Analysis of the Influence of MBR Process Microorganisms on Membrane Fouling

1 Introduction

Membrane bioreactor is a sewage treatment process that combines high-efficiency membrane separation technology with activated sludge process. MBR has the characteristics of high biodegradation efficiency and low sludge yield, small footprint, strong nitrification ability, and stable effluent quality. In addition, in the membrane bioreactor, various soluble and refractory organic substances in the raw sewage, as well as metabolites produced by microorganisms, can be respectively retained in the bioreactor, thereby improving the quality of the effluent. MBR system is a new type of high-efficiency biological treatment technology, especially it has great potential in waste water recycling and reclaimed water reuse, and has received widespread attention at home and abroad. However, due to problems such as membrane fouling and the impact of membrane module replacement, its operating costs and expenses have been greatly increased, thus hindering the widespread promotion and application of this technology.

2 Influencing factors of membrane fouling

After the MBR runs for a period of time, with the growth of microorganisms on the inner surface of the membrane and the attachment of the fouling layer on the outer surface of the membrane, the membrane module will be blocked by pollutants, and the membrane flux will gradually decrease until no more water is produced. The problem of membrane fouling shortens the service life of the membrane, which leads to the increase of the suction head of the pump and the increase of the aeration volume, which is the main reason for the high energy consumption of the MBR. Therefore, it is important to study the mechanism of membrane fouling during MBR operation and improve the MBR system to effectively reduce and control membrane fouling. Practical significance.

2.1 Causes of membrane fouling

At present, research on membrane bioreactors is mainly focused on membrane fouling. Relevant studies have shown that the accumulation process of membrane fouling substances is divided into two steps: (1) Initial fouling: the initial membrane flux decreases due to concentration polarization, and the mixing The dissolved substances in the solution cause the accumulation of solutes on the retention side of the membrane, resulting in a surface layer of the membrane surface with low permeability. (2) Long-term pollution: due to solute adsorption and particle deposition, the solute concentration on the membrane surface is high, resulting in the formation of a gel layer on the membrane surface, and the colloidal particles migrate to the membrane surface, thereby forming deposition, reducing hydraulic permeability and membrane flux. .

At present, there are different opinions on the formation mechanism of membrane fouling. However, the factors that affect membrane fouling mainly include the following aspects: microbial characteristics, operating conditions, and structural properties of the membrane. This paper mainly discusses the influence of microbial properties on membrane fouling.

2.2 Effect of microbial properties on membrane fouling

During the operation of the membrane bioreactor, the membrane fouling substances come from the sludge mixture, and its components mainly include microbial flora and their metabolites, organic macromolecules, small molecules, dissolved substances and solid particles in the wastewater. The above substances have a great influence on the membrane flux. Due to the interception effect of the membrane, the sludge in the reactor has a relatively high sludge concentration and sludge residence time. As the sludge concentration increases, the activated sludge is more likely to deposit on the membrane surface, thereby accelerating the membrane fouling rate, resulting in an increase in the membrane filtration resistance, thereby reducing the membrane flux; at the same time, the sludge concentration increases, and the sludge viscosity also decreases. As it increases, the membrane flux will also decrease; moreover, if the sludge concentration in the reactor is too high, it will also cause the viscosity of sewage to increase, which will affect the mass transfer efficiency of oxygen. Therefore, it is necessary to strictly control the sludge concentration in the reactor. If the sludge concentration is too high or too low, it will have an adverse effect on the quality of the effluent.

2.2.2 Effect of extracellular polymer on membrane fouling

Extracellular polymer (EPS) has a great influence on the flocculation performance, sedimentation performance, dehydration performance and heavy metal adsorption performance of sludge in the sludge treatment system. Extracellular polymers are polymeric compounds that are secreted by microorganisms and surrounded by the cell wall of microorganisms during their metabolism under certain environmental conditions. The main substances are proteins and polysaccharides (3). Extracellular polymers can not only accumulate in the membrane bioreactor, but also accumulate on the surface of the membrane, which will cause the increase of the mixed liquid viscosity and membrane filtration resistance. The extracellular polymer on the surface of the membrane directly changes the porosity and structure of the deposited layer. The extracellular polymer and fine particles are deposited and adsorbed on the membrane surface together to form a highly cohesive gel layer. Huangd et al. found that the extracellular For every 50 mg/L increase in polymer concentration, the membrane flux decreases by 70% (4); in activated sludge in various biological states, there is a linear relationship between extracellular polymer content and membrane fouling, and the extracellular polymer content The ratio of protein and sugar is different, the membrane flux of ultrafiltration is also different, and the membrane flux decreases with the increase of protein ratio. Excessively high or too low extracellular polymers will aggravate membrane fouling, so there is an optimal concentration of extracellular polymers to optimize sludge filtration performance.

2.2.3 Effect of dissolved organic matter products on membrane fouling

The contribution of dissolved organic matter products (SMPs) to membrane fouling has received increasing attention in recent years. Soluble microbial products with humus, polysaccharides, proteins and other substances as the main components are mainly produced in the matrix decomposition process and endogenous respiration process of microorganisms. The content of high molecular substances is high and the biodegradability is poor. Therefore, in membrane organisms Buildup will occur in the reactor. Soluble microbial products can easily block membrane pores and easily deposit on the surface of the membrane to form a gel layer; excessive accumulation of soluble microbial products may not only reduce the water quality stability of membrane filtered water, but may also affect sludge activity, causing Severe membrane fouling. The SMP in the membrane bioreactor is mainly composed of EPS and other substances released by microbial metabolism and cell crushing, or soluble EPS, and its molecular weight distribution is generally around 1,000 to 100,000. Studies have shown that EPS and There was a significant positive correlation between SMP and SMP increased sharply with the increase of EPS concentration. Therefore, EPS is the decisive factor causing the accumulation of SMP in the reactor.

2.2.4 Effect of filamentous bacteria on membrane fouling

Filamentous bacteria also have a great impact on membrane fouling. Too much or too little filamentous bacteria in sludge flocs can have adverse effects on the operation of the MBR system. This is because the density of filamentous bacteria has a great influence on the properties of activated sludge, and these properties are the fundamental factors affecting the membrane flux. If a very small amount of filaments are attached to the sludge flocs, their volume will be small, which will cause serious membrane pore blockage; and if there are many filaments attached to these flocs, an impermeable barrier will be formed on the membrane surface. Mud cake layer. Excessive filamentous bacteria can easily lead to a large number of extracellular polymers (EPS), resulting in low zeta potential and strong hydrophobicity, thus causing membrane fouling. Filamentous bacteria are closely related to the formation process, structure, and attachment form of pollutants on the membrane surface. Filamentous bacteria increase the adhesion strength of pollutants on the membrane surface in fixed forms such as adhesion and penetration of membrane materials. Filamentous bacteria in the form of a three-dimensional network structure play an important role in the formation of pollutants on the membrane surface and the structure of pollutants, which seriously affect the treatment capacity of membrane bioreactors and the efficiency of membrane cleaning. From this, we know that when the appropriate amount of filamentous bacteria is attached to the sludge floc, it is beneficial to the permeability of the membrane.

3. Conclusion

The main factor causing membrane fouling is sludge, too much or too little sludge will cause membrane fouling. In the operation of membrane bioreactor, there are a series of optimal values, such as economic aeration intensity, economic aeration rate, critical sludge concentration, etc. In the state of this series of optimal values, the membrane bioreactor can achieve the best treatment effect, and can reduce the degree of membrane fouling, prolong the cleaning cycle of the membrane and the replacement time of the membrane, thereby reducing the cost of the membrane bioreactor. energy consumption, reducing costs. But it should be pointed out that in the actual operation process, it is difficult to make the above various operating parameters in the best state.

Research on membrane fouling from the perspective of microorganisms will gradually become a research hotspot. Extracellular polymers (EPS) and soluble microbial products (SMP) have an important impact on membrane fouling, which can be adjusted by adjusting the solid residence time. (SRT), hydraulic retention time (HRT), and load to control microbial characteristics, thereby alleviating membrane fouling. Too much or too little filamentous bacteria will pollute the membrane, causing the formation of mud cake layer and clogging of membrane pores; at the same time, too much filamentous bacteria will cause sludge bulking, which will cause a large number of extracellular polymers to multiply, and will also damage the membrane pores. influence of membrane fouling.

In future research, more attention should be paid to the research on membrane fouling control, and the prevention and control of membrane fouling should be carried out on the basis of understanding the fouling mechanism, so as to reduce the cost of membrane bioreactor to the greatest extent.

Transferred from China Membrane Technology Network