Water Treatment & Disinfection Glossary

The Approved Code of Practice L8 — 'Legionnaires' disease: The control of Legionella bacteria in water systems' — published by the HSE. It provides practical guidance on how to comply with the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health Regulations 2002 (COSHH) in relation to Legionella risk.

The chemical ingredient in a biocidal product responsible for the biocidal action. Under BPR, active substances must be approved before products containing them can be authorised. Chlorine dioxide is an approved active substance for multiple product types including PT2, PT4, PT5, PT11, and PT12.

A chemical substance intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. Under the UK Biocidal Products Regulation (BPR), biocides must be authorised before they can be marketed or used. Chlorine dioxide is classified as a biocidal active substance used in multiple product types.

A structured community of microorganisms attached to a surface and enclosed in a self-produced matrix of extracellular polymeric substances (EPS). Biofilm forms in water systems, pipework, cooling towers, and spa jets, providing a protected environment where bacteria like Legionella can multiply. Chlorine dioxide is one of the few biocides that can penetrate and remove biofilm.

The GB Biocidal Products Regulation governs the authorisation, marketing, and use of biocidal products in Great Britain. Derived from EU Regulation 528/2012, GB BPR requires that all biocidal products are assessed for safety and efficacy before they can be sold. The regulation is enforced by the Health and Safety Executive (HSE).

Bromo-chloro-dimethylhydantoin (BCDMH) is a solid biocide tablet that releases both bromine and chlorine when dissolved in water. Commonly used in spa pools and hot tubs, bromine has a narrower effective pH range than chlorine dioxide and cannot penetrate biofilm. Bromine produces brominated disinfection by-products.

A compound formed when chlorine reacts with ammonia or organic nitrogen compounds in water. Chloramines (mono-, di-, and trichloramine) are responsible for the characteristic 'swimming pool smell' and cause eye and respiratory irritation. Trichloramine is volatile and particularly problematic in indoor pool environments. Chlorine dioxide does not form chloramines.

A yellowish-green gas with powerful oxidising properties used as a biocide for water disinfection. Unlike chlorine, it does not form trihalomethanes (THMs) or chloramines. Chlorine dioxide works through selective oxidation, targeting microorganisms without reacting indiscriminately with organic matter. It is effective across a wide pH range (4–10) and penetrates biofilm.

The primary inorganic by-product of chlorine dioxide disinfection. When chlorine dioxide reacts with target organisms, it is reduced to chlorite ion (ClO₂⁻). Chlorite levels are regulated in drinking water (typically 0.7 mg/L maximum in UK standards). Chlorite is considered significantly less harmful than the THMs and chloramines produced by chlorine.

An evaporative heat rejection device used in industrial and commercial HVAC systems. Cooling towers create conditions favourable for Legionella growth due to warm water temperatures and aerosol generation. They are regulated under ACOP L8 and HSG274 Part 1, requiring a written risk assessment, water treatment programme, and regular monitoring.

A protozoan parasite that causes cryptosporidiosis, a gastrointestinal illness. Cryptosporidium oocysts are highly resistant to chlorine disinfection but can be inactivated by chlorine dioxide and UV treatment. Pool and spa operators must consider Cryptosporidium risk in their water treatment strategy.

A section of pipework in a water system that has little or no flow. Dead legs allow water to stagnate, creating conditions for biofilm formation and Legionella colonisation. They should be removed where possible, or treated with a disinfectant residual that can reach them — chlorine dioxide's stable residual makes it effective in treating dead legs.

Chemical compounds formed as unintended consequences of water disinfection processes. DBPs include trihalomethanes (THMs), haloacetic acids (HAAs), chloramines, and bromate. The type and quantity of DBPs depend on the disinfectant used, water chemistry, and organic matter present. Chlorine dioxide produces significantly fewer DBPs than chlorine or bromine.

The independent regulator of drinking water quality in England and Wales. The DWI maintains lists of approved products and substances for use in public water supplies. Chlorine dioxide products used for drinking water treatment (PT5) must comply with DWI Regulation 31 requirements.

Hazard Analysis and Critical Control Points — a systematic preventive approach to food safety that identifies physical, chemical, and biological hazards in production processes. HACCP-compatible disinfectants like chlorine dioxide (PT4 authorised) are essential for food processing environments where food safety must be documented and controlled.

The UK government body responsible for the regulation and enforcement of workplace health and safety. The HSE is the competent authority for biocidal products regulation (GB BPR) in Great Britain and publishes key guidance including ACOP L8, HSG274, and HSG282 for Legionella risk management.

HSE technical guidance published in three parts: Part 1 covers evaporative cooling systems (cooling towers), Part 2 covers hot and cold water systems, and Part 3 covers other risk systems. HSG274 supplements ACOP L8 with detailed practical guidance for managing Legionella risk in specific system types.

HSE guidance document for the management of spa pool water quality. HSG282 covers the design, operation, and management of spa pools, hot tubs, and hydrotherapy pools, with specific guidance on microbiological control, water treatment, and monitoring requirements.

A genus of pathogenic gram-negative bacteria, most notably Legionella pneumophila, which causes Legionnaires' disease — a severe form of pneumonia. Legionella proliferates in warm water systems between 20–45°C and is commonly found in cooling towers, hot and cold water systems, spa pools, and shower systems. Control is mandated under ACOP L8.

A serious and potentially fatal form of pneumonia caused by inhaling water droplets containing Legionella bacteria. It is not transmitted person-to-person. Outbreaks are associated with poorly maintained water systems including cooling towers, hot tubs, and building water systems. Prevention requires effective water treatment and risk management under ACOP L8.

A chemical reaction involving the loss of electrons. In water treatment, oxidation is the mechanism by which disinfectants destroy microorganisms by disrupting their cell membranes and metabolic processes. Chlorine dioxide is a selective oxidant — it targets specific chemical structures in microorganisms rather than reacting indiscriminately with all organic matter.

A classification system under BPR that categorises biocidal products by their intended use. Relevant product types for water treatment include PT2 (disinfectants for private and public areas), PT4 (food and feed area disinfection), PT5 (drinking water disinfection), PT11 (cooling system preservatives), and PT12 (slimicides).

An opportunistic gram-negative bacterium that thrives in water systems, particularly in biofilm. Pseudomonas aeruginosa causes skin infections (folliculitis), ear infections, and can cause severe infections in immunocompromised individuals. It is a key target organism in swimming pool and spa pool water treatment, and is resistant to many conventional disinfectants when protected by biofilm.

A BPR Product Type 2 disinfectant is authorised for use in disinfection of private and public areas including swimming pools, spas, hot tubs, and surfaces. PT2 products must demonstrate efficacy against target organisms and meet safety standards for human exposure in treated environments.

A BPR Product Type 4 disinfectant is authorised for use in food and feed area hygiene. PT4 products are used to disinfect equipment, containers, utensils, surfaces, and pipework associated with food production, processing, transport, storage, and consumption. They must be safe for indirect food contact.

A BPR Product Type 5 disinfectant is authorised for use in drinking water disinfection. PT5 products are used to make water safe for human consumption by eliminating pathogenic microorganisms. They must comply with drinking water quality standards including the Drinking Water Inspectorate (DWI) requirements.

A formulation technology where chlorine dioxide is continuously generated from precursor chemicals in solution. ChloroKlean Plus L20 uses regenerative technology to provide a sustained release of active chlorine dioxide, offering prolonged antimicrobial activity compared to single-dose application methods.

The concentration of active disinfectant remaining in water after initial demand has been satisfied. A measurable residual throughout a water system indicates ongoing protection against microbial regrowth. Chlorine dioxide provides a stable, measurable residual that persists in distribution systems, unlike chlorine which is rapidly consumed by organic matter.

A document providing information on the properties and hazards of a chemical substance or mixture, required under REACH and CLP regulations. An SDS includes sections on identification, hazards, composition, first aid, handling and storage, exposure controls, and disposal. All ChloroKlean products are supplied with compliant SDS documentation.

Trihalomethanes are disinfection by-products formed when chlorine reacts with organic matter in water. Common THMs include chloroform, bromodichloromethane, and dibromochloromethane. THMs are regulated in drinking water due to potential health effects. Chlorine dioxide does not produce THMs, making it advantageous over chlorine in many applications.