Chemical cleaning solution for reverse osmosis membranes
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This post was last edited by Kang Zhaoyang on 2016-1-16 at 10:20. The cleaning of reverse osmosis membranes is a delicate and complicated task, and frequent cleaning can easily damage them. Since there is usually more than one membrane element in a single membrane housing, the degree of contamination of these elements varies as well; as a result, the cleaning effect is poor, and cross-contamination can easily occur. As a deep filtration method, reverse osmosis membrane elements inevitably have colloids, microorganisms, impurity particles, and insoluble salts deposited on their surfaces. Therefore, it is necessary to develop effective cleaning strategies for reverse osmosis membranes in various applications. The only difference between different equipment systems lies in the length of the cleaning cycles required. Reverse osmosis membrane; Offline cleaning of reverse osmosis membranes. Before carrying out reverse osmosis treatment, it is necessary to pre-treat the water supply in order to minimize contamination of the membrane surface as much as possible. The optimal operating conditions (product flow rate, pressure, recovery rate, and pH value) play a very important role in reducing fouling on the membrane surface. Once the pretreated feed water has a high SDI15 value (even if it is within the allowable range), as operation time increases, dirt such as suspended solids, colloids, and scale will accumulate on the surface of the reverse osmosis membrane. Dirt will lead to a decline in the performance of the membrane elements, manifested as a lower water production flow rate and/or a higher solute permeability and/or an increased pressure difference between the feed water and the concentrated water. Generally, depending on the level of contamination, different cleaning methods are used for reverse osmosis cleaning, including online cleaning and offline cleaning. Reverse osmosis membranes; Offline cleaning of reverse osmosis membranes; Online cleaning technology for reverse osmosis membranes. Online cleaning is a conventional cleaning method, primarily used for the maintenance and upkeep of reverse osmosis systems. 1. Cleaning conditions Your system requires immediate online cleaning when the following changes occur in the parameters of the reverse osmosis system: 1) Inter-stage pressure difference of the system: Pressure difference in the first stage ≤ 0.2 Mpa; pressure difference in the second stage ≤ 0.1 Mpa. 2) With the system operation parameters unchanged, the water production volume of the temperature correction system is restored to over 85% of the designed value. 3) The desalination rate of the produced water shall not vary by more than 5%. 2. Cleaning steps: ① Use a pump to transfer clean, free-of-free chlorine reverse osmosis product water from the cleaning tank (or corresponding water source) into the pressure vessel, and allow it to stay there for a few minutes. ②Prepare the cleaning solution in the cleaning tank using clean product water. ③Circulate the cleaning solution in the pressure vessel for 1 hour or the preset time. ④After cleaning is complete, drain the cleaning tank and rinse it, then fill it with clean product water for the next rinsing step. ⑤Use a pump to transfer the clean, free-of-free chlorine product water from the cleaning tank (or corresponding water source) into the pressure vessel and allow it to stand there for a few minutes. ⑥After flushing the reverse osmosis system, operate it with the product water discharge valve open until the product water is clean, free of foam, and free of cleaning agents (usually 15–30 minutes). Reverse osmosis membrane; Offline cleaning of reverse osmosis membranes. Techniques for offline cleaning of reverse osmosis membranes. The biggest problem with online cleaning is that the concentration of the cleaning solution at the end of the cleaning process gradually decreases. When the reverse osmosis system has been in operation for a certain period of time or when the reverse osmosis membranes are severely contaminated, online cleaning becomes ineffective; in such cases, it is necessary to perform offline cleaning of the reverse osmosis membrane elements. 1. Conditions for offline cleaning of reverse osmosis membranes ① When the pressure difference across a single stage in the reverse osmosis system is more than twice the value of that pressure difference at the beginning of system operation. ②The water production volume of the reverse osmosis system has decreased by more than 30%. ③The weight of a single reverse osmosis membrane element exceeds the normal value by more than 3 kilograms. ④The reverse osmosis system cannot have its performance restored through online cleaning. Reverse osmosis membrane, offline cleaning of reverse osmosis membranes. 2. Characteristics of offline cleaning: During online cleaning, it is difficult to target membranes at different locations effectively. Since there is usually more than one membrane element in a membrane housing, the degree of contamination among these elements varies. When cleaning membranes with different levels of contamination using the same concentration of cleaning solution, it inevitably results in some membranes being over-cleaned while others are under-cleaned; as a result, the cleaning effect is poor, and cross-contamination is likely to occur. Reverse osmosis membrane; offline cleaning of reverse osmosis membranes. Taking all the above factors into account, it is possible to monitor the cleaning status of the RO membrane during the cleaning process, and determine the extent of cleaning based on the actual conditions. By analyzing the main components of pollutants in the cleaning solution, the pollutants present in the system are identified, and corresponding adjustments are suggested to extend the service life of the customer’s system.
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I’ve learned it; it’s useful to draw on this experience. Thank you
The analysis is quite thorough and the content is rich
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Pollution of reverse osmosis membranes and cleaning methods – Continuing to support you here! ________________________________________ The methods described in this article are applicable to reverse osmosis membrane elements with diameters of 4 inches, 6 inches, 8 inches, and 8.5 inches. Note 1: Under no circumstances should water containing free chlorine come into contact with the composite membrane elements; such contact will lead to a decline in the performance of these elements, with no possibility of restoring their original performance. After sterilizing the pipelines or equipment, it is necessary to ensure that the feed water supplied to the reverse osmosis membrane elements is free of free chlorine. This should be confirmed through testing. Hydrogen sulfite solution should be used to neutralize any residual chlorine, and sufficient contact time must be ensured to guarantee complete reaction. Note 2: During the warranty period of the reverse osmosis membrane elements, it is recommended that each cleaning of the membranes be carried out in consultation with Hydranautics; at least for the first cleaning, Hydranautics’ on-site service personnel should be present. Note 3: Cationic surfactants should be avoided in the cleaning solution, as their use may cause irreversible contamination of the membrane elements. 1. Contaminants in reverse osmosis membrane elements: After operating for a period of time, reverse osmosis membrane elements can become contaminated by suspended substances or insoluble materials present in the feed water. The most common of these contaminants are calcium carbonate scale, calcium sulfate scale, metal oxide scale, silicon deposits, as well as organic or biological deposits. The nature of the pollutants and the rate of contamination are related to the feedwater conditions; contamination develops gradually, and if no measures are taken early on, it will damage the performance of the membrane elements within a relatively short period of time. Regularly testing the overall performance of the system is a good way to determine whether the membrane elements are contaminated; different contaminants can cause varying degrees of damage to the performance of these membrane elements. Table 1 lists the effects of common pollutants on membrane performance. 2. Removal of pollutants: The removal of pollutants can be achieved through chemical cleaning and physical flushing; sometimes it can also be accomplished by changing operating conditions. As a general rule, cleaning should be carried out when one of the following situations occurs. 2.1 At normal pressure, if the product water flow rate drops to 10–15% of the normal value. 2.2 To maintain a normal product water flow rate, the temperature-corrected feed water pressure was increased by 10 to 15%. 2.3 The water quality of the product decreases by 10 to 15%. The salt permeability increases by 10 to 15%. 2.4 Increasing the pressure by 10–15% results in a significant increase in the pressure difference between various sections of the RO system (although there may be no instruments to detect this change). 3. Common pollutants and their removal methods: 3.1 Calcium carbonate scale – When there is a malfunction in the scale-inhibitor addition system or in the acid addition system, which leads to an increase in the pH of the feed water, calcium carbonate may precipitate. It is important to detect the formation of calcium carbonate scale as early as possible to prevent the growing crystals from damaging the membrane surface. If detected early, calcium carbonate scale can be removed by lowering the pH of the feed water to between 3.0 and 5.0 and maintaining that level for 1–2 hours. For calcium carbonate scale with a longer precipitation time, it should be cleaned by circulating a citric acid cleaning solution or by soaking it overnight. Note: It must be ensured that the pH of any cleaning solution is not lower than 2.0, as this could cause damage to the RO membrane elements; extra care should be taken at higher temperatures. The maximum pH should not exceed 11.0. Ammonia water is used to increase the pH, while sulfuric acid or hydrochloric acid is used to lower it. 3.2 Calcium sulfate scale cleaning solution 2 (see Surface treatment) is the best method for removing calcium sulfate scale from the surface of reverse osmosis membranes. 3.3 Metal oxide scale: The methods described above for removing calcium carbonate scale can be used to easily remove the deposited hydroxides (such as iron hydroxide). 3.4 Silica scale: For silica scale that does not coexist with metal compounds or organic substances, it can generally only be removed through specialized cleaning methods; for detailed procedures, please contact Hydron Corporation. 3.5 Organic Deposits Organic deposits (such as microbial sludge or mold) can be removed using Cleaning Solution 3. To prevent their regrowth, a bactericidal solution approved by Hydron Corporation can be circulated and used for soaking in the system; generally, a prolonged soaking time is required for effective results. When the reverse osmosis unit is not in use for three days, it is advisable to carry out disinfection procedures. Please consult Hydron Corporation to determine the appropriate bactericide. 3.6 Cleaning Solutions It is recommended to use the cleaning solutions listed in Table 2 when cleaning reverse osmosis membrane elements. It is crucial to conduct chemical analysis of contaminants prior to cleaning; a detailed comparison of the analysis results helps ensure that the best cleaning agents and methods are selected. The cleaning method used and the resulting cleaning efficiency for each cleaning session should be recorded, providing a basis for identifying the optimal cleaning method under specific water supply conditions. For inorganic pollutants, it is recommended to use Cleaning Solution 1. For calcium sulfate and organic substances, it is recommended to use Cleaning Solution 2. For severe organic contamination, it is recommended to use Cleaning Solution 3. All cleaning can be carried out at a maximum temperature of 104°F (40°C) for 60 minutes. The amount of chemicals required is based on a per-100-gallon (379 liters) rate; when preparing the cleaning solution, the chemicals and water should be added in proportion. It is necessary to use reverse osmosis water free of free chlorine to prepare the solution, ensuring that it is well mixed. Chemical cleaning and water flushing of reverse osmosis membranes ________________________________________ During cleaning, the cleaning solution is circulated at low pressure and high flow rate on the high-pressure side of the membrane; at this time, the membrane element remains inside a pressure vessel, and a specialized cleaning device is required to carry out this process. General steps for cleaning reverse osmosis membrane elements: 1. Use a pump to pump clean, free-of-free chlorine reverse osmosis product water from the cleaning tank (or appropriate water source) into the pressure vessel and let it flow through for a few minutes. 2. Prepare the cleaning solution in the cleaning tank using clean product water. 3. Circulate the cleaning solution in the pressure vessel for 1 hour or for a pre-set time; for 8-inch or 8.5-inch pressure vessels, the flow rate should be 35–40 gallons per minute (133–151 liters per minute), for 6-inch pressure vessels it should be 15–20 gallons per minute (57–76 liters per minute), and for 4-inch pressure vessels it should be 9–10 gallons per minute (34–38 liters per minute). 4. After the cleaning is complete, drain the cleaning tank and rinse it, then fill it with clean product water to prepare it for the next rinsing step. 5. Use a pump to transfer the clean, free-of-free chlorine product water from the cleaning tank (or corresponding water source) into the pressure vessel and let it stand there for a few minutes. 6. After flushing the reverse osmosis system, operate it with the product water discharge valve open until the product water is clean, free of foam, and free of cleaning agents (this usually takes 15–30 minutes). Table 1. Characteristics of RO membrane fouling and treatment methods
Pollutant | General characteristics | Treatment method
--- | --- | ---
Calcium deposits (calcium carbonate and calcium phosphate; usually occur in the second stage of the system) | Significant decrease in desalination efficiency; increased system pressure; slight reduction in system water production | Clean the system using Solution 1
Oxides (iron, nickel, copper, etc.) | Significant decrease in desalination efficiency; substantial increase in system pressure; significant reduction in system water production | Clean the system using Solution 1
Various colloids (iron, organic substances, silica colloids) | Slight decrease in desalination efficiency; gradual increase in system pressure; gradual reduction in system water production | Clean the system using Solution 2
Calcium sulfate (usually occurs in the second stage of the system) | Significant decrease in desalination efficiency; slight to moderate increase in system pressure; slight reduction in system water production | Clean the system using Solution 2; use Solution 3 if fouling is severe
Organic deposits | Possible decrease in desalination efficiency; gradual increase in system pressure; gradual reduction in system water production | Clean the system using Solution 2; use Solution 3 if fouling is severe
Bacterial contamination | Possible decrease in desalination efficiency; significant increase in system pressure; significant reduction in system water production | Select one of the three solutions based on the type of contamination present. Note: It is necessary to identify the cause of fouling and eliminate the source of contamination; for assistance, please contact Hydranautics.
Table 2. Common cleaning solutions recommended for use
Cleaning solution | Components | Amount required to prepare 100 gallons (379 liters) of solution | pH adjustment
--- | --- | --- | ---
1 | Citric acid | Reverse osmosis product water (free of free chlorine): 17.0 pounds (7.7 kg) per 100 gallons (379 L); adjust pH to 3.0 using ammonia
2 | Sodium tripolyphosphate, disodium EDTA | Reverse osmosis product water (free of free chlorine): 17.0 pounds (7.7 kg); 7 pounds (3.18 kg) per 100 gallons (379 L); adjust pH to 10.0 using sulfuric acid
3 | Sodium tripolyphosphate, sodium dodecylbenzenesulfonate | Reverse osmosis product water (free of free chlorine): 17.0 pounds (7.7 kg); 2.13 pounds (0.97 kg) per 100 gallons (379 L); adjust pH to 10.0 using sulfuric acid
Pollutant | General characteristics | Treatment method
--- | --- | ---
Calcium deposits (calcium carbonate and calcium phosphate; usually occur in the second stage of the system) | Significant decrease in desalination efficiency; increased system pressure; slight reduction in system water production | Clean the system using Solution 1
Oxides (iron, nickel, copper, etc.) | Significant decrease in desalination efficiency; substantial increase in system pressure; significant reduction in system water production | Clean the system using Solution 1
Various colloids (iron, organic substances, silica colloids) | Slight decrease in desalination efficiency; gradual increase in system pressure; gradual reduction in system water production | Clean the system using Solution 2
Calcium sulfate (usually occurs in the second stage of the system) | Significant decrease in desalination efficiency; slight to moderate increase in system pressure; slight reduction in system water production | Clean the system using Solution 2; use Solution 3 if fouling is severe
Organic deposits | Possible decrease in desalination efficiency; gradual increase in system pressure; gradual reduction in system water production | Clean the system using Solution 2; use Solution 3 if fouling is severe
Bacterial contamination | Possible decrease in desalination efficiency; significant increase in system pressure; significant reduction in system water production | Select one of the three solutions based on the type of contamination present. Note: It is necessary to identify the cause of fouling and eliminate the source of contamination; for assistance, please contact Hydranautics.
Table 2. Common cleaning solutions recommended for use
Cleaning solution | Components | Amount required to prepare 100 gallons (379 liters) of solution | pH adjustment
--- | --- | --- | ---
1 | Citric acid | Reverse osmosis product water (free of free chlorine): 17.0 pounds (7.7 kg) per 100 gallons (379 L); adjust pH to 3.0 using ammonia
2 | Sodium tripolyphosphate, disodium EDTA | Reverse osmosis product water (free of free chlorine): 17.0 pounds (7.7 kg); 7 pounds (3.18 kg) per 100 gallons (379 L); adjust pH to 10.0 using sulfuric acid
3 | Sodium tripolyphosphate, sodium dodecylbenzenesulfonate | Reverse osmosis product water (free of free chlorine): 17.0 pounds (7.7 kg); 2.13 pounds (0.97 kg) per 100 gallons (379 L); adjust pH to 10.0 using sulfuric acid
What exactly is reverse osmosis cleaning solution?