By: Sunny Wu(Kate@aquasust.com)
Post Date: June.13th, 2022
Post Tags: Why the MBR membrane is easy to foul, and online backwashing is useless? What can we do?
MBR has been widely and maturely used in wastewater treatment because MBR replaces the secondary sedimentation tank, which can guarantee the effluent SS and high sludge concentration and save a lot of wastewater in operation some troubles, but the membrane contamination problem has also been troubling the development and operation of MBR! So in response to these problems, what exactly should MBR operators do to quickly find the root cause of membrane contamination and give precise strikes as a way to reduce cleaning frequency.
Table Content:
1. What is membrane contamination?
2. What are the types of membrane contamination?
3. The influence of factors of membrane contamination.
一,What is membrane contamination?
Membrane contamination usually refers to the process of adsorption and aggregation of substances in the mixture on the membrane surface (outside) and inside the membrane pores (inside), resulting in the blockage of membrane pores and the reduction of porosity, causing the decay of membrane flux and the increase of filtration pressure.
In the operation of membrane filtration, water molecules and fine materials continuously pass through the membrane, while some materials are retained by the membrane and block the membrane pores or deposited on the membrane surface, thus causing membrane contamination. It can be said that membrane contamination is caused by membrane retention. The direct manifestation of membrane contamination is the decrease of membrane flux or the increase of operating pressure.
Nutrient substrates, bacterial colloids, microbial cells, cell debris, microbial metabolites (EPS, SMP) and various organic and inorganic dissolved substances present in the activated sludge mixture system all contribute to membrane contamination.
The development of membrane contamination can usually be divided into 3 stages (there are also 2-stage statements).
(1) Initial contamination: It occurs at the initial stage when the membrane system is put into operation and the membrane surface interacts strongly with the colloids and organic matter in the mixture, and the contamination is in the form of adhesion, charge effect, and membrane pore blockage. Under the conditions of staggered flow filtration, fine bioflocs or extracellular polymers can still adhere to the membrane surface, while substances smaller than the membrane pore size will adsorb in the membrane pores and cause membrane contamination through the effects of concentration, crystallization precipitation and growth and reproduction.
(2) Slow contamination: Initially, the membrane surface is smooth, and large particles are not easily attached, mainly by EPS, SMP, biocolloids and other viscous substances adsorbed on the membrane surface through adsorption bridges, net trapping and other effects to form a gel layer, resulting in a slow rise in membrane filtration resistance, and the retention performance of pollutants in the mixture will be enhanced. The contamination of the gel layer is unavoidable and brings about the effect of a slow rise in membrane resistance. This is manifested as a slow rise in TMP in constant flow operation and a slow decay in flux in constant pressure mode.
(3) Rapid contamination: The gel layer formed in stage 2 is gradually dense with the deposition of contaminants under the action of continuous filtration pressure difference and permeable water flow, leading to the membrane contamination from quantitative to qualitative change, and the flocs in the mixture rapidly accumulate on the membrane surface and form sludge filter cake, and the trans-membrane pressure difference rises rapidly.
Contamination of the gel layer is unavoidable and brings about the effect of a slow rise in membrane resistance. This is manifested as a slow rise in TMP in constant flow operation and a slow decay in flux in constant pressure mode. Once a large amount of sludge floc is deposited on the membrane surface and a sludge cake layer is formed, the system is basically unable to operate normally. the main considerations for the MBR operation and maintenance process are to retard the gel layer contamination (maintain good hydraulic conditions, in-situ cleaning, control the rate of membrane contamination development, and extend the operation time of slow contamination) and to control the sludge cake layer contamination (rapid contamination).
二,What are the types of membrane contamination?
(1) Classification according to the composition of contaminants
a.Organic contamination
It mainly comes from macromolecular organic substances (polysaccharides, proteins, etc.), humic acids, microbial flocs, cell debris, etc. in the mixture. Among them, dissolved organic matter SMP and EPS account for 26%-52% of membrane contamination, although the proportion is very low for MLSS. Microbial growth and adsorption within the membrane pores and on the membrane surface are also important factors for membrane contamination.
b.Inorganic contamination
Formed by metal salts, inorganic salt ion bridging action. The common inorganic pollution of membrane is mainly carbonate, sulfate and silicate fouling substances of calcium, magnesium, iron and silicon, among which calcium carbonate, calcium sulfate and magnesium hydroxide are more.
(2) Classification according to the nature of pollutants
Reversible pollution (temporary pollution): can be removed through certain hydraulic measures to remove membrane pollution; such as backwashing through clean water, aeration shaking can be removed.
Irreversible pollution (long-term pollution): can not be removed by hydraulic cleaning measures to remove membrane pollution, can be removed by cleaning with oxidizers, acids, alkalis, reducing agents, etc.
Reversible and irreversible, both can be washed out. Any cleaning means can not be washed out is called irrecoverable pollution.
(3) Classification according to the location of contaminants
The material in the mixture is adsorbed, concentrated and crystallized in the membrane pore, and the formation of internal pollution is called internal pollution; the formation of aggregation and deposition on the membrane surface is called external pollution.
三,The influence of factors of membrane contamination
1. sludge mixture characteristics
The source of membrane contaminants in membrane bioreactor is activated sludge mixture, and the pollution of the membrane by sludge mixture is extremely complicated.
1)EPS and SMP
Extracellular polymer (EPS) and dissolved microbial products (SMP) are both microbial metabolites with roughly the same composition, and they have an important and complex impact on membrane contamination and are the most important pollutants in the MBR process.
Too high EPS concentration will increase the viscosity of the mixture which is not conducive to the diffusion of dissolved oxygen, making it difficult to oxygenate the sludge system, thus affecting the normal physiological activities of the bacterial colloid, and thus increasing the membrane filtration resistance. While too low EPS content will cause floc decomposition, which will be detrimental to the operation of MBR.
Therefore, there exists an optimal EPS value that makes the floc structure stable and does not cause a high tendency of membrane contamination.
It was found that most SMPs have molecular weights less than 1 KDa and greater than 10 KDa, and dissolved organic matter of small molecular weight, while passing through the membrane, tends to clog the membrane pores, causing membrane contamination and becoming the main residual organic matter in the effluent.
Meanwhile, the characteristics and composition of SMP are also affected by several operating parameters.
In general, the tendency of SMP contamination to the membrane in MBR decreases with increasing MLSS, decreasing organic loading, and increasing dissolved oxygen.
2)MLSS concentration of suspended solids in mixed liquor
MLSS concentration directly affects the viscosity of the mixture, viscosity rise is the main reason for the decline in filtration performance of the mixture caused by the rise in MLSS, if the wrong flow rate or aeration strength is not enough to flush out the solids attached to the surface of the membrane, will soon cause the generation of pollution layer.
3) Viscosity
The viscosity of mixed liquor is influenced by MLSS. When the MLSS concentration is higher than the critical value, the viscosity increases exponentially with the increase of solids concentration.
In hollow fiber MBR, the viscosity of the mixture affects the bubble size and the flexibility of the fiber membrane in the reactor. In addition, increased viscosity decreases the DO transfer efficiency of dissolved oxygen, and low dissolved oxygen concentration increases the tendency of membrane contamination.
4) Sludge hydrophilicity and hydrophobicity
The results of many studies have shown that hydrophilic dissolved organic matter in sludge plays a negative role in the occurrence of membrane contamination. However, it has also been found that highly hydrophobic flocculated sludge can also cause membrane contamination.
Both the hydrophobicity and surface charge of sludge are related to the composition and nature of extracellular polymers and the growth index of filamentous bacteria. The overgrowth of filamentous bacteria generates a large amount, which decreases the electrical potential, the irregular shape of flocculated sludge, and the increase of hydrophobicity, leading to severe membrane contamination.
5) Sludge particle size
The decrease of membrane flux is mainly caused by the particles around 2um. Generally speaking, the smaller the particle size, the easier the particles are deposited on the membrane surface, and the more dense the deposition layer formed, the less permeability, so the small particle size will aggravate the membrane pollution.
6) Sludge sedimentation index SVI
Although there is no direct effect on membrane contamination, the sludge settling index (SVI) can reflect the settling of organic substances in the mixture.
At present, organic substances that cannot be settled, such as colloids, dissolved organic matter, are generally considered to be the main contaminants of the membrane.
2,Operating conditions of MBR process
The operating conditions directly or indirectly affect the membrane contamination and the nature and composition of the sludge.
1) Sludge retention time (SRT)
Practical results show that increasing SRT can reduce the production of SMP and EPS, and the membrane contamination rate will be reduced.
However, an excessively long SRT can lead to high sludge concentration, which also brings about excessive viscosity and affects mass transfer and reactor hydrodynamics, leading to more serious membrane contamination. The SRT for membrane bioreactors in general municipal wastewater treatment is 5-20 days.
2) Hydraulic retention time (HRT)
Although HRT has no direct effect on membrane contamination, short HRT will provide more nutrients to microorganisms and make them grow rapidly, resulting in higher MLSS concentration and increased flux, which will increase the possibility of membrane contamination.
3) Temperature and pH
Comparing the temperature of different seasons, it is easy to find that reversible pollution is more serious in the low-temperature period and irreversible pollution develops more rapidly in the high-temperature period.
MBR operating pH range is generally 6-9, outside the range, the nitrifying bacteria in the reactor will be rapidly reduced, resulting in the inhibition of nitrification. When the pH is higher than its critical value, membrane contamination is rapid, and when the temperature rises, the maximum allowable pH decreases.
4) Dissolved oxygen (DO)
A low concentration of dissolved oxygen will reduce cell hydrophobicity and cause sludge floc decomposition, and when DO is lower than 1mg/l, SMP content rises sharply. Dissolved oxygen also affects the composition of EPS and SMP, and in high DO MBR systems, the ratio of proteins and polysaccharides increases and the microbial community composition is very different.
5) Membrane Fluxes
For all membrane processes, elevated fluxes can cause increased membrane contamination.
Balancing the choice of flux with minimizing membrane area, backwash and chemical cleaning intervals also have a direct impact on operating costs.
6) Staggered flow rate and aeration
In split membrane bioreactors, CFV is one of the methods to rapidly change membrane permeability.
In systems with high concentration and small pore size membranes, the increase of CFV can alleviate the deposition of contaminants on the membrane surface. However, for relatively large mixed liquor particulate matter, CFV enhancement has no or even opposite effect on flux elevation.
Aeration plays a very important role in the submerged MBR process: a, providing dissolved oxygen through aeration for normal growth and metabolism of microorganisms in the sludge; b, playing an agitating role to suspend the sludge and mix it fully in the mixed solution; c, loosening the membrane filaments of the hollow fiber membrane module and generating shear forces on the membrane surface to reduce the deposition of pollutants on the membrane surface and prevent the generation of membrane contamination to some extent.
3,The nature of the membrane and membrane component structure
1) Membrane pore size
Small pore size membrane, easy to retain the contaminants in the solution, and produce a deposited layer on the membrane surface, so that the membrane resistance increases. This type of pollution is generally reversible pollution, can be removed through the wrong flow, backwash, aeration, and other physical means, internal pollution is small.
Large pore size membrane, the membrane pore clogging is more serious in the early stage of filtration, with the formation of a dynamic membrane on the surface, the retention effect begins to improve. But pollutants are easily deposited and clogged on the surface and inside the membrane pore, forming irreversible pollution or even non-recoverable pollution, which becomes the main factor causing membrane performance degradation and life reduction in long-term operation.
2) Membrane Materials
For the contamination of different membrane materials in anaerobic MBR, the contamination trend of polyvinylidene fluoride (PVDF) membrane is significantly less than that of polysulfone (PS) and cellulose membranes under the same operating conditions.
It is worth mentioning that the composition of irreversible contaminants depends on the membrane material when polymers similar to the membrane material are present in the activated sludge organic fraction.
3) Degree of membrane surface roughness
The increase of membrane surface roughness increases the possibility of adsorption of contaminants on the membrane surface, but it also increases the degree of membrane surface deflection, which hinders the deposition of contaminants on the membrane surface, so the effect of roughness on membrane flux is the result of a combination of both factors.
4) Hydrophobicity
The hydrophobicity of membrane material also has an important influence on membrane contamination, comparing hydrophobic and hydrophilic ultrafiltration membranes, it is concluded that hydrophobic ultrafiltration membrane surface is more likely to adsorb dissolved substances and shows a greater tendency to contamination.
Currently, most of the ways to change membrane hydrophobicity are modifications of membrane materials. Such as changing the pore size, membrane surface roughness, adding inorganic materials to form a dynamic pre-coating on the membrane surface, etc.
4,Control measures of membrane contamination
The main factors for the formation of membrane contamination are: the inherent nature of the membrane, the nature of the mixture and the system operating environment, control and solving the membrane contamination should also take corresponding measures from these three aspects.
(1) The inherent nature of the membrane
The physical and chemical properties of the membrane are determined by the membrane material, and the anti-pollution ability of the membrane in the mixture is related to its material. It has been shown that the hydrophilicity of the membrane has a very important effect on the anti-pollution ability. Among the 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 difference in the modification process, the loss of hydrophilicity in the process of use will be fast and slow.
In addition, membrane anti-pollution ability is also related to membrane surface roughness, membrane surface charge, membrane pore size, etc. Generally speaking, the anti-pollution ability of membrane can be improved by choosing membrane materials with better hydrophilicity, improving the roughness of the membrane surface, choosing membrane materials with the same potential as the mixture, and suitable membrane pore size.
Inorganic membranes such as ceramic membranes: alumina, silicon carbide, titanium oxide, zirconium oxide, etc. as raw materials, high-temperature sintering, in the flux, strength, chemical stability convenience than organic membranes have obvious advantages.
(2) The nature of the mixed liquid
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, etc.
When the sludge concentration is low, the sludge adsorption and degradation ability of organic matter is insufficient, the concentration of organic matter in the mixture increases, the membrane pore blockage is serious, and the concentration of solute on the membrane surface is significantly increased due to the concentration of concentration polarization, which is easy to form a gel layer, resulting in increased filtration resistance; when the sludge concentration is higher than a certain value, the EPC concentration increases, the sludge viscosity grows rapidly, and the viscosity has an impact on the membrane flux and the size of bubbles in the mixture, and the sludge It is easy to deposit on the membrane surface and form a thicker sludge layer. It is generally believed that there is a critical value of sludge concentration, when the sludge concentration is higher than this value, the membrane flux will be adversely affected, so the sludge concentration can be chosen to control the membrane contamination effectively within a suitable range. Sludge expansion and sludge fines can cause serious membrane contamination.
The influent water quality of the MBR process also has a greater impact on the mixture components, which requires a certain degree of pretreatments, such as hair and garbage materials will wrap around the pattern, causing the membrane module to accumulate mud and thus leading to membrane contamination, which needs to be removed by different fine membrane grids before entering aerobic biochemical; mud and sand and other hard particles may damage the membrane filaments, which needs to be removed by the sand sink; oil causes uncleanable contamination to the membrane filaments. pollution, more than the requirements need to be removed by oil trap, air floatation, etc.; inorganic substances: may precipitate on the membrane surface, scaling, blocking the membrane pores. It can be controlled by flocculation and precipitation or pH adjustment to prevent it from precipitating. Other characteristic contaminants that have an impact on the membrane, like organic solvents, surfactants, defoamers, PAM, hardness, alkalinity, and temperature, should be paid special attention in specific cases.
(3) System operating environment
a.Sub-critical flux
The critical flux is defined as the existence of flux such that when the flux is greater than this value, the TMP increases significantly, while when the flux is less than this value, the TMP remains stable. This concept can help us to find a reference point between maximizing membrane flux and effective membrane contamination control. In the actual operation of membrane modules, the operating flux above the critical flux is referred to as super-critical flux operation, and the operating flux below the critical flux is referred to as sub-critical flux operation. In practice, the 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 the sub-critical flux operation mode, has a gradual increase in TMP.
b.Reasonable Aeration
In MBR, the purpose of aeration is not only to supply oxygen to microorganisms but also to make the rising bubbles and their generated disturbing water flow to clean the membrane surface and stop sludge aggregation to keep the membrane flux stable. At the same time, the jittering effect generated by the collision between bubbles and membrane fibers even makes the membrane fibers rub against each other, which can accelerate the shedding of membrane surface sediments and facilitate the mitigation of membrane pollution. When the aeration is too large, it will cause the particle size of the membrane surface deposit to decrease, making the structure of the filter cake denser, thus increasing the membrane filtration resistance; on the contrary, when the aeration is too small, the disturbance will be weakened and the pollution will be aggravated, so the appropriate aeration should be chosen.
c.Operation and stop alternation
According to the 3-stage theory of membrane contamination, the formation of contamination on the membrane surface requires a process. First, contaminants will adsorb, deposit and accumulate on the membrane surface. The operation mode of intermittent pumping aims to restore the membrane filtration performance by stopping the membrane filtration periodically so that the sludge deposited on the membrane surface can be dislodged from the membrane surface by the shear force caused by aeration and water flow. Generally, the longer the pumping time, the greater the accumulation of suspended solids on the membrane surface; the longer the stopping time, the more completely the sludge deposited on the membrane surface will fall off, and the more the membrane filtration performance can be recovered. In principle, the alternate operation and stopping method should be determined according to the membrane manufacturer's recommendation and the actual project operation to meet its own characteristics.
