Ask most swimmers what is in a swimming pool and they will say water and chlorine. That answer is so incomplete that it actively misleads. Almost every widespread belief about pools, including the belief that a strong smell means a well disinfected pool and the belief that a shower before swimming is a politeness rather than a chemical intervention, follows from thinking of a pool as a big tank with chlorine tipped into it.
A modern swimming pool is a water treatment plant with a large open tank attached to it. The tank is the part swimmers see. The plant is the part that determines what the tank is like, and it is running constantly, usually out of sight and usually behind a locked door.
The loop, which is the actual object
The defining feature of a treated pool is that the water does not sit still. It moves in a loop. Water leaves the pool, passes through a series of stages, and comes back. Everything that people call "the chemicals" happens inside that loop rather than in the pool itself.
The stages, in the order they generally occur:
- Removal. Water leaves the pool, mostly over the surface through a channel at the edge, because the surface is where most of the contamination sits. Some is drawn from lower down.
- Screening. Coarse material is caught before it reaches the pumps, because hair and debris will destroy a pump given the chance.
- Circulation. A pump moves the water. The rate at which it does so, and therefore how long the whole volume takes to pass through the plant, is one of the fundamental design decisions of the pool and is not something anyone can change on the day.
- Coagulation. A chemical is dosed that makes very fine suspended particles clump together into pieces large enough to be caught. Without this stage a filter simply passes the small stuff straight through, which is why a pool can be chemically fine and still look cloudy.
- Filtration. The water passes through a filter bed that traps the clumped material. Filters get dirty and are periodically cleaned by reversing the flow through them.
- pH correction. An acid or an alkali is dosed to hold the water within its working range. This matters enormously and is covered below.
- Disinfection. A disinfectant is dosed, most commonly a chlorine based one. This is the stage everybody has heard of.
- Return. Treated water goes back into the pool through inlets positioned to mix it through the whole tank rather than short circuiting straight back to the outlet.
Some pools add further stages, most commonly an ultraviolet or ozone stage, which does a job the chlorine stage does not do well. Some use disinfectants other than chlorine. The order and the presence of each stage is a design decision made when the pool was built or last refurbished, and the standard reference for how the whole thing should be put together in the United Kingdom is the code of practice maintained by the Pool Water Treatment Advisory Group, alongside the Health and Safety Executive's HSG179.
Why pH is doing more work than swimmers realise
If you only remember one non obvious fact about pool water, make it this one. The effectiveness of chlorine based disinfection depends heavily on pH. The same amount of chlorine in the water can be doing substantially different amounts of disinfection depending on how acidic or alkaline the water is, because the chemical form the chlorine takes changes with pH, and the two forms are not equally good at killing things.
This is why a plant operator will tell you that pH control is the first job and disinfection is the second. It is also why "they should just put more chlorine in" is usually the wrong instinct. A pool with pH drifting out of range does not have a chlorine problem to be solved with more chlorine. It has a pH problem, and adding chlorine to it produces the disinfection byproducts that make a hall smell without producing much extra disinfection.
pH does not sit still on its own. Everything that goes into the water pushes it about: the dosing chemicals themselves, the water supply, the air, and the swimmers. So it is corrected continuously, usually automatically, and checked manually as well.
There is no such thing as "the chlorine level" as a single sufficient description of a pool. There is free chlorine, which is the part still available to disinfect, and combined chlorine, which is the part that has already reacted and is now largely a problem rather than a solution. A pool can have plenty of chlorine in it and very little of it doing anything useful. That distinction is the subject of the next article and it explains the smell.
The water is also full of whatever swimmers brought
The loop is designed around an assumption that the water is being continuously contaminated, because it is. Every swimmer introduces material: skin cells, sweat, natural skin oils, traces of urine, cosmetic and personal care products, hair products, sunscreen, and whatever was on their feet. This is not a moral observation, it is a design parameter. Pool plant is specified on the assumption of a certain bather load, and the treatment has to keep up with it.
Two things follow. First, a pool is at its most chemically stressed when it is busiest, which is usually when children's lessons and public sessions overlap. Second, anything that reduces what swimmers carry into the water reduces the load on the plant, which is the real reason for the pre swim shower and the reason personal products matter more than most swimmers assume.
Does the treatment loop differ between an indoor pool and an outdoor one?
The loop is the same in principle. Indoors the additional problem is the air: the byproducts of disinfection do not blow away, they sit in the hall and are managed by ventilation. Indoor plant is therefore judged partly on what the air is like, not only the water.
Sunlight is absent, so nothing is being broken down by ultraviolet from above, and nothing is being warmed or cooled by weather. The system is more stable and more enclosed.
An outdoor pool has the same loop plus an open sky. Sunlight degrades free chlorine, so outdoor pools have to account for that. Rain dilutes and shifts the water. Leaves, insects, grass and airborne dust arrive continuously.
Byproducts largely disperse rather than accumulating, which is why an outdoor lido rarely smells the way a badly ventilated indoor hall can. The trade off is a less stable system and a genuine ultraviolet exposure problem for the people in and around it.
Disinfection is not sterilisation, and was never meant to be
A treated pool is not sterile and cannot be. It is a large open body of water with people in it. The purpose of disinfection is to control the microbial population continuously so that the risk of transmission is kept low, not to produce a sterile tank. This is worth stating plainly because it explains why some things are still possible in a well run pool, and why the response to certain contamination events is to clear the pool rather than to add more disinfectant.
It also explains why some organisms are a bigger operational problem than others. Chlorine based disinfection deals with many organisms quickly. Some are much more resistant, which is precisely why additional treatment stages exist and why contamination incidents have specific written procedures attached to them rather than being handled by judgement.
Almost none of this is the swimmer's decision
Everything described above is decided and executed by people the swimmer will probably never meet. The circulation rate was fixed by the design. The disinfection system was chosen by the operator. The dosing is automated and supervised. The testing regime is set against guidance. A swimmer cannot adjust any of it, cannot inspect it from the poolside, and mostly should not try.
Whether the water in this pool is being treated properly today
Cannot decide it and cannot verify it from the poolside. Can look at the water, notice the smell of the hall, and ask the operator for the day's records. Can decline to get in. Can raise a concern, and should, rather than assuming somebody else already has.
Cannot treat the water either, but should be checking the environment before the session and should know what to do if it looks wrong. A session leader who takes a group into water they have not looked at has skipped a step that belongs to them.
Owns it entirely. Owns the plant, the dosing, the testing regime, the records, the competence of the person operating it, and the decision to close the pool. This is not a shared responsibility that gets divided at the poolside.
Where a duty attachesHealth and Safety Executive, HSG179 Health and safety in swimming pools, together with the code of practice from Pool Water Treatment Advisory Group, and the general duties on employers and controllers of premises under the Health and Safety at Work etc. Act 1974.
What to take from this
Three things. First, when you read anything about pools, translate "chlorine" into "the disinfection stage of a treatment loop" and see whether the claim still stands up. It often does not. Second, the things a swimmer can actually influence are small in number and are all about what goes into the water, which makes them worth doing properly. Third, the things a swimmer cannot influence are held by named people with written duties, which means the right move when something looks wrong is to ask them rather than to speculate.
We do not publish any of the numbers involved. Not circulation rates, not disinfectant concentrations, not pH ranges, not turnover times. Those belong in the operator's records against current guidance, and a figure carried out of that context into a different pool is worse than no figure at all. If you want the values, the sources are Pool Water Treatment Advisory Group and Health and Safety Executive, HSG179 Health and safety in swimming pools, and for a specific pool, the operator.