Since the emergence of MBR technology, it has been widely used worldwide due to its small footprint, good effluent quality, high organic load rate, and low sludge production, especially in urban sewage treatment. However, due to the need for effective control of membrane fouling during operation, it is necessary to adopt measures such as increasing the cross flow rate and aeration, which consumes a large amount of energy during MBR operation. So, what should MBR operators do to address these issues? Only by quickly identifying the root cause of membrane fouling and providing precise strikes can we reduce the frequency of cleaning. How does membrane fouling occur?
Strictly speaking, membrane fouling refers to the phenomenon of membrane surface coverage and pore blockage caused by adsorption or deposition of particles, colloidal particles, or solute macromolecules that process materials during operation due to physical, chemical, or mechanical interactions with the membrane.
The phenomenon of membrane fouling is very complex, including multiple mechanisms. Among them, concentration polarization is the main reason for the formation of the filter cake layer on the surface, and the main sedimentary particles include suspended solids, colloids, and microbial communities. Organic and inorganic pollution refers to the pollution caused by the adsorption of organic and inorganic substances on membrane surfaces and pores. Biological pollution is the biofilm produced by the attachment and growth of microbial communities on the surface of a membrane. The scaling phenomenon occurs when the concentration of salt dissolved on the membrane surface exceeds its solubility, and is not the main cause of membrane fouling. Membrane pollution is generally used to summarize all the phenomena that cause the decline of membrane permeation flow. According to different cleaning methods, membrane pollution can be divided into: 1. Reversible pollution that causes the decline of flux in a short time due to concentration polarization, membrane pore pollution and the formation of gel layer. Pollution that can be quickly removed by surface cleaning methods such as backwashing, aeration, cross flow, etc., generally refers to short-term pollution. 2. The irreversible pollution caused by the long-term interaction between material particles and membrane materials cannot be removed by physical cleaning methods, but can be restored through chemical cleaning to restore flux pollution, generally referring to long-term pollution. 3. Pollution that cannot be removed by any cleaning method during long-term operation is called irreversible pollution.
What factors affect membrane fouling?
1、 The source of membrane pollutants in a membrane bioreactor is the activated sludge mixture, and the fouling of the membrane by the sludge mixture is extremely complex.
1. EPS and SMP
Extracellular polymeric substances (EPS) and soluble microbial products (SMP) are both microbial metabolites with roughly the same composition. They have important and complex effects on membrane fouling and are the main pollutants in MBR processes. Excessive EPS concentration can increase the viscosity of the mixed solution, which is not conducive to the diffusion of dissolved oxygen, making it difficult to oxygenate the sludge system and affecting the normal physiological activity of bacterial flocs, thereby increasing membrane filtration resistance. However, if the EPS content is too low, it can cause the decomposition of flocs, which is detrimental to the operation of MBR. Therefore, there exists an optimal EPS value that stabilizes the flocculent structure and does not cause a high tendency towards membrane fouling. Research has found that the majority of SMP molecules with molecular weights less than 1kDa and greater than 10KDa, small molecular weight dissolved organic matter, can easily clog membrane pores while passing through the membrane, causing membrane fouling and becoming the main residual organic matter in the effluent. Meanwhile, the characteristics and composition of SMP are also influenced by multiple operating parameters. Generally speaking, COD in the water circle believes that the fouling trend of SMP on the membrane in MBR weakens with the increase of MLSS, the decrease of organic matter loading, and the increase of dissolved oxygen.
2. The concentration of suspended solids in the mixed solution, MLSSMLSS concentration, directly affects the viscosity of the mixed solution. The increase in viscosity is the main reason for the decrease in filtration performance of the mixed solution caused by the increase in MLSS. If the cross flow rate or aeration intensity is not sufficient to flush out the solids attached to the membrane surface, it will quickly cause the formation of a fouling layer.
3. The viscosity of a mixture is affected by MLSS, and when the MLSS concentration is higher than the critical value, the viscosity increases exponentially with the increase of solid concentration. In hollow fiber MBR, the viscosity of the mixed solution affects the size of bubbles and the flexibility of the fiber membrane in the reactor. In addition, an increase in viscosity will decrease the efficiency of dissolved oxygen (DO) transfer, and a low dissolved oxygen concentration will exacerbate the trend of membrane fouling.
4. Many studies have shown that hydrophilic dissolved organic matter in sludge has a negative effect on membrane fouling. However, some studies have found that highly hydrophobic flocculent sludge can also cause membrane fouling. The hydrophobicity and surface charge of sludge are related to the composition and properties of extracellular polymers, as well as the growth index of filamentous bacteria. Excessive proliferation of filamentous bacteria can produce a large amount, leading to a decrease in potential, irregular shape of flocculent sludge, enhanced hydrophobicity, and severe membrane fouling.
5. The decrease in membrane flux due to the size of sludge particles is mainly caused by particles around 2um. Generally speaking, the smaller the particle size, the easier it is for particles to deposit on the membrane surface, resulting in a denser sediment layer and lower permeability. Therefore, a smaller particle size can exacerbate membrane fouling.
6. Although the sludge settling index (SVI) does not directly affect membrane fouling, it can reflect the settling properties of organic matter in the mixed solution. Currently, organic substances that cannot settle, such as colloids and dissolved organic matter, are widely regarded as the main pollutants in membranes.
2、 Operating conditions for MBR process
The operating conditions directly or indirectly affect membrane fouling and the properties and composition of sludge.
7. The actual results of sludge retention time (SRT) show that increasing SRT can reduce the production of SMP and EPS, and the membrane fouling rate will also decrease accordingly. However, excessively long SRT can lead to high sludge concentration, high viscosity, and affect mass transfer and reactor fluid dynamics, resulting in more severe membrane fouling. The SRT of membrane bioreactor in general urban sewage treatment is 5-20 days.
8. Although hydraulic retention time (HRT) has no direct effect on membrane fouling, short HRT provides more nutrients to microorganisms, leading to rapid growth and increasing MLSS concentration and flux, thereby increasing the likelihood of membrane fouling.
9. Comparing temperature and pH in different seasons, it is not difficult to find that reversible pollution is more severe during the low temperature period, and irreversible pollution develops more rapidly during the high temperature period. The pH range for MBR operation is generally 6-9. Beyond this range, the number of nitrifying bacteria in the reactor will rapidly decrease, leading to inhibition of nitrification. When the pH value is higher than its critical value, membrane fouling occurs rapidly, and when the temperature increases, the maximum allowable pH value will decrease.
10. Low concentrations of dissolved oxygen (DO) can reduce cell hydrophobicity and cause sludge floc decomposition. When DO is below 1mg/l, SMP content sharply increases. Dissolved oxygen can also affect the composition of EPS and SMP components. In high dissolved oxygen MBR systems, the ratio of protein to polysaccharide will also increase, and the composition of microbial communities will be very different.
11. For all membrane processes, an increase in membrane flux can lead to an exacerbation of membrane fouling. Balancing the selection of flux with minimizing membrane area, backwashing, and chemical cleaning time intervals directly affects operating costs.
12. Cross flow rate and aeration in a split membrane bioreactor, cross flow rate (CFV) is one of the methods to rapidly change membrane permeability. In systems with high concentration and small pore size membranes, an increase in CFV can alleviate the deposition of pollutants on the membrane surface. However, in the case of relatively large particles in the mixed liquid, the enhancement of CFV has no or even the opposite effect on the increase of flux. Aeration plays a very important role in the submerged MBR process: a、 Provide dissolved oxygen through aeration to facilitate the normal growth and metabolism of microorganisms in the sludge; b、 Plays a stirring role, suspending the sludge and thoroughly mixing it in the mixed solution; c、 Loosen the hollow fiber membrane module fibers and generate shear forces on the membrane surface, reducing the deposition of pollutants on the membrane surface and preventing membrane fouling to a certain extent. 3、 The properties of membranes and the structure of membrane components
13. The pore size of the membrane is small, and it is easy to intercept pollutants in the solution, resulting in a deposition layer on the membrane surface and increasing membrane resistance. This type of pollution generally belongs to reversible pollution and can be removed through physical methods such as cross flow, backwashing, aeration, etc. The internal pollution is relatively small. Large aperture membranes have severe pore blockage in the early stages of filtration, and as the surface dynamic membrane forms, the retention effect begins to increase. However, pollutants are prone to sedimentation and blockage on the surface and inside of membrane pores, forming irreversible and even irreversible pollution, which becomes the main factor causing membrane performance degradation and reduced lifespan during long-term operation. 14. The fouling trend of polyvinylidene fluoride (PVDF) membrane was significantly lower than that of polysulfone membrane (PS) and cellulose membrane under the same operating conditions, targeting the fouling situation of different membrane materials in anaerobic MBR. It is worth mentioning that when there are polymers similar to membrane materials in the organic components of activated sludge, the composition of irreversible pollutants depends on the membrane material. 15. The increase in membrane surface roughness increases the possibility of pollutant adsorption on the membrane surface, but at the same time, it also increases the flexibility of the membrane surface, hindering the deposition of pollutants on the membrane surface. Therefore, the effect of roughness on membrane flux is the result of the combined action of two factors. 16. The hydrophobicity of hydrophilic and hydrophobic membrane materials also has a significant impact on membrane fouling. Comparing hydrophobic ultrafiltration membranes with hydrophilic ultrafiltration membranes, it was found that hydrophobic ultrafiltration membranes are more likely to adsorb soluble substances on their membrane surface, exhibiting a greater tendency towards fouling. At present, the main way to change the hydrophobicity of membranes is to modify the membrane materials. Such as changing the pore size, membrane surface roughness, and adding inorganic materials to form dynamic pre coatings on the membrane surface.
How to control MBR membrane fouling?
1. At present, MBR process design generally adopts constant flux operation to control membrane fouling through other operational methods while meeting the requirements of sewage treatment capacity as much as possible. The concept of critical flux was first proposed in 1995. Its definition is that below this flux, the transmembrane pressure does not increase with the extension of filtration time, and there is a good linear relationship between transmembrane pressure and flux. The selection of critical flux plays an important role in operation. If the critical flux is exceeded, fouling will occur, and the transmembrane pressure difference (TMP) increases with the extension of filtration time. Membrane components operating below the critical flux can greatly delay membrane fouling.
2. Aeration rate and intensity
Generally speaking, enhancing aeration intensity is beneficial for improving membrane permeability and reducing membrane fouling. Under prolonged low aeration, pollutants quickly deposit on the membrane surface. However, strong aeration can also damage sludge flocs. It will change the size and distribution of sludge particles, release more colloids and dissolved organic matter (EPS and SMP), and exacerbate membrane fouling. Therefore, it is crucial to find the optimal aeration intensity for COD in the water circle. The effect of aeration intensity on membrane permeability is influenced by various factors, such as mixture concentration, mixture viscosity, and operating flux. Some scholars have proposed to plot the changes in aeration intensity, transmembrane pressure difference (TMP), and flux in one graph to find the optimal aeration intensity.
3. Selection of membrane fouling cleaning methods
The cleaning methods for membrane fouling mainly include physical cleaning and chemical cleaning. Physical cleaning includes intermittent operation and backwashing with clean water. During MBR operation, backwashing is an effective way to remove reversible fouling and delay membrane fouling. The effect of backwashing on delaying membrane fouling was studied in a pilot submerged MBR, and it was found that at the same flux, low-frequency high-intensity backwashing was more effective in reducing membrane fouling than high-frequency cleaning. It is worth mentioning that during intermittent operation, pollutants on the membrane surface loosen and fall off due to gravity, and the effect is more pronounced under aeration. Therefore, combining intermittent operation with backwashing can more effectively alleviate membrane fouling. Chemical cleaning and physical cleaning cannot prevent irreversible pollution from occurring, and flux must be restored through chemical cleaning methods. Chemical cleaning includes maintenance cleaning and strong (recovery) cleaning. The commonly used cleaning agents currently include NaClO solution for removing organic pollutants, citric acid for removing inorganic pollutants, etc. As is well known, MBR suppliers have their own strong cleaning formulas, with the main differences being the concentration of cleaning agents and cleaning methods. Taking MBR supplier Z company as an example,
4. The addition of adsorbents in MBR results in a very complex composition of the mixed solution. To improve the characteristics of the mixed liquid, the most commonly used method is to add powdered activated carbon PAC into the reactor to form biologically activated carbon. A certain experiment compared the effects of three different PAC dosages (0, 0.75, 1.5g/l) on membrane fouling in membrane bioreactors. The results showed that the fouling layer on the membrane surface decreased with the increase of PAC dosage, while irreversible fouling reached its lowest point at a dosage of 0.75g/l, and there was not much difference in irreversible fouling resistance between 0 and 1.5g/l. It is not difficult to find that activated carbon has its own adsorption capacity, and it will quickly reach adsorption saturation during operation, which not only cannot alleviate membrane fouling, but even exacerbates it. Therefore, during MBR operation, regular sludge discharge and the addition of fresh PAC are necessary to effectively improve MBR performance. At the same time, in order to save operating costs, it is possible to try using low dosing concentration PAC to improve the performance of MBR operation. The results show that under long SRT and high HRT operating conditions, low PAC dosage can improve operational efficiency and effectively save costs.
