Generally, the main reason for performance degradation of RO membrane is that its surface has been polluted, like, surface scaling, blockage of pores or caused by Physical and chemical changes. The physical changes are caused by compaction effect which will result in decrease of water permeability of RO membrane. And the chemical changes are caused by the fluctuation of PH value, like cellulose acetate membrane hydrolysis and free chlorine will also make the performance degradation of directional polyamide membrane. The blockage of RO membranes is caused by the surface deposition and microbial growth.The microbes will not only block the pores but also corrode the cellulose acetates.
There should maintain certain amount of residue chlorine, around 0.1~0.5mg/L, while for Aromatic polyamide membrane, the residue chlorine shall be kept less than 0.1mg/L. Cleaning of RO membranes is subtle and troublesome. Currently the quality of domestic made membranes is not so good and frequent cleaning will do damage to the membranes.
The method of cleaning RO membrane is to clean it regularly with 0.1% aquae formalinata for at least one hour. For the polluted membrane, clean it with 2% Ammonium citrate solution (PH=4~8), or use sodium bisulfite, calgon and dilute hydrochloric acid to prevent from scaling of manganese, ion and carbonate.Sometimes enzyme detergent is used to clean organics. The cleaning pressure is 0.34~0.98MPa(3.5~10kgf/cm2), and flowing velocity is 2~3 times that of water treatment plant.
Sectional view of reverse osmosis membrane
Daily Commissioning of RO System
The commissioning of RO system consists of two parts, one is for the pre-treatment and the other is for RO main unit. Pre-treatment mainly consists of cleaning of silica sand and active carbon, to rinse out the fine sand,carbon and other impurities and control the inlet water capacity. The commissioning of RO unit is mainly to adjust the inlet and outlet water flowing capacity and pressure. Below are the points on how to do daily commissioning of RO unit:
1. Test and analyze the inlet water. Only when the testing results are satisfactory can the unit be connected with water for testing.
2. Adjust the pressure control system of high pressure pump.
3. Check if all the pipe connections are well connected, if the pressure meters are all ready, and if the low pressure pipe connections are tightly connected or any shortage.
4. Open all the pressure meters, general water inlet valve, waste water discharge valve, and pure water discharge valve.
5. Start pre-treament units, and adjust the capacity of pure water larger than general water inlet.
6. Wash the silica sand filter and active carbon filter from top to downward and from bottom to top, until the water looks clean.
7. Start high pressure pump, and start slowly the general water inlet valve, controlling the pressure of general water inlet less than 0.5MPa, keeping washing for 5 minutes, and check all the high and low pressure pipelines and meters if they are working properly.
8. Adjust the water inlet valve and waste water discharge valve to make the inlet water pressure 1.0~1.4MPa.
9. Check the water conductivity. Start the pure water valve when the conductivity is satisfactory, and close pure water discharge valve.
10. The commissioning of RO unit is all in manual mode. It can be switched to auto mode only when it works properly and controlled by online meters and PLC.
11. Start the regeneration system to check it if works properly.
The most important point of commissioning of RO unit: never let the pure water valve closed while the high pressure pump is still on, otherwise it will cause great damage.
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