ASM3 - Activated Sludge Model No. 3

Storage-based metabolism model

Overview

ASM3 represents a conceptual improvement over ASM1 by explicitly modeling internal storage of substrates. It provides a more accurate representation of actual biomass metabolism, particularly for systems with varying loads.

When to Use

Recommended For

• High-rate activated sludge
• Systems with variable loading
• SBR processes
• Research applications

Compared to ASM1

Aspect	ASM1	ASM3
Storage	Implicit	Explicit
Decay concept	Death-regeneration	Endogenous respiration
Complexity	Lower	Higher
Dynamic accuracy	Good	Better

State Variables

ASM3 uses 12 state variables:

Soluble Components

Symbol	Description	Unit
S_O2	Dissolved oxygen	mg O2/L
S_S	Readily biodegradable	mg COD/L
S_NH4	Ammonia nitrogen	mg N/L
S_N2	Dinitrogen	mg N/L
S_NOX	Nitrate + nitrite	mg N/L
S_ALK	Alkalinity	mmol/L
S_I	Soluble inert	mg COD/L

Particulate Components

Symbol	Description	Unit
X_S	Slowly biodegradable	mg COD/L
X_I	Particulate inert	mg COD/L
X_H	Heterotrophic biomass	mg COD/L
X_STO	Stored substrate	mg COD/L
X_A	Autotrophic biomass	mg COD/L

Key Concepts

Storage Mechanism

Two-step substrate utilization:

Step 1: Storage
S_S → X_STO (fast, high rate)

Step 2: Growth
X_STO → X_H (slower, growth limited)

Advantages

1. Better dynamic response - Captures feast-famine behavior
2. Realistic decay - Direct endogenous respiration
3. Improved oxygen uptake - Two distinct phases

Storage Rate

$$r_{STO} = k_{STO} \cdot \frac{S_S}{K_S + S_S} \cdot \frac{S_{O2}}{K_{O2} + S_{O2}} \cdot X_H$$

Growth from Storage

$$r_{growth} = \mu_H \cdot \frac{X_{STO}/X_H}{K_{STO} + X_{STO}/X_H} \cdot \frac{S_{O2}}{K_{O2} + S_{O2}} \cdot X_H$$

Parameters

Storage Parameters

Parameter	Symbol	Value	Unit
Storage rate	kSTO	5.0	1/d
Storage yield	YSTO	0.85	g/g
Max growth rate	μH	2.0	1/d
Storage half-sat	KSTO	1.0	g/g

Comparison with ASM1

Parameter	ASM1	ASM3
μmax,H	6.0	2.0
YH	0.67	0.63
bH	0.62	0.20

Note: Lower μmax in ASM3 because growth is from storage.

Oxygen Uptake

OUR Components

1. Storage OUR - Substrate → Storage
2. Growth OUR - Storage → Biomass
3. Endogenous OUR - Biomass decay

$$OUR_{total} = OUR_{storage} + OUR_{growth} + OUR_{decay}$$

Dynamic OUR Profile

Substrate added
     ↓
OUR ─────┐
         │     High OUR (storage phase)
         │───────────────┐
         │               │  Lower OUR (growth phase)
         │               └──────────────────
         │                   Endogenous OUR
Time ────┴─────────────────────────────────→

Configuration in ErgoPlanner

Enabling ASM3

1. Select biological reactor
2. Set Calculation Mode to Advanced
3. Select Biokinetic Model: ASM3
4. Configure storage parameters

When to Choose ASM3

• Variable loading conditions
• SBR with distinct phases
• High-rate systems
• Research/calibration needs

Processes

Heterotrophic Processes (7)

1. Aerobic storage of S_S
2. Anoxic storage of S_S
3. Aerobic growth on X_STO
4. Anoxic growth on X_STO
5. Aerobic endogenous respiration
6. Anoxic endogenous respiration
7. Aerobic respiration of X_STO

Autotrophic Processes (3)

8. Aerobic growth of nitrifiers
9. Aerobic endogenous respiration
10. Anoxic endogenous respiration

Hydrolysis (2)

11. Aerobic hydrolysis
12. Anoxic hydrolysis

Example: SBR Process

Cycle Phases

Phase	Duration	Process
Fill	1 h	Substrate addition
React	4 h	Aeration, storage, growth
Settle	1 h	Clarification
Decant	0.5 h	Effluent withdrawal
Idle	0.5 h	Sludge waste

ASM3 Behavior

Fill Phase:

• Rapid substrate storage
• High OUR

React Phase:

• Growth from stored substrate
• Moderate OUR
• Nitrification

Settle/Decant:

• Endogenous respiration only
• Low OUR

Calibration Tips

Key Parameters

1. kSTO - Storage rate (from batch tests)
2. YSTO - Storage yield
3. μH - Growth rate from storage
4. KSTO - Storage half-saturation

Calibration Procedure

1. Batch tests for storage kinetics
2. Respirometry for OUR components
3. Continuous data for growth parameters
4. Dynamic validation

Limitations

1. No biological P removal - Add PAO model
2. Single storage compound - Simplified
3. More parameters - Harder to calibrate
4. Similar steady-state - Differences mainly dynamic

Comparison: ASM1 vs ASM3

Criterion	ASM1	ASM3
Simplicity	★★★★	★★★
Dynamic accuracy	★★★	★★★★
Data availability	★★★★★	★★★
Research use	★★★★	★★★★
Design practice	★★★★★	★★★

Related Models

• ASM1 - Simpler, widely used
• ASM2d - With P removal
• ADM1 - Anaerobic systems

References

1. Gujer, W. et al. "ASM3" Water Science & Technology, 1999
2. Koch, G. et al. "Storage in ASM3" Water Research, 2000
3. Henze, M. "Activated Sludge Models" IWA Publishing