Rotary Drum Thickener

Continuously rotating perforated drum with polymer conditioning to concentrate biological sludge from 0.5-1.5% to 4-7% dry solids through drainage and compression.

Overview

Rotary drum thickeners (RDTs) combine chemical conditioning with mechanical drainage on a slowly rotating cylindrical screen. Polymer-flocculated sludge is applied to the drum surface, where gravity drainage and compression against the drum surface progressively thicken the sludge mat.

When to Use

• Waste activated sludge (WAS) from any MCRT
• Industrial biological sludge
• Mixed primary + WAS
• When space is very limited (compact footprint)
• Continuous, automated operation desired
• Alternative to DAF when air supply is problematic

Key Characteristics

• Very compact footprint (1-5 m² per unit)
• Polymer required (3-8 kg/tonne DS)
• Consistent performance across SVI range
• Lower power than DAF (no compressor)
• Good capture rate (92-97%)
• Can handle variable solids loading

Operating Modes

Basic Mode

• Fixed polymer dose
• Constant drum speed
• Default hydraulic loading rate

Advanced Mode

• Dynamic polymer optimization based on torque/clarity
• Variable drum speed control
• Real-time drainage zone modeling
• Automatic backwash cycle optimization

Mathematical Model

Governing Equations

Drainage Rate:

$$q_d = K_d \times (X_{cake} - X_{filtrate})$$

Where:

• $q_d$ = drainage rate (L/m²·min)
• $K_d$ = drainage coefficient (function of polymer dose)
• $X_{cake}$ = cake solids concentration (kg/m³)
• $X_{filtrate}$ = filtrate solids concentration (kg/m³)

Thickening Factor:

$$TF = \frac{C_{out}}{C_{in}}$$

Where:

• $TF$ = thickening factor (typically 4-8)
• $C_{out}$ = underflow concentration (% DS)
• $C_{in}$ = feed concentration (% DS)

Hydraulic Loading:

$$HL = \frac{Q}{A_{drum}}$$

Where:

• $HL$ = hydraulic loading (m³/m²·h)
• $Q$ = feed flow rate (m³/h)
• $A_{drum}$ = effective drum surface area (m²)
• Typical range: 10-30 m³/m²·h

Solids Loading:

$$SL = \frac{Q \times TSS}{A_{drum}}$$

Typical range: 80-200 kg/m²·h

Drum Speed:

$$V_{surface} = \pi \times D \times N$$

Where:

• $V_{surface}$ = surface velocity (m/min)
• $D$ = drum diameter (m)
• $N$ = rotation speed (rpm), typically 0.5-3 rpm
• Typical surface speed: 1-5 m/min

Polymer Dose:

$$P_{dose} = k_{poly} \times Q \times TSS$$

Where:

• $k_{poly}$ = specific polymer demand (0.003-0.008 kg/kg DS)
• Typical: 3-8 kg polymer/tonne dry solids

Design Parameters

Input Parameters

Parameter	Symbol	Unit	Range	Default	Description
Feed concentration	$TSS$	% DS	0.5-2.0	1.0	Inlet solids concentration
Target thickened DS	$C_t$	% DS	4-7	5.5	Desired cake concentration
SVI	-	mL/g	80-250	150	Sludge volume index
Polymer dose	$k_{poly}$	kg/tDS	3-10	5	Specific polymer demand
Drum diameter	$D$	m	1.5-3.0	2.5	Cylinder diameter
Drum length	$L$	m	2-6	4.0	Active length
Drum speed	$N$	rpm	0.5-3.0	1.5	Rotation speed
Screen opening	-	mm	0.3-0.8	0.5	Perforated plate opening

Output Parameters

Parameter	Unit	Description
Effective area	m²	Drum surface area (πDL)
Thickened sludge rate	m³/d	Cake discharge volume
Filtrate rate	m³/d	Drainage volume (return to plant)
Solids capture	%	Percentage to cake
Polymer consumption	kg/d	Daily polymer usage
Power consumption	kW	Drive motor and polymer system
Backwash frequency	cycles/h	Cleaning cycles per hour

Configuration Guide

Step-by-Step Setup

1. Select Drum Size

   Standard sizes:

   • Small: 1.5m D × 2m L = 9.4 m² (50-200 m³/d)
   • Medium: 2.5m D × 4m L = 31.4 m² (200-800 m³/d)
   • Large: 3.0m D × 6m L = 56.5 m² (500-1500 m³/d)

2. Calculate Required Area

   Method 1: Hydraulic Loading

   For Q = 300 m³/d = 12.5 m³/h
   Use HL = 20 m³/m²·h
   A = 12.5/20 = 0.625 m²
   → 1.5m × 2m drum provides 9.4 m² (ample capacity)

   Method 2: Solids Loading

   Solids load = 300 m³/d × 10 kg/m³ = 3,000 kg/d = 125 kg/h
   Use SL = 100 kg/m²·h
   A = 125/100 = 1.25 m²
   → 1.5m × 2m drum (9.4 m²) ✓

3. Determine Drum Speed

   • Low feed solids (0.5-0.8%): 2-3 rpm (fast, more cycles)
   • Medium feed solids (0.8-1.2%): 1.5-2 rpm
   • High feed solids (1.2-2.0%): 0.5-1.5 rpm (slow, longer drainage)

4. Size Polymer System

   • Polymer type: Cationic, high MW (8-12M Da)
   • Dose: 5 kg/tonne DS (typical)
   • Static mixer: 10-15 seconds contact time
   • Makeup: 0.2-0.5% active solution

5. Configure Backwash

   • Spray pressure: 3-5 bar
   • Cycle: Every 1-5 drum rotations
   • Duration: 5-15 seconds per cycle
   • Nozzles: Full drum width coverage

6. Design Cake Discharge

   • Doctor blade or scraper at 270-300° position
   • Screw conveyor for cake transport
   • Chute to downstream process (digester/dewatering)

Typical Design Values

Sludge Type	Feed DS	Polymer	Cake DS	HL	SL	Capture	Drum Speed
Extended aer. WAS	0.6-0.8%	6-8 kg/t	4.5-5.5%	15-25 m³/m²·h	80-150 kg/m²·h	93-96%	2-3 rpm
Standard WAS	0.8-1.2%	4-6 kg/t	5.0-6.0%	18-28 m³/m²·h	100-180 kg/m²·h	94-97%	1.5-2.5 rpm
Mixed sludge	1.2-2.0%	3-5 kg/t	5.5-7.0%	20-30 m³/m²·h	120-200 kg/m²·h	95-98%	1-2 rpm

Performance Criteria

Achievable Cake Concentrations

Feed Sludge	SVI (mL/g)	Feed DS (%)	Cake DS (%)	Volume Reduction (%)
Extended aer. WAS	180-250	0.6-0.8	4.5-5.0	83-88%
Standard WAS	120-180	0.8-1.2	5.0-5.5	78-85%
Standard WAS	80-120	1.0-1.5	5.5-6.5	75-82%
Mixed sludge	100-150	1.5-2.0	6.0-7.0	71-78%

Solids Capture

• Typical: 94-97%
• With optimized polymer: 96-98%
• Poor performance: <92% (indicates problems)

Filtrate Quality

• TSS: 100-300 mg/L (acceptable for return to plant head)
• Biological sludge: 150-250 mg/L
• Mixed sludge: 100-200 mg/L
• Higher than DAF, lower than gravity thickener

Troubleshooting

Issue	Cause	Solution
Low cake concentration	Polymer underdose	Increase dose, check polymer quality/age
	High drum speed (short drainage time)	Reduce speed, increase retention
	Screen blinding	Increase backwash frequency/pressure
	High feed rate	Reduce flow, add parallel unit
High filtrate TSS	Polymer underdose	Increase dose gradually
	Poor polymer mixing	Check static mixer, increase contact time
	Screen opening too large	Replace with finer screen
	Polymer overdose (deflocculation)	Reduce dose
Excessive polymer use	Overdosing	Optimize dose via jar test, use auto-control
	Poor sludge conditioning	Pre-thicken, improve upstream processes
Screen blinding	Inadequate backwash	Increase pressure/frequency, clean nozzles
	Grease/fats in sludge	Improve upstream screening, hot water wash
	Fine inorganic particles	Screen feed, accept higher backwash frequency
Uneven cake discharge	Doctor blade wear	Replace blade, check angle
	Drum not level	Level machine, check foundation
	Variable polymer distribution	Check mixing, polymer feed points
Variable performance	Feed flow fluctuations	Flow equalization, automated polymer control
	Temperature changes	Adjust polymer dose seasonally
	Feed characteristics change	Monitor SVI, adapt polymer/speed
High power consumption	Drum bearing issues	Lubricate, inspect bearings
	Cake buildup on drum	Improve backwash, manual cleaning
Vibration	Unbalanced drum (uneven cake)	Improve polymer distribution, check alignment
	Bearing wear	Replace bearings, check shaft alignment

Operation & Maintenance

Daily Checks

• Visual inspection of cake formation
• Monitor filtrate clarity
• Verify backwash operation
• Check polymer dosing pump
• Observe cake discharge consistency
• Review drum speed and torque

Weekly Maintenance

• Clean backwash nozzles
• Check screen condition (blinding, damage)
• Verify polymer makeup concentration
• Inspect doctor blade wear
• Review polymer consumption vs. solids
• Calibrate online instruments

Monthly Tasks

• Full drum inspection (screen integrity)
• Measure cake and filtrate TSS (lab)
• Check drive chain/gear condition
• Lubricate bearings
• Inspect spray manifold
• Review performance trends

Annual Tasks

• Replace doctor blade
• Overhaul drive system
• Clean/replace screen panels if damaged
• Full bearing inspection
• Check structural supports
• Review performance vs. design

Advanced Features

Auto-Polymer Control

Filtrate turbidity-based:

• Online turbidity sensor in filtrate
• PID control adjusts polymer dose
• Target: <300 NTU
• Can reduce polymer use by 15-25%

Torque-based control:

• Measure drum torque (cake thickness indicator)
• Increase polymer if torque too low (thin cake)
• Reduce polymer if torque too high (over-conditioned)

Variable Speed Drive

Adjust drum speed based on:

• Feed solids concentration (slower for thicker feed)
• Cake dryness target (slower for drier cake)
• Solids loading (faster during peaks)

Enhanced Drainage Zones

Some designs include:

• Gravity drainage zone (0-90°)
• Compression zone (90-180°)
• Vacuum-assisted zone (180-270°) for extra dewatering
• Cake discharge (270-360°)

Vacuum can improve cake DS by 0.5-1.5%.

Cost Comparison

Capital Cost

• Small unit (9 m²): £150-250k
• Medium unit (30 m²): £300-500k
• Large unit (60 m²): £500-800k

Operating Cost (per 1000 kg DS/d)

Item	Cost
Polymer (5 kg/t @ £2,000/t)	£10/d = £3,650/year
Power (2-5 kW × 24h @ £0.15/kWh)	£7-18/d
Backwash water	Minimal (recycled)
Maintenance labor	1-2 hours/week
Total	~£5,000-8,000/year

vs. DAF Thickener:

• Lower power (no compressor)
• Similar polymer usage
• Lower capital cost
• Smaller footprint

vs. Gravity Thickener:

• Higher polymer cost
• Higher capital cost
• Much smaller footprint
• Better for high-SVI sludge

Worked Example

Given

• Standard WAS: 250 m³/d @ 1.0% DS (10 kg/m³)
• SVI: 140 mL/g
• Target cake: 5.5% DS
• Polymer: 5 kg/tonne DS

Solution

Step 1: Calculate solids load

Solids = 250 × 10 = 2,500 kg/d = 104 kg/h

Step 2: Select drum size

Use SL = 120 kg/m²·h
A_required = 104/120 = 0.87 m²

Select: 2.5m D × 4m L = 31.4 m² (provides redundancy)
Actual SL = 104/31.4 = 3.3 kg/m²·h (very conservative, allows for peaks)

Step 3: Verify hydraulic loading

Q = 250 m³/d = 10.4 m³/h
HL = 10.4/31.4 = 0.33 m³/m²·h ✓ (well within 10-30 range)

Step 4: Determine drum speed

For 1.0% feed: Use 1.5-2.0 rpm
Select: 1.8 rpm
Surface speed = π × 2.5 × 1.8 = 14.1 m/min

Step 5: Cake production

Solids to cake: 2,500 kg/d (assume 96% capture)
Cake at 5.5% DS (55 kg/m³):
Q_cake = (2,500 × 0.96)/55 = 43.6 m³/d
Volume reduction = (250-43.6)/250 = 82.6%

Step 6: Filtrate

Filtrate = 250 - 43.6 = 206.4 m³/d
Solids in filtrate = 2,500 × 0.04 = 100 kg/d
TSS_filtrate = 100,000/206.4 = 485 mg/L ✓

Step 7: Polymer usage

Daily polymer: 2.5 tonnes DS/d × 5 kg/t = 12.5 kg/d
Annual: 4.56 tonnes/year
Cost @ £2,000/tonne: £9,100/year

Step 8: Power

Drive motor: 3 kW (2.5m drum)
Polymer system: 1 kW
Backwash pump: 1 kW
Total: 5 kW × 24 h = 120 kWh/d
Daily cost @ £0.15/kWh: £18/d = £6,570/year

Total operating cost: £15,670/year

Related Units

• Gravity-Thickener - Alternative for low-SVI sludge
• DAF-Thickener - Alternative flotation thickening
• Gravity-Belt-Thickener - Similar mechanical principle
• Anaerobic-Digester - Downstream stabilization
• Belt-Press - Downstream dewatering (similar technology)

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed.
2. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
3. HUBER Technology. HUBER Rotary Drum Thickener RoS 3Q, Technical Manual.
4. Andritz. Rotary Drum Thickener Design and Operation Guide.
5. CIWEM (2013). Sewage Sludge Treatment and Disposal, Manual of Practice.