Activated Sludge Model No. 1

Introduction

Activated Sludge Model No. 1 (ASM1) was created by the International Water Association (IWA) to create a mathematical model to predict the output of activated sludge models. The model aims to incorporate carbon oxidation, nitrification, and denitrification. The model quantifies the kinetics and stoichiometry of each process. The ASM1 property and reaction relationships are based on the relationships for the activated sludge biological reactor provided in Henze, M. et al. (1987).

Implementation

Figure 1 shows the process flow diagram for ASM1, 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 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 (ASM)

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

Figure 1. ASM1 flowsheet

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

• CSTR

• Aeration tank

• Secondary clarifier

Documentation for the property model can be found below.

• ASM1

Degrees of Freedom

The following variables are initially specified for simulating the ASM1 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	\(\text{atm}\)
Soluble inert organic matter (S_I) concentration	30	\(\text{g/}\text{m}^3\)
Readily biodegradable substrate (S_S) concentration	69.5	\(\text{g/}\text{m}^3\)
Particulate inert organic matter (X_I) concentration	51.2	\(\text{g/}\text{m}^3\)
Slowly biodegradable substrate (X_S) concentration	202.32	\(\text{g/}\text{m}^3\)
Active heterotrophic biomass (X_B,H) concentration	28.17	\(\text{g/}\text{m}^3\)
Active autotrophic biomass (X_B,A) concentration	0	\(\text{g/}\text{m}^3\)
Particulate products arising from biomass decay (X_P) concentration	0	\(\text{g/}\text{m}^3\)
Oxygen (S_O) concentration	0	\(\text{g/}\text{m}^3\)
Nitrate and nitrite nitrogen (S_NO) concentration	0	\(\text{g/}\text{m}^3\)
NH4 \(^{+}\) + NH \(_{3}\) Nitrogen (S_NH) concentration	31.56	\(\text{g/}\text{m}^3\)
Soluble biodegradable organic nitrogen (S_ND) concentration	6.95	\(\text{g/}\text{m}^3\)
Particulate biodegradable organic nitrogen (X_ND) concentration	10.59	\(\text{g/}\text{m}^3\)
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	1333	\(\text{m}^3\)
Reactor 4 volume	1333	\(\text{m}^3\)
Reactor 5 volume	1333	\(\text{m}^3\)
Reactor 3 injection rate for component j	0	\(\text{g/}\text{s}\)
Reactor 4 injection rate for component j	0	\(\text{g/}\text{s}\)
Reactor 5 injection rate for component j	0	\(\text{g/}\text{s}\)
Reactor 3 outlet oxygen (S_O) concentration	0.00172	\(\text{g/}\text{m}^3\)
Reactor 4 outlet oxygen (S_O) concentration	0.00243	\(\text{g/}\text{m}^3\)
Reactor 5 outlet oxygen (S_O) concentration	0.00449	\(\text{g/}\text{m}^3\)
Reactor 5 underflow split fraction	0.6	\(\text{dimensionless}\)
Reactor 3 oxygen mass transfer coefficient	7.6	\(\text{hr}^{-1}\)
Reactor 4 oxygen mass transfer coefficient	5.7	\(\text{hr}^{-1}\)
Secondary clarifier H2O split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier S_I split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier S_S split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier X_I split fraction	0.00187	\(\text{dimensionless}\)
Secondary clarifier X_S split fraction	0.00187	\(\text{dimensionless}\)
Secondary clarifier X_BH split fraction	0.00187	\(\text{dimensionless}\)
Secondary clarifier X_BA split fraction	0.00187	\(\text{dimensionless}\)
Secondary clarifier X_P split fraction	0.00187	\(\text{dimensionless}\)
Secondary clarifier S_O split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier S_NO split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier S_NH split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier S_ND split fraction	0.48956	\(\text{dimensionless}\)
Secondary clarifier X_ND split fraction	0.00187	\(\text{dimensionless}\)
Secondary clarifier S_ALK split fraction	0.48956	\(\text{dimensionless}\)
Separator recycle split fraction	0.97955	\(\text{dimensionless}\)
Recycle pump outlet pressure	101325	\(\text{Pa}\)

Future Refinements

The following modifications to ASM1 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)

References

[1] Henze, M., Grady, C.P.L., Gujer, W., Marais, G.v.R., Matsuo, T., “Activated Sludge Model No. 1”, 1987, IAWPRC Task Group on Mathematical Modeling for Design and Operation of Biological Wastewater Treatment. https://belinra.inrae.fr/doc_num.php?explnum_id=4467

[2] Alex, J. et al. Benchmark Simulation Model no.1 (BSM1). Lund University, 2008, 5-6. https://www.iea.lth.se/publications/Reports/LTH-IEA-7229.pdf