After a period of normal operation, the reverse osmosis membrane components may be contaminated by suspended solids or insoluble salts that may exist in the feedwater. The most common types of these pollutants are calcium carbonate precipitation, calcium sulfate precipitation, metal (iron, manganese, copper, nickel, aluminum, etc.) oxide precipitation, silicon sediment, inorganic or organic sedimentary mixtures, NOM natural organic substances, synthetic organic substances (such as scale inhibitors/dispersants, cationic polyelectrolytes) Microbial (algae, mold, fungi) and other pollutants.
The nature and speed of pollution depend on various factors, such as water quality and system recovery rate. Usually, pollution develops gradually, and if not controlled early, it will damage the membrane components in a relatively short period of time. When it is confirmed that the membrane components have been contaminated, or before long-term shutdown, or as regular daily maintenance, it is recommended to clean the membrane components.
Chemical cleaning or physical flushing is required when the reverse osmosis system (or device) experiences the following symptoms:
Under normal water supply pressure, the water production decreases by 10-15% compared to the normal value;
To maintain normal water production, the feedwater pressure after temperature correction increases by 10-15%;
The water quality of the produced water decreases by 10-15%, and the salt penetration rate increases by 10-15%;
Increase the water supply pressure by 10-15%;
The pressure difference between each section of the system has significantly increased.
The stable operating parameters mainly refer to the production water flow rate, production water back pressure, recovery rate, temperature, and TDS. If these operating parameters fluctuate, EFLOW suggests to check whether there is pollution, or whether the actual operation of reverse osmosis is normal when the key operating parameters change.
The overall performance of a timed monitoring system is the basic method to confirm whether membrane components have been contaminated. The impact of pollution on membrane components is gradual, and the degree of impact depends on the nature of the pollution. Table 1 "Characteristics and Treatment Methods of Reverse Osmosis Membrane Pollution" lists common pollution phenomena and corresponding treatment methods.
The cleaning cycle of the contaminated reverse osmosis membrane depends on the actual situation on site. EFLOW company suggests that the normal cleaning cycle is every 3-12 months.
When the membrane element is only slightly contaminated, it is important to clean the membrane element. Severe pollution can hinder the penetration of chemicals into the contaminated layer, affecting the cleaning effect.
The type of pollutant to be cleaned and how to clean it should be based on the on-site pollution situation. For complex situations where several types of pollution coexist, the cleaning method is to use alternating cleaning solutions with low and high pH values (low pH followed by high pH values).
Table 1 Pollution Characteristics and Treatment Methods of Reverse Osmosis Membrane
Types of pollution
Possible occurrence
pressure drop
Feed water pressure
Salt transmittance
Metal oxides (Fe, Mn, Cu, Ni, Zn)
One section, the front-end membrane element
Rapid increase
Rapid increase
Rapid increase
Colloids (organic and inorganic mixtures)
One section, the front-end membrane element
Gradually increasing
Gradually increasing
Mild increase
Mineral scale (Ca, Mg, Ba, Sr)
Terminal membrane element
Moderate increase
Mild increase
General increase
Polymeric silicon sediment
Terminal membrane element
General increase
increase
General increase
Biological pollution
Any position, usually the front-end membrane element
Significantly increased
Significantly increased
General increase
Organic pollution (insoluble NOM)
All segments
Gradually increasing
Add
reduce
Scale inhibitor contamination
The second stage is the most severe
General increase
increase
General increase
Oxidative damage (Cl 2, O zone, KmnO 4)
The most serious period
General increase
reduce
Add
Hydrolysis damage (beyond pH range)
All segments
General reduction
reduce
Add
Abrasion damage (toner)
The most serious period
General reduction
reduce
Add
Leakage of O-ring (internal connecting pipe or adapter)
Irregular, usually at the water supply adapter
General reduction
General reduction
Add
Rubber ring leakage (caused by water backpressure)
The most serious period
General reduction
General reduction
Add
Rubber ring leakage (caused by closing the production water valve during cleaning or flushing)
Endmost component
Increase (initial pollution and increased pressure difference)
Add
2. Analysis of pollution situation
Calcium carbonate scale:
Calcium carbonate scale is a mineral scale. When there is a malfunction in the scale inhibitor/dispersant adding system, or when the pH adjustment system for acid addition malfunctions and causes an increase in the pH of the feedwater, calcium carbonate scale may deposit. Early detection of calcium carbonate scale is extremely necessary to prevent crystal deposition on the surface of the membrane layer from damaging membrane components. The early detection of calcium carbonate scale can be removed by reducing the pH value of the feed water to 3-5 and running for 1-2 hours. For calcium carbonate scale that has been deposited for a long time, it can be cleaned and removed with a low pH citric acid solution.
Calcium sulfate, Barium sulfate, Strontium sulfate scale:
Sulfate scale is a mineral scale that is much harder than calcium carbonate scale and is difficult to remove. Sulfate scale can deposit when the scale inhibitor/dispersant addition system malfunctions or when sulfuric acid is added to adjust the pH. EFLOW believes that early detection of sulfate scale is extremely necessary to prevent crystal deposition on the surface of the membrane layer from damaging membrane components. Barium sulfate and Strontium sulfate scales are difficult to remove because they are difficult to dissolve in almost all cleaning solutions, so special attention should be paid to prevent the formation of such scales.
Metal oxide/hydroxide pollution:
Typical metal oxide and metal hydroxide pollution is iron, zinc, manganese, copper, aluminum, etc. This scale may be caused by the corrosion products of device pipelines and containers (tanks/troughs), or metal ions, chlorine, ozone, potassium, Permanganate oxidized in the air, or by the use of iron or aluminum coagulant aids in the pretreatment filtration system.
Polymerized silica scale:
The scale of silicon gel layer is caused by the Supersaturation state of soluble silicon and polymer, and it is very difficult to remove. It should be noted that the pollution of this type of silicon is different from the pollution of silicone bodies. Silica gel contamination may be caused by association with metal hydroxide or organic matter. The removal of silicon scale is difficult, and traditional chemical cleaning methods can be used. If traditional methods cannot solve the problem of removing this scale, please contact the technical department of EFLOW company. Existing chemical cleaning agents, such as Ammonium bifluoride, have been successfully used in some projects, but the operation hazards of this method and damage to equipment must be considered when using, and protective measures must be taken.
Colloid contamination:
Colloids are inorganic substances suspended in water or particles of organic and inorganic mixtures that do not precipitate due to their own gravity. Colloids typically contain one or more of the following major components, such as iron, aluminum, silicon, sulfur, or organic matter.
Insoluble Natural Organic Matter Pollution (NOM):
Non soluble natural organic matter pollution (NOM) is usually caused by the decomposition of nutrients in surface water or deep well water. The chemical mechanism of organic pollution is very complex. The main organic components are either Humic acid or griseofulvic acid. Insoluble NOM adsorbed on the membrane surface can cause rapid pollution of RO membrane elements. Once absorption occurs, the pollution process of gradually forming gel or block will begin.
Microbial deposition:
Organic sediment is generated by bacteria, fungi, molds, and other pollutants, which are difficult to remove, especially when the water supply channel is completely blocked. Blocking the water supply channel will make it difficult for clean water to enter the membrane element evenly and fully. To suppress the further growth of this sediment, it is important not only to clean and maintain the RO system, but also to clean pre-treatment, pipelines, and ends. When using oxidizing sterilization for membrane components, please contact the technical support department of EFLOW company to use approved fungicides.
3. Selection and use of cleaning solution
Choosing suitable chemical cleaning agents and a reasonable cleaning plan involve many factors. Firstly, contact the service personnel of EFLOW company to determine the main pollutants and select appropriate chemical cleaning agents. Sometimes, for a specific pollutant or pollution situation, it is necessary to use specialized chemical cleaning agents from RO pharmaceutical manufacturers, and follow the product performance and usage instructions provided by the pharmaceutical supplier when applying. Sometimes, according to specific circumstances, a single membrane element that has been contaminated can be removed from the reverse osmosis device for testing and cleaning tests to determine the appropriate chemical agent and cleaning plan.
To achieve the best cleaning effect, sometimes different chemical cleaning agents are used for combined cleaning.
A typical procedure is to clean in a low pH range to remove mineral scale pollutants, followed by a high pH cleaning to remove organic matter. Some cleaning solutions are added with detergents to help remove serious biological and organic debris scale. At the same time, other agents such as EDTA Chelation can be used to help remove colloid, organic matter, microorganism and sulfate scale.
It should be carefully considered that if inappropriate chemical cleaning methods and agents are chosen, the pollution situation will worsen.
4. Selection and usage guidelines for chemical cleaning agents
The selected specialized chemical agent must first ensure that it has been recognized by EFLOW and meets the requirements for use in Hydergy's membrane components. The guidance/suggestions from pharmaceutical suppliers should not conflict with the cleaning parameters and limited types of chemical agents recommended in this technical service announcement by EFLOW company;
If the specified chemical agent is being used, it should be confirmed that it has been listed in this EFLOW company technical service announcement and meets the requirements of EFLOW company membrane components (consult EFLOW company);
Adopting a combination method to complete the cleaning work, including appropriate parameters such as cleaning pH, temperature, and contact time, will be beneficial for enhancing the cleaning effect;
Clean at the recommended optimal temperature to achieve the best cleaning efficiency and extend the lifespan of membrane components;
Cleaning with the minimum number of chemical contact times is beneficial for extending the membrane life;
Carefully adjusting the pH range from low to high can extend the service life of membrane components. The pH range is 2-12 (do not exceed);
The typical and most effective cleaning method is to clean from a low pH to a high pH solution. However, the cleaning of oil contaminated membrane components cannot start from a low pH value, as oil will solidify at low pH;
The flow direction of cleaning and flushing should be maintained in the same direction;
When cleaning multi-stage reverse osmosis devices, the most effective cleaning method is to clean them in stages, which can control the optimal cleaning flow rate and cleaning solution concentration to prevent pollutants from entering downstream membrane components from the previous stage;
Washing detergent with product water with higher pH can reduce the generation of foam;
If biological contamination has occurred in the system, it is necessary to consider adding a disinfectant chemical cleaning step after cleaning. Fungicides must be added immediately after cleaning, or a certain amount can be added continuously during operation (such as once a week). It must be confirmed that the fungicides used are compatible with the membrane components, do not pose any risks to human health, and can effectively control biological activity with low cost;
To ensure safety, when dissolving chemicals, it is important to slowly add the chemicals to sufficient water and stir at the same time;
From a safety perspective, acid cannot be mixed with caustic (corrosive) substances. Before using the next solution, thoroughly rinse the remaining chemical cleaning solution from the RO system.
5. Selection of cleaning solution
Table 2- The cleaning solution provided by the conventional cleaning solution formula is to add a certain weight (or volume) of chemicals to 100 gallons (379 liters) of clean water (RO product water or water without free chlorine). The solution is prepared in proportion to the amount of chemicals and water used. The solvent is RO product water or deionized water, free of free chlorine and hardness. Before entering the membrane components, it is required to thoroughly mix and evenly adjust the pH value according to the target value and stabilize the temperature according to the target temperature value. The conventional cleaning method is based on the operation of circulating a chemical cleaning solution for one hour and soaking an optional chemical agent for one hour.
Table 2 Conventional cleaning solution formula (based on 100 gallons, or 379 liters)
Cleaning solution
Main components
Dosage
PH value of cleaning solution
Maximum cleaning solution temperature
one
Citric acid (100% powder)
17.0 pounds (7.7 kilograms)
Adjust the pH to 3.0-4.0 with ammonia water