Cause and solution of reverse osmosis membrane damage
I. Reasons for shortening the service life of RO
1. Improper operation of reverse osmosis equipment causes damage to membrane performance
1.1. Residual gas in reverse osmosis equipment runs under high pressure, forming an air hammer that will damage the reverse osmosis membrane. Two things often happen:
A. After the equipment is emptied, when it is re-run, the gas is not exhausted and the pressure is increased rapidly. The remaining air should be drained out at a pressure of 2 ~ 4bar, and then gradually run with increased pressure.
B. When the joint between the pretreatment equipment and the high pressure pump is not sealed well or leaked (especially when the microfilter and the pipeline leakage after it), when the pretreatment water supply is not enough, such as the blockage of the microfilter, the vacuum will suck some air into the place where the seal is not good. The microfilter should be cleaned or replaced to ensure that the pipeline does not leak. In a word, the pressure should be increased step by step in the flowmeter when there is no bubble in the operation, and the bubble should be gradually reduced to check the cause.
1.2. Incorrect shutdown method of reverse osmosis equipment
A. Rapid depressurization without thorough washing when shutting down. Because the concentration of inorganic salts on the membrane concentrated water side is higher than that of raw water, it is easy to scale and pollute the membrane.
B. Rinse with pretreatment water with added chemical reagents. Water containing chemical reagents may cause membrane contamination during equipment outage. When the reverse osmosis equipment is ready to shut down, it should stop adding chemical reagents, gradually reduce the pressure to about 3bar, rinse with pre-treated water for 10min, until the TDS of the concentrated water is very close to that of the raw water.
1.3. Poor disinfection and maintenance of reverse osmosis equipment leads to microbial contamination. This is a common problem in the use of composite polyamide membrane. At present, the pure water produced by many manufacturers exceeds the standard of microorganisms, which is caused by poor disinfection and maintenance.
The main performance is: when leaving the factory, the RO equipment does not use disinfectant maintenance, after the installation of the equipment did not disinfect the whole pipeline and pretreatment equipment; Discontinuous operation without taking disinfection and maintenance measures; No regular disinfection of pretreatment equipment and reverse osmosis equipment; Maintenance solution failure or insufficient concentration.
1.4. Poor monitoring of residual chlorine of reverse osmosis equipment, such as pump failure or liquid failure of adding NaHSO3, or membrane damage due to residual chlorine when activated carbon is saturated.
2, cleaning and cleaning method is not correct resulting in damage to the film performance of the equipment in the process of use, in addition to the normal attenuation of performance, due to pollution caused by the attenuation of equipment performance is more serious. The usual pollution mainly includes chemical scale, organic matter and colloid pollution, microbial pollution and so on. Different kinds of pollution show different symptoms.
The identification of pollution types should be based on raw water quality, design parameters, pollution index, operation records, equipment performance changes and microbial indicators.
(1) Colloidal pollution: When colloidal pollution occurs, it is usually accompanied by the following two characteristics:
A. In the pre-treatment, the microfilter is plugged quickly, especially the pressure difference increases quickly.
B, SDI value is usually above 2.5.
(2) Microbial contamination: When microbial contamination occurs, the total number of bacteria in the permeating water and concentrated water of RO equipment is relatively high, so it must not be maintained and disinfected as required at ordinary times.
(3) Calcium scale: can be judged according to the raw water quality and design parameters. For carbonate water, if the recovery rate is 75% and scale inhibitor is added during design, the LSI of the concentrated solution should be less than 1. When no scale inhibitor is added, the LSI of the concentrate should be less than zero, and calcium scale will not be produced in general.
(4) 1/4 inch PVC plastic pipe can be inserted into the component to test the performance changes of different parts of the component to judge.
(5) Determine the type of pollution according to the change of equipment performance.
(6) Calcium scale can be determined by pickling (such as citric acid and dilute HNO3) according to the cleaning effect and cleaning solution, which can be further confirmed by the analysis of the composition of cleaning solution.
(7) Chemical analysis of cleaning liquid: raw water, cleaning stock, cleaning liquid, three sample analysis. When the type of contamination cannot be determined, cleaning (3)+ disinfection +0.1%HCl(pH 3) is usually performed.
Part two, to prevent the damage of film performance
The new reverse osmosis membrane elements are usually stored in sealed plastic bags soaked with 1%NaHSO3 and 18% glycerol solution. In the case of plastic bags not broken, stored for about 1 year, will not affect its life and performance. When the plastic bag opened, should be used as soon as possible, so as to avoid NaHSO3 oxidation in the air, have adverse effects on the components. Therefore, the membrane should be opened as far as possible before use. After testing the reverse osmosis equipment, we used two methods to protect the film. The test machine runs for two days (15 ~ 24h), and then adopts 2% formaldehyde solution for maintenance; Or after operation for 2 ~ 6h, use 1% NaHSO3 water solution for maintenance (the air in the equipment pipeline should be drained to ensure that the equipment does not leak, close all the inlet and outlet valves). Both methods can get satisfactory results. The first method is more expensive and is used for long periods of idle time, while the second method is used for short periods of idle time.
