1. Introduction: reverse osmosis membrane cleaning
Reverse osmosis membrane cleaning is the first step to recover declined RO system performance. Eventually, in every reverse osmosis plant, membrane performance will decline due to one or more different fouling scenarios.
This results in a higher operating pressure, decreased permeate flow, and an increased salt passage. At this point, it becomes necessary to clean the RO membranes, restore operating performance and bring all operational parameters closer to design values.
Apart from fouling and low membrane performance situations, RO membrane cleaning is recommended just before any medium or long term shutdown or as part of the scheduled maintenance protocol, typically every 3 to 6 month.
Since RO membranes represent a significant proportion of the whole reverse osmosis investment, protecting the membranes from physical and chemical degradation should be a maintenance priority and the means to facilitate the cleaning process should be part of the initial system design.
It is important to clean the membranes before they are heavily fouled. Once membranes are very deteriorated, the effectiveness of the chemicals will be compromised as penetration into the foulant will be limited.
2. How to Reduce RO Membrane Cleaning Frequency
The best option to minimise membrane fouling and extend the periods between reverse osmosis membrane cleaning is improving the RO pre-treatment:
- Sediment filters or media filters will reduce the particles reaching the RO system. When colloids are present in the feedwater, an upstream injection of coagulant will facilitate the removal of colloidal particles.
- Activated Carbon Filtration can deliver good results when organics are present in the RO feed water, however care must be taken to prevent the growth of bacteria in this media.
- Water softeners are effective in the presence of hard water preventing the formation of Calcium Carbonate scales in the surface of the reverse osmosis membranes.
- Acid dosing is an alternative to water softening that help increasing the solubility of Calcium salts, hence reducing the risk of Calcium scaling in the RO membranes.
However, there are other easily automated tasks that can help reducing membrane fouling and scaling, hence reducing the reverse osmosis membrane cleaning frequency:
- Flushing particles from the membrane surface by opening the concentrate valve (auto-flushing)
- Recirculating permeate with little or no production to dissolve forming scales.
3. Reverse Osmosis Membrane Cleaning Methods
Once fouling or scaling is detectable and consequences are evident, RO membrane cleaning is the only available option to recover the system capacity.
When membrane degradation is high or took place over a short time period the best option is to extract one of the membranes and identify the main cause. Typical membrane productivity issues are related with:
• Calcium Carbonate or Sulfate scale
• Iron, Manganese or Aluminum fouling
• Silica scale
• Colloidal fouling
• Bacteria growth
When membrane degradation can’t be attributed to any particular reason but the present of a number of them, chemicals can be used in any of the sequences described below to achieve the optimum cleaning.
3.1 PH Chemical Cleaning
Depending on the type problem, RO membranes should be cleaned either at low PH, high pH, or a combination of both. When a combination of acid and caustic solutions is chosen, it must be performed separately, never mixing acids with caustics chemicals.
|Low (Acid)||Hydrochloric Acid (20%)|
|High (Caustic)||Sodium Hydroxide (10%)|
It must be noted that choosing the wrong cleaning chemical or applying an incorrect cleaning sequence can worsen the problem. It is always recommended to check the membrane specs and/or membrane manufacturer before selecting any cleaning method.
In general and for polyamide membranes, the typical reverse osmosis membrane cleaning will include one of the following treatments:
- Carbonate and Sulfate scale as well as inorganic colloidal fouling will be cleaned with low pH chemicals
- Organics and biofouling will be cleaned with high pH chemicals
- Bacteria will be removed either at high PH or by means of specific biocides
- In the presence of organics and mineral scale, it is common to start with a low pH chemical that cleans the mineral scale and continue with a high pH chemical to remove organics.
Although the allowed PH range for most composite polyamide reverse osmosis membranes is pH 10 to PH 12, we always recommend a conservative approach not exceeding the PH 4 to PH 10 range for chemical cleaning. In case that conservative range doesn’t deliver the expected results, it can be widen with care.
3.2 Cleaning Steps
The basic steps for any reverse osmosis membrane cleaning are:
- Open the concentrate valve
- Flush the system, ideally with permeate water
- Connect the membrane feed inlet to the recirculation pump
- Connect permeate and concentrate outlets to the chemical tank (a small proportion of cleaning chemical might permeate the membrane)
- Recirculate the cleaning solution for one hour while controlling the PH
- Rinse the membrane, ideally with permeate water, removing all traces of chemical
- Repeat the cycle when more than one cleaning chemical needs to be used
4. Off-site vs. On-site membrane cleaning in reverse osmosis plants
On-site RO membrane cleaning, also known as Clean-in-Place (CIP) is the most common cleaning method for commercial and industrial reverse osmosis systems. On-site cleaning is is performed in-situ, while and membranes stay inside the pressure vessels, and is faster and cheaper than off-site cleaning.
Off-site cleaning involves removing the membranes from the pressure vessels and shipping them to a different facility. Off-site RO membrane cleaning is usually more efficient, is performed by specifically trained technicians and results can be better documented. However the cost will be significantly higher.
All Watercore commercial and industrial reverse osmosis desalination plants are supplied clean-in-place ready, including all hydraulic connections required to facilitate the on-site cleaning of the system membranes.
Although backwashing is a common cleaning procedure in media filtration it must NEVER be used in reverse osmosis membrane cleaning.
Cleaning of the RO membranes by applying pressurised water from the permeate side, will lead to irreversible membrane damage.
6. Conclusions: importance of reverse osmosis membrane cleaning
Reverse osmosis membrane cleaning is an important procedure that sooner or later will be required by any RO system and should be part of the RO scheduled maintenance protocol.
Reverse osmosis membrane cleaning requires, in the first place, an identification of the foulant/scalant to decide which PH treatment is more appropriate. Then the cleaning station will be connected to the membranes and operated for a minimum of one hour.
Watercore can help:
- Identifying the reason for the low-performance of the RO system
- Recommending the most effective cleaning chemical for the RO membranes
- Designing and implementing the RO membrane cleaning protocol
- Supplying the chemicals and the cleaning stations