In the UK the Health and Safety Executives Approved Code of Practice ACOP L8 and Guidance document HSG274 Part 1 dealing with the control of Legionella bacteria and Legionnaires’ disease in cooling water systems describe issues which can influence the effectiveness of a water treatment programme developed to control legionella. These issues include the control of:
Each of these elements are interrelated and failure to control any one of them may lead to all occurring simultaneously, resulting in an environment that encourages the growth of legionella. When developing an effective monitoring and water treatment control programme, it should be remembered that scale formation and fouling are continuous physico-chemical processes, and inhibitors to control such processes should be added to the water system on a continual basis.
Treating cooling towers
All water treatment chemicals dosed to cooling tower systems should be automated to ensure precise and continuous protection is achieved. All components of the treatment programme should preferably be dosed using a high quality pump or eductor (sometimes referred to as an ejector) system, or by a suitable halogen dosing system such as a brominator. This will help to minimise the health and safety risks to operators and ensure that dosing frequencies and rates of chemical application are maintained as recommended.
The importance of dosage and control equipment in the overall control of water systems should never be underestimated.
Automating the water treatment process
Installing and using automated monitoring and control equipment in water systems is a major component in the control of legionella bacteria in water systems. The type of equipment used in water systems is varied and dependent upon the type of system to be treated.
Typically a water treatment programme for a cooling tower would require the use of a dosing system to accurately meter the chemical scale and corrosion inhibitors into the system at a rate proportional to the water make-up.
The level of concentration of salts in the cooling water is usually measured by conductivity and this measurement linked to a solenoid valve to actuate a controlled bleed-off of water from the cooling system. There may also need to be a polymeric dispersant added to the treatment programme in order to control fouling by suspended solids. Additionally, and importantly there should always be some form of automated biocide addition to the system.
Using biocides to control microbiological activity
Biocide addition and the equipment needed to treat cooling towers effectively will typically depend on the approach used, and this is usually a simple decision linked to the type of biocide recommended by your water treatment specialist.
Oxidising biocides, such as Chlorine, Bromine, Chlorine Dioxide, Peroxides or Ozone are usually dosed by specifically designed equipment. This equipment is usually controlled by measuring the reserve of chemical in the cooling water, either by direct species measurement or by the use of a REDOX measuring device to actuate or switch off the dosing equipment.
With this biocide approach it is possible to maintain conditions in the water system with biocide present at all times and to continually measure and record the reserve. This method of cooling tower water treatment has become increasingly popular over the last 20 years.
Using alternating biocides
Non-oxidising biocides are usually added to a water system using a twin dosage arrangement that is controlled by a timer. As an example, the timer would be set to add biocide “A” for 3 doses and an alternating biocide “B, would be added every 4th dose. Such an alternating approach helps to minimise the risk of micro-organisms developing a resistance to the chemical species added.
Using water softeners to reduce scale
In addition to control equipment the use of water softeners has become more prevalent as a method of keeping heat exchange surfaces free from scale formation. This has the additional benefit of reducing areas for pathogenic microorganisms to hide-out and proliferate.
In the fight to control Legionella bacteria, dosing and control equipment is recognised as a well established and proven part of all good water treatment programmes, helping to significantly reduce the inherent risks of operating water systems.
Legionella control specialists
Water Treatment Services offer a comprehensive range of legionella control and environmental solutions for the control of Legionella bacteria including water treatment solutions, water analysis and legionella risk assessments.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers, in-field water treatment service specialists and technicians we can offer cost effective legionella risk management, ACOP L8 compliance and engineering support services throughout the UK and Ireland.
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