Water tools

Ozone: transferred dose, CT from a decaying residual, and the dose for a coliform target

Ozone is made on site, transferred into the water at less than full efficiency, satisfies an immediate demand, and then decays while it disinfects. Each step is a short calculation.

Gas flowL/min

Water flowL/min

Ozone in feed gasmg/LOxygen-fed generators 6 to 16% by weight, about 80 to 220 mg/L.

Ozone in off-gasmg/L

Residual leaving the dosing zonemg/L

Residual decay constant1/minFrom a batch decay test at the design residual; falls as the residual rises.

Contact time receiving creditmin

Coliform in, per 100 mL

Coliform target, per 100 mL

Initial ozone demand qmg/L

Slope n of the dose response

Gas balance

Transferred dose

11.40mg/L

Transfer efficiency

95.0%

Contactors reach 90 to 99%.

CT credit

CT delivered by the decaying residual

1.85mg·min/L

CT without decay

8.70mg·min/L

Coliform target

Utilised dose for the coliform target

4.61mg/L

Applied dose at this transfer efficiency

4.85mg/L

How it works

D_{t r a n s f e r r e d} = \frac{Q_{g}}{Q_{l}} (C_{g, i n} - C_{g, o u t}) T E = \frac{C_{g, i n} - C_{g, o u t}}{C_{g, i n}}

(1)

C (t) = C_{0} e^{- k_{d} t} C T = \frac{C_{0}}{k_{d}} (1 - e^{- k_{d} t})

(2)

For wastewater coliform the ozone response follows a log-log form whose intercept equals the initial demand:

\frac{N}{N_{0}} = {(\frac{U}{q})}^{- n} U = q {(\frac{N_{0}}{N})}^{1 / n} D_{a p p l i e d} = \frac{U}{T E}

(3)

where

$D_{t r a n s f e r r e d}$: ozone transferred into the water, mg/L
$Q_{g}, Q_{l}$: gas and water flow, L/min
$C_{g, i n}, C_{g, o u t}$: ozone in feed gas and off-gas, mg/L
$T E$: transfer efficiency, fraction
$C (t)$: residual after time t, mg/L
$C_{0}$: residual leaving the dosing zone, mg/L
$k_{d}$: decay constant, 1/min
$C T$: dose delivered, mg·min/L
$N, N_{0}$: coliform after and before
$U$: utilised ozone dose, mg/L
$q$: initial ozone demand, mg/L
$n$: slope of the dose response
$D_{a p p l i e d}$: applied dose, mg/L

No CT credit is taken in the dosing compartment. Decay is faster at high pH, high organic carbon and high temperature, and slower with high alkalinity. Bench units must match the full-scale dosing-compartment residence time; dispersion in the contactor is handled separately. Over-dosing raises bromate.

Ozone: transferred dose, CT from a decaying residual, and the dose for a coliform target

How it works

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