Disinfection Objective to understand the principles of chlorination

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

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.

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

CHEMICAL METHODS Mostly Oxidising Agents Large Scale: (Municipal W. S. ) Chlorine 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.

Chlorination (1) Free Chlorine Gas i. e. Cl 2 + Pure water (a) Hydrolysis Cl 2 + H 2 O HOCl + HCl (b) Ionisation HOCl H+ + OCl- Hypochlorous Acid Hypochlorite Ion Form of Free Chlorine depends on p. H (Free Chlorine Residuals) Strong Disinfectant Weak Disinf.

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

(2) Combined Chlorine Cl 2 + NH 3 (1 - 50 PPM) Sequential substitution of H in NH 3 NH 2 Cl (Monochloramine) NHCl 2 (Dichloramine) NCl 3 (Nitrogen trichloride) (Trichloramine) Low p. H High Cl: NH 3 ratio NHCl 2 and NCl 3 become more abundant NHCl 2 Good disinfectant but nasty to taste in water. NCl 3 is particularly offensive

High Cl: NH 3 ratios also give increased rate of breakdown reactions Wt. ratio Cl: NH 3 < 5: 1 HOCl + NH 3 NH 2 Cl + H 2 O < 10: 1 HOCl + NH 2 Cl NHCl 2 + H 2 O > 10: 1 HOCl + NHCl 2 NCl 3 + H 2 O 2 NH 3 + 3 Cl 2 N 2 + 6 HCl Ultimately: 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 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

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

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 32 - 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.

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 + 2 HCl 2 Fe(HCO 3)2 + Cl 2 + Ca(HCO 3)2 2 Fe(OH)3 (s) + Ca. Cl 2 + 6 CO 2 (associated p. H rise. Useful for: iron removal; coagulant production. ) Mn. SO 4 + Cl 2 + 4 Na. OH Mn. O 2 (s) + 2 Na. Cl + 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.

Disinfection Problems (1) p. H 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.

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

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

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 where: k = the reaction rate constant N = number of viable organisms Integrate, gives: N 0 = number of organisms at time = 0 Nt = number of organisms at time = t K = Death Rate Constant i. e. rate of disinfection is Logarithmic where: ln (n/no) straight line time

Minimum Bactericidal Chlorine Residuals Based on Coliform Removal at 20 -25 o. C For Virus and Protozoan Cyst disinfection, greater residuals required. Taste problems

USA Alternative Strategy: Aim for oxidative disinfection of Coxsackie virus A 2 Use ‘K’ values in CT = K relationship for design of disinfection process. ‘CT’ i. e. ‘K’ values under different conditions are: p. H 0 -5 o. C 10 o. C 7 -7. 5 12 8 8 20 15 8. 5 30 20 9 35 22 best kill higher temp neutral p. H Poorest kill low temp high p. H

Practical Disinfection Can Control: Type of disinfectant Concentration of disinfectant Time of contact (pref. 10 - 60 min) o Mixing p. H. 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)

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) p. H (7) Mixing (8) NH 3 “Chlorine demand”