Aerobic Zone

Aerated biological reactor zone for carbonaceous oxidation, nitrification, and biological phosphorus uptake.

Overview

The aerobic zone is the primary treatment zone in activated sludge systems. Under aerobic conditions (DO > 1.5 mg/L), heterotrophs oxidize organic matter while autotrophs convert ammonia to nitrate.

Functions

• BOD/COD oxidation
• Nitrification (ammonia → nitrate)
• Phosphorus uptake (in BNR systems)
• Endogenous respiration

Biochemistry

Carbon Oxidation

$$C_{org} + O_2 → CO_2 + H_2O + \Delta biomass$$

Rate: $$r_s = \mu_{max} \times \frac{S}{K_s + S} \times \frac{DO}{K_O + DO} \times X$$

Nitrification

Step 1 - Nitritification (AOB): $$NH_4^+ + 1.5O_2 → NO_2^- + 2H^+ + H_2O$$

Step 2 - Nitratification (NOB): $$NO_2^- + 0.5O_2 → NO_3^-$$

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

Oxygen requirement: 4.57 mg O₂/mg NH₄-N

ASM1 Kinetics

Autotrophic growth: $$\mu_A = \mu_{A,max} \times \frac{S_{NH}}{K_{NH} + S_{NH}} \times \frac{S_O}{K_{O,A} + S_O}$$

Temperature correction: $$\mu_{A,max}(T) = \mu_{A,max}(20) \times 1.07^{(T-20)}$$

Phosphorus Uptake (ASM2d)

In aerobic conditions, PAOs take up excess phosphorus:

$$P_{uptake} = Y_P \times VFA_{stored}$$

Requires prior anaerobic VFA storage.

Design Equations

Minimum SRT for Nitrification

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

At 12°C: SRT_min ≈ 4-5 days Design SRT = 2-3 × SRT_min

Oxygen Demand

Carbonaceous: $$OD_C = \frac{Q \times (S_0 - S)}{f} \times (1 - 1.42 \times Y_{obs})$$

Nitrogenous: $$OD_N = 4.57 \times Q \times (TKN_0 - TKN_e - 0.12 \times P_x)$$

Total: $$OD_{total} = OD_C + OD_N - OD_{denit}$$

Where denitrification credit: 2.86 mg O₂/mg NO₃-N removed

Aeration Requirement

$$AOR = \frac{OD_{total}}{SOTE \times \alpha \times \beta \times \theta^{(T-20)}}$$

Parameters

Operating Parameters

Parameter	Unit	Default	Range	Description
DO setpoint	mg/L	2.0	1.5-4.0	Target DO
MLSS	mg/L	3500	2000-6000	Mixed liquor
SRT	days	12	5-25	Solids retention
HRT	hours	4	2-8	Hydraulic retention
Temperature	°C	15	10-25	Operating temp

Design Parameters

Parameter	Unit	Default	Range	Description
Volume	m³	calculated	-	Zone volume
Volume fraction	%	60	50-75	Of total biological
KLa	h⁻¹	10	5-30	Transfer coefficient
α factor	-	0.6	0.4-0.9	Alpha correction
SOTE	%	25	15-35	Fine bubble

Kinetic Parameters

Parameter	Symbol	Unit	Default	Range
Max autotrophic growth	μ_A,max	d⁻¹	0.8	0.3-1.5
Autotrophic decay	b_A	d⁻¹	0.15	0.05-0.25
NH₄ half-saturation	K_NH	mg/L	1.0	0.5-2.0
O₂ half-saturation (AOB)	K_O,A	mg/L	0.5	0.2-1.0

Aeration Systems

Fine Bubble Diffusers

• SOTE: 20-35%
• Air flow: 2-5 Nm³/m²/h
• Floor coverage: 10-15%
• Most energy efficient

Coarse Bubble

• SOTE: 8-12%
• Used for mixing
• Lower efficiency

Mechanical Aerators

• Surface aerators
• Brush rotors (oxidation ditch)
• 1.5-2.5 kg O₂/kWh

Control Strategies

Strategy	Description	Energy Saving
DO feedback	Maintain setpoint	Baseline
Ammonia-based	DO proportional to NH₄	10-20%
ABAC	Ammonia-based aeration control	20-30%
MPC	Model predictive	25-35%

Configuration

Example Configuration

{
  "type": "aerobicZone",
  "id": "main-aeration",
  "parameters": {
    "volume": 4000,
    "doSetpoint": 2.0,
    "mlss": 3500,
    "srt": 12,
    "aeration": {
      "type": "fineBubble",
      "diffuserType": "membrane",
      "floorCoverage": 0.12,
      "sote": 0.25,
      "kla": 10,
      "alpha": 0.6,
      "beta": 0.95
    },
    "control": {
      "strategy": "ammoniaFeedback",
      "doMin": 1.0,
      "doMax": 4.0,
      "ammoniaSP": 1.0
    }
  }
}

DO Profiles

Uniform DO

• Constant setpoint throughout
• Simple control
• May over-aerate

Tapered Aeration

• Higher DO at inlet
• Matches oxygen demand
• Energy efficient

Intermittent Aeration

• Alternating aeration/mixing
• Simultaneous nitrification/denitrification
• For SND applications

Troubleshooting

Issue	Cause	Solution
High effluent NH₄	Low SRT	Increase SRT
	Low DO	Increase aeration
	Low temperature	Extend volume
High effluent NO₃	No denitrification	Add anoxic zone
Foaming	Nocardia/Microthrix	Reduce SRT, add selector
Rising sludge	High NO₃ to clarifier	Increase denitrification

Related Topics

• Anoxic-Zone
• Anaerobic-Zone
• CSTR
• Aeration-Control
• DO-Control

References

1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
2. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.
3. Rosso, D. et al. (2008). "Aeration of Large-Scale WWTPs".