In this technical guide the industrial pre-treatment and water filtration specialists at Water Treatment Services examine the problems caused by membrane fouling. The guide looks at what causes it, how to detect it, and importantly, how to prevent or at least control it. It then goes on to consider the different techniques used to clean fouled membranes including mechanical and chemical methods, the benefits of pre-treatment of raw water sources, and concludes by considering aspects of good filtration system design.
Controlling membrane fouling in filtration systems
Many different industries incorporate membrane filtration systems in to a wide range of applications from the desalination of seawater to the creation of ultra-pure water for the manufacture of semiconductors and pharmaceuticals.
One of the main concerns businesses have when considering the introduction of a membrane filtration system is about the potential for the membranes to become fouled, and what steps can be taken to avoid this.
Most membrane filter units require a lot less maintenance than other filtration treatments.
They are however perhaps more liable to membrane fouling, but there are also lots of techniques and treatments which can be put in place to keep the filtration system running at peak performance for longer.
What are the different types of membrane filtration?
The main types of membrane filtration systems currently available commercially include:
All of these processes use semi-permeable membranes of different pore sizes which capture or filter-out fine particles found in the raw or feed water as it flows through the membrane.
What causes membrane fouling?
Fouling occurs when contaminants from the raw water start to build up on the surface of the membrane, and when this happens it begins to reduce or even stop the raw water flowing through the filter properly.
There are lots of different factors which can lead to excessive fouling of these types of membrane.
This could include issues with too many biological or organic particles in the raw water, or the material from which the membrane is made might not be the best choice for that particular application.
The process conditions, such as the water temperature or pressure may also cause fouling to build-up more quickly.
How to detect fouled membranes
Depending on the cause of the membrane fouling, problems may occur over time or sometimes suddenly.
In the early stages, the first signs of fouling might be decreased membrane flux, or rising energy costs.
As the fouling problems progress, an increased pressure might be needed to force the raw water through the membrane.
It is important to recognise that if left unchecked, increased pressure like this may permanently damage the membrane and other aspects of the filtration system which could be both disruptive and very costly.
How to prevent membrane fouling
Sometimes, but not always, membrane fouling is a reversible process… so it’s always better to try to prevent the problem occurring in the first place.
There are lots of different preventative measures which can be used to avoid membranes from becoming fouled and we’ll consider some of them here.
A proper membrane cleaning system and regime can go a long way in preventing fouling.
Cleaning intervals are usually monthly, but the frequency will be determined by the needs of the individual filtration system.
The type of cleaning required will depend on the system design, and what types of contaminants you are dealing with in the raw water.
Most membrane cleaning systems will use one or several of the most common cleaning methods which include:
Mechanical cleaning of filters
Mechanical cleaning is a method that uses physical force to scrub unwanted foulants off the membrane surfaces and flush them out of the system.
Vibration is a commonly used mechanical cleaning technique, along with forwards or backwards flushing which involves running water through the unit at a faster speed, or higher pressure than usual.
This type of flushing causes turbulence which helps to remove the fouling from the membrane.
Another option is air scouring, which adds air into the flushing process to produce even more turbulence.
Chemical cleaning of filtration membranes involves using acids, anti-scale products, detergents, dispersants and caustic chemicals into the system with the aim of loosening foulants from the surface of the membrane where they can be easily removed.
The type of membrane cleaning chemicals used will depend on the nature of the feed water contamination which should be clearly identified before starting.
The material the membrane is made from will also affect the selection of the membrane cleaner, as some chemicals might cause damage to certain types of membrane.
Pre-treatment of raw water
NF and RO systems are therefore more likely to benefit from pre-treatment of the feed water to avoid or at least minimise membrane fouling.
If the source water includes very high concentrations of contamination, the process may benefit from pre-treatment before the feed water reaches the membrane filters to reduce the risk of excessive fouling.
There are several different options for the pre-treatment of the source water, including coagulation in systems with colloidal particles, sedimentation (also known as gravity settling), flocculation, and additional early stage filtration using other mediums to get rid of larger particles before they clog the finer membrane pores.
In other filtration systems, water may be pre-treated with chemicals to adjust the pH levels, or softening by ion exchange to stop foulants being deposited on the membrane.
Good filtration system design
While there are several really effective techniques that can be used to limit membrane fouling, the best way of preventing it is by designing the filtration system properly in the first place.
There is a lot to consider when designing a membrane filtration system, and the following factors should be carefully considered when either installing a new system or updating a current one:
Membrane filter materials
There are a wide range of different polymers, ceramics and metallic materials that are used to make filtration membranes.
Ionic charge on the surface, pH tolerance range or hydrophobicity will all affect how resistant the membrane is to various types of fouling and what sort of maintenance regime will be required.
Membrane pore size
Using the correct pore size for the right application is also a key factor in making sure that contaminants are removed effectively from the source water.
Undersized pores will lead to rapid fouling on filter surfaces, whereas oversized pores will result in ineffective filtration.
Other factors such as the quality of the feed water, temperature of the water and concentration of salts can also affect the choice of pore size.
Membrane fouling can happen more quickly at different temperatures and water pressures too.
Designing a filtration system to balance these various factors can also help make sure that fouling does not build up on the surface of the membrane too quickly.
These factors are all very complex and it’s therefore advisable to involve an expert in water treatment, such as Water Treatment Services who can fully assess your filtration requirements and make recommendations about the most suitable forms of filtration.
During the early fact finding and feasibility stages of this process our water experts may choose to carry out treatability studies, look at the composition of the source or feed water, the required water quality standards to be achieved; and run pilot studies to come up with a solution to minimise fouling of the membranes and so optimise to efficiency of the filtration system.
Water pre-treatment training
The industrial and process water treatment specialists at Water Treatment Services have developed a series of professional water and wastewater treatment training courses including WTS 0030 “Pre-Treatment for Industrial & Process Waters”.
This specialist water pre-treatment training course comprises an intensive one day session designed for plant operators, environmental process engineers, maintenance and engineering contractors, and those with responsibility for the operation and management of water pre-treatment systems, including membrane filtration.
Expert membrane filtration solutions for business
Water Treatment Services offer a comprehensive range of pre-treatment and membrane filtration solutions for a range of business applications.
Find out how we can help improve water quality standards, reduce costs and improve environmental performance.
Our industrial water treatment experts can provide advice and support to help you identify the most appropriate strategies for managing your water needs.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers, wastewater specialists and technicians we offer cost effective environmental support solutions across the whole of the UK and Ireland.
Contact us today to learn how our water management solutions can help improve your pre-treatment and membrane filtration processes, to improve water quality standards and reduce costs.
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