It is not unusual to have to use a number of different cleaning chemicals in a specific
sequence to achieve the optimum cleaning. There are times that a low pH cleaning is
used first to remove foulants like mineral scale, followed by a high pH cleaning to
remove organic material. However, there are times that a high pH cleaning is used
first to remove foulants like oil followed by a low pH cleaning. Some cleaning
solutions have detergents added to aid in the removal of heavy biological and organic
debris, while others have a chelating agent like EDTA added to aid in the removal of
colloidal material, organic and biological material, and sulfate scale. An important
thing to remember is that the improper selection of a cleaning chemical or the
sequence of chemical introduction can make the foulant worse.
There are a number of precautions in cleaning chemical selection and usage for a
composite polyamide membrane:
• Follow the manufacturer s recommended chemical list, dosage, pH, temperature
and contact time guidelines.
• Use the leash harshest chemical cleaning to get the job done. This will optimize
the useful life of the membrane.
• Be prudent in the adjustment of pH at the low and high pH range to extend the
useful life of the membrane. A gentle pH range is 4 to 10, while the harshest is 2
• Don t mix acids with caustics. Thoroughly rinse the 1st cleaning solution from the
system before introducing the next solution.
• Flush out detergents with high pH permeate
• Verify that proper disposal requirements for the cleaning solution are followed.
If your system has been fouled biologically, you may want to consider the extra step of
introducing a sanitizing biocide chemical after a successful cleaning. Biocides can
be introduced immediately after cleaning, periodically (e.g. once a week), or
continuously during service. You must be sure however that the biocide is compatible
with the membrane, does not create any health risks, is effective in controlling
biological activity, and is not cost prohibitive.
RO cleaning procedures may vary dependent on the situation. The time required to
clean a stage can take from 4 to 8 hours.,
The basic steps of cleaning are:
Perform a low pressure flush with feed or permeate water to remove service
concentrate and foulants.
Make up the cleaning solution per the manufacturer s instructions.
Introduce the cleaning solution to the first stage for 60 minutes. You may want to
throttle the flow up slowly to minimize the plugging of the feed path with dislodged
foulant. Send the displaced water and up to 20% of the fouled cleaning solution to
drain before returning the cleaning solution back to the RO Cleaning Tank.
Readjust the pH to the target when it changes more than 0.5 pH units.
An optional soak and recirculation sequence can be used. The soak time can be
from 1 hour to overnight depending on the manufacturer s recommendations, but
be cautious that the proper temperature and pH be maintained and that this does
increase the chemical exposure time of the membrane.
A low pressure Cleaning Rinse with permeate water is required to remove all
traces of chemical from the Cleaning Skid and the RO Skid.
Once all the stages of a train are cleaned, the RO can be placed back into service.
It is not unusual for it to take from a few hours to a few days for the RO permeate
quality to stabilize, especially after high pH cleanings.