Clean the water tank of bacteria

The most common form of Legionella control is by means of maintaining a strict temperature regime, effectively keeping the cold water cold (below 20) and the hot water hot (above 50) thereby avoiding the range suitable for bacterial growth. Where this is not possible or a significant risk is identified at the assessment stage some other form of additional control is desirable. This generally takes the form of continual addition of a biocide (a substance which kills micro-organisms) the most common of which is chlorine dioxide.

When dosed at the correct levels, chlorine dioxide is safe for human consumption and excellent at eradicating many waterborne bacteria including Legionella. It also has the added benefit of being able to penetrate biofilm. This can be of particular importance in buildings with recurrent problems , as biofilm can line the sides of the pipework providing a protected environment for bacteria to grow undisturbed.
 During the course of routine sampling the bacteria will be detected and the water system disinfected, usually using free chlorine. This will kill all of the free floating bacteria but not those contained within the film. Two - three weeks down the line the bacteria will start to re-colonise the system.

Chlorine dioxide gets over this problem by means of a two pronged attack; stripping out the biofilm and providing 24 hr disinfection.

Continuous dosing does require significant capital investment (purchasing and install of the unit) and ongoing costs are incurred in terms of chemical supply, sampling and testing. However, where bacterial levels are controlled using biocide, the hot water system can be run at whatever temperature desired. For larger installations, cost savings have been identified by off setting the chemical costs with the energy savings made from not having to constantly heat the water to 60 degrees.

The majority of on-site chlorine dioxide generation is by means of mixing an acid and chlorite in a reaction chamber and then dosing proportionally into the water system. Recent advances have led to ‘one drum' solutions whereby a single chemical is passed through a catalyst - this greatly reduces COSHH implications.

At all times chlorine dioxide levels should not exceed 0.5 ppm in potable water supplies.
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Source : website www.nemco-utilities.co.uk 503.