Why is it so easy to scale on MBR membranes (membrane filtration)

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Why is it so easy to scale on the MBR membrane that it needs to be removed and washed in 1 to 2 months, and online backwashing is useless?

MBR has been widely and maturely applied in sewage treatment, as it replaces the secondary sedimentation tank, which can ensure effluent SS and high sludge concentration, saving many sewage engineers some troubles in operation. However, membrane pollution has always been a problem for the development and operation of MBR! So, what should MBR operators do to address these issues? To quickly identify the root cause of membrane fouling and reduce cleaning frequency.

Definition of membrane fouling

Membrane fouling usually refers to the process in which substances in the mixed solution adsorb and aggregate on the surface (external) and inside (internal) of the membrane, causing blockage of the membrane pores and reducing porosity, leading to a decrease in membrane flux and an increase in filtration pressure.

In the operation of membrane filtration, water molecules and small substances continuously penetrate the membrane, while some substances are intercepted by the membrane, blocking the membrane pores or depositing on the membrane surface, resulting in membrane fouling. It can be said that membrane interception caused membrane fouling. The direct manifestation of membrane fouling is a decrease in membrane flux or an increase in operating pressure.

The nutrient matrix, microbial micelles, microbial cells, cell fragments, microbial metabolites (EPS, SMP), and various organic and inorganic soluble substances present in the activated sludge mixture system all contribute to membrane fouling.

Operating conditions of MBR process

1. Sludge retention time (SRT)

The actual results indicate that increasing SRT can reduce the production of SMP and EPS, and the membrane fouling rate will also decrease accordingly.

However, excessive SRT can lead to high sludge concentration, high viscosity, and affect mass transfer and reactor hydrodynamics, leading to more severe membrane fouling. The SRT of membrane bioreactors in general urban sewage treatment is 5-20 days.

2. Hydraulic retention time (HRT)

Although HRT has no direct impact on membrane fouling, short HRT will provide more nutrients to microorganisms, leading to rapid growth of microorganisms, increasing MLSS concentration and flux, thereby increasing the possibility of membrane fouling.

3. Temperature and pH

Comparing the temperatures of 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 of MBR operation is generally 6-9, and outside the range, the number of nitrifying bacteria in the reactor will rapidly decrease, leading to inhibition of nitrification. When the pH value exceeds its critical value, membrane fouling occurs rapidly, while when the temperature increases, the maximum allowable pH value decreases.

4. Dissolved oxygen (DO)

Low concentration of dissolved oxygen can reduce cell hydrophobicity and cause the decomposition of sludge flocs. When DO is below 1mg/l, the 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 proteins to polysaccharides will also increase, and the composition of microbial communities will be very different.

5. Membrane flux

For all membrane processes, an increase in flux can exacerbate membrane fouling.

Achieving a balance between flux selection and minimizing membrane area, backwashing, and chemical cleaning time intervals also directly affects operating costs.

6. Cross flow rate and aeration

In a split membrane bioreactor, cross flow velocity (CFV) is one of the methods for quickly changing 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 mixed liquid particles, the enhancement of CFV has no or even opposite effect on the increase of flux.

Aeration plays a very important role in the submerged MBR process: a. providing dissolved oxygen through aeration for the normal growth and metabolism of microorganisms in the sludge; b. It plays a stirring role, causing the sludge to suspend and fully mix in the mixed solution; c. Make the hollow fiber membrane module membrane fibers loose and generate shear force on the membrane surface, reduce the deposition of pollutants on the membrane surface, and to some extent prevent the generation of membrane pollution.

The Properties of MBR Membranes and the Structure of Membrane Components

1. The pore size of the membrane

Small pore size membranes are prone to trapping pollutants in the solution, creating a deposition layer on the membrane surface and increasing membrane resistance. This type of pollution is generally reversible and can be removed through physical methods such as cross flow, backwashing, and aeration, with relatively small internal pollution.

Large pore membrane has severe pore blockage in the early stage of filtration, and as the surface dynamic membrane forms, the interception effect begins to increase. However, pollutants are prone to deposition and blockage on the surface and inside of membrane pores, forming irreversible or even irreversible pollution, becoming the main factor causing membrane performance degradation and reduced lifespan during long-term operation.

2. Membrane material

Regarding the pollution of different membrane materials in anaerobic MBR, the study found that under the same operating conditions, the pollution trend of polyvinylidene fluoride (PVDF) membrane was significantly lower than that of polysulfone membrane (PS) and cellulose membrane.

It is worth mentioning that when there are polymers in the organic components of activated sludge that are similar to the membrane material, the composition of irreversible pollutants depends on the membrane material.

3. Roughness of membrane surface

The increase in membrane surface roughness increases the possibility of adsorbing pollutants on the membrane surface, but it also increases the degree of membrane surface deflection, hindering the deposition of pollutants on the membrane surface. Therefore, the impact of roughness on membrane flux is the result of a combination of two factors.

4. Hydrophilicity and hydrophobicity

The hydrophobicity of membrane materials also has a significant impact on membrane fouling. A comparison was made between hydrophobic ultrafiltration membranes and hydrophilic ultrafiltration membranes, and it was found that hydrophobic ultrafiltration membranes are more likely to adsorb soluble substances on the membrane surface, exhibiting a greater tendency towards fouling.

At present, most ways to change the hydrophobicity of membranes are to modify the membrane materials. For example, changing the pore size, roughness of the membrane surface, and adding inorganic materials to form a dynamic pre coating on the membrane surface.

 Control measures for membrane fouling

The main factors contributing to membrane fouling include the inherent properties of the membrane, the properties of the mixture, and the operating environment of the system. Corresponding measures should also be taken to control and solve membrane fouling from these three aspects.

(1) The inherent properties of membranes

The physical and chemical properties of a membrane are determined by the membrane material, and the anti fouling ability of the membrane in the mixed solution is related to its material. Studies have shown that the hydrophilicity of membranes has a significant impact on their ability to resist pollution. Among organic membrane materials, some are hydrophilic materials such as PAN, while the majority are hydrophobic materials such as PVDF, PE, PS, etc. Hydrophobic organic materials must undergo hydrophilicity modification when applied. Due to differences in modification processes, the loss of hydrophilicity during use can vary in speed.

In addition, the anti fouling ability of a membrane is also related to its surface roughness, surface charge, and pore size. Generally speaking, membrane materials with better hydrophilicity can be selected to improve the surface roughness of the membrane, and membrane materials with the same potential as the mixed solution and appropriate pore sizes can be selected to improve the membrane's anti fouling ability.

Inorganic membranes, such as ceramic membranes, are made by sintering alumina, silicon carbide, titanium oxide, zirconia, and other raw materials at high temperatures. They have obvious advantages over organic membranes in terms of flux, strength, and chemical stability.

(2) Properties of the mixture

Membrane fouling is largely the result of the interaction between the membrane and the mixture, which includes properties such as sludge concentration and viscosity, particle distribution, dissolved organic matter concentration, and microbial metabolic product concentration.

When the sludge concentration is low, the adsorption and degradation capacity of the sludge to organic matters is insufficient, the concentration of organic matters in the mixed solution increases, and the membrane pores are severely blocked. The concentration of solutes on the membrane surface increases significantly due to concentration polarization, which is easy to form a gel layer, leading to increased filtration resistance; When the sludge concentration is higher than a certain value, the EPC concentration increases, and the sludge viscosity increases rapidly. The viscosity affects the membrane flux and the size of bubbles in the mixed solution. The sludge is prone to deposition on the membrane surface, forming a thicker sludge layer. It is generally believed that there is a critical value for sludge concentration, and when the sludge concentration exceeds this value, it will have an adverse impact on membrane flux. Therefore, it is possible to choose to control the sludge concentration within an appropriate range to effectively control membrane fouling. Sludge bulking and fine crushing can easily cause serious membrane fouling.

In addition, the influent water quality of the MBR process also has a significant impact on the composition of the mixed solution, requiring a certain degree of pre-treatment. For example, hair and garbage substances may entangle in a pattern, causing mud accumulation in the membrane components and resulting in membrane fouling. Different fine film grids need to be used to remove them before entering the aerobic biochemical process; Particles with higher hardness such as mud and sand may damage the membrane fibers and require the use of a grit chamber for removal; Oil causes uncontaminable pollution to the membrane filaments, which exceeds the requirements and needs to be removed through oil separation, air flotation, etc; Inorganic substances: May precipitate and scale on the membrane surface, blocking membrane pores. It can be controlled by flocculation precipitation or adjusting pH to prevent precipitation. Other characteristic pollutants that have an impact on the membrane, such as organic solvents, surfactants, defoamers, PAM, hardness, alkalinity, and temperature, should be given special attention in specific situations.

(3) System operating environment

Subcritical flux

The definition of critical flux is that there exists a flux above which TMP increases significantly; When the flux is less than this value, TMP remains stable and unchanged. This concept can help us find a reference point between maximizing membrane flux and effectively controlling membrane fouling. In the actual operation of membrane modules, when the operating flux is higher than the critical flux, it is called supercritical flux operation, and when the operating flux is lower than the critical flux, it is called subcritical flux operation. In practical applications, it is necessary to choose the appropriate operating flux. This operating flux value is within the subcritical range, and sometimes the operating flux is only about 50% of the critical flux. Of course, membrane fouling gradually increases its TMP in long-term MBR operation, even if subcritical flux operation mode is adopted.

Reasonable aeration

In MBR, the purpose of aeration is not only to provide oxygen to microorganisms, but also to clean the membrane surface and prevent sludge accumulation by the rising bubbles and their generated disturbed water flow, in order to maintain the stability of membrane flux. At the same time, the shaking effect caused by the collision between bubbles and membrane fibers can even cause mutual friction between membrane fibers, which can accelerate the detachment of sediment on the membrane surface and help alleviate membrane pollution. When the aeration is too large, it will lead to a decrease in the particle size deposited on the membrane surface, making the structure of the filter cake more dense, thereby increasing the membrane filtration resistance; On the contrary, if the aeration rate is too small, the disturbance will weaken and the pollution will worsen, so it is necessary to choose an appropriate aeration rate.

Operation stop alternation

According to the three-stage theory of membrane fouling, the formation of membrane surface fouling requires a process. Firstly, pollutants will adsorb, deposit, and accumulate on the membrane surface. The intermittent suction operation mode aims to stop membrane filtration regularly, so that the sludge deposited on the membrane surface falls off from the membrane surface under the shear force caused by aeration and water flow, and the filtration performance of the membrane can be restored. The longer the suction time, the greater the accumulation of suspended solids on the membrane surface; The longer the stop time, the more thorough the sludge deposition and detachment on the membrane surface, and the more the membrane filtration performance can be restored.