Sumo2N - Two-Step Nitrification Model

Extended biokinetic model with separate AOB and NOB populations for two-step nitrification

Overview

Sumo2N is an extension of ASM-type models that explicitly separates nitrification into two distinct biological steps: nitritification (ammonia to nitrite by AOB) and nitratification (nitrite to nitrate by NOB). This enables modeling of nitrite accumulation, N2O emissions, and advanced nitrogen removal strategies.

When to Use

Recommended For

• Shortcut nitrogen removal (nitritation-denitritation)
• Anammox integrated systems
• N2O emission modeling
• Sidestream treatment (high ammonia)
• Process optimization where nitrite control matters
• Temperature-sensitive nitrification studies

Not Recommended For

• Conventional nitrogen removal (use ASM1)
• Phosphorus-focused studies (use ASM2d)
• Anaerobic systems (use ADM1)
• Simple design calculations

State Variables

Sumo2N extends ASM1/ASM2d with explicit nitrite state. It uses 14 primary state variables:

Soluble Components

Index	Symbol	Description	Unit
0	S_I	Soluble inert organics	mg COD/L
1	S_S	Readily biodegradable substrate	mg COD/L
2	S_O	Dissolved oxygen	mg O2/L
3	S_NH	Ammonia nitrogen	mg N/L
4	S_NO2	Nitrite nitrogen	mg N/L
5	S_NO3	Nitrate nitrogen	mg N/L
6	S_N2O	Nitrous oxide	mg N/L
7	S_N2	Dinitrogen gas	mg N/L
8	S_ND	Soluble organic nitrogen	mg N/L
9	S_ALK	Alkalinity	mol HCO3/m3

Particulate Components

Index	Symbol	Description	Unit
10	X_I	Particulate inert organics	mg COD/L
11	X_S	Slowly biodegradable substrate	mg COD/L
12	X_BH	Active heterotrophic biomass	mg COD/L
13	X_AOB	Ammonia-oxidizing bacteria	mg COD/L
14	X_NOB	Nitrite-oxidizing bacteria	mg COD/L
15	X_P	Particulate products from decay	mg COD/L
16	X_ND	Particulate organic nitrogen	mg N/L

Processes

Sumo2N includes 12 biological processes:

Process Summary

#	Process	Description
1	Aerobic heterotrophic growth	COD oxidation to biomass
2	Anoxic growth on NO3	Denitrification NO3 → N2
3	Anoxic growth on NO2	Denitrification NO2 → N2O/N2
4	Aerobic AOB growth	Nitritification NH4 → NO2
5	Aerobic NOB growth	Nitratification NO2 → NO3
6	Decay of heterotrophs	Death-regeneration
7	Decay of AOB	Death-regeneration
8	Decay of NOB	Death-regeneration
9	Ammonification	Organic N → NH4
10	Hydrolysis of X_S	Particulate → soluble COD
11	Hydrolysis of X_ND	Particulate → soluble org-N
12	N2O production pathway	NO2 reduction intermediates

Two-Step Nitrification Theory

Step 1: Nitritification (AOB)

$$NH_4^+ + 1.5O_2 \xrightarrow{AOB} NO_2^- + 2H^+ + H_2O$$

Rate equation:

$$\rho_{AOB} = \mu_{AOB} \cdot \frac{S_{NH}}{K_{NH,AOB} + S_{NH}} \cdot \frac{S_O}{K_{O,AOB} + S_O} \cdot X_{AOB}$$

Oxygen consumption: 3.43 mg O2/mg NH4-N oxidized

Step 2: Nitratification (NOB)

$$NO_2^- + 0.5O_2 \xrightarrow{NOB} NO_3^-$$

Rate equation:

$$\rho_{NOB} = \mu_{NOB} \cdot \frac{S_{NO2}}{K_{NO2,NOB} + S_{NO2}} \cdot \frac{S_O}{K_{O,NOB} + S_O} \cdot X_{NOB}$$

Oxygen consumption: 1.14 mg O2/mg NO2-N oxidized

Overall Nitrification

$$NH_4^+ + 2O_2 → NO_3^- + 2H^+ + H_2O$$

Total oxygen: 4.57 mg O2/mg NH4-N (same as ASM1)

Kinetic Equations

Process 4: AOB Growth (Nitritification)

$$\rho_4 = \mu_{AOB} \cdot \frac{S_{NH}}{K_{NH,AOB} + S_{NH}} \cdot \frac{S_O}{K_{O,AOB} + S_O} \cdot \frac{S_{ALK}}{K_{ALK} + S_{ALK}} \cdot X_{AOB}$$

Process 5: NOB Growth (Nitratification)

$$\rho_5 = \mu_{NOB} \cdot \frac{S_{NO2}}{K_{NO2,NOB} + S_{NO2}} \cdot \frac{S_O}{K_{O,NOB} + S_O} \cdot \frac{S_{ALK}}{K_{ALK} + S_{ALK}} \cdot X_{NOB}$$

Process 3: Anoxic Growth on Nitrite

$$\rho_3 = \mu_H \cdot \eta_{g,NO2} \cdot \frac{S_S}{K_S + S_S} \cdot \frac{K_{O,H}}{K_{O,H} + S_O} \cdot \frac{S_{NO2}}{K_{NO2} + S_{NO2}} \cdot X_{BH}$$

Free Ammonia Inhibition (AOB)

$$I_{FA} = \frac{K_{I,FA}}{K_{I,FA} + S_{FA}}$$

Where free ammonia:

$$S_{FA} = \frac{S_{NH} \cdot 10^{pH}}{K_b/K_w + 10^{pH}}$$

Free Nitrous Acid Inhibition (NOB)

$$I_{FNA} = \frac{K_{I,FNA}}{K_{I,FNA} + S_{FNA}}$$

Where free nitrous acid:

$$S_{FNA} = \frac{S_{NO2}}{K_a \cdot 10^{pH} + 1}$$

Parameters

AOB Kinetic Parameters (20C)

Parameter	Symbol	Value	Unit	Description
Max growth rate	muAOB	0.9	1/d	Maximum specific growth rate
Decay rate	bAOB	0.17	1/d	Endogenous decay
NH4 half-saturation	K_NH,AOB	0.7	mg N/L	Ammonia affinity
O2 half-saturation	K_O,AOB	0.6	mg O2/L	Oxygen affinity
Yield	Y_AOB	0.15	g COD/g N	Biomass yield

NOB Kinetic Parameters (20C)

Parameter	Symbol	Value	Unit	Description
Max growth rate	muNOB	0.7	1/d	Maximum specific growth rate
Decay rate	bNOB	0.17	1/d	Endogenous decay
NO2 half-saturation	K_NO2,NOB	0.5	mg N/L	Nitrite affinity
O2 half-saturation	K_O,NOB	1.2	mg O2/L	Higher than AOB
Yield	Y_NOB	0.06	g COD/g N	Biomass yield

Inhibition Parameters

Parameter	Symbol	Value	Unit
Free ammonia inhibition	K_I,FA	10	mg N/L
Free nitrous acid inhibition	K_I,FNA	0.1	mg N/L

Temperature Correction

$$k(T) = k(20) \cdot \theta^{(T-20)}$$

Parameter	Organism	theta (θ)
muAOB	AOB	1.098
muNOB	NOB	1.063
bAOB	AOB	1.029
bNOB	NOB	1.029

Key insight: AOB has higher temperature coefficient than NOB, enabling nitrite accumulation at elevated temperatures (>28C).

Stoichiometric Parameters

Parameter	Symbol	Value	Unit
AOB yield	Y_AOB	0.15	g COD/g N
NOB yield	Y_NOB	0.06	g COD/g N
N content of biomass	i_XB	0.086	g N/g COD
O2 for NH4→NO2	-	3.43	g O2/g N
O2 for NO2→NO3	-	1.14	g O2/g N

Competitive Advantages

NOB vs AOB Competition

Factors favoring AOB (nitrite accumulation):

Factor	Mechanism
High temperature (>28C)	AOB theta > NOB theta
Low DO (<1 mg/L)	K_O,NOB > K_O,AOB
High FA (>1 mg/L)	FA inhibits NOB more
Low SRT	AOB grows faster
Intermittent aeration	NOB more sensitive

Minimum SRT

$$SRT_{min,AOB} = \frac{1}{\mu_{AOB} - b_{AOB}}$$

$$SRT_{min,NOB} = \frac{1}{\mu_{NOB} - b_{NOB}}$$

At 15C:

• SRT_min,AOB ≈ 3 days
• SRT_min,NOB ≈ 4 days

N2O Emission Pathways

AOB-mediated Denitrification

Under low DO, AOB can produce N2O:

$$2NO_2^- + 2H^+ + 2e^- \xrightarrow{AOB} N_2O + H_2O$$

Rate:

$$r_{N2O,AOB} = k_{N2O,AOB} \cdot \frac{K_{O,AOB}}{K_{O,AOB} + S_O} \cdot \frac{S_{NO2}}{K_{NO2,N2O} + S_{NO2}} \cdot X_{AOB}$$

Heterotrophic Incomplete Denitrification

$$NO_2^- → NO → N_2O → N_2$$

At low COD:NO2 ratio, N2O accumulates.

Emission Factor

Typical: 0.1-3% of N load emitted as N2O

Configuration in ErgoPlanner

Enabling Sumo2N

1. Select biological reactor (CSTR, PFR, or SBR)
2. Set Calculation Mode to Advanced
3. Select Biokinetic Model: Sumo2N
4. Enable N2O tracking if needed
5. Configure AOB/NOB parameters

Parameter Input

{
  "model": "sumo2n",
  "parameters": {
    "aob": {
      "muAOB": 0.9,
      "bAOB": 0.17,
      "KNH_AOB": 0.7,
      "KO_AOB": 0.6,
      "YAOB": 0.15
    },
    "nob": {
      "muNOB": 0.7,
      "bNOB": 0.17,
      "KNO2_NOB": 0.5,
      "KO_NOB": 1.2,
      "YNOB": 0.06
    },
    "inhibition": {
      "KI_FA": 10,
      "KI_FNA": 0.1
    },
    "n2o": {
      "enabled": true,
      "kN2O_AOB": 0.01,
      "kN2O_het": 0.05
    }
  }
}

Applications

1. Shortcut Nitrogen Removal (Nitritation-Denitritation)

NH4 → NO2 (partial nitrification)
       ↓
      N2 (denitritation)

Benefits:

• 25% less oxygen
• 40% less carbon
• Faster kinetics

Control strategy:

• DO < 1 mg/L
• Temperature > 30C
• SRT between SRT_min,AOB and SRT_min,NOB

2. Partial Nitritation-Anammox

NH4 → NO2 (partial, ~55%)
       ↓
NH4 + NO2 → N2 (anammox)

Requires:

• Stable nitrite production
• NOB washout
• Anammox bacteria retention

3. N2O Mitigation

Identify N2O hotspots:

• Low DO zones
• High nitrite zones
• Carbon-limited denitrification

Mitigation:

• Avoid DO < 0.5 mg/L
• Sufficient COD for complete denitrification
• Continuous (not intermittent) aeration

Example: Sidestream Nitritation

Configuration

Parameter	Value
Volume	500 m3
Temperature	35C
DO setpoint	0.8 mg/L
SRT	5 days
pH	7.5

Influent (Reject Water)

Parameter	Value
Flow	200 m3/d
NH4-N	800 mg/L
COD	200 mg/L

Expected Results

Parameter	Value
NH4-N conversion	55%
NO2-N accumulation	400 mg/L
NO3-N	<50 mg/L
Nitrite accumulation ratio	>90%

Calibration Guidelines

Key Parameters to Calibrate

1. muAOB - From batch nitrification tests
2. muNOB - From batch nitratification tests
3. K_O,AOB / K_O,NOB - Critical for DO control
4. K_I,FA - From inhibition tests
5. theta values - From temperature studies

Calibration Procedure

1. Determine AOB and NOB populations (qPCR, FISH)
2. Batch tests at controlled DO for kinetics
3. Temperature gradient experiments
4. Free ammonia/FNA inhibition curves
5. Dynamic validation with step changes

Troubleshooting

Issue	Cause	Solution
No nitrite accumulation	NOB active	Lower DO, raise T
Complete nitrification	SRT too high	Reduce SRT
Nitrite accumulation too high	NOB washout	Increase DO
High N2O emissions	Low DO operation	Increase DO setpoint
Unstable nitritation	Temperature fluctuation	Stabilize temperature
AOB washout	SRT too low	Increase SRT

Comparison: ASM1 vs Sumo2N

Aspect	ASM1	Sumo2N
Nitrifier populations	1 (X_BA)	2 (X_AOB, X_NOB)
N intermediates	None	NO2, N2O
Parameters	~19	~30
Complexity	Lower	Higher
Nitrite modeling	No	Yes
N2O emissions	No	Yes
Temperature sensitivity	Simple	Differential

Limitations

1. No phosphorus removal - Combine with ASM2d for BNR
2. Simplified N2O - Full pathway models more complex
3. No comammox - Single-step NH4→NO3 organisms
4. Uniform populations - No spatial stratification
5. More parameters - Harder to calibrate

Related Models

• ASM1 - Single-step nitrification
• ASM2d - With biological P removal
• ASM3 - Storage-based kinetics
• ADM1 - Anaerobic digestion

References

1. Hiatt, W.C. & Grady, C.P.L. (2008). "An Updated Process Model for Carbon Oxidation, Nitrification, and Denitrification." Water Environment Research.
2. Hellinga, C. et al. (1998). "The SHARON process." Water Science & Technology.
3. Picioreanu, C. et al. (2016). "Model-based evaluation of N2O emissions." Water Research.
4. Henze, M. et al. (2000). "Activated Sludge Models ASM1, ASM2, ASM2d and ASM3." IWA Publishing.
5. Wett, B. (2007). "Development of a process for DEAMMONIFICATION." Water Science & Technology.