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: SAFE drinking water i.e. < 1 Coliform/100 ml Standards: WHO Guidelines EEC Drinking Water Directives UK Water Regulations 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 –(b) Tubular, jacketed, u-v lamps –(c) Impounding and storage Reservoirs

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.

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.

Chlorine Demand Chlorine added to water is not necessarily available for disinfection. Lowland surface waters –chlorine demand of 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 ( 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

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 Cl 2 N 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

chlorine dose (mg/l) chlorine residual (mg/l) Breakpoint Chlorination pH min contact time 0.5 mg/l ammonia Marginal Chlorination Breakpoint Chlorination Superchlorination (+ Dechlorination) NH 2 Cl Breakpoint Cl 2 Total

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.

Chlorine also reacts with H 2 S, Fe(II), Mn(II) (groundwaters or hypolimnetic water H 2 S + 4 Cl H 2 O H 2 SO HCl H 2 S + Cl 2 S + 2HCl 2Fe(HCO 3 ) 2 + Cl 2 + Ca(HCO 3 ) 2 2Fe(OH) 3 (s) + CaCl CO 2 (associated pH rise. Useful for: iron removal; coagulant production.) MnSO 4 + Cl NaOH MnO 2 (s) + 2 NaCl + Na 2 SO 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) 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.

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

Superchlorination and Dechlorination Where contact time must be short or pollution loads very variable. (Free chlorine levels so high, have to be removed before supply.) Superchlorination Advantages:Complete oxidation NH 3 Correction of Tastes and Odours Removal 20-50% colour Short Contact Time Disadvantage:THM Dechlorination (a)Reducing chemicals e.g. SO 2 (Large supplies) or Na thiosulphate (Small supplies) SO 2 + Cl 2 + 2H 2 O H 2 SO HCl (b)Activated Carbon. As granular from (GAC), high rate filtration, or As powder (PAC), added then removed by rapid sand filtration.

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