MBR membrane fouling mechanisms and control factors

　MBR has been widely and maturely applied in wastewater treatment because it replaces the secondary sedimentation tank, ensuring effluent SS and high sludge concentration, saving wastewater operators a lot of trouble in operation. However, membrane fouling has always plagued the development and operation of MBR!

　　1. Definition of membrane fouling
　　Membrane fouling usually refers to the adsorption and aggregation of substances in the mixed liquor on the membrane surface (external) and inside the membrane pores (internal), causing pore blockage and reducing porosity, resulting in a decrease in membrane flux and an increase in filtration pressure.
　　In membrane filtration operation, water molecules and small substances continuously pass through the membrane, while some substances are retained by the membrane and block the membrane pores or deposit on the membrane surface, causing membrane fouling. It can be said that membrane retention leads to membrane fouling. The direct manifestation of membrane fouling is the decrease in membrane flux or the increase in operating pressure.  
　　Nutrients, microbial flocs, microbial cells, cell debris, microbial metabolites (EPS, SMP), and various organic and inorganic soluble substances in the activated sludge mixed liquor system all contribute to membrane fouling.

　　The development of membrane fouling can usually be divided into 3 stages (there is also a 2-stage statement):
　　(1) Initial fouling: Occurs in the initial stage of membrane system operation, where the membrane surface interacts strongly with colloids and organic matter in the mixed liquor. Fouling methods include adhesion, charge interaction, and pore blockage. Under cross-flow filtration conditions, small bio-flocs or extracellular polymers can still adhere to the membrane surface, while substances smaller than the pore size will be adsorbed in the membrane pores, causing membrane fouling through concentration, crystallization, precipitation, and growth and reproduction.
　　(2) Slow fouling: The membrane surface is initially smooth, and large particles are not easily attached. It is mainly caused by EPS, SMP, and biological colloids, which are viscous substances that adsorb to the membrane surface through adsorption bridges and network trapping, forming a gel layer, causing a slow increase in membrane filtration resistance and enhancing the retention performance of pollutants in the mixed liquor. Gel layer fouling is unavoidable, and its effect is a slow increase in membrane resistance. In constant flow operation, it is manifested as a slow increase in TMP, and in constant pressure mode, it is manifested as a slow decrease in flux.
　　(3) Rapid fouling: Under the continuous filtration pressure difference and permeate flow, the gel layer formed in the second stage gradually becomes denser with the deposition of pollutants, causing membrane fouling to change from quantitative change to qualitative change. The flocs in the mixed liquor rapidly aggregate on the membrane surface and form a sludge filter cake, causing a rapid increase in transmembrane pressure difference.
　　Gel layer fouling is unavoidable, and its effect is a slow increase in membrane resistance. In constant flow operation, it is manifested as a slow increase in TMP, and in constant pressure mode, it is manifested as a slow decrease in flux. Once a large amount of sludge flocs deposit on the membrane surface to form a mud cake layer, the system is basically unable to operate normally. The main precautions in the MBR operation process are to slow down gel layer fouling (maintain good hydraulic conditions, in-situ cleaning, control the development rate of membrane fouling, and extend the operation time of slow fouling), and control mud cake layer fouling (rapid fouling).

2. Types of membrane fouling
　　(1) Classification by pollutant composition
Organic fouling
　　Mainly comes from macromolecular organic matter in the mixed liquor (polysaccharides, proteins, etc.), humic acids, microbial flocs, cell debris, etc. Among them, although the proportion of soluble organic matter SMP and EPS is very low compared to MLSS, the membrane fouling caused by them accounts for 26%-52%. Microbial growth and adsorption in the membrane pores and on the membrane surface are also important factors in membrane fouling.

　　Inorganic fouling  
　　Formed by the bridging action of metal salts and inorganic salt ions. Common inorganic fouling of membranes is mainly scaling substances of carbonates, sulfates, and silicates of 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 fouling (temporary fouling): Membrane fouling can be removed through certain hydraulic measures; for example, it can be removed by water backwashing and aeration shaking.
　　Irreversible fouling (long-term fouling): Membrane fouling that cannot be removed by hydraulic cleaning measures can be removed by cleaning with oxidants, acids, alkalis, and reducing agents.
　　Both reversible and irreversible fouling can be washed out. Any cleaning method that cannot wash it out is called irreversible fouling.

(3) Classification by the location of pollutants
　　The materials in the mixed liquor adsorbed, concentrated, crystallized, and aggregated in the membrane pores are called internal fouling; the aggregation and deposition on the membrane surface are called external fouling.

3. Influencing factors and control of membrane fouling
　　The main factors for the formation of membrane fouling are: membrane inherent properties, mixed liquor properties, and system operating environment. The control and solution of membrane fouling should also take corresponding measures from these three aspects.

　　(1) Inherent properties of the membrane
　　The physical and chemical properties of the membrane are determined by the membrane material, and the anti-fouling ability of the membrane in the mixed liquor is related to its material. Studies have shown that the hydrophilicity of the membrane has a very important impact on the anti-fouling ability. In 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 hydrophilized during application. Due to the differences in the modification process, the loss of hydrophilicity during use varies.
　　In addition, the anti-fouling ability of the membrane is also related to the membrane surface roughness, membrane surface charge, and membrane pore size. In general, the anti-fouling ability of the membrane can be improved by selecting a more hydrophilic membrane material, improving the surface roughness of the membrane, selecting a membrane material with the same potential as the mixed liquor, and selecting a suitable membrane pore size.  
　　Inorganic membranes such as ceramic membranes: made from materials such as aluminum oxide, silicon carbide, titanium oxide, and zirconium oxide, and sintered at high temperatures. They have significant advantages over organic membranes in terms of flux, strength, and chemical stability.

　　(2) Properties of the mixed liquor
　　Membrane fouling is largely the result of the interaction between the membrane and the mixed liquor. The properties of the mixed liquor include sludge concentration and viscosity, particle distribution, dissolved organic matter concentration, and microbial metabolite concentration.
　　When the sludge concentration is low, the sludge's adsorption and degradation capacity for organic matter is insufficient, resulting in increased organic matter concentration in the mixed liquor, severe membrane pore blockage, and concentration polarization, which significantly increases the solute concentration on the membrane surface and easily forms a gel layer, leading to increased filtration resistance; when the sludge concentration is above a certain value, EPC concentration increases, and sludge viscosity increases rapidly. Viscosity affects both membrane flux and the size of bubbles in the mixed liquor. Sludge easily deposits on the membrane surface, forming a thicker sludge layer. It is generally believed that there is a critical value for sludge concentration. When the sludge concentration is higher than this value, it will have an adverse effect on membrane flux. Therefore, the sludge concentration can be controlled within an appropriate range to effectively control membrane fouling. Sludge expansion and sludge fragmentation can cause serious membrane fouling.
　　Also The influent water quality of the MBR process also has a significant impact on the components of the mixed liquor and requires a certain degree of pretreatment, such as: Hair and garbage will wrap around the membrane, causing sludge accumulation in the membrane module and thus leading to membrane fouling. Different fine membrane screens need to be used to remove them before entering aerobic biodegradation; sand and other hard particles may damage the membrane fibers and need to be removed using a sedimentation tank; oils cause irreversible pollution to the membrane fibers, and if they exceed the requirements, they need to be removed through oil separation and flotation; inorganic substances: may precipitate and scale on the membrane surface, blocking the membrane pores. This can be controlled by flocculation and sedimentation or by adjusting the pH to prevent precipitation. Other characteristic pollutants that affect the membrane, such as organic solvents, surfactants, antifoaming agents, PAM, hardness, alkalinity, and temperature, should be paid special attention to in specific situations.

　　(3) System operating environment
　　Subcritical flux  
　　The critical flux is defined as a flux such that when the flux is greater than this value, TMP increases significantly; while when the flux is less than this value, TMP remains stable. 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, operation with a flux higher than the critical flux is called supercritical flux operation, and operation with a flux lower than the critical flux is called subcritical flux operation. In practical applications, the appropriate operating flux must be selected. This operating flux value is in the subcritical range, and sometimes the operating flux is only about 50% of the critical flux. Of course, even with subcritical flux operation, TMP gradually increases in long-term MBR operation due to membrane fouling.

　　Reasonable aeration  
　　In MBR, the purpose of aeration is not only to supply oxygen to microorganisms but also to allow rising bubbles and the resulting turbulent water flow to clean the membrane surface and prevent sludge accumulation to maintain stable membrane flux. At the same time, the shaking action produced by the collision of bubbles with membrane fibers, even causing friction between membrane fibers, can accelerate the shedding of deposits on the membrane surface, which is beneficial for alleviating membrane fouling. Excessive aeration can lead to a decrease in the particle size of the particles deposited on the membrane surface, making the filter cake structure denser and thus increasing the membrane filtration resistance; conversely, insufficient aeration weakens the disturbance, and pollution will worsen, so the appropriate aeration rate should be selected.

　　Alternating operation and shutdown  
　　According to the three-stage theory of membrane fouling, the formation of fouling on the membrane surface requires a process. First, pollutants will adsorb, deposit, and accumulate on the membrane surface. The intermittent suction operation mode aims to periodically stop membrane filtration to allow the sludge deposited on the membrane surface to fall off under the shear force caused by aeration and water flow, restoring the membrane's filtration performance. Generally, the longer the suction time, the greater the degree of accumulation of suspended solids on the membrane surface; the longer the stop time, the more thorough the shedding of sludge deposited on the membrane surface, and the more the membrane filtration performance can be restored. In principle, the alternating operation and shutdown method that meets its own characteristics should be determined based on the recommendations of the membrane manufacturer and the actual operation of the project.