Nitrogen Balance

Tracking nitrogen species through treatment

Overview

Nitrogen balance tracks all nitrogen species as they transform through wastewater treatment. Understanding N transformations is essential for designing nitrogen removal systems.

Nitrogen Species

Forms of Nitrogen

Species	Symbol	Description
Ammonia	NH4-N	Reduced, soluble
Nitrite	NO2-N	Oxidized, intermediate
Nitrate	NO3-N	Fully oxidized
Organic N	Org-N	Bound in organics
Particulate N	Part-N	In biomass/solids
Dinitrogen	N2	Gas, denitrification product
N2O	N2O	Greenhouse gas

Aggregate Parameters

Parameter	Components
TKN	NH4-N + Organic-N
TN	TKN + NOx-N
NOx-N	NO2-N + NO3-N
TSN	Total soluble N

Nitrogen Transformations

Process Flow

Organic N (Influent)
       │
       ├── Ammonification → NH4-N
       │                      │
       │         ┌────────────┴────────────┐
       │         │                         │
       │    Assimilation              Nitrification
       │    (to biomass)              (AOB + NOB)
       │         │                         │
       │         ↓                         ↓
       │    Particulate N              NO3-N
       │    (WAS removal)                  │
       │                                   │
       │                            Denitrification
       │                                   │
       │                                   ↓
       │                                 N2 (gas)
       │                                   │
       └───────────────────────────────────┘

Key Reactions

Nitrification (aerobic): $$NH_4^+ + 1.5O_2 \xrightarrow{AOB} NO_2^- + H_2O + 2H^+$$ $$NO_2^- + 0.5O_2 \xrightarrow{NOB} NO_3^-$$

Denitrification (anoxic): $$NO_3^- + organic\ carbon \rightarrow N_2$$

Mass Balance

Overall Balance

$$N_{in} = N_{effluent} + N_{sludge} + N_{gas}$$

Component Tracking

Stream	NH4	NO3	Org-N	Part-N	N2
Influent	35	0	10	5	0
Primary eff	35	0	8	2	0
Anoxic eff	30	5	5	25	10
Aerobic eff	2	8	3	27	15
Secondary eff	1.5	6	0.5	0.5	-
WAS	-	-	-	20	-

ErgoPlanner Configuration

Influent Nitrogen

{
  "nitrogen": {
    "TKN": 45,
    "NH4_N": 35,
    "organic_N": 10,
    "particulate_N_fraction": 0.3
  }
}

Nitrogen in COD

Biomass contains nitrogen:

• fN,BM = 0.086 g N/g COD (typical)
• Affects sludge N content

Design Considerations

SRT for Nitrification

Minimum SRT depends on temperature:

Temperature	Min SRT (days)	Safety Factor
10°C	12	1.5-2.0
15°C	6	1.5-2.0
20°C	3	1.5-2.0

$$SRT_{min} = \frac{1}{\mu_{max,A} - b_A}$$

Denitrification Capacity

Carbon requirement: $$COD_{denit} = 2.86 \times NO_3\ removed$$

Internal recycle ratio: $$r = \frac{TN_{target}}{TN_{in} - TN_{target}} - 1$$

Example: MLE Process

Configuration

Parameter	Value
Flow	20,000 m³/d
Influent TKN	45 mg/L
Target TN	<10 mg/L
RAS	100%
IRR	200%

Results

Location	NH4-N	NO3-N	TN
Influent	35	0	45
Anoxic zone	25	2	32
Aerobic zone	1.5	12	18
Effluent	1.0	7	9

N Removal Paths

Pathway	kg N/d	% of Load
Denitrification	600	67%
WAS	180	20%
Effluent	180	20%

Troubleshooting

Issue	Cause	Solution
High NH4 effluent	Low SRT	Increase sludge age
High NO3 effluent	Low IRR	Increase recycle
Incomplete denit	Low C/N	Add carbon source
Rising sludge	Denit in clarifier	Reduce blanket depth

N2O Emissions

Sources

• AOB pathway (incomplete oxidation)
• Heterotrophic denitrification
• Chemical reactions

Factors Increasing N2O

Factor	Effect
Low DO	Increases AOB pathway
High NO2	Substrate for N2O
Low COD/N	Incomplete denit
Transients	Upset conditions

Emission Factors

Typical: 0.1-1% of N load as N2O-N

Best Practices

1. Maintain adequate SRT - Safety factor of 1.5-2.0
2. Optimize IRR - Balance energy vs N removal
3. Provide sufficient carbon - COD/N > 5 for denit
4. Control DO - 2 mg/L aerobic, <0.2 anoxic
5. Monitor regularly - NH4 and NO3 profiles

Related Topics

• COD Fractionation
• Phosphorus Balance
• ASM1
• Controllers

References

1. Metcalf & Eddy "Wastewater Engineering" 5th Ed
2. Henze, M. "Biological Nitrogen Removal"
3. WEF MOP 30 "Nutrient Removal"