---
title: "Industrial effluent treatment plants, wet scrubbers and odour suppression: Legionella risks in environmental control plant"
source_url: https://legionella.io/articles/industrial-effluent-treatment-plants-wet-scrubbers-odour-suppression-legionella/
canonical_url: https://legionella.io/articles/industrial-effluent-treatment-plants-wet-scrubbers-odour-suppression-legionella/
pillar: "Building Types & Use Cases"
summary: "HSG274 Part 3 names industrial effluent treatment, wet scrubbers and odour suppression as other risk systems. How to assess and record them."
primary_keyword: "industrial effluent treatment 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/
---

# Industrial effluent treatment plants, wet scrubbers and odour suppression: Legionella risks in environmental control plant

Some industrial Legionella risks sit in the gap between production, environmental compliance and facilities. They are not taps. They are not cooling towers. They may not even be described internally as water systems.

HSG274 Part 3 still names them. Its list of "other risk systems" includes air washers, wet scrubbers, particle and gas scrubbers, industrial effluent treatment plants, and fire, dust and odour suppression systems [1]. The shared issue is not the label on the plant. It is water that can be stored or recirculated, contaminated by the process, warmed by operation and then turned into aerosol.

That is the missing audit question for many factories and process sites: who has checked the environmental-control plant?

## What this article covers

This is not a general guide to sewers, public wastewater networks or environmental permitting. It is about site-owned plant that uses water to control, treat or suppress process emissions and then may expose workers or others to breathable droplets.

Typical examples include:

- industrial effluent treatment plant with tanks, sumps, spray bars, aeration, agitation or wash-down;
- wet scrubbers, air scrubbers, packed beds and gas-cleaning units using recirculated water;
- odour-control systems that humidify, scrub or mist an air stream;
- dust-suppression misting or spray systems around conveyors, yards, waste handling, loading bays or process lines;
- fire-suppression or deluge systems where standing or recirculated water can become contaminated and then aerosolise during use or testing.

Some of these systems are covered by environmental permits, process safety reviews or maintenance schedules. That does not automatically mean Legionella is being controlled. A permit may care about discharge quality. A production inspection may care about uptime. The Legionella risk assessment has to ask a different set of questions: is water able to support growth, and can that water become aerosol?

## Why HSG274 Part 3 matters here

HSE's HSG274 page describes the technical guidance as support for dutyholders who must comply with legal duties to control exposure to Legionella [2]. Part 3 is the part for risk systems other than evaporative cooling and hot and cold water [1].

The Part 3 list is deliberately broad. It says other risk systems may produce aerosols and gives examples including the scrubber, effluent-treatment and suppression systems above [1]. It then gives the core risk factors: Legionella in the system water, conditions that allow growth such as water between 20 and 45 C, stored or recirculated water, sludge or biofilm that can feed bacteria, aerosol generation and susceptible people who may be exposed [1].

That framework fits environmental-control plant very closely. Effluent tanks and scrubber sumps often receive process residues. Dust and odour systems may run outdoors where water warms seasonally and then stagnates. Wet scrubbers may recirculate water through packed media or nozzles where scale and deposits collect. The plant can look like an environmental asset while behaving like an aerosol water system.

## The field checklist

Use this checklist to decide whether a piece of environmental-control plant belongs in the Legionella risk assessment. The answer does not have to be dramatic. A simple "yes" to storage, recirculation and aerosol is enough to bring the system into scope for competent review.

| Question | What to look for | Why it matters |
| --- | --- | --- |
| Does the system store or recirculate water? | Tanks, sumps, wet wells, scrubber basins, settlement tanks, water loops, reclaim systems | Stagnant or recirculated water is one of the Part 3 risk factors [1] |
| Does it warm up in normal operation? | Solar gain, warm process streams, pumps, motors, exhaust air, heated production areas | Water in or near the 20-45 C range can support growth [1] |
| Does process contamination enter the water? | Dust, metal fines, organic matter, oils, sludge, scale, rust, biofilm, food or waste residues | Deposits and organic matter can support bacterial growth [1] |
| Can it create aerosol? | Spray nozzles, misting heads, aeration, agitation, pressure washing, packed beds, fans, exhaust stacks | Legionella risk depends on breathable droplets, not drinking the water [1][3] |
| Who can breathe the aerosol? | Operators, maintenance staff, drivers, contractors, neighbours, the public near exhaust or yard sprays | Exposure determines the practical risk and the controls needed [1] |
| Is it owned outside facilities? | Environmental manager, production engineer, contractor, wastewater operator, cleaning team | Split ownership is how these systems drop out of water-safety records |

The most useful output is a short register entry for every relevant asset: location, plant owner, water source, storage volume, recirculation route, aerosol point, exposure group, controls, records and review date.

## Effluent treatment plant: avoid the broad-wastewater trap

The phrase "industrial effluent treatment" can pull people into the wrong debate. Not every drain, sewer or effluent process creates the same Legionella exposure route. The source-backed concern here is narrower: plant that creates the conditions HSG274 Part 3 describes.

An enclosed balance tank with no aerosol route is not the same as a warm treatment tank with aeration, open access, spray wash-down and operators standing nearby. A remote settlement lagoon is not the same as an indoor process sump that is pressure-washed every Friday. The assessment should not simply stamp "wastewater" as high or low risk. It should trace the water, heat, contamination and aerosol route.

Maintenance tasks deserve special attention. A system that creates little aerosol during normal running may create a lot during drain-down, hose cleaning, descaling, nozzle clearing, filter changes or sludge removal. If the only person breathing the mist is a contractor on a shutdown weekend, that exposure still counts.

## Wet scrubbers, air scrubbers and odour plant

Wet scrubbers and odour-control units can be awkward because they are usually discussed as air-emissions equipment. Water may be there only to capture dust, particles, gases or odorous compounds, so the water side gets treated as supporting infrastructure.

For Legionella, the water side is the system. Check the sump, make-up water, bleed, dosing, pH control, scale, suspended solids, media condition, spray headers, nozzles, demisters, access doors and exhaust route. If water sits in a basin, recirculates over packing and is pulled through an air stream, you need to know whether aerosol can leave the unit during operation, inspection or cleaning.

Do not assume that process chemistry is enough protection. Some scrubber liquors will be hostile to bacterial growth; others will not. Some will change as the production process changes. A competent assessment should decide whether the water chemistry, cleaning frequency, turnover, temperature, deposits and aerosol controls are enough, rather than relying on the fact that the plant is not domestic plumbing.

## Dust and odour suppression

Dust and odour suppression systems often look low-tech: a water line, a pump, a few nozzles, maybe a tank and timer. That simplicity is why they get missed.

The risk changes with use pattern. A continuously used misting system with treated water and maintained nozzles is a different proposition from a yard spray fed from a tank that sits warm for days and runs only when dust complaints rise. Outdoor systems also see seasonal temperature swings, sunlight on pipework, dirty environments and long dead legs to remote nozzles.

For suppression systems, the assessment should cover the make-up water source, storage tank condition, turnover, filters, nozzles, dead legs, drain-down, winterisation, restart after inactivity and who is in the spray plume. Where systems are temporary or contractor-owned, require the same evidence you would expect for permanent plant: schematic, maintenance, cleaning, disinfection where appropriate and records.

## Controls and records

HSG274 Part 3 does not hand you one universal frequency for this mixed group. It says dutyholders should assess the risk, maintain records, keep systems adequately controlled and often use maintenance, cleaning, disinfection and monitoring where appropriate [1]. HSE's other-risk-systems page gives the same direction: risk assess, keep these systems clean, monitor them where appropriate, and maintain maintenance and monitoring records [3].

For environmental-control plant, a defensible written scheme usually answers these points:

- who owns the plant and who owns Legionella control;
- what normal operation looks like, including temperatures, flow, turnover and dosing;
- where sludge, scale, rust, deposits and biofilm can accumulate;
- how often tanks, nozzles, packing, filters and demisters are inspected and cleaned;
- when the system is drained, disinfected, purged or restarted after inactivity;
- what monitoring is used, such as temperature, visual condition, biocide residuals, general microbiology or Legionella sampling;
- what result or condition triggers escalation;
- what safe system of work applies when cleaning may create contaminated mist.

Testing should follow the assessment. HSE says the regularity of checks depends on the system and the outcome of the risk assessment, and that where Legionella monitoring is appropriate, sampling should follow BS7592 and use a UKAS-accredited laboratory taking part in a suitable proficiency scheme [4]. For these plant types, sampling is usually only one part of control. Physical cleanliness, water turnover, process stability and clear corrective actions matter just as much.

## How this fits with nearby articles

Start with [Legionella prevention in industrial facilities](https://legionella.io/articles/legionella-prevention-in-industrial-facilities/) if you are building the whole site register. That article ranks cooling towers, welfare systems, process water and low-use emergency assets.

Use [HSG274 Part 3 explained](https://legionella.io/articles/hsg274-part-3-explained-other-risk-systems-beyond-hot-and-cold-water/) if you need the general map of "other risk systems" beyond hot and cold water. Use [Aqueous tunnel washers and spray pre-treatment plant](https://legionella.io/articles/aqueous-tunnel-washers-and-spray-pre-treatment-plant-legionella-risk/) for paint or powder-coating pre-treatment lines, where HSE has a more specific manufacturing source. Vehicle and pressure-wash systems are a related but different route, covered in [Car washes and pressure washers](https://legionella.io/articles/car-wash-pressure-washer-legionella/).

The practical rule is simple: if environmental plant stores or recirculates dirty water and can put droplets into the air, do not leave it out because it is managed by production, environment or a contractor. Put it in the register, assess the route and record the controls.

## FAQ

### Is every wastewater or effluent system a Legionella risk?

No. The risk depends on conditions, not the label. HSG274 Part 3 names industrial effluent treatment plants as examples of other risk systems, but the assessment still has to find water storage or recirculation, growth conditions, deposits, aerosol generation and possible exposure [1]. A broad "wastewater equals Legionella" rule is not precise enough.

### Do wet scrubbers need a separate Legionella risk assessment?

They need to be included in the site Legionella risk assessment if they store or recirculate water and can create aerosol. That may be a dedicated assessment for a complex scrubber or a section within the wider site assessment for simpler plant. The important point is that the water side is visible, owned and controlled.

### Can environmental permit monitoring replace Legionella controls?

No. Permit monitoring may prove something about emissions or discharge quality, but it does not automatically prove that aerosolised water is controlled for Legionella. Use permit and process data where helpful, then add the Legionella-specific checks for water condition, deposits, aerosol release, exposure, cleaning, disinfection, monitoring and records.

## Sources

[1] HSE, HSG274 Part 3, "The control of legionella bacteria in other risk systems". https://www.hse.gov.uk/pubns/priced/hsg274part3.pdf
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] HSE, "Other systems that can create a risk of legionella". https://www.hse.gov.uk/legionnaires/other-risk-systems.htm
[4] HSE, "Testing and monitoring your water system for legionella". https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm