||The ACOP says that the risk from exposure to legionella should be prevented or controlled precautions should include the use of water treatment techniques.The following section on treatment programmes offers guidance on how to treat water in cooling systems.A complete water treatment programme based on the physical and operating parameters for the cooling system and a thorough analysis of the make-up water should be established. The components of the water treatment programme should be environmentally acceptable and comply with any local discharge requirements.
It is important to ensure that water treatment programmes have sufficient range of adjustments to cope with any potential variations in make-up water supply quality. This enables control to be maintained. Failure to take account of variations in quality may lead to the rapid development of uncontrolled microbiological conditions within the cooling system.
There are a number of factors which will influence the effectiveness of any treatment programme:
corrosion; scale formation; fouling; and microbiological activity.
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 corrosion, scale formation and fouling are continuous physico-chemical processes and inhibitors to control such processes should be added on a continuous basis.
All components of the treatment programme should be preferably be dosed by pump or educator (sometimes referred to as an ejector) systems or by a suitable halogen dosing system such as a brominator. This will minimise health and safety risks to operators and ensure that frequencies and rates of application are maintained as recommended.
In many cooling systems, a significant proportion of the construction material is mild steel which is susceptible to corrosion. Although heat transfer equipment may be made of more corrosion-resistant metals such as copper or copper alloys or stainless steel, these metals also need to be adequately protected. Corrosion of steel should be inhibited as it may lead to conditions which encourage the growth of legionella. There are two types of corrosion inhibitors available; anodic and cathodic and a treatment programme would generally use both types for optimum protection. The actual inhibitors used will depend on the type of system and its operation water quality, operating temperatures, construction materials and environmental constraints. In all cases, corrosion inhibitors need to be applied continuously since their effectiveness depends on the presence of clean metal surfaces. This highlights the need for pre-commissioning cleaning and subsequent passivation of the metal surfaces. Corrosion inhibitors are commonly applied at a point of good mixing, such as the suction side of the recirculating pump.
Scale is the localised precipitation of normally water-soluble inorganic salts. Its formation is influenced by the concentration of calcium and magnesium salts, alkalinity and pH, surface and bulk water temperatures and the concentration of the total dissolved solids.
Scale formation results in loss of heat transfer, reduced flow rates, loss of efficiency and deposition/corrosion. Legionella can be associated with such deposits. The scale protects the bacteria and so reduces the effectiveness of biocidal treatment.
Chemicals used to control scale are known collectively as scale inhibitors. The specific chemicals used will depend upon the type of scale predicted from the water chemistry and system operating conditions. In systems which contain scale, or have had a history of scaling problems, chemical analysis of the scale will ensure that the most effective treatment programme is selected.
There are a number of other methods of scale control including:
It is common practice to apply scale inhibitors to a point of good mixing such as the suction side of the recirculating pump.
The term fouling is normally applied to deposition of particulate material and debris such as:
In systems using make-up water, which has a high concentration of suspended solids, pre-clarification may be necessary. Where this is not feasible, side stream filtration can be used to remove particulate debris introduced into the cooling tower.
Fouling can be controlled or alleviated through:
The operating conditions of a cooling system provide an environment in which micro-organisms can proliferate. The water temperatures, pH conditions, concentration of nutrients, presence of dissolved oxygen, carbon dioxide, sunlight, together with large surface areas all favour the growth of micro-organisms such protozoa, algae, fungi and bacteria, including legionella.
Problems arise when micro-organisms are allowed to grow or flourish to excess; this can result in the formation of biofilms on system surfaces.
Both surface-adhering (sessile) and free-flowing (planktonic) bacteri need to be controlled for a complete and effective programme.
Biocides are used to control microbiological activity. They should prevent the proliferation of micro-organisms but are not required to disinfect systems. Biocides can be oxidising or non-oxidising. Control, i.e. the frequency and quality of additions, will depend on the microbiological activity of the system.
Biocides, when correctly selected, applied and controlled, as part of a comprehensive water treatment programme, have been shown to be effective in preventing the proliferation of legionella. Many factors will influence the selection of chemicals required for the treatment programme. However, the success of the treatment programme is dependent on:
Biocides are routinely applied at the tower sump or the suction side of the recirculating water pump but should be dosed so that the biocide will circulate throughout the cooling system. However, in air-conditioning systems where the tower can be bypassed, the biocide needs to be added to the suction side of the recirculating pump.
Specific surfactants (bio-dispersants) which function by wetting bio-films and aiding penetration of the biocides into the films are often used to supplement oxidising biocide programmes. In microbiologically dirty systems which contain or readily grow biofilms, the use of bio-dispersants can improve the efficiency of oxidising biocides. Most non-oxidising biocide formulations already contain surfactants to improve performance.
Hazard data sheets should be available for all chemicals used in treatment applied to cooling towers and an assessment drawn up to ensure that those handling and applying them do it safely.
Where a biocide has been selected specifically for the control of legionella the supplier should be able to present test data to demonstrate efficacy, which should include kill concentrations and contact times.
Regardless of the results of laboratory testing, to establish an effective biocide programme to control legionella, it should be remembered that an operating cooling system is subject to unpredictable recontamination both by legionella and sources of nutrients. Therefore, regular microbiological testing needs to be carried out to ensure that the biocide programme remains effective.
A variety of other methods of water treatments is available. One approach relies on the electrolytic dissolution of metals such as copper and silver, thereby generating biocidal ions in solution. Another is the introduction of ozone, which produces an oxidising biocide in the water. Physical methods such as irradiation by ultraviolet (UV) light can also be used, although this is only effective when the water is clear, so it may be necessary to install a water filtration system too. Also, since UV irradiation is not a dispersive technique, additional biocides may be required to control biofilms.
Each of the techniques described above has the potential advantages, that they could replace the use of chemical biocides. However, they should only be used if they are capable of achieving at least the equivalent biocidal effect to those of the traditional methods.