Activated Sludge Model No. 2d

Introduction

The Activated Sludge Model No. 2d (ASM2d) represents a variation of the ASM1 model to account for biological phosphorus with nitrification-denitrification. The ASM2d property and reaction relationships of an activated sludge model for biological nutrient removal from wastewater using an activated sludge biological reactor with biological phosphorus removal as provided in Henze, M. et al. (1999).

Implementation

Figure 1 shows the process flow diagram for ASM2d, where influent wastewater is fed to a series of activated sludge reactors and a secondary clarifier (secondary treatment). CSTRs are used to model the two anaerobic tanks and the two anoxic reactors in the activated sludge process and CSTRs with injection (which accounts for aeration tanks) are used to model the three aerobic reactors. Finally, the effluent is passed through a clarifier. Note that a pressure changer is required in the recycle stream to ensure the pressure inside the recycle loop, is bounded. As the inlet Mixer uses a pressure minimization constraint and there is no pressure drop in the reactors, if pressure is not specified at some point within the recycle loop then it becomes unbounded. The flowsheet relies on the following key assumptions:

  • supports steady-state only

  • property and reaction packages are provided for the activated sludge model (ASM2d)

  • the clarifier is modeled as a separator with split fractions by components

../../_images/ASM2d.png

Figure 1. ASM2d flowsheet

Documentation for each of the unit models can be found below. All unit models were set up with their default configuration arguments.
Documentation for the property model can be found below.

Degrees of Freedom

The following variables are initially specified for simulating the ASM2d flowsheet (i.e., degrees of freedom = 0):
  • feed water conditions (flow, temperature, pressure, component concentrations, and alkalinity)

  • volume of activated sludge reactors

  • component injection rates for aerobic reactors

  • secondary clarifier split fraction(s)

  • pressure changer outlet pressure (feeds into the activated sludge process)

Flowsheet Specifications

Description

Value

Units

Feed Water\(^1\)

Volumetric flow

18446

\(\text{m}^3\text{/day}\)

Temperature

298.15

\(\text{K}\)

Pressure

1

\(\text{atm}\)

Dissolved oxygen (S_O2) concentration

1e-6

\(\text{g/}\text{m}^3\)

Fermentable, readily bio-degradable organic substrates (S_F) concentration

30

\(\text{g/}\text{m}^3\)

Fermentation products, considered to be acetate (S_A) concentration

20

\(\text{g/}\text{m}^3\)

Ammonium plus ammonia nitrogen (S_NH4) concentration

16

\(\text{g/}\text{m}^3\)

Nitrate plus nitrite nitrogen (S_NO3) concentration

1e-6

\(\text{g/}\text{m}^3\)

Inorganic soluble phosphorus (S_PO4) concentration

3.6

\(\text{g/}\text{m}^3\)

Inert soluble organic material (S_I) concentration

30

\(\text{g/}\text{m}^3\)

Dinitrogen (S_N2) concentration

1e-6

\(\text{g/}\text{m}^3\)

Inert particulate organic material (X_I) concentration

25

\(\text{g/}\text{m}^3\)

Slowly biodegradable substrates (X_S) concentration

125

\(\text{g/}\text{m}^3\)

Heterotrophic organisms (X_H) concentration

30

\(\text{g/}\text{m}^3\)

Phosphate-accumulating organisms (X_PAO) concentration

1e-6

\(\text{g/}\text{m}^3\)

Poly-phosphate (X_PP) concentration

1e-6

\(\text{g/}\text{m}^3\)

Poly-hydroxy-alkanoate (X_PHA) concentration

1e-6

\(\text{g/}\text{m}^3\)

Autotrophic nitrifying organisms (X_AUT) concentration

1e-6

\(\text{g/}\text{m}^3\)

Metal-hydroxides (X_MeOH) concentration

1e-6

\(\text{g/}\text{m}^3\)

Metal-phosphate (X_MeP) concentration

1e-6

\(\text{g/}\text{m}^3\)

Total suspended solids, TSS (X_TSS) concentration

180

\(\text{g/}\text{m}^3\)

Alkalinity (S_ALK)

7

\(\text{mol/}\text{m}^3\)

Activated Sludge Process

Reactor 1 volume

1000

\(\text{m}^3\)

Reactor 2 volume

1000

\(\text{m}^3\)

Reactor 3 volume

1000

\(\text{m}^3\)

Reactor 4 volume

1000

\(\text{m}^3\)

Reactor 5 volume

1333

\(\text{m}^3\)

Reactor 6 volume

1333

\(\text{m}^3\)

Reactor 7 volume

1333

\(\text{m}^3\)

Reactor 5 injection rate for component j

0

\(\text{g/}\text{s}\)

Reactor 6 injection rate for component j

0

\(\text{g/}\text{s}\)

Reactor 7 injection rate for component j

0

\(\text{g/}\text{s}\)

Reactor 5 outlet oxygen (S_O) concentration

0.00191

\(\text{g/}\text{m}^3\)

Reactor 6 outlet oxygen (S_O) concentration

0.00260

\(\text{g/}\text{m}^3\)

Reactor 7 outlet oxygen (S_O) concentration

0.00320

\(\text{g/}\text{m}^3\)

Reactor 7 underflow split fraction

0.6

\(\text{dimensionless}\)

Reactor 5 oxygen mass transfer coefficient

240

\(\text{hr}^{-1}\)

Reactor 6 oxygen mass transfer coefficient

240

\(\text{hr}^{-1}\)

Reactor 7 oxygen mass transfer coefficient

84

\(\text{hr}^{-1}\)

Secondary clarifier H2O split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_A split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_F split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_I split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_N2 split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_NH4 split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_NO3 split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_O2 split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_PO4 split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier S_ALK split fraction

0.48956

\(\text{dimensionless}\)

Secondary clarifier X_AUT split fraction

0.022117

\(\text{dimensionless}\)

Secondary clarifier X_H split fraction

0.021922

\(\text{dimensionless}\)

Secondary clarifier X_I split fraction

0.021715

\(\text{dimensionless}\)

Secondary clarifier X_MeOH split fraction

0.022

\(\text{dimensionless}\)

Secondary clarifier X_MeP split fraction

0.022

\(\text{dimensionless}\)

Secondary clarifier X_PAO split fraction

0.022

\(\text{dimensionless}\)

Secondary clarifier X_PHA split fraction

0.02147

\(\text{dimensionless}\)

Secondary clarifier X_PP split fraction

0.02144

\(\text{dimensionless}\)

Secondary clarifier X_S split fraction

0.02221

\(\text{dimensionless}\)

Secondary clarifier X_TSS split fraction

0.02194

\(\text{dimensionless}\)

Separator recycle split fraction

0.97955

\(\text{dimensionless}\)

Recycle pump outlet pressure

101325

\(\text{Pa}\)

Future Refinements

The following modifications to ASM2d are planned for development:
  • Improving costing relationships in terms of detail, completeness, and reasonable validity

  • Accounting for temperature-dependence in the oxygen mass transfer coefficient (KLa) and oxygen concentration at saturation

  • Replacing the ideal-separator formulation in the secondary clarifier with the widely used double-exponential settling model (i.e., the Takacs model)

NOTE: The repository also contains flowsheets for ASM2d modified for phosphorous removal, for property and reaction reference: modified ASM2d

References

[1] M. Henze, W. Gujer, T. Mino, T. Matsuo, M.C. Wentzel, G. v. R. Marais, M.C.M. Van Loosdrecht, Activated sludge model No.2D, ASM2D, Water Science and Technology. 39 (1999) 165–182. https://doi.org/10.1016/S0273-1223(98)00829-4.