Complete Analysis of the Causes of Reverse Osmosis Equipment Clogging and Cleaning Procedures-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Complete Analysis of the Causes of Reverse Osmosis Equipment Clogging and Cleaning Procedures

Published Time:

2022-12-26

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1. Overview

During the operation of a reverse osmosis system, the reverse osmosis membrane surface will accumulate deposits due to the presence of pollutants such as silt, colloids, organic matter, and microorganisms in the raw water, and the concentration of sparingly soluble substances during the membrane separation process, resulting in membrane fouling. We all know that the pretreatment device of the reverse osmosis system is specifically designed to remove as many substances that cause membrane fouling as possible. However, even with a fairly complete pretreatment system, membrane fouling during operation cannot be completely avoided. Therefore, periodic cleaning operations to remove pollutants from the membrane system are required during equipment operation. This operation is called in-situ cleaning (CIP, Cleaning In Place) of the reverse osmosis system.

After the reverse osmosis membrane is fouled, the system's water production will decrease, and the salt passage rate will increase, indicating a decline in membrane performance. However, due to changes in other major factors affecting 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 to this.

Currently, most aromatic polyamide reverse osmosis composite membranes on the market have considerable stability and certain temperature resistance within a wide pH range, so users can perform very effective cleaning of the reverse osmosis system. Years of engineering practice have shown that if a reverse osmosis system that has already experienced a certain degree of fouling is not cleaned in time, it is very difficult to thoroughly remove pollutants that have been attached to the membrane surface for a long time.

Generally, when considering a membrane system cleaning plan, the following points should be noted:

■ The environmental impact of cleaning wastewater (EDTA, bactericides, etc.) should be minimized.
■ The cleaning process should maximize the removal of pollutants.
■ Membrane damage during cleaning should be minimized (agents with minimal impact on membrane performance should be considered first).
■ During actual cleaning operations, cleaning costs should be minimized while ensuring cleaning effectiveness.

2. Causes of Reverse Osmosis Membrane Fouling
■ Inadequate pretreatment
• The pretreatment equipment provided by the system is not suitable for the raw water quality and flow rate, or necessary processes and steps are not equipped in the system.
• The pretreatment equipment is malfunctioning, that is, the existing pretreatment equipment in the system has low removal capacity for SDI components, turbidity, colloids, etc., in the raw water, and the pretreatment effect is not ideal.
■ Improper equipment selection or incorrect material selection (pumps, piping, and others).
■ Malfunction of the system's chemical injection device (acids, flocculants/coagulants, antiscalants/dispersants, reducing agents, and others).
■ Intermittent operation of the equipment or failure to take appropriate protective measures after the system is shut down.
■ Unreasonable equipment operation and use by operation and maintenance personnel (recovery rate, water production, concentrate water volume, pressure difference, cleaning, and others).
■ Long-term accumulation of sparingly soluble precipitates in the membrane system.
■ Significant changes in raw water composition or fundamental changes in water source characteristics.
■ Significant microbial contamination in the reverse osmosis membrane system.

3. Analysis of Membrane Fouling Substances

■ First, carefully analyze the recent equipment operation records that reflect the equipment's operating status.
■ Analyze the raw water quality.
■ Confirm the results of previous cleaning.
■ Analyze the foreign substances left on the filter membrane during SDI value testing when the system is running.
■ Analyze the deposits on the filter cartridges of the security filters configured in the reverse osmosis system.
■ Check for foreign substances inside the piping where raw water enters the system and at the inlet of the reverse osmosis membrane.

※ Manifestations of various fouling substances during scaling
(1) Carbonate scale
Manifestation after scaling: Standard permeate flow rate decreases, or desalination rate decreases.
Cause: Increased concentration polarization on the membrane surface
(2) Iron/Manganese
Manifestation after fouling: Standard pressure difference increases (mainly occurs in the membrane elements at the front of the device), and may also cause a decrease in permeate flow rate. Manganese and iron usually coexist.
(3) Sulfate scale
If deposition occurs, it first affects the membrane elements at the end of the system with the highest salt concentration, manifested as a significant increase in the two-stage pressure difference. A special cleaning agent is required.
(4) Silicon
Particulate silicon: Clogs the water flow channels of the membrane elements, causing an increase in system pressure difference. 0.4% dichloroisocyanurate is effective for severely contaminated dissolved silicon scale.
Colloidal silicon: Similar to particulate silicon.
Dissolved silicon: Forms silicate precipitation, dichloroisocyanurate cleaning should be used.
(5) Suspended solids/organic matter
Fouling manifestation: Permeate flow rate decreases, and the first-stage pressure difference increases significantly. If the raw water SDI is greater than 4 or the turbidity is greater than 1, the possibility of organic matter pollution is high.
(6) Microorganisms
Fouling manifestation: Standard pressure difference increases or standard permeate flow rate decreases. Non-oxidizing bactericides plus alkali can be used for cleaning.
(7) Iron bacteria
Fouling manifestation: Standard pressure difference increases. EDTA sodium salt plus alkali can be used for cleaning.

4. Judgment and Selection of Cleaning Time for Reverse Osmosis System

The reverse osmosis membrane system should be cleaned when any of the following situations occur
■ Standardized equipment water production has decreased by 10~15%;
■ The operating pressure of the standardized membrane system has increased by 15%;
■ The salt passage rate of the standardized membrane system has increased by 10~15% compared to the initial normal value;
■ The operating pressure difference has increased by 15% compared to the initial operation;
(It is recommended to use the operating records obtained during the initial 25~48 hours of equipment operation as the basis for comparison after standardization.)
The performance parameters of the reverse osmosis equipment are related to many factors, such as pressure, temperature, pH value, system water recovery rate, and raw water salinity. Therefore, it is very important to use the normal technical parameters (product water flow rate, pressure, pressure difference, and system desalination rate) obtained during the initial commissioning test as a basis for comparison with the current system data after standardization. In addition, the choice of cleaning time also varies due to differences in raw water quality conditions and environmental characteristics in the regions where reverse osmosis equipment is used. Therefore, it is necessary to implement appropriate management measures according to the on-site conditions of the equipment. However, regardless of the circumstances, for any well-designed and well-managed reverse osmosis system, the minimum cycle for chemical cleaning should be guaranteed for more than 3 consecutive months of cumulative operation, and an operating time of about 6-12 months is best. Otherwise, improvements to the pretreatment equipment of the existing system or its operation and management must be considered.

5. Determination of cleaning tank volume and calculation of cleaning solution usage

The volume of the cleaning tank and the amount of cleaning solution can be calculated in the following ways:
1) Estimation using 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 of the pipeline is:
V2=L1πd2/4
Where: L1 = Total length of the cleaning pipeline
d = Diameter of the cleaning pipeline
Total volume of the cleaning tank (i.e., cleaning solution preparation amount):
V= 1.2(V1+ V2)
2) Calculate the volume of the cleaning tank and the preparation amount of the cleaning solution according to the model specifications and pollution level of the membrane element:
For normal pollution conditions: Generally, 8.5 liters of cleaning solution is prepared for each 4-inch membrane element; 34 liters of cleaning solution is prepared for each 8-inch membrane element to calculate the volume of the reverse osmosis cleaning tank.
For more serious pollution conditions: 16 liters of cleaning solution is prepared for each 4-inch membrane element; 55 liters of cleaning solution is prepared for each 8-inch membrane element, thus obtaining the volume of the cleaning tank and the preparation amount of the cleaning solution.

6. Membrane cleaning process

1) First, rinse the reverse osmosis membrane components and system pipelines with reverse osmosis product water (preferably reverse osmosis product water, or softened water or filtered water that meets the reverse osmosis feed water standard),
2) Prepare the cleaning solution with reverse osmosis product water, which should at least be qualified softened water, and ensure that it is mixed evenly; before cleaning, repeatedly confirm whether the pH value and temperature of the cleaning solution are suitable.
3) First, inject the cleaning solution into the reverse osmosis equipment at 1/2 of the normal cleaning flow rate and 40~60 PSI operating pressure, and remove the water remaining inside the membrane container. Drain the first part of the circulating cleaning solution 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 cleaning system supply pressure is equal to the pressure difference between the raw water and the concentrated water). Because the appropriate cleaning operating pressure can minimize the possibility of foreign substances accumulating on the surface of the reverse osmosis membrane.
4) During cleaning, first drain the water previously remaining inside the pressure vessel. Then circulate the concentrated water and product water generated during the cleaning process to the cleaning tank, and pay attention to maintaining the cleaning solution temperature stable. Before starting the circulation cleaning, first confirm whether the temperature and pH value of the cleaning solution meet the standards. And confirm the visual conditions such as the turbidity of the reflux cleaning solution: If the reflux cleaning solution has obviously 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 chemically cleaning the system, the general operation method is: First, the pressure vessel to be cleaned is cleaned in a low-flow (1/2 standard cleaning flow rate) circulation for 5~15 minutes, and then cleaned in a medium-flow (2/3 standard cleaning flow rate) circulation for 10~15 minutes.
6) Then stop the pump and close the valve, allowing the membrane elements to soak in the cleaning solution for about 1 hour. If the membrane pollution is more serious or the cleaning is difficult to remove pollutants, the soaking time of this process can be appropriately extended. To ensure the cleaning solution temperature during long-term soaking, a combination of repeated circulation and soaking can also be used. Generally speaking, the temperature of the cleaning solution should be at least maintained above 20℃ and below 40℃, and the suitable cleaning solution temperature can enhance the cleaning effect; please note: The cleaning solution with too low temperature may cause precipitation of the drug during the cleaning process. When the cleaning solution temperature is too low, the cleaning should be carried out after the cleaning solution temperature is raised to a more suitable temperature.
Flow control of each reverse osmosis pressure vessel during cleaning
Pressure vessel diameter
(inches) Standard cleaning flow rate through each reverse osmosis pressure vessel
GPM m3/hr
2.5 ～5 ～1.1
4 ～10 ～2.3
8 ～40 ～9

7) During normal cleaning, after soaking in the final cleaning solution, rinse again with the standard cleaning flow for 20-60 minutes. Then, rinse the reverse osmosis membrane components with the same volume of reverse osmosis product water and drain the rinse water into the sewer. After confirming that the rinse is clean, the reverse osmosis equipment can be restarted. We recommend that at least the product water produced within 15 minutes after restarting the system after chemical cleaning should be discharged. After carefully confirming the chemical analysis results of the on-site system product water quality, the system output water obtained from the system operation can be added to the product water tank. In addition, when using multiple chemicals for cleaning, in order to prevent chemical reactions between chemicals, it is best to drain the water discharged from the product water side before each cleaning.
※ For multi-stage equipment, it is recommended to clean stage by stage to avoid uncontrollable flow - that is, the flow rate of the first stage is too low or the flow rate of the last stage is too high. This can also prevent pollutants washed away in the first stage from flowing back into the next stage, causing secondary pollution.
8) To prevent microbial recontamination, after cleaning the system, the membrane system can be sterilized with a bactericidal solution allowed by the membrane manufacturer, and the operation method is the same as before. Please note: The rinsing after sterilization must be thorough to avoid bringing disinfectant into the product water.

7. Appendix General Cleaning Solution for Polyamide Composite Membrane Elements

Cleaning Solution
Pollutant 0.1% (W) NaOH or 1.% (W) Na4EDTA
[pH12/30℃ (maximum)] 0.1% (W) NaOH or 0.025% (W) Na-SDS
[pH12/30℃ (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 scale
(such as CaCO3) Best Can Can Can
Sulfate scale
(CaSO4 BaSO4) Best Can
Metal oxide
(such as iron) Best Can Can Can
Inorganic colloid (silt) Best
Silicon Can Best
Biofilm Can Best
Organic matter Can be used as the first step of cleaning Can be used as the first step of cleaning Best Can be used as the second step of cleaning Best
1、(W) indicates the weight percentage of the active ingredient;
2、In order, the chemical symbols for pollutants are: CaCO3 represents calcium carbonate; CaSO4 represents calcium sulfate; BaSO4 represents barium sulfate.
3、In order, the chemical symbols for cleaning chemicals are: NaOH represents sodium hydroxide; Na4EDTA represents tetrasodium ethylenediaminetetraacetate; Na-SDS represents sodium dodecylbenzenesulfonate, also known as sodium metasilicate; HCl represents hydrochloric acid; Na2S2O4 represents sodium dithionite; H3PO4 represents phosphoric acid; NH2SO3H represents sulfamic acid.
4、In order to effectively clean sulfate scale, it must be discovered and treated as early as possible. Since the solubility of sulfate scale increases with the increase in the salt content of the cleaning solution, NaCl can be added to the NaOH and Na4EDTA cleaning solution. When scaling for more than a week, the success rate of sulfate scale cleaning is questionable.
5、Citric acid is an optional cleaning agent for inorganic scale.
When cleaning RO membranes, 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%