By: Kate
Email:kate@aquasust.com
Date: 21th January 2025
1. Definition of membrane fouling
Membrane fouling usually refers to the process in which substances in the mixed liquid are adsorbed and aggregated on the membrane surface (external) and in the membrane pores (internal), causing the membrane pores to be blocked and the porosity to decrease, resulting in the attenuation of membrane flux and the increase of filtration pressure.
During the operation of membrane filtration, water molecules and fine substances continue to 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 interception leads to membrane fouling. The direct manifestation of membrane fouling is the decrease in membrane flux or the increase in operating pressure.
2. Types of membrane pollution
(1) Classification by pollutant composition
Organic pollution
Mainly comes from macromolecular organic matter (polysaccharides, proteins, etc.), humic acid, microbial flocs, cell fragments, etc. in the mixed liquid. Among them, although the proportion of dissolved organic matter SMP and EPS is very low compared to MLSS, the membrane pollution caused by them accounts for 26%-52%. The growth and adsorption of microorganisms in the membrane pores and on the membrane surface are also important factors in membrane pollution.
Inorganic pollution
Formed by metal salts and inorganic salt ion bridging. Common inorganic pollution of membranes is mainly scaling substances of carbonates, sulfates and silicates of calcium, magnesium, iron, silicon, etc., among which calcium carbonate, calcium sulfate and magnesium hydroxide are more.
(2) Classification by the nature of pollutants
Reversible pollution (temporary pollution): Membrane pollution can be removed by certain hydraulic measures; such as backwashing with clean water 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 pollution can be washed out. Pollution that cannot be washed out by any cleaning means is called irreversible pollution.
(3) Classification by location of pollutants
Internal pollution is formed by the adsorption, concentration, crystallization and aggregation of materials in the mixed liquid in the membrane pores; external pollution is formed by aggregation and deposition on the membrane surface.
3. Control measures for membrane pollution
The main factors for membrane pollution are: inherent properties of the membrane, properties of the mixed liquid and the system operating environment. Corresponding measures should be taken from these three aspects to control and solve membrane pollution.
(1) Inherent properties of the membrane
The physical and chemical properties of the membrane are determined by the membrane material. The anti-pollution ability of the membrane in the mixed liquid is related to its material. Studies have shown that the hydrophilicity of the membrane has a very important influence on the anti-pollution ability. Among 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 hydrophilic when used. Due to differences in the modification process, the loss of hydrophilicity during use has different speeds.
In addition, the anti-pollution ability of the membrane is also related to the surface roughness of the membrane, the surface charge of the membrane, the pore size of the membrane, etc. Generally speaking, the anti-pollution ability of the membrane can be improved by selecting a membrane material with better hydrophilicity, improving the roughness of the membrane surface, selecting a membrane material with the same potential as the mixed liquid and a suitable membrane pore size.
Inorganic membranes such as ceramic membranes: Made of alumina, silicon carbide, titanium oxide, zirconium oxide, etc., sintered at high temperature, they have obvious advantages over organic membranes in terms of flux, strength, and chemical stability.
(2) Properties of 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 ability to adsorb and degrade organic matter is insufficient. The organic matter concentration in the mixed liquor increases, the membrane pores are seriously blocked, and the concentration polarization causes the concentration of solutes on the membrane surface to increase significantly, which easily forms a gel layer and increases the 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 liquor. The sludge is easily deposited 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 the membrane flux. Therefore, the sludge concentration can be controlled within an appropriate range to effectively control membrane pollution. Sludge expansion and sludge fragmentation can easily cause serious membrane pollution.
In addition, the influent water quality of the MBR process also has a great influence on the components of the mixed liquor, and a certain degree of pretreatment is required. For example, hair and garbage will entangle the pattern, causing sludge accumulation in the membrane assembly and thus membrane pollution. Different fine membrane grids need to be used to remove them before entering the aerobic biochemical process; particles with high hardness such as mud and sand may damage the membrane fibers, and a sand settling tank is needed to remove them; oils cause pollution to the membrane fibers that cannot be cleaned, and if the requirements are exceeded, they need to be removed by oil separation, flotation, etc.; inorganic matter: may precipitate and scale on the membrane surface, blocking the membrane pores. It can be controlled by flocculation precipitation or pH adjustment to prevent precipitation. Other characteristic pollutants that affect the membrane, such as organic solvents, surfactants, defoamers, PAM, hardness, alkalinity, and temperature, should be paid special attention to in specific situations.
(3) System operating environment
Subcritical flux
The definition of critical flux is that there is such a flux that when the flux is greater than this value, TMP increases significantly; 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, when the operating flux is higher than the critical flux, it is called supercritical flux operation, and when the operating flux is less than the critical flux, it is called subcritical flux operation. In practical applications, a suitable 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, in a long-term MBR, even if the subcritical flux operation mode is adopted, the TMP of membrane fouling will gradually increase.
Reasonable aeration
In MBR, the purpose of aeration is not only to provide oxygen for microorganisms, but also to make the rising bubbles and the disturbed water flow they generate clean the membrane surface and prevent sludge aggregation, so as to maintain the stability of membrane flux. At the same time, the shaking effect caused by the collision between bubbles and membrane fibers even causes friction between membrane fibers, which can accelerate the shedding of membrane surface sediments and help alleviate membrane pollution. When aeration is too large, the particle size of particles deposited on the membrane surface will decrease, making the structure of the filter cake more compact, thereby increasing the membrane filtration resistance; on the contrary, when the aeration volume is too small, the disturbance will weaken and the pollution will increase, so it is necessary to choose a suitable aeration volume.
Alternation of operation and stop
According to the three-stage theory of membrane pollution, the formation of membrane surface pollution requires a process. First, pollutants will be adsorbed, deposited, and accumulated on the membrane surface. The intermittent suction operation mode aims to stop membrane filtration regularly so that the sludge deposited on the membrane surface can fall off from the membrane surface under the shear force caused by aeration and water flow, so that the filtration performance of the membrane can be restored. Generally, 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 deposited on the membrane surface falls off, and the more the membrane filtration performance can be restored. In principle, the operation and stop alternation method that meets its own characteristics should be determined based on the recommendations of the membrane manufacturer and the operation of the actual project.