Membrane technology is also employed in industrial processes and industrial wastewater treatment and it has recently expanded into the treatment of secondary and tertiary municipal wastewater as well as oil field-produced water.

After a certain time, the quality of the water starts to decrease and the performance of the system decrease.

Why does this happen? What causes this problem?

What is Membrane Fouling?

Membrane fouling is the process by which particles, colloidal particles, or solute macromolecules are deposited or adsorbed onto membrane pores or surfaces through physical and chemical interactions or mechanical action, resulting in smaller or blocked membrane pores.

Membrane fouling can cause significant flux drops and have an impact on the quality of the water produced.

Severe fouling may necessitate a thorough chemical cleaning or membrane replacement.

This raises a treatment plant’s operating costs.

Factors contribute to membrane fouling

Membrane filtration methods that use semi-permeable membranes to filter particles from liquids include microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and Nano-filtration (NF).

Membrane fouling happens when contaminants deposit on the surface of a filtration membrane, obstructing liquid flow through the membrane’s pores.

Excessive biological, colloidal and/or organic particles in the source water; an unsuitable choice of membrane material and/or inappropriate process conditions including flow rate, temperature and pressure can all cause fouling.

Types of fouling membranes

Membrane fouling is classified based on the foulant and the location of the fouling.

Particulate, organic, inorganic and biological microorganisms are the main types of foulants.

Membrane fouling is classified based on the foulant and the location of the fouling.

Due to pore adsorption and blockage, internal fouling occurs in most low-pressure membrane-based separation processes, such as UF and MF.

Surface fouling occurs in semi-permeable membranes that are denser and more compact, such as those used in NF and RO.

The sections that follow elaborate on membrane fouling based on the foulant type.

Particulate fouling

Fouling from Particulate and colloidal matter particulate fouling takes place when suspended solids and/or colloidal material clog or adhere to the surface of a membrane’s holes.

As particles accumulate on the membrane, they form a “cake” layer that stops water from passing through the membrane’s pores, resulting in symptoms such as higher-pressureifferential measurements and increased energy usage.

Particulate/colloid fouling is caused by the presence of non-biological and inorganic particles (e.g., silt or clay) in the feed water, which is more prevalent when the stream comes from a body of surface water.

Water treatment specialists typically measure a feed stream’s Silt Density Index (SDI) to determine the relative risk of particulate/colloidal fouling.

Measurements are especially important when installing RO systems because they have the smallest pores of any membrane filtration system and are thus much more susceptible to Particulate fouling.

Particulate fouling is usually avoided by using appropriate upstream coagulation and filtration.

Inorganic fouling

Inorganic fouling, or scaling, is the deposition of inorganic compounds on the membrane surface or within the pores of the membrane.

The deposits could be inorganic compounds that are poorly soluble in water or solutes that are abundant in water.

They combine to form supersaturated solutions, which eventually precipitate out of the solution and onto the membrane’s surface.

Scale formation on membrane surfaces can occur in two mechanisms such as crystallization and particulate fouling.

The former mechanism includes the ion precipitation and subsequent deposition on the membrane, whereas the latter involves the convective transport of particulates from the solution’s bulk to the surface of the membrane.

The main inorganic compounds that contribute to scaling in rivers, groundwater, seawater and municipal wastewater are hydroxides, sulfates, carbonates, calcium, magnesium, iron, ortho-phosphates, silicic acids and silica.

Organic fouling

This state is caused by the aggregation and deposition of relatively dense organic molecules such as polysaccharides, proteins, humic compounds, nucleic acids, lipids and amino acids.

Both surface and subsurface organic matter is common.

Water and wastewater are classified into natural organic matter and artificial organic matter (compounds and soluble microbial products).

NOM refers to naturally occurring heterogeneous mixtures formed by the decomposition of animal and plant remains.

Synthetic compounds are DOMs that are added or created artificially during the process of disinfection.

Finally, SMPs are byproducts of biological treatment processes that break down organic compounds biologically.

Several research studies indicate that the influence of organic matter on RO membrane fouling differs depending on the dominant type of organic foulant, feedwater chemistry, foulant-surface and foulant–foulant interaction

Bio-fouling

Bio-fouling is the growth of microorganisms, plants, algae, or other biological contaminants on or in the pores and surfaces of filtration membranes.

Biological and microbial foulants thrive in warm environments with low flow rates, where they can attach to the membrane and multiply while releasing an extracellular polymeric substance (EPS).

A biofilm is formed when microbes and EPS mix to produce a slimy gel layer.

Because of its chemical qualities, biofilm is resistant to conventional cleaning treatments such as backwashing or the application of biocides such as chlorine.

Although not a foulant, free chlorine attack is the most common cause of membrane oxidation.

It is permanent and irreversible. Bio-fouled membranes can be difficult to repair and, in some cases, must be replaced.

Indications of membrane fouling

Raising of Transmembrane pressure

Transmembrane pressure is the amount of pressure necessary to force a solvent past a semipermeable membrane.

The greater the concentration of the stream, the more pressure is required to force the penetrate through.

Similarly, if foulants attach to the membrane, significantly greater power is necessary to force the liquid through.

When transmembrane pressure rises sharply or in small increments over time, fouling, whether colloidal, bacterial, or organic, is frequently suspected and cleaning methods are implemented.

Mold and strong smells

Biological growth is usually to blame when the membranes begin to smell.

In this situation, the filters may have slime accumulating on the surfaces or mold evidence along the ends.

A temperature change is occasionally necessary to prevent further biological growth and a biocide or alkaline cleaner may be used in this situation, either once to clean the biological foulants or repeatedly to help keep them from growing again.

Accumulations on the membrane’s margins

If you detect solids accumulating on the membrane’s margins, this could be an early warning indication that something isn’t right.

Solids and components such as silt or rust will occasionally begin to collect at the scroll ends, but if this occurs more frequently than usual, it can be an indicator to examine before these foulants become a greater problem.

Depending on the foulant, an acid cleaning may be required in addition to frequent flushing or backwashing of the membranes.

If you notice this type of accumulation, make a note of it and speak with a specialist to ensure it isn’t a sign of a greater problem.

How can minimize membrane fouling?

Cleaning schedule

A regular cleaning schedule can assist to keep foulants from accumulating on the membrane.

To reap the most benefits, cleaning cycles should be conducted regularly or at other regular intervals.

Maintenance tactics can vary based on the architecture of the membrane filtration system and the impurities involved and can include one or more cleaning processes, such as:

Mechanical cleaning: is known to employ physical force to dislodge pollutants from the membrane and flush them out of the system.

Chemical cleaning is the process of loosening and removing foulants from the membrane surface using detergents, caustics, acids, antiscalants, or dispersants.

Pretreatment

If colloidal particles are present, pretreatment techniques may be required to remove bigger or coagulated particles which include coagulation, as well as gravity settling (sedimentation), flocculation and medium filtering.

Chemical pH adjustment and ion exchange are two more methods of pretreatment that can be used to reduce foulant adsorption or deposition on the membrane.

Well-planning and design

When upgrading a membrane or installing a new system, several variables play a part in the proper system function for a membrane filtering system. These are some examples:

Membrane composition: Membrane material properties such as surface ionic charge, hydrophobicity and pH tolerance range influence whether the membrane will be resistant to specific types of fouling and how well it will tolerate process conditions and the required maintenance regimen.

Membrane pore size: Pore size is the most important component in ensuring that a membrane filtration unit removes specified pollutants efficiently.

Furthermore, choosing the right membrane pore size can help to avoid fouling by maximizing permeate flux about other elements including feed water quality, temperature and salt concentration.

operating environments: Certain temperatures, pH, transmembrane pressure and flow rate ranges can exacerbate membrane fouling.

A well-designed system will balance these variables to prevent foulants from accumulating on the membrane’s surface.

References

[1] Lingling Liu, … Sheng-Lian Luo, in Nanomaterials for the Removal of Pollutants and Resource Reutilization, 2019, 2-Application of Nanotechnology in the Removal of Heavy Metal From Water, ScienceDirect online, available at https://www.sciencedirect.com/topics/engineering/membrane-fouling

[2] -SAMCO, October 17, 2018, What is membrane fouling and how can it be avoided? Online, available at https://www.samcotech.com/what-is-membrane-fouling-and-how-can-you-avoid-it/

[3] Ghaleb Adnan Husseini, May 18, 2021, Membrane fouling, encyclopedia, online, available at https://encyclopedia.pub/entry/9767

[4] SAMCO, November 2, 2018, what are the different types of membrane fouling, and what causes them? online, available at https://www.samcotech.com/types-of-membrane-fouling-and-causes/

[5] Signs Your Membranes Are Fouling and How to Clean Them online, available at: https://samcotech.com/signs-your-membranes-are-fouling-and-how-to-clean-the