Oxidation Ditch

Extended aeration activated sludge system using an oval or racetrack-shaped channel with mechanical surface aerators or brush rotors to provide aeration, mixing, and circulation for simultaneous nitrification-denitrification.

Overview

The Oxidation Ditch is a modified activated sludge process that uses a continuous loop channel with horizontal or vertical axis mechanical aerators. The carousel configuration creates alternating aerobic and anoxic zones as mixed liquor circulates around the channel, enabling simultaneous nitrification-denitrification (SND) in a single reactor without separate anoxic zones.

When to Use

• Small to medium wastewater treatment plants (<100,000 PE)
• Applications requiring simple, robust operation
• Extended aeration with long SRT (20-30 days)
• Nutrient removal without complex zone separation
• Plants with variable or seasonal loading
• Low operator skill environments

Key Characteristics

• Oval or racetrack channel configuration
• Mechanical surface aeration (brush rotors, discs)
• High channel velocity (0.25-0.35 m/s)
• Extended aeration (SRT 20-30 days)
• Low sludge production due to endogenous respiration
• Simultaneous nitrification-denitrification capability
• Simple operation and maintenance

Configuration Types

Channel Configurations

Configuration	Channels	Aerators	Applications
Single channel	1	2-4	Small plants <5,000 PE
Dual channel	2 parallel	4-8	Medium plants 5,000-50,000 PE
Carrousel	3+	Multiple	Large plants, BNR
Folded channel	Single, folded	Multiple	Space-constrained
Orbal (concentric)	3 concentric	3+	BNR optimization

Aerator Types

Horizontal Brush Rotors (Kessener Brush):

• Rotating horizontal shaft with bristles or paddles
• Typical diameter: 0.7-1.0 m
• Speed: 60-100 RPM
• Oxygen transfer: 1.5-2.5 kg O2/kWh
• Creates horizontal flow + surface aeration

Vertical Disc Aerators (Mammoth Rotor):

• Vertical axis rotating discs
• Large diameter: 2-4 m
• Speed: 20-40 RPM
• Higher oxygen transfer efficiency
• Lower channel velocity

Submerged Turbine Aerators:

• Submerged propeller with air injection
• Variable DO control
• Lower maintenance
• Less visual impact

Standard Configurations

Single Channel (Simple Oxidation Ditch):
        ┌─────────────────────────────────────────┐
        │                                         │
   ←→   │   [Rotor 1]            [Rotor 2]       │   ←→
        │      ↓                     ↓            │
        │    ─────────────────────────            │
        │   │                         │           │
        │   │    Circulation          │           │
        │   │    ─────────────────→   │           │
        │    ─────────────────────────            │
        │                                         │
        └─────────────────────────────────────────┘
                      ↑         ↓
                   Influent  To Clarifier

Carrousel Configuration (BNR):
        ┌─────────────────────────────────────────┐
        │ Zone 1 │ Zone 2 │ Zone 3 │ Zone 4      │
        │ Anoxic │ Aerobic│ Aerobic│ Aerobic     │
        │   ↓    │   ↓    │   ↓    │   ↓         │
        │        │[Rotor] │[Rotor] │[Rotor]      │
        │    ─────────────────────────────────    │
        │   │                                  │  │
        │   │         Circulation              │  │
        │   │         ←──────────────────      │  │
        │    ─────────────────────────────────    │
        └─────────────────────────────────────────┘
              ↑                              ↓
           Influent                    To Clarifier

Biokinetic Equations

Extended Aeration Kinetics

Low F/M Operation: $$F/M = \frac{Q \times S_0}{V \times X} = 0.03 - 0.10 \text{ kg BOD/kg MLSS/d}$$

High SRT Operation: $$SRT = \frac{V \times X}{Q_w \times X_w} = 20 - 30 \text{ days}$$

Net Yield (Observed): $$Y_{obs} = \frac{Y_H}{1 + b_H \times SRT}$$

At SRT = 25 days: $$Y_{obs} = \frac{0.67}{1 + 0.4 \times 25} = 0.06 \text{ kg VSS/kg BOD}$$

Minimal sludge production due to high endogenous respiration.

ASM1 Integration

Heterotrophic Growth: $$r_{X_H} = \mu_{H,max} \times \frac{S_S}{K_S + S_S} \times \frac{S_O}{K_O + S_O} \times X_H$$

Autotrophic Growth (Nitrification): $$r_{X_A} = \mu_{A,max} \times \frac{S_{NH}}{K_{NH} + S_{NH}} \times \frac{S_O}{K_{O,A} + S_O} \times X_A$$

Denitrification (Anoxic Zones): $$r_{NO3} = \mu_{H,max} \times \eta_g \times \frac{S_S}{K_S + S_S} \times \frac{S_{NO}}{K_{NO} + S_{NO}} \times \frac{K_O}{K_O + S_O} \times X_H$$

Where eta_g = 0.8 (anoxic reduction factor)

Simultaneous Nitrification-Denitrification (SND)

DO Gradient in Channel:

As mixed liquor circulates, DO varies:

• Near aerator: DO = 2-4 mg/L (aerobic, nitrification)
• Away from aerator: DO = 0.2-0.8 mg/L (anoxic, denitrification)

SND Efficiency: $$\eta_{SND} = \frac{N_{removed} - N_{assimilated}}{N_{nitrified}} \times 100%$$

Typical SND efficiency: 30-50% of TN removal without separate anoxic zone

Floc Anoxic Core: At low bulk DO (0.5-1.5 mg/L):

• Outer floc: aerobic (nitrification)
• Inner floc: anoxic (denitrification)

$$\delta_{O2} = \sqrt{\frac{2 \times D_{O2} \times S_O}{q_{O2} \times X_f}}$$

At DO = 0.5 mg/L, floc penetration ~50-100 um - creates anoxic core

Temperature Correction

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

Parameter	theta	10C Factor	5C Factor
mu_H,max	1.07	0.51	0.36
mu_A,max	1.10	0.39	0.24
b_H	1.04	0.68	0.56
K_L*a	1.024	0.79	0.69

Long SRT compensates for cold weather kinetics.

Channel Velocity and Oxygen Transfer

Channel Velocity Requirements

Minimum Velocity (Solids Suspension): $$v_{min} = 0.25 \text{ m/s}$$

Design Velocity: $$v_{design} = 0.30 - 0.35 \text{ m/s}$$

Maximum Velocity (Excessive Shear): $$v_{max} = 0.45 \text{ m/s}$$

Flow Rate in Channel: $$Q_{channel} = v \times A_{cross-section}$$

Where A = depth x width of channel

Power for Circulation

Power for Velocity Maintenance: $$P_{circulation} = \frac{\rho \times g \times Q_{channel} \times h_f}{\eta}$$

Where:

• h_f = head loss around channel (m)
• eta = mechanical efficiency (0.6-0.8)

Head Loss Estimation: $$h_f = f \times \frac{L}{D_h} \times \frac{v^2}{2g} + \sum K \times \frac{v^2}{2g}$$

For oxidation ditch bends, K ~ 0.3-0.5 per 180-degree turn

Oxygen Transfer

Standard Oxygen Transfer Rate (SOTR): $$SOTR = K_La \times C^*_{s,20} \times V$$

Actual Oxygen Transfer Rate (AOTR): $$AOTR = SOTR \times \alpha \times \frac{(\beta \times C^_{s,T} - C_L)}{C^_{s,20}} \times \theta^{(T-20)}$$

Brush Rotor Oxygen Transfer:

Immersion Depth	OTR (kg O2/kWh)	Channel Velocity
100 mm	1.2-1.5	High
150 mm	1.5-2.0	Medium
200 mm	2.0-2.5	Lower
250 mm	2.2-2.8	Lowest

Trade-off: Deeper immersion = more oxygen, less velocity

Disc Aerator Performance: $$OTR = 1.8 - 2.5 \text{ kg O}_2\text{/kWh (installed)}$$

DO Control Strategy

Zone	Target DO	Function
Immediately after aerator	2-4 mg/L	Nitrification
Mid-channel	0.5-1.5 mg/L	SND
Pre-aerator	0.2-0.5 mg/L	Denitrification

Variable Frequency Drive (VFD) Control:

• Adjust rotor speed based on DO setpoint
• Different setpoints for different rotors (Carrousel)
• Seasonal adjustment for temperature

Design Criteria

Dimensional Parameters

Parameter	Unit	Range	Typical
Length	m	30-150	60-100
Width	m	5-15	8-12
Depth	m	2.5-4.5	3.0-3.5
Length:Width ratio	-	3:1 to 10:1	5:1 to 8:1
Channel velocity	m/s	0.25-0.45	0.30-0.35
HRT	hours	12-36	18-24
SRT	days	15-30	20-25

Loading Parameters

Parameter	Unit	Range	Notes
F/M ratio	kg/kg/d	0.03-0.10	Very low
Volumetric load	kg BOD/m3/d	0.1-0.3	Extended aeration
MLSS	mg/L	3000-6000	Typically 4000-5000
Sludge load	kg BOD/kg MLSS/d	0.03-0.08	Low
Oxygen requirement	kg O2/kg BOD	1.5-2.0	High due to endogenous

SRT Design for Nitrification

At design temperature (worst case, typically 10C):

$$SRT_{design} = SF \times \frac{1}{\mu_{A,max}(T) - b_A}$$

Where SF = safety factor (2-3)

Example at 10C: $$\mu_{A,max} = 0.8 \times 1.10^{-10} = 0.31 \text{ d}^{-1}$$ $$SRT_{min} = \frac{1}{0.31 - 0.09} = 4.5 \text{ days}$$ $$SRT_{design} = 3 \times 4.5 = 13.5 \text{ days}$$

Oxidation ditch typically operates at SRT = 20-30 days - very conservative for nitrification.

Parameters

Reactor Parameters

Parameter	Symbol	Unit	Default	Range	Description
Volume	V	m3	calculated	-	Total channel volume
Channel length	L	m	80	30-150	Total loop length
Channel width	W	m	8	5-15	Channel width
Depth	D	m	3.5	2.5-4.5	Operating depth
Number of channels	n	-	1	1-3	Parallel loops
Channel velocity	v	m/s	0.30	0.25-0.45	Circulation speed

Aerator Parameters

Parameter	Symbol	Unit	Default	Range	Description
Aerator type	-	-	Brush	Brush/Disc/Turbine	Type selection
Number of aerators	n	-	2	2-6	Per channel
Rotor diameter	D	m	0.9	0.7-1.0	Brush rotor
Rotor length	L	m	4	2-8	Per rotor
Rotor speed	N	RPM	80	60-100	Variable
Immersion depth	d	mm	175	100-250	Adjustable
OTR	-	kg O2/kWh	2.0	1.5-2.5	Transfer rate
Power per rotor	P	kW	15	5-50	Installed

Biological Parameters

Parameter	Symbol	Unit	Default	Range	Description
MLSS	X	mg/L	4500	3000-6000	Mixed liquor
MLVSS/MLSS	-	-	0.75	0.70-0.80	VSS fraction
SRT	-	days	25	20-30	Extended
HRT	-	hours	20	12-36	Long retention
F/M	-	kg/kg/d	0.05	0.03-0.10	Very low
DO (avg)	-	mg/L	1.5	0.5-3.0	Variable around loop

Nutrient Removal Parameters

Parameter	Unit	Default	Range	Description
TN removal	%	70-85	60-90	Via SND
NH4-N effluent	mg/L	2	<1-5	Near complete
NO3-N effluent	mg/L	8	5-15	Limited denit
TP removal	%	20-30	15-40	Biological only

Configuration

Example Configuration

{
  "type": "oxidationDitch",
  "id": "ox-ditch-1",
  "parameters": {
    "geometry": {
      "numberOfChannels": 1,
      "channelLength": 80,
      "channelWidth": 10,
      "operatingDepth": 3.5,
      "freeboard": 0.5,
      "bendRadius": 5
    },
    "aerators": {
      "type": "brushRotor",
      "numberOfAerators": 3,
      "rotorDiameter": 0.9,
      "rotorLength": 5,
      "rotorSpeed": 75,
      "immersionDepth": 175,
      "vfdEnabled": true,
      "otrPerKw": 2.0
    },
    "hydraulics": {
      "channelVelocity": 0.32,
      "circulationTime": 8,
      "mixingIntensity": "medium"
    },
    "biological": {
      "mlss": 4500,
      "srt": 25,
      "doSetpoint": 1.5,
      "doVariation": "gradient",
      "biokinetics": {
        "model": "ASM1",
        "muHMax": 6.0,
        "muAMax": 0.8,
        "yH": 0.67,
        "yA": 0.24,
        "bH": 0.4,
        "bA": 0.15
      }
    },
    "nutrientRemoval": {
      "sndEnabled": true,
      "targetTnRemoval": 0.75,
      "anoxicZoneFraction": 0.25
    },
    "control": {
      "doControl": "cascade",
      "doSensors": 3,
      "ammoniaFeedback": true
    }
  }
}

Worked Example

Given

• Design population: 25,000 PE
• Flow: 5,000 m3/d average, 7,500 m3/d peak
• Influent: BOD 250 mg/L, TSS 280 mg/L, TKN 45 mg/L, TP 8 mg/L
• Effluent targets: BOD <20 mg/L, TSS <30 mg/L, NH4-N ❤️ mg/L, TN <15 mg/L
• Minimum temperature: 10C
• Single oxidation ditch with secondary clarifier

Design Calculations

Step 1: Select SRT for Nitrification

At 10C: $$\mu_{A,max} = 0.8 \times 1.10^{(10-20)} = 0.31 \text{ d}^{-1}$$ $$SRT_{min} = \frac{1}{0.31 - 0.09} = 4.5 \text{ days}$$ $$SRT_{design} = 3 \times 4.5 = 13.5 \text{ days}$$

Select SRT = 25 days (extended aeration, conservative)

Step 2: Calculate Reactor Volume

Using F/M approach with F/M = 0.05 kg BOD/kg MLSS/d: $$V = \frac{Q \times S_0}{F/M \times X} = \frac{5,000 \times 0.25}{0.05 \times 4.5} = 5,556 \text{ m}^3$$

Round to V = 5,500 m3

Check HRT: $$HRT = \frac{5,500 \times 24}{5,000} = 26.4 \text{ hours}$$ - appropriate for extended aeration

Step 3: Channel Dimensions

Select depth D = 3.5 m, width W = 10 m

Cross-sectional area: A = 3.5 x 10 = 35 m2

For velocity = 0.30 m/s: $$Q_{channel} = 0.30 \times 35 = 10.5 \text{ m}^3\text{/s}$$

Circulation time around loop: $$t_{circ} = \frac{V}{Q_{channel}} = \frac{5,500}{10.5} = 524 \text{ s} = 8.7 \text{ min}$$

Total length of channel: $$L = \frac{V}{W \times D} = \frac{5,500}{10 \times 3.5} = 157 \text{ m}$$

This gives a racetrack with straight lengths of ~65 m each and semicircular ends of radius 5 m.

Step 4: Oxygen Demand

Carbonaceous oxygen demand: $$OD_C = \frac{Q \times (S_0 - S)}{f} = \frac{5,000 \times (250 - 20) \times 0.001}{1.6} = 719 \text{ kg O}_2\text{/d}$$

Nitrogenous oxygen demand: $$OD_N = 4.57 \times Q \times (TKN - N_e) = 4.57 \times 5,000 \times (45 - 3) \times 0.001 = 960 \text{ kg O}_2\text{/d}$$

Denitrification credit (assume 60% denit via SND): $$OD_{credit} = 2.86 \times Q \times NO_3_{denit} = 2.86 \times 5,000 \times 0.6 \times (42 \times 0.9) \times 0.001 = 325 \text{ kg O}_2\text{/d}$$

Endogenous respiration (high at long SRT): $$OD_{endo} = f_d \times b_H \times SRT \times P_x$$

Estimate Px: $$P_x = Y_{obs} \times Q \times S_0 = 0.06 \times 5,000 \times 0.25 = 75 \text{ kg/d}$$ $$OD_{endo} = 1.42 \times 0.4 \times 25 \times 75 = 1,065 \text{ kg O}_2\text{/d}$$

Net oxygen demand: $$OD_{net} = 719 + 960 - 325 + 1,065 = 2,419 \text{ kg O}_2\text{/d}$$

Step 5: Aerator Sizing

With brush rotors at 2.0 kg O2/kWh and 75% efficiency at field conditions: $$SOTR = \frac{OD_{net}}{0.6 \times 0.95 \times 0.85} = 4,993 \text{ kg O}_2\text{/d}$$

Power required: $$P = \frac{4,993/24}{2.0} = 104 \text{ kW}$$

Select 3 brush rotors at 40 kW each = 120 kW installed (15% margin)

Rotor specifications:

• Diameter: 0.9 m
• Length: 5 m each
• Speed: 60-90 RPM (VFD controlled)
• Immersion: 150-200 mm (adjustable)

Step 6: Check Channel Velocity

Each rotor provides circulation power. At 120 kW total, estimate 40% for circulation, 60% for oxygen: $$P_{circ} = 0.4 \times 120 = 48 \text{ kW}$$

Velocity check using empirical correlation: $$v \approx 0.8 \times (P_{circ}/V)^{0.35} = 0.8 \times (48/5,500)^{0.35} = 0.29 \text{ m/s}$$ - adequate

Step 7: Clarifier Sizing

At MLSS = 4,500 mg/L:

SOR at average flow: $$SOR = \frac{5,000}{A} = 0.8 \text{ m/h target}$$ $$A = 5,000/24/0.8 = 260 \text{ m}^2$$

Diameter for circular clarifier: $$D = \sqrt{4 \times 260/\pi} = 18.2 \text{ m}$$

Select D = 20 m (314 m2)

SLR check: $$SLR = \frac{(Q + Q_{RAS}) \times MLSS}{A} = \frac{(5,000 + 3,750) \times 4.5}{314 \times 24} = 5.2 \text{ kg/m}^2\text{/h}$$ - acceptable

Step 8: Sludge Production and WAS

$$P_x = Y_{obs} \times Q \times S_0 + X_{inert}$$ $$P_x = 0.06 \times 5,000 \times 0.25 + 5,000 \times 0.05 \times 0.001 = 75 + 0.25 = 75 \text{ kg/d}$$

Very low sludge production due to extended aeration.

WAS flow: $$Q_{WAS} = \frac{P_x}{X_{WAS}} = \frac{75}{10} = 7.5 \text{ m}^3\text{/d}$$

Step 9: Design Summary

Parameter	Value	Unit
Oxidation ditch volume	5,500	m3
Channel length	157	m
Channel width	10	m
Channel depth	3.5	m
MLSS	4,500	mg/L
SRT	25	days
HRT	26	hours
F/M	0.05	kg/kg/d
Aerator power (installed)	120	kW
Number of rotors	3	-
Channel velocity	0.29	m/s
Clarifier diameter	20	m
Sludge production	75	kg/d

Troubleshooting

Issue	Possible Cause	Diagnostic	Solution
Poor nitrification	Low DO	Measure DO profile	Increase rotor speed/immersion
	Low SRT	Calculate actual SRT	Reduce WAS
	Cold temperature	Check temp correction	Accept reduced performance
Poor settling	Filamentous bulking	Microscopy	Selector, increase DO
	High SVI	Measure SVI	Adjust F/M, add polymer
Floating sludge	Denitrification in clarifier	N2 bubble formation	Reduce RAS detention
Low channel velocity	Insufficient power	Measure velocity	Increase rotor speed
	Worn rotor blades	Visual inspection	Replace blades
Uneven DO	Aerator imbalance	DO profile	Balance rotor speeds
	Dead zones	Flow pattern analysis	Modify baffles
High energy	Excessive aeration	Compare OD vs capacity	Reduce rotor speed
	Inefficient rotors	Check OTR	Maintenance/upgrade
Foam	Nocardia	Microscopy	Reduce SRT, chlorinate
	High F/M spike	Check loading	Flow equalization
Poor TN removal	Insufficient anoxic time	DO profile	Lower avg DO, VFD control
	No carbon for denit	Check C/N ratio	Consider external carbon

Advantages and Limitations

Advantages

Aspect	Benefit
Simplicity	Minimal operator attention required
Reliability	Robust to shock loads
Nitrification	Excellent due to long SRT
Sludge quality	Well-stabilized, easy dewatering
Sludge quantity	Low production (50-60% of conventional)
Construction	Simple, low-cost earthwork possible
SND capability	Nutrient removal without recycles

Limitations

Aspect	Limitation
Footprint	Large area requirement
Energy	Higher than high-rate processes
TN removal	Limited to 70-85% without modifications
P removal	Poor without chemical addition
Peak capacity	Limited flexibility
Cold climate	Large volume needed for same performance

Related Topics

• CSTR - Continuously Stirred Tank Reactor
• SBR - Sequencing Batch Reactor
• Carrousel - Multi-zone oxidation ditch
• Orbal - Concentric channel system
• Extended-Aeration
• Nitrification-Fundamentals

References

1. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Ed. McGraw-Hill.
2. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed.
3. EPA (1993). Nitrogen Control Manual. EPA/625/R-93/010.
4. Pasveer, A. (1959). "A contribution to the development in activated sludge treatment." Journal WPCF, 31(2), 225-231.
5. Rittmann, B.E., and McCarty, P.L. (2001). Environmental Biotechnology: Principles and Applications. McGraw-Hill.
6. von Sperling, M. (2007). Activated Sludge and Aerobic Biofilm Reactors. IWA Publishing.