The water-contact surface is the copper pipe, the drinking water hose, but also the surface of drinks dispensers that come into direct contact with the drinking water. The nutrients are present in the water in the form of humic acids, microorganisms and breakdown products of biofilms. The formation of a biofilm, even in newly laid and cleaned pipes, takes place very quickly and can be divided into six phases.
Fighting biofilms
Fight biofilms and bacteria with biostream® products: chlorine dioxide penetrates the biofilm matrix.
Behind the water meter, drinking water enters pipe networks where microorganisms settle and form biofilms, ideal hiding places for Legionella, Pseudomonas and other waterborne germs. Chlorine dioxide fights exactly these biofilms where conventional products fail.
Biofilms as a source of pathogenic germs
Behind the water meter, the water enters a pipe network, for example a building installation, in which, in most cases, no refurbishment has taken place since construction. The microorganisms in these pipe systems can lead to the formation of biofilms, which show up in the presence of Legionella, Pseudomonas or other waterborne germs (for example, as CFU counts).
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The problem: the biofilm
Like an anchor, the biofilm protects microorganisms from being flushed out of the pipe system and offers them "shelter from the flow" for unimpeded growth. It performs this role for non-pathogenic as well as pathogenic germs.
A biofilm needs only the following factors to form:
- a water-contact surface
- enough water
- microorganisms
- nutrients
How chlorine dioxide destroys biofilms
Chlorine dioxide is known for its efficacy against biofilms. It targets the structure and function of these complex microbial communities through several mechanisms.
1. Penetrating the matrix
EPS
Chlorine dioxide penetrates the extracellular polymeric substance (EPS), the main component of the biofilm matrix, and gets into biofilms better than chlorine (sources under Where the key figures come from).
2. Oxidising the germs
O₂
As a strong oxidising agent, chlorine dioxide oxidises key cellular components such as proteins, lipids and nucleic acids. This leads to the structural collapse of the cells and the death of the microorganisms.
3. Destroying the structure
↯
Chlorine dioxide breaks down biofilm-forming compounds such as polysaccharides within the EPS, disrupts the physical structure of the biofilm and makes it more susceptible to mechanical removal.
4. Preventing new formation
∅
With regular use at suitable concentrations, the initial attachment of microorganisms is inhibited. This makes it harder for a new biofilm to form.
5. Broad spectrum of action
Chlorine dioxide is effective against a wide range of microorganisms, including bacteria, viruses, fungi and algae that are commonly found in biofilms. This breadth of efficacy is particularly beneficial for environments with diverse microbial populations.
Applications
Chlorine dioxide is used in various industries and sectors to fight biofilms, including in water treatment, in food processing plants, in the reprocessing of medical devices and in cooling systems. Regular, targeted use helps maintain the integrity and safety of these systems and reduces the risk of infections and other biofilm-related problems.
Making germs in drinking water visible
Bacterial smears on agar plates make visible what is hidden in the drinking water. Incubation on culture media serves to detect bacteria and biofilms and shows how effectively the disinfection actually works.
How do you recognise a biofilm?
A biofilm is usually there long before anyone sees it. The first signs are non-specific and are therefore often explained differently, for example by the season or a new batch of drinks. Typical signs are:
- Slimy coating on seals, hose ends, taps or in the tank, which feels greasy and comes back after wiping.
- Musty or earthy smell in the first water in the morning or after a longer period of stagnation.
- Change in taste of drinks, even though the product and preparation are unchanged.
- Cloudiness or flakes in the first water after stagnation.
- Recurring positive water samples despite regular disinfection. This is the clearest signal, because it shows that the source is not being reached.
The last point is the most important. If a system is unremarkable for a few weeks after every disinfection and then shows problems again, this is rarely down to the dosing. It is because the disinfectant treated the water, not the coating on the pipe wall.
Why must cleaning happen before disinfection?
The order is already in the title of the relevant code of practice: DVGW code of practice W291 is called "Cleaning and Disinfection of Water Distribution Systems". Clean first, then disinfect. This order is not a formality.
Every organic deposit in the pipe consumes disinfectant before it reaches the germs. The technical term for this is demand. Anyone who disinfects a heavily coated system directly is therefore dosing against the dirt, not against the microorganisms. The result is a disinfection that has taken place on paper but achieves little in the water.
In practice, this means: flush mechanically and loosen whatever can be loosened, and only then prepare the disinfectant. How to calculate the quantity for your system volume is set out in the dosing instructions.
Where do biofilms form first?
Biofilms grow where water stands still. Flowing water carries loose cells away; standing water lets them settle. So almost always the same points are affected:
- Dead legs, that is, sections of pipe that were disconnected after a conversion but not removed. They are never flushed through and act as a reservoir for the whole system.
- Rarely used outlets such as guest bathrooms, washbasins in side rooms, or taps that are only needed seasonally.
- Water tanks and boilers, in which some residue always remains.
- Hoses that are stored wet, for example after an event.
- Systems after an interruption in operation, from the dispensing system after the weekend to the motorhome after winter.
A single dead leg can repeatedly recolonise an entire installation. Anyone searching for the cause without success should therefore look for disconnected pipes first, before questioning the disinfection method.
Why is disinfecting once not enough?
When a well-established biofilm is broken up, the bacteria protected within it are released into the water. A sample taken directly after the first treatment can therefore turn out worse than before. This is not a sign that the method does not work, but that it has worked. In practice, this effect is known as initial worsening.
The consequence: where microbial contamination is confirmed or after a longer period of stagnation, the treatment is carried out twice. The first pass loosens the coating, the second captures the released germs. Anyone who takes a sample after the first pass and judges the result a failure abandons the remediation at precisely the point where it starts to work.
For the same reason, a thorough flush belongs after every disinfection, before the system goes back into operation.
What role do materials play?
Not every material behaves the same way. Plastics can provide microorganisms with nutrients and favour the growth of a biofilm. There is a dedicated test method for exactly this: DVGW code of practice W270 examines how strongly a material that comes into contact with drinking water promotes microbial growth.
In practice, this has two implications. First: for new installations and when replacing hoses, it is worth checking the material test, because an unsuitable hose keeps rebuilding the biofilm. Second: even a tested material does not stay germ-free on its own. The test states that the material does not additionally promote growth, not that no biofilm forms.
We have described the application for drinking water hoses separately: hose disinfection under KTW W270. Which other codes of practice apply is set out under Approval and standards.
Biofilm is not the same as limescale
The two are regularly confused, and the confusion leads to the wrong measure. Limescale is a mineral deposit, hard, light-coloured and can be scratched off. It forms from the water itself and is not alive. A biofilm is a living layer of microorganisms and their excretions, soft, slimy and usually greyish to brownish.
The practical difference: acid helps against limescale, but not against biofilm. Anyone who descales a slimy pipe ends up with a descaled pipe that still has a biofilm. Conversely, a disinfectant does not dissolve a limescale coating. In practice, the two often occur together, because a rough limescale layer offers the biofilm extra surface to grip. In that case, both steps are needed, in this order: descale first, then clean, then disinfect.
A simple test helps tell them apart: limescale feels dry and rough and dissolves in vinegar or citric acid with bubbles forming. A biofilm feels slippery, can be wiped off, and is back again after a short time.
Further reading
- Presentation on biofilms (in German), structure, formation and control in context
- Drinking water basics (in German), the summary of the active substance and its use
Context and evidence
Efficacy tested. In 1,620 individual tests at the Institute of Food Science and Human Nutrition of Leibniz University Hannover (led by Dr. rer. nat. H.-D. Werlein), biostream® ZERO was shown to have an efficacy of 99.9 % against germs occurring in drinking water, at a contact time of 5 minutes.
Hydrogen peroxide is not a disinfectant in the drinking water list. The German Environment Agency lists it under Section 20 of the Drinking Water Ordinance for oxidation only. Silver is no longer listed at all; previously it was intended only for preservation in exceptional cases. Chlorine dioxide is listed there for disinfection. For the disinfection of installations whose disinfection water is discarded afterwards, DVGW codes of practice W 291 and W 551-3 name hydrogen peroxide and hypochlorite in addition to chlorine dioxide.
Permitted in drinking water. Chlorine dioxide is included in the list of permitted treatment substances and disinfection methods kept by the German Environment Agency (UBA) under Section 20 of the Drinking Water Ordinance (TrinkwV). Its production and use are governed, among others, by DIN EN 12671 and DVGW code of practice W 224; for the disinfection of installations, DVGW W 291 and W 551-3 name chlorine dioxide as a proven chemical.
Biofilm in practice: the guides
How you remove a biofilm in your system depends on where it sits. These pages describe the procedure together with the rules that apply in each case:
- Fighting Legionella in the drinking water installation
- Flushing and disinfection: the order for system disinfection
- Cleaning and disinfecting the water tank in a motorhome or boat
- Cleaning and disinfecting dispensing systems and beer lines
- Disinfecting the water lines of the treatment unit
- Cleaning and disinfecting drinking water hoses
Use biocides safely. Always read the label and product information before use. All statements on efficacy are based on the many years of experience of biostream® GmbH, as well as on information from specialist literature, legislation, codes of practice and positive customer references. Material compatibility applies when the recommended dosage and contact time are observed.