CE 370

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CE 370

• Disinfection

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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

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Classification of Disinfectants

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

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Disinfection Rate

• Disinfection rate by a chemical agent obeys
  Chicks 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)

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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 gt 1, then disinfection greatly depends on
  the concentration of the disinfectant
• If n lt 1, then disinfection greatly depends on
  the time of contact
• K depends on type of microorganism and
  environmental factors such as pH and temperature

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Disinfection Methods

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

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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

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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

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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

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Chlorination Reaction

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

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Chlorination Reaction

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

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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

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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

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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)

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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

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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

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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

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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 1.11

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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

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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)

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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

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Current Research Trends

• Identification of byproducts of the reaction of
  ozone with organic materials
• Ozone reaction with pesticides may produce a more
  toxic material

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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

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Preparation of ClO2

• Acid and sodium chlorite
• Chlorine gas and sodium chlorite (excess
  chlorine)
• Sodium hypochlorite and sodium chlorite

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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

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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

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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

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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

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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

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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

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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

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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