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| Chemical Dosing and Control Systems |
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The Approved Code of Practice & Guidance: Legionnaires’ disease (L8) describes a number of areas which will influence the effectiveness of water treatment programmes:
They are interrelated and failure to control any one may lead to all occurring simultaneously, resulting in an environment that encourages the growth of legionella. In setting up an effective monitoring and control system, it should be remembered that scale formation and fouling are continuous physico-chemical processes, and inhibitors to control such processes should be added on a continual basis.
The importance of dosage and control equipment in the overall control of water systems should never be underestimated. Installing and using 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 meter scale and corrosion inhibitor into the system at a rate proportional to the water make up. The level of concentration of salts is usually measured by conductivity and this measurement linked to a solenoid valve to actuate a controlled bleed off of water from the system. There may need to be a polymeric dispersant to control fouling by suspended solids and there should always be some form of automated biocide addition.
Non oxidising biocides are added by a twin dosage system that is controlled by a timer. The timer would be required to add biocide A for 3 doses and an alternating biocide B, would be added every 4th dose to minimise the risk of micro organisms developing a resistance to the chemical species added. 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, and hence reducing areas for pathogenic micro organisms to harbour. In the fight to control legionella bacteria, dosing and control equipment is recognised as a well established and proven part of significantly reducing the inherent risks of Article by: |
All chemicals dosed to cooling tower systems should be automated. All components of the treatment programme should be preferably dosed by pump or eductor (sometimes referred to as an ejector) systems, or by a suitable halogen dosing system such as a brominator. This will minimise the health and safety risks to operators and ensure that frequencies and rates of application are maintained as recommended.
Biocide addition and the equipment needed depend on the approach used, and it is usually a simple decision linked to the type of biocide prescribed by the water treatment specialist. Oxidising biocides, such as Chlorine, Bromine, Chlorine Dioxide, Peroxides or Ozone are usually dosed by specifically designed equipment, and the 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 has become increasingly popular over the last 15 years. |
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