Complete Mix Activated Sludge for BOD removal Handbook

Design tool for wastewater treatment biological reactors


Activated sludge design involves performing mass balances on key constituents and the application of fundamental kinetic relationships, the mass balance can be determined dynamically (over time) or based in the equilibrium (steady-state). This tool uses the steady-state design approach and should provide good enough designs for domestic or municipal wastewater. Industrial effluents can be treated using the same algorithms but the kinetic coefficients will require manual adjustments as well as careful biodegradability and toxicity analysis of the feed stream.

This model sizes the biological reactor using BOD/COD as target contaminants. It should be used when there are no requirements for nitrites, nitrates or ammonia in the effluent.

Design tool

Web Based Excel Interface



Quick calculation instructions

  1. Plant design inputs: Flow, temperature and altitude (impacts the oxygenation rate).
  2. Biological reactor design inputs:
    • Solids Retention Time (SRT) will determinate the final product quality. For BOD removal only SRT typical values are between 4 and 10 days, for extended aeration the SRT varies from 18 to 30 days [2].
    • Tank depth: Higher will improve the oxygen transfer but is limited by construction costs. Typical depths are between 4 and 5.5m for diffused aerators [2].
    • Aeration tank volume: If a number is set then the value will be used as the tank volume and the MLSS will be adjusted according to the SRT. If this is set to false the volume will be determined by the algorithm (recommended).
    • Aerator height: How high is the aerator from the bottom of the tank.
    • Mixed Liquor Suspended Solids (MLSS): This is the most important design parameter for the reactor and together with the SRT, define the reactor volume. Typical values [2]:
      • 1500 to 3500mg/L for SRT lower than 10 days
      • 2500 to 4000mg/L for SRT between 20 and 30 days
    • Dissolved oxygen concentration typical values are between 1.5 and 2mg/L.
  3. Clarifier design inputs:
    • Use the default values unless you have other guidelines.
    • Typical values for the MLSS in the return line [2]:
      • 8000 to 12000mg/L
  4. Wastewater quality inputs:
    • All parameters from this list are the minimum required for sizing the plant.
    • TDS impacts the aeration efficiency.
    • Minimum recommended nutrient concentrations BOD:N:P (mg/L) for biodegradability [2]:
      • 100:5:1 for SRT lower than 10 days
      • 100:3:0.5 for SRT between 20 and 30 days
    • You can use the default values for the "Expected Suspended solids in the product" and "bCOD to BOD ratios" unless you have more accurate values.
  5. Biochemical constants (advanced)
    • Use this section to adjust the biochemical/kinetic constants.
    • Actual values are valid for domestic and municipal wastewater.
    • Coefficients are based in bCOD instead of BOD for maximum compatibility with dynamic computation models. Be aware that several constants reported in the literature are BOD based and need to be converted before use.
  6. Aeration constants (advanced)
    • Use this section do adjust the aeration devices efficiency and parameters.
    • Default aerator: Fine bubble membrane.

Calculation model description

    1. bCOD, nbCOD, nbsCODe, nbVSS and iTSS parameters are calculated according to the wastewater inputs.
    2. Endogenous decay coefficient and maximum specific growth rates correction for the temperature [1].
    3. Soluble bCOD calculation from the SRT and coefficients [1].
    4. Effluent soluble BOD calculation from bCOD.
    5. Biomass production calculation from the SRT, and endogenous decay coefficients [1].
    6. Production of TSS and VSS [1].
    7. Volume of the reactor is calculated based on the user specified MLSS. If the user defined the tank volume then the MLSS will be adjusted to accommodate the biomass into the specified volume.
    8. HRT, MLVSS, FM, BODload and yields are determined from mass balance relations.
    9. Oxygen consumption is calculated [1]
    10. Alpha coefficient for the aerator is calculated from the MLSS in the tank [5].
    11. Atmospheric pressure [1] and oxygen saturation [3,4] determination
    12. Standard Oxygen Transfer Rate determination [1].
    13. Air flow calculation from the air density[1].
    14. Activated sludge return rate and waste flow by mass balance relations.
    15. Clarifier area determination
    16. Final BOD from effluent suspended solids and soluble BOD [1].

Known limitations and important notes

  • This model does not estimate suspended solids removal in the primary clarifier. It assumes the wastewater inputs already consider the primary removal.
  • Biochemical and aeration constant inputs are assumed at 20°C and then corrected to the process temperature.
  • TDS effects in the biomass are not considered. TDS inputs are used only for oxygen transfer efficiency calculations.

Literature references

[1] Metcalf & Eddy, AECOM - Wastewater Enginering: Treatment and Resource Recovery, 5th Edition, McGraw-Hill 2014
[2] Marcos Von Sperling, Lodos Ativados, 2ed, Departamento de Engenharia Sanitária e Ambiental - UFMG, Belo Horizonte - MG - Brasil 2002
[3] Benson, B.B., and Daniel Krause, Jr, 1980, The concentration and isotopic fractionation of gases dissolved in freshwater in equilibrium with the atmosphere. 1. Oxygen: Limnology and Oceanography, vol. 25, no. 4
[4] Benson, B.B., and Daniel Krause, Jr, 1984, The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere: Limnology and Oceanography, vol. 29, no. 3
[5] Racault. Y.A.-E. Stricker. A. Husson, and S.Gillot (2010) "Effect of Mixed Liquor Suspended Solids on the Oxygen Transfer Rate in Full-Scale Membrane Biorreactors,"Proceedings of the WEF 83rd ACE", New Orleans, L A.