system, the shortest cycle of chemical cleaning should be ensured to run continuously for a cumulative period of more than 3 months, and the optimal operating time is generally around 6-12 months. Otherwise, it is necessary to consider improving the pre-treatment equipment or operational management of the original system.

5. Determination of the volume of the cleaning box and calculation of the amount of cleaning solution used

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

1) Estimate using the empty volume of pressure vessels and pipelines:

The empty volume of a pressure vessel is:

V1=N π R2L

Among them: N=number of pressure vessels during each cleaning

R=radius of pressure vessel

L=effective length of pressure vessel

The empty volume of the pipeline is:

V2=L1 π d2/4

Among them: L1=is the total length of the cleaning pipeline

D=diameter of the cleaning pipeline

Total volume of cleaning tank (i.e. amount of cleaning solution prepared):

V=1.2 (V1+V2)

2) Calculate the volume of the cleaning tank and the amount of cleaning solution prepared based on the model specifications and degree of contamination of the membrane components:

For normal pollution situations: Generally, 8.5 liters of cleaning solution are prepared for each 4-inch membrane element;Calculate the volume of the reverse osmosis cleaning tank by preparing a cleaning solution of 34 liters per 8-inch membrane element.

For situations with severe pollution: prepare 16 liters of cleaning solution for each 4-inch membrane element;Prepare a cleaning solution of 55 liters for each 8-inch membrane element, and obtain the volume of the cleaning tank and the amount of cleaning solution prepared from this.

6. Membrane cleaning process

1) Firstly, rinse the reverse osmosis membrane components and system pipelines with reverse osmosis product water (preferably reverse osmosis product water, or softened or filtered water that meets the reverse osmosis inlet water standards),

2) The reverse osmosis product water should be at least qualified softened water to prepare cleaning solution, and ensure uniform mixing; Before cleaning, it is necessary to repeatedly confirm whether the pH value and temperature of the cleaning solution are suitable.

3) Firstly, inject cleaning solution into the reverse osmosis equipment using 1/2 of the normal cleaning flow rate and an operating pressure of 40-60PSI, and remove any water remaining inside the membrane container. And drain the part of the cleaning solution that has just started to circulate back to prevent dilution of the cleaning solution.

During normal cleaning, the pressure control criterion for the cleaning system is to use the pressure that almost prevents the system from producing pure water (i.e., the supply pressure of the cleaning system is equal to the pressure difference between the raw water and concentrated water). Because the appropriate cleaning operating pressure can minimize the possibility of foreign substances re accumulating on the reverse osmosis membrane surface.

4) When cleaning, first drain the water that was previously stored inside the pressure vessel. Then, circulate the concentrated water and output water generated during the cleaning process to the cleaning tank, and pay attention to maintaining a stable temperature of the cleaning solution.Before starting the cycle cleaning, it is necessary to first confirm whether the temperature and pH value of the cleaning solution have met the standards. And confirm the turbidity and other visual conditions of the reflux cleaning solution: if the reflux cleaning solution has significantly changed color or become turbid, the cleaning solution should be prepared again; If the pH change of the reflux cleaning solution exceeds 0.5, it is best to readjust the pH value or replace the cleaning solution.

5) When performing chemical cleaning on the system, the general operating method is to first cycle clean the pressure vessel that needs to be cleaned with low flow rate (1/2 standard cleaning flow rate) for 5-15 minutes, and then cycle clean with medium flow rate (2/3 standard cleaning flow rate) for 10-15 minutes.

6) Then stop the pump and close the valve to soak the membrane components in the cleaning solution for approximately one hour.If the membrane fouling is severe or the pollutants are difficult to remove during cleaning, the soaking time of this process can be appropriately extended.To ensure the temperature of the cleaning solution during long-term soaking, a combination of repeated cycles and soaking can also be used. Generally speaking, the temperature of the cleaning solution should be maintained at least 20 ℃ and below 40 ℃, and an appropriate cleaning solution temperature can enhance the cleaning effect; Please note that cleaning solutions with low temperatures may cause drug precipitation during the cleaning process. When the temperature of the cleaning solution is too low, cleaning should be arranged after raising the temperature of the cleaning solution to a more suitable temperature.

Flow control of each reverse osmosis pressure vessel during cleaning

Pressure vessel diameter

Standard cleaning flow rate per reverse osmosis pressure vessel (in inches)

GPM m3/hr

2.5-5-1.1

4-10-2.3

8-40-9

7) During normal cleaning, after soaking in the cleaning solution, the cleaning can generally be completed by circulating it again at a standard cleaning flow rate for 20-60 minutes. Then rinse the reverse osmosis membrane components with the same volume of reverse osmosis product water and discharge the rinse water into the sewer. After confirming that the flushing is clean, the reverse osmosis equipment can be restarted. We suggest that at least the product water produced within 15 minutes after chemical cleaning and re operation of the system should be discharged. After carefully confirming the chemical analysis results of the on-site system product water quality, the system produced water obtained from system operation should be transferred to the product water tank. In addition, when using multiple drugs for cleaning, to prevent chemical reactions between chemicals, it is best to also drain the water discharged from the product water side before each cleaning.

If it is a multi-stage equipment, it is recommended to clean it in stages to avoid situations where the flow cannot be controlled - that is, the first stage flow is too low or the last stage flow is too high. This can also prevent the contaminated sediment washed off in the first stage from flowing back into the next stage, forming secondary pollution.

8) If you want to prevent microbial contamination again, after cleaning the system, you can use a sterilization solution allowed by the membrane manufacturer to sterilize and clean the membrane system. The operation method is the same as before. Please note that the rinsing after sterilization and cleaning must be thorough to avoid bringing disinfectant into the product water.

7. Appendix: General cleaning solution for polyamide composite membrane components

Cleaning solution

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

[pH 12/30 ℃ (maximum)] 0.1% (W) NaOH or 0.025% (W) Na-SDS

[pH 12/30 ℃ (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

(such as CaCO3) It is best to be able to

Sulfate scale

(CaSO4 BaSO4) Ideally

Metal oxides

(such as iron) It is best to be able to

Inorganic colloids (sludge) are best

Silicon can be the best

Microbial membranes can be the best

Organic matter can be cleaned in the first step, preferably in the second step

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

2. The chemical symbol for pollutants in sequence is: CaCO3 represents calcium carbonate; CaSO4 represents calcium sulfate; BaSO4 represents barium sulfate.

3. The chemical symbol for cleaning in sequence is: NaOH represents sodium hydroxide; Na4EDTA represents tetrasodium ethylenediaminetetraacetic acid; HCl represents hydrochloric acid; Na2S2O4 represents sodium bisulfite; H3PO4 represents phosphoric acid; NH2SO3H represents amine bisulfite.

4. In order to effectively clean sulfate scale, it is necessary to detect and treat it as early as possible. As the solubility of sulfate scale increases with the salt content of the cleaning solution, NaCl can be added to the cleaning solution of NaOH and Na4EDTA. When the scale forms for more than a week, the success of cleaning sulfate scale is questionable.

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

When cleaning the RO membrane, it is best to check the pH value. The pH value of the acid is around 2%, and the concentration is 2%. The pH value of the alkali is around 12%, and the concentration is 0.5%.

Shenzhen Hongjie Water Technology Co., Ltd. is an integrated high-tech enterprise specializing in product research and development, production, sales, engineering design, installation and commissioning, technical consulting, and sales of supporting materials in the fields of industrial water treatment and drinking water treatment. Main products: A high-tech manufacturer specializing in the design, manufacturing, installation, commissioning, and maintenance of water treatment equipment, including pure water equipment, ultra pure water equipment, reverse osmosis equipment, GMP purified water equipment, EDI deionized water equipment, domestic sewage equipment, industrial wastewater equipment, softened water equipment, and reclaimed water reuse equipment.