The fouling by organic matter is due to the accumulation of it, and that is greater the greater its concentration. And biological fouling occurs when microorganisms affect the membrane and find biodegradable organic matter. The effectiveness of chemical cleaning depends largely on the choice of cleaner or cleaners. There are specific compounds for each type of fouling. In general, acid solutions facilitate the dispersion of the inorganic pollutants that constitute the first two groups; while the constituents of the third and fourth group are removed with alkaline solutions. When the chemistry of feed water is known, the nature of fouling can be predicted. The first time a cleaning is required, it is recommended to contact a specialist. The industrial Ro plant membranes is known as the perfect cleaner .
Before cleaning, there are important points to consider:
· Review the manufacturer's recommendations of the cleaner: dose, pH, temperature, rest times and agitation.
· Verify the compatibility of the cleaner (s) with the membrane manufacturing materials.
· Be careful when preparing the solution with the cleaner to avoid leaving the recommended pH range. A range between 4 and 10 causes no damage to the membranes. If the degree of fouling requires more aggressive solutions, remember that pH values between 2 and 4, or between 10 and 12, can cause damage to the membrane if cleaning for a certain time is exceeded.
· Do not mix acidic solutions with caustics. Be sure to rinse each cleaning solution thoroughly before introducing the next solution.
· In multistage systems, it is advisable to wash each stage separately.
· Perform cleaning in an area with adequate ventilation.
· All materials that form the cleaning system must be resistant to chemical corrosion.
The tank must be of adequate size to contain the cleaning solution. It is recommended to use a volume of 5 gallons per 4 × 40 ”membrane and 20 gallons per 8 × 40” membrane. The tank in which the solution is prepared must be easily accessible. The arrangement of pipes must allow recirculation and this must be discharged into the bottom of the tank to minimize foaming. The pump must be adequate to provide the required flow and pressure. The maximum recommended pressure is 60 psi. A pressure that is too high does not allow the release of solids or impurities that are detaching from the membrane surface. The flow should not be greater than 40 gpm per 8 × 40 ”membrane, and 10 gpm for a 4 × 40” membrane. It is recommended to use a cartridge filter of 5 to 10 microns, to retain the solids released in the cleaning. It must be placed in the recirculation line to the tank. It is important to monitor the differential pressure at this point, and replace the cartridge upon reaching a differential pressure of 15 psi. It is essential to have sampling points and pressure gauges that allow pH and pressure measurements to be taken at the discharge of the pump and in the recirculation return line. The change in pH is expected: it means that the contaminants are reacting with the cleaning solution. When detecting a change greater than 5 pH units, adjust to the initial pH value of the solution with the chemical cleaner.
The most used indicator to determine when to stop cleaning is when the pH no longer undergoes changes, which means that dispersion of the contaminants no longer occurs.
OI cleaning procedures vary depending on the particularities of each system.
The basic cleaning steps are:
· Fill the tank with the volume of water necessary for the size and number of membranes to be cleaned. Use permeated or at least softened water.
· Add the cleaning agent according to the manufacturer's instructions. Use a mechanical stirrer or re circulate in the tank, to ensure that the solution is completely homogeneous. IT IS IMPORTANT TO ALWAYS ADD THE CLEANING AGENT TO THE WATER AND NOT TO THE REVERSE, TO AVOID REACTIONS THAT MAY RISK YOUR SAFETY.
· If possible, heat the solution without exceeding 40 ° C, for more effective results. Consult the membrane temperature restrictions.
· With the rejection valve completely open, enter the cleaning solution. Regulate the feed rate with a valve in the discharge of the pump, with the intention that the rate of entry of the solution is gradual. Recirculate for 30 to 60 minutes.
· Monitor pH and pressure at least every 15 minutes, at the entrance and exit of the membrane. It is recommended to save this information in a maintenance log of the equipment.
· When the contamination is excessive, it is recommended to leave the membranes flooded in the cleaning solution and at rest for the necessary time, which can vary between 45 min and 8 hours. Consider that the solution is highly aggressive and prolonged exposure could damage the membrane. Another option is, after the flood at rest, make a second recirculation, follow the same recommendations in points 4 and 5.
· To rinse, use permeated or softened water at low pressure and send it to the appropriate disposal system. An effective way to determine when to stop rinsing is to use an SDT meter. The criterion for terminating the rinse is when the value of SDT is equal at the input and output.
· To put the RO back into service, first return the rejection valve to its usual position, put into service and monitor the water quality. It is normal for it to take a while for the permeated water to stabilize.
· If even after cleaning you do not get the expected quality or flow for the system, it is necessary to contact a specialist to rule out that the membrane has an irreversible fouling or is broken.
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