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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
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(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 constant N = number of viable organisms Integrate, gives: N 0 = number of organisms at time = 0 N t = number of organisms at time = t K = Death Rate Constant i.e. rate of disinfection is Logarithmic time ln (bacteria) straight line
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log(n/n 0 ) 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/n 0 ) v. time 1/2
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(2) Concentration of Disinfectant Various concentrations of disinfectant can achieve the same effective kill, with various times of contact. Empirical relationship: C n t = K (K is a constant for any given organism) C = Concentration of disinfectant t = time of contact log(n/n 0 ) 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)
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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 (O 3 ) n > 1.0 (e.g. 3.0)
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C n t = K K value indicates the relative resistance of organisms to disinfection [Cl 2 ] 1 10 time (min) for 99% kill 1 0.1 Coxsackie virus A2 Adenovirus Escherichia coli Polio Spore forming bacteria e.g. for HOCl, and 99% kill Ct = 230 Giardia (protozoan) Ct = 6.3 Coxsackie virus A2 Ct = 1.2 polio virus 1 Ct = 0.24E. coli Ct = 0.1 Adenovirus 3 e.g. for Ozone, and 99% kill Ct = 1 Giardia (protozoan) Ct = 0.02E. coli Same slope because Same “n”
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Minimum Bactericidal Chlorine Residuals Based on Coliform Removal at 20-25 o C For Virus and Protozoan Cyst disinfection, greater residuals required. Taste problems
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USA Alternative Strategy: Aim for oxidative disinfection of Coxsackie virus A2 Use ‘K’ values in C n t = K relationship for design of disinfection process. pH0-5 o C10 o C 7-7.5 12 8 best kill higher temp 8 20 15 neutral pH 8.5 30 20 9 35 22 Poorest kill low temp high pH
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(3) Effect of Temperature Van’t Hoff-Arrhenius relationship WhereE= Activation energy T 1, T 2 = Absolute temperatures (Kelvin) t 1, t 2 = 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 temperature but should design disinfection stage taking ambient temperature into account. In general, higher temps more rapid disinfection.
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Practical Disinfection Can Control:Type of disinfectant Concentration of disinfectant Time of contact (pref. 10 - 60 min) o Mixing pH. Cannot Control:Temperature Organics / NH 3 / 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)
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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, H 2 S, Reduced Fe + Mn “Chlorine demand” (6)pH (7)Mixing (tank design) (8)NH 3 “Chlorine demand”
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