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CHE 370 Waste Treatment Processe

VIEWS: 27 PAGES: 32

									      CHEE 370
Waste Treatment
     Processes


        Disinfection



                       1
What we have covered so far:      •CSTR no Recycle
                                  •The Activated Sludge Process
                                  •Trickling Filters
               Type II Settling

Bar Racks                              Type III Settling




  Type I Settling


                                                  To reduce the
                                                  sludge volume
                                                  sent to the AD

  Sludge stabilization by
  methanogenic bacteria                  •Fertilizer
                                         •Incineration
                                         •Landfills
Disinfection
   Selective destruction of disease-causing
    organisms
   In WW treatment, we are concerned with:
       Some bacteria
       Viruses
       Amoebic cysts
   Overall objective is to prevent waterborne
    diseases such as typhoid, cholera, and dysentery
   Disinfection is not the same thing as sterilization
   Textbook Chapter 12; Table 12-28

                                                          3
Mechanisms of Disinfection
1.   Damage the cell wall
2.   Alteration of cell permeability (i.e. phenol)
3.   Alteration of colloidal nature of protoplasm
     (i.e. heat)
4.   Inhibition of enzyme activity (i.e. chlorine or
     any oxidant)



                                                       4
Methods of Disinfection
   Chemical agents:
       Chlorine and its compounds, bromine, iodine,
        ozone, phenol, alcohols, soaps and detergents,
        hydrogen peroxide, acids, bases.
   Most common disinfectants are oxidizing
    chemicals
   Chlorine is the most commonly used
    disinfectant in North America

                                                         5
Methods of Disinfection
   Physical Agents:
       Heat (boiling), UV
       The use of UV systems has been increasing
        dramatically in the last few years


   Mechanical Means:
       Filtration, ultrafiltration, nanofiltration



                                                      6
Selecting the Appropriate Type
of Disinfection
Considerations:
 Germicidal and viricidal effect
       Needs to be sufficient to achieve the treatment objectives
   Stable Residual
       Important in drinking water treatment (or cases of water
        reuse) to prevent re-contamination during distribution
   Safety
       Some chemicals are highly toxic, such as chlorine
   Formation of disinfection by-products

                                                                     7
Measuring the Effectiveness of
Disinfection
    Total Coliform
        Species ferment lactose and produce CO2 gas when
         incubated at (35 ± 0.5) °C for (24 ± 2) h
        Species produce a colony within (24 ± 2) h to (48 ±
         3) h when incubated in a medium that facilitates growth

    Fecal Coliform
        Species produce gas or colonies when incubated at
         the higher temperature of (44.5 ± 0.2) °C for (24 ±
         2) h

    Total coliform is used as the indicator for drinking
     water and wastewater effluent
                                                                   8
Disinfection Using Chlorine
        Chlorine can be added as a:
    1.    Gas (Cl2(g)) - lots of plants use this method.
          Chlorine gas is highly toxic.
    2.    Sodium hypochlorite (NaOCl) - Commonly
          known as bleach
    3.    Calcium hypochlorite (Ca(OCl)2) - solid


        Fairly complex chemistry in water and
         wastewater
                                                           9
Chlorine in Water
        There are three basic reactions occurring when
         chlorine is added to water:
1.       Dissolution: Cl2(g) Cl2 (l)
         Very fast reaction
2.       Hydrolysis: Cl2(l) + H2O  HOCl + H+ + Cl-
         Very fast reaction
         Chlorine reacts with water to generate hypochlorous acid,
          hydrogen ions (acid), and chloride
3.       Dissociation: HOCl  H+ + OCl-
         Hypochlorous acid is a weak acid that can dissociate into
          hydrogen ions and hypochlorite.

                                                                      10
Speciation of Chlorine as a
Function of pH




                              11
Influence of Chlorine Species
 Hypochlorous acid is a much stronger oxidant
  than hypochlorite (40 - 80 x the killing efficiency)
 For a given total chlorine concentration:

CT=Cl2(l) + HOCl + OCl- = Free Residual Chlorine

   Lower disinfection efficiencies are obtained at
    high pH
   Either longer exposure times or higher dosages
    are required if hypochlorite is the predominant
    species

                                                         12
Reactions of Hypochlorous
Acid with Ammonia
   Hypochlorous acid is a very strong oxidant that
    can react with ammonia present in the effluent
       Can be very significant if the plant is not nitrifying

   The reactions result in the formation of different
    species, with differing levels of disinfection
    (oxidation) capabilities
       Need to account for these side reactions to provide
        adequate disinfection



                                                                 13
Reactions of Hypochlorous
Acid with Ammonia
   NH3 + HOCl  NH2Cl (monochloramine) + H2O
   NH2Cl + HOCl  NHCl2 (dichloramine) + H2O
   NHCl2 + HOCl  NCl3 (nitrogen trichloride) + H2O

   Free Residual Chlorine= Cl2(l) + HOCl + OCl-
   Combined Residual = Summation of all
    Chloramines
   Total Residual Chlorine = Free Residual Chlorine +
    Combined Residual Chlorine
   All concentrations are expressed as Cl2
                                                         14
Breakpoint Chlorination


                  Dose




                          15
Breakpoint Chlorination

   If a target residual chlorine concentration
    is to be maintained, it is necessary to
    provide a dose such that the demand is
    satisfied and the desired residual is
    obtained
   Breakpoint chlorination leads to the
    effective removal of nitrogen from the
    effluent wastewater

                                                  16
Kinetics of Bacterial Death
   Several factors influence the rate of bacterial kill:
       The type of disinfectant (we are considering chlorine)
       Temperature (moderate effect)
       pH (strong effect)
       Presence of organic matter (can have a strong effect)
       Concentration and types of organisms (strong effect)
       The concentration at which chlorine is present
       The contact time between the chlorine and the
        organisms to be killed


                                                                 17
Effect of Contact Time
    The most common expression to reflect the
     effect of contact time is Chick’s Law:

    dN
         kN
     dt
    N  Number of organisms per unit volum e surviving at time t
    k  Rate constant for given organism, in a given system with a given disinfecta nt
    Integratin g the expression :
    N  N 0e kt



                                                                                     18
Effect of Concentration
   The rate of kill will be maximum if both the
    contact time and the disinfectant
    concentration are high
   It is possible to have adequate kill rates if C is
    low, but t is high (and vice-versa)
   It is common to assess the product of C*t
    when dealing with disinfection systems


                                                    19
Estimating the Kill Efficiency
   A correlation often used to estimate the amount of
    residual chlorine required to achieve a certain kill
    efficiency is:

                          1  0.23Ct 
                      Nt                3
   Collins Model
                      N0
   N = number of organisms
   [C] = total chlorine residual, mg/L; [t] = time, minutes

                                                               20
Practice Problem
   Consider a chlorination tank with a detention time
    of 30 minutes. We want to destroy 99.9999% of
    the coliforms in the effluent. What residual chlorine
    concentration is required?




                                                            21
MOE Guidelines
   MOE Guidelines for Chlorine Contact
    Chambers
    1.   30 minutes for average annual flow
    2.   15 minutes for peak hourly flow
        Design the chamber based on the larger volume




                                                     22
MOE Guidelines
   MOE regulates the chlorine concentration in
    the plant effluent
       Total Residual Chlorine (TRC) ≤ 0.02 mg/L
   If the required residual chlorine
    concentration for adequate disinfection is
    higher than what the MOE allows, it is
    necessary to perform dechlorination
       Dechlorination involves the addition of sulphur
        dioxide, which leads to the conversion of
        hypochlorous acid to chloride
                                                          23
Chlorine as a Disinfectant
   Advantages:
       Reliable
       Cheap
       Simple
       Provides a stable residual

   Limitations:
       Extremely toxic and corrosive
       Can influence water taste and odor
       Forms trihalomethanes by reacting with organic
        matter in the WW - these compounds are known
        carcinogens                                    24
UV Disinfection
   Physical method of inactivating pathogens
   Mechanism of UV Disinfection:
       Radiation with a wavelength of ~ 260 nm
        penetrates the cell wall and cell membrane of
        microorganisms
       The radiation is absorbed by cell material such as
        DNA and RNA, promoting changes that prevent
        cell replication
           DNA - interferes with cell replication
           RNA - interferes with protein and enzyme production

                                                                  25
UV Disinfection - Mechanism
   Schematic of the effect of UV on DNA




                                           26
UV Disinfection - Dosage
   For chlorine disinfection, the “Ct product” is
    an indication of disinfecting potential
   For UV disinfection, a similar quantity is UV
    dosage:
           D  I t
           D  UV Dosage (mW  s/cm 2 )
           I  Radiation Intensity (mW/cm 2 )
           t  Contact Time (seconds)

                                                     27
UV Design Factors
    There is an “intensity field” in an UV reactor, as
     each lamp radiates in all directions
        The water, non-biological materials, and other lamps
         will absorb the emitted radiation
        Effect is termed “dissipation”
    Bacteria are present in flocs in the effluent
        The higher the TSS concentration in effluent, the
         higher the dosage required to reach all the bacteria
    It is necessary to perform pilot tests for UV
     systems to determine the effectiveness
                                                                28
Full-Scale UV System




                       29
UV Disinfection
   Advantages of UV disinfection over
    chlorine disinfection:
       No toxic by-products of disinfection are known
        to be formed
       Short detention times
           UV disinfection requires a six-to-10-second contact
            time, compared to a 15-to-30-minute contact time
            for chlorine
       UV disinfection presents no dangers in terms
        of handling chemicals

                                                                  30
Ozone
   More commonly used for the disinfection of
    water supplies, rather than for municipal WW
    treatment
   As ozone is unstable, a gap electrode is used
    to generate O3 from O2 on site
       3 O2 -> 2 O3
       Using air: 0.5 - 3% ozone by weight
       Using pure oxygen: 1 - 6% ozone by weight

                                                    31
Ozone as a Disinfectant
   Advantages:
       Does not react with ammonia
       No bad taste or colouration
       No chemical residue
       Aerates water to near O2 saturation

   Limitations:
       Must reach a threshold concentration to facilitate
        disinfection
       Problems can result if there are high hydraulic loadings
       Organics can interfere with the process
       Raises the electricity costs

                                                                   32

								
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