Anoxic Zone

Biological reactor zone operating without dissolved oxygen but with nitrate as terminal electron acceptor for denitrification.

Overview

The anoxic zone is a key component of biological nutrient removal (BNR) systems. Under anoxic conditions (DO < 0.5 mg/L, NO₃ present), facultative heterotrophs use nitrate instead of oxygen for respiration, converting it to nitrogen gas.

Purpose

• Nitrogen removal via denitrification
• Carbon utilization for nitrate reduction
• Selector function for settling improvement
• Alkalinity recovery (50% of nitrification consumption)

Denitrification Theory

Biochemistry

$$2NO_3^- + 10H^+ + 10e^- → N_2 + 4H_2O + 2OH^-$$

Simplified with organic matter:

$$5CH_2O + 4NO_3^- → 2N_2 + 4HCO_3^- + CO_2 + 3H_2O$$

ASM1 Kinetics

$$\mu_{H,denit} = \mu_{H,max} \times \eta_g \times \frac{S_s}{K_s + S_s} \times \frac{S_{NO}}{K_{NO} + S_{NO}} \times \frac{K_O}{K_O + S_O}$$

$$r_{NO_3} = -\frac{(1 - Y_H) \times \mu_{H,denit} \times X_{B,H}}{2.86 \times Y_H}$$

Where:

• η_g = anoxic reduction factor (0.6-0.9, typical 0.8)
• 2.86 = O₂ equivalents per NO₃-N

Denitrification Rate

Specific rate:

$$SDNR = \frac{\Delta NO_3}{X \times t}$$

Typical values:

• With readily biodegradable COD: 3-6 mg NO₃-N/g MLVSS/h
• With slowly biodegradable COD: 1-3 mg NO₃-N/g MLVSS/h
• Endogenous: 0.5-1.5 mg NO₃-N/g MLVSS/h

Carbon Requirement

$$COD_{required} = NO_3 \times \frac{2.86}{1 - Y_H}$$

For Y_H = 0.67: $$COD = NO_3 \times 8.6$$

Or BOD ≈ NO₃ × 4-5

Design Equations

Volume from SDNR

$$V_{anoxic} = \frac{Q \times \Delta NO_3}{SDNR \times X}$$

Volume from HRT

$$HRT = \frac{V}{Q}$$

Typical: 1-3 hours (pre-anoxic)

Recirculation Requirement

For pre-anoxic denitrification:

$$\eta_{TN} = \frac{R + r}{R + r + 1}$$

Where:

• R = internal recycle ratio (typically 2-4× Q)
• r = RAS ratio (typically 0.5-1× Q)

For 80% TN removal: R + r ≈ 4

Mass Balance

Nitrate removed = Nitrate loaded - Nitrate in effluent

$$NO_{3,removed} = Q \times (R + r + 1) \times NO_{3,in} - Q \times NO_{3,out}$$

Parameters

Operating Parameters

Parameter	Unit	Default	Range	Description
DO setpoint	mg/L	<0.3	0-0.5	Target DO
NO₃ in	mg/L	variable	5-30	From nitrification
SDNR	mg/g/h	4	1-6	Specific rate
HRT	hours	1.5	0.5-3	Retention time
Temperature	°C	15	10-25	Operating temp

Design Parameters

Parameter	Unit	Default	Range	Description
Volume	m³	calculated	-	Zone volume
Volume fraction	%	25	15-35	Of total bio
Internal recycle	×Q	3	1-6	Nitrate return
RAS recycle	×Q	0.75	0.5-1.5	Sludge return
Mixers	W/m³	8	5-15	Mixing power

Kinetic Parameters

Parameter	Symbol	Unit	Default	Range
Anoxic factor	η_g	-	0.8	0.6-0.9
NO₃ half-sat	K_NO	mg/L	0.5	0.2-1.0
DO inhibition	K_O	mg/L	0.2	0.1-0.5

Alkalinity Recovery

Denitrification produces alkalinity:

$$Alk_{recovered} = 3.57 \text{ mg CaCO}_3 / \text{mg NO}_3\text{-N}$$

This recovers ~50% of alkalinity consumed in nitrification (7.14 mg/mg).

Configuration Options

Pre-Anoxic (MLE Process)

Influent → [Anoxic] → [Aerobic] → Clarifier → Effluent
              ↑           |
              └───────────┘ (Internal Recycle)

Advantages:

• Uses influent COD
• Simple configuration
• Reliable performance

Post-Anoxic

Influent → [Aerobic] → [Anoxic] → Clarifier
                          ↑
                    [Carbon Add]

Advantages:

• No recirculation limits
• Very low TN possible
• Requires external carbon

4-Stage Bardenpho

[Anoxic 1] → [Aerobic 1] → [Anoxic 2] → [Aerobic 2]

Achieves TN < 3 mg/L

Mixing Requirements

Purpose

• Keep biomass suspended
• Distribute nitrate uniformly
• Prevent DO intrusion

Design Criteria

Parameter	Value
Velocity gradient G	40-80 s⁻¹
Power	5-15 W/m³
Mixer type	Submersible, banana blade
Number	1 per 500-1000 m³

Avoid surface aeration - must not introduce oxygen!

Configuration

Example Configuration

{
  "type": "anoxicZone",
  "id": "pre-anoxic",
  "parameters": {
    "volume": 1500,
    "doSetpoint": 0.2,
    "position": "pre-aerobic",
    "mixing": {
      "type": "submersible",
      "power": 10,
      "numberPerTank": 2
    },
    "recycle": {
      "internalRecycle": 3.0,
      "rasRecycle": 0.75
    },
    "kinetics": {
      "sdnr": 4,
      "etaG": 0.8
    }
  }
}

Troubleshooting

Issue	Cause	Solution
High effluent NO₃	Insufficient anoxic volume	Increase recycle or volume
	Low carbon	Add rbCOD
	DO intrusion	Check mixers, reduce aeration upstream
Low denitrification	Low temperature	Account in design
	pH extremes	Maintain 6.5-8.5
Rising sludge in clarifier	Denitrification in clarifier	Reduce SRT, increase RAS
NO₂ accumulation	Incomplete denitrification	Add carbon, reduce NO₃ load

Related Topics

• Aerobic-Zone
• Anaerobic-Zone
• MLE-Process
• Bardenpho-Process
• CSTR

References

1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
2. Henze, M. et al. (2008). Biological Wastewater Treatment. IWA Publishing.
3. WEF (2010). Nutrient Removal. WEF MOP 34.