An Overview of Activated Sludge Background Design Process Control by ixl26840

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									An Overview of Activated Sludge
           Background
           Design
           Process Control
                Julie Burland
                Rowena Patawaran
                Patrick Radabaugh
Activated Sludge Process Flow
Diagram
Microbiology of Activated Sludge

 Bacteria
 Protozoa – Single Celled
 Animals
 Metazoa – Multicelled
 Animals
Definitions

 Electron Donor             fs
 Electron Acceptor          fe



   ED + feEA ----> fsBUGS + feCO2 + xH2O
Energy Production/Utilization

             Electron Donor
            (Food – COD, BOD5)




                              fs
                                   Cell Synthesis


                       fe


      Electron Acceptor
            (O2, NO3-, CO2)
Relative Energy and Cell Synthesis
For Typical Electron Acceptors in
Wastewater Treatment

                                    O2 – aerobic conditions
1.00                                NO3- - anoxic conditions
0.75                     Relative   SO4-2 – anaerobic
                         Energy     Organic Compounds –
0.50                     fs(max)    fermentation
0.25                                CO2 – anaerobic
0.00                                (methanogenic)
       Ae
       An ic
       S u ic
       M e te R
          ro b
          ox
          lfa
          th a e d u
               no
                  g e in g
                     n ic
                        c
Bacterial Growth Kinetics




               Monod Kinetics
Mass Balance Approach to
Activated Sludge Design
Definitions

Y - yield                 MLSS - mixed liquor
K - rate of substrate     suspended solids
utilization               SRT - solids retention
b - endogenous decay      time
rate                      HRT - hydraulic
Xr, Xd, Xsyn - influent   residence time, Vol/Q
solids data
Governing Activated Sludge Design
Equations


                        * (X                             )
                 SRT
 MLSS        =                 R   + X   D   + X   SYN
                 HRT

                  1
 SRT         =
               Y *k −b
       lim



XR = Influent Inert Solids Concentration
XD = Influent Biodegradable Solids Concentration
XSYN = Bug Growth and Bug Bones
Design and Previous Equations

 Engineer controls HRT = Vol basins/Q
 Engineer selects SRT to achieve CBOD
 removal and/or nitrification
 Three variables. Other variables in equations
 determined from lab work, standard values,
 design criteria relevant to project.
Biological Nutrient Removal (BNR)


 Nitrogen
 Phosphorus
 Nitrogen Removal

Nitrification
          +
       NH 4 + O2 + HCO3− ⇒ BUGS + NO3− + H 2 O


Denitrification
       e − Donor + NO3− ⇒ BUGS + N 2 + HCO3−
Biological Phosphorus Removal

                       Polyphosphates
    VFAs                                          Cells




           Anaerobic                    Aerobic



                                                          O2

    PHB                                           PHB
                          Soluble P
    PHV                                           PHV
Reduction in Yield with Increasing
Cell Age
  Yield




            Sludge Age
Increase in Aeration Basin Oxygen
Demand With Increasing SRT
  Amount




                 Sludge Age
            O2 Demand - lbs/day   Solids, lbs/day
Increase in MLSS Concentration
with Sludge Age


                  MLSS

                  Inert TSS
  Concentration




                  Biodegradable
                  TSS
                  Total XSYN




                                  Sludge Age
Reduction in Active Mass with
Increased SRT
  Concentration




                  Active Fraction
                  of Biomass

                  Inert Fraction of
                  Biomass




                                      Sludge Age
Example Activated Sludge Design

  Influent Conditions         Design Assumptions

BOD5 – 300 mg/l             Primary Clarifiers
TSS – 300 mg/l                 Remove 50% of inf. TSS
VSS – 270 mg/l                 Remove 25% of inf. BOD5
NH3 – 35 mg/l               Solids Hydrolyis Rate –
Flow – 1 mgd                   0.1/day
Temperature - 10°C (winter) Plant must nitrify
Determination of SRT

At 10°C – Nitrifiers
  Y = 0.05 mg VSS/mg NH3
  k = 8 mg NH3/mg VSS-day
  b = 0.05 1/day

SRTlim = (0.06*8-0.05)-1 = 2.9 days

Use SRT = 6 days for design
Aeration Basin Volume
Determination

Pick Design MLSS = 2,500 mg/L
Determine X from influent data and kinetic
  equations
      XR = 15 mg/L
      XD = 135 mg/L
      XSYN = 152 mg/L
      Xtotal = 302 mg/L
Aeration Basin Volume
Determination Continued…

Solve
   Q * SRT
V=         * ( X R + X D + X SYN )
    MLSS
Volume = (3,785,412*6*302)/2,500
Volume = 2,743,666 liters
Volume = 725,000 gallons
Longmont Activated Sludge



              integra engineering
Activated Sludge System

       Aeration Basin   Secondary
                         Clarifier
Equipment required for Activated
Sludge Process


  Aeration Basins
  Blowers and Diffusion System
  Secondary Clarifiers
  Return Sludge Pumps (RAS) pumps
  Waste Sludge Pumps (WAS) pumps
  Control Instrumentation (air valves, flow
  meters, DO probes, pressure gages, etc)
Aeration Basins and Gates
Aeration Basins   Gates for flow
                  sequence
Blowers
 Aeration Basin Air Piping and
 Diffusion Grids
Aeration Header




Aeration Drop Leg



                    Diffuser Grids
Secondary Clarifiers
Secondary Clarifier Design
RAS and WAS pumps


  RAS pumps     WAS pumps
Process Control

An art of balance!

Process control   Adjustment achieved with:
adjustment
HRT               Putting more basins on/off line
RAS rate          VFD on RAS pump
WAS rate          VFD on WAS pump
Air rate          providing more/less air in aeration basins




Any adjustment influences MLSS and SRT.
 Air Control
 Example: Longmont WWTP


Modulating Air valve                                DO Probe
                              Maintains setpoint




VFD on blower     Maintains setpoint
                                               Pressure Gauge
Sludge Testing


(1) Settlometer Test: rate of            Time = 0     Time = X       Time = 30 min
    settling over 30 min
Represents settling speed of
    sludge                                Sample
                                                    Settled solids
                                                                     Settled solids



(2) Sludge Volume Index (SVI) Test: Settled solids over 30 min/[TSS]
A low SVI # indicates a sludge that settles well                Time = 30 min
SVI # can be used to determine [TSS] and [VSS] in RAS line



                                                                       Settled solids
Other Process Testing


  For nitrification: pH, alkalinity, presence of nitrate and nitrite



Not usually performed tests:

  MLVSS: represent microorganism content
  SOUR: determine how fast organisms use up oxygen
Activated Sludge Startup-Feb 2003




               Young Sludge Foam
Activated Sludge has become a conventional
treatment process for wastewater




                            Thank you

								
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