Water tools

Disinfectant CT for a target log inactivation, with temperature

Disinfection is a rate process: concentration times time, against a lethality coefficient that depends on the organism, the disinfectant and the temperature. Either model fits most data; the lag model handles resistant organisms.

Kinetic model

Lethality coefficient Λ (base e)L/mg·minFree chlorine on E. coli about 3.75 at 2 to 5 °C; chloramine about 0.04; chlorine dioxide 3.3.

Target log inactivation

Residual concentrationmg/L

CT to evaluate (optional)mg·min/L

Temperature the coefficient was measured at°C

Operating temperature°C

Activation energykJ/molFree chlorine on E. coli 27 at pH 8.5, 50 at pH 9.8; chloramine 50 to 84; ozone and chlorine dioxide on protozoa about 40 to 80.

CT required at the operating temperature

0.36mg·min/L

Contact time at the residual

7.3min

Lethality at the operating temperature

19.00L/mg·min

CT at the measurement temperature

0.66mg·min/L

Batch kinetics in clean water. Real contactors need the t10 or a residence-time model; protozoan cysts and oocysts need markedly higher values than bacteria.

How it works

Chick-Watson, first order in the product of concentration and time:

\ln \frac{N}{N_{0}} = - Λ C t C t = \frac{2.303 L R V}{Λ}

(1)

Collins-Selleck, no effect below a lag and a power law above it:

\frac{N}{N_{0}} = {(\frac{C t}{b})}^{- n} (C t > b) C t = b 10^{L R V / n}

(2)

Temperature by Arrhenius, giving a higher lethality in warmer water:

Λ_{2} = Λ_{1} \exp [\frac{E_{a}}{R} (\frac{1}{T_{1}} - \frac{1}{T_{2}})]

(3)

where

$N, N_{0}$: organisms after and before contact
$Λ$: lethality coefficient, L/mg·min
$C$: disinfectant residual, mg/L
$t$: contact time, min
$C t$: dose, mg·min/L
$L R V$: log inactivation
$b$: lag coefficient, mg·min/L
$n$: slope of the log-log model
$Λ_{1}, Λ_{2}$: lethality at temperatures T₁ and T₂, K
$E_{a}$: activation energy, J/mol
$R$: gas constant, 8.314 J/mol·K

The dilution coefficient is taken as 1, which the evidence supports. Chick-Watson suits goals up to about 3 log; the log-log form suits lag or tailing and 4 log and more. Chloramine kills slowly but holds its residual; free chlorine and chlorine dioxide decay and need the decay accounted for at long contact times.

Disinfectant CT for a target log inactivation, with temperature

How it works

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