Cleaning method of ultrafiltration membrane

Physical cleaning method of ultrafiltration membrane

Physical method: Generally, filters with a pore size of 5~10 μm are installed before ultrafiltration to remove solid suspended solids and colloids such as iron and aluminum. The fouled membrane can be cleaned by the following methods.

  1. Hydraulic method, reducing the operating pressure and increasing the circulating volume of the retained liquid (ie high-speed water flushing) is conducive to increasing the flux; the use of liquid flow pulses can quickly remove membrane fouling, especially the combination of washing liquid pulses and recoil, Satisfactory results will be obtained. For example, the internal pressure type hollow fiber membrane can be cleaned in the following two ways. One is to backflush the washing liquid through the membrane in reverse to remove the dirt deposited on the inner wall of the fiber. Note that the washing liquid must not contain suspended matter to prevent the spongy bottom layer of the hollow fiber membrane from being blocked. For example, two ultrafilters can be run in parallel during backwashing, and the effluent of one ultrafilter can be used to backwash the other ultrafilter. This should be performed at a lower operating pressure to avoid membrane rupture. The backwashing time It usually takes 20 to 30 minutes. The other is to close the outlet of the permeate by circulating washing, and use the feed liquid and the permeate for cleaning. Since the flow rate of the feed liquid in the hollow fiber cavity is high, the flow pressure drop is large. After closing the outlet of the permeate, the pressure between the fibers is roughly equal to the average value of the pressure in the fibers. The inner pressure of the inlet section of the hollow fiber is higher, resulting in filtrate; the outer pressure of the outlet section of the fiber is higher, and the filtrate flows back into the fiber. In the inner cavity, the permeate circulates inside and outside the hollow fiber. The returned filtrate flow and the high-speed feed liquid flow can remove the deposited dirt. Recently, a development trend is to use two sets of internal pressure hollow fiber membranes in combination. Ultrafiltrate is used to backflush another set of hollow fiber membranes, and the exchange is carried out after a period of time. Generally, it is working for 10 minutes and backflushing for 1min. This method of backflushing while working can well prevent the blockage of the membrane pores and make the membrane pass through. Keep working at a high level. This method of operation breaks through the view that it is necessary to wait until the membrane is fouled before it stops working for cleaning. It does not need cleaning agents and does not need to remove the membrane assembly. It is a very good method. The control part of the switch has higher requirements, otherwise the life of the membrane will be affected.
    For another example, the external pressure hollow fiber membrane can be washed by the isobaric method. When flushing, the first step is to depressurize operation, close the ultrafiltrate outlet and increase the flow rate of the raw solution inlet. At this time, the inner cavity pressure of the hollow fiber rises until it reaches the same operating pressure as the outer cavity of the fiber, so that the pressure difference between the inner and outer sides of the membrane is zero, and the retention The solute molecules on the membrane surface will be suspended in the solution and discharged with the concentrated water. Isobaric flushing is suitable for hollow fiber membranes. For hollow fiber membrane modules, the negative pressure cleaning method can also be used, that is, the functional surface of the membrane is in a negative pressure state by means of suction, so as to remove pollutants and restore the performance of the membrane. The advantage is that when the outside pressure of the membrane is atmospheric pressure, the maximum pressure difference between the inside and outside of the membrane is one atmospheric pressure, the membrane is not easily damaged, and its cleaning effect is better than isobaric cleaning.
  2. Gas-liquid pulses pass high-pressure gas (air or nitrogen) into the gap of the membrane filter device to form gas-liquid pulses. The gas pulse expands the pores in the membrane, allowing contaminants to be flushed away by the liquid. The effect of this method is good, and the gas pressure is generally 0.2~0.5MPa, which can restore the membrane flux to more than 90%. In addition, there are electric field filtration, pulse electric pulse cleaning, pulse electrolytic cleaning, electro-osmosis backwashing, sponge ball mechanical scrubbing and other methods.

Chemical cleaning method of ultrafiltration membrane

Chemical cleaning: When the membrane fouling is serious and the flux cannot be recovered by physical methods, chemical cleaning agents must be used for cleaning. Chemical cleaning is essentially a multiphase reaction between sediment and cleaning agent. According to Bright’s theory, chemical cleaning is divided into 6 processes:

  1. Mechanical cleaning before chemical cleaning;
  2. The cleaning agent spreads to the dirt surface;
  3. Penetration and diffusion into the fouling layer;
  4. Cleaning reaction (including physical and chemical processes: melting, mechanical stress and thermal stress, wetting, infiltration, swelling, shrinkage, solvation, emulsification, deflocculation and adsorption. The chemical process is: hydrolysis, peptization, Saponification, dissolution, integration, integration and suspension, these reactions always produce some soluble product, or at least some disperse. The reaction product weakens the bond between the fouling particles and the membrane surface):
  5. The cleaning reaction product is transferred to the inner surface;
  6. The product is transferred to the inner surface.

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Further reading:

Backwashing Design UF Membrane

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