How to Clean Reverse Osmosis Membrane - By Wes Byrne
The vast majority of RO membrane systems suffer from some degree of fouling, usually as related to the inability to completely filter out suspended particles in their inlet water. This will result in the need for greater pressure in producing the same rate of purified water, and it may also cause the RO salt rejection percentage to decline. If the fouling solids are not effectively removed from the membrane in a chemical cleaning process, the losses in membrane performance may accumulate in magnitude, eventually resulting in the need to replace the membrane.
Apply the RO Performance Variables
There are numerous misconceptions about RO cleaning within the RO industry. One of them is that there is only one right way to clean, but it is more accurate to say that the only right way is one that results in the full restoration of original membrane performance. Different types of RO fouling and different RO system designs often result in the need for different cleaning methods to fully remove the fouling solids. Cleaning effectiveness should be quantifiably evaluated using three RO performance variables; the normalized permeate flow rate, the normalized feed-to-concentrate pressure differential, and the salt rejection. Normalization involves standardizing the variable for the effect of any changes in operating variables. (For more information, see the Purewatercloud Volume III - RO Monitoring training course from the Encyclopedia of Water Treatment.)
Another misconception is that frequent cleaning will damage the membrane elements and shorten their life expectancy. Permanent membrane damage only occurs when aggressive chemicals are used during the cleanings, such as cleaning solutions of an extreme pH, less than 1.5 or greater than 12.0, especially if the solution aggressiveness is enhanced with a solution temperature greater than 95 F. Many membrane manufacturers will now allow these extreme pH solutions, but that does not mean that it is safe for the membrane. Extreme pH solutions should only be used if required for restoring original performance.
If frequent cleaning means that the extent of fouling is not allowed to become severe, such as by keeping the normalized permeate flow rate from declining by more than 15% or keeping the normalized pressure differential from increasing by more than 25%, then aggressive cleaning chemicals will usually not be needed to restore original performance. These frequent cleanings at less than aggressive pH values will not impact the membrane life. Rather, it will extend it if they are able to maintain the original membrane performance.
Clean Individual Vessel Stages Separately at Low Pressure
RO systems of a substantial size usually have staged sets of membrane vessels in order to obtain good water agitation without excessive pressure losses, as well as to minimize the size of its high pressure pump. After the first stage of membrane vessels remove permeate water from the inlet flow, fewer membrane vessels are used in downstream stages in order to maintain a similar level of water agitation within those vessels. For example, the membrane vessels might be staged as 2 parallel vessels whose concentrate is plumbed into the inlet of a third vessel, or it might be staged as 4 parallel vessels plumbed to 3 vessels plumbed to 2 vessels, the best vessel array being dependent on what results in the optimum system performance. Unfortunately, this staging increases the difficulty of effectively cleaning the RO system.
One RO equipment manufacturing company handled this challenge by cleaning the RO system at high pressures, often greater than 200 psi for brackish water applications. Cleaning at high pressure results in the production of permeate water during the cleaning circulation, which better balances the flow hydraulics between the RO vessel stages. This type of cleaning design reduces system cost and makes cleaning easier, but it is not particularly effective. With the permeation of water through the membrane, fouling solids are being compressed against the membrane surface by the water moving to and then permeating through the membrane. This reduces the ability of the cleaning solution to lift the fouling solids off the membrane surface and the cleaning often fails, especially in the inlet end of the membrane vessel staging where the pressure and water permeation is the highest. Cleaning effectiveness was somewhat improved by including soaking steps to allow the fouling solids to decompress by simply letting the unpressurized RO sit in the cleaning solution for a period of time.
The best cleaning action occurs when the permeate flow rate can be minimized while also obtaining a high degree of agitation by passing the solution at a high crossflow velocity across the membrane surface. This is achieved within an RO system by providing only enough pressure to achieve that high crossflow rate, which can only be accomplished by cleaning each membrane stage separately.
Some staged RO systems are designed to be cleaned all-at-once using low pressure. This then results in having to limit cleaning flow rate based on the number of membrane vessels in the last stage of the system in order to prevent damage to the membranes, such as crushing them or causing the membrane polymer layer to delaminate off its backing material. This flow limitation then results in the upstream vessels not receiving ideal cleaning flow and agitation as might be needed to remove stable fouling solids.
Optimum cleaning hydraulics will occur when the crossflow can be maximized while the water permeation through the membrane is minimized. This requires that the membrane stages be cleaned individually. But from a time perspective, it still makes sense to initially fill the entire RO system with cleaning solution. Fouling solids can start to hydrate within the soaking stages while the solution is being circulated through a membrane stage. Each stage may still need its own circulation step to fully remove its fouling solids but its time requirement may be reduced.
Best Cleaning Flow Rate When Cleaning Individual RO Stages:
- 10 gpm for 4-inch diameter membrane if the stage pressure differential does not exceed 60 psi
- 40 gpm for 8-inch membrane if the stage pressure differential does not exceed 60 psi
- or the maximum allowed by the membrane manufacturer if the pressure differential is < 60 psi
Best Cleaning Pressure: The lowest possible and never exceed 60 psi (as based on no return pressure)
Monitor the Cleaning Flow Rate(s) and Pressure
When the fouling has caused a restriction in the flow of water through the membrane elements within a stage, it will cause its normalized feed-to-concentrate pressure differential to increase. This means that there will be a measurable increase in the inlet pressure required to push a specific cleaning flow rate through that membrane stage. Its reduction can be monitored during the cleaning while maintaining a constant cleaning flow rate. When it has returned to normal/clean value, it serves as an indication that whatever was blocking the flow channels has been fully removed.
Fouling solids that lay out on the membrane surface may only affect the normalized permeate flow rate and possibly the RO salt percent rejection. It is not possible to monitor the membrane water permeation during cleaning, which means the extent of cleaning effectiveness may not be known until the RO is rinsed and re-started. The cleaning time requirement is dependent on the severity of the fouling, but the specific time needed is often just a guess. If the fouling is not allowed to accumulate as based on the previously recommended maximum changes in the performance variables, each cleaning step might be successful in as little as 30 to 60 minutes.
Increasing the cleaning solution temperature will increase the rate at which the cleaning solution dissolves/suspends fouling solids. When relying on an undersized electric immersion heater, the time required to heat the solution may not be worth it. When using extremely low or high pH solutions, high temperature (such as greater than 95 F) may cause the solution to become too aggressive toward the membrane polymer. Be sure to follow the guidelines of the membrane manufacturer.
The system pumps will tend to add heat to the system during circulation. This can result in damage if the solution temperature is allowed to rise excessively. This can easily occur in an environment where the ambient temperature is extremely warm.
Start the Cleaning with an Alkaline or an Acidic Cleaning Solution?
Some companies promote that every cleaning should begin with an alkaline cleaning and complete with an acidic cleaning. This approach has some advantages, although it may not be optimal in every fouling situation. Because most fouling of the standard polyamide thin-film membrane includes some percentage of biological fouling solids, as related to the removal of chlorine in its inlet water, beginning with an alkaline cleaning will usually remove a high percentage of the biological solids while beginning with a strongly acidic cleaning can cause the biological solids to stabilize via increased crosslinking of its sugary solids. Also, finishing with a strongly acidic cleaning step will cause the RO membrane to essentially tighten where its rejection will temporarily improve, although at the expense of a slight reduction in water permeation.
In some cases where the fouling is primarily biological or includes only silt/clay within the fouling solids, an alkaline cleaning step alone may be sufficient to fully restore performance, more likely when the fouling is not too severe. Calcium carbonate scale may be removed with just an acidic cleaning solution. If the solution pH is not allowed to be much lower than 3.5, the potential for stabilizing biological solids is minimal.
Some RO units foul mostly with iron solids, in which case a strong chemical reducing agent may be fully effective at chemically reducing the iron from its oxidized/ferric state to its soluble ferrous state. This solution will be able to remove a greater amount of iron solids than a standard acidic cleaning solution.
Some stable iron bacterial fouling solids are best removed by first pulling out the iron with a strong chemical reducing agent. Following this step (after rinsing) with a standard alkaline cleaning solution then results in this cleaning step being far more effective in removing the remaining biological materials.
Generic or Formulated Cleaning Products?
One membrane manufacturer representative was once fully convinced that RO units could be fully cleaned using only a diluted hydrochloric acid solution step and a diluted sodium hydroxide solution step, and this belief was promoted to the RO industry. But while the cost of these cleanings were dramatically reduced as opposed to using packaged cleaning formulations, they were often not fully effective at restoring RO performance. Cleaning RO membrane is not that chemically different than cleaning cars, dishes, or laundry, such that the inclusion of surfactants (and other cleaning agents) within the solution will improve the wetting out the fouling solids and thus the effectiveness of the cleaning. However with minimal fouling, it may be found that the diluted acid/alkaline chemical is effective.
Mixing and pH
Purified water should be used to dilute cleaning chemicals. The presence of alkalinity in the dilution water will buffer the solution pH, and alkaline cleaners may cause hardness in the water to come out of solution. It is critical that the solution be well mixed, especially when using a solution with an aggressive pH. Never add more chemical to a cleaning tank while the solution is circulating through the RO as the cleaner is likely of much higher density than the solution. Before it has a chance to fully mix, it will quickly drop to the bottom of the tank and get sucked into the cleaning pump intake.
pH meters are often inaccurate at high pH measurement. Be sure to verify pH readings when using aggressive cleaning pH values.
Special Cleaning Procedures and Rinsing
Another questionable concept was that cleaning flow rates should be gradually increased over a several hour time span and that rinsing should be carefully performed in a similar manner with temperature adjusted water. These procedures would typically double the cleaning time as compared to a more efficient process that does not include these steps. However, the vast majority of RO systems are successfully cleaned without these extra procedures. It might be argued that if an RO system is so badly fouled that these extra steps are necessary, then it is highly unlikely that the cleaning is going to be fully effective, regardless of whether these extra steps are followed.
Rinsing with purified water does reduce the potential for solids to come back out of solution/suspension and re-foul the RO in some other location, but again this rarely occurs. A reasonable level of security against this possibility would be gained by displacing the spent cleaning solution held up within the RO with purified water and then rinsing it with low pressure raw water for something like 10 minutes. The raw water will usually rinse up the system faster because its natural contaminants will better pull up surfactant from the system surfaces.
Surfactants/detergents have an affinity for attaching to the RO system surfaces, so a drawback of their use in a cleaning solution is the extra rinsing time it will take to get it completely out of the permeate side of the RO system. Certain applications can be extremely sensitive to even extremely low concentrations of a surfactant, such as semiconductor manufacturing. With its permeate stream diverted to drain, the RO may need to be rinsed for a number of hours before the surfactant is completely removed. Stagnant regions, such as the plugged permeate end of each vessel, might be better rinsed by removing those plugs for a period of time.
The residual presence of surfactant may not make a noticeable effect on permeate conductivity, so some other means of verifying its absence may be required. Some means of sensitively measuring the concentration of organics in the water may be needed. Some facilities without one of these analytical tools may simply divert the permeate stream for 24 hours before allowing the RO to go back into normal service.
What If the Cleaning Does Not Restore Original Performance?
If the RO system has rinsed up and the normalized permeate flow rate is still lower or the normalized pressure differential is still higher that they were when the membrane is new, it is certainly a concern that not all of the fouling solids were removed. If the last cleaning step was performed with a strongly acidic solution, the RO membrane may be temporarily restricted in its permeate flow rate, which means the normalized permeate flow rate may be reduced by up to 15% temporarily. It may take a day or two to fully recover from this effect.
If fouling solids were left behind on the membrane surface, suspended solids in the incoming water will often have a greater affinity for attachment to the existing fouling solids as opposed to the clean membrane, especially when using low-fouling membrane. Thus, the fouling rate may increase.
In the case of an elevated pressure differential, the flow channels are partially blocked. These blockages may not be uniform between parallel membrane vessels, which can then cause an imbalance in crossflow that will accelerate fouling in the membrane vessel(s) with restricted channels.
In these cases, a more aggressive cleaning should be considered, such as using a more extreme cleaning pH or an entirely different cleaning solution. It is possible that the cleaning simply needed more circulation time to be fully effective.
Offsite Membrane Cleaning
Purewatercloud offers offsite membrane cleaning when it may not be possible to perform a successful cleaning onsite, possibly due to water discharge issues or limited downtime. We can assist with determing how best to clean the membranes and can identify membranes that should be replaced.