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									      CHEE 370
Waste Treatment
     Processes


           Last Lecture
    Final Exam Review


                          1
Wastewater Constituents
   Suspended solids
   Biodegradable organics
   Micro-organisms
   Nutrients
   Refractory organics
   Heavy metals
   Dissolved inorganics
   Physical contaminants (temperature, colour, odour, solids)




                                                                 2
Measurement of Solids
                                                          Evaporation
                                                          and drying at
                                                          105 ºC



               Evaporation
               and drying at
               105 ºC




           Ignition at 500 ºC




                               TS = TVS + TFS
                           TS = organic + inorganic   2.3 Metcalf & Eddy
The BOD Test
COHNS + O2 + bacteria (C5H7NO2)
  more C5H7NO2 + CO2 + H2O + NH3 + other

   An INDIRECT measure of the organic content
   Measures the amount of oxygen consumed as the
    bacteria found in the WW use the organic material as a
    carbon/energy source for the production of more bacteria


              BODt=UBOD(1 - e-kt)
                                                           4
BODt=UBOD(1 - e-kt)
Chemical Oxygen Demand
(COD)
   An INDIRECT measure of the organic
    content
   Mass of oxygen theoretically required to
    completely oxidize an organic compound to
    carbon dioxide
   Measured by mixing the WW with a very
    strong chemical oxidant

           CODin = CODout + O2consumed
                                                6
Total Organic Carbon (TOC)
   A DIRECT measure of the organic content
   In theory - based on the chemical formula
   In practice - organic carbon is converted to
    carbon dioxide, which can then be measured

Theoretical Oxygen Demand (ThOD)
   An INDIRECT theoretical measure of organic
    content
   Calculated using stoichiometric equations
   Considers both carbonaceous and                7
    nitrogenous oxygen demand
Additional Characterization
   Micro-organisms
       Total coliform, fecal coliform

   Toxicity
       Acute toxicity (LC50), Chronic toxicity

   Nutrients
       TKN, NH3, TP, ortho-phosphate

   Flowrates
       Hydraulic flowrates (peak and min), Loadings
                                                       8
WWTP for a Large Community
Screening - Headloss

                        1 V 2  v 2 
                    hL             
                        C  2g 
   hL = headloss (m)
   C = empirical discharge coefficient to account for turbulence and
    eddy losses (clean screen = 0.7; clogged screen = 0.6)
   V = velocity through the openings (m/s)
   v = approach velocity in upstream channel (m/s)

   g = acceleration due to gravity (9.81 m/s2)



                                                                        10
Types of Settling

 Type I: Discrete Settling
 Type II: Flocculent Settling

 Type III: Hindered or Zone Settling




                                        11
Type I Settling
Discrete Settling

   Settling of discrete, non-flocculating particles
       Found in grit removal tanks!
   Particles settle as individual entities at a
    constant velocity
   Minimal interaction between particles
   Applies only to particles in a suspension with
    a low solids concentration

                                                   12
Type I Settling
Critical Settling Velocity
   vo= critical settling velocity
   A particle starting at the top of the inlet zone with a
    settling velocity of vo will just reach the bottom of the tank
    at the beginning of the outlet zone




                                                                13
Type I Sedimentation
Analysis
   Use a batch settling column

   Withdraw samples from a fixed height “h” at time
    intervals and measure the solids concentration

   Calculate the weight fraction remaining

   Calculate the settling velocity for the particles at each
    of the time intervals (vs = h/t)

   Plot the weight fraction remaining versus velocity
    (cumulative distribution curve)
                                                                14
                     fo
                1
F  (1 f o ) 
                vo
                      v df
                          s
                              Total Removal
                     o
Type II Settling
Flocculant Particle Settling

   Particles coalesce as they settle

   Rate of settling (vs) changes with time
       Particles change in size, shape and weight as
        they settle
       Larger particles have a higher vs




                                                        16
Type II Settling
Analysis
   Use a batch settling column with multiple sampling ports
   Withdraw samples from each port at time intervals and
    measure the solids concentration
   Calculate the weight fraction removed
   Prepare a plot of sampling depth versus time and
    indicate the weight fraction removed for each of the
    samples
   Draw the “equal percent removal” lines at intervals of
    10% on the plot

                                                             17
r3


r2




r1
Type I and Type II Settling
Clarifier Dimensions

   A = L x W = surface area of the basin (m2)

   Default aspect geometry:
       L = 4W
       A = 4W2




                                                 19
Type I and Type II Settling
Scouring Velocity
   Re-suspension of particles due to large horizontal velocities
    (u)

                                Q
                            u
   Where:
                               HW
       u = horizontal velocity (m/s)
       Q = water flowrate (m3/s)
       HW = cross-sectional area (entry area) in the direction of flow
        (m2)

               To prevent scouring, u < (9 * vo)

                                                                          20
Type III Settling
Hindered or Zone Settling
   Occurs in solutions with a very high solids
    concentration

   Strong cohesive forces between the particles
    cause them to settle collectively as a zone

   Distinct interface between the settled
    particles and the clarified effluent

                                                  21
Type III Settling
Clarifier Design

   Secondary clarifiers need to be designed for
    two purposes:

     Clarification
     Thickening




                                                   22
Type III Settling
Secondary Clarifier Design
1.       Calculate the area required for clarification

                                  Qe
                             vo 
                                  Ac
        Where:
         vo = initial zone settling velocity at the feed concentration (X),
          [m/h], (function of X)
         Ac = surface area for clarification [m2]

          
         Qe = overflow rate of clarified liquid [m3/h]


                                                                               23
 Type III Settling
 Secondary Clarifier Design
 2.       Calculate the area required for thickening
 •        Find the gravity mass flux


                         Gg  v i X i
         Where:
           Gg = gravity flux [M/L2•T] (kg/(m2•h))
           vi = settling velocity at solids concentration Xi [L/T] (m/h)
           Xi = local concentration of solids [M/L3] (kg/m3)




                                                                            24
Type III Settling
Secondary Clarifier Design
2.       Calculate the area required for thickening
•        Find the bulk mass flux due to underflow pumping

                                                 Qu
            Gu  ub Xi                      ub 
                                                 A
        Where:
          ub = bulk downward velocity of the solids [L/T] (m/h)
          Qu = underflow flowrate [L3/h] (m3/h)
          A = surface area of settling tank [L2] (m2)
                                                                 25
Type III Settling
Secondary Clarifier Design
2.   Calculate the area required for thickening
•    Find the total mass flux
    Plot G, Gg, Gu


           G  Gg  Gu
           G  X i v i  X i ub
                                                  26
                     QX o
Limiting Flux   AT 
                     GL



         




                            27
Type III Settling
Secondary Clarifier Design
   Identify which area is greater:
       Area for clarification
       Area for thickening


   Use the larger area to size the clarifier
   Adesign = 1.75*Acalculated
   For an ideal clarifier, L = 4W


                                                28
Designing for a Specific Underflow Solids Concentration (Not Given ub)
  Batch Bacterial Growth Curve
                                1.       Lag Phase
                                         Acclimation to environment

                                2.       Exponential Growth Phase
                                         Multiplication at max rate
                                         Rapid utilization of S

                                3.       Stationary Phase
                                         Growth is offset by death
                                         Steady state

                                4.       Death Phase
                                         Depletion of S
                                         Decrease in X due to cell
                                          death                     30
Metcalf and Eddy; Figure 7-10
WW Treatment
Bacterial Growth Rate

                    dX
                        X  kd X
                    dt
   Where:
       X = biomass concentration (mass/volume)
         = specific growth rate (time-1)
    
    

       kd = endogenous decay coefficient (time-1)

                                                     31
Bacterial Growth in Biological WW Treatment
Monod Kinetics
                              Specific growth rate
                               increases as the
                               concentration of the
                               limiting substrate S
                               increases
                                   = specific growth rate
                                   (time-1)
                                  max = maximum specific
                                   growth rate (time-1)

            m axS
                                  S = concentration of the
                                   growth limiting substrate
                                 (M/V)

            S  KS                Ks = half saturation
                                   constant (M/V)
                                                               32
Michaelis-Menten Continued
   Derivation carried out in class!
                     [S ]
      vo  vm ax
                 S   K M
   Now analogously, if our “product” are cells
    the above specific rate of cell formation is
    as described as:
              m axS
        
              S  KS                               33
Estimation of Kinetic
Parameters
   It is possible to estimate the kinetic
    parameters (Ks, kd, max, Y) from bench-scale
    CSTR process data in order to design
    biological waste treatment facilities

   Perform tests starting with a known limiting
    substrate concentration So
       Measure X and S at various residence times ()

                                                         34
CSTR With No Recycle
Problems
   If the kinetic parameters (Ks, kd, max, Y) are known, and
    you are given So, Q, and one additional variable (i.e. S,
    U, V), then you can solve for the rest




                                                                 35
  AS Design Equations
     V                   VX                 Ks  S
        c  c  Q X  Q X  c  S(   k ) k K
     Qo               E E   w R          max   d   d s


        c Y(So  S)        K s(1 c kd )             So  S
   X                S                        U
         1 kd c       c (  m ax  kd ) 1           X
 1  S                       
          max          1                      (K s  So )(1 R  R)
                kd         YU  kd  w 
    c Ks  S          c                      So (  max  kd ) kd K s
               
                                         
                                      maxXSV              Qr    XR
   BiomassWasted  QE X E  QwX R           kd XV R        
                                     Ks  S                Qo    X
                                    
                m axSX
                                                    m axS
    RXR  (1 R)X              kd X  0 R  (1 R)             kd  0
                      Ks  S                              Ks  S
               F Qo  So   S                            
                                                       So  S
                         o               
                                           Efficiency        100%
               M V  X  X                              So
    COD Mass Balance
    Oxygen Consumed
    When working in COD units, you can always perform a mass
     balance

COD substrate in = COD substrate out + COD biomass out + O2
                         consumed

      O2 consumed = COD substrate in - COD substrate out -
                       COD biomass out

     O2 consumed = COD substrate consumed - COD biomass
                             out


                                                                37
Conversion Factor (f)
   Required to determine the oxygen requirements if
    the substrate concentration is expressed in terms of
    BOD5
                       BOD5 converted
                   f 
                       UBODconverted

   If you are given the influent substrate concentration
    in terms of BOD5 and UBOD, you can calculate “f” to
         
    determine the effluent substrate concentration in
    UBOD units
                                                        38
AS - Aeration Requirements
   Air supply requirements can be expressed in a
    variety of units
       kg O2/day, kmol O2/day, m3 O2/day, m3 air/day
   Conversion factor: 22.4 m3 gas/kmol (@ STP)
   Air contains ~21 % O2
   If the oxygen transfer efficiency of the aeration
    system is known or can be estimated, the air
    requirements may be determined

    ALWAYS DESIGN AERATION SYSTEMS WITH A
              SAFETY FACTOR OF 2
                                                        39
SVI Determination
   Take a sample of MLSS from the aeration basin
   Settle for 30 min (usually in a 2 L container with a
    diameter larger than a graduated cylinder)
   Determine the volume and mass of the settled solids

             Y         Y, settled volume of sludge (mL)
       SVI            X, mass of settled solids (g)
             X         Typical range: 50 - 150 mL/g

                                 6
                          10
                     Xr              Units of mg/L
                          SVI                              40
 TF Design Equation
               Se                  n 
                                A 
                   exp  z    
                        K
               Si      
                              Q  
                                       
    z = depth of the packing media/bed [ft]
    Q = applied flow (Qo + Qr) [MG/D]
    A = filter bed cross-sectional area = π•r2 [Acres]

    n, K = constants; f(packing media);
        See table 6.11


                                                          41
AD Model Assumptions
   Design based on the rate-limiting step - breakdown of
    volatile fatty acids (VFAs)
   Non-biodegradable fractions of COD remain unchanged
    by the digestion process
   Heterotrophic bacteria only decays and the COD
    associated with decay will be accumulated as VFAs
    available to the methanogens
   Complete hydrolysis and fermentation of biodegradable
    organic matter -> fully available to methanogens
   Use the kinetics for the growth of the methanogens to
    determine the minimum SRT, then use this value with a
    safety factor to determine the operating conditions   43
Minimum SRT Calculation

                                K vfa  Svfa,available
        m in 
                Svfa,available  (m ax,m  k d ,m )  K vfa  k d ,m

   Where:
        umax,m = maximum specific growth rate for the
         methanogens
        Kd,m = decay rate for the methanogens



                                                                        44
Factor of Safety for Growth
   It is necessary to provide a factor of safety for
    methanogen growth (prevent “stuck” digester) and
    headspace
   Use a factor of safety of at least 2.5

                   design  2.5  min
   The ministry of the environment requires at least 15
    days SRT at 35 C
       Compare with your calculation and select the larger value

                                                                  45
Heterotroph Mass Balance
   Assume there is no growth - only decay
   Perform a mass balance on the digester for the
    heterotrophic bacteria:

                        X H ,o
                XH 
                     1 kd , H   c
   As the SRT increases, the amount of active
    heterotrophic biomass in the effluent decreases

                                                    46
     Debris Mass Balance
        Debris (XD) can enter the digester in the influent
         (XDo) stream and is also generated during biomass
         decay
        Perform a debris mass balance on the digester :
                                      k   
            X D  X Do  f d  X H,o   d ,H   c
                                                    
                                      kd , H  c 
                                      1
        Where fd = debris fraction of the degraded biomass
         (fd ranges from ~ 0.08 - 0.20)

                                                            47
VFAs for Methanogens
   Multiple Sources:
       Soluble biodegradable COD (Ss)
       Biodegradable particulate COD (Xs)
       Decay of heterotrophic biomass

                                                 k   
Svfa,available    Ss  X S  (1 f d )  X H ,o  d ,H   c
                                                               
                                                 kd , H  c 
                                                 1

                                                             48
 Effluent VFA and Formation of
 Methanogenic Bacteria
    CSTR without recycle

                      K vfa  (1  c k d ,m )
           Svfa   
                     c (m ax,m  k d ,m ) 1

                     (Svfa,available  Svfa )
          X m  Ym 
                      1  c  k d ,m
                                                 49
Methane Production
   COD balance can be performed in order to
    determine the amount of methane produced
   CODin = Q(SSo + XSo + XHo + XDo)
   CODout = Q(Svfa + XH + Xm + XD)

          CODin = CODout + CODmethane produced

              CH4 + 2O2  CO2 + 2H2O
   64 g COD/mol CH4
                                                 50
Methane Production
   Use the ideal gas law to calculate the volume
    produced per day (V=nRT/P)
   Textbook example 7-9, p. 633 - Effect Of Temp!
   Volume of methane produced per day:

                            mCH 4 RT
                  FCH 4 
                             64P
   Where mCH4 is mass-COD of CH4 produced/time
                                                     51
Breakpoint Chlorination


                  Dose




                          52
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

                                                               53
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                                    54
Nitrification
   In a suspended growth (AS) process, low-rate
    extended aeration conditions lead to nitrification
   Involves two types of autotrophic bacteria:
       Nitrosomonas oxidizes ammonia to nitrite
        NH3 + 3/2 O2  HNO2 + H20
       Nitrobacter oxidizes nitrite to nitrate
        HNO2 + 1/2 O2  HNO3
       Overall conversion:
        NH3 + 2 O2  HNO3 + H20


                                                         55
Nitrifying Systems
   The AS design algorithm can be used to develop
    strategies to remove ammonia from the effluent
    through nitrification
   Ammonia is used as the substrate in the AS design
    model (in place of the biodegradable organics in the
    influent (BOD))
   The systems are designed to facilitate longer solids
    retention times
       Accounts for the slower growth the the nitrifying bacteria

   In practice, typically a plug flow system is used
    (results in coupled PDEs)                                        56
One-Step Process




Two-Step Process




                                                   57
                   Metcalf and Eddy; Figure 7-19
Summary of the BEPR Process
   In the anaerobic zone of the system:
       PAOs take up the VFAs from the liquid phase
       Phosphorus is released to the liquid phase due to polyphosphate
        cleavage to provide energy for VFA transport
       Glycogen is utilized and transformed into PHAs to provide enough
        reducing power to drive the transformation of VFAs into PHAs


   At the end of the anaerobic period:
       No VFAs left
       Large phosphorus concentration in the liquid phase
       PAOs will have low intracellular glycogen and high PHA contents
                                                                      58
    Summary of the BEPR Process
   In the aerobic zone of the system:
       PAOs use stored PHA as a substrate for growth and for replenishing the
        glycogen pool
       PAOs use stored PHA for phosphorus uptake and replenishment of the
        polyphosphate pool
       The amount of phosphorus taken up in the aerobic phase is higher than
        the amount of phosphorus released in the anaerobic phase
       Net P removal from the liquid phase


   At the end of the aerobic period:
       More PAOs will be present
       Intracellular glycogen content is high, polyphosphate content is high,
        PHA content is low
       Soluble phosphorus concentration is very low (can be zero)
                                                                            59
The BEPR Process




                   60

								
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