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ASM2d - Activated Sludge Model No. 2d ​

Extended model for biological phosphorus removal

Overview ​

ASM2d extends ASM1 to include biological phosphorus removal by phosphorus accumulating organisms (PAOs). It models the anaerobic-aerobic cycling required for enhanced biological phosphorus removal (EBPR).

When to Use ​

  • A2O, UCT, Bardenpho processes
  • Biological phosphorus removal
  • Combined N and P removal
  • EBPR optimization
  • Systems without P removal (use ASM1)
  • Purely chemical P removal
  • Anaerobic digestion (use ADM1)

State Variables ​

ASM2d uses 19 state variables:

Soluble Components ​

SymbolDescriptionUnit
S_FFermentable substratemg COD/L
S_AAcetate/VFAmg COD/L
S_NH4Ammonia nitrogenmg N/L
S_NO3Nitrate nitrogenmg N/L
S_PO4Orthophosphatemg P/L
S_ALKAlkalinitymmol/L
S_O2Dissolved oxygenmg O2/L
S_ISoluble inertmg COD/L
S_N2Dinitrogenmg N/L

Particulate Components ​

SymbolDescriptionUnit
X_HHeterotrophic biomassmg COD/L
X_PAOPAO biomassmg COD/L
X_PHAStored PHA in PAOmg COD/L
X_PPPolyphosphate in PAOmg P/L
X_AUTAutotrophic biomassmg COD/L
X_SSlowly biodegradablemg COD/L
X_IParticulate inertmg COD/L
X_TSSTotal suspended solidsmg TSS/L

PAO Metabolism ​

Anaerobic Phase ​

PAOs take up VFA (acetate) and store as PHA:

  • Release orthophosphate (P release)
  • Consume stored polyphosphate (poly-P)
  • No oxygen or nitrate required

$$\text{VFA} + \text{Poly-P} \xrightarrow{\text{anaerobic}} \text{PHA} + \text{PO}_4^{3-}$$

Aerobic Phase ​

PAOs oxidize PHA for growth and P uptake:

  • Luxury uptake of phosphorus
  • PHA consumed for energy
  • Poly-P reformed

$$\text{PHA} + \text{O}_2 + \text{PO}_4^{3-} \xrightarrow{\text{aerobic}} \text{Biomass} + \text{Poly-P}$$

Anoxic Phase ​

Similar to aerobic but using nitrate:

  • Denitrifying PAOs (dPAOs)
  • Reduced efficiency factor

Key Parameters ​

PAO-Specific Parameters ​

ParameterSymbolValueUnit
Max PAO growth rateqPHA3.01/d
PHA half-saturationKPHA0.01g/g
Max P uptake rateqPP1.51/d
Max poly-P contentKMAX0.34g P/g COD
P/PHA ratioYPO40.4g P/g COD
PAO yieldYPAO0.625g/g
Anoxic factorηPAO0.6-

Temperature Effects ​

P removal is temperature sensitive:

  • Optimal: 20-25°C
  • Reduced at <15°C
  • PAOs outcompeted at >30°C

Process Configuration ​

A2O Process ​

         ┌─────────────┐
         │   ANAEROBIC │ ← VFA uptake, P release
         │     Zone    │
         └──────┬──────┘
                ↓
         ┌─────────────┐
  IRR →  │    ANOXIC   │ ← Denitrification
         │     Zone    │
         └──────┬──────┘
                ↓
         ┌─────────────┐
         │   AEROBIC   │ ← P uptake, nitrification
         │     Zone    │
         └──────┬──────┘

Typical Sizing ​

ZoneHRT% Volume
Anaerobic0.5-2 h10-15%
Anoxic1-3 h20-30%
Aerobic4-8 h55-70%

Configuration in ErgoPlanner ​

Enabling ASM2d ​

  1. Select biological reactor
  2. Set Calculation Mode to Advanced
  3. Select Biokinetic Model: ASM2d
  4. Ensure anaerobic zone present

Critical Configuration ​

For EBPR to work:

  • [ ] Anaerobic zone with no O2 or NO3
  • [ ] VFA/rbCOD available in influent
  • [ ] Adequate SRT (>5 days)
  • [ ] Temperature >12°C

Performance Factors ​

VFA Requirements ​

EBPR requires readily available VFA:

P RemovalRequired VFA:P
80%>15:1 mg VFA/mg P
90%>20:1 mg VFA/mg P
95%>25:1 mg VFA/mg P

Competing Processes ​

Factors that reduce EBPR:

  • Nitrate in anaerobic zone (GAO competition)
  • Low VFA availability
  • Excessive aeration
  • Temperature extremes
  • Long SRT (>25 days)

Example: A2O Process ​

Configuration ​

ParameterValue
Total Volume10,000 m³
Anaerobic1,500 m³
Anoxic2,500 m³
Aerobic6,000 m³
IRR200%
RAS100%

Influent ​

ParameterValue
Flow25,000 m³/d
COD450 mg/L
VFA40 mg/L
TKN45 mg/L
TP10 mg/L

Results ​

ParameterEffluent
COD40 mg/L
NH4-N0.8 mg/L
TN8 mg/L
TP0.8 mg/L

Troubleshooting EBPR ​

IssueCauseSolution
Poor P removalLow VFAAdd primary fermenter
P release at clarifierAnaerobic conditionsReduce RAS detention
Unstable P removalGAO competitionLower temperature, pH
Secondary P releaseLong sludge storageQuick dewatering

Limitations ​

  1. No N2O emissions - Not modeled
  2. Simplified glycogen - Not explicit state
  3. Single PAO - No GAO competition
  4. No side reactions - Simplified biochemistry
  • ASM1 - N removal only
  • ASM3 - Storage-based
  • ADM1 - Anaerobic systems

References ​

  1. Henze, M. et al. "Activated Sludge Models ASM1, ASM2, ASM2d and ASM3"
  2. Smolders, G.J.F. "A Metabolic Model for EBPR"
  3. Comeau, Y. "EBPR Biochemical Model"