Chemical cleaning is essentially the use of chemical reagents and sediments, dirt, corrosion products and other pollutants that affect flux rate and water quality to remove pollutants from the membrane. These chemical reagents include acids, bases, chelating agents, oxidizing agents, and products manufactured according to formula.
1. Acid and alkali solution Acid is effective in removing calcium-based scales such as calcium carbonate and calcium phosphate, iron oxide and metal sulfide. Alkaline cleaning solutions include phosphates, carbonates and hydroxides. These solutions can loosen, emulsify and disperse the precipitate. In order to remove the wet oil, grease, dirt and biological substances, surfactants are usually added to increase the descaling properties of the alkaline cleaning agent. When removing deposits that are particularly difficult to remove, such as silicates, alkaline cleaners and acid cleaners are used alternately.
2. Chelating agent In addition to strong acids and alkalis, chelating agents are also used to remove deposits from contaminated films. Commonly used chelating agents are ethylenediaminetetraacetic acid (EDTA), phosphocarboxycarboxylic acid, gluconic acid and citric acid. Among them, gluconic acid is usually effective in chelating iron ions in strong alkaline solutions, and EDTA is often used to dissolve alkaline earth metal sulfates.
3. Oxidant When NaOH or surfactant does not work, it can be cleaned with chlorine. The optimum pH is 10~11.
Physical cleaning uses mechanical methods to remove contaminants from the surface of the membrane. This method has the characteristics of not introducing new contaminants and simple cleaning steps. However, this method is only effective for the membrane in the early stage of pollution, and the cleaning effect cannot be sustained. Physical cleaning includes a variety of methods, such as positive direction washing, variable direction washing, penetrating liquid reverse pressure washing, vibration, exhaust water filling method, air injection, automatic sponge ball cleaning, hydraulic method, gas-liquid pulse and circulating washing, etc.
1. Backwashing refers to the method of blowing gas or liquid from the permeable side of the membrane to remove contaminants on the membrane surface. Note that it should be carried out at a lower operating pressure (around 132kPa) to avoid membrane rupture. The recoil time generally takes 20~30min.
2. Static immersion plus hydraulic backwashing For the membrane modules that have long-term continuous operation and the water permeability decreases and regeneration is difficult, when the operation is stopped, pure water is immersed and allowed to stand for more than 10h, and then hydraulic backwashing is an effective method to increase flux. .
3. For mechanical scraping, flexible foam plastic balls and sponge balls (diameter slightly larger than the inner diameter of the film tube) can be used for the tube-type components. The inner pressure tube membrane is cleaned, and the foam and sponge balls are repeatedly passed through the surface of the film by hydraulic force in the tube. Mechanically remove contaminants. This method can remove almost all soft scales, but it is not only difficult to remove hard scales but also easy to damage the membrane surface. Therefore, this method is particularly suitable for cleaning the surface of contaminated membranes with organic colloid as the main component.