Centrifuge (Decanter Centrifuge)

Overview

A decanter centrifuge, or solid bowl centrifuge, is a high-speed mechanical device that separates solids from liquids using centrifugal force. In wastewater treatment, centrifuges are widely used for sludge dewatering, producing cake solids of 22-32% while maintaining high solids capture rates. The continuous discharge design makes centrifuges suitable for automated, 24/7 operation with minimal operator intervention.

Common Applications:

• Municipal sludge dewatering (WAS, digested, mixed)
• Industrial waste solids separation
• Lime-stabilized biosolids
• High-solids applications requiring cake > 25%
• Plants requiring compact, automated systems

Typical Performance:

• Feed solids: 2-6%
• Cake solids: 22-32% (non-digested), 18-28% (digested)
• Solids capture: 90-96%
• Polymer dose: 2-10 kg/tonne dry solids
• G-force: 2,000-3,500 × gravity
• Throughput: 10-100 m³/h per unit

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Process Description

The decanter centrifuge consists of a horizontal, cylindrical bowl rotating at high speed (2,000-4,000 rpm) with an internal screw conveyor rotating at a slightly different speed (differential speed). Polymer-conditioned sludge is fed into the rotating bowl, where centrifugal force separates solids from liquids.

Operating Principle:

1. Feed Introduction:

   • Conditioned sludge enters through hollow rotating shaft
   • Distributed into bowl via feed ports

2. Centrifugal Separation:

   • Bowl rotation creates 2,000-3,500 G-force
   • Solids settle to bowl wall (pool depth)
   • Clarified liquid forms inner layer (pond depth)

3. Solids Conveyance:

   • Internal screw conveyor rotates at differential speed (1-50 rpm difference)
   • Solids conveyed up beach (conical section) toward cake discharge
   • Further dewatering occurs on beach

4. Discharge:

   • Dewatered cake discharged from small-diameter end
   • Centrate (liquid) overflows weir at large-diameter end

Key Design Elements:

• Bowl diameter: 300-600 mm (typical municipal)
• Length-to-diameter ratio (L/D): 3-4
• Pool depth: Adjustable via weir plates
• Beach angle: 5-15° for solids conveyance
• Differential speed: Controls solids residence time

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Operating Modes

Basic Mode

Simplified empirical model based on G-force, differential speed, and polymer dose.

Model Equations:

G-force:

G = \frac{\omega^2 \cdot r}{g} = \frac{(2\pi N / 60)^2 \cdot r}{9.81}

Cake solids prediction:

C_{cake} = C_{max} \cdot \left(1 - e^{-k_G \cdot G/1000}\right) \cdot \left(1 - e^{-k_D \cdot D_{polymer}}\right)

Solids capture:

SC = SC_{base} + k_{capture} \cdot \ln(G) + k_{polymer} \cdot D_{polymer}

Throughput capacity:

Q_{max} = k_{size} \cdot D_{bowl}^2 \cdot L_{bowl} \cdot \sqrt{G}

Where:

• G = G-force (multiples of gravity)
• ω = Angular velocity (rad/s)
• N = Rotational speed (rpm)
• r = Bowl radius (m)
• g = Gravitational acceleration (9.81 m/s²)
• C_max = Maximum achievable cake solids (28-35%)
• k_G = G-force effectiveness (0.3-0.5)
• k_D = Polymer effectiveness (0.4-0.6)
• D_polymer = Polymer dose (kg/tonne DS)
• SC_base = Baseline capture efficiency (0.85-0.90)
• k_capture = G-force capture coefficient (0.02-0.04)
• k_size = Size/capacity constant

Advanced Mode

Detailed model incorporating settling velocity, beach dewatering kinetics, differential speed effects, cake transport, and energy consumption.

Extended Model:

Particle settling velocity (Stokes):

v_s = \frac{(\rho_s - \rho_l) \cdot d_p^2 \cdot G \cdot g}{18 \cdot \mu}

Critical particle diameter (captured):

d_{crit} = \sqrt{\frac{18 \cdot \mu \cdot Q}{(\rho_s - \rho_l) \cdot G \cdot g \cdot A_{settling}}}

Pool depth (radial distance):

r_{pool} = r_{bowl} - h_{pool}

Beach residence time:

t_{beach} = \frac{L_{beach}}{\Delta N \cdot \tan(\alpha) \cdot 2\pi}

Cake moisture reduction on beach:

M_{cake} = M_{pool} \cdot e^{-k_{beach} \cdot t_{beach} \cdot G}

Conveying capacity (Sigma theory):

\Sigma = \frac{2\pi}{g} \cdot \int_{r_{pool}}^{r_{bowl}} r^2 \cdot L(r) \cdot \omega^2 \, dr

Clarification efficiency:

E_{clarification} = \frac{\Sigma \cdot v_s}{Q_{feed}}

Power consumption:

P = P_{bowl} + P_{differential} + P_{feed}

Bowl power:

P_{bowl} = \frac{\pi \cdot \mu \cdot \omega^3 \cdot r_{bowl}^4 \cdot L_{bowl}}{2 \cdot h_{gap}}

Differential power (torque):

P_{diff} = \tau_{diff} \cdot \Delta \omega = F_{friction} \cdot r_{avg} \cdot \Delta N \cdot 2\pi / 60

Where:

• v_s = Settling velocity (m/s)
• ρ_s, ρ_l = Solids and liquid density (kg/m³)
• d_p = Particle diameter (m)
• μ = Liquid viscosity (Pa·s)
• d_crit = Critical particle size for capture
• A_settling = Settling area (m²)
• h_pool = Pool depth (m)
• r_pool, r_bowl = Pool radius, bowl radius (m)
• L_beach = Beach length (m)
• ΔN = Differential speed (rpm)
• α = Beach angle (radians)
• M_pool, M_cake = Moisture at pool, in cake (%)
• k_beach = Beach dewatering rate constant
• Σ = Sigma factor (settling capacity, m²)
• L(r) = Length at radius r
• τ_diff = Differential torque (N·m)
• Δω = Differential angular velocity (rad/s)
• F_friction = Frictional force on solids
• h_gap = Gap between bowl and screw

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Input Parameters

Parameter	Symbol	Typical Range	Units	Description
Feed Flow Rate	Q_feed	10-100	m³/h	Liquid sludge flow
Feed Solids	S_feed	2-6	%	Total solids concentration
Feed VS/TS Ratio	VS/TS	0.60-0.80	-	Organic fraction
Sludge Type	-	-	-	WAS, primary, digested, mixed
Polymer Dose	D_polymer	2-10	kg/tonne DS	Active polymer dosage
Polymer Type	-	-	-	Cationic, anionic
Bowl Speed	N_bowl	2,500-4,000	rpm	Main bowl rotation speed
Differential Speed	ΔN	5-50	rpm	Bowl-screw speed difference
Pool Depth	h_pool	20-60	mm	Radial liquid depth
Feed Temperature	T	10-30	°C	Sludge temperature

Advanced Parameters:

Parameter	Symbol	Typical Range	Units	Description
Bowl Diameter	D_bowl	350-600	mm	Inner diameter
Bowl Length	L_bowl	1,200-2,400	mm	Cylindrical + conical length
L/D Ratio	L/D	3.0-4.5	-	Length to diameter ratio
Beach Angle	α	5-15	degrees	Conical section angle
Beach Length	L_beach	400-800	mm	Dewatering zone length
G-Force	G	2,000-3,500	× g	Centrifugal acceleration
Particle Size (d50)	d_50	10-100	μm	Median particle diameter
Sludge Viscosity	μ	10-100	mPa·s	Dynamic viscosity
CST	CST	100-800	s	Capillary suction time

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Output Parameters

Parameter	Symbol	Units	Description
Cake Flow Rate	Q_cake	m³/h	Dewatered cake volume
Cake Solids	C_cake	%	Cake total solids concentration
Cake Production Rate	m_cake	kg DS/h	Dry solids in cake
Centrate Flow Rate	Q_centrate	m³/h	Separated liquid
Centrate Solids	S_centrate	mg/L	Suspended solids in centrate
Solids Capture	SC	%	Percentage of solids in cake
Solids Loading Rate	SLR	kg DS/h	Dry solids throughput
Hydraulic Loading	HL	m³/h	Volumetric throughput
Polymer Consumption	m_polymer	kg/h	Active polymer used
Power Consumption	P_total	kW	Total electrical power
Specific Energy	E_specific	kWh/tonne DS	Energy per dry solids
G-Force	G_actual	× g	Actual centrifugal force
Torque	τ	N·m	Differential torque

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Configuration

{
  "unitId": "CENT-101",
  "unitType": "Centrifuge",
  "operatingMode": "Advanced",
  "design": {
    "manufacturer": "Alfa Laval / Andritz / Flottweg / GEA",
    "model": "ALDEC / D-Series / Z-Series / CF-Series",
    "bowlDiameter": 450,
    "bowlLength": 1800,
    "L_D_ratio": 4.0,
    "beachAngle": 8,
    "beachLength": 600,
    "driveType": "Dual Motor (Bowl + Back-Drive)",
    "gearboxType": "Planetary"
  },
  "operatingParameters": {
    "bowlSpeed": 3200,
    "differentialSpeed": 15,
    "poolDepth": 35,
    "targetCakeSolids": 26,
    "targetSolidsCapture": 0.93,
    "hydraulicCapacity": 45
  },
  "polymerSystem": {
    "polymerType": "Medium-High MW Cationic",
    "makedownConcentration": 0.5,
    "doseRange": [3, 8],
    "injectionPoint": "Inline static mixer",
    "mixingTime": 20
  },
  "controls": {
    "bowlSpeedControl": "VFD",
    "differentialSpeedControl": "VFD or Hydraulic Back-Drive",
    "poolDepthControl": "Adjustable weir plates (manual)",
    "polymerDoseControl": "Flow-paced PID",
    "feedRateControl": "VFD on feed pump",
    "torqueMonitoring": "Current sensing with trip",
    "vibrationMonitoring": "Accelerometers with alarm"
  },
  "monitoring": {
    "cakeSolidsMonitoring": "Periodic lab testing",
    "centrateSSMonitoring": "Online turbidity",
    "torqueMonitoring": "Continuous (motor current)",
    "vibrationMonitoring": "Continuous (bearing sensors)",
    "temperatureMonitoring": "Bearing and gearbox",
    "polymerFlowMonitoring": "Magnetic flow meter"
  },
  "protection": {
    "highTorqueShutdown": true,
    "highVibrationAlarm": true,
    "highTemperatureAlarm": true,
    "lowFeedShutdown": true,
    "imbalanceDetection": true
  }
}

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Design Criteria

Sizing Guidelines

Hydraulic Capacity:

Q_{max} = k \cdot D_{bowl}^2 \cdot L_{bowl} \cdot \sqrt{\frac{G}{1000}}

Where k = 0.15-0.25 (empirical constant)

Solids Loading:

SLR_{max} = Q_{max} \cdot S_{feed} \cdot 10

Power Requirement:

P_{total} \approx 0.3 \cdot D_{bowl}^{2.5} \cdot N_{bowl}

Typical Design Values

Parameter	Range	Recommendation
Bowl Speed	2,500-4,000 rpm	3,000-3,500 rpm for municipal
G-Force	2,000-3,500 × g	2,500-3,000 × g typical
Differential Speed	5-50 rpm	10-20 rpm for good cake dryness
Pool Depth	20-60 mm	30-40 mm for balanced performance
Hydraulic Loading	10-80 m³/h	30-50 m³/h per unit
Solids Loading	300-1,500 kg DS/h	500-800 kg DS/h typical
Polymer Dose (WAS)	4-8 kg/tonne DS	5-7 kg/tonne DS typical
Polymer Dose (Digested)	3-6 kg/tonne DS	4-5 kg/tonne DS typical
Cake Solids (WAS)	22-28%	24-26% typical
Cake Solids (Digested)	18-25%	20-23% typical
Solids Capture	90-96%	> 92% target
Specific Energy	20-50 kWh/tonne DS	30-40 kWh/tonne DS typical

Performance Factors

Sludge Type Characteristics:

Sludge Type	Expected Cake Solids	Polymer Dose	Typical G-Force
Primary	28-35%	2-4 kg/tDS	2,000-2,500 × g
WAS (non-digested)	22-28%	5-8 kg/tDS	2,500-3,000 × g
Mixed (50/50)	24-30%	4-6 kg/tDS	2,200-2,800 × g
Anaerobically Digested	18-25%	4-6 kg/tDS	2,500-3,200 × g
Aerobically Digested	16-22%	5-8 kg/tDS	2,800-3,500 × g
THP + AD	24-32%	3-5 kg/tDS	2,500-3,000 × g

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Process Control

Bowl Speed and G-Force

Control Strategy:

• Set bowl speed to achieve target G-force (2,500-3,000 × g)
• Higher G-force → better clarification, higher cake solids
• Lower G-force → gentler operation, less wear
• Use VFD for precise control

G-Force Calculation:

G = \frac{(2\pi N_{bowl} / 60)^2 \cdot (D_{bowl}/2)}{9.81}

Differential Speed

Control Strategy:

• Higher differential → faster solids conveyance, wetter cake
• Lower differential → longer beach time, drier cake, higher torque
• Typical range: 10-20 rpm for municipal sludge

Effects:

• ΔN = 5-10 rpm: Maximum cake dryness, risk of overload
• ΔN = 15-25 rpm: Balanced performance
• ΔN = 30-50 rpm: High throughput, wetter cake

Pool Depth

Control Strategy:

• Adjusted via weir plate position (manual change)
• Shallow pool → shorter residence time, lower capture
• Deep pool → longer residence time, better capture, wetter cake

Optimization:

• Typical pool depth: 30-40 mm
• Adjust based on solids capture target and cake dryness

Polymer Dose

Control Strategy:

• Flow-paced dosing proportional to feed flow
• Optimize through pilot testing or daily adjustments
• Monitor centrate turbidity for feedback

Automatic Control:

D_{polymer}(t) = D_{base} + K_P \cdot (Turb_{centrate} - Turb_{SP})

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Performance Monitoring

Key Performance Indicators

Indicator	Target Range	Action if Outside Range
Cake Solids	22-28% (WAS)	Adjust differential speed, polymer dose
Solids Capture	> 92%	Increase polymer, reduce throughput
Centrate TSS	< 1,000 mg/L	Increase polymer dose, reduce loading
Polymer Dose	3-8 kg/tDS	Optimize through testing
G-Force	2,500-3,000 × g	Adjust bowl speed
Differential Speed	10-20 rpm	Adjust for cake dryness vs. capacity
Torque	< 80% max	Reduce feed rate or differential speed
Vibration	< 5 mm/s RMS	Balance bowl, check bearings
Power Consumption	25-45 kWh/tDS	Check for mechanical issues

Sampling and Testing

Continuous:

• Centrate turbidity (online)
• Torque (motor current)
• Vibration (accelerometers)
• Bowl and differential speed

Hourly:

• Visual cake inspection

Daily:

• Feed solids (%)
• Cake solids (%)
• Centrate TSS
• Polymer dose verification

Weekly:

• Detailed mass balance
• CST or SRF
• Bearing temperature trends

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Troubleshooting

Problem	Possible Cause	Solution
Low cake solids (< 20%)	Differential speed too high	Reduce to 10-15 rpm
	Insufficient G-force	Increase bowl speed
	Pool depth too deep	Reduce pool depth (change weir)
	Poor flocculation	Optimize polymer dose/type
High centrate solids (> 1,500 mg/L)	Polymer underdose	Increase polymer dose
	Excessive G-force (shear)	Reduce bowl speed slightly
	Pool depth too shallow	Increase pool depth
	Feed rate too high	Reduce hydraulic loading
High torque / overload trip	Differential speed too low	Increase to 15-20 rpm
	Excessive solids loading	Reduce feed rate
	Poor flocculation (sticky cake)	Optimize polymer
	Beach plugged with solids	Flush with water, inspect
Excessive vibration	Bowl imbalance	Re-balance bowl
	Worn bearings	Replace bearings
	Feed pulsation	Install pulsation dampener
	Solids buildup in bowl	Clean bowl interior
Poor flocculation	Polymer type mismatch	Switch polymer (charge, MW)
	Inadequate mixing	Improve inline mixer design
	Polymer degradation	Check polymer age, storage
High polymer consumption	Overdosing	Conduct jar test, reduce dose
	Poor sludge quality	Pre-thicken or condition
Cake discharge problems	Cake too dry (sticky)	Increase differential speed
	Beach angle too shallow	(Design limitation, consult manufacturer)
Bearing overheating	Lubrication failure	Check oil level, replace
	Excessive load	Reduce feed rate or solids loading
	Coolant system failure	Inspect cooling system

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Advantages and Limitations

Advantages

• High cake solids: 22-32% (best among mechanical dewatering)
• Compact footprint: Small floor space requirement
• Automated operation: Minimal operator intervention
• High G-force: Excellent clarification and dewatering
• Continuous discharge: Steady-state operation
• Enclosed system: Reduced odor and aerosol release
• Fast startup: Ready to operate within minutes

Limitations

• High capital cost: $300,000-$800,000 per unit
• High energy consumption: 25-45 kWh/tonne DS
• Maintenance intensive: Bearings, gearbox, wear parts
• Noise and vibration: Requires vibration isolation
• Polymer sensitive: Performance depends on good flocculation
• Wear on bowl and screw: Abrasive sludges cause wear
• Complex controls: Requires skilled maintenance staff

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Maintenance

Routine Maintenance (Daily/Weekly)

• Monitor vibration and temperature trends
• Check polymer system (dose, concentration)
• Inspect cake and centrate quality
• Verify torque levels
• Check for unusual noises
• Inspect seals for leaks

Periodic Maintenance (Monthly/Quarterly)

• Lubricate bearings (per manufacturer schedule)
• Check gearbox oil level and condition
• Inspect wear surfaces (screw flights, bowl)
• Calibrate instrumentation
• Test vibration sensors and alarms
• Clean feed and discharge areas

Major Maintenance (Annual/Biennial)

• Overhaul gearbox
• Replace bearings (if needed)
• Inspect and resurface wear surfaces
• Balance bowl assembly
• Replace seals and gaskets
• Inspect electrical systems
• Load test and verify capacities

Critical Wear Parts:

• Tungsten carbide tiles on screw flights and beach
• Bowl wear-resistant coating
• Main bearings (expected life: 20,000-40,000 hours)
• Gearbox oil (change annually)

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Economic Considerations

Capital Costs (Order of Magnitude)

• Small unit (D = 350mm): $250,000-$400,000
• Medium unit (D = 450mm): $400,000-$600,000
• Large unit (D = 550mm): $600,000-$900,000

Includes:

• Centrifuge unit with drives
• Polymer system
• Feed pump and piping
• Cake conveyor
• Controls and instrumentation

Operating Costs

Annual Costs (per tonne DS):

• Polymer: $100-$350 (varies with sludge type)
• Electricity: $40-$90 (higher than belt press)
• Maintenance parts: $60-$120
• Labor: $30-$60
• Total: $230-$620 per tonne DS

Comparison with Belt Press:

• Higher capital cost (+40-60%)
• Higher energy cost (+100-150%)
• Higher cake solids (+3-7% absolute)
• Lower polymer consumption (-20-40%)
• Lower footprint (-60-70%)

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Related Process Units

• Gravity Belt Thickener: Pre-thickening before centrifuge
• Polymer Feed System: Conditioning for flocculation
• Sludge Storage Tank: Equalization before dewatering
• Cake Conveyor: Transport of dewatered cake
• Centrate Treatment: Return to plant headworks with equalization
• Thermal Dryer: Further drying to > 90% solids

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References

1. WEF Manual of Practice No. 8 (2009). Design of Municipal Wastewater Treatment Plants, 5th Edition. Water Environment Federation. Chapter 21: Residuals Processing.

2. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Edition. McGraw-Hill. Chapter 14: Biosolids Processing.

3. Novak, J.T. (2006). "Dewatering of Sewage Sludge". Drying Technology, 24(10), 1257-1262.

4. Amirtharajah, A. & O'Melia, C.R. (1990). "Coagulation Processes: Destabilization, Mixing, and Flocculation". Water Quality and Treatment, 4th Edition. McGraw-Hill.

5. Records, A.M. & Sutherland, K. (2001). Decanter Centrifuge Handbook. Elsevier.

6. Leung, W.W.-F. (1998). Industrial Centrifugation Technology. McGraw-Hill.

7. Water Environment Federation (2010). Operation of Municipal Wastewater Treatment Plants, 6th Edition, MOP 11. Chapter 19: Dewatering.

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