Filtration is an effective means of disinfecting water and is used by Water Utilities, industry and medical practices to either reduce or remove microbial load. In some cases filtration can be used to sterilise water. In most instances filtration will improve any other disinfection processes being used. Filtration may play an important role in a Water Safety and Risk Management Plan.There are different locations where different types of filtration can be applied. So which filter goes where – and why?

Types of Filter

Filters are classified by their ‘pore size’. This is the size of the holes that things can pass through. So a 5 micron filter will block anything above 5 microns in diameter. However, this can be a bit misleading because most things aren’t round so they don’t have a diameter. So filtration pore size is more of a guide than a rule. This means that particles that are less than 5 microns at their widest point can pass through the filters depending on what angle they hit the filter surface.

Often filtration will enhance the effects of other disinfection processes. This is the result of filters reducing the microbial load, organic and inorganic particles. Filtration can significantly improve the effectiveness of all commonly applied potable water disinfection processes.

Media filters

Filters materials are quite diverse. Some are ‘sediment or media type’ filters. These use fine sand to filter water. As the water runs through the sand traps particles in the gaps between the grains of sand. This is the basis of filtration for most water utilities and for swimming pools. Mostly, these filters won’t operate below 5 micron pore size. As they trap the material they need a ‘backflush’ regularly. Backflushing is reversing the flow of water through the filter to ‘wash out’ the collected material.

Cartridge Filters

Cartridge type filters are made from woven or moulded materials. These can be organic or inorganic materials that ‘sieve’ the water. Often they use ‘tangential flow’. This means the water passes ‘sideways’ across the filter. So the sideways flow increases the surface area for water to pass through the filter. This has two advantages. Firstly filters take longer to clog. Secondly, they can cope with a higher flow rate of water.

Most of the cartridge filters, like the ones for domestic drinking fountains (left), need replacing. This is because backflushing is not an option. Replacing as advised by manufacturers is a very good option. Usually these filters have pore sizes down to 5 microns or less. Industrial sized filters like those used in wastewater treatment and desalination use similar principles. The big difference is that these filters can backflush and can physically clean their surfaces.

Fine filters

Filtration to 0.22 microns or less (ultra-filtration) is an effective means of removing bacteria from water. These filters will trap almost all bacteria – but viruses can pass through. They have been used to sterilise media in microbiology laboratories for decades. Their advantage to microbiologists is they can filter solutions that would otherwise be damage by heat or chemical treatments. eg egg white. Because these filters are so fine they clog rapidly if either the water is not of a high quality or large volumes of water are passing through them.

Source: Sigma-Aldrich

Filter Locations

Just as the real estate people say ‘location, location, location!’ Installing the filter in the right place is an essential for getting the right result. Sometimes a filter in the wrong place can affect disinfection or water flows and end up causing more problems than it solves.

At the boundary

Water is goes through filters at most Water Utilities in Australia. However as it travels down pipelines sediment and debris and surviving microorganisms settle out on to the surfaces. Though water is still safe at the point of entry to your site it may have a considerable organic and inorganic content. This of course, if food for the microorganisms colonising your water system. The picture on the left is a new filter (far left) and the other two filters (centre and right) after 2 months of use. These filters are installed at the boundary entry point of the water supply to an aged care facility in central New South Wales.




In the Distribution

In some cases filters are installed in the water distribution line. This is usually for systems that need good quality water such as spring water bottling plants. They are also necessary for UV disinfection systems. For UV to work it must be able to penetrate ‘clear’ water. So filtering makes the process work better. UV without filtration is probably a waste of time and money.


At the Point of Use

Point of use filters are a recent addition to the filtration story. These filters have a pore size of .22 micron or less. They come as tap or shower fittings. They attach directly onto the existing tap-ware. This allows use of the potable water that is bacteriologically sterile. Naturally they have a niche market. They are a good option in high risk areas such as people receiving chemotherapy (cancer treatments) or organ transplants. They can also be a temporary barrier in facilities where Legionella bacteria is present and needs controlling.

These filters will clog depending on water quality. Backflushing is not an option. They will need replacing every 30 or 60 days depending on the brand. Of course if the water quality is poor it may be they need more regular replacement

Filtration and Disinfectants

Using a filter does not kill bacteria. Any organisms getting past the filter are free to multiply. Filtration will not remove dissolved nutrients, only particles. So filtration is disinfectant but has no residual effect. This means it can remove bacteria, but cannot kill them. However this can be useful if it is in combination with chemicals that can kill bacteria. By reducing the particles and incoming microorganisms it makes whatever disinfection process you use more effective. Most chemicals used in potable water systems will react with organic matter whether it is live or dead. Reducing this ‘organic load’ improves disinfection and saves on chemical use.

The other benefit of filtering is reducing the nutrient coming into your building. If you can cut off the supply route for food then you can control the population. Less nutrient means less biofilm.

Many filters use activated carbon as the ‘medium’. Activated carbon (charcoal) removes organic material and inorganic chemicals. It will also remove any chemical disinfectant in the water system. This is not an issue if the filter is close to the outlet. Filters more than a meter away from the outlet leave the rest of the pipe unprotected. This can cause disease outbreaks in aged and health care premises.

‘Ice and chilled water from machines should not be provided to patients with high risk of micro-aspiration and susceptibility to Legionella. Instead, pre-boiled tap water should be chilled or frozen in conventional style fridges or freezers for these patients. Ice and chilled water can be provided to other patients from these machines. However, carbon filters should not be used on the inlets of these machines in care facilities.’ Source: Public Health Fact Sheet #3-14
Legionnaires’ disease: managing the risks associated with chilled water and ice machines in care facilities (2014)

Filtration Considerations

Filtration has no residual effect other than lowering the nutrient load entering the system. dissolved organic matter will pass through them. Depending on the pore size microorganisms will pass through them too.

Flushing filters and replacing them routinely is important to stop them becoming reservoirs for microbial biofilms. As part of their operation filters reduce water pressure and flow rates. In some cases this can cause stagnation.

High nutrient load water supplies that require frequent backflushing or filter changes may elevate the cost of operation. Backflushing filters means locating them where there is access to sewer for discharge or backflush water can collect and be disposed of safely.

So a filter in the right place for the right reasons is an asset. A filter in the wrong place for the wrong reasons is a liability. Good advice from a water treatment specialist will save money and heartache.

Further Reading

Australian Government 2011 (updated 2013). Australian Drinking Water Quality Guidelines.

Kim et al (2002) Literature Review – Efficacy of various disinfectants against Legionella in water systems. Water Research 36 (4433-4444).

New Zealand Government (2012) Ministry of Health, The Prevention of Legionellosis in New Zealand: Guidelines for the Control of Legionella Bacteria

South Australian Health Dept. Public Health Fact Sheet #3-14 Legionnaires’ disease: managing the risks associated with chilled water and ice machines in care facilities (2014)

US EPA 2015. Technologies for Legionella Control: Scientific Literature review

World Health Organisation (2007) Legionella and Prevention of Legionellosis