Storage and Maintenance of Reverse Osmosis Membrane Elements

In order to prevent the growth of organisms on the membrane surface during storage and the degradation of performance during subsequent operations, the membrane elements must be preserved in a specially formulated solution.


　1.1 New Membrane Elements　　
(1) Membrane elements are factory tested for water passage and treated with a 1% sodium sulfite solution for storage, and then vacuum-packed in oxygen-insulated bags.　　
(2) Membrane elements must be kept wet at all times. Even if they need to be opened temporarily to confirm the quantity in the same package, this must be done without damaging the plastic bag, and they should be kept in this state until the time of use; (3) It is preferable to store the membrane elements in a moist state.　　
(3) Membrane elements should preferably be stored at a low temperature of 5~10℃. When stored in an environment where the temperature exceeds 10℃, a well-ventilated place should be chosen and direct sunlight should be avoided, and the storage temperature should not exceed 35℃; (4) If the membrane element is frozen, it should be stored in a condition that does not damage the plastic bag.　　
(4) Membrane elements can be physically broken if freezing occurs, so insulation measures should be taken to prevent them from freezing; and　　
(5) When stacking the membrane elements, the packing box should not exceed 5 layers, and make sure the carton box is kept dry.

　　1.2 Used membrane elements　　
(1) Membrane elements must be kept in a dark place at all times, at a storage temperature not exceeding 35°C, and protected from direct sunlight; (2) There is a risk of freezing at temperatures below 0°C, so anti-freezing measures must be taken; (3) In order to prevent the membrane elements from freezing, they must be stored in a dark place at all times.　　
(3) In order to prevent the growth of microorganisms in the membrane element during short-term storage, transportation, and system standby, it is necessary to soak the element in a protective solution of sodium sulfite (food-grade) with a concentration of 500 to 1,000 ppm, pH 3 to 6, prepared with pure water or reverse osmosis produced water. Usually, Na2S2O5 is used, which reacts with water to form bisulfite: Na2S2O5 + H2O-2NaHSO3.　　
(4) After soaking the membrane element in the preservation solution for about 1 hour, remove the membrane element from the solution and package it in an oxygen barrier bag, seal the bag and label it with the date of packaging.　　
(5) After repacking the membrane element to be preserved, the preservation conditions are the same as those of the new membrane element.　　
(6) The concentration and pH of the preservation solution should be maintained in the above range, and should be checked periodically, and the preservation solution should be reconstituted if there is a possibility of deviation from the above range; (7) The preservation solution should be maintained in the above range under all circumstances.　　
(7) Under no circumstances should the membrane be allowed to dry out during preservation.　　
(8) Alternatively, a formaldehyde solution with a concentration (mass percent concentration) of 0.2 to 0.3% can be used as the preservation solution. Formaldehyde is a stronger microbicide than bisulfite and does not contain oxygen in its composition.


　2 Reverse osmosis system downtime maintenance　　
2.1 Matters to be considered for preservation are as follows　　
(1) After shutdown, flush the system with treated reverse osmosis supply water, softened water, or product water.　　
(2) To maintain system performance, the membrane element must be kept wet at all times.　　
(3) Sterilization is required to prevent the growth of bacteria in the pressure vessel.　　
(4) If the membrane elements are contaminated, system failure may occur, so chemical cleaning is performed before preservation, which removes dirt from the membrane elements and limits bacterial growth to the minimum possible; and　　
(5) The permissible temperature and pH ranges for preservation in a pressure vessel are as follows: temperature range: 0~35℃, pH range: 3-7　　
(6) The preservation solution is an acidic sodium bisulfite (SBS) solution. The water in which the preservation solution is prepared must be such that it cannot contain residual free chlorine or other oxidizing agents.
　　

2.2 Short-term storage (less than 48 hours)　　
If the downtime of the reverse osmosis system is longer than one day but shorter than two days, take the following measures to preserve the system:　　
(1) Flush the system with RO feed water while exhausting air from the system.　　
(2) Close the feed and concentrate valves when the pressure vessel is filled with water.　　Repeat the above steps every twelve hours.
2.3 Long-term preservation (greater than 48 hours)　　
If the downtime of the reverse osmosis equipment is greater than 48 hours, the following procedures can be taken for the preservation of the system:　　
(1) Prior to all preservation, the system needs to be cleaned to remove all contaminants and fouling deposited on the membrane surface (this operation is employed only if the membrane element is contaminated or expected to be contaminated). Prior to long-term system shutdown, use the recommended standard cleaning procedures or consult Toray for individual cleaning and disinfection procedures. After successful completion of cleaning and disinfection, save as soon as possible, with a maximum of 12 hours between the most recent cleaning/disinfection.　　(2) Preservation is accomplished by using a cycle of 500-1,000 ppm sodium bisulfite solution with the cleaning system. In this manner, the membrane elements will be thoroughly wetted in the preservation solution. Circulating the solution throughout the system in this manner will minimize the amount of residual air after the secondary circulation is complete (approximately 1 hour). Ensure that the system does not contain air for sealing during preservation and that it is airtight.　　
(3) Close all valves on the supply side and brine side. Any contact of the sodium bisulfite solution with outside air (oxygen) will result in the oxidation of the sodium bisulfite to sulfate, and the pH will continue to drop. After all the bisulfite has been consumed, the remaining oxygen will not be absorbed, and the state of the organism will become unstable.
(4) The pH of the preserved reverse osmosis system needs to be adjusted periodically (at least once a month) to ensure that the pH of the solution does not fall below 3. If the pH of the preservation solution is found to be below 3, then the preservation solution must be replaced. The preservation solution must be replaced once a month.　　
(5) During downtime, the storage temperature should not exceed 35°C but should not fall below 0°C to prevent freezing.
(6) A formaldehyde solution with a mass percent concentration of 0.2-0.3% may also be used as a preservation solution instead of sodium bisulfite. Formaldehyde is a stronger microbiocide than bisulfite and contains no oxygen in its composition.　

　Note: Because formaldehyde has a certain degree of toxicity, so the use of formaldehyde, must pay attention to the formaldehyde use of safety regulations; membrane elements in formaldehyde solution for preservation before, must be used in normal conditions for at least 24 hours, otherwise may lead to a significant reduction in the flow rate of water production