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CE 370 Disinfection. Disinfection Definition: is the process of destruction of living pathogenic microorganisms Definition: is the process of destruction.

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Presentation on theme: "CE 370 Disinfection. Disinfection Definition: is the process of destruction of living pathogenic microorganisms Definition: is the process of destruction."— Presentation transcript:

1 CE 370 Disinfection

2 Disinfection Definition: is the process of destruction of living pathogenic microorganisms Definition: is the process of destruction of living pathogenic microorganisms Disinfection depends on: Disinfection depends on: The physico-chemistry of the disinfectant The physico-chemistry of the disinfectant The cyto-chemical nature and physical state of the pathogens The cyto-chemical nature and physical state of the pathogens The interaction of the above The interaction of the above Temperature Temperature pH pH Electrolytes Electrolytes Interfering substances Interfering substances

3 Classification of Disinfectants Oxidizing agents (ozone, halogens) Oxidizing agents (ozone, halogens) Cations of heavy metals (silver, gold, mercury) Cations of heavy metals (silver, gold, mercury) Organic compounds Organic compounds Gaseous agents Gaseous agents Physical agents (heat, UV, pH) Physical agents (heat, UV, pH)

4 Disinfection Rate Disinfection rate by a chemical agent obeys Chick’s Law: Disinfection rate by a chemical agent obeys Chick’s Law: dN/dt = rate of cell destruction (number / time) dN/dt = rate of cell destruction (number / time) k = rate constant k = rate constant N = number of living cells remaining at time t N = number of living cells remaining at time t k depends on: k depends on: Microorganisms species Microorganisms species Disinfectant nature Disinfectant nature Disinfectant concentration Disinfectant concentration Environmental factors (pH and temperature) Environmental factors (pH and temperature)

5 Disinfection Rate Another equation is: Another equation is: C = concentration of disinfectant at time t = 0 C = concentration of disinfectant at time t = 0 tc = time of contact required to kill a given percentage of the microbes tc = time of contact required to kill a given percentage of the microbes K, n = exponential constants K, n = exponential constants n depends on the nature of disinfectant n depends on the nature of disinfectant If n > 1, then disinfection greatly depends on the concentration of the disinfectant If n > 1, then disinfection greatly depends on the concentration of the disinfectant If n < 1, then disinfection greatly depends on the time of contact If n < 1, then disinfection greatly depends on the time of contact K depends on type of microorganism and environmental factors such as pH and temperature K depends on type of microorganism and environmental factors such as pH and temperature

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8 Disinfection Methods Chlorination Chlorination Chlorine dioxide Chlorine dioxide Ozonation Ozonation UV irradiation UV irradiation High pH High pH Other halogens (iodine and bromine) Other halogens (iodine and bromine)

9 Chlorination Chlorine is widely used Chlorine is widely used Effective at low concentration Effective at low concentration Cheap Cheap Forms residual if applied in sufficient dosages Forms residual if applied in sufficient dosages Chlorine is applied as: Chlorine is applied as: Gas (most common) Gas (most common) hypochlorite hypochlorite Chlorine is a strong oxidizing agent Chlorine is a strong oxidizing agent It oxidizes enzymes necessary for metabolism It oxidizes enzymes necessary for metabolism

10 Chlorination Reaction Chlorine gas reacts with water to form Chlorine gas reacts with water to form Hypochlorous acid (HOCl) Hypochlorous acid (HOCl) Hydrochloric acid (HCl) Hydrochloric acid (HCl) Hypochlorous acid dissociates to hypochlorite ion Hypochlorous acid dissociates to hypochlorite ion The dissociation of the HOCl is a function of pH The dissociation of the HOCl is a function of pH

11 Chlorination Reaction Hypochlorite salts Hypochlorite salts Are available in dry form Are available in dry form They should be dissolved in water They should be dissolved in water OCl -1 seeks equilibrium with H + ion OCl -1 seeks equilibrium with H + ion Therefore, acid may be needed to be added Therefore, acid may be needed to be added Both hypochlorous acid and hypochlorite ions are effective disinfectants Both hypochlorous acid and hypochlorite ions are effective disinfectants

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13 Chlorination Reaction Reaction with ammonia Reaction with ammonia At pH > 6.0, the monochloramine predominates At pH > 6.0, the monochloramine predominates At pH about 5.0, the dichloramine predominates At pH about 5.0, the dichloramine predominates Chloramines are effective against bacteria but not viruses Chloramines are effective against bacteria but not viruses

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15 Chlorination Reaction Reaction with organics Reaction with organics Reaction with phenol produces chlorophenols Reaction with phenol produces chlorophenols Reaction with humic substances produoces trihalomethanes Reaction with humic substances produoces trihalomethanes Chloroform (CHCl 3 ) Chloroform (CHCl 3 ) Bromodichloromethane (CHCl 2 Br) Bromodichloromethane (CHCl 2 Br) Dibromochloromethane (CHClBr 2 ) Dibromochloromethane (CHClBr 2 )

16 Dosages, Demand and Residuals Dosage: the amount of chlorine added Dosage: the amount of chlorine added Demand: the amount of chlorine needed to oxidize materials Demand: the amount of chlorine needed to oxidize materials Residual: the amount of chlorine remaining after oxidation Residual: the amount of chlorine remaining after oxidation

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19 Dosages, Demand and Residuals Contact time Contact time When increased, more microorganisms are killed When increased, more microorganisms are killed When increased, the demand increases When increased, the demand increases When increased, the amount of chlorinated by-products increase (if precursors are available) When increased, the amount of chlorinated by-products increase (if precursors are available) Free chlorine residuals Free chlorine residuals Chlorine gas + hypochlorous acid + hypochlorite ion Chlorine gas + hypochlorous acid + hypochlorite ion Combined chlorine residuals Combined chlorine residuals Chloramines + other chlorine reactive forms Chloramines + other chlorine reactive forms

20 Dosages, Demand and Residuals Free chlorine residuals are faster acting than combined residuals Free chlorine residuals are faster acting than combined residuals Free chlorine residuals have greater disinfecting capacity than combined residuals (especially for viruses) Free chlorine residuals have greater disinfecting capacity than combined residuals (especially for viruses)

21 Dechlorination Chlorinated effluents can have negative impacts on receiving environment Chlorinated effluents can have negative impacts on receiving environment Chemicals used for dechlorination include: Chemicals used for dechlorination include: Sulfur dioxide (most common) Sulfur dioxide (most common) Sodium sulfite Sodium sulfite Sodium bisulfite Sodium bisulfite Sodium thiosulfate Sodium thiosulfate Hydrogen peroxide Hydrogen peroxide ammonia ammonia

22 Dechlorination Activated carbon is: Activated carbon is: Well established in the dechlorination process Well established in the dechlorination process It has limited use due to its high cost It has limited use due to its high cost Requires long contact times Requires long contact times It can be used for dechlorination and removal of organic compounds at the same time It can be used for dechlorination and removal of organic compounds at the same time

23 Sulfur Dioxide (SO 2 ) General characteristics General characteristics Gas Gas Colorless Colorless Has strong pungent odor Has strong pungent odor Neither flammable nor explosive Neither flammable nor explosive With sufficient moisture, it is corrosive to most metals With sufficient moisture, it is corrosive to most metals Has solubility in water of 18.6% at 32  F (0  C) Has solubility in water of 18.6% at 32  F (0  C) When reacts with water, it forms a weak solution of sulfurous acid (H 2 SO 3 ) When reacts with water, it forms a weak solution of sulfurous acid (H 2 SO 3 ) Sulfurous acid dissociates as follows: Sulfurous acid dissociates as follows:

24 Sulfur Dioxide (SO 2 ) Free and combined chlorine forms react readily with sulfite ion (SO 3 -2 ) as follows: Free and combined chlorine forms react readily with sulfite ion (SO 3 -2 ) as follows: Required mass ratio of sulfur dioxide to chlorine is 1.1:1 Required mass ratio of sulfur dioxide to chlorine is 1.1:1

25 Ozonation General characteristics of ozone General characteristics of ozone O 3 O 3 Powerful oxidant Powerful oxidant More powerful than hypochlorous acid More powerful than hypochlorous acid Unstable in aqueous solutions Unstable in aqueous solutions Has a half-life of 20 to 30 minutes in distilled water Has a half-life of 20 to 30 minutes in distilled water Widely used in drinking water treatment Widely used in drinking water treatment Is produced on-site and can not be stored Is produced on-site and can not be stored Ozone production Ozone production 1. Air is refrigerated to remove moisture (-40 to -60  C) 2. Air is dried through desiccants (silica gel and activated alumina) 3. Air is passed between oppositely charged plates

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30 Ozonation Performance Some investigators reported total destruction of polioviruses in distilled water at a dose of 0.3 mg/l and a contact time of 3 minutes Some investigators reported total destruction of polioviruses in distilled water at a dose of 0.3 mg/l and a contact time of 3 minutes Presence of SS may increase the dose and contact time Presence of SS may increase the dose and contact time For good results, SS should not be very high For good results, SS should not be very high Ozone is never used as a terminal treatment (no residual ozone) Ozone is never used as a terminal treatment (no residual ozone)

31 Ozonation Advantages When compared to chlorination: When compared to chlorination: Ozone degrades to O 2, so no toxic residues Ozone degrades to O 2, so no toxic residues O 2 level in effluent is usually at saturation O 2 level in effluent is usually at saturation Does no increase TDS in effluent Does no increase TDS in effluent Less cost with pure oxygen activated sludge process Less cost with pure oxygen activated sludge process Ozone has been used to remove phenols, cyanides and heavy metals Ozone has been used to remove phenols, cyanides and heavy metals Ozone can render some refractory organics biodegradable Ozone can render some refractory organics biodegradable

32 Current Research Trends Identification of byproducts of the reaction of ozone with organic materials Identification of byproducts of the reaction of ozone with organic materials Ozone reaction with pesticides may produce a more toxic material Ozone reaction with pesticides may produce a more toxic material

33 Chlorine Dioxide (ClO 2 ) ClO 2 was originally used to remove taste and odor from water ClO 2 was originally used to remove taste and odor from water General characteristics of ClO 2 General characteristics of ClO 2 More powerful oxidant than chlorine More powerful oxidant than chlorine Does not react with water Does not react with water Can be easily removed from water by aeration Can be easily removed from water by aeration Readily decomposed by exposure to UV radiation Readily decomposed by exposure to UV radiation Does not react with ammonia Does not react with ammonia Maintains a stable residual Maintains a stable residual

34 Preparation of ClO 2 Acid and sodium chlorite Acid and sodium chlorite Chlorine gas and sodium chlorite (excess chlorine) Chlorine gas and sodium chlorite (excess chlorine) Sodium hypochlorite and sodium chlorite Sodium hypochlorite and sodium chlorite

35 Applications of ClO 2 Disinfection ClO 2 has longer lasting residual than HOCl ClO 2 has longer lasting residual than HOCl In wastewater, ClO 2 use is limited to phenolic wastes and the control of sulfide in wastewater collection systems In wastewater, ClO 2 use is limited to phenolic wastes and the control of sulfide in wastewater collection systems ClO 2 does not produce measurable amounts of THMs (trihalomethanes) or TOXs (total organic halogens) ClO 2 does not produce measurable amounts of THMs (trihalomethanes) or TOXs (total organic halogens) Cost of equipment and sodium chlorite are high Cost of equipment and sodium chlorite are high

36 Ultraviolet (UV) Irradiation Was used in drinking water disinfection Was used in drinking water disinfection In wastewater treatment, and when compared to chlorination and Ozonation, UV was found In wastewater treatment, and when compared to chlorination and Ozonation, UV was found More effective More effective More economical More economical UV irradiation is gaining prominence UV irradiation is gaining prominence

37 Advantages and Disadvantages of UV Advantages (in wastewater treatment) Advantages (in wastewater treatment) Effective in pathogen inactivation Effective in pathogen inactivation Ability to achieve disinfection goals Ability to achieve disinfection goals Viable applications to wide range of wastewater qualities Viable applications to wide range of wastewater qualities Cost effective Cost effective Relative simplicity Relative simplicity Absence of residuals Absence of residuals Disadvantages (in water treatment) Disadvantages (in water treatment) Lack of residuals Lack of residuals

38 Types of UV Installations There are two types: There are two types: Overhead bulb radiating downward through a shallow depth of water (2.5 to 5.1 cm) Overhead bulb radiating downward through a shallow depth of water (2.5 to 5.1 cm) Submerged UV lamb encased in a quartz tube Submerged UV lamb encased in a quartz tube The submerged type was found more effective The submerged type was found more effective The efficiency of the process is not significantly affected by turbidity and suspended solids The efficiency of the process is not significantly affected by turbidity and suspended solids UV performs well with viruses UV performs well with viruses

39 Performance of UV Irradiation Dose-response for three organisms showed similar trends: Dose-response for three organisms showed similar trends: y = log reduction y = log reduction x = log UV dose x = log UV dose Dose (D) = radiation intensity  detention time Dose (D) = radiation intensity  detention time

40 Performance of UV Irradiation For total and fecal coliforms: For total and fecal coliforms: D = 24,800 x – 60,000 (fecal coliforms) D = 24,800 x – 60,000 (fecal coliforms) D = 20,000x – 48,200 (total coliforms) D = 20,000x – 48,200 (total coliforms) Where x = log reduction Where x = log reduction

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42 High pH Treatment Lime can destruct bacteria at high pH values Lime can destruct bacteria at high pH values In this case, no residual will remain after neutralization In this case, no residual will remain after neutralization Studies showed that Studies showed that higher removal of viruses was obtained with higher pH values higher removal of viruses was obtained with higher pH values Optimum pH in the range of 11.2 to 11.3 Optimum pH in the range of 11.2 to 11.3 Optimum contact time in the range of 1.56 to 2.40 hours Optimum contact time in the range of 1.56 to 2.40 hours Complete destruction of viruses was obtained at pH of 11.0 and contact time of 5.0 hours and 10 minutes Complete destruction of viruses was obtained at pH of 11.0 and contact time of 5.0 hours and 10 minutes

43 High pH Treatment The process requires an additional residual disinfectant (minimum amount) The process requires an additional residual disinfectant (minimum amount) High pH treatment can also remove ammonia and phosphorous High pH treatment can also remove ammonia and phosphorous Recarbonation of the treated water might be needed before discharge Recarbonation of the treated water might be needed before discharge Lime stabilization of sludge is implemented at pH of 12 for 30 minutes (higher destruction of bacteria at higher pH values) Lime stabilization of sludge is implemented at pH of 12 for 30 minutes (higher destruction of bacteria at higher pH values) No information about viruses No information about viruses Little effect on parasites Little effect on parasites


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