Frequently Asked Questions

Legionella Problem Legionella Problem

Legionella can be potentially fatal to everyone.

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10 Questions & answers about Chloridos™ and Chlorine Dioxide

Q1. What is Chlorine Dioxide?
Q2. How is Chlorine Dioxide Produced?
Q3. Is Chlorine Dioxide Safe to Drink?
Q4. How does Chlorine Dioxide Differ from Chlorine?
Q5. Can Chlorine Dioxide be used for Refrigeration Cooling Towers?
Q6. Can Chlorine Dioxide be used in Breweries and Dairies?
Q7. What other benefits does Chlorine Dioxide offer?
Q8. Why bother dosing Chlorine Dioxide in Hospitals and Older buildings?
Q9. Can Chlorine Dioxide be dosed into hot water systems?
Q10. Why dose Chlorine Dioxide in new, modern buildings?

A1. What is Chlorine Dioxide?

Chlorine Dioxide is a powerful oxidising disinfectant discovered by Sir Humphrey Davie in 1811. It is a slightly soluble gas that dissolves in water to give a Green coloured solution, that can be easily dosed where required.

A2. How is Chlorine Dioxide Produced?

There are several methods to produce Chlorine Dioxide. The most common is to react 2 Binary chemicals of Sodium Chloride and an Acid activator in equal amounts to produce a 2% (20,000p.p.m.) Chlorine Dioxide solution. Although this can be dosed directly into a water pipe, the ChloridosTM generator utilises a dilution stream to carry the chemical at lower concentrations, which is safer. The ChloridosTM generator can also safely produce Chlorine Dioxide solutions of up to 5% (50,000p.p.m.) using either higher strength or stabilised reagents.

A3. Is Chlorine Dioxide Safe to Drink?

Yes. Chlorine Dioxide has been used to treat drinking water since 1915. The recommended safe upper limit for potable water in Europe is 0.5p.p.m. For a well managed system, a continuous dose of between 0.15 0.4p.p.m will successfully inhibit most pathogens.

A4. How does Chlorine Dioxide Differ from Chlorine?

Chlorine has been used as a disinfectant for water supplies for over 150 years. It is a persistent disinfectant that survives for several days in distribution pipework. Although versatile, the active component HypoChlorous Acid is neutralised by high pH values, to become less effective. Above a pH value of 8.5, less than 10% of the biocide is effective!! Chlorine Dioxide is unaffected by high pH values as high as 12.5! Chlorine is also very reactive with other compounds to form Tri-Halo-Methane's, which cause Mucus irritation, and are regarded as potential carcinogens. Chlorine Dioxide is an oxidant, similar to Ozone and works in a completely different way to Chlorine. It destroys cell membrane walls in seconds, thus destroying pathogens. Laboratory and field tests have shown that lower concentrations of Chlorine Dioxide are faster and more effective at reducing bacteriological counts than higher concentrations of Chlorine. Chlorine Dioxide is also a "Broad Spectrum Biocide", being more effective against Algae's, fungi's and spores than ordinary Chlorine.

Chlorine Dioxide graph

A5. Can Chlorine Dioxide be used for Refrigeration Cooling Towers?

Yes. Chlorine Dioxide will not react with Ammonium compounds, unlike Chlorine based compounds which will produce Ammonium Chloride, a white liquid/solid compound that will cause blockages and problems.

A6. Can Chlorine Dioxide be used in Breweries and Dairies?

Definitely yes. Chlorine Dioxide is less reactive with "phenolic compounds", typically found in "humic" Scottish waters. These give a distinctive "TCP" type taste that will taint beers and spirits. This property ensures that the majority of large scale distilleries and breweries use Chlorine Dioxide in preference to Chlorine as a Secondary disinfectant. For breweries, Chlorine Dioxide is typically used for "Cleaning in Place (C.I.P.)" processes. This involves flushing the pipes & vats with a caustic solution, rinsing then flushing with a 2p.p.m. Chlorine Dioxide solution, followed by a final rinse. The tradition method of using 4% Nitric, phosphoric or Per-Acetic Acid is less safe form a COSHH perspective, and requires more flushing water before beer can be produced again.

A7. What other benefits does Chlorine Dioxide offer?

One of the most beneficial properties of Chlorine Dioxide is penetration and removal of biofilms in pipes and surfaces. Biofilms are thought to provide a "safe haven" for Legionella and other pathogens to reproduce, away from the main disinfecting water stream. Biofilms are also great insulators. 1mm of biofilm will provide the same drop-off of thermal efficiency as 3mm of scale on an heat exchanger.

Legionella Biofilms

A8. Why bother dosing Chlorine Dioxide in Hospitals and Older buildings?

Old hospitals and buildings have specific problems:

  • Large oversized metal roof tanks, containing scale, rust and biofilms
  • Long runs of old pipework with branches and "dead-legs"
  • Oversized tanks with low utilisation are difficult to keep within recommended temperature limits, thus encouraging bacterial growth
  • Hospitals house vulnerable patients who have reduced tolerance to Legionella, MRSA and other pathogens
Routine biological testing, often find high levels of Legionella pneumophila bacteria in hospital water systems. The standard response is to disinfect the system with 50p.p.m. of free residual Chlorine for at least 1 hour. This is fine for a short term remedy, but a more permanent solution is required. Field studies have shown that dosing Chlorine Dioxide continuously at 0.3 0.45mg/L to the incoming water supply, effectively removes biofilms and reduces the biological loads within 4 weeks after dosing commences.

A9. Can Chlorine Dioxide be dosed into hot water systems?

Yes. Hot water systems usually rely on high temperatures >65 degrees to "pasteurise" bacterial loads. Because of the need to reduce energy consumption (cost and Carbon footprint), and risk of severe scalding, this method is becoming less desirable. Investigations have often found that the source of Legionella infestation originate from the hot water system, because Legionella is a "thermally tolerant" bacteria. Because Chlorine Dioxide "gasses-off" at higher temperatures, it is not applied directly into the calourifier but dosed to the incoming cold water feed. This prevents "seeding of bacteria" from the incoming water supply, which can then colonise the hot water system. Once microbial loads have been reduced, then hot water temperatures can also be reduced, thus saving energy and costs.

A10. Why dose Chlorine Dioxide in new, modern buildings?

Because of fire suppression regulations, and security of supply from low pressure incoming mains, modern buildings have to store large amounts of water in a tank. Problems occur within variable usage systems with the water stagnating. The background temperatures can rise above 20degrees centigrade in summer, and the background residual free Chlorine levels can be depleted to below 0.1p.p.m. This reduces the primary disinfection levels so low, that microbial loading can occur. The effective answer is to treat the incoming mains water with Chlorine Dioxide, proportional to incoming flow. This will effectively apply a secondary disinfectant to the stored water to reduce bacteria and biofilm growth. The Chloridos™ advance generator also has a unique "boost" facility to dose disinfectant during low usage periods at a pre defined setpoint.


We trust you find the above information useful as a training and marketing aid. Gaffey Industrial Division staff have over 100 years of combined experience and knowledge in this field. If you have any other questions, or required further information then please contact Ian Walker, or Phil Gaffey on 01254 350180.

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