NovaSolver›Swimming Pool Chlorine Decay & CT Value Simulator Back
Pool Disinfection
Swimming Pool Chlorine Decay & CT Value Simulator
From free residual chlorine, pH, water temperature and bather load, this tool computes the HOCl efficacy fraction, chlorine decay rate, daily sodium hypochlorite demand, CT value and THM by-product concentration in real time. It then checks compliance against WHO/CDC MAHC and Japanese pool sanitation standards so you can design safe and efficient pool water management.
Parameters
Pool type
Outdoor adds UV decay; thermal adds degassing
Pool volume V
m³
Bathers per hour
/h
Active Bather Unit ≈ 50 mg/min chlorine demand each
Free residual chlorine
mg/L
MHLW range 0.4-1.0 mg/L
pH
pKa ≈ 7.5; HOCl ≈ 50% at pH 7.5
UV intensity (outdoor only)
W/m²
Water temperature
°C
Turnover time
hr
Time to filter one pool volume
Results
—
Free chlorine (mg/L)
—
HOCl fraction (%)
—
Decay rate (/h)
—
CT value (mg·min/L)
—
Dose (g/h)
—
THM (μg/L)
—
Pool cross-section — chlorine gradient, circulation, UV
Pool water with bathers, chlorine dosing unit, circulation pump and UV/sun source (outdoor only). Colour encodes HOCl efficacy (blue = high, red = low).
CT required for 3-log Giardia inactivation. Cryptosporidium needs CT ≈ 25,500 — impractical with chlorine alone.
Pool Chlorine CT Value — Disinfection, WHO, MHLW
🙋
Whenever I go to a public pool, it smells really chlorinated. How much chlorine is actually in there? Too much and too little both sound bad.
🎓
Good question. In Japan, MHLW's swimming pool sanitation standard sets free residual chlorine to 0.4–1.0 mg/L with pH 7.2–7.8; WHO and the CDC MAHC recommend essentially the same range. The 0.4 mg/L floor is needed to suppress E. coli and to deliver Giardia 3-log within reasonable contact time. The 1.0 mg/L ceiling limits THM by-products and reduces eye/airway irritation. Here is the counter-intuitive part though: that strong "chlorine smell" you notice is almost never the free chlorine itself — it's combined chlorine (mostly chloramines) formed when chlorine reacts with sweat and urine. A well-managed pool barely smells. A strongly smelling pool is actually under-disinfected.
🙋
Wait — strong smell means too little disinfection? That's the opposite of what I expected. And what is the "CT value" on the left? It's the largest number on screen.
🎓
CT is concentration C (mg/L) × contact time t (min), in mg·min/L. Think of it as the total disinfection budget your water can deliver. WHO and CDC use it as the design metric for water treatment: at 25°C and pH 7.5, killing 99.9% of Giardia needs CT ≈ 121. So 1 mg/L of chlorine for 60 minutes gives CT = 60 in the raw sense — but the effective CT is weighted by the HOCl fraction. Slide the pH from 7.4 up to 8.2 and watch the HOCl % and the CT value both fall sharply. Same chlorine, much weaker punch.
🙋
Why does the pH matter so much? The formula says "pKa 7.5"?
🎓
In water, free chlorine splits between HOCl (hypochlorous acid) and OCl⁻ (hypochlorite ion). HOCl is roughly 80× more biocidal than OCl⁻, and the pKa is 7.5. At pH 7.5 you have 50% HOCl; at pH 8.5 you have only about 9%. So the same nominal 1 mg/L of "free chlorine" delivers very different real disinfection — by a factor of three to five — depending on pH. Outdoor pools see strong sunlight that breaks chlorine down AND tends to raise pH through CO₂ stripping, so they often lose efficacy through the afternoon. That is the scientific reason behind the old advice "don't swim in the evening unless they have re-dosed".
🙋
I've seen news reports about Cryptosporidium outbreaks at pools. Why doesn't chlorine kill it? The tool shows CT 25,500 which is a totally different order of magnitude…
🎓
Cryptosporidium parvum survives as oocysts — a dormant form with a thick protective wall — and is extraordinarily chlorine-resistant. Giardia is killed at CT around 240; Crypto needs CT around 25,500. At a normal pool concentration of 1 mg/L that would mean roughly 18 days of continuous exposure, which is obviously not feasible. The 1993 Milwaukee water outbreak hit 400,000 people; the 2007 Utah pool outbreak hit 1,900. After those, the CDC MAHC explicitly recommends supplementary UVC (254 nm) or ozone in parallel with chlorination. In Japan, leisure pools and children's pools have been moving the same direction.
🙋
So pool water is actually quite engineered. The THM byproduct stat shows 110 μg/L at the default — is that okay?
🎓
Trihalomethanes (THMs) form when chlorine reacts with organic matter — sweat, urine, sunscreen, hair proteins. Chloroform is the classic one and is a probable carcinogen. Tap-water standards put total THM under 100 μg/L. Pool water has no formal limit, but exceeding the tap-water value means lifeguards and competitive swimmers inhale volatilised THMs all day. Several studies link competitive pool environments to elevated asthma rates among swimmers and lifeguards, attributed to THMs and cyanogen chloride. Practical rules: keep chlorine high but not excessive, secure water turnover per bather, and dilute regularly with fresh make-up water. Try pushing bathers/h to 80 and you'll see THM climb hard.
Frequently Asked Questions
CT value is the product of disinfectant concentration C (mg/L) and contact time t (min), with units mg·min/L. It quantifies the disinfection capacity needed to achieve a given log-reduction of pathogens, and is used by WHO and CDC as a water-treatment design basis. For example, at 25°C and pH 7.5, Giardia 3-log (99.9%) inactivation needs CT ≈ 121, whereas Cryptosporidium needs CT ≈ 25,500. Because pool hygiene is delivered as "concentration × time", any extended period at low chlorine starves CT and raises infection risk.
The Japanese Ministry of Health, Labour and Welfare's "Swimming Pool Sanitation Standards" require free residual chlorine 0.4-1.0 mg/L, pH 5.8-8.6 (7.2-7.8 in practice) and KMnO4 demand under 12 mg/L. The 0.4 mg/L floor secures E. coli suppression and timely Giardia 3-log inactivation. The 1.0 mg/L ceiling limits THM and other disinfection by-products and reduces eye/respiratory irritation. WHO Guidelines for Safe Recreational Water Environments (Vol. 2) and the CDC MAHC recommend essentially the same range.
Free chlorine in water exists as an equilibrium between hypochlorous acid HOCl and hypochlorite ion OCl⁻; HOCl is roughly 80× more biocidal than OCl⁻. Because pKa ≈ 7.5, HOCl is about 50% at pH 7.5, 22% at pH 8.0 and only 9% at pH 8.5. So the same "1 mg/L of free residual chlorine" delivers 76% HOCl at pH 7.0 but only 22% at pH 8.0 — effectively cutting biocidal power by more than two thirds. This tool computes the HOCl fraction from pH and feeds it into the effective CT value.
Cryptosporidium parvum exists as oocysts protected by a thick wall and is extremely resistant to free chlorine and monochloramine. The CT needed for 3-log inactivation is about 25,500 mg·min/L; at a typical pool concentration of 1 mg/L, that means about 425 hours (roughly 18 days) of contact — not feasible with chlorine alone. The 1993 Milwaukee waterborne outbreak (400,000 cases) and the 2007 Utah pool outbreak (1,900 cases) were both Cryptosporidium events, and the CDC strongly recommends supplementary UVC (254 nm) or ozone treatment.
Real-world Applications
Municipal and school pools: public pools across Japan measure free chlorine, pH and temperature by DPD method hourly or at least every two hours, and log the results. Using a tool like this to pre-plan sodium hypochlorite (NaOCl) dosing against expected bather load avoids the typical "chlorine collapse" in the afternoon peak and over-dilution overnight. A 400 m³ standard pool typically consumes 5-10 L/day of 12 % NaOCl.
Leisure pools, spas and hot baths: spas and thermal pools at 35-40 °C consume chlorine 2-3× faster than ordinary indoor pools due to faster decay and higher bather density. Standard practice is HACCP-style management with 2-3 hour turnover, continuous ORP monitoring and automated NaOCl dosing to hold ≈ 0.6 mg/L at all times, combined with Legionella controls (≥ 0.2 mg/L residual at all times).
Competitive pools and swim schools: FINA targets a tight 25 °C, 0.4-1.0 mg/L free chlorine and pH 7.2-7.6. Because elite swimmers and coaches inhale pool air for hours, modern facilities now combine secondary disinfection (UV or ozone), strict pre-shower discipline and high-performance filtration (DE filters, UF membranes) to suppress chloramines. This tool's THM stat lets operators see the trade-off between chlorine level and by-product risk.
Water-treatment plants and municipal supply: the same CT concept drives contact-tank design in drinking-water plants. The US EPA Surface Water Treatment Rule provides CT tables for Giardia 3-log and virus 4-log targets, parameterised by temperature, pH and chlorine concentration. A swimming pool can be viewed as a closed-loop, real-time application of the same design language, which makes CT a shared engineering vocabulary across pool, spa and drinking-water sectors.
Common Misconceptions & Pitfalls
The biggest myth is the "strong chlorine smell means good disinfection" belief. The opposite is true: that sharp smell is mostly combined chlorine (mono-, di- and trichloramine), formed when free chlorine reacts with sweat, urine and skin oils. A well-balanced pool is almost odour-free; a smelly pool is one running short of free chlorine to keep up with bather load. Always read BOTH "free" and "total" chlorine from a DPD test. If combined chlorine exceeds about 0.4 mg/L, the operator needs fresh make-up water dilution or breakpoint chlorination, not simply more chlorine.
Next, "keep pH at 7.0 because chlorine works best in acidic water" is too simplistic. Lower pH does push more HOCl, but below about 6.8 you get rapid metal pipe and concrete corrosion plus serious skin and mucosal irritation. Above pH 7.8, in addition to HOCl loss, the Langelier saturation index goes positive and scale precipitation and cloudiness become an issue. The practical sweet spot is pH 7.4-7.6, which delivers ≥ 50% HOCl while avoiding corrosion and scaling. You can verify on the slider: the compliance verdict stays green only inside that band.
Finally, do not assume that "hyperchlorination" is a one-shot fix for Cryptosporidium. Yes, 20 mg/L over 12+ hours can deliver 3-log inactivation, but the pool must be closed during and partially refilled or neutralised (e.g. with sodium thiosulfate) before reopening — both operationally and water-wise expensive. The CDC MAHC reserves hyperchlorination strictly for fecal/AFR events and recommends continuous secondary disinfection — UVC 254 nm at 40 mJ/cm² (3-log) or ozone (CT ≈ 5) — together with operational measures: barring symptomatic bathers, enforcing diaper-aged toilet breaks and pre-swim showers. Chlorine alone is never the whole answer.
How to Use
Enter pool volume in cubic meters (e.g., 100 m³ for a standard 25 m lap pool)
Input bather load as swimmers per hour (typical: 50–200 for public pools)
Set free residual chlorine concentration in mg/L (maintain 1.0–3.0 mg/L per CDC standards)
Enter pH value (optimal range 7.2–7.6 for maximum HOCl efficacy)
Record water temperature in °C (affects decay kinetics and disinfection rates)
Run simulation to obtain CT value, decay rate, and trihalomethane formation
Worked Example
Olympic 2,500 m³ pool at pH 7.4, 25°C, receiving 120 bathers/hour. Initial free chlorine: 2.0 mg/L. Simulator calculates HOCl fraction at 51%, decay rate 0.18 /h (chlorine depletion ~0.36 mg/L/hour due to oxidant demand). CT value reaches 85 mg·min/L after 40 minutes contact time. Required chlorine dose: 14.4 g/hour to maintain residual. Predicted THM formation: 68 μg/L at equilibrium.
Practical Notes
Higher pH (above 7.8) shifts equilibrium toward less effective OCl⁻, reducing CT efficacy by 40–50% versus pH 7.2
Bather load increases organic nitrogen load; 200 bathers/hour in 100 m³ pool accelerates chlorine decay 2.5× versus 50 bathers/hour
Cold water (10°C) extends CT requirements; warm pools (32°C) reduce contact time needed but increase THM formation risk