UASB - Upflow Anaerobic Sludge Blanket

High-rate anaerobic reactor using a dense sludge blanket and gas-solid-liquid separator for efficient organic removal with biogas recovery.

Overview

The UASB reactor treats wastewater anaerobically by flowing upward through a dense blanket of granular or flocculent sludge. Biogas production provides mixing, and the distinctive three-phase separator captures gas while returning sludge to the blanket.

When to Use

• High-strength industrial wastewater (>1000 mg COD/L)
• Warm climates (>20°C)
• Energy recovery applications
• Brewery, food, pulp & paper industries
• Pre-treatment for municipal in tropics

Key Characteristics

• No external energy for mixing (biogas driven)
• Positive energy balance (biogas production)
• Low sludge production (0.05-0.1 kg VS/kg COD)
• Compact footprint
• Temperature sensitive

Anaerobic Biochemistry

Four-Stage Process

graph TD
    A[Complex Organics] -->|Hydrolysis| B[Simple Organics]
    B -->|Acidogenesis| C[VFAs, H2, CO2]
    C -->|Acetogenesis| D[Acetate, H2, CO2]
    D -->|Methanogenesis| E[CH4 + CO2]

Key Reactions

Acetoclastic Methanogenesis: $$CH_3COOH → CH_4 + CO_2$$

Hydrogenotrophic Methanogenesis: $$4H_2 + CO_2 → CH_4 + 2H_2O$$

ADM1 Simplified Kinetics

Acidogenesis: $$r_{su} = k_m \times X_{su} \times \frac{S_{su}}{K_s + S_{su}}$$

Methanogenesis: $$r_{CH4} = k_{m,ac} \times X_{ac} \times \frac{S_{ac}}{K_{s,ac} + S_{ac}} \times I_{pH}$$

pH Inhibition

$$I_{pH} = \exp\left(-3 \times \left(\frac{pH_{UL} - pH}{pH_{UL} - pH_{LL}}\right)^2\right)$$

For pH < pH_UL (upper limit ≈ 8.0, lower limit ≈ 6.0)

Design Equations

Organic Loading Rate (OLR)

$$OLR = \frac{Q \times COD_{in}}{V}$$

Typical: 5-15 kg COD/m³/d (granular sludge)

Upflow Velocity

$$v_{up} = \frac{Q}{A}$$

Design range: 0.5-1.5 m/h Maximum: 2.0 m/h (with good granulation)

Hydraulic Retention Time

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

Typical: 4-12 hours (high-strength) Extended: 12-24 hours (dilute waste)

Solids Retention Time

$$SRT = \frac{M_{sludge}}{M_{wasted}}$$

Target: 30-100 days (slower growth)

Biogas Production

$$Q_{biogas} = \frac{COD_{removed} \times 0.35}{T/273 \times P/101.3}$$

At STP: 0.35 m³ CH₄/kg COD removed

Sludge Production

$$P_x = Y_{obs} \times COD_{removed}$$

Y_obs = 0.05-0.15 kg VS/kg COD (much lower than aerobic)

Design Criteria

Loading Rates

Wastewater Type	OLR (kg COD/m³/d)	COD Removal
Brewery	10-20	85-95%
Soft drinks	8-15	80-90%
Pulp & paper	5-12	75-85%
Municipal	2-5	70-80%
Dilute waste	2-4	60-75%

Temperature Effects

Temperature	Relative Activity	Notes
<15°C	<30%	Too cold
20°C	50%	Minimum practical
25°C	70%	Acceptable
30°C	85%	Good
35°C	100%	Optimum (mesophilic)
55°C	120%	Thermophilic peak

$$k(T) = k(35) \times \theta^{(T-35)}$$

Where θ ≈ 1.07 for mesophilic

Three-Phase Separator

Function

1. Separate biogas from liquid
2. Settle and return sludge
3. Produce clear effluent

Design Parameters

Parameter	Value
Gas hood angle	50-60°
Settler area	>40% of cross-section
Aperture velocity	<2 m/h
Deflector clearance	100-150 mm
Gas collection efficiency	>98%

Parameters

Reactor Parameters

Parameter	Unit	Default	Range	Description
Volume	m³	calculated	-	Reactor volume
Height	m	5	4-6	Total height
Sludge bed height	m	2.5	2-3	Blanket depth
OLR	kg COD/m³/d	10	2-20	Organic loading
HRT	hours	8	4-24	Retention time
Upflow velocity	m/h	1.0	0.5-2	Hydraulic velocity
Temperature	°C	35	25-55	Operating temp

Sludge Parameters

Parameter	Unit	Default	Range	Description
Sludge concentration	kg VS/m³	50	30-80	In bed
Sludge blanket depth	m	2.5	2-3	Active zone
SMA	g COD/g VS/d	0.5	0.3-1.5	Specific activity
SVI	mL/g	15	10-30	Settleability

Biogas Parameters

Parameter	Unit	Default	Range	Description
CH₄ content	%	70	60-80	Methane fraction
CO₂ content	%	28	20-35	Carbon dioxide
H₂S content	ppm	1000	100-5000	Hydrogen sulfide
Gas yield	m³/kg COD	0.35	0.3-0.4	At STP

Alkalinity & pH Control

Bicarbonate Alkalinity

Required to buffer VFA production:

$$Alk_{required} = \frac{VFA_{peak}}{1.4}$$

Typical: 2000-4000 mg CaCO₃/L

pH Control

Maintain pH 6.8-7.4

Dosing options:

• NaOH, NaHCO₃ for low alk waste
• Lime (Ca(OH)₂) economical but scaling risk
• Recycle effluent (has alk from CO₂)

Configuration

Example Configuration

{
  "type": "uasb",
  "id": "uasb-reactor",
  "parameters": {
    "volume": 500,
    "height": 5,
    "sludgeBedHeight": 2.5,
    "olr": 12,
    "temperature": 35,
    "upflowVelocity": 1.0,
    "sludge": {
      "concentration": 50,
      "sma": 0.5,
      "granular": true
    },
    "gasSeparator": {
      "type": "threePhaseSeparator",
      "hoodAngle": 55,
      "settlerFraction": 0.45
    },
    "phControl": {
      "setpoint": 7.0,
      "alkalinityTarget": 3000,
      "dosingChemical": "NaHCO3"
    }
  }
}

Worked Example

Given

• Flow: 1,000 m³/d
• Influent COD: 5,000 mg/L
• Temperature: 30°C
• Target: 90% COD removal

Design

Step 1: COD Load $$Load = 1000 \times 5 = 5,000 \text{ kg COD/d}$$

Step 2: Design OLR

At 30°C, reduce from optimum: OLR = 10 × 0.85 = 8.5 kg COD/m³/d

Step 3: Volume Required $$V = \frac{5,000}{8.5} = 588 \text{ m}^3$$

Round to 600 m³

Step 4: Dimensions At H = 5m: $$A = 600/5 = 120 \text{ m}^2$$

Diameter = 12.4m → Use 12.5m

Step 5: Upflow Velocity Check $$v_{up} = \frac{1000/24}{120} = 0.35 \text{ m/h}$$ ✓ (<1.5)

Step 6: Biogas Production COD removed = 5,000 × 0.90 = 4,500 kg/d

At 30°C: $$Q_{biogas} = 4,500 \times 0.35 \times \frac{303}{273} = 1,750 \text{ m}^3/d$$

CH₄ = 1,750 × 0.70 = 1,225 m³/d

Energy = 1,225 × 35.8 MJ/m³ = 43,900 MJ/d = 12.2 MWh/d

Startup Procedure

Seeding

• Source: Existing UASB, digester sludge
• Amount: 10-20 kg VS/m³ reactor
• Granular preferred for faster startup

Gradual Loading

Week	OLR (% of design)	Notes
1-2	10-20%	Monitor VFA, pH
3-4	30-40%	Check granulation
5-8	50-70%	Adjust nutrients
9-12	80-100%	Full operation

Troubleshooting

Issue	Cause	Solution
High VFA	Overloading	Reduce OLR
	Low pH	Add alkalinity
Low gas	Temperature drop	Heat influent
	Toxicity	Identify inhibitor
Sludge washout	High velocity	Reduce flow
	Poor granulation	Improve conditions
Foaming	Surfactants	Antifoam, dilute

Related Topics

• Anaerobic-Digester
• EGSB - Expanded Granular Sludge Bed
• IC-Reactor - Internal Circulation
• Biogas-Utilization

References

1. Lettinga, G. et al. (1980). "Use of UASB Reactor Concept". Biotechnology & Bioengineering.
2. van Lier, J.B. et al. (2015). Anaerobic Sewage Treatment. IWA Publishing.
3. Chernicharo, C.A.L. (2007). Anaerobic Reactors. IWA Publishing.
4. Batstone, D.J. et al. (2002). ADM1. IWA Scientific & Technical Report No. 13.