How to prevent legionella in water systems

Present in all water sources, this bacteria is harmless at low concentrations. But, if allowed to thrive, there’s a real danger of serious, even fatal, infections.

Legionella is a fact of life for building services. There is no vaccine against legionellosis – preventing the growth and spread of legionella is the only safeguard. This is a longstanding legal duty, with heavy fines for non-compliance.

So, when installing or upgrading cold and hot water systems, it’s vital for duty holders to choose the right treatment system as part of their Legionella control strategy.

Water treatment has evolved

Technology for water treatment and control systems has advanced significantly since the UK Health and Safety Executive’s Approved Code of Practice (ACOP) first appeared. To ensure that they are the best fit for your intended application, research the alternatives carefully and seek expert advice.

For detailed guidance on all aspects of legionella control, consult ACOP L8. This summarised guide focuses on water treatment options only, relating to ACOP Part 2, sections 2.8 – 2.118.

Controlling legionella

  • Legionella flourishes between 20-45°C: keep water below 20°C or above at least 50°C if possible.

  • Legionella loves stagnant water: design and maintain systems to eliminate it.

  • Sediment, scale, corrosion, and biofilm: eliminate these Legionella habitats and nutrients.

  • Ensure adequate disinfection.

  • Assess and manage risks with a control strategy: maintain equipment, monitor temperature, biocide levels and other system aspects – and take action when required.

Treatment options

Temperature control vs biocides

Legionella remains dormant below 20°C and above 60°C is killed by scalding, so controlling water temperature can control legionella. There are drawbacks as this may:

  • be dangerous for end users (scalding risk)
  • consume excessive energy
  • be impractical or impossible to achieve.

Biocides and other techniques can be used with, or instead of temperature control. All listed below are proven as effective but any treatment system must be carefully matched to each application, whether used individually or in combination.


Chlorination in its various forms is the proven method for high volume disinfection, persisting in the water to provide ongoing protection. Chlorine dosing amount and frequency must be carefully calculated, controlled and monitored to balance disinfection effectiveness against the formation of undesirable chemical by-products and free chlorine levels. Free chlorine residuals up to 2.0 mg/l, but typically 0.5mg/l or less, provide effective continuous disinfection in water distribution systems.

Sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(ClO)2)

Hypochlorite can be added as an aqueous solution, in granules, by mixing precursors or created onsite by electrolytic chlorination. It dissolves in water to form hypochlorous acid (HOCl) and hypochlorite ions (OCl).

  • Highly effective disinfection that persists in the water and greatly inhibits biofilm formation. A higher initial dose may be needed to remove pre-existing bio-films.
  • Forms disinfection by-products (DBPs) that can affect water smell and taste in higher concentrations.
  • Aqueous hypochlorite solution decomposes quickly while electrolytically-generated hypochlorite chlorine solution is more durable, lasting for many months.

Find out more about electrolytic chlorination systems.

Chlorine dioxide (ClO2)

Chlorine dioxide is renowned for eliminating biofilm. It is dosed into or blended with water to control both legionella and stop biofilm growth in both hot and cold water. Less reactive than pure chlorine, ClO2 is a more selective oxidant and doesn’t create as many DBPs (disinfection by-products) as chlorination so the treated water is more palatable.

ClO2 is a much more efficient disinfectant than free chlorine because it is:

  • less dependant on pH levels than chlorine.
  • ten times more soluble in water.
  • does not hydrolyse.

More information on chlorine dioxide systems.


Formed by combining ammonia and sodium hypochlorite, monochloramine is more stable and does not dissipate as rapidly as free chlorine. 

  • Penetrates biofilm much faster than chlorine making it more effective.
  • Creates low amounts of DBPs and low THMs (trihalomethanes).

Copper-silver ionisation

Copper and silver ionisation is strongly biocidal in both hot and cold water, attacks biofilms and offers residual protection. It is widely used in hospitals in the UK and USA.

  • Maintaining adequate silver ion concentrations in hard water systems is difficult – regular electrode cleaning required.
  • No agreed safe level of silver ions for domestic water systems.
  • Effectiveness can decrease at higher pH levels.
  • May leave grey residue, possible bio-resistance.

Stabilised silver hydrogen peroxide

A relatively new and powerful biocide that’s also effective against biofilms. It is not pH-dependant and has no odour, taint or smell.

  • May be unsuitable for regular use in hospitals.
  • Hydrogen peroxide is hazardous to handle and store.
  • Expensive precursors.

Point-of-use filters

Used primarily as a temporary measure at the point of use (shower heads, taps), filters are useful for trapping high concentrations of legionella or dislodged biofilm.

  • Only treat water at point of filter contact, no residual protection in rest of system.
  • Requires regular cleaning or replacement.

Ultraviolet light

Common in healthcare and lower volume supply, UV treatment is chemical-free, fast, compact, simple to operate, clean and environmentally-friendly. 

  • Point of contact treatment only with no residual protection. 
  • Water should be pre-filtered or contaminants will absorb UV.
  • Lights quickly lose intensity and require regular cleaning.
  • Typically inadequate as a standalone solution, requires a second treatment method in tandem.

Picking a treatment solution

The optimum treatment(s) for legionella control varies with water system design and age, building size and water chemistry, as well as the need to control other organisms or contaminants.

As well as control efficacy, it’s vital to consider any solution’s practical and commercial aspects:

  • Compliance with appropriate legislation/standards.
  • Bulky equipment that doesn’t fit the space available.
  • Complex installation procedures.
  • Frequent, costly servicing.
  • Expensive or hard-to-source chemical precursors.
  • Hazardous chemicals with handling and storage challenges.
  • Ability to adapt or upgrade older water systems instead of complete replacement.

Whether you’re looking to disinfect a spa pool or treat millions of litres of drinking water a day, we manufacture systems that provide safe environments that you can trust.

Water treatment innovation from Gaffey

Gaffey is the UK’s market leader for in-situ disinfection systems. With decades of experience in building services, we design our in-situ technology to avoid the problems seen with traditional reaction and blending equipment.

Our service runs from installation through the entire lifetime of the product, with our UK-manufactured equipment backed by expert technical support and a completely UK-based supply chain.

Cost-effective with simple, rapid installation and minimal ongoing maintenance, our robust, compact and fully automated units fit easily into both new and existing water systems, and can scale to almost any treatment volume – just add extra modules or choose a skid system.

Consuming only low cost, inert salt and water, Hyprolyser® electrochlorination technology is a safer, cleaner and more environmentally friendly way to produce hypochlorite in-situ. Reg31-approved, it’s simple to use and ultra-reliable with no regular cleaning required.

Our WRAS-approved ChloriDOS® iOX® chlorine dioxide blending systems consume fewer chemicals on-site, offer online monitoring and employ super-safe and accurate auto-mixing under vacuum to minimise H&S issues in material handling.

ChloriDOS® MC (monochloramine) is our latest system, delivering a durable biocide with minimum by-products for the most demanding applications, like hospital hot water distribution.

Cutting-edge chlorination solutions

Innovative and safe with both purchase and rental options, Gaffey systems are proven in numerous treatment projects.

  • Effective – extremely potent biocides.
  • Efficient – high yield chemistry reduces chemical consumption and DBPs.
  • Economical – inexpensive precursors, minimal servicing.
  • Low risk – 100% UK-based supply chain.
  • Safe – standards-compliant, use low hazard or harmless chemical precursors.
  • Sustainable – safe chemical transportation and on-site generation minimises environmental impact.

Our treatment systems are proven to perform worldwide. Read how our customers and partners deployed Gaffey disinfection technology – safely, reliably and at minimum cost. 

At Gaffey, our experts have decades of experience in designing, specifying, installing and maintaining water treatment systems. 

We can help you to solve your toughest treatment problems.

Call: 01254 350180

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