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 NH4 and
NO3 as target contaminants. It should be used when the
treatment objective is convert Ammonia to Nitrates, convert Nitrates
to N2 and remove BOD at the same time. The algorithm
considers both AOB and NOB kinetics for Nitrification instead of the
"AOB only" approach commonly used before 2015. The denitrification
occurs in a pre-anoxic tank.
Quick calculation instructions
- Plant design inputs: Flow, temperature and altitude (impacts the
- Biological reactor design inputs:
Clarifier design inputs:
- 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 .
- Aerator height: How high is the aerator from the bottom of the
- Aeration tank volume: If a number is set 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).
- 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 :
- 2500 to 4000mg/L for SRT between 20 and 30 days
- Dissolved oxygen concentration typical values are between 1.5
- Safety factor for TKN: Accounts for the variation of the
Ammonia concentration in the wastewater. Recommended value:
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
- Effluent Nitrite concentration: The Nitrite (NO2)
concentration in the treated effluent is very low but in some
cases the nitrification is limited by NOB and this concentration
can be high even with low Ammonia in the product. If there are
no regulations for Nitrites it is advised to keep the same value
as set for Ammonia in the product.
- Effluent Nitrate (NO3) concentration will define
the size of the anoxic compartment.
- 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.
- To prevent the model from using the NOB route for
nitrification calculations, set the maximum specific growth for
NOB bacteria to 100.
- 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 design temperature .
- Calculation of the specific growth rate for AOB and NOB ;
- The program will pick the lower specific growth rate and use
to calculate the SRT.
- Final SRT is adjusted with the safety factor.
- Soluble bCOD calculation from the SRT and coefficients .
- Effluent soluble BOD calculation from bCOD.
- Biomass production is calculated. An optimization algorithm is
used to find the exact NOX production for the calculated SRT.
- Production of TSS and VSS is calculated .
- 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 for the aerobic treatment is calculated 
- Internal recirculation rate is calculated based in the total
NOx and the desired effluent Nitrates concentration 
- Nitrate concentration feeding the anoxic compartment is then
- An optimization model is used to adjust the hydraulic
retention time of the anoxic tank to the desired Nitrate removal
rate. The Standard Denitrification Rate (SDNR) is calculated
using the Food-to-Microorganism ration and the fraction of
readily biodegradable COD in the wastewater .
- If the user defined the tank volume, the SDNR will be
calculated and the difference between the input nitrates
concentration and the nitrates removed will be displayed to the
user. Ideally this number should equal zero or be negative.
- Oxygen credits from the denitrification subtracted from the
oxygen requirements in the aeration tank .
- 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 .
- Alkalinity requirements check .
- Mixing power to the anoxic tank is calculated. Please not that
this is not the electric power but rather the energy that must
be dissipated in the fluid to perform the mix.
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.
- This model calculates the rates for AOB and NOB bacteria and
then picks the slower (critical) reaction to calculate the design
SRT . When the design is based in the NOB kinetics, the final
Ammonia concentration will be lower than the desired value
specified in the inputs. When the design is limited by AOB, the
final Nitrite concentration will be lower than the desired value.
- Standard denitrification rates (SDNR) are valid for internal
recirculation ratios up to 4.
 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.
 EPA/600/R-10/100 EPA Nutrient Control Design
Manual, August 2010