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Activated Sludge Design (Complete Mix Reactor). Activated Sludge Principles . Wastewater is aerated in a tank Bacteria are encouraged to grow by providing Oxygen Food (BOD) Nutrients Correct temperature Time As bacteria consume BOD, they grow and multiply

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Presentation Transcript
activated sludge principles
Activated Sludge Principles
  • Wastewater is aerated in a tank
  • Bacteria are encouraged to grow by providing
      • Oxygen
      • Food (BOD)
      • Nutrients
      • Correct temperature
      • Time
  • As bacteria consume BOD, they grow and multiply
  • Treated wastewater flows into secondary clarifier
  • Bacterial cells settle, removed from clarifier as sludge
  • Part of sludge is recycled back to activated sludge tank, to maintain bacteria population
  • Remainder of sludge is wasted
kinetics of microbial growth
Kinetics of Microbial Growth
  • Biochemical reaction
  • Biomass concentration.
  • The concentration of biomass, X (mg/L), increases as a function of time due to conversion of food to biomass:

Where m is the specific growth rate constant (d-1). This represents the mass of cells produced/mass of cells per unit of time.

monod kinetics
Monod Kinetics
  • Growth rate
      • Growth rate constant, m , is a function of the substrate concentration, S.
      • Two constants are used to describe the growth rate
        • mm (mg/L) is the maximum growth rate constant (the rate at which the susbtrate concentration is not limiting)
        • Ks is the half-saturation constant (mg/L) (i.e., concentration of S when m = mm/2
slide7
Biomass production
  • Where kd represents the endogenous decay rate (d-1) (i.e., microorganism death rate).
        • Substituting the growth rate constant:
slide8
Substrate utilization

Where Y is the yield factor (mg of biomass produced/mg of food consumed)

  • Y range:
        • Aerobic: 0.4 - 0.8 mg/mg
slide9
Food to microorganism ratio (F/M)
      • Represents the daily mass of food supplied to the microbial biomass, X, in the mixed liquor suspended solids, MLSS
      • Units are Kg BOD5/Kg MLSS/day
activated sludge design equations
Activated Sludge Design Equations

Mass balance of biomass production

  • Influent biomass + biomass production = effluent biomass + sludge wasted
  • Substitute biomass production equation
  • Assume that influent and effluent biomass concentrations are negligible and solve
mass balance of food substrate
Mass balance of food substrate
  • Influent substrate + substrate consumed = effluent susbtrate + sludge wasted substrate
  • Substitute substrate removal equation
  • Assume that no biochemical action takes place in clarifier. Therefore the substrate concentration in the aeration basin is equal to the substrate concentrations in the effluent and the waste activated sludge. Solve:
overall equations
Overall equations
    • Combine the mass balance equations for food and biomass:
  • The cell residence time is:
  • and the hydraulic retention time is,q = V/Qo
          • Substitute and rearrange:
  • Compute the F/M ratio