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Pool Chemistry 101-A: Glossary of Terms

Updated: Feb 26

There are a lot of different terms used when discussing the chemistry of swimming pools and spas or hot tubs. Some can sound quite technical and complex and many will appear in our conversations with customers and our articles, so here's a list of some of the most common and important terms explained:


Algaecide - Group of chemicals that target algae growth in water

Active Oxygen - An oxidising chemical based on hydrogen peroxide (H2O2)

Automatic Dosing System - A system which constantly measures and adds chemicals


Biofilm - A chlorine-resistant growth which can sometimes be found in pipes

Bromamine - Disinfection byproduct formed when using bromine as a sanitiser. Less harmful and irritating than chloramines and are as effective at disinfection as hypobromous acid


Calcium Hardness (CH) - the total amount of calcium present in water, commonly reffered to simply as "hardness"

Chloramine - Disinfection byproduct formed when using bromine as a sanitiser. Can be harmful and irritating, do not aid disinfection, and can lower the effectiveness of free chlorine

Clarifier - Chemical added to water to resolve slight cloudiness

Colony Forming Unit (CFU) - Viable (able to multiply) microorganisms

Combined Chlorine - General term for chloramines

Cyanuric Acid (CYA) - Chlorine stabiliser used to prevent dissipation of chlorine by UV radiation


Disinfectant - Any chemical such as bromine or chlorine added to the water to counteract pathogens and other compounds in water

Disinfection Byproduct (DBP) - General term for chloramines, bromamines, Trihalomethanes and other compounds produced when sanitisers "do their job"

Feeder - General term for a tablet holder connected to the swimming pool or spa plumbing which gradually adds sanitiser to maintain residual levels

Floater - A tablet holder that floats on the surface of the water to slowly release sanitiser and maintain residual levels

Flocculant - Chemical that binds smaller particles together to aid filtration or allows removal via vacuuming

Flush - Chemical or process used to clean pipework and remove biofilm

Folliculitis - Term for "hot tub rash" caused by Pseudomonas Aeruginosa

Free Chlorine - Amount of dissolved chlorine available for disinfection


Hypobromous Acid - the active disinfectant produced when bromine is dissolved in water

Hypochlorous Acid - the active disinfectant produced when chlorine is dissolved in water


Oxidiser - product or chemical used to break down compounds and DBPs

Ozone - O3 gas used as secondary disinfection. Sanitises, is a potent oxidiser, and has a flocculant effect


Pathogen - microorganisms such as bacteria and viruses that can cause disease or infection

pH - the scale used to measure the quantity of hydrogen ions present (represented as how acidic or alkali a solution is)

Photometer - A digital testing tool that uses light frequencies to measure water chemistry

Pseudomonas Aeruginosa - Bacteria found in water which causes hot tub rash/folliculitis


Reagent - A tablet or liquid added to a water sample to produce a colour, which is then compared to a colour chart or placed into a photometer to assess water chemistry

Residual - the maintained level above zero of sanitisers such as chlorine or bromine


Sanitiser - Another general term for disinfectant

Secondary Disinfection - General term for systems including UV and ozone generators that support the action of sanitisers, can inactivate chlorine-resistant microorganisms, and sometimes have added benefits

Shock - The chemical or action used to increase sanitiser levels and oxidise compounds within water

Stabiliser - Another term for cyanuric acid


Total Alkalinity (TA) - the overall quantity of dissolved alkali in the water

Total Chlorine (TC) - Free Chlorine + Combined Chlorine

Total Dissolved Solids (TDS) - Overall quantity of dissolved solid compounds in the water. High TDS affects sanitiser efficacy and clarity

Total Viable Count (TVC) - Concentration of undefined microorganisms present in water - measure of the colony forming units per ml.

Trihalomethanes (THMs) - type of disinfection byproduct


UV - Ultraviolet frequencies of light as given off by the sun or a UV lamp

UV System - Secondary disinfection system which uses UV lamps to disinfect water (and in some cases oxidise compounds)


Is there a term that you think we missed or would you like more information on water chemistry? Call us on 0333 577 9692 or email enquiries@energiseleisure.co.uk


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