13-1 Disinfection_F11DISINFECTION (3rd DC 240; 4th DC 294) Definition Disinfection - Killing or inactivating pathogens - To destroy organisms that cause diseases in humans and domestic animals. Pathogens: Disease-producing microorganisms Sterilization: destruction of all living organisms - Drinking water need not be sterile Purpose of Disinfection - To protect public health by preventing the spread of water born diseases by disinfecting drinking and wastewater to an acceptable level. Three categories of human enteric pathogens: 1) Bacteria 2) Viruses 3) Protozoa (amebic cysts) - Purposeful disinfection must be capable of destroying all three. Diseases and Pathogens (Examples) Water Born Disease Typhoid Bacillary dysentery Cholera Giardiasis Cryptosporidiosis Amoebiasis Adenovirus infection Gastroenteritis Gastroenteritis Pathogens Salmonella typhi Shigella dysenteriae Vibrio cholera Giardia lambia Cryptosporidium parvumis Naglaria Adenovirus Astrovirus Parvovirus Note Bacteria Bacteria Bacteria Protozoa Protozoa Protozoa Virus Virus Virus Removal of Pathogenic Microorganisms by Water Treatment Processes - Removal of pathogenic microorganisms and other microorganisms in conventional water treatment processes include: a. Coagulation/flocculation b. Sedimentation c. Filtration d. Inactivation by chemicals, (e.g., lime, soda ash) e. Natural die-away of organisms in an unfavorable environment 1 13-1 Disinfection_F11 Disinfection by chemical disinfectants Considerations as a water disinfectant 1) Effectiveness - ability to kill the types and numbers of organisms within reasonable (practicable): a) contact time b) temperature (over an expected range) c) water quality variations 2) Availability - constant and readily available supply of disinfectant at reasonable cost and in form conveniently, safely and accurately applied. a) b) c) d) 3) Toxicity reasonable cost ($) readily, safely available constant supply convenient form - cannot render the water toxic or objectionable (aesthetically or otherwise) for human consumption a) non-toxic to humans and domestic animals b) palatable, non-objectionable 4) Handling a) convenient, safe, and easy to store, transport, and handle. b) conveniently, safely, easy, and accurately apply 5) Persistency (maintaining killing power) a) persist in residual concentrations as safeguard against recontamination 6) Assay technique (for process control) a) practical, easy, reproducible b) reasonable cost c) rapid and accurate 2 ClO2 4) Ozone. 1 atm b. almost completely. to form hypochlorous acid: . humic substances) Nitrites Iron and manganese Hydrogen sulfide and cyanide.= 4 x 10-14 {Cl2} {H2O} { } = γ[ ] { } = activity [ ] =M γ = activity coefficient 3 . Cl2(g) + H2O HOCl + H+ + Cl- (1) (2) {HOCl} {H+} {Cl-} Keq = -----------------------. HOCl .Hypochlorous acid is a weak acid and dissociate poorly at pH< 6. NH2Cl C dichloramine. HOCl C hypochlorite. Iodide (I -) Chemistry of Chlorination Chlorine a. OCl2) Chloramine C monochloramine.13-1 Disinfection_F11 Disinfectants 1) Chlorine. Iodine (I2). Cl2 C hypochlorous acid.Chlorine gas hydrolizes in water rapidly. NHCl2 3) Chlorine dioxide. Chlorine gas is soluble in water solubility = 7160 mg/L at 20° C. Chlorine reacts with: 1) Water 2) Ammonia and 3) Other compounds: a) b) c) d) e) Organic nitrogen Organic compounds (phenol. O3 5) Others C Bromine (Br). Basic reactions with water Formation of Hypochlorous acid. then adding chlorine.= -------------{Cl2} {H+} {Cl−} {HOCl}/{Cl2} increases.Chloramine can be formed by first adding a small quantity of ammonia to the water.HOCl is approximately 80─100 times as effective as OCl−. . Reaction with ammonia . as {H+} increases.5 {HOCl} 2.Hypochlorous acid ionizes to (dissociates into) hypochlorite ion and hydrogen ions in the reversible reaction: HOCl H+ + OCl− (3) (4) {H+} {OCl−} Ka = ----------------. .log {H+} Formation of Hypochlorite ion. 1. Chloramines . as {H+} decreases Note: pH = .7 x 10−8 {OCl−} ---------.6.= ----------------{HOCl} {H+} {OCl− } / {HOCl} decreases.7 x 10-8 = 10−7.Ammonium ion exists in equilibrium with ammonia and hydrogen ion NH4+ NH3 + H+ 2.Chlorine exists predominantly as HOCl at pH 4 . Chlorine reacts with ammonia in water to form chloramines NH3 NH2Cl NHCl2 + + + HOCl HOCl HOCl NH2Cl NHCl2 dichloramine + H2O + H2O + H2O (3) (4) (5) monochloramine NCl3 trichloramine (nitrogen trichloride) 4 .= 2.13-1 Disinfection_F11 {HOCl} 4 x 10-14 ---------. OCl− . Difference between the amount of Cl2 added to a water and the quantity of free and combined available chlorine remaining at the end of a specified contact period.2) mg/L = 0. Chlorine Residuals and Chlorine Demand a. Example (Sato): Chlorine added to water (chlorine dosage) is 7 mg/L. hypochlorite ion is somewhat less effective.is the amount of chlorine that reacts with inorganics (Fe2+. . OCl. .Hypochlorous acid is primary disinfectant. and NO3 -) and organic impurities.3 mg/L.1 mg/L. NCl3. Thus the power of free chlorine residual decreases with increasing pH. Free available chlorine residual (FAR): HOCl.Residual existing in chemical combination with ammonia (chloramine) or organic nitrogen compounds. c. Chlorine Demand = (Chlorine added to water) . free chlorine residual is 0. .3 + 0. NHCl2.4 mg/L 5 . monochloramine is 0.1 mg/L + (0.6 mg/L = 6. Mn2+.Residual chlorine existing in water as hypochlorous acid or hypochlorite ion.2 mg/L. Chlorine demand . 1) What is total available chlorine residual (TAR)? 2) What is the chlorine demand? Solution Total Available Chlorine Residual (TAR) = FAR + CAR = 0. NO2-.Effectiveness of CAR is significantly less than that of FAR.This amount must be satisfied before free available chlorine is formed.(TAR) .(TAR) = 7 mg/L – 0. organic N-Cl .6 mg/L Chlorine Demand = (Chlorine added to water) . Combined available chlorine residual (CAR): NH2Cl. b.13-1 Disinfection_F11 3. and dichloramine is 0. Total Available Chlorine Residual (TAR): TAR = FAR + CAR d. 000 2 Saturday 180.000 3 Day of Week Sunday Monday Tuesday Wednesday Thursday Friday Saturday (Solution) 1) Calculate weekly average: (Spores/L) Day of Week Raw Finished Sunday 200.000 10 180.000 8 170.000 2 150.714 6.000 2 Wednesday 150.000 16 145.000 4 Tuesday 170. The city data are as follows: (Spores/L) Raw Finished 200.13-1 Disinfection_F11 Removal of microbes (or spores) in a treatment plant a) Log removal (LR) in a plant is given by Influent concentrations LR = log (----------------------------------) Effluent concentrations b) Percent (%) removal in a treatment plant is 100 % removal = 100 .000 16 Monday 145.000 10 Friday 180.000 8 Thursday 170.---------10LR Example 4-29 (4th DC 297) A city measured the concentration of aerobic spores in its raw and finished water as an indicator of plant performance.000 3 AVERAGE 170.000 2 180. Spores are often plentiful in water supplies and are conservative indicators of how well a plant is able to remove Cryptosporidium.43 6 .000 4 170. --------.996 104.42 6.714 Log Removal (LR) = log (--------------. coli.90 % 103 Example (Sato): What is the log reduction of E. coli that is equivalent to 99.= 99.--------.42 Example (Sato): What is the equivalent percent reduction for a 3 log reduction of E.43 100 % Removal = 100 . 7 . coli? (Solution) 100 % removal = 100 .= 99.90% reduction? (Solution) 100 % removal = 100 .-------.-------10LR 100 % removal = 100 .= 99.13-1 Disinfection_F11 170.) = 4.90 % 10LR Solve for LR. LR = 3 There is a 3 log inactivation of E. time-1 (1st order rate constant) Factors affecting the kill 1) Form of chlorine 2) pH 3) Concentration 4) Contact time 5) Type of organism 6) Temperature .Under ideal conditions.13-1 Disinfection_F11 Disinfection Kinetics (4th DC 295) . − ∆N ∞N ∆t ⇒ − dN = kN dt where N = the number of organisms k = rate constant.Chlorine concentration and contact time relationship can be expressed by: Cn tp = constant where C = concentration of chlorine. the rate of die-off follows Chick's law. min. Polio melitus would have a much higher “constant” value than E. which is specific for type of microorganisms being killed. when an exposed microorganism contains a single site vulnerable to a single unit of disinfectant. constant = constant for a given system (experimentally derived). For example. mg/L n = constant for a given system (experimentally derived). tp = time required for given percent kill. Chick’s law (Chick. coli. 8 . related to its power to kill.the number of organisms destroyed in a unit time is proportional to the number of organisms remaining (First-order reaction). dependent on type of disinfectant species being used. 1908) . 13-1 Disinfection_F11 9 . pH. ° C .1758 pH 2. • EPA used empirical data and a safety factor to develop the table.5-log inactivation of Giardia is required for an untreated surface water. . Table 4-20 (4th DC 298) • Table 3-20 (3rd DC 244) or Table 4-20 (4th DC 244) presents the required CT values (in mg/L • min) for inactivation of Giardia cysts by free chlorine at 10° C. pH. (114) (3rd DC 243. the numbers in the table do not exactly match Eq.log {H+} temp = temperature. • • 10 .widely used in the Surface Water Treatment Rule (SWTR) as a criteria for cyst and virus disinfection. for a conventional coagulation plant. 3-log inactivation is required Generally. and water temperature are known.Microorganisms kill by disinfectants appears to follow the CT concept.The relationship in Eqn 114 means that the combination of concentration and time (CT) required to produce a 3-log reduction in Giardia cysts by free chlorine can be estimated if the free chlorine concentration.7519 temp -0. and water temperature are known.” .9847 C0. 0.“The product of disinfectant concentration (C) and contact time (T) yields a constant.13-1 Disinfection_F11 Water Disinfection (Chlorine Disinfection) (3 DC 243. Empirical expression: CT = 0. 4 DC 296) .1467 where C = disinfectant concentration (mg/L) T = contact time between the microorganism and the disinfectant (min) pH = . Therefore.If the free chlorine concentration. 114. 4th DC 296) Table 3-20 (3rd DC 244). the combination of concentration and time (CT) required to produce a 3-log reduction in Giardia cysts by free chlorine can be estimated. rd th The CT concept . 0 MGD. Steps 1) Determine log inactivation required by disinfection nactivation 3 . Contact ontact time = 50 min. temperature = 10 10° C. pH = 7.8 mg/L to minimize THM formation.5 = 0.5 log inactivation 2) Determine contact time At a peak hourly flow rate = 5 MGD contact time T = 50 min 11 . -------------------------------------------------------Approach C Use the CT (concentration • time) concept: C Apply Surface Water Treatment Rule (SWTR): C For disinfection of Giardia.5 log inactivation is allowed for treatment prior to disinfection. C The chlorine dosage must not exceed 1.2.13-1 Disinfection_F11 Example : Determine the required residual chlorine concentration? Given: Water treatment for the disinfection of Giardia At Peak hourly flow rate. b) 2. a) 3 log inactivation required (plant efficiency). Q = 5. 400).13-1 Disinfection_F11 3) Determine CT from Table 420 (4th DC 298) Noting that the chlorine dosage must not exceed 1. Breakpoint chlorination Definition: .247). th 4-47 (4 DC 302). or Fig.4 0.Chlorination of a water to the extent that all the ammonia is converted to N2 (or a higher oxidation state). p.8 mg/L to minimize THM formation. Fig. p. 4th DC 302) When chlorine is added to water containing reducing agents and ammonia. Breakpoint chlorination is required . 11. you yield a curve similar to Figure 11-13 11 (3rd DC.40 mg/L The residual chlorine concentration needed is 0.to obtain a free chlorine residual for better disinfection (if ammonia is present in a water supply) Breakpoint chlorination Curve (3rd DC247.The application of chlorine to water to the point where free residual chlorine is available. 12 .4 0.40 mg/L. CT = 20 (mg/L)(min) at pH = 7 4) Determine chlorine dosage C = CT / T = 20 (mg/L)(min) / (50 min) = 0. .16 : VH. 13-1 Disinfection_F11 13 . hydrogen sulfide. A-B: a. b. a free available chlorine residual begins to develop (point C on the curve). This results in the creation of oxidized nitrogen compounds (e. At D (Break Point) . which in turn reduce the chlorine residual. manganese ions) c. 14 . As the free available chlorine residual increases. c. all added residual is free available chlorine. B-C: a. When all the ammonia has been reacted with. The addition of chlorine in excess of that required up to point C results in the formation of chloramines. cyanide. C-D: a. nitrogen.Once most of the chloramines are oxidized. Chloramines established show an available chlorine residual and are effective as disinfectants. c. Reducing agents (those common to water and wastewater include nitrites. Zone III.13-1 Disinfection_F11 Zone I. Zone II. b. the previously produced chloramines are oxidized. The chlorine dosage at B is the amount required to meet the demand exerted by the reducing agents. Chlorine reacts first with reducing agents present and develops no measurable residual. nitrous oxide. b. Beyond the breakpoint.some resistant chloramine can still be present beyond D.g. . D-E: a. nitrogen trichloride). but their relative importance is small.. ferrous ions. additional chlorine applied to the water creates an equal residual (as indicated by the rising curve at point D) Zone IV. 13-1 Disinfection_F11 Chemical Reactions of Chlorine in Natural Waters ================================================================= Rxn No.E.D: NH2Cl + HOCl ↔ NHCl2 + H2O NH2Cl + NHCl2 ↔ N2(g) + 3H+ + 3Cl NHCl2 + 2HOCl + H2O ↔ NO3 .E: Cl 2(g) + H2O ↔ HOCl + H + + Cl HOCl + H2O ↔ OCl. D . New Orleans. R. Reaction ------------------------------------------------------------------------------------------------------------------Zone I. B .+ 5H + + 3Cl Zone IV. Kinetics of Breakpoint Chlorination in Continuous flow systems.C: NH3 + HOCl ↔ NH2Cl + H2O Zone III. C . LO (June. and Selleck. 1976). A . Paper presented at 96th Annual Conference AWWA.M. 15 .B: 2Fe(HCO3)2 + Ca(HCO3 -)2 + Cl2 ↔ 2Fe(OH)3 + CaCl2 + 6CO2 MnSO4 + 4NaOH + Cl2 ↔ MnO2 + 2Na2SO4 + 2NaCl + 2H2O H2S + Cl2 ↔ S + 2H+ + 2Cl Zone II. B.+ H3O+ hypochlorous acid hypochlorite N-trichloride ammonia formation dichloramine N removal nitrate formation monochloramine NHCl2 + HOCl ↔ NCl3 + H2O NH2Cl + NH2Cl ↔ NH3 + NHCl2 ----------------------------------------------------------- Reference: Saunier. Solution: 1) Fast response . AOB consume nearly all of the free ammonia in the tank.13-1 Disinfection_F11 Chloraminated Potable Water Storage Tanks (Opflow.14-16. ammonia. most of the residual chloramine in the tank can be destroyed in just a few days.. December 2008) Problem: Ammonia-Oxidizing Bacteria (AOB) Trouble arises when AOB converts free ammonia to nitrite and then to nitrate.g.e. causing an acceleration of chloramines into chlorine and more free ammonia (by auto-decomposition). DO).+ H+ + H2O NH2Cl + H2O As the AOB continue to consume free ammonia. p. nitrite. 2) Boost the chlorine dosage .Twice-weekly water testing when water temperature reach 15ºC or higher (on total chlorine.Interrupting the rapid formation of nitrite and corresponding chloramine loss as soon as possible .5 mg/L of chlorine requires 3. AOB start growing rapidly. to boost 1 Mgal of water by 0. nitrate. free chlorine. NH3 + 2 O2 NH3 + HOCl NO3.3 gal of 12. pH.5 % sodium hypochlorite 3) Adequate mixing throughout the tank 16 . When chloraminated water reaches 15ºC (59ºF). i. humic and fulvic acids. activated carbon can be applied during early treatment stages to adsorb humic substances. humic acids.Moving the point of chlorine application to later stages in water treatment (the easiest method for reducing THM formation) . UV.g. 3) Use an alternative disinfectant.Chlorine reacts with humic substances commonly found in raw surface waters to forms trihalomethanes (principally.. adsorption using activated carbon. by aeration. in addition.) 2) Improve the removal of precursors prior to chlorination. chloroform and bromodichloromethane). e. fulvic acids .Alternatives for reducing the production of THMs 1) Change the point of chlorine application .Not before coagulation process .e.Breakpoint chlorination prior to chemical coagulation is no longer common practice (bacause of trihalomethane formation.Decaying vegetation produces the humic substances. O3 17 .THMs are formed by the reaction between chlorine and dissolved organics (humic substances) during the chlorination of surface waters. .. improved coagulation can be implemented to enhance the removal of organic substances.Disinfection Byproducts (DBP) Common THMs: CHCl3 chloroform CHBr3 bromoform CHCl2Br bromodichloromethane CHClBr2 dibromochlormethane Formation and Control of Trihalomethanes Formation . 0. Precursor + HOCl time* THMs * reaction time (detention time) Precursors Dissolved organics.13-1 Disinfection_F11 Trihalomethanes (THMs) . .. Water Quality Standard (MCL) Total trihalomethanes (THMs). referred to as precursors. e.g.If necessary.1 mg/L (interim) (Source: Safe Drinking Water Act of 1996) 2. Control of THM Formation (Remedial Actions) . min ( log10 slope = Therefore. -6 . mg/L t = residence time.log1 ( log100 . (1+0.The reductions of coliform organisms in a chlorinated primary treated effluent can be expressed by the relationship: Nt = (1 + 0.23 Ct t ) where No = number of coliform organisms at time t = o N t = number of coliform organisms at time t C t = total (amperometric) chlorine residual at time t. min 18 .23 Ct t ) −3 No .log1) ) = −6 − 0 = − 3 2−0 log Nt = ( −3) log(1 + 0.13-1 Disinfection_F11 Wastewater Disinfection (Chlorine Disinfection) Plot of Nt No vs. mg/L t = residence time.23 Ct t ) No Nt = (1 + 0.23 Ct t ) −3 No where No = number of coliform organisms at time t = o N t = number of coliform organisms at time t C t = total (amperometric) chlorine residual at time t. 78 mg / L 0.13-1 Disinfection_F11 Example: What is the required total chlorine residual (mg/L)? Given: No = 10.23 t 0.23 Ct t ) −3 No solve for Ct −1/ 3 Nt 200 −1 −1 No 10.23 (15) −1/ 3 Pocatello WWTP – Chlorine contact basin --------------------------------------------------------------------------Chlorine dose 8 mg/L Chlorine demand 5 mg/L At the end of the chlorine contact basin 1 mg/L Discharge to the Portneuf River after dechlorination using SO2 <0.1 mg/L --------------------------------------------------------------------------- Chlorine contact basin in Pocatello WWTP (2011) 19 .000 Ct = = = 0.000 org/100 mL N t = 200 org/100 mL t = 15 min (Solution) Nt = (1 + 0. 300 lb/d of sodium hypochlorite is required. how many pounds per day of sodium hypochlorite are required? (0.65 Thus.68 lb/d Thus.= 300 lb/d 0. 3. How many pounds per day of chlorine are required to disinfect the plant’s water supply? Conversion formula is: (mg/L)(MGD)(8. 430.77 lb/d of calcium hypochlorite required.77 lb/d 0. If calcium hypochlorite (65 percent available chlorine) is used.13-1 Disinfection_F11 Calculating Chlorine Dosage 1.34) = lb/d (2.= 430. Disinfection at a treatment plant requires 280 lb/d of chlorine. If sodium hypochlorite with 10 percent available chlorine is used. A water supply requires 30 lb/d of chlorine for disinfection.68 lb/d of chlorine required.1 Thus.0 mg/L. how many pounds per day of calcium hypochlorite will be required? (0.34) = 11.0 mg/L)(0.7 MGD)(8. 11.65)(X lb/d) = 280 lb/d 280 lb/d X lb/d = ------------.000 gpd. The flow rate at the plant is 700.1)(X lb/d) = 30 lb/d 30 lb/d X lb/d = ------------. 20 . The chlorine dosage at a water treatment plant is 2. 2. + H2O Cl.+ NH4 + + H + ----------------------------------------------------------------------------------NH2Cl + SO2(g) + 2H2O NH4 + + SO42. Other dechlorination compounds: C sulfite.+ 2H+ .+ 2H + -----------------------------------------------------------------------SO2 + HOCl + H2O Cl.+ SO4 2.+ SO4 2.+ 4 H+ 21 . For free chlorine residual SO 2(g) + H2O HSO3 .the oxidation-reduction reaction between the monochloramine residual and sulfur dioxide is very rapid.+ H+ HOCl + HSO3Cl. SO2 in aqueous solution .+ 3H+ For monochloramine SO2(g + H2O HSO3 . . b.added at the discharge end of the chlorination chamber. sodium metabisulfite 2SO4 2.Any excess sulfur dioxide applied reduces dissolved oxygen as: 2SO2 + O2 + 2H2O c.13-1 Disinfection_F11 Dechlorination a. Usually accomplished by adding sulfur dioxide.+ SO42.+ Cl.+ H + NH2Cl + HSO3 . Dechlorination is required to detoxify a discharge after chlorination. 13-1 Disinfection_F11 Ultraviolet Radiation (4th DC 303) Source: http://www.com/us/index.wedeco.315 200 .1.200 UVA: causes changes to the skin that lead to tanning UVB: can cause skin burning and is prone to induce skin cancer UVC: is extremely dangerous since it is absorbed by proteins and can lead to cell mutations or cell death 22 .400 280 .wedeco.com/us/index.280 100 .php?id=91294 Source: http://www.700 315 .000 400 .php?id=91294 Table 4-22 (4th DC 304): Spectral ranges of interest in photochemistry Range name Near infrared Visible Ultraviolet UVA UVB UVC Vacuum ultraviolet (VUV) Wavelength range (nm) 700 . mW/cm2 t = average exposure time.7 nm. EPA requirements.S.7 nm (primarily between 200 nm and 300 nm). • UV light is most strongly absorbed by DNA at 253. s Sato’s experimental unit 23 . disinfection • UV has been found to be very effective for the disinfection of Cryptosporidium. • U. • Low-pressure Hg arcs at 253. and viruses. The average UV dose is calculated by: D=It where D = UV dose I = average intensity. EPA has established UV dose requirements − See Table 4-23 (4th DC 305) shows the U. UV dose .S.The inactivation of microorganisms by UV is directly related to UV dose. Giardia.13-1 Disinfection_F11 Note: • Ozone absorbs UV radiation in the 220-330 nm region. Thus.can shield pathogens from UV light or scatter UV light to prevent it from reaching the target microorganisms. and suspended solids . LR) The survival fraction is calculated by: Survival fraction = log (N / No) where N = organism concentration after inactivation No = organism concentration before inactivation • The major factor affecting the performance of UV disinfection systems is influent water quality: 1) Particles. it is recommended that UV systems be installed downstream of the filters. turbidity. 24 . requiring higher levels of UV to achieve the same dose. 2) Organic and inorganic compounds − Some organic compounds and inorganic compounds (such as iron and permanganate) can reduce UV transmittance by absorbing UV energy.13-1 Disinfection_F11 Survival Fraction (similar to Log removal. thus reducing its effectiveness as a disinfectant. p. 36-39 (2011) 25 .13-1 Disinfection_F11 UV application to Combined Sewer Overflow (CSOs) Source: Muller et al. Water Environment & Technology. Osaka.cfm • Ozone reacts with natural organics in source waters. . − converting natural organics to smaller assimilable organic carbon (AOC) molecules that can be bacterial nutrients. o Bromate is regulated at 10 ug/L (due to cancer risk) o http://www.html http://water.epa.100-111 (2008). iron.Chlorine is added after ozonation 26 . odorous. p. 11. No. colorless gas. . Haloacetic acids) − can form bromate if source waters contain bromide. and protozoa (including Giardia and Cryptosporidium) . 100. Ozonation: − does not form THMs (or HAASs.Murano Water Treatment Plant.epa.must be generated onsite via air or oxygen electrolytic processes.13-1 Disinfection_F11 Ozone (Journal AWWA.is also used to control manganese. Japan . Example of O3 treatment . Vol. viruses. Thus the systems using ozone often include biologically active filters (BAFs) or other biologically active processes to reduce AOC levels prior to entering the distribution system in order to prevent biological re-growth in the distribution system. . strong oxidant).is used as a disinfectant to kill bacteria. and naturally occurring taste-and-order substances. Ozone (O3) .gov/enviro/html/icr/gloss_dbp.is a highly reactive (unstable.gov/drink/contaminants/basicinformation/disinfectionbyproducts.