Temperature reduction in autumn and winter: operation and maintenance of RO membrane

There is a feature in the production of pure water by reverse osmosis: the actual water yield of reverse osmosis membrane is greatly affected by temperature The water yield of most laboratory ultra pure water machines or RO pure water equipment is marked according to the standard water yield of RO membrane at the inlet water temperature of 25 ℃ With the decrease of temperature, the viscosity of water increases, the diffusivity of water weakens, and the water yield decreases with the decrease of temperature Under the same pressure, when the temperature drops by one degree centigrade, the water yield can be reduced by 3-4% The following equation is the relationship between the water yield flux and the temperature: TCF = exp [K × (1 ÷ (273 + T) - 1 ÷ 298], where TCF is the temperature correction factor, K is the membrane material constant, t is the inlet water temperature centigrade The equation takes 25 ℃ as the reference, TCF = 1 reverse osmosis membrane temperature correction factor TCF Calculation method: after correction, flow = measured flow × correction coefficient corresponding to feed water temperature In order to prevent the growth of membrane surface organisms during storage and performance degradation in subsequent operations, membrane elements must be stored in a special solution 1 The new membrane element (1) must be kept in a wet state after the membrane element is filled with water Even if it needs to be opened temporarily in order to confirm the quantity of the same package, it must be carried out under the condition of not damaging the plastic bag, and this state should be kept until the use; (2) the membrane element should be better kept at a low temperature of 5 ~ 10 ℃ When it is stored in an environment with a temperature of more than 10 ℃, a well ventilated place shall be selected, and direct sunlight shall be avoided, and the storage temperature shall not exceed 35 ℃; (3) if the membrane element is frozen, it will be physically damaged, so thermal insulation measures shall be taken, and it shall not be frozen; (4) when stacking the membrane element, the packing box shall not exceed 5 layers, and the carton shall be kept dry 2 Used membrane elements (1) the membrane elements must always be kept in a dark place, the storage temperature shall not exceed 35 ℃, and direct sunlight shall be avoided; (2) there is a risk of freezing when the temperature is below 0 ℃, so anti freezing measures shall be taken; (3) In order to prevent the growth of microorganisms during short-term storage, transportation and standby of the system, it is necessary to prepare sodium bisulfite (food grade) protective solution with concentration of 500-1000ppm and ph3-6 with pure water or reverse osmosis water to soak the components (4) after immersing the membrane element in the preservation solution for about 1 hour, take the membrane element out of the solution and pack it in the oxygen isolation bag, seal and label the bag, and mark the packing date; (5) after repacking the membrane element to be preserved, keep the same condition with the new membrane element; (6) The concentration and pH of the preservation solution should be kept in the above range, and it should be checked regularly If it is possible to deviate from the above range, the preservation solution should be reconditioned; (7) in any case, the membrane should not be kept in a dry state (8) in addition, formaldehyde solution with concentration (mass percentage concentration) of 0.2-0.3% can also be used as preservation solution Formaldehyde is a stronger microbial killer of bisulfite sodium, and the composition does not contain oxygen Shutdown and maintenance of reverse osmosis system 1 The items to be considered during storage are as follows: (1) after shutdown, use the treated reverse osmosis supply water, softened water or product water to wash the system; (2) in order to maintain the performance of the system, the membrane elements must be kept wet all the time; (3) in order to prevent the growth of bacteria in the pressure vessel, carry out disinfection treatment; (4) If the membrane element is polluted, system failure may occur, so chemical cleaning shall be carried out before storage, so that the dirt can be removed from the membrane element, and the growth of bacteria is limited to a small possibility; (5) the allowable temperature and pH range during storage in the pressure vessel are as follows: temperature range: 0 ~ 35 ℃, pH range: 3-7 (6) The preservation solution was acid sodium bisulfite (SBS) solution The water in which the preservation solution is prepared must be free of residual free chlorine or other oxidants 2 Short term storage (less than 48 hours): if the reverse osmosis system is shut down for more than one day but less than two days, the following measures shall be taken to save the system: (1) flush the system with RO water, and discharge the air in the system at the same time; (2) when the pressure vessel is filled with water, close the water inlet and concentrated water valves Repeat the above steps every 12 hours 3 For long-term storage (more than 48 hours), if the shutdown time of reverse osmosis equipment is more than 48 hours, the following procedures can be adopted for system storage: (1) before all storage, the system needs to be cleaned to remove all pollutants and dirt deposited on the membrane surface Before long-term system shutdown, the recommended standard cleaning procedures shall be used After successful cleaning and disinfection, it is necessary to store as soon as possible, and the interval between cleaning and disinfection for the nearest time is 12 hours (2) preservation is accomplished by circulation of 500-1000ppm sodium bisulfite solution from the cleaning system In this way, the membrane element will be thoroughly wetted in a preservation solution In this way, the solution can be circulated in the whole system, and the amount of residual air can be reduced to a small amount after the completion of the secondary circulation (about 1 hour) Ensure that the system does not contain air for sealing during storage, and has good air tightness (3) close all valves at the supply end and brine end Any contact of sodium bisulfite solution with external air (oxygen) will lead to the oxidation of sodium bisulfite to sulfate, and the pH value will continue to decline After all the bisulfite is consumed, the remaining oxygen will not be absorbed, so the biological state will become unstable (4) the pH value of the preserved RO system needs to be adjusted regularly (at least once a month) to ensure that the pH value of the solution will not be lower than 3 If the pH value of the preservation solution is found to be lower than 3, the preservation solution must be replaced The preservation solution must be replaced once a month (5) during shutdown, the storage temperature shall not exceed 35 ℃, but shall not be lower than 0 ℃, so as to prevent freezing (6) formaldehyde solution with mass percentage concentration of 0.2-0.3% can also be used instead of sodium bisulfite as preservation solution Formaldehyde is a stronger microbial killer of bisulfite, and its composition does not contain oxygen Note: due to the toxicity of formaldehyde, it is necessary to pay attention to the safety regulations for the use of formaldehyde when using formaldehyde; the membrane components must be used in normal conditions for at least 24 hours before being stored in formaldehyde solution, otherwise the water production flow may be greatly reduced