Spray tunnel washers identified as Legionnaires risk

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.

Odour Management – A brief overview

Typically, odours are detected at very low concentrations of chemicals and compounds in air. The human nose is very sensitive with on average over 5 million scent receptors. Humans can detect concentrations as low as a few parts per billion (ppb), or less in air. This illustrates the difficulty of quantifying odour objectively.

The basic sensory attributes are

  • Detection – Concentration of an odour when first detectable
  • Recognition – Human ability to differentiate between odours, e.g. wine or vinegar
  • Intensity – Perceived strength at differing concentrations, e.g. faint, distinct, strong
  • Hedonic Tone – Pleasantness / offensiveness, e.g. pleasant, unpleasant, offensive
  • Odour Quality or Character – Association & complexity, e.g. the many “tones” and associations we have with an odour such as flowers, coffee, waste, sewage, etc.

The concentration at which an odour is just detectable by a panel of selected human “sniffers” is defined as the detection threshold and has an odour concentration of 1 European odour unit per cubic metre (1 ouE m -3 or 1 ouE/m3 )

  • 1 ouE m -3 is the point of detection
  • 5 ouE m -3 is a faint odour
  • 10 ouE m -3 is a distinct odour

People react to odours in different ways. Rural odours may be pleasant to one person but an anathema to another. Common odour sources and some of their possible issues are…

Sewage Treatment

Increase treatment volumes & flow rates e.g. from storm conditions, increased proximity of sensitive development such as housing, inadequate maintenance of odour control systems.

Food Processing and Commercial Kitchens

Extraction system design e.g. inadequate discharge height, absence of odour control at source, poor “filtration” system maintenance.

Plants and Solvents

Odour control system’s design, building leakage and poor positioning of vents (including garages and workshops)

Animals, Livestock and Poultry

Proximity of sensitive development such as housing, waster management on-site, poor dispersions on odours during early morning and evening.

How do we combat odours?

If we split the types of sources we have those which may be enclosed; i.e. in a shed or factory or those which are open field.

The historical method of dealing with enclosed odour sources are:

  1. ABSORPTION – Extract the air from the enclosure and allow it to be pulled through an odour absorbing material such as Carbon granules. The downside of this is that the carbon gets filled with water and odourous chemicals and needs regular renewing. High pressure fans need lots of electrical power to run them. High ongoing costs.
  2. MASKING – Use other, sweeter smelling chemicals to override the noxious ones. Costly in chemical use and often locals don’t like this smell either.
  3. BIOBED – Large structures are built to enclose vast amounts of wet, high surface area materials, (clinker, tree bark) that are impregnated with particular bacteria strains. This sits on a porous plenum into which the odourous air is pumped. As the odours percolate upwards they are “eaten” by the bacteria and less odourous chemicals escape. This is a high capital outlay and requires the bacteria to be looked after and kept damp. Drying out will kill the bacteria and odd excesses of gasses like ammonia may result in the death one strain of bacteria leaving another to thrive that may not be as useful in odour management.
  4. ELECTRICAL DISCHARGE – Ozone and other vigorous radicals can be created electrically. These are projected into the space where the odour is present. The odourous molecules are broken up by the aggressive radicals and odour is reduced. An effective solution but lots of regular maintenance is required as the ozone levels slowly decrease with use.
  5. SIMPLEST – Extraction of the foul air through a spray wash column. A scrubber. This relies on solubility and as most scrubbers re-circulate the water they are a source of high bacterial growth, fungal growth as the water becomes warmer. As the water becomes warmer it absorbs less ammonia and odours. They are also an item which requires regular legionella testing if biocides or UV-C systems are not employed.

The iPURtech method is an attempt to find an ideal scenario.

  1. Destroy the odours not capture or mask them.
  2. Reduce maintenance by making the process very simple.
  3. Use as little energy as possible.
  4. Require little site space.
  5. Take known technologies and design around them.
  6. A system that can re-circulate air within a shed or extract the air to atmosphere.

We use a mix of catalysts anchored to a high surface area media to destroy organic molecules including odours. The catalyst is not activated by heat, as in car exhausts, but by a particular wavelength of Ultra Violet energy. We burn / oxidise odour molecules in a cold environment.

Each iPURtech Air Purification System has a series of catalyst cartridges set between layers of UV lamps. The surface area of each layer being sufficient to allow a calculated contact time of the air passing through the system. The initial stage of the process includes set(s) of filters to remove dust or any other materials which could coat the catalyst. These may be hosed clean at intervals, (monthly) depending on the environment. The unit offers little air resistance so fans are low powered. A typical large unit will take less than 3 kw of lamp energy.