Environmental Water Treatment Knowledge: Meaning and Role of Membrane Fouling
Membrane fouling refers to the phenomenon in membrane filtration where fine particles, colloidal particles, or macromolecular solutes in water, due to physico-chemical interactions or mechanical action with the membrane, become adsorbed and deposited on the membrane surface or within membrane pores, causing the membrane pore size to shrink or become blocked and producing an irreversible change in the membrane's permeate flux and separation characteristics.
The reliability of GE-standard membranes is one of the key factors currently hindering the widespread application of membrane technology, and fouling is the decisive factor affecting that reliability. According to investigations, for ultrafiltration, fouling remains the primary problem; eliminating fouling would improve ultrafiltration process efficiency by more than 30%, reduce investment by 15%, and enhance separation performance while broadening the scope of ultrafiltration. A detailed analysis of the types of membrane fouling and their causes will help to adopt appropriate measures to mitigate or eliminate its adverse effects.
Pressure-driven membrane separation technologies include reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). Based on the interactions between different membranes and the particulates in water, it is known that precipitation fouling has a particularly significant impact on RO and NF.
When the salt concentration in the raw water exceeds its solubility, precipitates or scaling form on the membrane. A widely concerning group of foulants is the precipitates of calcium, magnesium, iron, and other metals, such as hydroxides, carbonates, and sulfates. [1]
Adsorption of organic matter onto the membrane surface is usually the dominant factor affecting membrane performance. Over time, the adsorption or accumulation of foulants within membrane pores leads to reduced pore size and increased membrane resistance, which is difficult to recover. Humic acid and other natural organic matter (NOM), even at relatively low concentrations, have a far greater impact on permeability than clay or other inorganic colloidal particles.
Organic-matter characteristics related to membrane fouling include their affinity for the membrane, molecular weight, functional groups, and configuration. Organic polyelectrolytes with negatively charged functional groups (such as humic and fulvic acids) experience electrostatic repulsion from negatively charged membrane surfaces. Polysulfone, cellulose acetate resin, ceramic, and thin-film composite membranes used in water and wastewater treatment all carry a certain degree of negative charge. Generally, the greater the membrane surface charge density, the stronger the membrane hydrophilicity. However, hydrophobic interactions can increase the accumulation of NOM on the membrane, leading to more severe adsorption fouling.
Specific components of NOM that cause membrane fouling can be identified. Using pyrolysis gas chromatography (GC)/mass spectrometry (MS) fractionation techniques, polysaccharides and polyhydroxy aromatic compounds have been identified as the two main components in surface water and karst groundwater.
In addition to its direct adsorption fouling of membranes, NOM also plays an important role in the adhesion and deposition of colloids on the membrane. Analysis of the types and relative concentrations of organic pollutants appearing in the deposition layer from natural water bodies shows that polyphenolic compounds, proteins, and polysaccharides adhere together with colloids and deposit onto the membrane, forming a gel layer on the membrane surface. Therefore, the stability of the adsorption fouling and the gel layer formed by organic matter in water affects the efficiency of purely hydraulic cleaning. Hydraulic cleaning methods include backwashing, rapid pulsing, or cross-flow reverse flushing. Reagents used for chemical membrane cleaning must be able to effectively dissolve the organic compounds in the gel layer. Therefore, chemical cleaning solutions for membranes usually consist of caustic substances and enzymes.
(1) Understand the microbial colonies in the biofilm to identify suitable organisms for experimental simulation and adhesion bioassays. Non-growth-based molecular genetic assays are recommended, such as ribosomal RNA gene fragment analysis, gene probe bioassays, and fluorescence in situ hybridization.
Biofilms composed of bacteria, fungi, and other microorganisms can directly (through enzymatic action) or indirectly (through local pH or reduction-potential effects) degrade membrane polymers or other RO unit components, resulting in shortened membrane life, destruction of membrane structural integrity, and even major system failures and penalties.
Assimilable organic carbon (AOC) is considered the growth potential of biofilms. Therefore, the AOC indicator can characterize the possibility and extent of biofilm formation. Research has confirmed that bacteria adhere to different polymers at greatly varying rates. For example, polyamide membranes are more susceptible to bacterial contamination than cellulose acetate membranes. Therefore, separation membranes made of polymers with reduced bio-affinity and easy cleanability will hinder biofilm growth. To develop biofilm fouling control technologies, researchers must first understand the surface molecular structure of the separation membrane polymers and the mechanism by which adherent biological cells interact with the membrane. The basic research necessary for better control of membrane biofouling includes the following six aspects.
Biofouling refers to the phenomenon in which microorganisms accumulate at the membrane-water interface, thereby affecting system performance. The interior of membrane modules is damp and dark, providing an ideal environment for microbial growth; therefore, once the biological activity level of the raw water is high, membrane biofouling is highly likely to occur. Membrane biofouling occurs in two stages: adhesion and growth. When no biocide is added to the solution or the dosage is insufficient, adherent cells grow and reproduce under the nourishment of influent nutrients, forming a biofilm. Secondary adhesion or entrainment on the primary biofilm further develops the biofilm. Aging biofilm bacteria mainly decompose into proteins, nucleic acids, polysaccharide esters, and other macromolecular substances, which strongly adsorb onto the membrane surface and cause membrane surface modification. The modified membrane surface more readily attracts other types of microorganisms. An important characteristic of microorganisms is their ability to make rapid biochemical and genetic adjustments in response to changing nutrients, hydrodynamic, or other conditions. Therefore, biofouling is a more serious problem than inactive colloidal fouling or mineral scaling.
(2) The adhesion process must be studied at the molecular and atomic level to better understand the influence of physicochemical forces during cell adhesion.
(3) The effect of the modified membrane on bacterial adhesion and initial biofilm formation requires further study. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) measurements can help analyze the problem.
(4) During the biofouling process, the interaction between bacterial exopolymers (such as alginates) and membrane materials is not yet fully understood. Theoretically, molecular simulation can quickly and inexpensively predict membrane biofouling. At the same time, simulation techniques can be used to identify new chemical substances that interfere with cell adhesion.
(5) The structural integrity of the biofilm itself relies on intermolecular forces between cells, which are related to the interaction between cells and adjacent extracellular polymeric substances (EPS). To date, the magnitude and nature of the forces between cells in the biofilm remain unclear. Combining molecular simulation techniques with appropriate experimental methods (such as X-ray diffraction) can help analyze the problem.
(6) There is currently a lack of understanding of the physiological ecology of biofilms. Some studies have pointed out that brominated furan (derived from seabed algae) can hinder bacterial adhesion and weaken the fouling impact of the biofilm's source solution.
Biofouling can be controlled by continuous or intermittent disinfection of the influent. However, the degradability of the disinfectant to the membrane must be considered. Research shows that monochloramine is a biofilm disinfectant superior to chlorine disinfection, which can greatly reduce trace organic oxides and inhibit bacterial growth. Continuous dosing of 3-5 mg/L monochloramine into wastewater can inhibit biofilm growth (without oxidative damage to the membrane) and extend the operating cycle.
In membrane desalination systems, the addition of low-concentration (0.5-1.0 mg/L) copper sulfate can inhibit algal growth. Some surfactants and other chemical reagents can interfere with bacterial adhesion on membrane polymers. In addition, physical means can be used, such as strengthening cross-flow velocity and increasing gas backwash, to prevent microbial adhesion.
The three types of fouling - precipitation fouling, adsorption fouling, and biofouling - sometimes occur simultaneously, and the occurrence of one type of fouling may accelerate another. During membrane treatment, the raw water composition should be analyzed to identify the main cause of membrane fouling, so as to better eliminate its effects and extend the membrane's service life. [1]
