By: Kate
Email:kate@aquasust.com
Date: 20th December 2024
Misunderstanding 1: The higher the MBR membrane flux design, the better.
Correct answer: The flux of the membrane is determined by the membrane material and structure. For a specific membrane treatment unit, its porosity and flux have an upper limit. Filtration is essentially a physical process. The membrane material must sacrifice the throughput while reducing the pore size, while also taking into account the strength and ensuring long-term operation, which results in the flux being a fixed value. Therefore, during the design and application process, the design should be carried out according to the parameters recommended by the membrane manufacturer.
Misunderstanding 2: When using MBR, the effluent quality must be guaranteed. Correct answer: The MBR process is essentially a combination of activated sludge method and membrane filtration. It replaces the secondary sedimentation tank with membrane filtration on the basis of the traditional activated sludge method, which brings advantages such as good interception and high sludge concentration, but this does not mean that the MBR process can cure all diseases. For the removal of pollutants, membrane filtration has a very obvious effect on SS, while the removal of organic matter still depends on the biodegradability of activated sludge and organic matter. For sewage with poor biodegradability or sewage that has been fully biochemically treated in the previous stage, MBR has limited effect.
Misunderstanding 3: MBR process does not require sludge discharge. Correct answer: Due to the excellent interception effect of membrane filtration, the activated sludge in the MBR process can reach a higher concentration, realizing the separation of hydraulic retention time and sludge age, and the influence of sludge concentration on effluent can be ignored. However, this advantage only increases the space for biochemical development. Not affecting the effluent cannot be used as a reason to support not discharging sludge. The question of whether to discharge sludge still has to go back to activated sludge. If the sludge is not discharged, the activated sludge will age, which will affect the biochemical performance and aeration. The MBR process in the project can indeed reduce sludge discharge, but it does not mean that it will not be discharged.
Misunderstanding 4: To avoid membrane blockage, the sludge concentration should be reduced as much as possible.
Correct answer: Membrane blockage is a common problem in MBR applications. It is the sludge that blocks the membrane, especially when the sludge concentration is too high. Therefore, there is a misunderstanding that the sludge concentration should be reduced to avoid membrane blockage, which is exactly another misunderstanding. Sludge with too low or too high concentrations will quickly block the membrane. The correct approach is to keep the concentration within a suitable range. In addition, aeration has the effect of flushing the membrane surface, and it should also be kept at a corresponding retention amount.
Misunderstanding 5: The membrane is responsible for the effluent.
Correct answer: The principle of the membrane itself is filtration, which is a physical process. The removal of pollutants is mainly SS. Its significance to MBR is to improve the biochemical performance, but it cannot filter soluble organic matter, so it cannot be responsible for indicators such as COD. The removal of organic matter still depends on the design and operation management of the entire process chain.
Misunderstanding 6: MBR is only suitable for domestic sewage
Correct answer: MBR process is a combination of activated sludge method and membrane filtration, which is derived from the combination of traditional activated sludge method and secondary sedimentation tank. In this sense, the MBR process is applicable to the scene using activated sludge method. However, for some sewage that is easy to cause membrane blockage, pretreatment must meet the requirements.
Misunderstanding 7: Flux
Misunderstanding: The improvement of membrane flux in design and operation enhances the competitiveness of the technology. Fact: In the past 10 years, the improvement of membrane flux in design and operation has been very limited, and the price of membrane components has fallen. It is a fact.
Misunderstanding8: Energy consumption
Misunderstanding: The energy consumption of MBR is less than 1 kWh/ton of water. Fact: For large sewage treatment plants, it can be achieved through exquisite design and superb optimized operation, but it is not for small sewage treatment plants. And it is even more impossible for sewage treatment plants that are not fully loaded. Analysis: MBR technology cannot be viewed one-sidedly. No matter what process it is, as long as it is not fully loaded, any technology will cause high operating energy consumption. If it is expected that the system cannot be operated at full capacity, a modular design can be adopted to operate the modules effectively according to different water volumes, which can reduce the operating costs. From a technical point of view, MBR technology is essentially more suitable for modular design.
Misunderstanding 9: Process competitiveness
Misunderstanding: MBR will replace the traditional activated sludge process Fact: Unless there is a revolutionary breakthrough in technology, it is impossible. The MBR process is suitable for medium-sized sewage treatment plants (5000-100.000PE), and the purpose is to require reuse or reduce the floor space. In fact, the so-called medium-sized sewage plants abroad can only be regarded as small sewage plants in China, with a scale of about 1,000 tons/day to 20,000 tons/day.
Analysis: No technology can completely subvert other technologies, but it is undeniable that MBR replacing the secondary sedimentation tank does bring the best effluent water quality, providing a feasible solution for my country under the current pollution environment. As for what water volume MBR is suitable for, what needs to be evaluated is the balance between advantages and disadvantages.
Misunderstanding 10: Decentralized sewage treatment
Misunderstanding: MBR is a good solution to decentralized sewage treatment
Fact: MBR technology is not economical for a population equivalent of less than 5,000 (about 1,000 tons/day).
Analysis: What is the most critical difficulty in the development of decentralized sewage treatment in my country? It is not an economic issue, but an operational issue. Can we say that artificial wetlands are not suitable for rural domestic sewage treatment in the southern region? Definitely not, this kind of ecosystem is definitely fine. I do not think that MBR technology is the only way to solve decentralized sewage. Good effluent water quality, fast startup capability and unattended power operation all play a role in solving local pollution and reducing human operation capabilities.
Misunderstanding 11: Impact on process performance
Misunderstanding: Membranes can improve process performance Fact: In addition to good performance in disinfection performance and turbidity, other performance has nothing to do with membranes. Moreover, the use of membranes has stringent requirements on the water volume changes in the system. The flexibility of operation must have a large enough space, otherwise the membrane will be difficult to cope with the changes in water volume.
Analysis: The so-called MBR technology is the effective combination of microbial technology and membrane separation technology. In essence, it is microbial technology and membrane separation technology separately. It is difficult to reflect its effectiveness when they operate independently. For example, the earliest MBR technology: the former MBR technology of aerobic + secondary sedimentation + ultrafiltration, its main function is to have good performance on disinfection performance and turbidity. The popular built-in or external MBR technology now undoubtedly combines microbial technology with membrane separation technology to achieve the effect of 1+1>2.
Misunderstanding 12: Disinfection
Misunderstanding: Ultrafiltration disinfection performance is better than microfiltration disinfection performance Fact: This is not the case for bacteria. Ultrafiltration and microfiltration can achieve 6Log removal.
Analysis: The pore size of the filter membrane used in most MBR technologies is basically between 0.05-0.4 microns, between ultrafiltration and microfiltration. The interception effect on microorganisms is obvious.
Misunderstanding 13: Trace organic pollutants
Misunderstanding: MBR can better remove trace organic pollutants. Fact: Under the same operating parameters, there is no significant difference between MBR and traditional activated sludge process in this regard.
Analysis: The retention of macromolecular organic matter by MBR technology and the degradation effect of sufficient residence time are difficult to match by activated sludge.
Misunderstanding 14: Sludge production
Misunderstanding: The sludge production of MBR process is small. Fact: Due to the retention of SS by the membrane, the sludge production of MBR process is higher than that of traditional activated sludge process. Low sludge production requires a sufficiently long sludge age, at which time the investment and operating costs will increase significantly. Analysis: The retention of SS by the membrane has no direct relationship with the sludge production. The microorganisms in the MBR system are in the endogenous respiration stage, and the low sludge production is an obvious fact.
Misunderstanding 15: Membrane fouling
Misunderstanding: Polysaccharides and proteins are the manifestations of membrane fouling. Fact: There is no consistent conclusion in this research. These parameters are only indirect representations, reflecting the flocculation characteristics of organisms. Analysis: Membrane pollution is not only related to the biochemical system. If the membrane performance is not good, no matter how good the biochemical system is, it will be of no help.