Generally, membrane fouling refers to the physical and chemical changes of certain components in the filtrate on the membrane surface or in the membrane pores, which cause the adsorption or deposition of pollutants on the membrane surface or in the membrane pores. It will lead to a decrease or even a block of the membrane pore size, resulting in decreasing performance of the ceramic flat sheet membrane's filtration.
Generally, the transmembrane pressure difference (TMP) is measured to determine whether the diaphragm is contaminated. There may be two reasons for the increase in TMP:
1) As the reaction progresses, the filtrate in the main reaction tank is continuously concentrated, and its concentration gradually increases so that the pressure difference between the inner and outer surfaces of the membrane gradually increases, which means, the TMP increases. But at this time the diaphragm is not yet contaminated, and the diaphragm still has a certain degree of permeability;
2) After the filtration reaction has progressed to a certain level, the contaminants in the filtrate cause the membrane to block. At this time, the membrane is contaminated and must be cleaned.
In order to eliminate the possibility of the increase of TMP caused by the increase in the concentration of the concentrated solution, the membrane flux is maintained unchanged, when the TMP changes from the normal working 0-30Kpa to more than 60Kpa, it is considered that the membrane is contaminated and needs to be removed for cleaning.
Because the pH of the raw water is between 7.2 and 8.0, the metal ions in the ionic state are constantly in contact with oxygen during the entire treatment process and are oxidized into oxides or hydroxides. And after the ceramic membrane filtration treatment, a small number of solid oxides and the feed liquid in the raw water are continuously concentrated to form new oxides, which adhere to the surface of the membrane or adhere to the particles originally attached to the surface of the membrane, causing membrane pollution. However, the content of metal elements in the membrane pollutants is relatively small, and although the raw water is purified by processes such as degreasing tanks and flotation machines, it still contains a large number of oily substances. Oily substances are more viscous and are easier to adhere to the surface of the diaphragm, which is the main reason for the blockage of the diaphragm and the decrease of the membrane flux. In the process of ceramic flat membrane oily wastewater, the main pollutants are oil pollutants. Under acidic conditions, oils and fats will undergo hydrolysis reaction, and the generated carboxylic acids and alcohols and other small molecular organic substances can be separated from the membrane surface, leading to the membrane flux being restored.
Generally, online chemical cleaning is used to clean the flat membrane. The cleaning cycle depends on the contamination of the membrane.
Preparation of cleaning agent
A. Alkaline lotion: prepare a mixed aqueous solution of 2000-5000mg/L sodium hypochlorite and 1000mg/L sodium hydroxide, or a 0.5% sodium hypochlorite solution separately (the concentration of sodium hypochlorite solution is the same amount of chlorine (Especially when used as a disinfectant)as the oxidizing power of chlorine compounds:))
B. Pickling solution: prepare 1000mg/L oxalic acid solution. When cleaning on the spot, the amount of cleaning solution is the same as above.
According to the actual requirements of the project, two on-site chemical cleaning methods can be adopted, as shown in Figure 1. The first method is gravity injection, the cleaning tank is above the liquid level of the membrane tank (the cleaning system is on the membrane tank); The second method is a pump-lift injection, and the cleaning tank is below the liquid level of the membrane tank (the cleaning system is on the ground or below the liquid level).
When on-site chemical cleaning cannot solve the problem, it is necessary to remove the membrane element from the module and wipe the membrane surface with a brush or low-pressure water gun. This is generally rare.
The less metal element content in the contaminants on the surface of the ceramic diaphragm indicates that metal oxides and hydroxides are not the main cause of the contamination of the ceramic diaphragm. Oils and other organic substances are the main cause of membrane blockage and membrane flux decline.
Using physical methods such as hydraulic washing and brush cleaning to treating the contaminated ceramic membrane can only clean up large-size contaminants adhering to the surface of the ceramic membrane. The recovery effect of the membrane flux still cannot meet the requirements, and chemical cleaning for further cleaning is required.
After actual verification, for ceramic membranes polluted after treatment of oily wastewater, acidic cleaning fluids have better cleaning effects than alkaline and strong oxidant cleaning fluids.