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Disinfection

Disinfection. Objective to understand the principles of chlorination, and the factors that influence its efficiency in the disinfection of water . Literature Chemistry for Environmental Engineering - Sawyer et al Water Supply - Twort et al Water and Wastewater Engineering - Fair et al

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Disinfection

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  1. Disinfection • Objective to understand the principles of chlorination, and the factors that influence its efficiency in the disinfection of water. • Literature Chemistry for Environmental Engineering - Sawyer et al Water Supply - Twort et al Water and Wastewater Engineering - Fair et al Handbook of Chlorination - White

  2. DISINFECTION “The removal of Pathogenic micro-organisms from Water” (-not necessarily removal of ALL micro-organisms) AIM: to produce SAFE drinking water i.e. < 1 Coliform/100 ml Standards: 1984 & 1993 WHO Guidelines 1980 EEC Drinking Water Directives 1989 UK Water Regulations Treated water ENTERING Distribution system mustconform Treated water IN distribution system should have < 3 coliforms /100 ml but NEVER any E.coli /100 ml Therefore must maintain RESIDUAL disinfectant in Distribution System to control growth or contaminant bacteria.

  3. Disinfection Methods PHYSICAL (1) Boiling - Household use, temporary, expensive, emergency measure. - Kills bacterial, viruses + other microorganisms. (2) U-V light - effective for bacteria + viruses if Turbidity is low (a) Simple storage in glass containers - effective but not very practical (b) Tubular, jacketed, u-v lamps - Need power supply - Used in operating theatres + isolated communities. (c) Impounding and storage Reservoirs

  4. CHEMICAL METHODS Mostly Oxidising Agents Large Scale: Chlorine (Municipal W.S.) Sodium / Calcium hypochlorite Chloramine Chlorine dioxide Ozone Small Scale: Silver Iodine Potassium permanganate Chlorine compounds Used impregnated in ceramic filters or as tablets For household use, camping etc.

  5. Chlorination (1) Free Chlorine Chlorine Gas i.e. Cl2 + Pure water (a) Hydrolysis Cl2 + H2O HOCl + HCl (b) Ionisation HOCl H+ + OCl- (Free Chlorine Residuals) Hypochlorous Hypochlorite Acid Ion Form of Free Chlorine depends on pH Strong Disinfectant Weak Disinf.

  6. Chlorine Demand Chlorine added to water is not necessarily available for disinfection. Lowland surface waters • chlorine demand of 6 - 8 mg/l • Chlorine Reacts with: • Ammonia • breakpoint chlorination • Organic Matter • Dissolved, colour • particulate • Metal ions • pipe materials • from source water

  7. (2) Combined Chlorine Cl2 + NH3 (1 - 50 PPM) Sequential substitution of H in NH3 NH3 NH2 Cl (Monochloramine) NHCl2 (Dichloramine) NCl3 (Nitrogen trichloride) (Trichloramine) Low pH NHCl2 and NCl3 become more High Cl:NH3 ratio abundant NHCl2 Good disinfectant but nasty to taste in water. NCl3 is particularly offensive

  8. High Cl:NH3 ratios also give increased rate of breakdown reactions Wt. ratio Cl:NH3 < 5:1 HOCl + NH3 NH2Cl + H2O < 10:1 HOCl + NH2Cl NHCl2 + H2O > 10:1 HOCl + NHCl2 NCl3 + H2O Ultimately: 2 NH3 + 3 Cl2 N2 + 6 HCl Mole ratio 2 : 3 givescomplete oxidation = Breakpoint ie. Wt. ratio 1 : 7.6 givescomplete oxidation = Breakpoint Other products of oxidation include: - NO3- (Nitrate ion) - Organo- chloramines (protein amino groups) If NH3 concentration in water (including organic nitrogen) is known can calculate amount HOCL required for “breakpoint” Theoretically Chlorine requirement = Wt. NH3-N x 7.6 in practice (Margin of safety) = Wt. NH3-N x 10

  9. Breakpoint Chlorination pH 7.0 30 min contact time 0.5 mg/l ammonia Total 6 5 4 3 2 1 Cl2 Breakpoint chlorine residual (mg/l) NH2Cl 0 1 2 3 4 5 6 7 8 chlorine dose (mg/l) Breakpoint Chlorination Superchlorination (+ Dechlorination) Marginal Chlorination

  10. Chlorination Practice Combined Residual (a) Simple, Marginal chlorination Suitable for Upland waters (b) Ammonia-chlorine treatment. (Add NH3, then HOCl) Suitable for groundwaters Ensures combined residuals in distribution. Free Residual (a) Breakpoint chlorination Suitable for Lowland surface waters. (b) Superchlorination + Dechlorination(SO2, S2O32- or Act. Carbon. ) • For industrially polluted surface waters destroys tastes + odours + colour • Short contact time or pollution load variable (wells). Desirable to have chlorine Residual in the Distribution System (in U.K.) Combined chlorine preferable. Most persistent.

  11. Chlorine also reacts with H2S, Fe(II), Mn(II) (groundwaters or hypolimnetic water H2S + 4 Cl2 + 4 H2O H2SO4 + 8 HCl H2S + Cl2 S + 2HCl 2Fe(HCO3)2 + Cl2 + Ca(HCO3)2 2Fe(OH)3(s) + CaCl2 + 6 CO2 (associated pH rise. Useful for: iron removal; coagulant production.) MnSO4 + Cl2 + 4 NaOH MnO2 (s) + 2 NaCl + Na2SO4 + 2 H2O (precipitate takes 2-4 hours to form, longer for complex Mn ions) Where H2S, Mn or Fe present: previous practice used PRECHLORINATION + FILTRATION But T.H.M. problems, therefore now discouraged.

  12. Disinfection Problems (1) pH influences effectiveness (2) THM formation (CARCINOGEN) 1 ug/l MAC (EC) and 100 ug/l MCL (USEPA) ug/l = ppb Therefore Chlorination practice now modified - Discourage PRECHLORINATION - Aim to remove THM PRECURSORS using O3 + GAC/PAC before final chlorination Alternative Strategy: replace Cl2 by other oxidants or remove micro-organisms by more efficient clarification.

  13. Taste and Odour (1) From Chlorine Residuals Acceptable maximum levels of Chlorine and Chloramines Residual Max Level (mg/l) Free Chlorine 20 Monochloramine 5 Dichloramine 0.8 Nitrogen Trichloride 0.02 (2) From Chlorinated Organics Chlorophenols (3) From Natural Products Fungal and algal metabolites acceptable thresholds will be lower for high purity water

  14. Operational Factors Affecting Chlorination Practice • Form of Chlorine • Storage and decomposition • Mixing Efficiency • baffled mixing chambers • Temperature • slower at low temps • seasonal variation significant • pH • Concentration • Time

  15. 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

  16. ln (n/no) 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 where: k = the reaction rate constant N = number of viable organisms Integrate, gives: where: 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

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

  18. USA Alternative Strategy: Aim for oxidative disinfection of Coxsackie virus A2 Use ‘K’ values in CT = K relationship for design of disinfection process. ‘CT’ i.e. ‘K’ values under different conditions are: 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

  19. 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)

  20. 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 (8) NH3  “Chlorine demand”

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