Analysis of the Impact of MBR Process Microorganisms on Membrane Fouling

Introduction

  A membrane bioreactor (MBR) is an advanced wastewater treatment process that combines efficient membrane separation technology with the activated sludge method. MBRs boast high biological degradation efficiency, low sludge production, a small footprint, strong nitrification capabilities, and consistently stable effluent quality. Additionally, in an MBR, various soluble and hardly degradable organic compounds from the raw wastewater, along with metabolic byproducts produced by microorganisms, are effectively retained within the bioreactor, further enhancing the purity of the treated water. MBR systems represent a cutting-edge, highly efficient biological treatment technology, particularly notable for their significant potential in wastewater resource recovery and reclaimed water reuse—areas where they have garnered widespread attention both domestically and internationally. However, challenges such as membrane fouling and the associated costs of replacing membrane components have led to substantially higher operational expenses, thereby hindering the broader adoption and implementation of this innovative technology.

Factors affecting membrane fouling

  After MBR operation for a period, the membrane components become clogged as microorganisms grow on the inner membrane surface and fouling layers accumulate on the outer side. This leads to a gradual decline in membrane flux—eventually resulting in no water production at all. Membrane fouling not only shortens the membrane's lifespan but also increases the pump's suction head and elevates aeration requirements, making it the primary driver of high energy consumption in MBR systems. Therefore, investigating the mechanisms behind membrane fouling during MBR operation and implementing targeted improvements to the system are crucial steps toward effectively reducing and controlling fouling. Such efforts are vital for maintaining optimal MBR performance, accurately estimating operational costs, and guiding the scalable design of MBR processes—making them highly relevant to both practical applications and long-term sustainability.

   2.1 Causes of Membrane Fouling

  Currently, research on membrane bioreactors has primarily focused on membrane fouling. Relevant studies indicate that the accumulation of foulants occurs in two distinct stages: (1) Initial fouling: Concentration polarization leads to a decline in initial membrane flux, while soluble substances in the mixed liquor cause solute buildup on the retention side of the membrane, resulting in the formation of a thin, low-permeability layer on the membrane surface. (2) Long-term fouling: Solute adsorption and particle deposition elevate the concentration of solutes at the membrane surface, promoting the development of a gel layer that eventually forms on the membrane surface. Meanwhile, colloidal particles migrate toward the membrane surface, leading to further deposition and ultimately reducing both hydraulic permeability and membrane flux.

  Currently, there are various theories regarding the mechanisms behind membrane fouling. However, when it comes to the factors influencing membrane fouling, they can generally be categorized into the following key areas: microbial characteristics, operational conditions, and the structural properties of the membrane. This article primarily explores how microbial properties affect membrane fouling.

   2.2 The Impact of Microbial Properties on Membrane Fouling

  During the operation of a membrane bioreactor, membrane fouling substances originate from the mixed sludge liquor, which primarily consists of microbial communities and their metabolic byproducts, as well as organic macromolecules, small molecules, dissolved compounds, and solid particles present in the wastewater. These substances significantly impact membrane permeability. Due to the membrane's retention effect, the reactor maintains a high sludge concentration and extended sludge retention time. As the sludge concentration increases, activated sludge becomes more prone to depositing on the membrane surface, accelerating the rate of membrane fouling. This, in turn, elevates membrane filtration resistance and ultimately reduces membrane permeability. Additionally, higher sludge concentrations lead to increased sludge viscosity, further diminishing membrane flux. Moreover, excessively high sludge concentrations can also elevate wastewater viscosity, impairing oxygen transfer efficiency. Therefore, it is crucial to tightly control the sludge concentration within the reactor, as both excessively high and low levels can negatively affect the quality of the treated effluent.

   2.2.2 The Impact of Extracellular Polymeric Substances on Membrane Fouling

  Extracellular polymeric substances (EPS) significantly influence the sludge's flocculation, settling, dewatering, and heavy-metal adsorption properties in sludge treatment systems. EPS are polymeric compounds secreted by microorganisms during their metabolic processes under specific environmental conditions, forming a protective layer outside the microbial cell walls. These substances primarily consist of proteins and polysaccharides (3). EPS not only accumulate within membrane bioreactors but also tend to build up on the membrane surface, leading to increased mixed-liquor viscosity and enhanced membrane filtration resistance. On the membrane surface, EPS directly alter the porosity and structure of the fouling layer, as they co-deposit with fine particles and adsorb onto the membrane, ultimately forming a highly adhesive gel layer. For instance, Huang et al. found that for every 50 mg/L increase in EPS concentration, membrane flux decreased by 70% (4). Moreover, there is a linear relationship between EPS content and membrane fouling across different biological states of activated sludge. Importantly, the ratio of proteins to carbohydrates in EPS varies depending on the sludge type, which in turn affects ultrafiltration performance: as the protein proportion rises, membrane flux tends to decline. Both excessively high and excessively low levels of EPS can exacerbate membrane fouling; thus, there exists an optimal EPS concentration that maximizes sludge filtration efficiency.

   2.2.3 Impact of Dissolved Organic Matter Products on Membrane Fouling

  In recent years, the contribution of soluble microbial products (SMP)—comprising substances such as humic acids, polysaccharides, and proteins—to membrane fouling has received increasing attention. SMP primarily arises during microbial substrate decomposition and endogenous respiration processes, with high-molecular-weight components that are poorly biodegradable. As a result, these substances tend to accumulate in membrane bioreactors. SMP is highly prone to clogging membrane pores and readily deposits on the membrane surface, forming a gel-like layer. Moreover, excessive accumulation of SMP can not only compromise the stability of the effluent quality but also negatively impact sludge activity, leading to severe membrane fouling. In membrane bioreactors, SMP mainly consists of extracellular polymeric substances (EPS) released through microbial metabolism and cell lysis—or what’s referred to as soluble EPS—with a typical molecular weight distribution ranging from 1,000 to 100,000 daltons. Studies have shown a significant positive correlation between EPS and SMP: as EPS concentrations rise, SMP levels increase dramatically. Thus, EPS emerges as the key factor driving the accumulation of SMP within the reactor.

   2.2.4 The Impact of Filamentous Microorganisms on Membrane Fouling

  Filamentous bacteria significantly influence membrane fouling as well. Both an excessive or insufficient presence of filamentous bacteria within sludge flocs can negatively impact the operation of MBR systems. This is because the density of filamentous bacteria profoundly affects the properties of activated sludge—properties that, in turn, are critical determinants of membrane flux. If sludge flocs contain very few filamentous structures, their size remains small, leading to severe clogging of membrane pores. Conversely, if the flocs harbor an overabundance of filaments, they can form an impermeable sludge cake layer on the membrane surface. Moreover, an excess of filamentous bacteria often triggers the rapid proliferation of extracellular polymeric substances (EPS), resulting in low ZETA potential and enhanced hydrophobicity—both of which contribute to membrane fouling. Filamentous bacteria are closely linked to the formation, structure, and attachment mechanisms of membrane surface contaminants; they can adhere to or even penetrate membrane materials, thereby strengthening the adhesion of pollutants to the membrane surface. Additionally, the three-dimensional, network-like architecture of filamentous bacteria plays a crucial role in shaping the structure and composition of membrane-bound contaminants, severely compromising the treatment efficiency of membrane bioreactors as well as the effectiveness of membrane cleaning processes. Thus, we can conclude that maintaining an optimal level of filamentous bacteria attached to sludge flocs actually promotes membrane permeability.

Conclusion

  The primary factor causing membrane fouling is sludge—both excessively high or too low sludge concentrations can lead to membrane contamination. In the operation of membrane bioreactors, there are a set of optimal values, such as the economically viable aeration intensity, the ideal aeration volume, and the critical sludge concentration. Operating under these optimal conditions allows the membrane bioreactor to achieve the best treatment performance while simultaneously minimizing membrane fouling, extending both the cleaning cycle and the lifespan of the membranes. This, in turn, reduces energy consumption and lowers operational costs. However, it’s important to note that in practical operation, it’s often challenging to maintain all these key parameters at their ideal levels simultaneously.

  Research on membrane fouling from a microbiological perspective is gradually becoming a growing research hotspot. Extracellular polymeric substances (EPS) and soluble microbial products (SMP) significantly influence membrane fouling, and their impact can be managed by adjusting key operational parameters such as solid retention time (SRT), hydraulic retention time (HRT), and organic loading rates—thereby helping to mitigate membrane contamination. Both an excessive or insufficient presence of filamentous bacteria can lead to membrane fouling, triggering the formation of sludge cake layers and clogging of membrane pores. Moreover, an overabundance of filamentous bacteria often causes sludge bulking, which in turn promotes the rapid proliferation of EPS, further exacerbating membrane fouling.

  Future research should place greater emphasis on studying membrane fouling control, aiming to prevent and mitigate membrane contamination based on a thorough understanding of the underlying fouling mechanisms—and ultimately reducing the costs of membrane bioreactors to the greatest extent possible.

Translated from China Membrane Technology Network