Water tools

What a disinfectant dose does to E. coli, Cryptosporidium, norovirus and phage

A dose that gives three logs on E. coli does almost nothing to Cryptosporidium with chlorine, yet UV at the same E. coli dose takes the oocysts out. Compare the four organisms at any dose.

Disinfectant

Dose (CT in mg·min/L, or UV in mJ/cm²)

E. coli

3.00log

Cryptosporidium oocysts

3.17log

Murine norovirus

1.10log

MS2 coliphage

0.17log

Basis

3-log doses

E. coli 5.5, Crypto 5.2, norovirus 15, MS2 97 mJ/cm²

How it works

Λ = \frac{\ln 1000}{D_{3 \log}} \frac{N}{N_{0}} = e^{- Λ D} L R V = \frac{3 D}{D_{3 \log}}

(1)

where

$Λ$: lethality coefficient per unit dose
$D_{3 \log}$: dose that gives three logs on the organism
$D$: dose applied, mg·min/L or mJ/cm²
$N, N_{0}$: organisms after and before
$L R V$: log inactivation

First-order kinetics in a well-mixed batch for every pair, scaled from the dose each organism needs for three logs. Free chlorine and chloramine doses that control E. coli leave Cryptosporidium essentially untouched; UV at the E. coli dose inactivates the oocysts well but viruses only modestly. E. coli is therefore a variable indicator depending on the disinfectant. The chlorine value for Cryptosporidium assumes hypochlorous acid is the only active species; peracetic acid on Cryptosporidium is too variable to tabulate.

What a disinfectant dose does to E. coli, Cryptosporidium, norovirus and phage

How it works

Related reading