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CE 370

CE 370. Disinfection. Disinfection. Definition: is the process of destruction of living pathogenic microorganisms Disinfection depends on: The physico-chemistry of the disinfectant The cyto-chemical nature and physical state of the pathogens The interaction of the above Temperature pH

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CE 370

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  1. CE 370 Disinfection

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

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

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

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

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

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

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

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

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

  11. Chlorination Reaction • Reaction with organics • Reaction with phenol produces chlorophenols • Reaction with humic substances produoces trihalomethanes • Chloroform (CHCl3) • Bromodichloromethane (CHCl2Br) • Dibromochloromethane (CHClBr2)

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

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

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

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

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

  17. Sulfur Dioxide (SO2) • General characteristics • Gas • Colorless • Has strong pungent odor • Neither flammable nor explosive • With sufficient moisture, it is corrosive to most metals • Has solubility in water of 18.6% at 32 F (0 C) • When reacts with water, it forms a weak solution of sulfurous acid (H2SO3) • Sulfurous acid dissociates as follows:

  18. Sulfur Dioxide (SO2) • Free and combined chlorine forms react readily with sulfite ion (SO3-2) as follows: • Required mass ratio of sulfur dioxide to chlorine is 1.1:1

  19. Ozonation • General characteristics of ozone • O3 • Powerful oxidant • More powerful than hypochlorous acid • Unstable in aqueous solutions • Has a half-life of 20 to 30 minutes in distilled water • Widely used in drinking water treatment • Is produced on-site and can not be stored • Ozone production • Air is refrigerated to remove moisture (-40 to -60 C) • Air is dried through desiccants (silica gel and activated alumina) • Air is passed between oppositely charged plates

  20. 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 • Presence of SS may increase the dose and contact time • For good results, SS should not be very high • Ozone is never used as a terminal treatment (no residual ozone)

  21. Ozonation Advantages • When compared to chlorination: • Ozone degrades to O2, so no toxic residues • O2 level in effluent is usually at saturation • Does no increase TDS in effluent • Less cost with pure oxygen activated sludge process • Ozone has been used to remove phenols, cyanides and heavy metals • Ozone can render some refractory organics biodegradable

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

  23. Chlorine Dioxide (ClO2) • ClO2 was originally used to remove taste and odor from water • General characteristics of ClO2 • More powerful oxidant than chlorine • Does not react with water • Can be easily removed from water by aeration • Readily decomposed by exposure to UV radiation • Does not react with ammonia • Maintains a stable residual

  24. Preparation of ClO2 • Acid and sodium chlorite • Chlorine gas and sodium chlorite (excess chlorine) • Sodium hypochlorite and sodium chlorite

  25. Applications of ClO2 Disinfection • ClO2 has longer lasting residual than HOCl • In wastewater, ClO2 use is limited to phenolic wastes and the control of sulfide in wastewater collection systems • ClO2 does not produce measurable amounts of THMs (trihalomethanes) or TOXs (total organic halogens) • Cost of equipment and sodium chlorite are high

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

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

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

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

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

  31. High pH Treatment • Lime can destruct bacteria at high pH values • In this case, no residual will remain after neutralization • Studies showed that • higher removal of viruses was obtained with higher pH values • Optimum pH in the range of 11.2 to 11.3 • 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

  32. High pH Treatment • The process requires an additional residual disinfectant (minimum amount) • High pH treatment can also remove ammonia and phosphorous • 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) • No information about viruses • Little effect on parasites

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