Why MBR Membranes Are Prone to Scaling and How to Handle It

Why do MBR membranes easily scale up, requiring cleaning every couple of months, even when online backwashing is ineffective?

MBR (Membrane Bioreactor) technology has been widely and successfully applied in wastewater treatment. By replacing the secondary sedimentation tank, MBR ensures high effluent quality and high sludge concentration, reducing many operational headaches for wastewater treatment personnel. However, membrane fouling has always been a challenge for the development and operation of MBR systems. So, how can MBR operators quickly identify the causes of membrane fouling and address them effectively to reduce cleaning frequency?

Definition of Membrane Fouling

Membrane fouling typically refers to the process in which substances in the mixed liquor accumulate and adsorb on the membrane surface (outside) and inside the membrane pores (inside). This leads to pore blockage, reduces porosity, causes membrane flux decline, and increases filtration pressure.

In membrane filtration operations, water molecules and small particles continuously pass through the membrane, while some substances are retained by the membrane, blocking the pores or depositing on the membrane surface, leading to fouling. Essentially, membrane fouling occurs as a result of membrane filtration. The direct manifestation of membrane fouling is the decrease in membrane flux or an increase in operating pressure.

The substances in the activated sludge mixed liquor, such as nutrients, microbial flocs, microbial cells, cell fragments, metabolic by-products (EPS, SMP), and various organic and inorganic dissolved substances, all contribute to membrane fouling.

Stages of Membrane Fouling

Membrane fouling generally develops in three stages (some classifications refer to two stages):

1. Initial Fouling: This occurs in the early stages of membrane system operation. Membrane surfaces interact strongly with colloids, organic matter, etc., in the mixed liquor, causing fouling through adhesion, charge effects, pore blockage, etc. In cross-flow filtration conditions, small biological flocs or extracellular polymers still adhere to the membrane surface, while substances smaller than the membrane pore size can adsorb in the pores. Through concentration, crystallization, and growth, fouling occurs.

2. Slow Fouling: Initially, the membrane surface is smooth, and large particles do not easily adhere. The primary fouling materials are EPS, SMP, and biological colloids, which adsorb on the membrane surface and form a gel-like layer. This increases filtration resistance slowly, enhancing the membrane’s ability to retain pollutants in the mixed liquor. Gel layer fouling is inevitable, causing a gradual rise in membrane resistance. In constant-flow operation, this appears as a slow increase in TMP (transmembrane pressure), while in constant-pressure mode, it results in a slow decline in flux.

3. Rapid Fouling: The gel layer formed in the second stage becomes more compact under continued filtration pressure and permeation flow, causing fouling to transition from gradual to drastic. Large quantities of flocs quickly accumulate on the membrane surface, forming a sludge cake, and cross-membrane pressure increases rapidly.

Gel layer fouling is inevitable and causes a gradual increase in membrane resistance. In constant-flow operation, this is evident as a gradual rise in TMP, while in constant-pressure mode, it manifests as a slow decline in flux. Once large amounts of sludge flocs deposit on the membrane surface, forming a sludge cake, the system can no longer operate normally.

The main focus during MBR operation and maintenance is to delay gel layer fouling (by maintaining good hydraulic conditions, in-situ cleaning, controlling the rate of membrane fouling development, and extending the slow fouling phase) and control sludge cake fouling (rapid fouling).

Types of Membrane Fouling

1. Based on the Composition of Fouling Materials

   • Organic Fouling: This primarily comes from macromolecular organic substances (such as polysaccharides, proteins), humic acids, microbial flocs, and cell fragments in the mixed liquor. While soluble microbial products (SMP) and extracellular polymeric substances (EPS) account for a small portion of MLSS (mixed liquor suspended solids), they contribute significantly to membrane fouling (26%-52%). The growth and adsorption of microorganisms inside the membrane pores and on the membrane surface also play a significant role in fouling.

   • Inorganic Fouling: This results from metal salts and inorganic ions that form bridges, such as calcium, magnesium, iron, and silica, resulting in scaling, particularly calcium carbonate, calcium sulfate, and magnesium hydroxide.

2. Based on the Nature of the Fouling

   • Reversible Fouling (Temporary Fouling): This type of fouling can be removed through certain hydraulic measures, such as backwashing with clean water or aeration.

   • Irreversible Fouling (Permanent Fouling): This fouling cannot be removed using hydraulic cleaning methods and requires cleaning with chemicals such as oxidants, acids, bases, or reducing agents.

   Both reversible and irreversible fouling can be cleaned, but irreversible fouling involves permanent damage to membrane performance.

3. Based on the Location of the Fouling

   • Internal Fouling: This occurs when materials in the mixed liquor accumulate, crystallize, and aggregate inside the membrane pores.

   • External Fouling: This happens when materials aggregate and deposit on the membrane surface.

Factors Affecting Membrane Fouling

1. Properties of the Mixed Liquor

   The source of fouling substances in MBR systems is the activated sludge mixed liquor, which has complex properties affecting fouling:

   • EPS and SMP: EPS and SMP are microbial metabolic by-products that play a critical and complex role in fouling. Excessive EPS increases the viscosity of the mixed liquor, making oxygen diffusion difficult and affecting microbial floc activity, which leads to increased filtration resistance. If EPS levels are too low, the flocs break down, negatively impacting MBR operation.

   • MLSS Concentration: The concentration of MLSS directly impacts the viscosity of the mixed liquor. As MLSS increases, the viscosity increases exponentially, which reduces filtration efficiency. If cross-flow velocity or aeration intensity is insufficient to wash off solids attached to the membrane surface, fouling will occur.

   • Viscosity: The viscosity of the mixed liquor is influenced by MLSS and has a direct effect on bubble size and membrane flexibility, as well as oxygen transfer efficiency. Higher viscosity results in a higher fouling tendency.

   • Hydrophobicity of Sludge: Studies suggest that the hydrophobicity of sludge plays a significant role in membrane fouling. High hydrophobicity can cause severe fouling, particularly when excess filamentous bacteria lead to irregular floc shapes.

   • Particle Size of Sludge: Smaller particles, around 2 microns, are more likely to deposit on the membrane surface, forming dense layers and increasing filtration resistance.

   • Sludge Settling Velocity (SVI): While SVI does not directly affect fouling, it can indicate the settling characteristics of the organic materials in the mixed liquor. Colloids and dissolved organics are major contributors to fouling.

2. MBR Operational Conditions

   • Sludge Retention Time (SRT): Increasing the SRT can reduce the production of SMP and EPS, thereby reducing fouling. However, excessively long SRT can lead to higher sludge concentrations, increasing viscosity and worsening fouling.

   • Hydraulic Retention Time (HRT): Although HRT does not directly affect fouling, shorter HRT provides more nutrients to microorganisms, leading to faster microbial growth, which can increase fouling potential.

   • Temperature and pH: Low temperatures cause reversible fouling, while high temperatures accelerate irreversible fouling. MBR typically operates within a pH range of 6-9; extreme pH values may inhibit nitrifying bacteria, causing fouling to increase.

   • Dissolved Oxygen (DO): Low DO levels increase the hydrophobicity of microbial cells, causing sludge floc breakdown. When DO levels fall below 1 mg/L, SMP levels rise sharply, exacerbating fouling.

   • Membrane Flux: Increased flux raises fouling potential in all membrane processes. Balancing flux, membrane area, and backwashing/chemical cleaning intervals is key to minimizing fouling.

3. Membrane Properties and Membrane Module Structure

   • Pore Size: Smaller pore membranes tend to retain more contaminants, creating denser fouling layers, which are harder to remove. Larger pore membranes may have higher initial blockage but form dynamic membranes that can improve filtration performance over time.

   • Membrane Material: Hydrophobic membranes, such as PVDF, tend to be more prone to fouling. Ceramic membranes, on the other hand, are more resistant to fouling and have advantages in terms of chemical stability and strength.

   • Surface Roughness: The roughness of the membrane surface increases its ability to adsorb contaminants but also introduces some flexibility, reducing the likelihood of contaminants sticking permanently.

   • Hydrophobicity vs. Hydrophilicity: Membranes made from hydrophilic materials generally have a better resistance to fouling.

Control Measures for Membrane Fouling

Fouling is primarily influenced by the inherent properties of the membrane, the mixed liquor characteristics, and the operating conditions. Therefore, controlling fouling should focus on these three aspects:

1. Membrane Properties: Choose membranes with better hydrophilicity, surface roughness, and suitable pore size to enhance fouling resistance. Ceramic membranes are often a good choice for their strength and chemical stability.

2. Mixed Liquor Properties: Control MLSS concentration and viscosity, as well as the composition of organic and inorganic materials, to reduce fouling. Pre-treatment steps like filtration can remove larger particles and prevent clogging.

3. System Operating Environment: Use appropriate fluxes to stay within sub-critical values to