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 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.
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.
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.