Membrane fouling
Encyclopedia
Membrane fouling is a process where solute
or particles deposit onto a membrane
surface or into membrane pores in a way that degrades the membrane's performance. It is a major obstacle to the widespread use of this technology
. Membrane fouling
can cause severe flux
decline and affect the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement. This increases the operating costs of a treatment plant
. There are various types of foulants: colloid
al (clays, floc
s), biological (bacteria
, fungi), organic (oils, polyelectrolyte
s, humics) and scaling (mineral precipitates).
Fouling can be divided into reversible and irreversible fouling based on the attachment strength of particles to the membrane surface. Reversible fouling can be removed by a strong shear force of backwashing. Formation of a strong matrix of fouling layer with the solute during a continuous filtration process will result in reversible fouling being transformed into an irreversible fouling layer. Irreversible fouling is the strong attachment of particles which cannot be removed by physical cleaning.
, it can be minimised by strategies such as cleaning, appropriate membrane selection and choice of operating conditions.
Membranes can be cleaned physically, biologically or chemically. Physical cleaning includes sponges, water jets or backflushing using a permeate. Biological cleaning uses biocide
s to remove all viable microorganisms, whereas chemical cleaning involves the use of acids and bases
to remove foulants and impurities.
Another strategy to minimise membrane fouling is the use of the appropriate membrane for a specific operation. The nature of the feed water must first be known; then a membrane that is less prone to fouling with that solution is chosen. For aqueous filtration
, a hydrophilic
membrane is preferred.
Operating conditions during membrane filtration are also vital, as they may affect fouling conditions during filtration. For instance, crossflow filtration is always preferred to dead end filtration, because turbulence
generated during the filtration entails a thinner deposit layer and therefore minimises fouling.
Solute
Solute may refer to:* Solute, UMIK or UBOOK desolving in a substance,forming INT/INTY* Solute , a group of Paleozoic echinoderms...
or particles deposit onto a membrane
Artificial membrane
An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A...
surface or into membrane pores in a way that degrades the membrane's performance. It is a major obstacle to the widespread use of this technology
Membrane technology
The membrane technology covers all process engineering measures for the transport of substances between two fractions with the help of permeable membranes...
. Membrane fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
can cause severe flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
decline and affect the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement. This increases the operating costs of a treatment plant
Sewage treatment
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...
. There are various types of foulants: colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
al (clays, floc
Flocculation
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
s), biological (bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, fungi), organic (oils, polyelectrolyte
Polyelectrolyte
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions , making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes and polymers , and are sometimes called polysalts. Like salts, their...
s, humics) and scaling (mineral precipitates).
Fouling can be divided into reversible and irreversible fouling based on the attachment strength of particles to the membrane surface. Reversible fouling can be removed by a strong shear force of backwashing. Formation of a strong matrix of fouling layer with the solute during a continuous filtration process will result in reversible fouling being transformed into an irreversible fouling layer. Irreversible fouling is the strong attachment of particles which cannot be removed by physical cleaning.
Factors affecting membrane fouling
Factors that affect membrane fouling:- Membrane properties such as pore size, hydrophobicityHydrophobeIn chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
, pore size distribution and membrane material. - Solution properties such as concentration, the nature of the components and particle size distribution.
- Operating conditions such as pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
, temperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, flow rate and pressure.
Measure of membrane fouling
Flux and transmembrane pressure (TMP) are the best indicators of membrane fouling. Under constant flux operation, TMP increases to compensate for the fouling. On the other hand, under constant pressure operation, flux declines due to membrane fouling.Fouling control
Even though membrane fouling is an inevitable phenomenon during membrane filtrationMembrane technology
The membrane technology covers all process engineering measures for the transport of substances between two fractions with the help of permeable membranes...
, it can be minimised by strategies such as cleaning, appropriate membrane selection and choice of operating conditions.
Membranes can be cleaned physically, biologically or chemically. Physical cleaning includes sponges, water jets or backflushing using a permeate. Biological cleaning uses biocide
Biocide
A biocide is a chemical substance or microorganism which can deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means. Biocides are commonly used in medicine, agriculture, forestry, and industry...
s to remove all viable microorganisms, whereas chemical cleaning involves the use of acids and bases
Bases
Bases may refer to:*Bases , a military style of dress adopted by the chivalry of the sixteenth century.*Business Association of Stanford Entrepreneurial Students...
to remove foulants and impurities.
Another strategy to minimise membrane fouling is the use of the appropriate membrane for a specific operation. The nature of the feed water must first be known; then a membrane that is less prone to fouling with that solution is chosen. For aqueous filtration
Filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
, a hydrophilic
Hydrophile
A hydrophile, from the Greek "water" and φιλια "love," is a molecule or other molecular entity that is attracted to, and tends to be dissolved by water. A hydrophilic molecule or portion of a molecule is one that has a tendency to interact with or be dissolved by, water and other polar substances...
membrane is preferred.
Operating conditions during membrane filtration are also vital, as they may affect fouling conditions during filtration. For instance, crossflow filtration is always preferred to dead end filtration, because turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
generated during the filtration entails a thinner deposit layer and therefore minimises fouling.