Author: Site Editor Publish Time: 2022-07-12 Origin: Site
For each membrane unit, people want it to function to the maximum, hoping for the highest desalination rate, maximum water permeability and the longest possible life, to achieve the above three points, the water quality is crucial, so the raw water entering the membrane unit must have good pretreatment. Reasonable pretreatment is very important for the long-term safe operation of the reverse osmosis plant. With pretreatment to meet the requirements of reverse osmosis feed water quality, you can ensure that the flow of produced water remains stable; the desalination rate is maintained at a certain value for a long time; the product water recovery rate can remain unchanged; operating costs are minimised; and the membrane has a long service life. Specifically, reverse osmosis pre-treatment is designed to do the following.
(1) To prevent contamination on the membrane surface, i.e. to prevent suspended impurities, microorganisms, colloidal substances, etc. from attaching to the membrane surface or fouling the water flow channel of the membrane element.
(2) To prevent scaling on the surface of the membrane. In the operation of reverse osmosis plant, due to the concentration of water, there are some insoluble salts deposited on the membrane surface, so to prevent the generation of these insoluble salts.
(3) Ensure that the membrane is protected from mechanical and chemical damage so that it performs well and lasts long enough to be used.
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Despite all the pre-treatment measures, deposits and scaling can occur on the membrane surface after long term use, blocking the membrane pores and reducing water production, so regular cleaning of contaminated membranes is necessary. However, reverse osmosis membrane systems cannot wait until the pollution is very serious before cleaning, which will increase the difficulty of cleaning, but also make the cleaning steps increase and cleaning time extended. It is important to get the timing right and to remove the fouling in a timely manner.
Maintenance of new reverse osmosis membranes : New reverse osmosis membrane elements are usually stored in sealed plastic bags after being infiltrated with a 1% aqueous solution of NaHSO3 and 18% glycerol. If the plastic bag is not broken, storage for about 1 year will not affect its life and performance. When the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the components due to the oxidation of NaHSO3 in the air. Therefore the film should be opened as far as possible before use.
Residual gas is retained during start-up and shutdown, allowing the system to operate under high pressure. Pressure gauges before and after the filter in the system are used to monitor the pressure drop across the cartridge, while primary and final pressure gauges are used to monitor the pressure drop across the RO membrane module. The inlet and thick water valves are adjusted to ensure operating pressure and recovery rate. If the flow rate of produced water or total flow rate decreases during operation, or if the differential pressure between the primary and secondary stages increases significantly compared to the differential pressure at the beginning of operation (based on data from the initial operation of a new RO membrane module), the system will need to be flushed or cleaned to ensure the safe performance of the membrane module.
(1) Rapid pressure drop during shutdown without thorough flushing. As the concentration of inorganic salts on the concentrated water side of the membrane is higher than that of the raw water, it is easy to scale and contaminate the membrane. When preparing to shut down, gradually lower the pressure to about 3 bar and flush with pre-treated water for 14 to 16 minutes.
(2) When preparing to shut down the machine, add chemical reagents, which will make the chemicals stay in the membrane and membrane shell, causing membrane pollution and affecting the service life of the membrane. The dosing should be stopped.
