09 chlorination
- 1. Disinfection
- 2. Biological water quality • Assessed by MPN test • Multiple tube fermentation technique • Membrane filtration technique • Improvement by physical separation or removal of pathogens • Filtration (and membrane filtration, UF!), Coagulation- flocculation, and Settling • Improvement by disinfection: deactivation (render harmless) of pathogens (causing water borne diseases) Disinfection two types • Primary disinfection: achieving desired level of microbial kills or inactivation • Secondary disinfection: maintaining disinfectant residual in finished water to prevent regrowth of microorganisms Biological water quality and Disinfection
- 3. Disinfectants • Ideal disinfectants – Versatile (effective against all types of pathogens), Fast-acting (effective within short contact times), and Robust (effective in the presence of interfering materials) – Easy to handle (non-toxic, non-flammable, non-explosive , soluble) and Compatible with various materials/surfaces in WTPs (pipes, equipments) – Economical (cheap) – Toxic to microorganisms well below the toxic thresholds to humans and higher animals – Should persist enough to prevent re-growth in the distribution system – Should not form the undesirable disinfection byproducts (DBPs) No disinfectant used is ideal disinfectant
- 4. Disinfectants • Commonly used disinfectants are – Chemical agents (disinfectants): Chlorine, hypochlorite, Chlorine dioxide (twin oxide) and Chloramines; Ozone; Peroxide, etc. – Irradiation (UV radiation) and Heating (to boiling!) – Sonification, electrocution, etc. • Different disinfectants are effective at different concentrations/ doses • Tolerance levels against a disinfectant are different for different organisms – Cysts, encysted bacteria, bacterial spores, etc. could be more resistant or more tolerant to disinfectant • Factors such as turbidity offer sanctuary and provide shield against disinfection • Disinfectants through reacting with water and its constituents (metals, ammonia, organics, etc.) can form disinfection byproducts (DBPs), and cause taste and odour problems
- 5. Disinfection kinetics kt N N kN dt dN 0 ln N is the number of microbes ‘t’ is contact time ‘k’ is disinfection rate constant Contact time (Chick’s law) When the disinfectant level is held constant Equal susceptibility and uniform dispersion of the microbes is assumed 21 12 1 2 ln TRT TTE K K a Disinfection is temperature sensitive Activation energy (Ea) R is gas constant (8.314 J/mol.K) Kills double with 10°C temp. raise Disinfection is function of both time and concentration (C) tCk N N n 0 0 ln Chick-Watson law Disinfection is pH sensitive Ozone is more effective at lower pH Chlorine (OCl-/HOCl) is more effective at lower pH ClO2 is more effective at higher pH
- 6. Disinfection K0 for 99% kills at 5°C
- 7. CT values for inactivation using ClO2
- 8. Chlorination – Chemistry Objectives of chlorination: Disinfection, H2S control, sludge bulking control, odour control, etc. Chlorine as gas and sodium/calcium hypochlorite are used – Large systems use chlorine gas The chlorine based disinfectants release HOCl or OCl- pH and temperature determine the equilibrium relationship – HOCl levels are higher at lower pH (at <5 pH all chlorine is HOCl, 76% HOCl at 7.0 pH, 33% at 7.8 pH and ~0% at >10 pH) – HOCl fraction increases with decreasing temperature Sum of HOCl and OCl- is known as free residual chlorine HOClHOHCl 22 OClCaOClCa 2)( 2 2 OClNaNaOCl OClHHOCl HOCl OClH Ka HOCl is more effective than OCl- as disinfectant
- 10. Chlorine reacts with reduced materials (Fe2+, Mn2+, H2S, organics, NH3, etc.) Breakpoint Chlorination
- 11. • Chlorine gas is highly oxidizing, toxic, corrosive and hazardous yellow-green gas (supplied as liquid chlorine in bullets) • Can be lethal to human beings at 0.1% (volume) concentration • Heavier than air and spreads slowly at ground level • Effective against all types of microbes as both primary and secondary disinfectant • Leaves combined and free residual chlorine in the treated water and this can be responsible for secondary disinfection • Chlorine handling requires specialized equipment, care and skill • A separate storage room (not connected to other rooms) with doors opening to outside, and with view windows for inspection • Chlorinator installed in the rooms with direct emergency access to outside air • Self contained breathing apparatus and chlorine cylinder repair kit must be readily accessible • Masks, air tanks, chlorine detection devices etc. needed Disinfection by Chlorine gas
- 12. • Liquid chlorine is drawn out, vapourized (supply latent heat!) and dosed into water by an injector • Highly pressurized water is passed through a venturi – vacuum created sucks chlorine gas into the water stream • Provisions are made for proper mixing of the chlorine dosed and for the requisite contact time • pH control may be necessary for effective disinfection (chlorine gas on dissolution also reduces the pH) • Alternatively the chlorine gas is dissolved in water to form chlorine solution and this in turn is dosed/injected into water – Solubility is 750 mg/l at usual pH and temperature • Dechlorination of the chlorinated water is often needed for the removal of the residual chlorine – SO2, Na2SO3, sodium metabisulfite, activated carbon can be used Disinfection by Chlorine gas
- 13. Chlorine gas application system
- 14. Chlorine dose • Dose range is known from break point chlorination and water flow rates – Water characteristics and residual chlorine desired in water influence the dose • Feedback control involving residual chlorine monitoring is followed Injection and initial mixing • Withdrawn from the bullet as gas and applied either directly or indirectly as aqueous solution – Evaporators are used if withdrawal is >180 kg/day – Evaporators are used for gasification when withdrawal rate is >18 kg/day for 68 kg cylinders and >205 kg/day for 908 kg cylinders • Black steel piping is used for dry chlorine (liquid or gas) conveyance and PVC piping (schedule-80) for chlorine solution • Diffuser (plastic pipe with perforations) is used for chlorine dosing • Mechanical mixing, static inline mixing or hydrodynamic mixing is used for the mixing of chlorine with water Design of chlorination
- 16. Chlorine contact basin (WHO recommends 15 mg-min/L) • Contact basin is eliminated if travel time of water in the supply pipeline is greater than the needed chlorine contact time • Long plug flow chambers (with 20:1 to 40:1 aspect ratio) are used • Compacted into rectangular tanks with baffles • Hydraulic dead zones and hydraulic short-circuiting are associated with the chlorine contact basins – Submerged baffles, guide vanes and rounded corners are used to avoid the problems Design of chlorination Corrections to C-t Temp. and pH correction Df of contact tank MPN number 6-10% of the total cross-sectional area is openings in the submerged baffles
- 18. • A separate storage room (not connected to other rooms) with doors opening to outside • Ventilation at floor level to storage chamber with capacity of 60 air changes per hour • Fixed glass viewing window for checking leaks prior to entry • Fan controls at the room entrance • Protect storage and feed facilities from fire hazards • Provide leak detection equipment connected to alarm system • Protect cylinders from direct sun light during summer warm climates • Spill control and containment and emergency caustic scrubbing system to neutralize leaks – Containment vessels to provide total enclosure of cylinder – In the event of cylinder failure gas is contained within and processed at normal rate through chlorination facilities Chlorine storage facilities
- 19. • More expensive than Cl2 and highly corrosive • Available as a solution with 5-15% available chlorine level • Decomposes on storage – Should not be stored beyond a month and should be stored in cool, dark and dry areas • Easier to handle than calcium hypochlorite and chlorine gas • Preferred for highly populated areas and small scale applications • The sodium hypochlorinator includes a solution tank, dosing pumps, tubing and diffuser – The solution is injected into water supply pipe at controlled rate • On dissolution sodium hypochlorite forms OCl- (less effective as disinfectant than HOCl) • Sodium hypochlorite can be generated onsite by electrolysis of sodium chloride solution (electro-chlorinators!) – Hydrogen is given off here as a byproduct OClNaNaOCl Disinfection by sodium hypochlorite
- 20. • A white solid soluble in water and has about 65% available chlorine – Can be purchased in granular, powdered and tablet forms • Very corrosive, has a strong odour and readily absorbs moisture and generates chlorine gas • Should be kept away from organic materials (can generate heat and cause fire or explosion) • Packed calcium hypochlorite is very stable • Can be dissolved in water to prepare a solution with 1-2% available chlorine and injected as solution • Tablets of calcium hypo can be directly dissolved in water at atmospheric pressure • On dissolution calcium hypochlorite forms OCl- (less effective as disinfectant than HOCl) OClCaOClCa 2)( 2 2 Disinfection by calcium hypochlorite
- 21. • Very strong oxidant (comparable to Cl2) and very expensive • Highly soluble in water (10 times to Cl2) • React with household materials and produce offensive odours • Can transfer from solution to gas form and become explosive • Volatile and subjected to photo-decomposition • Unstable at higher concentrations (>15%) and under pressure • Used for disinfection, as primary disinfectant (alternative to Cl2) • Disinfection is brought about by oxidation • Does not react with ammonia - Forms halogenated organics and chlorite (toxic to humans), but not THMs and HAAs • Superior for manganese oxidation • Insensitive to pH over a broad range (4-8) • ClO2 is also used in a pretreatment (>5.0 min contact time) • Destructs TTHM and HAA precursors, oxidizes manganese and controls taste and odour (from algae – diatoms) • Typical dose: 0.6 to 1.7 ppm (2.0 to 5.0 ppm is typical for Cl2) Disinfection by Chlorine Dioxide
- 22. Chlorine dioxide application system
- 23. Disinfection by TwinOxide • Developed and made in Netherlands • An advanced delivery system of aqueous ClO2 solution (0.3%) • Delivered as a powder kit of two components having very long shelf life (5 years) – Component A: 64% - sodium chlorite and 36% - other ingradients – Component B: sodium bisulfate • Components A & B are mixed onsite in specified volume of tap water at neutral pH , and left for 3 hrs to produce aqueous ClO2 solution (concentrate) – 4 kg product produces 100 L of 0.3% of the concentrate – The concentrate should be stored in UV proof sealed container, in cool dark room (half-life when stored in dark at 22°C is 30-60 days) • Works as disinfectant in the pH range of 4 to 10 – Imparts no smell, taste or colour – Generates no chlorine, chlorate, chlorite or chloride – Non-explosive and lightly corrosive
- 24. Disinfection by Chloramines • Weak disinfectant – effective bactericide but less effective against viruses and protozoa – Produces fewer disinfection byproducts (DBPs) – An effective and appropriate secondary disinfectant • Formed by chlorinating ammonia containing water or by adding ammonia (anhydrous ammonia, ammonium sulfate, or ammonium hydroxide) to the water containing chlorine – Into the water supply main chlorine is injected and then ammonia is injected and adequate mixing and contact time is provided – Chloramines formation reactions are 99% complete within a few minutes • Formation of nitrogen trichloride is undesirable – NCl3 is harmful to humans and imparts disagreeable taste and odour – Chlorine to ammonia ratio of 5:1 is not exceeded and pH of water is not allowed to drop below 5 do not allow NCl3 formation
- 25. • Chlorination is highly sensitive to inorganic and organic loads and forms harmful disinfection byproducts (DBPs), like, tri-halo- methanes (THMs), halo-acetic acids (HAAs), etc. • Oxidation products of organics, some are carcinogenic, and some cause taste and odor problems • IS 10500: 2012 prescribes limits for total tri-halo-methanes (TTHMs) in drinking water • Limits prescribed: 0.08 ppm for TTHM, 0.06 ppm for HAA and 1.0 ppm for Chlorite • Factors affecting the DBPs formation • Types and concentrations of organic materials • Dose of chlorine and reaction/contact time • Temperature and pH of water • Solutions to the disinfection byproducts (DBPs) • Reducing the organics concentration in the water prior to chlorination (adsorption on activated carbon) • Use of alternate disinfectants that form no undesirable DBPs (substitution of chloramines, a less effective disinfectant, to chlorine) • Removal of the DBPs formed from the water after chlorination Disinfection/Chlorination byproducts (DBPs)
- 26. Disinfection by Ozonation • Formed by passing dry air or oxygen through a system of high voltage electrodes • Unstable (half life is <15 min.) hence generated onsite • Ozonation system includes – Air (or oxygen) preparation as feed (pure oxygen use has higher production density and requires relatively lesser energy) – Electrical power supply – Ozone generation by using a corona discharge cell – Ozone contact chamber - requires shorter contact time than Cl2 – Ozone exhaust gas destruction • Used as a primary disinfectant (leaves no disinfecting residue and hence requires a secondary disinfectant) – Preferred for waters containing colour and organics – Has low solubility in water hence needs rigorous mixing • Capital cost of ozonation systems is higher; Operation and maintenance is complex; Electricity amounts to 26-43% of O&M cost • Forms no undesirable products with organics
- 28. Disinfection by UV radiation • Special lamp is used for UV radiation and disinfection – Thin sheets of turbidity free water are exposed to UV radiation (~30 microwatts/cm2) – Energy intensity and contact time are important • Destroys genetic material - effective wavelengths is UV-B • Used as primary disinfectant (attractive for small water systems) – May not be effective in inactivating protozoan cysts – Not suitable for water with high levels of suspended solids, turbidity, colour and/or soluble organic matter – A secondary disinfectant must be used to prevent re-growth of microbes • Requires shorter contact times - produces no known toxic residuals