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 must conform 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 Gasi.e. Cl 2 + Pure water (a)HydrolysisCl 2 + H 2 O HOCl + HCl (b)IonisationHOCl H + + OCl - (Free Chlorine Residuals) HypochlorousHypochlorite AcidIon 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 Cl 2 + NH 3 (1 - 50 PPM) Sequential substitution of H in NH 3 NH 3 NH 2 Cl (Monochloramine) NHCl 2 (Dichloramine) NCl 3 (Nitrogen trichloride) (Trichloramine) Low pH NHCl 2 and NCl 3 become more High Cl:NH 3 ratio abundant NHCl 2 Good disinfectant but nasty to taste in water. NCl 3 is particularly offensive

8. High Cl:NH 3 ratios also give increased rate of breakdown reactions Wt. ratio Cl:NH 3 < 5:1HOCl + NH 3 NH 2 Cl + H 2 O < 10:1HOCl + NH 2 Cl NHCl 2 + H 2 O > 10:1HOCl + NHCl 2 NCl 3 + H 2 O Ultimately: 2 NH 3 + 3 Cl 2 N 2 + 6 HCl Mole ratio 2 : 3 gives complete oxidation = Breakpoint ie.Wt. ratio 1 : 7.6 gives complete oxidation = Breakpoint Other products of oxidation include: - NO 3 - (Nitrate ion) - Organo- chloramines (protein amino groups) If NH 3 concentration in water (including organic nitrogen) is known can calculate amount HOCL required for “breakpoint” Theoretically Chlorine requirement = Wt. NH 3 -N x 7.6 in practice (Margin of safety) = Wt. NH 3 -N x 10

9. 654321654321 0 1 2 3 4 5 6 7 8 chlorine dose (mg/l) chlorine residual (mg/l) Breakpoint Chlorination pH 7.0 30 min contact time 0.5 mg/l ammonia Marginal Chlorination Breakpoint Chlorination Superchlorination (+ Dechlorination) NH 2 Cl Breakpoint Cl 2 Total

10. Chlorination Practice Combined Residual (a)Simple, Marginal chlorination Suitable for Upland waters (b)Ammonia-chlorine treatment. (Add NH 3, then HOCl) Suitable for groundwaters Ensures combined residuals in distribution. Free Residual (a)Breakpoint chlorination Suitable for Lowland surface waters. (b)Superchlorination + Dechlorination (SO 2, S 2 O 3 2- 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 H 2 S, Fe(II), Mn(II) (groundwaters or hypolimnetic water H 2 S + 4 Cl 2 + 4 H 2 O H 2 SO 4 + 8 HCl H 2 S + Cl 2 S + 2HCl 2Fe(HCO 3 ) 2 + Cl 2 + Ca(HCO 3 ) 2 2Fe(OH) 3 (s) + CaCl 2 + 6 CO 2 (associated pH rise. Useful for: iron removal; coagulant production.) MnSO 4 + Cl 2 + 4 NaOH MnO 2 (s) + 2 NaCl + Na 2 SO 4 + 2 H 2 O (precipitate takes 2-4 hours to form, longer for complex Mn ions) Where H 2 S, 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 O 3 + GAC/PAC before final chlorination Alternative Strategy: replace Cl 2 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 ResidualMax Level (mg/l) Free Chlorine20 Monochloramine5 Dichloramine0.8 Nitrogen Trichloride0.02 (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. (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: 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 (n/no) straight line

17. Minimum Bactericidal Chlorine Residuals Based on Coliform Removal at 20-25 o C 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: 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

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 / 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)

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, H 2 S, Reduced Fe + Mn  “Chlorine demand” (6)pH (7)Mixing (8)NH 3  “Chlorine demand”