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The internationally recognised targets that separate a well-run pool from a guess — the numbers a completed pool is actually run to, every day it stays open.
A pool isn't safe simply because chlorine has been added, or because the water looks clear. The only way to know is to measure — against fixed numbers, on a fixed schedule, every day the pool is open.
This page sets out the operating targets we benchmark against: the ranges published by the Pool Water Treatment Advisory Group (PWTAG), recognised by the UK's Health and Safety Executive, Environmental Health Officers, and UK courts as the standard for responsible pool management.
Although PWTAG is written for pool operators rather than pool builders, it reinforces a principle we've built our practice around: a well-designed pool is inherently easier to run safely. Poor hydraulic design, inadequate filtration, and a compromised plant room often create operational problems that no amount of maintenance can fully undo.
We hold our own commissioning and handover documentation to these numbers — because the foundations of safe operation are established during design, not after.
Chlorine alone cannot compensate for poor engineering, or poor operation. Failure of any one of these can compromise water quality — and, where necessary, justify closing the pool until it's corrected.
Two families of chemistry keep a pool safe: enough disinfectant to kill what enters the water, and a pH balance that lets that disinfectant actually work.
| What's Measured | Safe Target | In Plain Terms |
|---|---|---|
| Free chlorine (standard pools) | 1.0 mg/l | Below this, germs survive; above 3 mg/l it starts irritating skin and eyes. |
| Free chlorine (stabilised/salt pools) | 2.5–5 mg/l | Pools using cyanuric acid stabiliser need a higher working range to compensate. |
| Combined chlorine | as close to 0 as possible | The source of that sharp "chlorine smell." A strong smell means too much of this, not too much disinfectant. |
| pH | 7.2 – 7.4 | Controls how effective the chlorine actually is. |
| Total alkalinity | 80 – 200 mg/l | Keeps pH from swinging wildly between tests. |
| Calcium hardness | 75 – 150 mg/l | Too low corrodes tiles and metalwork; too high causes scaling. |
| Total dissolved solids | source + 1,000 mg/l max | Rising TDS makes water aggressive toward finishes and equipment. |
| Water clarity | below 0.5 NTU | The point at which a pool must close. |
"mg/l" (milligrams per litre) is the same measure most test strips and digital testers already report.
Chloramine odour has a name: it's combined chlorine, the parameter behind that sharp smell.
TDS Can't Be Chemically Corrected
Once TDS climbs past source water + 1,000 mg/l, no chemical fixes it — every chemical added raises TDS further. The only real remedy is a partial drain and refill with fresh water, alongside investigating the actual cause: excessive dosing, inadequate dilution, heavy bather loads, or poor operational practice.
Two pools can hold identical chlorine readings and be in completely different states of disinfection — because pH decides what fraction of that chlorine is actually able to kill anything.
A pool tested at pH 8.0 needs roughly double the chlorine reading of a pool at pH 7.5 to achieve the same disinfection. A "high but stable" chlorine number means little if pH has drifted upward unnoticed.
Water clarity is one of the easiest indicators of whether a treatment system is functioning correctly. Cloudy water can point to inadequate filtration, excessive bather load, hydraulic deficiencies, or poor chemical balance — often before any other symptom appears.
A pool can look clear and still carry a hygiene load. PWTAG treats loss of clarity as a serious operational issue, not a maintenance inconvenience — recommending closure wherever safe water quality can't be demonstrated. See the 0.5 NTU threshold above →
Automatic controllers measure some of this continuously. They do not measure all of it.
| Test | Frequency | Note |
|---|---|---|
| Free & combined chlorine, pH | Before opening · every 2 hrs · after closing | Combined chlorine must be tested by hand — auto-controllers cannot measure it. |
| Alkalinity, hardness, TDS, stabiliser | Weekly | Slower-moving parameters, checked less frequently. |
| Sample location | 100–300mm deep, far from inlets | Never sampled from the auto-controller's own sensing point. |
A number that isn't logged, at a fixed interval, isn't a control system — it's a hope.
Some organisms — most notably Cryptosporidium, a chlorine-resistant parasite — survive normal chlorine levels almost entirely unaffected. Physical filtration, done correctly, is what actually removes it.
Filtration is not simply equipment selection — it's an engineering exercise involving hydraulic design, flow velocity, circulation, and filter sizing. It's why we cap flow velocity well under conventional international rates on every project.
| Parameter | Target |
|---|---|
| Coagulant dose | ≈0.1 ml per m³ of flow |
| Coagulation pH | below 7.5 |
| Backwash trigger | manufacturer's pressure-loss point, or weekly |
| Backwash flow rate (standard sand) | ≈30 m/hr |
A filter running past its rated pressure loss is no longer filtering to specification — even if the water still looks clear.
Increasing chlorine alone doesn't remove a biofilm. Effective removal means physically cleaning pool surfaces, filters, balance tanks, pipework, and equipment — then disinfecting. Good hydraulic circulation, effective filtration, routine maintenance, and correct water chemistry are what prevent one from forming in the first place.
Contamination incidents happen even in well-run pools. Every well-designed pool has a written response for the moments chemistry alone can't handle.
Scoop it out immediately, disinfect the scoop, and continue normal operation. The risk is low if the material is removed intact.
A chlorine-resistant parasite may be present — filtration, not chlorine, is what removes it. The standard procedure:
Used only when filtration alone cannot resolve a confirmed contamination event. Dose and duration are calculated together, as one figure:
pH is held at 7.5 or below throughout, under supervision, with the filter run to waste afterward rather than back through the pool.
Reduce dosing once free chlorine exceeds 3 mg/l. Stop chlorination entirely at 5 mg/l. If still climbing, bathers leave the water until the fault is corrected.
PWTAG focuses on operation — but many of the problems it describes originate years earlier, at the design stage.
Undersized filtration systems.
Poor hydraulic circulation.
Compromised plant rooms.
Incorrect equipment selection.
Insufficient maintenance access.
Every one of these increases operating cost, reduces water quality, and makes compliance harder to hold — long after the design decision that caused it is forgotten. Our own design philosophy runs the other direction:
We specify, build, and commission to these numbers — not to what a checklist requires us to claim.
The chemistry targets above only hold if the filtration behind them is doing its job. The chart below shows how particulate removal efficiency falls as filtration velocity increases, and how glass media outperforms silica sand at every velocity band.
Particulate removal efficiency vs. filtration velocity — glass media vs. silica sand. Hover over the chart for exact readings.
Sand-curve shape and the 25 m/h threshold reflect PWTAG's published filtration rate guidance (low-rate ≤10 m/h, medium-rate 10–25 m/h, high-rate 25–50 m/h; high-rate ≈10–25% as effective as medium-rate). Source: pwtag.org. Glass-media curve reflects directional industry consensus on finer particle capture, not a single controlled study.
Cartridge filters typically capture particles down to 10–20 microns — finer than sand (20–40 microns), coarser than glass (3–10 microns). Shown as a single reference value, not a velocity curve: cartridge systems use a much larger surface area at a fundamentally different, lower flux rate than sand or glass beds, and PWTAG does not publish an efficiency-vs-velocity relationship for them.