I. Pollutants of Reverse Osmosis Membrane Elements

After a period of normal operation, reverse osmosis membrane elements will be polluted by suspended or insoluble substances that may exist in the feed water. The most common of these pollutants are calcium carbonate scale, calcium sulfate scale, metal oxide scale, silicon deposits, and organic or biological deposits.

The nature and speed of pollution are related to the feed water conditions. Pollution develops slowly. If measures are not taken early, pollution will damage the performance of the membrane elements in a relatively short time.

Regularly checking the overall system performance is a good way to confirm that membrane element pollution has occurred. Different pollutants will cause different degrees of damage to the membrane element performance. Table 1 lists the effects of common pollutants on membrane performance.

II. Removal of Pollutants

Pollutants can be removed by chemical cleaning and physical flushing. Sometimes, it can also be achieved by changing operating conditions. As a general principle, cleaning should be carried out when any of the following situations occur.

1. Under normal pressure, the product water flow rate drops to 10%~15% of the normal value.

2. In order to maintain the normal product water flow rate, the feed water pressure after temperature correction has increased by 10%~15%.

3. Product water quality decreases by 10%~15%. Salt passage rate increases by 10%~15%.

4. Operating pressure increases by 10%~15%.

5. The pressure difference between the RO sections increases significantly (there may be no instrument to monitor this sign).

 

III. Common Pollutants and Their Removal Methods:

1. Calcium carbonate scale

When the antiscalant addition system fails or the acid addition system fails, resulting in an increase in the feed water PH, calcium carbonate may precipitate. The occurrence of calcium carbonate scale precipitation should be detected as early as possible to prevent the growing crystals from damaging the membrane surface. If calcium carbonate scale is detected early, it can be removed by lowering the feed water PH to 3.0~5.0 and operating for 1~2 hours. For calcium carbonate scale that has precipitated for a longer time, a citric acid cleaning solution should be used for circulating cleaning or overnight soaking.

Note: Ensure that the PH of any cleaning solution does not fall below 2.0, otherwise it may damage the RO membrane elements, especially at higher temperatures. The highest PH should not exceed 11.0. Use ammonia water to increase PH, and use sulfuric acid or hydrochloric acid to lower PH.

2. Calcium sulfate scale

Cleaning solution 2 (see surface treatment) is the best way to remove calcium sulfate scale from the surface of the reverse osmosis membrane.

3. Metal oxide scale

The methods described above for removing calcium carbonate scale can be easily used to remove precipitated hydroxides (e.g., iron hydroxide).

 

4. Silicon scale

For silicon scales that are not coexisting with metal compounds or organic matter, they can generally only be removed by special cleaning methods.

5. Organic deposits

Organic deposits (e.g., microbial sludge or mildew) can be removed using cleaning solution 3. To prevent re-growth, a bactericidal solution can be circulated and soaked in the system. Generally, it requires a longer soaking time to be effective. If the reverse osmosis device is shut down for three days, disinfection treatment is recommended.

6. Cleaning solution

It is recommended to use the cleaning solutions listed in Table 2 when cleaning reverse osmosis membrane elements. It is very important to perform chemical analysis of the pollutants before cleaning. Detailed analysis of the analysis results can ensure the selection of the best cleaning agent and cleaning method. The cleaning method and cleaning effect obtained should be recorded for each cleaning to provide a basis for finding the best cleaning method under specific feed water conditions.

For inorganic pollutants, cleaning solution 1 is recommended. For calcium sulfate and organic matter, cleaning solution 2 is recommended. For severe organic pollution, cleaning solution 3 is recommended. All cleaning can be performed at a maximum temperature of 104°F (40°C) for 60 minutes. The required amount of supplies is calculated per 100 gallons (379 liters). When preparing the cleaning solution, add the chemicals and cleaning water proportionally. Use reverse osmosis product water without free chlorine to prepare the solution and mix thoroughly.

During cleaning, circulate the cleaning solution at low pressure and high flow rate on the high-pressure side of the membrane. At this time, the membrane element is still installed in the pressure vessel and a special cleaning device is required to complete this work.

IV. General Steps for Cleaning Reverse Osmosis Membrane Elements:

1. Use a pump to pump clean, free chlorine-free reverse osmosis product water from the cleaning tank (or corresponding water source) into the pressure vessel and drain for a few minutes.

2. Prepare the cleaning solution in the cleaning tank with clean product water.

3. Circulate the cleaning solution in the pressure vessel for 1 hour or a preset time. For 8-inch or 8.5-inch pressure vessels, the flow rate is 35~40 gallons/minute (133~151 liters/minute). For 6-inch pressure vessels, the flow rate is 15~20 gallons/minute (57~76 liters/minute). For 4-inch pressure vessels, the flow rate is 9~10 gallons/minute (34~38 liters/minute).

4. After cleaning is complete, drain the cleaning tank and rinse it. Then fill the cleaning tank with clean product water for the next rinsing step.

5. Use a pump to transfer clean product water free of free chlorine from the cleaning tank (or corresponding water source) into the pressure vessel and drain for several minutes.

6. After rinsing the reverse osmosis system, operate the reverse osmosis system with the product water drain valve open until the product water is clean and free of foam or cleaning agent (usually 15-30 minutes).

V. Reverse Osmosis Membrane Fouling Characteristics and Treatment Methods

Note: The cause of contamination must be identified and the source of contamination eliminated.

Table 2. Commonly Used Cleaning Solutions

 

This is just a formula. According to the on-site analysis, it can be matched according to the specific situation.

 

Several Commonly Used Formulas for Reverse Osmosis Membrane Cleaning

Cleaning Formula 1

1%-2% citric acid solution or 0.4% HCl solution, suitable for iron contamination and carbonate crystal blockage;

Cleaning Formula 2

0.2% NaClO + 0.1% NaOH solution, suitable for cleaning ultrafiltration membrane components contaminated by organic matter and active organisms;

Cleaning Formula 3

0.3% H2O2 + 0.3% NaOH solution, suitable for cleaning ultrafiltration membrane components contaminated by glutamic acid fermentation broth;

Cleaning Formula 4

1% formaldehyde solution, suitable for bacterial contamination of ultrafiltration;

Cleaning Formula 5

HNO3: 0.5% aqueous solution, suitable for lead phosphate blockage of ultrafiltration during electrophoretic paint treatment (this cleaning must be performed after other conventional chemical cleaning);

Cleaning Formula 6

Cleaning agent formula: 20% Na2CO3, 7% Na3PO4, 3% NaOH, 0.5% EDTA, mainly used for membrane fouling caused by colloidal pollutants;

Cleaning Formula 7

Cleaning agent formula: 9% sodium dodecylbenzenesulfonate, 9% surfactant, 0.4% NaOH, 0.15 anhydrous sodium carbonate, 11% sodium phosphate, 10% sodium silicate. Pay attention to pH control during cleaning. Some membranes are not suitable for cleaning with high-pH cleaning solutions. Choose carefully. Mainly used for cleaning membrane fouling caused by oily wastewater;

Cleaning Formula 8

Cleaning agent formula: 3% H3PO4, 0.5% disodium ethylenediaminetetraacetate, 0.5% LBOW special cleaning agent, mainly used for cleaning pollution caused by protein and oil pollutants.

Cleaning Formula 9

Cleaning agent formula: 20% H2SO4, mainly used for pollution caused by silica scale crystallization in ultrafiltration systems.

RO membrane elements are the most important part of the reverse osmosis system. The quality of daily maintenance directly affects the quality of the system's effluent. Here, the cleaning methods of reverse osmosis membranes are summarized, and the system describes the pollutants that may appear in the operation of reverse osmosis membranes and the corresponding cleaning methods.

 

Methods/Steps

1. Bacterial Contamination

General characteristics: Desalination rate may decrease, system pressure drop increases significantly, system water production decreases significantly

Cleaning method: pH 10, 2% sodium tripolyphosphate solution, 0.8% EDTA tetrasodium (replace with 0.25% Na-DDBS in severe cases), temperature 40°C; 0.1% NaOH and 0.03% SDS, pH=11.5.

2. Calcium Sulfate Contamination

General characteristics: Desalination rate decreases significantly, system pressure drop slightly or moderately increases, system water production slightly decreases. Cleaning method: pH 10, 2% sodium tripolyphosphate solution, 0.8% EDTA tetrasodium (replace with 0.25% Na-DDBS in severe cases), temperature 40°C; sometimes an aqueous NaOH solution with pH less than 10 can also be used for cleaning.

3. Organic Matter Precipitation

General characteristics: Desalination rate may decrease, system pressure drop gradually increases, system water production gradually decreases

Cleaning method: pH 10, 2% sodium tripolyphosphate solution, 0.8% EDTA tetrasodium (replace with 0.25% Na-DDBS in severe cases), temperature 40°C.

4. Oxide/Hydroxide (Iron, Nickel, Copper) Contamination

General characteristics: Desalination rate decreases significantly, system pressure drop increases significantly, system water production decreases significantly

Cleaning method: Adjust pH to 4 with ammonia water, 2% citric acid solution, temperature 40°C, sometimes a 0.5% hydrochloric acid aqueous solution with pH 2-3 can also be used for cleaning.

5. Inorganic Salt Precipitate Contamination

General characteristics: Desalination rate decreases significantly, system pressure drop increases, system water production slightly decreases

Cleaning method: 2% citric acid solution, adjust pH to 4 with ammonia water, temperature 40°C, or a 0.5% hydrochloric acid aqueous solution with pH 2-3 can also be used for cleaning.

6. Various Colloids (Iron, Organic Matter and Silica Colloids) Contamination

General characteristics: Desalination rate slightly decreases, system pressure drop slightly increases, system water production gradually decreases.

Cleaning method: Adjust pH to 10 with sulfuric acid, 0.2% sodium tripolyphosphate (STTP) solution, temperature 40°C; sometimes an aqueous NaOH solution with pH less than 10 can also be used for cleaning.

RO Reverse Osmosis Membrane Cleaning Plan

The RO reverse osmosis membrane is the core of a household pure water machine and represents the most advanced water filtration technology in the world. Currently, domestic pure water machine manufacturers primarily rely on imports, as domestic technology cannot yet produce them. Domestic RO reverse osmosis membranes only purchase pre-rolled membrane sheets from overseas. Generally, the lifespan of an RO membrane is 2-3 years. In northern regions with relatively high TDS values, this lifespan is affected. RO membranes are expensive, making replacement costly and inconvenient. Luo Gong, a senior expert from the technology department of Shenzhen Qinuo Water Purifier, a leading manufacturer of China's top ten household pure water machine brands, provides an analysis of RO cleaning solutions:

1. Cleaning Introduction: The pretreatment process design of the reverse osmosis system in a pure water machine removes as many pollutants from the source water as possible, but not all. Therefore, reverse osmosis system contamination is common. Fortunately, most contaminants can be removed through regular chemical cleaning. As long as the pretreatment is working normally and there are no uncontrollable factors, such as changes in source water quality or unavoidable microbial contamination, the cleaning frequency can be kept as low as possible. Sometimes, operational errors can cause membrane fouling, such as excessively high recovery rates or malfunctions in the chemical dosing system. When the membrane is fouled, the water production will decrease, the desalination rate will decrease, and the pressure difference between the inlet and concentrate will increase. Membrane cleaning can use various acidic and alkaline agents (under pH 2-pH 12 conditions), with the cleaning solution temperature up to 45℃. Many contaminants, especially those of a sticky nature, accumulate, compress, and thicken over time, increasing cleaning difficulty and significantly extending cleaning time. If the following symptoms are present, clean the membrane element immediately: a 10-15% reduction in standardized water production, a 0.5% decrease in desalination rate, and a 15% increase in pressure difference ΔP (compared to the initial 24-48 hour pressure difference).

2. Required Cleaning Chemicals and Equipment 1.

Cleaning tank: Chemical cleaning tanks are usually made of PE or FRP material and can withstand a pH range of 1-12.

3. Cleaning heating device: The cleaning temperature should be as high as possible, such as 20-40℃. Chemical cleaning should not be carried out below 10℃ because the cleaning efficiency is very low; therefore, a heating device is required during cleaning.

3. Cleaning agents: Cleaning uses strong acids and alkalis (e.g., phosphoric acid, citric acid, sodium hydroxide, etc.). Therefore, the selection of chemicals is crucial. Sometimes, improper selection of agents can cause membrane fouling or even damage.

4. Cleaning pump: The cleaning pump must be acid and alkali resistant, with a flow rate of 12-15 m3/h and a head of 20-30 meters.

5. During cleaning, appropriate pipes, valves, and pressure gauges must be installed. Connect the pump outlet to the membrane inlet, the concentrate pipe to the chemical tank, and install a ball valve before connecting to the source water tank. Connect the chemical tank and the pump.

4. Cleaning Procedure

1. The cleaning procedure is a circulation cleaning process, with the concentrate outlet pipe connected to the cleaning chemical tank.

5. Fill the cleaning tank with pure water produced by the pure water machine. The cleaning solution must meet the volume requirements of the pressure vessel and piping. Add 1%-2% of the cleaning agent to the cleaning tank. The pH value for acidic solutions should be greater than 1, and the pH value for alkaline solutions should not exceed 13.

4. It is best to heat the cleaning solution during cleaning to improve cleaning efficiency.

6. Inject the heated solution into the RO membrane of the pure water machine using a pump, with a flow rate of 10-15 m3/h and a head of 20-30 meters. Ensure these parameters are met and do not exceed them. During the initial cleaning, do not recirculate the first tank of solution back to the tank; instead, directly discharge it, as it contains a large amount of contaminants that will directly affect the effectiveness of the solution and lead to poor cleaning. After the initial discharge of contaminants, the solution can be recirculated back to the original tank for continuous cleaning. During cleaning, fully open the concentrate discharge valve to minimize pressure and close the product water valve to prevent the solution from flowing into the pure water tank.

6. A single cleaning procedure generally involves 1-2 hours of circulation cleaning, followed by a 30-minute soak (sometimes a 1-hour or longer soak can be very effective), and then another 1-2 hours of circulation cleaning. The total cleaning time should be at least 4 hours.

7. After cleaning, rinse the membrane with pure water produced by the reverse osmosis system or clean water (filtered with SDI <3, free of residual chlorine and microorganisms, and conductivity <10000 μs/cm) for at least half an hour.

8. After cleaning, restart the household pure water machine system. However, the initial water may need to be discharged until the conductivity and pH of the product water return to normal. Note that if the machine is shut down for more than 24 hours after cleaning, soak it in a protective solution (1% sodium hydrosulfite solution).

9. When the system is running again, add an antiscalant. Because the conductivity of the pretreatment system is very high, exceeding 1000 μs/cm, this water quality easily causes membrane fouling, leading to increased conductivity, decreased water production, and increased pressure.