Design tool for wastewater treatment biological
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.
Quick calculation instructions
- Plant design inputs: Flow, temperature and altitude (impacts the
- Biological reactor design inputs:
Clarifier 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
- 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 .
- 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
- 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 :
- 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
Wastewater quality inputs:
- Use the default values unless you have other guidelines.
- Typical values for the MLSS in the return line :
Biochemical constants (advanced)
- 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
- 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.
Aeration 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.
- Use this section do adjust the aeration devices efficiency and
- Default aerator: Fine bubble membrane.
Calculation model description
- bCOD, nbCOD, nbsCODe, nbVSS and iTSS parameters are
calculated according to the wastewater inputs.
- Endogenous decay coefficient and maximum specific growth rates
correction for the temperature .
- Soluble bCOD calculation from the SRT and coefficients .
- Effluent soluble BOD calculation from bCOD.
- Biomass production calculation from the SRT, and endogenous
decay coefficients .
- Production of TSS and VSS .
- 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
- HRT, MLVSS, FM, BODload and yields are determined from mass
- Oxygen consumption is calculated 
- Alpha coefficient for the aerator is calculated from the MLSS
in the tank .
- Atmospheric pressure  and oxygen saturation [3,4]
- Standard Oxygen Transfer Rate determination .
- Air flow calculation from the air density.
- Activated sludge return rate and waste flow by mass balance
- Clarifier area determination
- Final BOD from effluent suspended solids and soluble BOD .
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.
 Metcalf & Eddy, AECOM - Wastewater Enginering: Treatment and
Resource Recovery, 5th Edition, McGraw-Hill 2014
 Marcos Von Sperling, Lodos Ativados, 2ed,
Departamento de Engenharia Sanitária e Ambiental - UFMG, Belo
Horizonte - MG - Brasil 2002
 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
 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
 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.