Why do bacteria return after water disinfection?

It is easy to think that the problem is solved when a water sample after treatment shows good results.

In some systems, that is the case.

But in other cases, the bacteria return after days, weeks, or months.

This does not necessarily mean that the disinfection was ineffective.

It may mean that the treatment succeeded in reducing the microorganisms in the water volume itself, but that the root cause still remains somewhere in the system.

Common causes are:

• Biofilm
• Stagnant water
• Dead legs
• Poor circulation
• Inaccessible parts of the system
• Too low or uneven disinfection level
• New contaminants
• Lack of cleaning
• Short answer: why do bacteria return?

Bacteria can return after disinfection if microorganisms still remain on the surfaces of the system or if the same conditions that created the problem in the first place still exist.

It may involve:

• Biofilm in pipes and tanks
• Areas with low water turnover
• Water that becomes stagnant
• Inadequate treatment of the entire system
• Recontamination after treatment

It is therefore important to distinguish between treating the water and sanitizing the entire water system.

What happens during a one-time disinfection?

A one-time disinfection is often aimed at quickly reducing the microbiological load.

• It can be effective for:
• Reducing bacteria in the water phase
• Lowering the total microbiological load
• Improving water samples
• Restoring control after a deviation

But if bacteria are established in biofilm or inaccessible parts of the system, they can survive longer than the bacteria that are freely suspended in the water.

After treatment, they can then begin to spread back into the water phase.

Why is biofilm so important?

Biofilm is one of the most common explanations for recurring microbiological problems.

Biofilm consists of microorganisms that attach to a surface and are surrounded by a protective structure.

It can be found in:

• Pipes
• Tanks
• Hoses
• Valves
• Pumps
• Filters
• Heat exchangers
• Taps
• Showers
• Other water-bearing components

Microorganisms in biofilm behave differently than microorganisms that are freely suspended in water.

This can make them harder to reach with a treatment.

Can a good water sample be misleading?

Yes, a single sample can provide a snapshot.

If the sample is taken shortly after a treatment, the result may show that the water phase is well controlled.

But the sample does not always show exactly what is happening inside the pipes.

If biofilm remains, microorganisms may later begin to be released into the water again.

This does not mean that the sample was wrong.

It means that the sample described the situation at that exact time.

What does recolonization mean?

Recolonization means that microorganisms begin to establish themselves again after a treatment.

It can happen in two ways.

Microorganisms remain in the system

Biofilm or other microbiological reservoirs partially survive the treatment.

New microorganisms enter

The system is recontaminated via, for example:

• Refilling
• Raw water
• Hoses
• Taps
• Service work
• Ventilation or open tanks
• Other external sources

It is therefore important to both treat the existing load and reduce the risk of new contamination.

What does stagnant water mean?

Stagnant water is water that is not regularly circulated.

It can be found in:

• Dead legs
• Rarely used taps
• Spare lines
• Tanks
• Closed sections
• Long pipe sections

Water that stands still has different conditions than water with good circulation.

The temperature can change, and the disinfection effect can decrease.

At the same time, microorganisms have more time to interact with the surfaces of the system.

What is a dead leg?

A dead leg is a part of a pipe system where the water no longer circulates normally.

It can occur when:

• A tap is removed
• An installation is rebuilt
• A spare line is left behind
• An old pipe is not completely removed
• Water can then stand still for a long time.

It creates an area where microbiological growth can have better conditions.

When the flow later changes, microorganisms can spread back to the rest of the system.

Why does circulation matter?

Good circulation helps to:

• Circulate water
• Reduce stagnant zones
• Distribute treated water
• Even out temperatures
• Maintain microbiological control

Poor circulation, on the other hand, can create local areas where the treatment does not work as well.

A system can therefore look well treated at one measurement point while other parts have significantly worse conditions.

Why does the treatment not always reach the entire system?

There are several possible causes.

The treatment solution may not reach:

• Dead legs
• Low points
• Rarely used taps
• Closed valves
• Filter housings
• Spare sections
• All surfaces of the tank
• Complex pipe sections

If you only treat the main part of the water volume, some parts of the system can remain as microbiological reservoirs.

Can too low a concentration be a problem?

Yes.

Too low a concentration can mean that microorganisms are not sufficiently reduced.

But more is not always better.

The right treatment depends on:

• Disinfection method
• Water quality
• Organic material
• Contact time
• Temperature
• pH, depending on the method
• Material in the system
• Microbiological load

The right dosage and the right contact time are more important than just increasing the concentration without control.

What does contact time mean?

Contact time is the time the disinfectant needs to be in contact with the water and the surfaces of the system.

A treatment can be too short even if the concentration is high.

In the same way, a lower concentration over sufficient time can sometimes work better than a very short treatment.

It is therefore the combination of concentration and contact time that is central.

Can dirt and organic material reduce the effect?

Yes.

Organic material and other substances in the water can react with the disinfectant.

This can mean that part of the treatment is consumed before it reaches the microorganisms.

It is therefore important to assess:

• Turbidity
• Sediment
• Organic load
• Biofilm
• Other reactive substances
• A heavily loaded system may need to be cleaned or pretreated before the disinfection becomes effective.

What happens if a filter is dirty?

Filters can collect:

• Particles
• Organic material
• Sediment
• Microorganisms

If the filter is not maintained, it can become a place where microbiological load builds up.

This means that a system can be treated effectively, but then recontaminated from an old filter.

Therefore, filters always need to be included in a system review.

Can a refilling hose cause recontamination?

Yes.

It is common to focus on the tank and pipes but forget the hose used during filling.

A hose that:

• Stays warm
• Is damp inside
• Lies on the ground
• Is rarely used
• Is not cleaned
• can itself contain microbiological load.

Then clean water can be contaminated already during refilling.

What happens after service work?

Service and rebuilding often involve opening the system.

Then new microorganisms or particles can enter.

It is therefore common that larger systems need to be:

• Flushed
• Cleaned
• Disinfected
• Checked

after major interventions.

Otherwise, a previously stable system can develop new microbiological problems.

Are the same bacteria always back?

Not necessarily.

A recurring problem can be due to:

• The same microorganisms that survived in biofilm
• New microorganisms from the water source
• Recontamination from equipment
• Changes in operating conditions

It is therefore important not to assume that every deviation has exactly the same cause.

With recurring problems, microbiological analysis and system review may need to be combined.

Why can the problem return after weeks?

After a treatment, the amount of microorganisms can be very low.

But if the basic conditions are still favorable, the population can gradually build up again.

It can take time.

Therefore, a system can first show good sample results and then deteriorate after a few weeks.

This is typical for problems where:

• Biofilm remains
• Water stands still
• The temperature is favorable
• Disinfection is not maintained
• New microorganisms are introduced

Is a one-time test enough?

Not always.

A single sample can be valuable, but it only provides information about a certain place and time.

With recurring problems, it may be necessary to have:

• Repeated samples
• Sampling at multiple locations
• Temperature checks
• Flow checks
• Tank inspections
• Biofilm assessment
• Filter checks
• System mapping

The more complex the system is, the more important the overall picture becomes.

What is the difference between sanitizing and maintenance?

This is central.

Sanitizing

Aims to reduce an already established microbiological load.

It may require:

• Higher treatment level
• Longer contact time
• Cleaning
• Flushing
• Treatment of the entire system

Maintenance

Aims to keep the system stable after sanitizing.

It can involve:

• Continuous dosing
• Regular flushing
• Temperature control
• Good circulation
• Sampling
• Cleaning

They are therefore two different ways of working.

Why is sanitizing not always enough?

If the system's root problem is not addressed, the same conditions will remain.

Example:

• The dead leg still remains
• The water still stands still
• The temperature is still favorable
• The filter is still contaminated
• The refilling routine is still poor
• Biofilm still exists in parts of the system

Then sanitizing is only a temporary restoration.

What is continuous treatment?

Continuous treatment means that a controlled amount of disinfectant is continuously added.

The purpose is to maintain microbiological control over time.

It may be relevant in systems where there is:

• Continuous supply of new water
• Recurring microbiological load
• Large distribution systems
• Risk of biofilm
• Critical hygiene requirements

Continuous treatment does not, however, replace the need for good system design and maintenance.

How does Xinix work with recurring problems?

Xinix works with solutions for both sanitizing and continuous microbiological control.

FreeBact® Water is used for the disinfection of drinking water in different volumes.

For larger and continuous systems, FreeBact® Dosage has been developed for continuous dosing in, among others:

• Properties
• Industries
• Pools
• Spas
• Hospitals
• Other technical water systems

With a recurring microbiological problem, the system first needs to be assessed.

The goal is not just to get a good sample immediately after treatment.

The goal is to understand why the problem occurred and how the system is kept stable over time.

Common questions

Why are the bacteria back even though the tank was cleaned?

The problem may be in other parts of the system, for example pipes, hoses, pumps, or taps.

Can biofilm survive disinfection?

Biofilm can be significantly harder to treat than microorganisms in free water. The result depends on the method, concentration, contact time, and how established the biofilm is.

Is it enough to increase the dosage?

Not always. Higher dosage does not automatically solve problems with poor circulation, biofilm, or dead legs.

Can new water become contaminated immediately?

Yes, if it is filled into a system where microorganisms or biofilm already remain.

Does the entire system need to be treated?

If the goal is system disinfection, the treatment needs to reach as large a part of the system as possible, not just the main tank.

Why is the water sample good at first and bad later?

It may be because the water phase was treated, but microorganisms were later reintroduced from biofilm or other parts of the system.

Can continuous dosing prevent the problems from coming back?

Continuous treatment can be an important part of long-term microbiological control, but the system's construction and operation also need to be correct.

Can an old filter cause bacterial problems?

Yes. Filters that are not maintained can collect material and microorganisms and become a source of recontamination.

Summary

When bacteria return after disinfection, it is often a sign that the problem is larger than the water volume itself.

Common causes are:

• Biofilm
• Stagnant water
• Dead legs
• Poor circulation
• Inadequate system treatment
• Filters and components that have not been cleaned
• New contamination
• Lack of long-term control

The most important principle is therefore:

Do not just treat the sample, understand the system.

A successful sanitizing should preferably be followed by measures that reduce the risk of the same problem recurring.

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Contact

Thank you for following along all the way here!
If you have any questions & thoughts, feel free to contact us.

Alexandra Németh, Vice CEO & Marketing Manager, Xinix Global AB
alexandra@xinix.se
Mobile: 070-78 601 678