Cleaning and maintenance can be a vital part of having an RO water treatment system in your facility. Membranes can become fouled by several contaminants including algae, viruses, fungi and bacteria. If left untreated, these contaminants can affect the entire system, forcing a complete shut-down that can significantly impact your operation.
It's worth pointing out that whilst there many RO’s that need cleaning and cleaning regularly, there are many RO units that operate without ever being cleaned. It’s all down to the quality of the feed water and the quality of the pre treatment - for more information go to http://www.wychwood-water.com/process/reverse-osmosis/.
When It’s Time To Clean RO System Membranes
There are many ways to tell if it’s time for RO membrane cleaning. Most manufacturers recommend that your system’s membranes undergo cleaning when scheduled maintenance is set to take place or just before a long-term system shut-down.
Symptoms of fouled membranes include a 10-15% decrease in normalised permeate quality and permeate flow, and an observed increase in normalised pressure of 10-15% between concentrate and feed headers.
Why It’s Important To Clean Membranes Properly
If RO membrane cleaning doesn’t take place on a regular basis, the system can become contaminated quickly. Bacteria, which occurs naturally in water, is always a threat to RO systems. However, it can also be present as the result of inadequate membrane cleaning and system disinfection. Therefore, understanding how to clean membranes properly and thoroughly is of the utmost importance.
Options For RO System Cleaning
The permeate water from your RO system can be used to flush contaminants. To do this, simply soak RO elements during a period of system standby to help loosen any precipitates and help to dissolve scale. Doing this is helpful because it will reduce the need to use chemicals.
Cleaning For Specific Fouling Agents
The cleaning method used for your system will depend on the foulant or combination of foulants that are present. Fouling agents like silica scale, metal oxides, organic material and sulphate scale may require more than one chemical, applied in a specific sequence to remove them from the system.
Should a system have mineral scale and organic contaminants to be dealt with, a two-stage process is required. The mineral scale is removed using a cleaner having a low ph. Following this treatment, a high pH chemical is used to remove the organic contaminants.
The type and pH of chemicals, as well as the order in which they are applied, can all vary according to the foulant being dealt with. Heavy biological and organic foulants will require a cleaning solution with added detergents to remove them. Other cleaning agents will contain chelating agents for the removal of colloidal, sulphate scale or biological foulants.
Biological Fouling May Require An Extra Step
An extra step may be required if your RO membranes have become fouled by biological contaminants. Typically, a biocide is used for this purpose. Biocides can be applied once periodic cleaning has been completed. They can also be added on a continuous basis as the system runs.
One important factor to consider is compatibility between the biocide and the membrane, as using the wrong biocide can result in membrane damage. Other equally-important factors include ensuring the biocide won’t cause health issues, will effectively control biological contaminants and is at a cost that’s within reach for your facility.
Prevention Of Membrane Fouling
In addition to proper cleaning methods, there are also ways to prevent or at least delay the fouling of system membranes.
One way to prevent RO membrane scaling is to ensure that feed water is softened constantly to <4ppm hardness. Lower energy consumption and higher flow can be attained by using XLE thin-film composite membranes. Particle contamination and fouling can both be reduced by installing 5-micron absolute rated depth filters. Organic matter and free chlorine can be removed more effectively when granular activated carbon is installed in the automatic backwashing unit.
RO System Cleaning Schedule
Although time required to clean an RO system will vary from facility to facility, you should expect to spend between 4 and 8 hours per system stage. In general, there are several basic RO cleaning steps.
The system must first be flushed with permeate or feed water at low pressure for initial foulant and contaminant removal. Then, the properly-mixed cleaning solution should be introduced to the first stage for one hour. The cleaning solution should be returned to the RO cleaning tank, but not before draining displaced water and approximately 20% of the fouled cleaning solution.
A sequence for soaking and recirculation can be executed following the application of cleaning solution, with soaking taking as little as one hour or as long as overnight. Finally, permeate water can be sent through at low pressure to remove traces of cleaning chemicals before restarting the system. It may take up to a few days for the quality of permeate water to return to normal.
Although the cleaning of your RO system may be thorough and use all of the right chemicals, a second cleaning may be required to get the results you wish to see.
Get Professional Assistance With RO Membrane Cleaning
Ensuring your RO membranes and system are correctly and thoroughly cleaned is not always an easy task. Our team at Wychwood have created The Guide to Industrial Water Systems to help. In it, you will find the information you need to ensure your RO system is producing the quality of manufacturing water needed by your facility. You can click here to claim your free copy.