The inclusion of chlorine dioxide based biological control systems, such as HydroDOS®, in the HSE’s L8 approved code of practice, underlines the importance of this technology as a primary method in the control of legionella and other bacteria in water systems.
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Chlorine dioxide precursors are manufactured to meet the requirements of BS:EN standards
Meter and chlorine dioxide residual monitoring control offered as standard.
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The use of chlorine dioxide in water treatment is very wide spread, with vast quantities being used on a daily basis for the bleaching of both wood pulp and flour. For water treatment the earliest recorded use was in 1944 when chlorine dioxide was used in a processing stage to treat water from the Niagara Falls which served New York and it achieved large scale disinfection use in Brussels, Belgium in 1956, where it became preferred to the use of chlorine for the treatment of municipal water.
Its use in the treatment of water for building services has grown significantly in recent years, this has been driven by several factors including, increased awareness of biological related health issues, the need to conserve energy and the increasing quality and simplicity of use of chlorine dioxide systems, such as the HydroDOS® range of equipment. In use chlorine dioxide is termed a ‘dispersive’ treatment, this means that the chlorine dioxide is dosed into the water system and travels around the entire water system, providing a ‘residual’ level of treatment. This is very important as it means that the applied chlorine dioxide can continue to kill bacteria in all areas of the system that it reaches and not just at the point of use and this helps to explain why chlorine dioxide is so effective.
Chlorine dioxide (chemical formula ClO2) in its pure form exists at room temperature as a gas and dissolves sparingly in water, the equipment that we have developed however means that there is no requirement to handle this as a gas, instead two liquid ‘precursor’ chemicals are reacted together, the resultant solution being the chlorine dioxide required, in water. It has already been stated that one of the key benefits of this technology is that it is a dispersive technology, however there are several more features that give this material some unique benefits, these include:
- It is approved for the control of legionella bacteria in water systems by the HSE’s L8 approved code of practice (>0.1ppm)
- It is a very powerful disinfectant
- In addition to controlling legionella, it is fatal to a very wide range of bacteria, E.Coli, Coliforms, Pseudomonas, Cryptosporidium, Giardia Cysts, Algae and Amoebae
- Chlorine dioxide will kill both planktonic (free swimming bacteria) and sessile organisms (biofilms), this is very important as the majority of bacteria will live in the biofilms, rather than the free swimming form. Chlorine dioxide will destroy and remove biofilms
- It is approved for use in drinking water, by the Drinking Water Inspectorate (DWI) (<0.5ppm Total Oxidant)
- Bacteria cannot develop resistant strains
- Very simple to test for and monitor
- Chlorine dioxide can be used as an alternative to ‘temperature regime’ for legionella control, this has additional important benefits. For designs where chlorine dioxide is employed it is not necessary to operate the system at the traditional/conventional temperature levels (this excludes NHS premises), particularly for hot water systems where hot water should be generated at 60°C, the return temperature needs to be kept above 50°C and every hot tap should reach 50°C within one minute of operation. By utilising the benefits of chlorine dioxide, it is possible to generate and distribute hot water at much lower temperatures, for example 40-45°C if desired. For this to be possible though, it is necessary to operate the chlorine dioxide regime as stipulated by the HSE’s code of practice (please see the guidance notes overleaf ). There are some very significant environmental and financial benefits that can be gained by operating this regime, these are:
- Many of the more environmentally friendly methods of hot water generation can produce significant quantities of warm water, around the 40-45°C mark at high levels of efficiency. The production of water at higher temperatures may lead to the heater being much less efficient. A good example of this would be a ground source heat pump where, at the lower production temperatures, there is a reasonable coefficient of performance, however these drop off rapidly when the temperatures are elevated, alternatively a secondary heating source may be introduced, such as a gas or electric fired unit. By using the chlorine dioxide regime it is therefore possible for the user to save fuel, save on the cost of the fuel and significantly reduce carbon dioxide emissions, helping to fulfil many of the users’ environmental and commercial targets.
- By distributing the water at lower temperatures, it will be possible to reduce or totally eliminate the need for thermostatic mixing valves in the design. Not only are these valves relatively expensive and may give rise to operational problems, but they have been associated with several cases of Legionnaires disease, so their removal from a design is desirable in most cases.
- By reducing the temperature of hot water generation the propensity to form scale is correspondingly reduced. This has several benefits, first the heat transfer surfaces will stay cleaner for longer, ensuring the associated heat generation plant always operates at its optimum efficiency, helping to maintain the low carbon footprint and also by reducing scale formation, nutrition and habitat for bacteria is further reduced, meaning cleaner and healthier systems.
Guidance Notes for Operating the Chlorine Dioxide Control Regime in Hot and Cold Water Systems
The advice given in these notes is extracted from, Legionnaires disease, The control of legionella bacteria in water systems, Approved Code of Practice and Guidance, ISBN 0-7176-1772-6, published by the Health and Safety Executive and also known as the L8. The advice given is correct at the date of publication, but may be subject to revision by the HSE as appropriate and users are recommended to ensure these guidelines have not been superseded prior to design of a system.
In common with all water systems, a water services risk assessment is required; this is independent of which of the four regimes of bacterial control is employed. Additionally for all systems the HSE prescribe a standard for system maintenance, cleanliness and visual inspections. The HSE make recommendations on the scope and frequency of the risk assessment, this should be performed by a competent person, if this is not in place please consult a member of the Legionella Control Association for advice. Specifically for chlorine dioxide use, the HSE make the following recommendations:
Where biocides (this includes chlorine dioxide) are used to treat water systems they, like the temperature regime will require meticulous control if they are to be equally effective.
It is recommended that the control system is checked for the correct operation weekly, to ensure that any potential lapses in the application of the biocide does not leave the system vulnerable.
The Drinking Water Inspectorate prescribes a maximum value for the total oxidants in drinking water supplies, which is the combined chlorine dioxide, chlorite and chlorate concentration.
The level of total oxidants in a system should not exceed 0.5mg/l (0.5ppm).
To monitor the chlorine dioxide regime the following advice is given:
- The quantity of chemicals in the reservoir; (checking the number of drums used in the HydroDOS® system on a regular basis)
- The rate of addition of chlorine dioxide to the water supply; (water meter readings can be checked against the chemical use in a) above alternatively the chlorine dioxide readout can be recorded on a regular basis, or this can be fed to a BMS for a permanent record)
- On a monthly basis, the concentration of chlorine dioxide should be measured at the sentinel taps – the concentration should be at least 0.1mg/l; and
- On an annual basis, the chlorine dioxide concentration at a representative number of outlets – the concentration should be at least 0.1mg/l.
It is recommended that monitoring for legionella is carried out ‘in water systems treated with biocides where storage and distribution temperatures are reduced from those recommended in the section on the use of temperature to control legionella. This should be carried out on a monthly basis. The frequency of testing should be reviewed after a year and may be reduced when confidence in the efficacy of the biocide regime has been established’.