Author: Site Editor Publish Time: 2022-11-11 Origin: Site
MBR has been widely and maturely used in wastewater treatment, because MBR replaces secondary sedimentation tank, can guarantee the effluent SS and high sludge concentration, save a lot of sewage division in the operation of some troubles, but, membrane pollution problems have also been plagued by the development and operation of MBR! So what can MBR operators do about these problems? To quickly find the root cause of membrane contamination and give a precise blow to reduce the frequency of cleaning.
The main factors in membrane contamination are: the intrinsic nature of the membrane, the nature of the mixture and the operating environment of the system, and the corresponding measures should be taken to control and solve membrane contamination from these three aspects.
Inherent properties of membranes
The physical and chemical properties of membranes are determined by the membrane material, and the resistance of membranes to contamination in a mixture is related to their material. It has been shown that the hydrophilicity of the membrane has a very important influence on the resistance to contamination. Among organic membrane materials, some are hydrophilic materials such as PAN and most are hydrophobic materials such as PVDF, PE, PS etc. Hydrophobic organic materials must be modified hydrophilically when applied, and due to the differences in the modification process, the loss of hydrophilicity during use can be fast or slow.
In addition, the membrane's ability to resist contamination is also related to the membrane surface roughness, membrane surface charge and membrane pore size. Generally speaking, the membrane's ability to resist contamination can be improved by selecting a membrane material with better hydrophilicity, improving the roughness of the membrane surface, choosing a membrane material with the same potential as the mixture and a suitable membrane pore size.
Membrane contamination is largely the result of the interaction between the membrane and the mixture. The nature of the mixture includes sludge concentration and viscosity, particle distribution, dissolved organic matter concentration, microbial metabolite concentration and so on.
When the sludge concentration is low, the sludge is not sufficient for organic matter adsorption and degradation, the concentration of organic matter in the mixture increases, the membrane pore blockage is serious, and the concentration of solutes on the membrane surface increases significantly, resulting in an increase in filtration resistance. The sludge is easily deposited on the membrane surface, forming a thicker sludge layer. It is generally accepted that there is a critical value of sludge concentration above which the membrane flux will be adversely affected, so sludge concentration can be selected to be controlled within a suitable range to effectively control membrane contamination. Sludge swelling and sludge fines can cause serious membrane contamination.
Sub-critical flux is defined as the existence of a flux above which the TMP increases significantly, and a flux below which the TMP remains stable. This concept can help us to find a reference point between maximising membrane flux and effective control of membrane contamination. In practice, membrane modules are referred to as super-critical flux operation when the operating flux is above the critical flux and sub-critical flux operation when the operating flux is below the critical flux. In practice, an appropriate operating flux must be selected. This operating flux value is in the sub-critical range and sometimes the operating flux is only about 50% of the critical flux. Of course, membrane contamination in a long-running MBR, even with sub-critical flux operation mode, results in a gradual increase in TMP.
In MBR, the purpose of aeration is not only to supply oxygen to the microorganisms, but also to keep the membrane flux stable by allowing the rising bubbles and the resulting disturbing water flow to clean the membrane surface and stop sludge accumulation. At the same time, the jittering effect of the bubbles and membrane fibres colliding with each other can even cause the membrane fibres to rub against each other, accelerating the shedding of membrane deposits and facilitating the mitigation of membrane pollution. When the aeration is too large, it will lead to a reduction in the particle size of the membrane surface deposits, making the structure of the filter cake more dense, thus increasing the membrane filtration resistance; on the contrary, when the aeration is too small, the disturbance will weaken and the pollution will increase, so the appropriate aeration should be chosen.