What are the common contaminants of RO membrane elements?
The nature of contaminants and their fouling rate depend on feed water conditions. Fouling develops slowly, and if not addressed early, it can significantly damage membrane performance in a relatively short period. Regular system performance checks are a good way to detect fouling. Different contaminants affect membrane performance to varying degrees.
- Suspended Solids: Suspended solids are commonly found in surface water and wastewater, with particle sizes greater than 1 µm (colloids may be smaller than 1 µm). They can settle in an unstirred solution (colloids remain suspended). After pre-treatment, the parameters should be: turbidity < 1 NTU, SDI (Silt Density Index) < 5 for 15 minutes.
- Colloidal Contaminants: Colloidal pollutants are widely present in surface water or wastewater and typically exist at the front end of the RO system. These particles are smaller than 1 µm and remain suspended in solution. They can be organic or inorganic compounds, such as silica, iron, aluminum, and sulfur (inorganic) or tannic acid, lignin, and humic substances (organic). After pre-treatment, parameters should be: turbidity < 1 NTU, SDI < 5 for 15 minutes.
- Organic Contaminants: Organic pollutants are commonly found in surface water or wastewater and are usually adsorbed onto the membrane surface. These natural humic organic substances come from decaying plants and often carry a charge. While there are no strict regulations for total organic carbon (TOC) in feed water, attention should be paid if the TOC concentration exceeds 2×10⁻⁵.
- Biological Contaminants: Biological pollutants, such as bacteria, biofilms, algae, and fungi, are typically found in surface water or wastewater. They initially form contaminants at the front end of the RO system, and then spread throughout the entire system. A warning level for biological activity is when bacterial count exceeds 10,000 CFU/mL, thus biological activity must be controlled.
How to identify and diagnose RO system faults?
When diagnosing an RO system fault, ask the following questions:
- Is the RO system operating abnormally?
- Has the RO system been idle for too long during normal shutdowns?
- Are the pre-treatment and chemical dosing systems operating normally?
- Is the system being used under the appropriate feed water temperature, TDS, or pH conditions?
- Is the water flow rate and recovery rate appropriate?
- Is the pressure drop (feed/concentrate) normal?
- Are all instruments and gauges calibrated?
- Have the permeate flow rate and water quality been standardized?
- Have water quality measurements been taken for each stage and each pressure vessel?
- Are there any damaged seals in the pressure vessels?
- Is the feed water security filter contaminated?
- Are the RO membrane elements fouled or damaged?
- Have the feed water, concentrate, and permeate samples been analyzed?
- Have water quality data been compared with the design calculations?
- Based on standardized permeate quality, flow, and pressure drop changes, can the possible contaminants be identified?
- Have the suspected contaminants and scale been cleaned?
- Have cleaning solutions been analyzed for contaminants, color, and pH changes?
- Have non-destructive tests been performed on the RO membrane elements, and cleaning plans determined?
- As a last resort, a membrane dissection and experimental analysis can be conducted to identify the contaminants.
How to locate faults in RO systems and membrane elements?
After “normalizing” the permeate flow and salt rejection, faults can be located. This can be done through online and offline investigations.
- Online Investigation:
If a high salt rejection is observed in a pressure vessel, measure the conductivity of the permeate from each membrane element to locate the origin of the issue. Use a plastic or stainless steel pipe to take samples of the permeate at different locations along the product water pipe, marking each sampling point to correspond with specific membrane elements. Start sampling at the furthest point in the product water pipe and measure the conductivity. Then, sample in stages, moving backwards, to observe the changes in conductivity.
As feed water flows through the pressure vessel, the concentration gradually increases, which will lead to a rise in permeate concentration. If there is a significant difference (around 10%) in conductivity between adjacent membrane elements, the issue is likely to be localized. A sudden jump in conductivity indicates mechanical leakage.
Additionally, comparing the ratio of divalent ions to monovalent ions in the permeate can help to predict leaks. - Offline Investigation:
Non-destructive offline research for spiral-wound membrane elements is primarily conducted using vacuum testing. If the vacuum drop exceeds 20 kPa per minute (i.e., 6 inHg), it indicates that the membrane element has severe leakage and is no longer usable.
If this test does not reveal the issue, destructive analysis (dissection) may be required. The membrane element can be examined for internal damage, and tests can be performed to analyze the contaminants.
4. Common RO Membrane Fouling Phenomena
The common fouling phenomena in reverse osmosis (RO) membranes include:
- Membrane Degradation:
Membrane degradation can occur due to:- Hydrolysis (e.g., for cellulose acetate membranes exposed to excessively low or high pH values).
- Oxidation (e.g., exposure to oxidizing agents such as chlorine (Cl₂), hydrogen peroxide (H₂O₂), or potassium permanganate (KMnO₄)).
- Mechanical Damage: Caused by factors like feed water backpressure, protruding membrane rolls, overheating, or abrasion from fine carbon or sand particles.
- Precipitate Deposition:
If antiscalant measures are not taken or are improperly applied, precipitate deposition can occur. Common types of precipitates include:- Calcium carbonate scaling (CaCO₃)
- Calcium, barium, and strontium sulfate scaling (CaSO₄, BaSO₄, SrSO₄)
- Silica scaling (SiO₂)
- Colloidal Deposition:
Colloidal fouling is typically caused by metal oxides (such as Fe, Zn, Al, Cr) and other various colloidal particles. - Organic Deposits:
Organic fouling can arise from:- Natural organic matter (humic substances and fulvic acid)
- Oils (e.g., leakage from pump seals or newly installed pipelines)
- Excessive use of antiscalants or iron precipitates
- Excess cationic polymers (originating from pre-treatment filters)
- Biological Fouling:
Microorganisms can form biofilms on the surface of the membrane, and bacteria may corrode cellulose acetate membranes. These microorganisms include algae, fungi, and other pathogens.
5. What are the Symptoms of RO Membrane Fouling?
When fouling occurs in an RO system, the following symptoms are typically observed:
- Increased Pressure Difference between the feed water and concentrate (brine).
- Variation in Feed Water Pressure: The pressure changes as fouling develops.
- Changes in Standardized Permeate Flow Rate: A decrease in the flow rate of permeate is a common sign of fouling.
- Changes in Standardized Salt Rejection Rate: The salt rejection rate may decrease, indicating membrane fouling.
6.RO Fault Diagnosis Checklist
| Fault | Possible Cause | Solution |
|---|---|---|
| Low Permeate Flow | – Fouling of membranes (e.g., scaling, biofouling, organic fouling) | – Perform chemical cleaning based on fouling type. – Optimize pretreatment processes. |
| – Insufficient feedwater pressure | – Increase feedwater pressure to the recommended range. | |
| – Membrane damage | – Inspect and replace damaged membranes. | |
| High Salt Rejection | – Damaged membranes causing internal leakage | – Replace membranes showing physical damage. |
| – Misalignment or poor sealing of O-rings | – Inspect and reseat O-rings properly. | |
| – High salt concentration in feedwater | – Improve pretreatment processes or use blending techniques. | |
| High Pressure Drop | – Blockages in membranes caused by fouling | – Conduct backwashing or chemical cleaning. |
| – Clogged pre-filters | – Replace pre-filters regularly. | |
| Low Recovery Rate | – Improper system design or settings | – Review system configuration and optimize recovery rate settings. |
| – Membrane fouling reducing permeability | – Perform thorough cleaning and improve pretreatment. | |
| High Permeate TDS | – Membrane aging or damage | – Replace aging or damaged membranes. |
| – Improper flushing leading to residual salts | – Perform proper flushing after cleaning or shutdown. | |
| System Shutdown | – High-pressure pump malfunction | – Inspect and repair the high-pressure pump. |
| – Low feedwater supply or pressure | – Check and maintain adequate feedwater supply. | |
| – Electrical or control system failure | – Inspect control system and troubleshoot electrical components. | |
| Scaling Issues | – Insufficient antiscalant dosing | – Increase antiscalant dosing to the recommended level. |
| – Poor feedwater pretreatment | – Improve pretreatment, such as softening or removing specific ions causing scaling. | |
| Biofouling Issues | – Inadequate biocide dosing or disinfection | – Apply biocides at the recommended dosage. – Improve cleaning schedules. |
| – Organic matter or microbial growth in feedwater | – Optimize pretreatment to remove organic loads. | |
| Chemical Attack | – Incorrect cleaning chemical concentration or type used | – Use appropriate cleaning chemicals according to the membrane manufacturer’s guidelines. |
| – pH or temperature outside membrane tolerance | – Maintain feedwater pH and temperature within operational limits. |
7. When is chemical cleaning needed?
The removal of contaminants can be achieved through chemical cleaning, physical flushing, or sometimes by altering operational conditions. As a general rule, cleaning should be performed under the following circumstances:
- When the permeate flow decreases to 85%–90% of the normal value under normal pressure.
- When feedwater pressure increases by 10%–15% (temperature-corrected) to maintain normal permeate flow.
- When permeate water quality decreases by 10%–15%, and salt passage increases by 10%–15%.
- When operating pressure increases by 10%–15%.
- When the differential pressure between RO stages increases significantly.
8. What are the commonly used cleaning solutions?
Specific cleaning solutions should be verified with the respective membrane element manufacturer before use
| Cleaning solution formulation | Ingredient | The amount added when the 397L(100gal) solution is configured | PH adjustment |
| 1 | Citric Acid Reverse osmosis Product Water(no free chlorine) Sodium Tripolyphosphate EDTA | 7.7kg 379L | Adjust PH to 3.0 with ammonia |
| 2 | Tetrasodium salt reverse osmosis Product Water(no free chlorine) | 7.7kg 3.18L 379L | Adjust PH to 10.0 wiht sulfuric acid |
| 3 | Sodium tripolyphosphate sodium dodecyl benzulfonate reverse osmosis Product Water | 7.7kg 0.97L 379L | Use sulfuric acid to adjust PH to 100 water for porters |
9.What are the common contaminants and their removal methods?
Calcium carbonate scale:
- Early-stage calcium carbonate scale can be removed by lowering the feedwater pH to 3.0–5.0 and operating for 1–2 hours.
- For longer-deposited calcium carbonate scale, use a citric acid cleaning solution for circulation or overnight soaking.
- Ensure the cleaning solution’s pH does not drop below 2.0 to avoid damage to the RO membrane, especially at higher temperatures. The maximum pH should not exceed 11.0. Ammonia can be used to increase pH, while sulfuric acid or hydrochloric acid can lower it.
Calcium sulfate scale:
- Cleaning solution 2 can remove calcium sulfate scale from the RO surface.
Metal oxide scale:
- Can be removed easily using methods for calcium carbonate scale, particularly for hydroxide deposits like ferric hydroxide.
Silica scale:
- Silica scale that is not co-deposited with metals or organic matter requires specialized cleaning methods. Contact the membrane or chemical supplier for details.
Organic deposits:
- Organic deposits (e.g., microbial slime or mold) can be removed using Cleaning Solution 3. To prevent regrowth, circulate and soak the system with a biocide approved by the membrane supplier. Prolonged soaking is usually required. For RO systems unused for more than 3 days, disinfection is recommended. Consult the membrane supplier to select suitable biocides.
Cleaning solutions:
- For inorganic fouling, use Cleaning Solution 1.
- For calcium sulfate and organic matter, use Cleaning Solution 2.
- For severe organic contamination, use Cleaning Solution 3.
- All cleaning solutions should be used at a maximum temperature of 40°C for 60 minutes. Cleaning solutions should be prepared by adding chemicals and cleaning water at the correct proportions. Use chlorine-free RO permeate water for preparation and mix thoroughly.
10.What are the characteristics of RO membrane fouling and the treatment methods?
These methods are effective for conventional fouling. For special contaminants, consult professional chemical suppliers.
| Pollutant | General characteristics | Treatment method |
| Calcium deposits(calcium carbonate and calcium phosphate,usually occurring in the second stage of the system) | The desalting rate decreased obviously and the system Pressure drop increased The water yield of the system decreased slightly | Solution 1 Cleaning system |
| Oxides(iron,copper,nickel,etc.) | The desalting rate decreased obviously The system pressure drop increase obviously The water yield of the system decreased significantly | Solution 1 Cleaning system |
| Various colloids(iron,organic and silica) | Desalting is slightly reduced The system pressure drop increase gradually The amount of water produced by the system gradually decrease | Solution 2 Cleaning system |
| Calcium sulfate | The desalting rate decreased obviously Slight or moderate increase in system pressure drop The water production of the system gradually decrease | Solution 2 Cleaning system Clean with solution 3 when the pollution is seriouus |
| Organic deposit | The desalting rate maybe reduce. The system pressure drop increase gradually The water production of the system gradually decreases | Solution 2 Cleaning system Clean with solution 3 when the pollution is seriouus |
| Bacterial contaminant | The desalting rate can be reduce The system pressure drop increase obviously The water yield of the system decreased significantly | Select the cleaning solution of the porter according to the possible type of contamination Choose 1 of the 3 solutions to clear |
11.What are the general procedures for RO membrane cleaning?
Cleaning involves circulating cleaning solutions at low pressure and high flow rates on the high-pressure side of the membrane. The membranes remain installed in the pressure vessels, and a dedicated cleaning system is required.
Steps:
- Use a pump to flush the pressure vessels with clean, chlorine-free RO permeate water from a cleaning tank (or other source) for several minutes.
- Prepare the cleaning solution in the cleaning tank using clean permeate water.
- Circulate the cleaning solution through the pressure vessels for 1 hour or the predetermined time. For:
- 8-inch or 8.5-inch vessels: Flow rate of 133–151 L/min.
- 6-inch vessels: Flow rate of 57–76 L/min.
- 4-inch vessels: Flow rate of 34–38 L/min.
- After cleaning, drain the cleaning tank and flush the system. Fill the cleaning tank with clean permeate water for the next step.
- Use a pump to flush the pressure vessels with clean, chlorine-free permeate water for several minutes.
- After flushing the RO system, operate the system with the product water discharge valve open until the permeate water is clean, foam-free, and free of cleaning agents (typically 15–30 minutes).
12.How to protect RO membranes for short-term storage?
Short-term storage applies to RO systems that are idle for 5–30 days. Membranes remain installed in the pressure vessels. The procedure is as follows:
- Flush the RO system with feedwater, ensuring all air is purged from the system.
- Fill the pressure vessels and related pipelines with water, then close the valves to prevent air from entering the system.
- Repeat the flushing process every 5 days as described above.
13.How to protect RO membranes for long-term storage?
Long-term storage applies to RO systems that are idle for over 30 days. Membranes remain installed in the pressure vessels. The procedure is as follows:
- Clean the membranes in the system.
- Prepare a biocide solution using RO permeate water and flush the RO system with the solution. Refer to the membrane supplier’s technical documents for selecting biocides and preparation methods.
- Fill the RO system with the biocide solution, close the valves, and ensure the system is fully filled.
- If the system temperature is below 27°C, repeat steps 2 and 3 every 30 days using a fresh biocide solution. If the temperature exceeds 27°C, replace the biocide solution every 15 days.
- Before restarting the RO system, flush it with low-pressure feedwater for 1 hour, followed by high-pressure feedwater flushing for 5–10 minutes. During flushing, keep the product water discharge valve fully open. Before resuming normal operation, check and ensure that the permeate water contains no biocide.
14.How to reduce faults and cleaning frequency in RO systems?
To minimize faults and reduce cleaning frequency, the following measures should be implemented:
- Design the RO system based on a comprehensive water quality analysis.
- Determine the SDI value of the RO feedwater before system design.
- Make appropriate design adjustments if feedwater quality changes.
- Ensure sufficient pretreatment.
- Select appropriate membrane elements.
- Choose conservative water flux rates.
- Select a reasonable water recovery rate.
- Design sufficient crossflow and brine flow rates.
- Standardize operational data.