Hyperchlorination Math: CT Values and the CYA Drain Table

Under the CDC's 2024 Model Aquatic Health Code (MAHC, if your state adopted it), a diarrheal incident in a pool without stabilizer needs a CT value of 15,300 mg-min/L: 20 ppm free chlorine for at least 12.75 hours, or any concentration and time that multiplies to the same number. Once cyanuric acid (CYA) is in the water, that multiplication stops being valid, and you use a fixed table after getting CYA to 15 ppm or less.

How do you turn a CT value into hours?

The Annex defines the CT value as free chlorine (C, mg/L) times time (T, minutes), and uses 15,300 for Cryptosporidium (Annex 6.5.3.2). Divide 15,300 by the concentration:

Free chlorine	Minutes	Hours
20 ppm	765	12.75
30 ppm	510	8.5
40 ppm	382.5	6.4

The Annex's own table prints only the 1, 10 and 20 ppm rows. The Code (6.5.3.2) says "at least 20.0 ppm" for 12.75 hours "or an equivalent time and concentration", so the 30 and 40 ppm rows are our arithmetic from that sentence. The straight-line trade holds only without CYA. Two limits:

• The time counts from the moment testing shows the pool at target (Annex 6.5.2.3; for stabilized pools, Code 6.5.3.2.1 item d). Until the farthest test point reads 20 ppm, the clock has not started.
• The Code says to "maintain" the level. Ask your health department whether it accepts adding up concentration times minutes across a dip. Do not assume it does.

Formed stool and vomit are lighter: 2.0 ppm for 25 minutes (6.5.3.1, 6.5.3.3), with the time doubled when the water has CYA (6.5.3.1.1, 6.5.3.3.1). CDC's fecal incident guidelines give 2 ppm for 25 to 30 minutes.

What has to be true before the clock starts?

The Code (6.5.2.3) lists five preparation items: pH 7.5 or lower; water at 77°F (25°C) or higher; filtration running while the pool reaches and holds the level; free chlorine tested at multiple points for the whole hold; and non-stabilized chlorine products only. The closure covers the affected venue and any venue on the same recirculation system (6.5.2.1.1), so a spa on the pool's system closes too.

A refill with cold fill water can pull an outdoor pool under 77°F, so test temperature before you dose. Many conventional kits cannot read 20 ppm. The Annex (6.5.2.3) prefers a FAS-DPD titration kit, diluted with chlorine-free water if needed, or strips whose range includes 20 ppm.

What changes when the pool has CYA?

CYA slows the kill. The Annex says 3-log inactivation was reached at 16 ppm CYA and not at 50 or 100 ppm; at 50 ppm CYA and 20 ppm chlorine, 1-log took an average of 61.9 hours. So the Code (6.5.3.2.1) gives three routes:

1. Lower CYA to 15 ppm or less (drain, refill from an approved source), then hold one of the listed levels for its table time.
2. Circulate through secondary treatment to cut oocysts below 1 per 100 mL (4.7.3.3.2.4).
3. Drain the venue completely.

The Annex says only the listed concentrations are warranted, because the data are not linear, so do not multiply across levels. Table 6.5.3.2.1 gives the hours:

Starting CYA (ppm)	Drain and refill	20 ppm	30 ppm	40 ppm
15 or less	0%	28 h	18 h	8.5 h
30	50%	25.2 h	16.2 h	7.7 h
50	70%	23.2 h	14.9 h	7.0 h
70	79%	21.8 h	14.0 h	6.6 h
90	83%	20.7 h	13.3 h	6.3 h

The larger drain also dilutes the parasite (0.30 log at 50%, 0.78 log at 83%), and the Annex shortens the hold in proportion. A drain only counts once you refill: CYA falls only when fresh water goes in.

How much do you drain?

With CYA-free fill water, the fraction to replace is 1 minus (15 divided by starting CYA). At 40 ppm: 1 − 15/40 = 0.625, so 62.5%. Round up for margin.

The table shows what a bigger drain buys. Going from the 30 ppm row (50% drain) to the 90 ppm row (83%) cuts the 20 ppm hold from 25.2 to 20.7 hours, 4.5 hours for replacing a third more of the pool. Going from 20 to 40 ppm on the 30 ppm row cuts it from 25.2 to 7.7. Drain only what 15 ppm requires, then use concentration to buy time. Above 90 ppm CYA, the Annex says a complete drain may be more practical, unless a high water table makes emptying an in-ground pool risky.

Worked example: a 60,000 gallon pool with 40 ppm CYA

Illustrative numbers: an outdoor apartment pool has a diarrheal incident at 2:00 pm. CYA tests 40 ppm, pH 7.4, water above 77°F.

1. Drain: 62.5% is the minimum. We choose 70% (42,000 gallons). Refilled with CYA-free water, CYA is 40 × 0.30 = 12 ppm.
2. Drain and refill time, at example rates of 150 gpm out and 100 gpm in: 42,000 ÷ 150 = 280 min, plus 42,000 ÷ 100 = 420 min. Total 700 min, 11 hours 40 minutes.
3. Hold: a 40 ppm start sits between the 30 and 50 rows. We use the 30 ppm row, the longer, as the conservative choice. The 70% drain would support the shorter 50 ppm row, but ask your health department first.

Dose: pool volume is 60,000 × 3.785 = 227,125 L. Chlorine needed = (target − 1.0 ppm after refill) × 227,125 L. We assume the chlorine reads 1.0 ppm after the refill. As example strengths, say the liquid label reads 12.5% available chlorine by weight at a density of 1.2 kg/L (150 g/L), and the granular calcium hypochlorite label reads 65%. Use the strength on your own product label.

Target	Chlorine needed	Liquid at 150 g/L	Granular at 65%	Hold	Reopen, from 2:00 pm
20 ppm	4,315 g	28.8 L (7.6 gal)	14.6 lb	25.2 h	41 h
30 ppm	6,587 g	43.9 L (11.6 gal)	22.3 lb	16.2 h	32 h
40 ppm	8,858 g	59.1 L (15.6 gal)	30.0 lb	7.7 h	24 h

The reopen column adds 30 minutes to close and test, the 11 hours 40 minutes, 2 hours to dose and confirm several test points, the hold, and 2 hours to bring chlorine down to the label limit and retest. The 2-hour figures are our assumptions, and chlorine demand and sun make real doses higher. Here, 40 ppm reopens about 24 hours after the incident and uses about twice the chlorine of the 20 ppm plan, which would keep the pool closed about 17 hours longer.

How do you choose between 20, 30 and 40 ppm?

Hold window: hours between reaching target and your wanted opening time.

Situation	Choice
No CYA, window 12.75 h or more	20 ppm, hold 12.75 h
No CYA, window 8.5 to 12.75 h	30 ppm, hold 8.5 h
No CYA, window 6.4 to 8.5 h	40 ppm, hold 6.4 h
CYA 15 ppm or less	Use 28 h, 18 h or 8.5 h, whichever the window allows
CYA over 15 ppm, can refill	Drain to 15 ppm or less, then use the row for your starting CYA
CYA over 15 ppm, cannot refill, no secondary treatment	Drain the venue completely
Window shorter than every hold	Stay closed. The Code gives no shortcut.

Pick the lowest level whose hold fits; each step up costs more product.

Which product, and how much do you keep?

The Code allows non-stabilized chlorine only. The Annex advises against storing it just for incidents, because chlorine products degrade. It suggests buying it at a pool supply store, or unscented household bleach labelled as EPA registered for drinking water disinfection. So the SOP question is each pool's 40 ppm dose and where the on-call supervisor gets it. Pools on stabilized tablets need a named supplier.

Dose formula: gallons × 3.785 × (target − current ppm) ÷ (grams of available chlorine per liter of product, from the label) = milliliters of product. At the example 150 g/L, each 1 ppm in 10,000 gallons takes about 250 mL.

After the hold, bring chlorine below the label maximum (5.7.3.1.1.5). Free chlorine above 10 ppm with swimmers present is an imminent health hazard (6.6.3.1 item 3). Follow your product label and SDS for dechlorination. CDC's guidelines say to backwash the filter thoroughly after reaching the CT value, directly to waste, and not to return the backwash through the filter.

What do you record?

The bodily fluids log (6.4.1.8.3) has 13 fields. The hyperchlorination ones: stabilizer and its concentration at the incident, residual and pH at the incident, the remediation used with contact time, residual and pH at reopening, stabilizer at reopening, and the reopening date and time. The response plan belongs in the EAP (6.3.4.5.6 item 5), gets reviewed at least annually (6.5.1.4), and its supplies are verified at least weekly (6.5.1.3). See fecal incident records.

What does the overnight order look like for 30 pools?

Give the on-call supervisor a one-page phone order with these fields:

• Pool, incident time and type
• CYA reading and date, free chlorine, pH, water temperature
• Shared recirculation system: every venue closed
• Drain share, fill source, refill finish time
• Product, label strength, dose in gallons or pounds
• Target ppm, time the last test point hit target, hold end time
• Verifier name, reopening residual and pH

The supervisor who gives the order should not be the person who verifies the readings. Poolean keeps closures and the reopening steps your team has recorded on each pool's record (see water testing). For limits after reopening, see commercial pool chlorine range and the reopening checklist. To talk it through, book a 30-minute call.

The short version

• Without CYA, hours = 15,300 ÷ (ppm × 60). That gives 12.75 hours at 20 ppm, 8.5 at 30 and about 6.4 at 40, counted from the moment the whole pool is at target.
• With CYA, use only the table levels: 28, 18 or 8.5 hours at 15 ppm CYA or less. Above 15, drain and refill first, use secondary treatment, or drain completely.
• A bigger drain barely shortens the hold. Concentration is the lever, so know each pool's 40 ppm dose and where to buy it overnight.
• Record CYA and residual at the incident and at reopening.