3.8 DISINFECTION

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3.8 DISINFECTION
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Disinfection

• Disinfection is a process designed for the
  deliberate reduction of the number of pathogenic
  microorganisms
• While other water treatment processes, such as
  filtration, coagulation-flocculation-sedimentation
  , may achieve pathogen reduction, this is not
  generally their primary goal
• The concept of disinfection preceded the
  recognition of bacteria as the causative agent of
  disease.

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Selecting Disinfectant

• Effective to kill bacteria
• Nontoxic to humans or domestic animals
• Nontoxic to fish
• Easy and safe to store, transport, and denspense
• Low cost
• Easy and readily analysis in water
• Provides residual protection in water

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Regulatory Issues

• SWTR
• Amendments to the Safe Drinking Water Act require
  that all surface water suppliers in the US filter
  and/or disinfect to protect the health of their
  customers.
• The amount of disinfection credit to be awarded
  is determined by with the CT concept, CT being
  defined as the residual disinfectant
  concentration (C, mg/l) multiplied by the contact
  time (T, min) between the point disinfectant
  application and the point of the residual
  measurements.
• SWRT Guidance Manual provides table of CT values
  for different disinfectants

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Disinfectant By-product Requirements

• Along with disinfection requirements, since 1974
  there have been explicit regulations on
  disinfection by-products first to the respect
  of trihalomethanes and more recently with respect
  to haloacetic acids, bromate, and other possible
  by-products
• The combination of the requirement to achieve
  disinfection along with the requirement to
  minimize disinfection by-products has led to an
  increasing spectrum of options being considered

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Kinetics of Disinfection

• The kinetics of disinfection is described by
  first order law from studies by Chick (1908)
• dN/dt -kN
• where N is the number of microorganisms
• t is time
• k is the dieoff coefficient
• The dieoff coefficient is a function of
  disinfectant,
• type of microorganism and conditions in water
• k ?Cn
• C is the concentration of disinfectant
• n is a constant of dilution
• ? is inactivation constant

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• In equation n is commonly assumed to be
• equal to 1. Combining two equations we have
• the Chick-Watson disinfection model
• ln N/No - ?Cnt
• N0 is actual number of microorganisms

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Chlorine

• Chlorine was first prepared by Scheelle in 1774,
  but was not regarded as a chemical element until
  1808.
• Early uses of chlorine included the use for
  wastewater treatment in1825 and its use as a
  prophylactic agent during the European cholera
  epidemic of 1831
• Disinfection of water by chlorine first occurred
  in 1908 at Bubbly Creek (Chicago) and Jersey City
  Water Treatment Company. By 1918 , over 1000
  cities, treating more than 3 billion gallon per
  day (1.1107 m3/day) of water were employing
  chlorine as a disinfectant

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Chlorine Disinfection action

• Microorganism kill by disinfectants is assumed to
  follow CT concept
• CT 0.9847C0.1758 pH2.7519 T-0.1467

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Basic Chemistry

• Chlorine and chlorine compounds reactions in
  water
• Cl2(g)H2OHOClHCl-
• The reaction is pH dependent and complete within
• few milliseconds.
• At pH gt 1.0 equilibrium displaced to the right
• Hypochloric acid is a weak acid and dissociates
  poorly at pH lt 6
• 6lt pHlt8.5 there occurs sharp change from
  undissociated HOCl to almost complete
  dissociation
• HOCl H OCl-
• pK7.537 _at_ 75oC

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• OCl- 2e- 2HCl- H2O
• Hypochlorite salts dissociate in water to yield
  hypochlorite ions
• NaOClNa OCl-
• Ca(OCL)2Ca2 2OCl-
• 1 mole of hypochlorite is electrochemically
  equivalent to 1 mole of
• elemental Cl-, and may be said to contain 70.91
  g of available
• chlorine (MW of Cl element is 35. 453).
• The salts contain 1 and 2 mole of hypochlorite
  per mole of
• chemical and, as result, 70.91 and 141.9 g
  available chlorine per
• mole respectively. The MWs are 74.5 and 143 so
  the pure preparations
• of the 2 compounds contain 95.8 and 99.2 of
  chlorine hence they
• effective means of supplying chlorine.
  

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• Advantages
• Well known
• Economical
• Disadvantages
• Produces trihalomethanes
• Slows the nervous system in people
• Chlorinated hydrocarbons are considered health
  hazards
• Corrosive

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Chlorine / Ammonia Reactions

• Chloramination (for ability of prolong the
  stability of residual disinfectant and for its
  diminished propensity to produce chlorphenolic
  taste and odor) was first employed in Ottawa,
  Canada, and Denver, Colorado, in 1917. Shortage
  of ammonia during World War II and recognition of
  superiority of free chlorine as a disinfectant,
  reduced the popularity of chloramination process

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Chlorine Ammonia Reactions

• When chlorine added to water that contains
  ammonia the ammonia reacts with HOCl to form
  various chloramines, which like HOCl, retain the
  oxidizing power of the chlorine
• NH3HOCl NH2H2O - monochloramine
• NH2ClHOClNHCl2H2O - dichloramine
• NHCl2HOCl NCl3H2O - trichoramine

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Chlorine Dioxide

• Was first produced from reaction of potassium
  chlorate and hydrochloric acid by Davy in 1811
• It has widely been used as a bleaching agent in
  pulp and paper manufacture
• However not until the industrial scale
  preparation of sodium chlorite , from which
  chlorine dioxide can more readily be generated,
  did its widespread use occur

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• Chlorine dioxide (ClO2) is neutral compound of
  chlorine in the IV state. It has a boiling
  temperature at 11oC at atmospheric pressure
• As chloride dioxide exerts its oxidizing power,
  it degrades into chlorite (ClO2-) and to a lesser
  extend to chlorate (ClO3-)
• Chlorite ion can oxidize hemoglobin and cause
  hemolytic anemia, which is a decrease in the
  blood concentration of hemoglobin

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• Chlorine Dioxide ClO2
• Advantages
• Oxidize all metals and organic matter
• Oxygen converts organic matter to carbon dioxide
  and water
• Kills protozoans and bacteria
• Generated on site
• Disadvantages
• Dangerous around activated carbon
• Must monitor chlorite
• Can produce odors
• Corrosive

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Ozone

• Ozone was discovered in 1783 by Van Marum and
  named by Schonbein in 1840.
• Ozone was first applied for as a potable water
  water disinfectant in 1893 in Oudshoorm,
  Netherlands
• In the US, ozone was firest employed for taste
  and odor control at NY citys Jerome Park
  Reservoir in 1906
• Since the 1993 the Milwaukee Criptosporodium
  outbreak, there has been an upsurge in interest
  in ozone as a disinfectant

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• Ozone is created by an electrical discharge in a
  gas containing oxygen. Ultraviolet irradiation at
  wavelengths lt200 nm of a gas containing oxygen is
  an alternative method The equation for ozone
  formation is
• 3O2 2O3

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• Ozone
• Kills by direct affect on DNA
• 30,000 times faster than chlorine
• Kills all pathogens but has no residual
• for system
• Corrosive

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UV Radiation

• The biocide effect of ultraviolet radiation has
  been known since it was established that
  short-wavelength UV was responsible for microbial
  decay often associated with sunlight
• By the early 1940s, design guidelines for UV
  disinfection were proposed, but historically,
  however, it has met little enthusiasm in public
  water treatment because of the lack of a residual
  following application
• UV has been accepted for treating potable water
  on passenger ships

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Other Agents

• A variety of other methods can be used to effect
  of inactivation of microorganisms
• Heat, extremes in pH, metals (silver, cooper),
  surfactants, permanganate, electrom beam
  radiator.
• Heat is useful only in emergencies in boil
  water orders, and uneconomical
• An alkaline pH (during lime softening) may
  provide some microbial inactivation, but is not
  usually sufficient as a sole disinfectant
• Potassium permanganate has been reported to
  achieve some disinfecting effects, but the
  magnitude has not been well characterized
• Feasibility of high-energy electrons for
  disinfection of potable water is still uncertain