Legionella deaths can often hit the headlines. The source of an outbreak in Edinburgh in 2012 was never discovered although two people died and dozens became very ill. A recent legionella death in Stoke-on-Trent was associated with a spa-bath display in a shopping centre.
Why should we be concerned in the metal finishing industry?
Legionella bacteria are commonly found in natural water systems such as rivers and lakes where their numbers are usually low and they pose little risk. They grow over a temperature range of 20°C to 45°C. Below this temperature they survive dormant and as the temperature rises above 50°C they are progressively killed off and will not survive long at temperatures above 60°C. Their ideal growth temperature is around human body temperature, 37°C so that even in a hot chemical bath system the first rinse is a perfect environment for bacteria. Whilst legionella are widely found in the natural environment the risk comes when they contaminate man-made water systems, entering at low levels in the incoming water supply or in air-borne aerosols. Many aqueous spray plants not only provide the ideal environment for legionella to multiply but also involve water sprays which can release dangerous aerosols into the atmosphere.
As well as a water temperature of 20°C to 45°C legionella bacteria need a supply of nutrients which are usually provided by other microbes which are commonly found in the water. In particular there is a strong relationship between biofilms (microbial slimes), amoebae and the growth of legionella. Biofilms flourish in warm, stagnant water. As a general rule legionella likes dirty systems which are fouled with corrosion products and scale and dislikes clean water systems.
Am I describing your wash tunnel?
The cost of gas is continually rising so the reduction in process temperatures will also give rise to more legionella outbreaks.
Employees at a large engineering firm in the Midlands of England were diagnosed with Legionnaires disease; the only common factor was their employment at the company. After investigation and sampling, the source of their infection was found to be an aqueous tunnel washer (pre-treatment plant) of the type that is typically found at motor vehicle and ‘white goods’ manufacturing facilities. The plant was of the type used to pre-treat components prior to painting (e.g. with wet paint or powder coating).
The management “owned up” to their inadequacies and invited the HSE to make a full investigation which would help others who had similar equipment. The details are on the HSE web site under “legionella in aqueous tunnel washers.”
To summarise: Under general health and safety law, you have to consider the risks from legionella that may affect your staff or members of the public and take suitable precautions. As an employer or a person in control of the premises (eg a landlord), you must:
- identify and assess sources of risk;
- prepare a scheme (or course of action) for preventing or controlling the risk;
- implement and manage the scheme – appointing a person to be managerially responsible, sometimes referred to as the ‘responsible person’;
- keep records and check that what has been done is effective; and
- if appropriate, notify the local authority that you have tunnel spray washers on site.
The first course of action when identifying any risk is to invite a competent company to produce a risk assessment. In the case of spray tunnel washers the results will inevitably indicate a “high risk”. i.e. Storage of tepid, water in a dirty environment and the permanent presence of water droplets in the air.
What to do?
The common practice for companies who have identified the problem is to add a biocide to the tanks on a regular basis in order to keep the background level of bacteria in control. This is combined with a daily “dip slide test” which gives a bacteria reading a day or so later when the slide is incubated at 30- 35C. Some companies raise the temperature of the bath to 60C.once a week. This does kill bacteria but they do multiply and return to previous levels within days. It’s also a waste of energy and will not deal with the most serious area of contamination. i.e. the tepid and dirty first rinse stage.
The choice of biocide will depend on a number of factors including:
- Compatibility with chemicals used in the pre-treatment process
- Requirement to change the formulation as the bacteria become immune
- Availability of competent staff who are trained and equipped to use these chemicals Possibility of chemical attack and degradation of the plant itself
- Possible effect on the components and interference with the surface finish
- The chemistry of the rinse waters (e.g. pH, reduction potential and temperature)
- Plus the water board do not like excess biocide in the wastewater as it kills their useful bacteria in the sewage farm.
The alternative is to use a non-biocide approach and fit a catalytic reactor source specifically designed to cope with cloudy liquids. The “cloudy liquids definition” is important as a standard UV system will not work in this industry because its UV can only penetrate “clear” water. An iPURtech Liquid Purification System will not need regular attention or trained staff to operate it and will save on the regular purchase and storage associated with biocides. It also kills fungus and the bacteria which can never become immune to its rays.