Kinetics of Disinfection

Ideally: All cells equally mixed with disinfectant

All cells equally susceptible to disinfectant.

Disinfectant concentration unchanged in contact tank.

No interfering substances present

Then: Disinfection is a function of:

(1) Time of Contact

(2) Concentration of Disinfectant

(3) Temperature of Water

(1) Time of Contact

Chicks Law “The number of organisms destroyed in unit time is proportional to the number remaining

Rate of Kill

k = the reaction rate constantN = number of viable organisms

Integrate, gives:

N0 = number of organisms at time = 0

Nt = number of organisms at time = t K = Death Rate Constant

i.e. rate of disinfection is Logarithmic

kNdt

dN

KtN

Nt

0ln

time

ln (bacteria)straight line

log(n/n0)

log(n/n0)

t

t

Time of Contact

Deviations from log. death rate are common

Increasing rate of kill: diffusion barrier (cell coat)

Decreasing rate of kill:Variable resistance, or clumping

May be necessary to modify plots Straight line graphs.

e.g. plot log (n/n0) v. time1/2

(2) Concentration of Disinfectant

Various concentrations of disinfectant can achieve the same effective kill, with various times of contact.

Empirical relationship: Cn t = K

(K is a constant for any given organism)

C = Concentration of disinfectant

t = time of contact

log(n/n0)

3 mg/l 2 mg/l 1 mg/l

t

log(0.01)

x

x

x

log(time for 99% kill)

log[chlorine] slope = - 1/n

for given % kill

“n” = coefficient of dilution (order of reaction characteristic of each disinfectant type (e.g. HOCl)

Coefficient of Dilution ‘n’( order of reaction)

n > 1 - efficiency decreases considerably if disinfectant is diluted slightly.

(or increases considerably as disinfectant concentration is increased slightly)

n < 1 - time of contact more important the dosage.

n = 1 - 1st order reaction

- time and concentration of equal importance.

Examples of ‘n’ values:

Chlorine (HOCl) n 1.0 (e.g. 0.86)

Ozone (O3) n > 1.0 (e.g. 3.0)

Cnt = K

K value indicates the relative resistance of organisms to disinfection

[Cl2]

1 10 time (min) for 99% kill

1

0.1

Coxsackie virus A2

Adenovirus

Escherichiacoli

PolioSpore formingbacteria

e.g. for HOCl, and 99% kill

Ct = 230 Giardia (protozoan)Ct = 6.3 Coxsackie virus A2Ct = 1.2 polio virus 1Ct = 0.24 E. coliCt = 0.1 Adenovirus 3

e.g. for Ozone, and 99% kill

Ct = 1 Giardia (protozoan)

Ct = 0.02 E. coli

Same slope becauseSame “n”

Minimum Bactericidal Chlorine ResidualsBased on Coliform Removal at 20-25oC

pH MinimumFree Chlorine

(mg/l)after 10 min

MinimumCombined Chlorine

(mg/l)after 60 min

6.0 0.2 1.0

7.0 0.2 1.5

8.0 0.4 1.8

9.0 0.8 > 3.0

10.0 0.8 > 3.0

For Virus and Protozoan Cyst disinfection,greater residuals required. Taste problems

USA Alternative Strategy:

Aim for oxidative disinfection of

Coxsackie virus A2

Use ‘K’ values in Cnt = K relationship for design of disinfection process.

pH 0-5oC 10oC

7-7.5 12 8 best kill higher temp

8 20 15 neutral pH

8.5 30 20

9 35 22

Poorest killlow temphigh pH

(3) Effect of Temperature

Van’t Hoff-Arrhenius relationship

Where E = Activation energyT1 , T2 = Absolute temperatures (Kelvin)t1 , t2 = times for equal % kill

at fixed disinfection concentration

at different temperatures.

Where E 7,000 cal (30 kJ) Diffusion type process(very fast reaction)

Where E 15,000 cal (60 kJ) Chemical Reation(denaturation of protein)

(equilibration of HOCl/OCl-)

In practice, cannot control temperaturebut should design disinfection stage taking ambient temperature into account.

In general, higher temps more rapid disinfection.

21

12

2 TT303.2

T-TE

t

tlog

1

R

Practical Disinfection

Can Control: Type of disinfectantConcentration of disinfectantTime of contact (pref. 10 - 60 min)o MixingpH.

Cannot Control: TemperatureOrganics / NH3 / Interfering subs.

Chlorine demand(measure Free Available Chlorine after set period of time)

Primary requirements: Adequate contact time before distribution

Adequate mixing / turbulence(Difficult to achieve, especially in small

systems)

Summary:

Factors which influence disinfection:

(1) Number and nature of pathogens

(2) Type and Concentration of Disinfectant.

(3) Temperature (High temps. Increase kill rate)

(4) Contact Time (Longer contact better kill)

(5) Presence of organic particulates, H2S, Reduced Fe + Mn “Chlorine

demand”

(6) pH

(7) Mixing (tank design)

(8) NH3 “Chlorine demand”