Anaerobic Zone

Oxygen-free and nitrate-free biological reactor zone for volatile fatty acid fermentation and biological phosphorus release by PAOs.

Overview

The anaerobic zone is essential for biological phosphorus removal (Bio-P or EBPR). Under strictly anaerobic conditions (no O₂ or NO₃), phosphorus-accumulating organisms (PAOs) release stored phosphorus while taking up volatile fatty acids (VFAs).

Purpose

• VFA uptake and storage by PAOs
• Phosphorus release (prerequisite for luxury uptake)
• Fermentation of rbCOD to VFAs
• Selector for PAO growth

Key Requirements

• DO = 0 mg/L
• NO₃ = 0 mg/L
• VFAs present (or fermentable COD)

Biological Phosphorus Removal Theory

PAO Metabolism

Anaerobic Phase:

1. PAOs uptake VFAs (acetate, propionate)
2. Store as intracellular PHA (poly-hydroxyalkanoates)
3. Energy from polyphosphate hydrolysis
4. Phosphorus released to bulk liquid

Aerobic Phase (downstream):

1. PHA oxidized for energy and growth
2. Luxury phosphorus uptake
3. Polyphosphate storage rebuilt
4. Net P removal in waste sludge

ASM2d Kinetics

VFA uptake rate: $$r_{VFA} = q_{max} \times \frac{S_{VFA}}{K_{VFA} + S_{VFA}} \times \frac{K_O}{K_O + S_O} \times \frac{K_{NO}}{K_{NO} + S_{NO}} \times X_{PAO}$$

P release rate: $$r_P = Y_{PO4} \times r_{VFA}$$

Where Y_PO4 ≈ 0.4 mol P/mol VFA-C

VFA Requirements

$$VFA_{required} = \frac{P_{removal}}{Y_{P/VFA}}$$

Typical: 10-20 mg VFA/mg P removed

Design Equations

Volume for P Release

$$V_{anaerobic} = \frac{Q \times P_{release}}{r_{release} \times X_{PAO}}$$

HRT Requirement

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

Typical: 0.5-2 hours

• Short HRT: High VFA, limited fermentation
• Long HRT: Allows fermentation

Fermentation Potential

If VFAs insufficient, rbCOD can ferment in-zone:

$$VFA_{generated} = f_{ferm} \times rbCOD \times (1 - e^{-k_{ferm} \times HRT})$$

Fermentation rate k_ferm ≈ 0.1-0.3 h⁻¹

Mass Balance

Phosphorus released must equal uptake potential:

$$P_{release} = X_{PAO} \times V \times q_{P,release}$$

Later in aerobic:

$$P_{uptake} = P_{release} + P_{luxury}$$

Parameters

Operating Parameters

Parameter	Unit	Default	Range	Description
DO	mg/L	0	0	Must be zero
NO₃	mg/L	0	0	Must be zero
VFA (inlet)	mg/L as COD	>50	30-150	Acetic + propionic
P release	mg/L	10-30	5-50	During anaerobic
pH	-	7.0	6.5-7.5	Optimal range

Design Parameters

Parameter	Unit	Default	Range	Description
Volume	m³	calculated	-	Zone volume
Volume fraction	%	15	10-25	Of total biological
HRT	hours	1.0	0.5-2.0	Retention time
Mixers	W/m³	8	5-15	Mixing power
RAS return point	-	post-anoxic	-	Avoid NO₃

Kinetic Parameters (ASM2d)

Parameter	Symbol	Unit	Default	Range
Max VFA uptake	q_max	g/g/d	3.0	2-5
VFA half-sat	K_VFA	mg/L	4.0	2-10
P/VFA yield	Y_PO4	mol/mol	0.4	0.3-0.5
PHA yield	Y_PHA	g/g	1.5	1.2-1.8

Nitrate Exclusion

Nitrate entering the anaerobic zone:

1. Consumes VFAs (denitrifiers outcompete PAOs)
2. Reduces P release
3. Leads to poor EBPR

Strategies

1. RAS to post-anoxic: Return sludge enters after anoxic zone
2. Step-feed anoxic: Dedicated anoxic before anaerobic
3. Denitrifying PAO selection: DPAOs can use NO₃

Configuration Options

A2O Process

Influent → [Anaerobic] → [Anoxic] → [Aerobic] → Clarifier
                            ↑           |
                            └───────────┘ (Nitrate Recycle)
                 ↑
              [RAS to Anoxic]

UCT Process (Improved)

             [Anoxic] ←← [Internal Recycle]
                ↓
Influent → [Anaerobic] → [Anoxic] → [Aerobic]
                ↑
           [RAS to Anoxic] (no NO₃ to anaerobic)

Modified Bardenpho (5-Stage)

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

Mixing Requirements

Purpose

• Keep biomass suspended
• Distribute VFAs uniformly
• No surface agitation (prevents O₂ intrusion)

Design Criteria

Parameter	Value
Velocity gradient G	30-60 s⁻¹
Power	5-12 W/m³
Mixer type	Submersible, slow speed
Submergence	Deep mounting

Configuration

Example Configuration

{
  "type": "anaerobicZone",
  "id": "ebpr-anaerobic",
  "parameters": {
    "volume": 750,
    "hrt": 1.0,
    "doSetpoint": 0,
    "nitrateTarget": 0,
    "position": "first",
    "mixing": {
      "type": "submersible",
      "power": 8,
      "submergence": "deep"
    },
    "vfaSource": {
      "type": "influent",
      "supplemental": false
    },
    "monitoring": {
      "pRelease": true,
      "vfa": true,
      "orp": true
    }
  }
}

Monitoring

ORP (Oxidation-Reduction Potential)

ORP (mV vs Ag/AgCl)	Zone Condition
> +100	Aerobic (O₂ present)
0 to +100	Anoxic (NO₃ present)
-100 to 0	Anaerobic (transition)
< -100	Truly anaerobic
< -300	Sulfate reducing

Target: -50 to -200 mV for EBPR

Key Indicators

Parameter	Good	Poor
P release	15-30 mg/L	<10 mg/L
VFA uptake	>90%	<70%
NO₃ intrusion	0 mg/L	>2 mg/L
DO intrusion	0 mg/L	>0.2 mg/L

Troubleshooting

Issue	Cause	Solution
Low P release	Insufficient VFA	Add fermentate/chemicals
	NO₃ intrusion	Improve denitrification
	Short HRT	Increase zone volume
Secondary P release	RAS too long in clarifier	Reduce SRT
Poor effluent P	Low PAO population	Check anaerobic conditions
	VFA to aerobes	Exclude O₂
High effluent VFA	Overloading	Reduce VFA addition

VFA Sources

Influent VFAs

• Raw wastewater: 30-80 mg/L
• After primary: 50-150 mg/L

Fermentation

• Primary sludge fermentation
• Side-stream fermenters
• In-line fermentation

Chemical Addition

• Acetic acid (most effective)
• Sodium acetate
• Molasses (slower)
• Glycerol

Related Topics

• Anoxic-Zone
• Aerobic-Zone
• A2O-Process
• UCT-Process
• EBPR-Fundamentals

References

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