Activated Sludge Processes by 32X84688

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									Activated Sludge Processes

          CE - 370
                     Basic Process
   The basic AS process consists of
       A   reactor in which the microorganisms responsible for
         treatment are kept in suspension and aerated
        Liquid-solids separation, usually sedimentation tank
        A recycle system for returning solids removed from the
         liquid-solids separation unit back to the reactor
   Important feature of the AS process is:
                 of flocculent settleable solids that can be
        Formation
        removed by gravity settling
   Activated Sludge process utilizes:
            Fluidized microorganisms
            Mixed growth microorganisms
            Aerobic conditions
   Microorganisms
            Use organic materials in wastewater as substrates
            Thus, they remove organic materials by microbial respiration and
             synthesis
   MLSS
            Ranges between 2000 and 4000 mg/l
   Flows
       Feed wastewater (Q)
       Waste activated sludge (Qw)
       Recycled activated sludge (R)
            Prior to entering aeration tank
            OR immediately after entering
   Oxygen Supply
     Diffused compressed air
     Mechanical surface aeration
     Pure oxygen

   Purposes of aeration
     Provides oxygen required for aerobic bio-oxidation
     Provides sufficient mixing for adequate contact
      between activated sludge and organic substances
   In order to maintain the desired MLSS in the
    aeration tank, R/Q ratio must be calculated
              Calculate (R / Q) Ratio
   Calculate the Sludge Density Index (SDI)
          Sample MLSS from downstream of aeration tank
          Determine SS in MLSS
          Place 1 liter of the MLSS in 1-liter graduate cylinder
          Settle the sludge for 30 minutes
          Measure volume occupied by settled sludge
          Compute SS in settled sludge in mg/l
          SS represents SDI
          The test approximates the settling that occurs in final clarifier
   If SDI = 10,000 mg/l and MLSS must be 2,500 mg/l
   Then, Q(0) + R(10,000) = (Q+R)(2500)
   R/Q = (2500)/(7500) = (1/3) = 33.3 %
   So, R is 33.3% of feed wastewater (Q)
   Sludge Volume Index (SVI) = 1/ SDI
        Is the volume in ml occupied by 1 gram of settled
         activated sludge
        It is a measure of settling characteristics of sludge
        Is between 50 and 150 ml/gm, if process is operated
         properly
   Why Qw?
        Microbes  utilize organic substances for respiration and
         synthesis of new cells
        The net cell production (Qw) must be removed from the
         system to maintain constant MLSS
        Qw is usually 1 to 6 % of feed wastewater flowrate (Q)
   Common organic materials in municipal wastewater
    are:
          Carbohydrates (C, H, O0
          Fats (C, H, O)
          Proteins (C, H, O, N, S, P)
          Urea (C, H, O, N)
          Soaps (C, H, O)
          Detergents (C, H, O, P)
          Traces of
                Pesticides
                Herbicides
                Other agricultural chemicals

   Activated sludge can be represented by:
          C5H7O2N
          Has a molecular weight of 113
                                 Design
   To design of AS, the following must be determined:
       Volume of reactor
            Number of basins
            Dimensions of each basin
       Volume of reactor is determined from:
            Kinetic relationships
            Space loading relationships
            Empirical relationships
       Sludge production per day (Xw), kg/day
       Oxygen required per day (Or), kg/day
       Final clarifier
            Number of basins
                  Biological Kinetics
   1. Michaelis – Menten Concept


                     1 dS       S 
                          ks      
                               K S 
                     X dt       m   

       (1/X)(ds/dt) = specific rate of substrate utilization
       (ds/dt) = rate of substrate utilization
       ks = maximum rate of substrate utilization
       Km = substrate concentration when the rate of utilization is half
        maximum rate
       S = substrate concentration
             1 dS       S 
                  ks      .......()
                       K S        1
             X dt       m   
   If S is very large, Km can be
    neglected,      therefore    S
                                         1 dS
    cancels out and the reaction
    is zero order in substrate. K             k s  K ......( )
                                                              2
    is the rate constant for zero-
                                         X dt
    order reaction.

   If S is relatively small, it can
    be      neglected     in     the
    denominator         and      the     1 dS k s
    reaction is first-order in                  ( S )  KS ......( 3)
                                         X dt K m
    substrate. K is the rate
    constant for the first-order
    reaction
   Rearrange and integrate Equation (2)
                          St               t
                      S0
                               dS   K X  dt
                                          0

                       yields
                      St  S 0   K X t
                      or
                      St  S0  K X t......( )
                                           4

          X = average cell mass concentration during the biochemical
           reaction, that is X = (X0 + Xt)/2
          St = substrate concentration at time t
          S0 = substrate concentration at time t = 0
   Rearrange and integrate Equation (3)

                    StdS         t
                  S0 S   K X 0 dt
                  yields
                  ln St  ln S 0   K X t
                  or
                  ln St  ln S 0  K X t......( )
                                              5

          X = average cell mass concentration during the biochemical
           reaction, that is X = (X0 + Xt)/2
          St = substrate concentration at time t
          S0 = substrate concentration at time t = 0
   Equations (4) and (5) are in the form of
     y = mx + b
     Plotting St on y-axis versus Xt on the x-axis on
      arithmetical paper produce a straight line with a
      slope of –K
     Plotting St on y-axis versus Xt on the x-axis on
      semilog paper produce a straight line with a slope
      of -K
   The substrate could be
         The BOD5
         Biodegradable part of COD
         Biodegradable fraction of TOC
         Biodegradable of any other organic matter
   Rate Constant, K
        Depends   on the specific wastewater
        For domestic wastewater, it ranges between 0.1 to 1.25
         liter/(gram MLSS)(hr) using BOD5
        Should be determined using lab-scale or pilot-scale
         studies
        In the absence of studies, K between 0.1 and 0.4
         liter/(gram MLSS)(hr) is recommended
Example on Biochemical Kinetics
    Food to Microorganism Ratio (F/M)
   F/M ratio is equal to the specific rate of substrate
    utilization (1/X)(dS/dt)


                     F   S
                       
                     M Xt
   The units of F/M ratio are (mass substrate) / (mass
    microbes)  (time)
           (kg BOD5/kg MLVSS-day)
     Mean Cell Residence Time (c)
   It is defined as:


                           X
                      c 
                           Xw
           X = active biological solids in the reactor
            X = active biological solids in the waste activated sludge flow
   Units of c is days
   Mean cell residence time is sometimes referred to as
    sludge age
                  F/M Ratio and c
   Both parameters are used characterize the
    performance of the activated sludge process
       A  high F/M ratio and a low c produce filamentous
         growth that have poor settling characteristics
        A low F/M ratio and a high c can cause the biological
         solids to undergo excessive endogenous degradation and
         cell dispersion
   For municipal wastewater
        c should be at least 3 to 4 days
        If nitrification is required, c should be at least 10 days
                     F/M Ratio and c
   Relationship between c and F/M ratio can be derived
    by starting with the equation of cell production, as
    follows:

                      X    S
                         Y     ke X
                      t    t
          (X/t) = rate of cell production, mass/time
          Y = cell yield coefficient, mass cell created/mass substrate removed
          ke = endogenous decay, mass cells/(total mass cells)  (time)
          X = average cell concentration, mass
                F/M Ratio and c
   Divide by X

               X / t    S / t
                       Y          ke
                 X          X
   c is the average time a cell remains in the system,
    thus

                           X
                    c 
                         X / t
                F/M Ratio and c
   The F/M ratio is the rate of substrate removal per unit
    weight of the cells, thus

                       F S / t
                         
                       M   X
   Thus
                      1    F
                        Y    ke
                     c    M
               F/M Ratio and c
   Since F/M was also expressed as:

                    F   S
                      
                    M X t
   Then,

                      1    S
                        Y       ke
                     c    X t
           Types of Reactors


 Plug-flow reactors
 Dispersed plug-flow reactors
 Completely-mixed reactors
     Plug-flow and Dispersed-flow
               Reactors
 In plug-flow reactors, there is negligible
  diffusion along the flow path through the
  reactor
 In dispersed-flow reactors, there is significant
  diffusion along the flow path through the
  reactor
 Both types of reactors are used in conventional
  and tapered aeration activated sludge
    Conventional Activated Sludge
 Rectangular aeration tank
 F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day)
 Space loading = 0.3 to 0.6 (kg BOD5/day-m3)
 c = 5 to 15 (days)
 Retention time (aeration tank) = 4 to 8 (hours)
 MLSS = 1500 to 3000 (mg/l)
 Recycle ratio (R/Q) = 0.25 to 1.0
 Plug-flow and Dispersed-flow
 BOD removal = 85 to 95 (%)
            Tapered Aeration
 It is a modification of the conventional process
 F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day)
 Space loading = 0.3 to 0.6 (kg BOD5/day-m3)
 c = 5 to 15 (days)
 Retention time (aeration tank) = 4 to 8 (hours)
 MLSS = 1500 to 3000 (mg/l)
 Recycle ratio (R/Q) = 0.25 to 1.0
 Plug-flow and Dispersed-flow
 BOD removal = 85 to 95 (%)
Oxygen Demand versus Reactor
Length for Municipal Wastewater
Quarter of Reactor   O2 Demand/total O2 for
                        entire reactor (%)
       1st                      35

       2nd                    26

       3rd                    20

       4th                    19
                         Performance
                                 St     K X
                                    e
                                 S0
           is the detention time for the plug-flow reactor


   The volume of the plug-flow or dispersed-flow
    reactor is given by:


                          V  (Q  R) 
     Completely Mixed Reactors
 Usually circular and square aeration tanks
 F/M = 0.1 to 0.6 (kg BOD5/kg MLSS-day)
 Space loading = 0.8 to 2.0 (kg BOD5/day-m3)
 c = 5 to 30 (days)
 Retention time (aeration tank) = 3 to 6 (hours)
 MLSS = 2500 to 4000 (mg/l)
 Recycle ratio (R/Q) = 0.25 to 1.5
 Completely mixed
 BOD removal = 85 to 95 (%)
              Design Parameters
   The retention time and reactor volume for completely
    mixed reactors can be determined by:


                          Si  St
                       
                          K X St


                       V  Q 
          Process Modifications
 Objective of modifications
 Modifications
      Stepaeration
      Modified aeration
      Contact stabilization
      High-rate aeration
      Extended aeration
     Objectives of Modification


Several modifications of the activated sludge
process were made to attain a particular or
design objective
               Step Aeration
 It was developed to even out the oxygen
  demand of the MLSS throughout the length of
  the reactor
 It uses plug-flow and dispersed plug-flow
  reactors with step inputs of the feed flow (Q)
 Design Parameters
      = 3-5 hrs; c = 5-15 days; R/Q = 25-75%; MLSS =
      2000-3500 mg/l; BOD5 and SS removal = 85-95%; F/M
      = 0.2-0.4 kg/kg-day; space loading = 0.6-1.0 kg
      BOD5/day-m3
            Modified Aeration
 Designed to provide a lower degree of
  treatment than the other activated sludge
  processes
 It uses plug-flow and dispersed plug-flow
  reactors
 Design Parameters
      = 1.5-3 hrs; c = 0.2-0.5 days; R/Q = 5-15%; MLSS =
      200-500 mg/l; BOD5 and SS removal = 60-75%; F/M =
      1.5-5.0 kg/kg-day; space loading = 1.2-2.4 kg
      BOD5/day-m3
          Contact Stabilization
 Designed to provide two reactors, one for the
  sorption of organic matter and for the bio-
  oxidation of the sorbed materials
 It uses plug-flow and dispersed plug-flow
  reactors
 Design Parameters
      = 0.5-6 hrs; c = 5-15 days; R/Q = 50-150%; MLSS =
      1000-10000 mg/l; BOD5 and SS removal = 80-90%;
      F/M = 0.2-0.6 kg/kg-day; space loading = 1.0-1.2 kg
      BOD5/day-m3
          High-Rate Aeration
 Designed to provide a lower degree of
  treatment than the other activated sludge
  processes
 It uses completely mixed reactor
 Design Parameters
      = 2-4 hrs; c = 5-10 days; R/Q = 100-500%; MLSS =
      4000-10000 mg/l; BOD5 and SS removal = 75-90%;
      F/M = 0.4-1.5 kg/kg-day; space loading = 1.6-16 kg
      BOD5/day-m3
            Extended Aeration
 Designed to minimize waste activated sludge
  production by providing a large endogenous
  decay of the sludge mass
 It uses plug-flow and dispersed plug-flow
  reactors
 Design Parameters
      = 18-36 hrs; c = 20-30 days; R/Q = 75-150%; MLSS
      = 3000-6000 mg/l; BOD5 and SS removal = 75-95%;
      F/M = 0.05-0.15 kg/kg-day; space loading = 0.16-0.4 kg
      BOD5/day-m3
          Pure Oxygen Process
 Designed to reduce retention time, decrease
  the amount of waste activated sludge, increase
  sludge settling characteristics and reduce land
  requirement
 It uses completely mixed reactors
 Design Parameters
      = 1-3 hrs; c = 8-20 days; R/Q = 25-50%; MLSS =
      3000-8000 mg/l; BOD5 and SS removal = 85-95%; F/M
      = 0.25-1.0 kg/kg-day; space loading = 1.6-3.2 kg
      BOD5/day-m3
Effect of Temperature on Growth Rate
   Arrhenius relationship

                          K2
                                   T2 T1

                          K1
       K1 = reaction rate constant at temperature T1
       K2 = reaction rate constant at temperature T2
        = temperature correction coefficient
       T1 = temperature of MLSS for K1
       T2 = temperature of MLSS for K2
Effect of Temperature on Endogenous
   Degradation Rate Constant (ke)
   The relationship

                             ke2
                                        T2 T1

                             ke1
       ke1 = endogenous degradation rate constant at temperature T1
       ke2 = endogenous degradation rate constant at temperature T2
        = temperature correction coefficient
       T1 = temperature of MLSS for ke1
       T2 = temperature of MLSS for ke2
         Other Kinetic Relationships
   2. The Monod Equation

                              S 
                       max    
                             K S
                              s  
        = growth rate constant, time-1
       max = maximum growth rate constant, time-1
       S = substrate concentration in solution
       Ks = substrate concentration when the growth rate constant
        is half the maximum rate constant.
   Monod observed that the microbial growth is
    represented by:

                   dX
                       X
                   dt
       dX/dt = rate of cell production
       X = number or mass of microbes present
        = growth rate constant
Generalized substrate consumption and biomass growth with time.

								
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