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Holding Time Index

Also known as HTI · half-life of a treatment chemical

HTI=ln⁡2  VB\text{HTI} = \frac{\ln 2 \; V}{B}

Worked example: 30,000 gal bleeding 20 gpm → HTI 17.33 h — press Try an example to run it live, then adjust anything.

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Holding Time Index explained

VBHTI

A bled system is a stirred tank being continuously diluted, so any chemical in it decays exponentially — and the holding time index is that decay's half-life. The 0.693 is ln 2, exactly the same constant that governs radioactive half-life; formula.expert carries the full ln 2 = 0.693147. A 30,000 gallon tower bleeding 20 gpm turns over in 1500 minutes, so its HTI is 0.693 × 1500 ≈ 1040 minutes, about 17 hours: stop feeding and half your inhibitor is gone by tomorrow morning.

HTI is the number that sets biocide strategy. A slug of oxidising biocide only works if it stays above its kill threshold long enough, and regulators treat this seriously — US EPA biocide labels are written around contact time, and a system with a four-hour HTI simply cannot hold a twelve-hour contact without continuous feed or a bleed lockout on the controller. The classic field error is computing HTI from the bleed valve alone and forgetting drift and leaks; on a leaky tower the real half-life can be a third of the calculated one, which is why residuals mysteriously vanish overnight.

Holding Time Index formula

HTI=ln⁡2  VB\text{HTI} = \frac{\ln 2 \; V}{B}
Where
  • HTI\text{HTI}= Holding time index (h)
  • VV= System volume (L)
  • BB= Blowdown rate (L/min)

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