In demineralized water treatment equipment, reverse osmosis membrane is the most advanced and economical technology at present. In the daily operation of reverse osmosis equipment, it is often found that the desalination rate of reverse osmosis pure water equipment drops too fast. What are the reasons for the rapid decline of desalination rate of pure water equipment?
1. Desalination rate decreases due to high pressure difference
The increase of pressure difference is often accompanied by the rapid decline of desalination rate. Under normal flow rate, the rise of differential pressure is usually caused by impurities, pollutants and scale entering the partition of the water flow channel of the membrane element, which leads to the decline of water production flow.
When the set feedwater flow is exceeded, excessive differential pressure will also occur. When the feedwater pressure rises too fast during startup, the differential pressure of water hammer will be large. If the membrane has been polluted, especially microbial pollution, the differential pressure will also increase. The pressure difference between feed water and concentrated water represents the hydraulic resistance, which is related to the flow rate and temperature of feed water. It is necessary to maintain a certain flow rate of permeate and concentrated water. appear
The possibility of high pressure difference includes scale, microbial pollution, scale inhibitor precipitation, filter medium leakage of the filter, and water supply/concentrated water seal damage.
2. Online chemical cleaning is unreasonable
Ultrapure water equipment is inevitably polluted in operation. Pretreatment and addition of various agents can only minimize the possibility of RO being polluted, but cannot completely eliminate it. Therefore, it is necessary to fully demonstrate and confirm what kind of pollutant is in the long-term reverse osmosis system after a certain period of operation. According to the characteristics of polyamide membrane, appropriate cleaning agent can be selected according to the corresponding dirt:
a. Hydrochloric acid (36% - 38%), prepared into 0.12% dilute solution to remove metal oxides.
b. Sodium hydroxide, prepared into a 0.1% dilute solution to remove silica, microbial membrane, organic matter, etc., with a pH of about 12. The function is to peel off the hydrolytic damage of the organic microbial mucosa. For the silica colloidal scale, the sodium silicate formed is soluble, so as to remove the scale.
c. Tetrasodium ethylenediaminetetraacetate, as a chelating agent, is widely used in industrial cleaning. The 1% aqueous solution pH10.5-11.5 is added with a concentration of 0.5% - 1%.
d. Sodium dodecyl sulfonate, an anionic surfactant, aims to disperse organic compounds in the solution, which can reduce the surface tension of the solution and cause positive adsorption, so that the concentration of solute molecules on the solution surface is greater than that in the solution. Sodium dodecyl sulfonate is the most important surfactant for reverse osmosis cleaning, with the concentration of 0.025%.
e. Formaldehyde, formaldehyde has a strong killing effect on bacteria, fungi, viruses, spores and protozoa, with the addition concentration of 0.5% - 35.
3. Poor control of residual chlorine
As a bactericide, sodium hypochlorite is widely used in the pretreatment of pure water equipment. In the reverse osmosis system, in order to prevent microbial pollution of reverse osmosis, the reverse osmosis inlet water shall be chlorinated. Use a colorimeter to measure the residual chlorine, and control the mass concentration of residual chlorine at the inlet of the sand filter to be 0.5mg/L, not less than 0.3mg/L, and less than 0.1mg/L at the cartridge filter before reverse osmosis. The outstanding problem of polyamide film is to prevent it from being oxidized. The residual chlorine value of influent water and strong oxidation have adverse effects on it, which must be strictly controlled. Therefore, it is extremely important to regularly detect the residual chlorine value of the reverse osmosis inlet water.
Reasons for rapid decline of desalination rate of pure water equipment.
2022 09/26