This paper introduces the common pollution and cleaning methods that affect the performance of composite membranes. This paper applies to reverse osmosis membrane elements with diameters of 4 inches, 6 inches, 8 inches and 8.5 inches.
Note 1: Under no circumstances should the water containing free chlorine contact with the composite membrane element. If such contact occurs, the performance of the membrane element will be reduced, and its performance can no longer be restored. After sterilization of the pipeline or equipment, it should be ensured that the feed water sent to the reverse osmosis membrane element is free of free chlorine, which should be confirmed by laboratory test. Hydrogen sulfite solution should be used to neutralize the residual chlorine, And ensure sufficient contact time to ensure complete reaction.
Note 2: During the warranty period of reverse osmosis membrane elements, it is recommended that each osmosis membrane cleaning should be carried out after consultation with Hydergy. At least during the first cleaning, the on-site service personnel of Hydergy should be on site
Note 3: Cationic surfactant should be avoided in the cleaning solution, because it may cause irreversible pollution of membrane elements.
1. Pollutants of reverse osmosis membrane elements
After a period of normal operation, the reverse osmosis membrane elements will be contaminated by suspended substances or insoluble substances that may exist in the feedwater. The most common of these pollutants are calcium carbonate scale, calcium sulfate scale, metal oxide scale, silicon sediment and organic or biological sediment.
The nature and pollution rate of pollutants are related to the water supply conditions. The pollution develops slowly. If measures are not taken early, the pollution will damage the performance of membrane elements in a relatively short time.
Regular detection of the overall performance of the system is a good way to confirm that the membrane element is polluted. Different pollutants will cause different degrees of damage to the performance of the membrane element. Table 1 lists the effects of common pollutants on membrane performance.
2. Removal of pollutants
The removal of pollutants can be achieved by chemical cleaning and physical washing, and sometimes by changing the operating conditions. As a general principle, cleaning should be carried out when one of the following situations occurs.
2.1 Under normal pressure, if the product water flow drops to 10~15% of the normal value.
2.2 In order to maintain the normal product water flow, the feed water pressure after temperature correction is increased by 10-15%.
2.3 The product water quality is reduced by 10~15%. The salt transmittance increased by 10-15%.
2.4 Increase the service pressure by 10-15%
2.5 The pressure difference between RO sections increases significantly (there may be no instrument to monitor this sign).
3. Common pollutants and their removal methods:
3.1 Calcium carbonate scale
In case of failure of the scale inhibitor adding system or the acid adding system, which causes the PH of the feedwater to rise, then calcium carbonate may deposit. When it comes out, the occurrence of calcium carbonate scale precipitation should be found as early as possible to prevent the growth of crystals from damaging the membrane surface. If calcium carbonate scale is found early, it can be removed by reducing the PH of the feedwater to 3.0~5.0 for 1~2 hours. For calcium carbonate scale with longer precipitation time, citric acid cleaning solution shall be used for circulating cleaning or overnight soaking.
Note: It should be ensured that the PH of any cleaning solution is not lower than 2.0. The cup may cause damage to the RO membrane element, especially when the temperature is high. The highest PH should not be higher than 11.0. Check to use ammonia to improve PH, and use sulfuric acid or hydrochloric acid to reduce PH.
3.2 Calcium sulfate scale
Cleaning solution 2 (see surface treatment) is the best method to remove calcium sulfate scale from the surface of reverse osmosis membrane.
3.3 Metal oxide scale
The method of removing calcium carbonate scale described above can be used to easily remove the deposited hydroxide (such as iron hydroxide).
3.4 Silica scale
For the silicon scale that is not symbiotic with metallization or organic matter, it can be removed only by special cleaning methods.
3.5 Organic sediment
Organic sediment (such as microbial slime or mildew) can be removed with cleaning solution 3. In order to prevent re-propagation, a sterilization solution approved by Hydergy can be used to circulate and soak in the system. Generally, it takes a long time to soak to be effective. If the reverse osmosis device is out of service for three days, it is better to use disinfection treatment. Please consult with Hydergy to determine the appropriate sterilization agent.
3.6 Cleaning fluid
It is recommended to use the cleaning solution listed in Table 2 when cleaning the reverse osmosis membrane elements. It is very important to determine the chemical analysis of pollutants before cleaning. The detailed analysis and comparison of the analysis results can ensure the selection of the best cleaning agent and cleaning method. The cleaning method and the cleaning effect obtained during each cleaning should be recorded to provide a basis for finding the best cleaning method under the specific water supply conditions.
For inorganic pollutants, it is recommended to use cleaning fluid 1. For calcium sulfate and organic matters, it is recommended to use cleaning solution 2. For serious organic pollution, it is recommended to use detergent 3. All cleaning can be carried out at a maximum temperature of 104 degrees Fahrenheit (40 degrees Celsius) for 60 minutes. The amount of supplies required is calculated by adding the amount per 100 gallons (379 liters). When preparing the cleaning solution, add drugs and cleaning water in proportion. Reverse osmosis product water without free chlorine should be used to prepare the solution and mix it evenly.