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Reverse Osmosis Membranes
Hydranautics Technical Articles
Reverse Osmosis Cleaning
Reverse Osmosis Cleaning
Reverse Osmosis Cleaning
Cleaning Your RO
Eventually the day comes when your RO system will require cleaning. Cleaning is
recommended when your RO shows evidence of fouling, just prior to a long term
shutdown, or as a matter of scheduled routine maintenance. Fouling characteristics
that signal you need to clean are a 10-15% decrease in normalized permeate flow, a
10-15% decrease in normalized permeate quality, or a 10-15% increase in
normalized pressure drop as measured between the feed and concentrate headers.
RO cleaning frequency due to fouling will vary by site. A rough rule of thumb as to an
acceptable cleaning frequency is once every 3 to 12 months. If you have to clean
more than once a month, you should be able to justify further capital expenditures for
improved RO pretreatment or a re-design of the RO operation. If cleaning frequency is
every one to three months, you may want to focus on improving the operation of your
existing equipment but further capital expenditure may be harder to justify.
One RO design feature that is commonly over-looked in reducing RO cleaning
frequency is the use of RO permeate water for flushing foulants from the system.
Soaking the RO elements during standby with permeate can help dissolve scale and
loosen precipitates, reducing the frequency of chemical cleaning.
What you clean for can vary site by site depending on the foulant. Complicating the
situation frequently is that one more than one fulant can be present. Typical foulants
ï¿½ Calcium carbonate scale
ï¿½ Sulfate scale of calcium, barium or strontium
ï¿½ Metal oxides of iron, manganese, aluminum, etc.
ï¿½ Silica scale
ï¿½ Colloidal deposits (inorganic or mixed inorganic/organic)
ï¿½ Organic material of natural origin or man-made origin
ï¿½ Biological (bioslime, mold, or fungi)
There are a number of factors involved in the selection of a suitable cleaning chemical
(or chemicals) and proper cleaning protocol. The first time you have to perform a
cleaning, it is recommended to contact the manufacturer of the equipment, the ROelement manufacturer, or a RO specialty chemical supplier. Once the suspected
foulant(s) are identified, one or more cleaning chemicals will be recommended.
These chemical(s) can be generic and available from a number of suppliers or can
be private-labeled proprietary cleaning solutions. The proprietary solutions can be
more expensive but may be easier to use and you cannot rule out the advantage of the
intellectual knowledge supplied by these companies. An invaluable service offered by
some service companies is that they will determine the proper cleaning chemicals
and protocol by testing at their facility an element pulled from your system.
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:
ï¿½ Follow the manufacturer s recommended chemical list, dosage, pH, temperature
and contact time guidelines.
ï¿½ Use the leash harshest chemical cleaning to get the job done. This will optimize
the useful life of the membrane.
ï¿½ 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
ï¿½ Don t mix acids with caustics. Thoroughly rinse the 1st cleaning solution from the
system before introducing the next solution.
ï¿½ Flush out detergents with high pH permeate
ï¿½ 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.
The successful cleaning of an RO on-site requires a well designed RO cleaning skid.
Normally this skid is not hard piped to the RO skid and uses temporary hosing for
connections. It is recommended to clean a multi-stage RO one stage at a time to
optimize crossflow cleaning velocity. The source water for chemical solution make upand rinsing should be RO permeate, DI water or at least soft water. Components
must be corrosion proof. Major cleaning system components are:
ï¿½ RO Cleaning Tank: This tank needs to be sized properly to accommodate the
displacement of water in the hose, piping, and pressure vessels. The tank should
be designed to allow 100 % drainage, easy access for chemical introduction and
mixing, a recirculation line from the RO Cleaning Pump, proper venting, overflow,
and a return line located near the bottom to minimize foam formation when using a
ï¿½ RO Cleaning Pump: This pump needs to be sized to develop the proper crossflow
velocity to scrub the membrane clean. The cleaning rate for a 8-inch diameter
vessel is 30 to 40 gpm and for a 4-inch diameter vessel is 8 to 10 gpm. The
maximum recommended pressure is 60 psi to minimize the production of
permeate during cleaning and reduce the convective redeposition of foulant back
on to the membrane surface.
ï¿½ RO Cleaning Cartridge Filter: Normally 5 to 10-micron and is designed to remove
foulants that have been displaced from the cleaning process.
ï¿½ RO Tank Heater or Cooler: The optimal temperature for cleaning is 35 to 45 o C.
One cannot forget that heat is generated and imparted by the RO Cleaning Pump
ï¿½ RO Tank Mixer: This is recommended to get optimal mixing of chemical, though
some designers rely solely on the slow introduction of chemical while maintaining
a recirculation through the RO Cleaning Pump back to the tank.
ï¿½ Instrumentation: Cleaning system instrumentation should be included to monitor
flow, temperature, pressure, and tank level.
ï¿½ Sample Points: Sample valves should be located to allow pH and TDS
measurements off the RO Cleaning Pump discharge and the concentrate side
recirculation return line.
ï¿½ Permeate Return Line: A small amount of the cleaning solution can permeate
through the membranes and so a permeate side return line is recommended.
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:
1. Perform a low pressure flush with feed or permeate water to remove service
concentrate and foulants.
2. Make up the cleaning solution per the manufacturer s instructions.
3. 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.
4. 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.
5. A low pressure Cleaning Rinse with permeate water is required to remove all
traces of chemical from the Cleaning Skid and the RO Skid.
6. 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.
It is exciting to have a successful cleaning and watch your pressures and permeate
quality improve. On the flip side it is frustrating to have an unsuccessful cleaning,
though its no reason to become suicidal. If the cleaning did not provide the results
you were hoping for, you may want to consider talking to those suppliers who offer offsite
services rather than proceed with a trial-and-error approach on site. Pull one or
two elements from the front or back end and ship them to a service company. A
service company can determine the optimal cleaning procedure and also report how
effective the cleaning was in restoring flow and salt rejection.
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