Characteristics and offline cleaning of severely polluted reverse osmosis membranes
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Time of issue:2020-01-09
【 Summary Description 】When a reverse osmosis system is severely polluted, the first step is to analyze the cause of the pollution, find a solution, and clean the reverse osmosis system through appropriate channels in the shortest possible time. The longer the time, the more difficult it will be to clean.
Reverse osmosis system designs incorporate a certain degree of redundancy to ensure that, in emergencies, insufficient water supply due to decreased water production or desalination rate, or increased pressure difference in the reverse osmosis system, does not threaten safe production. However, this redundancy can sometimes mask hidden faults, potentially leading to severe fouling of the reverse osmosis membrane elements.
1. Concept of Severe Fouling of Reverse Osmosis Membrane Elements
Suspended solids, colloids, organic matter, microorganisms, and other particles in the reverse osmosis system feed water can cause surface adhesion and deposition fouling of the RO membrane. Chemical scaling can also occur due to factors such as concentration polarization, where the product of ion concentrations at the membrane surface exceeds the solubility product. Severe fouling is defined as a single-stage pressure difference exceeding twice the initial single-stage pressure difference at system startup, a decrease in reverse osmosis system water production of more than 30%, or a single reverse osmosis membrane element weight exceeding the normal value by more than 3 kg.
Severe fouling is often a combination of severe physical and chemical fouling. In some cases, both occur simultaneously and may recur even after multiple cleanings.
2. Offline Cleaning Requirements for Severely Fouled RO Membranes
Offline cleaning of severely fouled RO membrane elements is required when the following occurs:
(1) Membrane fouling meets the "severe fouling" criteria;
(2) Online cleaning fails to meet the system's rated standards;
(3) Water shortages prevent online cleaning, or online cleaning equipment is unavailable;
(4) The type of fouling is complex, and online cleaning may cause cross-contamination (where contaminants from the upstream section of the reverse osmosis system are carried into the downstream section during online cleaning, fouling downstream membrane elements);
(5) Fouling recurs after multiple cleanings.
一一:
3. Offline Cleaning Methods and Steps for Reverse Osmosis Membrane Elements
1) Replace the membrane elements to be cleaned in the reverse osmosis system with high-performance spare membrane elements to ensure continuous operation of the reverse osmosis system and maintain the stability of the entire production process.
2) Reverse Osmosis Membrane Element Performance Testing (This step is crucial):
Individually test the performance indicators of each membrane element, including desalination rate, water production, pressure difference, and weight, and record the pre-test data.
Desalination rate, water production, and pressure difference testing conditions: Comply with the standards provided by different membrane manufacturers.
3) Understand the current operating status of the system before cleaning.
4) Collect and record the various parameter indicators of the operating reverse osmosis system.
5) Determine the cleaning process based on the user's raw water full analysis report, performance test results, and the understood system information.
6) Contaminant identification. First, make a preliminary determination based on the analysis results in 5), then further verify using special equipment and instruments to determine the specific contaminant type.
7) Determine the required cleaning formula based on the analysis results in 5) and 6). Once the contaminants on the RO membrane are identified, we can select a suitable formula or two from the manufacturer's series of formulas; or select a special formula (when the RO membrane is fouled by special contaminants, ordinary formulas are less effective, or special formulas are more economical from an economic perspective). Currently, there are many specialized cleaning agents for reverse osmosis membrane elements, such as Kaiyuan Hengye's KY series cleaning agents and bactericides. In my experience, they are effective and economical compared to traditional agents.
8) Perform experimental cleaning using the above cleaning agents and physical treatment cleaning methods on specialized reverse osmosis cleaning equipment to select the appropriate cleaning formula and procedure.
9) Determine the cleaning method and treat all the membrane elements.
10) Test the cleaned membrane elements on the testing platform and record the results. Those that do not meet the requirements will be returned to the cleaning equipment for reprocessing.
11) Organize cleaning data and write a cleaning summary report.
Importance of Offline Cleaning in the Process of Reverse Osmosis Pure Water Equipment Membrane Element Cleaning
In current reverse osmosis membrane element cleaning, offline cleaning, as an effective method for thoroughly eliminating reverse osmosis membrane element fouling, has been widely accepted by reverse osmosis users and is becoming an increasingly important cleaning method.
However, more and more owners are also increasingly confused:
1. During offline cleaning of reverse osmosis membrane elements, the water treatment system must be shut down or intermittently operated, which is often difficult for industrial enterprises to achieve. Even if achieved, it is done at the expense of normal production, posing significant risks to enterprise profits and safe production.
2. Due to the lack of spare membrane elements, the cleaning process must be accelerated, resulting in incomplete cleaning and persistent fouling risks.
3. After offline cleaning, the system's initial operating parameters are good, but the causes of contamination discovered during the cleaning process and the solutions adopted are not fully verified, and contamination often recurs. Even with warranties, the reverse osmosis system soon enters a cycle of fouling-cleaning-fouling, shortening the lifespan of the membrane elements and potentially jeopardizing the owner's interests for various reasons.
一一:
Reverse osmosis pure water equipment membrane elements were developed for offline cleaning. Their good performance makes spare membrane elements almost as good as new ones. During the use of spare membranes:
1. Effectively address and verify the problems found during the cleaning process.
2. Some measures used in RO pretreatment may be inappropriate and may damage the membrane elements. However, since these are spare (old) membrane elements, the potential losses are smaller, which is beneficial to both parties;
3. After taking effective measures to solve the problem, a period of observation of the operating parameters is needed to verify whether the measures taken to address the identified problem are appropriate and whether they can ensure the long-term stable operation of the system. With first-hand data, the post-cleaning pollution problem will no longer be an issue, and it will be more than just words on paper.
In the pollution control of reverse osmosis, the most fundamental measures lie in the design, manufacturing, installation process of the water treatment system centered on reverse osmosis, the selection of various consumables for the reverse osmosis system, and the control of operation and management levels. A good grasp of these aspects plays a crucial role in the safe and healthy operation of the reverse osmosis system.
Of course, when the reverse osmosis system is seriously polluted, the first measure is to analyze the cause of the pollution, find solutions to the pollution, and clean the reverse osmosis system through appropriate channels in the shortest possible time, because as time goes on, the cleaning difficulty increases.
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Shijiazhuang Tianwang Environmental Protection Technology Co., Ltd.
Shijiazhuang Tianwang Environmental Protection Technology Co., Ltd. is a high-tech enterprise specializing in the research and development, manufacturing and sales of water treatment equipment.
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