Biofilms in water systems: formation, risks and solutions

A biofilm is, simply put, a layer of microorganisms such as bacteria, germs, fungi, protozoa or algae, embedded in a slimy matrix and attached to a surface – especially in stagnant water.

These microbial communities are highly adapted to their environment. The microorganisms within a biofilm benefit from each other’s metabolic properties and protective mechanisms.

How do biofilms form?

Certain types of bacteria can produce a slimy matrix (known as extracellular polymeric substances, EPS) that allows them to attach to surfaces. This layer forms the structural basis of biofilms. 

As the structure grows, it makes it easier for other microorganisms to settle. The biofilm spreads laterally and develops into a three-dimensional structure with multiple layers.

In some areas, such as water pipes, biofilms can form within just a few days – even if the water is of high quality.

Where do biofilms form?

Biofilms can form, for example, on stones in a streambed – but also on the inner surfaces of water pipes. This is a key factor in drinking water hygiene.

In the human body, biofilms may develop on implants, which is usually associated with health complications.

They can also serve useful functions, such as preventing the colonisation of harmful germs and bacteria in the gastrointestinal tract.

Why biofilms can be problematic

Although biofilms play a beneficial role in certain natural processes – like soil and water self-purification or the binding of carbon dioxide – they pose major risks in other environments.

Pathogenic microorganisms found in biofilms include Legionella pneumophila and Pseudomonads such as Pseudomonas aeruginosa. These can cause various infections in humans, such as pneumonia, urinary tract infections or wound infections.

Other typical biofilm-associated pathogens include Sphingomonas and Mycobacterium.

Biofilm in water pipes and drinking water installations

Water pipes and drinking water installations naturally support microbial processes due to their structure and operating conditions – including constant moisture, temperature-favourable environments and internal pipe surfaces that promote attachment.

Operational factors such as stagnation, insufficient temperatures or nutrient-releasing materials can further encourage biofilm formation.

Biofilm formation in new building water systems

A 2024 study found that even in new buildings, stable biofilms can develop in water pipes within just eight weeks.

Bacteria naturally present in drinking water begin adhering to the internal surfaces of new pipe materials within just a few hours after commissioning – especially when water stagnates or when temperatures support the growth of potentially pathogenic microorganisms such as Legionella spp. (though Legionella was not specifically analysed in the referenced study).

Within the first few days, a thin organic layer (conditioning film) forms on the pipe walls. This layer facilitates the adhesion of further bacteria and thus promotes early-stage biofilm development.

The continuous inflow of drinking water constantly introduces new microorganisms into the system, which can colonise existing biofilms and further reinforce them.

Expansion and spread of biofilm

Low flow velocity or dead legs in drinking water installations also promote biofilm formation. This, in turn, makes it easier for bacteria to spread.

Within water distribution systems, parts of the biofilm can later detach due to the force of flowing water – once the slimy layer has reached a certain thickness – and reattach at other locations in the pipework. This leads to further spread of the biofilm.

Contact with contaminated water

Humans can come into contact with these bacteria through taps (see the following image: biofilm in the faucet aerator), showerheads or siphons.

Waterborne germs can also enter medical devices – such as endoscopes – during final rinsing after cleaning, allowing contamination to occur even after reprocessing.

Removing biofilm

The biofilm structure – a so-called extracellular matrix – provides protection against chemical and thermal disinfection.

This makes it almost impossible to completely remove an established biofilm.

Only a combination of mechanical removal, chemical disinfection and structural prevention allows for effective and long-term control of biofilm in water installations.

However, complete elimination is rarely achievable under real-world conditions.

Preventing biofilm formation

Targeted material selection, structural planning and operational measures – such as avoiding water stagnation and using the drinking water installation as intended – can significantly reduce the risk of biofilm formation and help maintain hygienic standards.

Risk reduction relies primarily on preventive actions, including strict hygiene protocols, regular monitoring and optimised system design.

Thanks to improved materials, planning and operating techniques, the risk of biofilm formation and Legionella contamination in modern buildings is considerably lower than in the past – but it still cannot be entirely ruled out.

Point-of-use filters prevent contact with contaminated water

Where bacteria and germs in drinking water may pose a risk to immunosuppressed or immunocompromised patients, any contact with the water must be consistently avoided.

To prevent exposure to contaminated water, various types of water filters can be used:

• Single-use filters for taps or showerheads
• Reusable sterile filters for medical environments – for example in dental units, endoscope reprocessing or TEE probes

Thanks to their hollow fibre membrane technology, these sterile water filters retain all waterborne germs and bacteria – including Legionella pneumophila and Pseudomonads.