UK guidance for managing water quality, Legionella risk, and public safety in decorative water features.

PWTAG TN67 addresses a critical public health gap: decorative water features in public spaces are often treated as purely aesthetic installations, yet they present significant microbiological risks. Water features that generate aerosols through spray, jets, or cascading flow can transmit Legionella pneumophila and Pseudomonas aeruginosa to anyone within the spray zone. Children frequently paddle in open-access fountains, and ground-based jets in dry plaza installations invite direct body contact. TN67 recognises that these features require water treatment standards comparable to swimming pools despite being classified as non-interactive.

The inspection methodology combines two complementary frameworks. PWTAG TN67 provides the water chemistry targets and operational requirements specific to decorative water features, including the elevated chlorine range of 3-5 mg/L (higher than the 1-3 mg/L range for swimming pools) justified by the intermittent circulation and higher contamination risk inherent in outdoor features. BS 8580-1:2019 provides the Legionella risk assessment framework that evaluates the physical condition of the asset, the presence of biofilm and sediment as nutrient sources for bacterial colonisation, the aerosol generation potential, and the temperature profile of the water system.

The inspection workflow follows a structured five-stage process: first, the inspector classifies the feature type and its inherent risk profile based on public access level and aerosol generation potential. Second, water chemistry measurements are taken immediately upon arrival, before any physical disturbance of the water body. Third, the basin, jets, and surrounding structure are visually inspected for biofilm, sediment, algae, and structural defects. Fourth, the plant room equipment is checked including circulation pumps, chemical dosing systems, UV treatment, and filtration. Finally, management records are audited to verify that bacteriological testing, Legionella monitoring, and risk assessments are current.

The guidance is published by the Pool Water Treatment Advisory Group (PWTAG) and serves as the primary reference for water treatment in public swimming pools and water features across the United Kingdom.

The cornerstone of PWTAG TN67 compliance is maintaining water chemistry within defined safe limits. The standard mandates a free chlorine residual of 3.0-5.0 mg/L for chlorine-sanitised features, significantly higher than the 1.0-3.0 mg/L range required for swimming pools. This elevated range compensates for the higher organic loading from environmental contamination (leaves, bird droppings, dust, litter) and the intermittent operation schedules common in decorative features. The pH target of 7.0-7.4 is tighter than many pool standards and reflects the need to maintain chlorine in its most effective form as hypochlorous acid.

For features using bromine as the primary sanitiser, the target total bromine concentration is 4.0-6.0 mg/L, with a corresponding pH range of 7.4-8.2 that reflects the different dissociation curve of hypobromous acid. The inspector measures free chlorine, total chlorine, and pH using a DPD (diethyl-p-phenylenediamine) test kit or electronic photometer. Combined chlorine, calculated as the difference between total and free chlorine, should not exceed half the free chlorine reading. Elevated combined chlorine indicates the formation of chloramines, which reduce disinfection effectiveness and can cause respiratory irritation in the spray zone.

Water clarity is a critical pass/fail parameter under TN67. If the water is cloudy or opaque to the point where the basin bottom is not visible, the feature must be drained and closed to the public until clarity is restored. This is not merely aesthetic, as turbid water prevents visual detection of hazards, indicates inadequate filtration, and correlates with elevated bacterial counts. Water temperature is recorded because temperatures above 20 degrees Celsius create conditions favourable to Legionella proliferation, requiring enhanced monitoring and potentially more aggressive chemical treatment.

For related water quality management standards in drainage systems, see the CIRIA C753 SuDS inspection standard.

Legionella pneumophila, the bacterium responsible for Legionnaires' disease, thrives in warm stagnant water between 20 and 45 degrees Celsius and is transmitted through inhalation of contaminated aerosol droplets. Decorative water features present an ideal transmission route because they combine warm recirculated water with active aerosol generation from jets, sprays, and cascading flow. BS 8580-1:2019 (Water Quality - Risk Assessments for Legionella Control) provides the risk assessment methodology that PWTAG TN67 references for evaluating and managing this hazard.

The risk assessment evaluates four interconnected factors. First, aerosol generation potential is classified from High (spray and mist from jets) through Medium (splashing) and Low (laminar flow) to None (still water). Features with High aerosol risk require the most stringent chemical treatment and monitoring regimes. Second, the physical condition of the asset is assessed for biofilm, which provides the protective matrix within which Legionella colonies establish and resist chemical treatment. Widespread biofilm is a critical failure condition that requires immediate shutdown and deep cleaning. Third, sediment and organic debris are evaluated as nutrient sources that support bacterial growth. Heavy sludge accumulation in a basin or balance tank indicates inadequate maintenance and significantly elevates the Legionella risk profile.

Fourth, the management framework is audited to verify that Legionella testing is performed at least monthly, that results are recorded and trended, and that a current risk assessment (less than two years old) exists for the installation. The BS 8580 risk assessment status in the inspection form captures whether the risk assessment is current, expired, or non-existent. A missing or expired risk assessment is itself a non-conformance that must be flagged regardless of the water quality findings. The last Legionella test result is recorded with four outcome levels: Not Detected, Detected below 1000 cfu/L, Detected above 1000 cfu/L, or Unknown. Any detection above 1000 cfu/L triggers immediate corrective action including thermal or chemical shock treatment, system review, and re-sampling.

The full BS 8580 methodology is available through the British Standards Institution (BSI).

The physical inspection component of a PWTAG TN67 assessment evaluates the water feature asset from two perspectives: the visible condition of the basin and its surroundings, and the operational status of the treatment plant equipment. The basin inspection uses a structured assessment aligned with BS 8580 condition categories. General cleanliness is classified as Satisfactory or Unsatisfactory. Biofilm and slime presence is graded on a three-point scale: None Visible, Minor Localised Slime, or Widespread Biofilm. The distinction between minor and widespread is operationally critical because widespread biofilm indicates systemic failure of the disinfection regime and requires shutdown for deep cleaning, while minor localised slime can typically be addressed through targeted removal and chemical adjustment.

Sediment and debris accumulation is classified into four categories: None, Organic Debris (leaves and litter), Scale and Staining, or Heavy Sludge. Each category has different remediation requirements. Organic debris requires removal and review of site management practices. Scale and staining indicate water chemistry imbalances, particularly elevated calcium hardness or pH. Heavy sludge is a critical finding that requires drain-down, mechanical cleaning, and investigation of filtration effectiveness. Algae growth is assessed across four types: None, Green Algae, Black Algae, or Mustard Algae. Black algae is particularly concerning as it develops deep root structures that resist surface treatment and requires aggressive intervention including acid washing.

The plant room inspection covers the full treatment chain. Circulation status is recorded as Running, Off (Scheduled), Off (Fault), or Reduced Flow. TN67 recommends 24-hour continuous circulation for features with public access, and any unscheduled stoppage increases contamination risk. Pump strainers are checked for blockage or damage. Chemical dosing lines are inspected for leaks, crystallisation, and air locks, all of which compromise disinfection delivery. The UV system status is recorded as Operational, Fault Light On, Bulb Replacement Due, or Not Applicable. TN67 specifies a minimum UV dose capability of 40 mJ per square centimetre for effective secondary disinfection against Cryptosporidium, which is resistant to chlorine treatment.

For similar structural condition assessment methods in infrastructure assets, see the EN 13508-2 sewer inspection standard. Browse all available inspection standards in the standards directory.

The final stage of a PWTAG TN67 inspection audits the site management records to verify that the operator is maintaining the required documentation and testing regime. The logbook availability check confirms that a current operational logbook exists on site, recording daily water chemistry readings, chemical dosing adjustments, equipment maintenance, and any incidents or complaints. The absence of a logbook is a significant non-conformance that suggests reactive rather than proactive water management.

Bacteriological testing frequency is verified by recording the date of the last bacteriological test. TN67 recommends monthly testing for features with public access and aerosol generation potential. The test panel typically includes Legionella pneumophila, E. coli (as a faecal indicator organism), Pseudomonas aeruginosa (an opportunistic pathogen that thrives in warm water and biofilm), and Total Viable Count (TVC) as a general indicator of microbial load. The inspection form captures sample details including a unique Sample ID, the sample location (Basin, Jets, Balance Tank, or Make-up Water), and the specific tests requested for each sample.

The risk assessment status is classified as Current (less than two years old), Expired, or None Available. BS 8580 requires risk assessments to be reviewed at least biennially and updated whenever significant changes occur to the installation, the water source, the operating regime, or following any Legionella detection event. An expired or missing risk assessment represents a failure of the management system regardless of the current water quality results. Defects identified during the inspection are logged with a description, a category (Hygiene, Mechanical, Chemical, Structural, or Health and Safety), a priority level (Immediate/Safety Critical, High, Medium, or Low), and mandatory photographic evidence.

For further guidance on Legionella control requirements, refer to the Health and Safety Executive (HSE) Legionella guidance.