HPO - High Purity Oxygen Activated Sludge

High-rate activated sludge process using pure oxygen (90-99.5% O2) in covered tanks for enhanced oxygen transfer and intensified biological treatment.

Overview

High Purity Oxygen (HPO) activated sludge, also known as pure oxygen activated sludge, uses enriched oxygen (typically >90% O2) instead of air for aeration. The process operates in covered, staged reactors that maintain high dissolved oxygen levels (8-25 mg/L), enabling significantly higher organic loading rates and shorter detention times compared to conventional air-based systems.

When to Use

• Industrial wastewater with high organic loads
• Space-constrained sites requiring compact treatment
• Retrofitting overloaded conventional plants
• High-strength wastewaters (>500 mg BOD/L)
• Pulp and paper, refinery, food processing
• Plants with on-site oxygen availability
• Cold climate applications (enclosed system)

Key Characteristics

• 3-5x higher oxygen transfer rates than air
• 50-75% smaller tank volumes than conventional
• Higher MLSS concentrations (4,000-10,000 mg/L)
• Covered tanks with gas recirculation
• Superior oxygen utilization (85-95%)
• Consistent effluent quality
• Reduced off-gas and odor emissions

Oxygen Transfer Theory

Oxygen Solubility Enhancement

Henry's Law for oxygen: $$C_s^* = H \times p_{O2}$$

Where:

• C_s* = saturation concentration (mg/L)
• H = Henry's constant (mg/L/atm)
• p_O2 = oxygen partial pressure (atm)

For pure O2 vs air:

• Air: p_O2 = 0.21 atm, C_s* = 9.2 mg/L (at 20 deg C)
• Pure O2: p_O2 = 1.0 atm, C_s* = 44 mg/L (at 20 deg C)

Saturation increase: 4.8x

Oxygen Transfer Rate

$$OTR = K_La \times (C_s^* - C)$$

Where:

• OTR = oxygen transfer rate (kg O2/m3/h)
• K_La = volumetric mass transfer coefficient (1/h)
• C = bulk DO concentration (mg/L)

Driving Force Enhancement

For air at C = 2 mg/L: $$\Delta C_{air} = 9.2 - 2 = 7.2 \text{ mg/L}$$

For pure O2 at C = 8 mg/L: $$\Delta C_{HPO} = 44 - 8 = 36 \text{ mg/L}$$

Driving force increase: 5x

Alpha Factor Considerations

$$\alpha = \frac{K_La \text{ (wastewater)}}{K_La \text{ (clean water)}}$$

HPO systems typically achieve higher alpha (0.6-0.9) due to:

• Enclosed headspace reducing surfactant effects
• Higher bubble residence time
• Better mixing conditions

Process Configuration

Covered Reactor System

        Gas Recirculation
         ←─────────────────────────────┐
         │                             │
    ┌────┴────┐  ┌────────┐  ┌────────┐  ┌────┴────┐
    │ Stage 1 │→ │ Stage 2│→ │ Stage 3│→ │ Stage 4│→ To Clarifier
    │         │  │        │  │        │  │         │
    └────┬────┘  └───┬────┘  └────┬───┘  └────┬────┘
         │           │            │           │
    O2 Feed      Surface       Surface     Exhaust
                 Aerators      Aerators     Gas

Staging Benefits

Multiple stages in series provide:

• Plug-flow hydraulics for better kinetics
• Oxygen gradient (higher at inlet)
• Efficient oxygen utilization
• Process flexibility

Typical: 3-4 stages

Gas-Liquid Contact

Equipment	Description	Oxygen Utilization
Surface aerators	Covered mechanical mixers	80-90%
Submerged turbines	Sparged + mixing	85-95%
Jet aerators	High-velocity injection	90-95%
Hollow fiber	Membrane dissolution	>95%

Design Equations

Oxygen Requirement

$$OR = \frac{Q \times \Delta S}{Y_{obs}} + 4.57 \times Q \times \Delta N - 2.86 \times Q \times N_{denit}$$

Where:

• OR = oxygen requirement (kg O2/d)
• Delta S = BOD removed (mg/L)
• Y_obs = observed yield (kg cells/kg BOD)
• Delta N = nitrogen nitrified (mg/L)
• N_denit = nitrogen denitrified (mg/L)

Aeration Tank Volume

$$V = \frac{Q \times S_0 \times \theta_c \times Y}{X \times (1 + k_d \times \theta_c)}$$

HPO allows 50-75% volume reduction vs air systems.

F:M Ratio

$$F:M = \frac{Q \times S_0}{V \times X}$$

HPO typical: 0.4-1.5 kg BOD/kg MLSS/d (vs 0.2-0.6 conventional)

Sludge Retention Time

$$\theta_c = \frac{V \times X}{Q_w \times X_w + Q_e \times X_e}$$

HPO typical: 1-4 days (vs 5-15 conventional)

Oxygen Utilization Efficiency

$$OUE = \frac{OR}{O2_{feed}} \times 100%$$

Target: 85-95%

Gas Purity Maintenance

$$\frac{dC_{O2}}{dt} = \frac{O2_{feed} - OR - O2_{exhaust}}{V_{headspace}}$$

Maintain >85% O2 in headspace

Design Criteria

Loading Rates

Parameter	HPO	Conventional	Units
F:M	0.4-1.5	0.2-0.6	kg BOD/kg/d
Volumetric load	1.5-4.0	0.5-1.2	kg BOD/m3/d
MLSS	4,000-10,000	2,000-4,000	mg/L
HRT	1-3	4-8	hours
SRT	1-4	5-15	days

Oxygen System Design

Parameter	Design Value	Range	Units
O2 purity	93	90-99.5	%
DO setpoint	8-12	6-25	mg/L
Gas space O2	85-95	80-98	%
Vent rate	5-15	-	% of feed
O2 utilization	90	85-95	%

Reactor Configuration

Parameter	Design Value	Range
Number of stages	4	3-6
L:W ratio	2:1	1:1-3:1
Depth	4.5-6	4-8 m
Freeboard (gas)	0.3-0.5	0.2-1.0 m
Cover material	FRP/concrete	-

Parameters

Reactor Parameters

Parameter	Unit	Default	Range	Description
Volume	m3	calculated	-	Total reactor
Stages	-	4	3-6	Number of stages
Stage volume ratio	-	equal	1:1-2:1	First to last
Depth	m	5	4-8	Liquid depth
Cover type	-	FRP	FRP/concrete	Material
Headspace	m	0.4	0.2-0.6	Gas zone

Biological Parameters

Parameter	Unit	Default	Range	Description
MLSS	mg/L	6000	4000-10000	Mixed liquor
F:M	kg/kg/d	0.8	0.4-1.5	Food to micro
SRT	d	2	1-4	Solids retention
Yield	kg/kg	0.5	0.4-0.6	Observed yield
k_d	1/d	0.06	0.04-0.08	Decay rate

Oxygen System Parameters

Parameter	Unit	Default	Range	Description
O2 purity	%	93	90-99.5	Feed oxygen
O2 feed rate	kg/d	calculated	-	Oxygen supply
DO setpoint	mg/L	10	6-25	Per stage
Gas recirculation	%	20	10-40	Of headspace
Vent rate	%	10	5-20	CO2 purge
OUE target	%	90	85-95	Utilization

Aeration Equipment

Parameter	Unit	Default	Range	Description
Aerator type	-	surfaceAerator	surface/submerged	Equipment
Power	W/m3	30	20-50	Mixing power
Speed	rpm	60	40-100	Rotational
Immersion	%	50	30-70	Depth fraction

Oxygen Supply Systems

On-Site Generation

Technology	Purity	Capacity	Application
PSA	90-95%	20-200 t/d	Medium plants
VSA	90-93%	50-500 t/d	Large plants
Cryogenic	99.5%	>100 t/d	Very large
Membrane	30-50%	Small	Oxygen enrichment

Delivered Liquid Oxygen (LOX)

• Purity: 99.5%
• Storage: Cryogenic tanks
• Vaporizers required
• Suitable for: Small plants, backup

Cost Comparison

Method	Capital	O2 Cost	Best Application
LOX	Low	High	<5 t/d
PSA	Medium	Medium	5-50 t/d
VSA	High	Low	>50 t/d
Cryogenic	Very high	Very low	>200 t/d

Configuration

Example Configuration

{
  "type": "hpo",
  "id": "hpo-industrial-treatment",
  "parameters": {
    "reactor": {
      "totalVolume": 2000,
      "stages": 4,
      "stageConfiguration": "equal",
      "dimensions": {
        "length": 20,
        "width": 10,
        "liquidDepth": 5,
        "headspace": 0.4
      },
      "cover": {
        "material": "FRP",
        "gasSealing": "pressurized"
      }
    },
    "biological": {
      "mlss": 6000,
      "fm": 0.8,
      "srt": 2.5,
      "temperature": 25
    },
    "oxygenSystem": {
      "supplyType": "PSA",
      "purity": 93,
      "feedRate": 2500,
      "doSetpoints": [15, 12, 10, 8],
      "headspaceO2": 90,
      "ventRate": 12,
      "oueTarget": 90
    },
    "aeration": {
      "type": "coveredSurfaceAerator",
      "unitsPerStage": 2,
      "powerPerUnit": 75,
      "speed": 60,
      "immersion": 0.5
    },
    "safetyFeatures": {
      "o2Monitors": true,
      "explosionVents": true,
      "coMonitor": true,
      "purgeSystem": "nitrogen"
    }
  }
}

Worked Example

Given

• Industrial wastewater flow: 15,000 m3/d
• Influent BOD: 800 mg/L
• Influent TSS: 400 mg/L (after primary)
• Target: BOD <30 mg/L
• Temperature: 25 deg C
• Space constraint: Max 600 m2 footprint

Design

Step 1: BOD Load $$L_{BOD} = 15,000 \times 0.8 = 12,000 \text{ kg BOD/d}$$

Step 2: Oxygen Requirement Assuming Y_obs = 0.5 kg VSS/kg BOD, carbonaceous only:

$$OR = \frac{12,000}{0.5} \times 1.42 + 12,000 \times 0.1 = 17,040 + 1,200 = 18,240 \text{ kg O}_2\text{/d}$$

Wait - let me recalculate properly: O2 for synthesis: 12,000 x (1 - 1.42 x 0.5) = 12,000 x 0.29 = 3,480 kg/d O2 for endogenous: Estimate 30% of synthesis = 1,044 kg/d

Actually, simpler approach: O2 = 1.1-1.4 x BOD removed = 1.2 x 12,000 = 14,400 kg O2/d

Step 3: HPO Reactor Volume

Using volumetric loading = 2.5 kg BOD/m3/d: $$V = \frac{12,000}{2.5} = 4,800 \text{ m}^3$$

Hmm, that's large. Let's use higher loading (3.5 kg/m3/d for HPO): $$V = \frac{12,000}{3.5} = 3,430 \text{ m}^3$$

Step 4: Dimensions Check At depth 6m: $$A = 3,430/6 = 572 \text{ m}^2$$ < 600 m2 constraint

4 stages: Each 143 m2, dimension 12m x 12m x 6m

Step 5: MLSS and SRT F:M = 0.8 kg BOD/kg MLSS/d

$$MLSS = \frac{Q \times S_0}{V \times F:M} = \frac{15,000 \times 0.8}{3,430 \times 0.8} = 4,370 \text{ mg/L}$$

Round to 4,500 mg/L (conservative)

HRT = 3,430 / (15,000/24) = 5.5 hours

SRT = V x MLSS / (waste sludge) Estimate SRT = 2-3 days for HPO

Step 6: Oxygen System Sizing

O2 requirement = 14,400 kg/d At 90% OUE: $$O2_{feed} = \frac{14,400}{0.90} = 16,000 \text{ kg O}_2\text{/d} = 667 \text{ kg/h}$$

PSA system at 93% purity: Capacity required = 16,000 / 0.93 = 17,200 kg O2 product/d = 17.2 t/d

Step 7: Vent Gas Calculation

CO2 production approx 0.9 x O2 consumption = 0.9 x 14,400 = 12,960 kg CO2/d

To maintain 90% O2 in headspace: Vent rate = 10% of feed = 1,600 kg/d (carries CO2 + some O2)

Step 8: Aeration Power

Surface aerators at 30 W/m3: $$P_{total} = 30 \times 3,430 = 103 \text{ kW}$$

Plus PSA power (approx 0.3 kWh/kg O2): $$P_{PSA} = 0.3 \times 667 = 200 \text{ kW}$$

Step 9: Comparison with Conventional

Conventional AS for same load:

• Volume: 12,000 / 0.8 = 15,000 m3 (4.4x larger)
• Area at 5m: 3,000 m2 (5.2x larger)
• Aeration: 0.8 kWh/kg O2 x 14,400/24 = 480 kW

HPO savings:

• Volume: 77% reduction
• Area: 81% reduction
• Power: Similar (303 vs 480 kW) but 37% lower

Step 10: Summary

Parameter	Value	Units
Reactor volume	3,430	m3
Footprint	572	m2
Number of stages	4	-
MLSS	4,500	mg/L
HRT	5.5	hours
O2 feed rate	16,000	kg/d
PSA capacity	17.2	t/d
Total power	303	kW

Safety Considerations

Oxygen-Enriched Atmosphere Hazards

Hazard	Risk Level	Mitigation
Fire/explosion	High	O2 monitors, vent systems
Oxidizer contact	Medium	Material selection
Asphyxiation	Low	Confined space procedures
Pressure buildup	Medium	Relief valves

Design Safety Features

1. Continuous O2 monitoring in headspace
2. CO detector for incomplete combustion
3. Explosion relief panels or vents
4. Nitrogen purge system for maintenance
5. Flame arrestors on vents
6. Earthing/grounding of all equipment
7. Oil-free components in O2 contact areas

Material Compatibility

Component	Acceptable Materials
Piping	Stainless steel, copper
Seals	Viton, PTFE, silicone
Lubricants	O2-compatible only
Coatings	Epoxy (O2-rated)
Avoid	Carbon steel, oils, greases

Operational Protocols

• Maintain headspace O2 <95% (fire risk increases sharply above)
• Regular leak testing
• Hot work permits
• Confined space entry procedures
• Emergency vent procedures

Troubleshooting

Issue	Cause	Solution
Low DO in final stage	Insufficient O2 feed	Increase oxygen
	Overloading	Reduce flow, add stage
Poor OUE	Gas leakage	Check seals, covers
	High vent rate	Reduce, check CO2
	Poor mixing	Increase aerator speed
High effluent BOD	Short SRT	Reduce wasting
	Overloading	Increase volume, O2
Foaming	Low MLSS	Increase SRT
	Surfactants	Source control
Poor settling	Low SRT	Increase
	Filaments	Selector, chlorination
Rising CO2	Metabolism	Increase vent rate
	Poor stripping	Check aerators
Headspace O2 drop	High load	Increase O2 feed
	System leak	Pressurize, find leak

Commercial Technologies

Technology	Manufacturer	Features
UNOX	Linde	Original HPO system
OACES	Air Products	Staged system
MAROX	Union Carbide	Surface aerators
Bio-Oxygen	FMC	Jet aeration

Advantages and Limitations

Advantages

1. Compact: 50-75% smaller footprint
2. High rate: 3-5x conventional loading
3. Stable: Consistent DO, enclosed system
4. Odor control: Covered reactors
5. Cold climate: Enclosed, heated if needed
6. Retrofit: Upgrades overloaded plants

Limitations

1. Capital cost: Oxygen system expensive
2. Operating cost: Oxygen production energy
3. Complexity: More sophisticated operation
4. Safety: Oxygen-enriched atmosphere risks
5. Nitrification: May be limited at short SRT
6. Maintenance: Covers, seals, oxygen equipment

Related Topics

• CSTR - Conventional Activated Sludge
• High-Rate-Activated-Sludge - Air-based high rate
• Oxygen-Transfer - Aeration fundamentals
• Industrial-Wastewater - High-strength treatment
• Cryogenic-Oxygen - Oxygen production

References

1. EPA (1977). Process Design Manual for High Purity Oxygen Activated Sludge.
2. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery. 5th Ed.
3. WEF/ASCE (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.
4. Albertson, O.E. & DiGregorio, D. (1975). "Pure oxygen activated sludge". Journal WPCF.
5. Air Products (2010). OACES System Design Manual.
6. Daigger, G.T. & Littleton, H.X. (2014). "Simultaneous biological nutrient removal: A state-of-the-art review". Water Environment Research.