--- title: "Aqueous tunnel washers and spray pre-treatment plant: the Legionella risk HSE singled out" source_url: https://legionella.io/articles/aqueous-tunnel-washers-and-spray-pre-treatment-plant-legionella-risk/ canonical_url: https://legionella.io/articles/aqueous-tunnel-washers-and-spray-pre-treatment-plant-legionella-risk/ pillar: "Building Types & Use Cases" summary: "HSE singled out aqueous tunnel washers after a manufacturing outbreak. How to assess spray pre-treatment plant, tanks, aerosols and records." primary_keyword: "aqueous tunnel washer Legionella" date_published: 2026-07-08 date_reviewed: 2026-07-08 author: "Legionella.io editorial team (REMOTE TECH LTD)" reviewed_against: "HSE L8 and HSG274 guidance" region: "United Kingdom" license: "CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Quote, adapt or republish with attribution to REMOTE TECH LTD and a link to source_url." license_url: https://creativecommons.org/licenses/by/4.0/ --- # Aqueous tunnel washers and spray pre-treatment plant: the Legionella risk HSE singled out Some process-water risks are so specific that a generic "industrial water" paragraph is not enough. Aqueous tunnel washers are one of them. HSE has a dedicated Sector Information Minute on Legionella in aqueous tunnel washers. It was written after two employees at a large engineering firm developed Legionnaires' disease and the source was traced to an aqueous tunnel washer used as pre-treatment plant before painting [1]. That does not mean every washer is a crisis. It does mean this equipment deserves its own line in the risk assessment, not a passing mention under "process water". ## What equipment are we talking about? The plant HSE describes is the kind used to pre-treat components before wet paint or powder coating. Components travel through a tunnel with a series of sprayed wash, rinse and chemical-treatment stages. Each section drains back into a holding tank and the water or solution is recirculated through spray nozzles [1]. That layout is common in vehicle-component, architectural metalwork, powder-coating and white-goods manufacturing. The risk is not the paint process itself. It is the combination of warm recycled water, large tanks, contamination from parts, scale, rust, biofilm, convoluted pipework and fine aerosol escaping where people load, unload, inspect or clean the plant. This is different from a small enclosed parts washer or a hand lance. The HSE SIM is mainly about large tunnel washers and similar spray-wash plant where recycled water may rise above ambient temperature and be sprayed [1]. Smaller washers still need proportionate assessment, but do not blindly copy a tunnel-washer regime onto equipment with a different scale and exposure pattern. ## Why tunnel washers can grow and spread bacteria The trap is the rinse water. Some stages may be deliberately hot, while nominally cold rinse stages can warm up through drag-out from heated chemical stages and heat transfer from the components. In the HSE investigation, rinse water in holding tanks built up to a range HSE described as suitable for Legionella growth [1]. The water is also dirty in a very particular way. Oil, metal particles and process residues wash off the components. Mild-steel tanks can rust. Limescale and sludge collect in tanks, pipework and nozzles. The washer creates exactly the niches that bacteria and biofilm exploit, then turns the same water into spray. The aerosol route is the decisive point. Legionnaires' disease is caught by breathing in contaminated water droplets, not by swallowing process water [2]. HSE's tunnel-washer memo highlights fine aerosol generation inside the tunnel, escape from the tunnel openings and monorail gaps, exposure to workers near loading and unloading points, and extra aerosol during cleaning with hoses or pressure washers [1]. ## The control file a tunnel washer should have Use this as an evidence index for the responsible person and competent adviser. It is not a dosing schedule. **Plant scope** - Identify every tunnel washer, spray pre-treatment line and large recirculating washer on site. - Keep the plant schematic, tank list, spray-section layout, ventilation route and manufacturer's operating guidance with the risk assessment. - Record which teams own production, maintenance, water treatment, cleaning, local exhaust ventilation and environmental discharge. **Risk assessment** - Include process plant as well as domestic hot and cold water systems in the Legionella risk assessment [1]. - Check tanks, header tanks, pipework, dead legs, spray nozzles, filters, drains and areas where sludge, scale, rust or biofilm collect. - Consider workers at the plant and people beyond the immediate area if aerosols are extracted or blown outside [1]. - Include downtime, holiday shutdowns, weekend stops and restart conditions. **Control measures** - Monitor temperatures in the relevant tanks and sections. - Consider whether process stages can be kept outside the growth range, where product quality and the process allow [1]. - Manage scale, corrosion, suspended solids, filtration, flow, stagnation and material build-up. - Specify cleaning that physically removes sludge, scale and biofilm rather than only rinsing surfaces. - Choose any biocide or disinfection approach with a competent provider, because compatibility with the pre-treatment chemistry, plant materials and surface finish matters [1]. **Aerosol control** - Check whether spray escapes at entry, exit, conveyor or monorail gaps. - Review spray-jet angles, strip curtains, air curtains, extraction and local ventilation. - Treat pressure washing or jet cleaning of contaminated plant as a high-exposure maintenance task, not routine housekeeping. - Record the respiratory protective equipment and safe system of work for cleaning tasks that may generate contaminated mist [1]. **Monitoring and records** - Trend general bacterial results, such as dip slides, rather than treating one isolated result as the whole story [1]. - Use Legionella sampling where the assessment and HSE SIM indicate it is needed, and set the frequency with the competent adviser and control history [1][3]. - Keep records of temperature checks, cleaning, dosing, sampling, corrective actions, shutdown controls, training and responsible-person decisions. - Graphing or tabulating results is useful because drift matters more than a single number [1]. ## How this differs from ordinary process-water advice The industrial overview in [Legionella prevention in industrial facilities](https://legionella.io/articles/legionella-prevention-in-industrial-facilities/) is the starting point: anything that stores, warms and sprays water belongs in the asset register. Aqueous tunnel washers need the next level of attention because HSE has named the plant type, described a real outbreak investigation and listed risk factors that are easy to miss from a normal plumbing survey [1]. The closest sibling is [metalworking fluids and machine coolant sumps](https://legionella.io/articles/metalworking-fluids-and-machine-coolant-sumps-an-overlooked-legionella-risk/). Both are manufacturing systems that recirculate water and make mist. The difference is that a tunnel washer often has bigger tanks, multiple spray stages, cross-contamination between stages, difficult-to-clean pipework and potentially wider aerosol escape. Food and drink manufacturers should also recognise the pattern. The same blind spot appears when a food-safety system checks product water but ignores warm non-product spray, as covered in [Legionella in food and drink manufacturing](https://legionella.io/articles/food-and-drink-manufacturing-legionella/). The label changes; the aerosol logic does not. ## Where sites usually under-control the risk The common failure is assuming the chemical process controls the biology. A hot degrease tank or a surface-treatment chemical may discourage growth in one stage, but carry-over can warm and contaminate a rinse, and substitution or process changes can alter the biological balance. Treat every tank and rinse stage as its own water environment. The second failure is cleaning the visible tunnel and leaving the reservoirs. HSE is explicit that swilling down may not remove sludge and biofilm, and that effective cleaning can require draining, physical removal of deposits and periodic chemical clean-and-scour work [1]. If cleaning never reaches the holding tanks, pipework corners and nozzle internals, the plant can look better without being controlled. The third failure is downtime. A line that is tolerable during steady production can go out of control over a holiday stop, then aerosolise the first shift's water on restart. The shutdown plan should say whether tanks are drained, treated, cleaned, sampled or purged before operators return. ## FAQ ### Is an aqueous tunnel washer legally a cooling tower? No. A tunnel washer is not a cooling tower, and the cooling-tower notification duty is a separate issue. The Legionella duty still applies because it is a water system that can store, warm, recirculate and aerosolise contaminated water. HSE's SIM says these process plants should be included in the Legionella risk assessment [1]. ### Do we need Legionella samples from every tank? Not automatically. HSE's tunnel-washer SIM expects microbiological monitoring and says Legionella sampling should be carried out at regular intervals for the type of plant it describes [1]. The exact sampling points and frequency should come from the site assessment, plant design, water-treatment programme, cleaning history and competent advice, not from a copied template. ### Can our paint or powder supplier choose the biocide? They should be involved, but not alone. The biocide has to work against the microbiological risk and be compatible with process chemistry, plant materials, product quality, discharge limits and surface finish [1]. That usually needs the plant manufacturer, paint or powder supplier, water-treatment provider and site EHS lead in the same decision. ## Next step Walk the line with the production lead and the water-treatment adviser. Mark every tank, spray stage, warm rinse, sludge trap, nozzle bank, opening where mist escapes, and cleaning task that uses a hose or pressure washer. If any of those items is absent from the Legionella risk assessment, that is the gap to close first. ## Sources [1] HSE, "Legionella in aqueous tunnel washers - SIM 03/2009/06". https://www.hse.gov.uk/foi/internalops/sims/manuf/3_09_06.htm [2] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html [3] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm