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Kinetics of Disinfection

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

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Kinetics of Disinfection

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  1. 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

  2. ln (bacteria) straight line time (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: 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

  3. Time of Contact Deviations from log. death rate are common log(n/n0) Increasing rate of kill: diffusion barrier (cell coat) t log(n/n0) Decreasing rate of kill: Variable resistance, or clumping t  May be necessary to modify plots  Straight line graphs. e.g. plot log (n/n0) v. time1/2

  4. log(n/n0) log(0.01) 2 mg/l 3 mg/l 1 mg/l (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 t for given % kill x log[chlorine] slope = - 1/n “n” = coefficient of dilution (order of reaction characteristic of each disinfectant type (e.g. HOCl) x x log(time for 99% kill)

  5. 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)

  6. Cnt = K K value indicates the relative resistance of organisms to disinfection e.g. for Ozone, and 99% kill Ct = 1 Giardia (protozoan) Ct = 0.02 E. coli e.g. for HOCl, and 99% kill Ct = 230 Giardia (protozoan) Ct = 6.3 Coxsackie virus A2 Ct = 1.2 polio virus 1 Ct = 0.24 E. coli Ct = 0.1 Adenovirus 3 1 0.1 [Cl2] Coxsackie virus A2 Escherichia coli Spore forming bacteria Polio Adenovirus Same slope because Same “n” 1 10 time (min) for 99% kill

  7. Minimum Bactericidal Chlorine ResidualsBased on Coliform Removal at 20-25oC For Virus and Protozoan Cyst disinfection, greater residuals required.  Taste problems

  8. 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 kill low temp high pH

  9. (3)Effect of Temperature Van’t Hoff-Arrhenius relationship Where E = Activation energy T1 , 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 temperature but should design disinfection stage taking ambient temperature into account. In general, higher temps  more rapid disinfection.

  10. Practical Disinfection Can Control: Type of disinfectant Concentration of disinfectant Time of contact (pref. 10 - 60 min) o Mixing pH. Cannot Control: Temperature Organics / 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)

  11. 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”

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