Holding Time Index

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.