Why is MBR membrane prone to scaling? How to handle it?

Why is it so easy to scale on MBR membranes that they have to be removed and washed every month, and online backwashing is useless?

MBR has been widely and maturely applied in wastewater treatment, as it replaces the secondary sedimentation tank and can ensure effluent SS and high sludge concentration, saving many sewage engineers some troubles in operation. However, membrane fouling has always been a problem that troubles the development and operation of MBR! 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 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 (outside) and inside the membrane pores (inside), causing membrane pore blockage and reducing porosity, resulting in a decrease in membrane flux and an increase in filtration pressure.

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

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

The development of membrane fouling can usually be divided into three stages (there are also two stages):

(1) Initial pollution: It occurs during the initial operation of the membrane system, where strong interactions occur between the membrane surface and colloids, organic matter, etc. in the mixed solution. The modes of pollution include adhesion, charge interaction, membrane pore blockage, etc. Under cross flow filtration conditions, small biological flocs or extracellular polymers can still adhere to the membrane surface, while substances smaller than the membrane pore size will adsorb in the membrane pores, causing membrane fouling through concentration, crystallization precipitation, and growth and reproduction.

(2) Slow pollution: at the initial stage, the membrane surface is smooth, and large particles are not easy to attach. Viscous substances such as EPS, SMP, and biocolloids are mainly adsorbed on the membrane surface through adsorption bridges, net traps, etc. to form a gel layer, causing a slow rise in membrane filtration resistance, which will enhance the retention performance of pollutants in the mixed solution. The pollution of gel layer is inevitable, and the effect is the slow rise of membrane resistance. In constant current operation, it shows a slow increase in TMP, and in constant pressure mode, it shows a slow decay in flux.

(3) Rapid pollution: the gel layer formed in the second stage is gradually dense with the deposition of pollutants under the effect of continuous filtration pressure difference and permeable flow, leading to the membrane pollution from quantitative change to qualitative change. The flocs in the mixed solution quickly gather on the membrane surface and form sludge cake, and the transmembrane pressure difference rises rapidly.

The pollution of gel layer is inevitable, and the effect is the slow rise of membrane resistance. In constant current operation, it shows a slow increase in TMP, and in constant pressure mode, it shows a slow decay in flux. Once a large amount of sludge flocs deposit on the membrane surface and form a mud cake layer, the system is basically unable to operate normally. The main precautions in the MBR operation and maintenance process are to delay the pollution of the gel layer (maintain good hydraulic conditions, clean in situ, control the development rate of membrane pollution, and extend the operation time of slow pollution), and control the pollution of the mud cake layer (rapid pollution).

Types of membrane fouling

(1) Classification by pollutant composition

Organic pollution:

Mainly derived from macromolecular organic compounds (polysaccharides, proteins, etc.), humic acids, microbial flocs, cell debris, etc. in the mixed liquid. Although soluble organic compounds SMP and EPS account for a very low proportion of MLSS, they account for 26% -52% of membrane fouling caused by them. The growth and adsorption of microorganisms within membrane pores and on membrane surfaces are also important factors in membrane fouling.

Inorganic pollution:

Formed by the bridging effect of metal salts and inorganic salt ions. The common inorganic pollutants in membranes are mainly carbonate, sulfate, and silicate scaling substances such as calcium, magnesium, iron, and silicon, among which calcium carbonate, calcium sulfate, and magnesium hydroxide are more common.

(2) Classification by the nature of pollutants

Reversible pollution (temporary pollution): Membrane pollution can be removed through certain hydraulic measures; It can be removed through water backwashing and aeration shaking.

Irreversible pollution (long-term pollution): Membrane pollution that cannot be removed by hydraulic cleaning measures can be removed by cleaning with oxidants, acids, alkalis, reducing agents, etc.

Both reversible and irreversible can be washed out. What cannot be washed out by any cleaning method is called irreparable pollution.

(3) Classification by the location of pollutants

The material in the mixed solution that adsorbs, concentrates, crystallizes, and aggregates within the membrane pores is called internal pollution; The accumulation and deposition on the surface of the membrane form external contamination.

Factors affecting membrane fouling

1. Characteristics of sludge mixture

The source of membrane fouling substances in membrane bioreactors is activated sludge mixture, which causes extremely complex membrane fouling.

(1) EPS and SMP

Extracellular polymers (EPS) and soluble microbial products (SMP) are both microbial metabolites with roughly the same composition, and they have important and complex impacts on membrane fouling, making them the main pollutants in the MBR process.

If the concentration of EPS is too high, it will increase the viscosity of the mixed solution and hinder the diffusion of dissolved oxygen, making it difficult for the sludge system to oxygenate, thereby affecting the normal physiological activity of microbial communities and increasing membrane filtration resistance. If the EPS content is too low, it can cause the decomposition of flocs, which is unfavorable for the operation of MBR.

Therefore, there exists an optimal EPS value that stabilizes the floc structure and does not cause a high trend of membrane fouling.

Research has found that most SMPs have molecular weights less than 1KDa and greater than 10KDa. Small molecular weight dissolved organic matter can easily block 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, the trend of membrane fouling by SMP in MBR weakens with the increase of MLSS, the decrease of organic loading, and the increase of dissolved oxygen.

(2) Mixed liquid suspended solid concentration MLSS

The concentration of MLSS directly affects the viscosity of the mixed solution, and 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 wash away the solid attached to the membrane surface, it will quickly cause the formation of a fouling layer.

(3) Viscosity

The viscosity of the mixed liquid is affected by MLSS. 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 mixture affects the size of bubbles and the flexibility of the fiber membrane in the reactor. In addition, an increase in viscosity will lead to a decrease in the efficiency of dissolved oxygen (DO) transfer, and low dissolved oxygen concentration will exacerbate the trend of membrane fouling.

(4) Hydrophilicity of sludge

Many research results indicate that hydrophilic dissolved organic compounds in sludge have a negative impact on membrane fouling. However, studies have also 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 serious membrane fouling.

(5) Sludge particle size

The decrease in membrane flux is mainly caused by particles around 2um. Generally speaking, the smaller the particle size, the easier it is for the particles to deposit on the membrane surface, resulting in a denser deposition layer and lower permeability. Therefore, if the particle size is too small, it will exacerbate membrane fouling.

(6) Sludge settling index (SVI)

Although it does not have a direct impact on membrane fouling, the sludge settling index (SVI) can reflect the settling properties of organic matter in the mixed liquid.

Organic substances that cannot be settled at present, such as colloids and dissolved organic matter, are widely considered as the main pollutants of membranes.

2. Operating conditions of MBR process

The operating conditions directly or indirectly affect membrane fouling and the properties and composition of sludge.

(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, 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 bioreactors in general urban sewage treatment is 5-20 days.

(2) Hydraulic retention time (HRT)

Although HRT does not have a direct impact on membrane fouling, a short HRT will provide more nutrients to microorganisms, leading to rapid growth, increased MLSS concentration, and increased 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. Outside the range, the 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 as the temperature increases, the maximum allowable pH value decreases.

(4) Dissolved oxygen (DO)

Low concentrations of dissolved oxygen can reduce cell hydrophobicity and cause sludge floc decomposition. When DO is below 1mg/l, the SMP content increases sharply. Dissolved oxygen can also affect the composition of EPS and SMP. In high dissolved oxygen MBR systems, the ratio of proteins and 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.

Balancing the selection of flux with minimizing membrane area, backwashing, and chemical cleaning time intervals 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 rapidly changing membrane permeability.

In high concentration and small pore membrane systems, increasing CFV can alleviate the deposition of pollutants on the membrane surface.

However, in cases where the particle size of the mixed liquid is relatively large, 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. Play a stirring role, suspend the sludge, 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, reducing the deposition of pollutants on the membrane surface and to some extent preventing membrane fouling.

3. The properties of membranes and the structure of membrane components

(1) The pore size of the membrane

Small pore membranes are prone to trapping pollutants in the solution, creating a deposition layer on the surface of the membrane, which increases membrane resistance. This type of pollution generally belongs to reversible pollution, which can be removed through physical methods such as cross flow, backwashing, and aeration, and internal pollution is relatively small.
Large pore membranes
 have severe
pore blockage
in
 the
early stages of
filtration, and as the surface dynamic membrane forms, the interception effect begins to increase. However, dirt is prone to sedimentation and blockage on the surface and inside of membrane pores, forming irreversible or even irreversible pollution, which becomes the main factor causing membrane performance degradation and reduced lifespan during long-term operation.
(2)
Regarding
 the
fouling of different membrane materials in anaerobic MBR,
 the
study
 found that
under
 the
same operating conditions,
 the
fouling 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) The
increase
 in membrane surface roughness increases
 the
possibility of adsorbing pollutants
 on the
membrane surface, but
 at the
same time
, it
also increases the degree of membrane surface deflection, hindering the deposition of pollutants on the membrane surface. Therefore, the influence of roughness on membrane flux is the result of a combination of two factors.

(4) Hydrophilicity

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 prone to adsorbing soluble substances on the membrane surface, exhibiting a greater tendency towards fouling.

At present, most ways to change the hydrophobicity of membranes are by modifying the membrane material. If the pore size is changed, the roughness of the membrane surface is increased, and inorganic materials are added to form a dynamic pre coating on the membrane surface.

4. Control measures for membrane fouling

The main factors leading to membrane fouling include inherent membrane properties, mixed liquid properties, and system operating environment. 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 a 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. In 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 during application. Due to differences in modification processes, the loss of hydrophilicity during use varies in speed.

In addition, the anti fouling ability of the membrane is also related to the surface roughness, surface charge, and pore size of the membrane. 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 from raw materials such as alumina, silicon carbide, titanium oxide, zirconia, etc., and sintered at high temperatures. They have significant advantages over organic membranes in terms of flux, strength, and chemical stability.

(2) The properties of the mixture

Membrane fouling is largely the result of the interaction between the membrane and the mixture. The properties of the mixture include sludge concentration and viscosity, particle distribution, dissolved organic matter concentration, microbial metabolite concentration, etc.

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 exceeds 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 sedimentation 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 effect on membrane flux. Therefore, it is possible to choose to control sludge concentration within an appropriate range to effectively control membrane fouling. Sludge bulking and sludge fragmentation can easily cause serious membrane fouling.

The inlet water quality of the MBR process also has a significant impact on the composition of the mixed liquid, requiring a certain degree of pre-treatment. For example, the entanglement pattern of hair waste substances can cause mud accumulation in the membrane components, leading to membrane fouling. Different fine film grilles need to be used to remove them before entering aerobic biochemistry; High hardness particles such as mud and sand may damage the membrane fibers and require the use of a sedimentation tank for removal; Oil causes uncleanable pollution to the membrane fibers, and exceeding the requirements requires removal through oil separation, air flotation, etc; Inorganic substances: may precipitate and scale on the surface of the membrane, blocking membrane pores. It can be controlled by flocculation precipitation or pH adjustment 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 components, operating flux above the critical flux is called supercritical flux operation, and operating flux below the critical flux is called subcritical flux operation. In practical applications, it is necessary to choose the appropriate operating flux. This operating flux value is in the subcritical range, sometimes the operating flux is only about 50% of the critical flux. Of course, membrane fouling in long-term MBR, even with the use of subcritical flux operation mode, gradually increases its TMP.

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. 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 fouling. When the aeration is too high, it will lead to a decrease in the particle size of the deposited particles on the membrane surface, making the structure of the filter cake more compact, thereby increasing the membrane filtration resistance; On the contrary, when the aeration rate is too small, disturbance weakens and pollution worsens, 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 aggregate on the surface of the membrane. The intermittent suction operation mode aims to periodically stop membrane filtration, so that the sludge deposited on the membrane surface can detach from the membrane surface under the shear force caused by aeration and water flow, restoring the filtration performance of the membrane. 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. In principle, the operation and shutdown alternation mode that meets its own characteristics should be determined based on the recommendations of the membrane manufacturer and the actual operation of the project.

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