Gravity Belt Thickener

Continuous moving belt with polymer conditioning to concentrate biological sludge from 0.5-1.5% to 4-6% dry solids through gravity drainage.

Overview

Gravity belt thickeners (GBTs) use a porous moving belt to dewater polymer-conditioned sludge through gravity drainage. Sludge is flocculated inline with polymer, applied to the belt, and progressively drained as it travels through the drainage zone. The thickened sludge is discharged at the end while filtrate drains through the belt.

When to Use

• Waste activated sludge (WAS) from any MCRT
• Mixed primary + WAS
• Industrial biological sludge
• Continuous automated operation desired
• Space-limited applications
• When gentle treatment is needed (minimal shear)

Key Characteristics

• Compact footprint (2-10 m² per unit)
• Polymer required (3-7 kg/tonne DS)
• Low power consumption (belt drive only)
• Good capture rate (90-96%)
• Simple, reliable operation
• Minimal operator attention required

Operating Modes

Basic Mode

• Fixed polymer dose
• Constant belt speed
• Default hydraulic loading

Advanced Mode

• Dynamic polymer optimization
• Variable belt speed control
• Plow frequency adjustment
• Real-time drainage modeling

Mathematical Model

Governing Equations

Drainage Rate:

$$q_d = K_d \times \Delta P \times t$$

Where:

• $q_d$ = drainage rate (L/m²·min)
• $K_d$ = drainage coefficient (function of polymer dose and sludge properties)
• $\Delta P$ = pressure head (sludge mat thickness)
• $t$ = drainage time (function of belt speed and length)

Drainage Time:

$$t_d = \frac{L_{drain}}{V_{belt}}$$

Where:

• $t_d$ = drainage time (min)
• $L_{drain}$ = effective drainage length (m), typically 3-6m
• $V_{belt}$ = belt speed (m/min), typically 0.5-3 m/min

Hydraulic Loading:

$$HL = \frac{Q}{W \times L_{drain}}$$

Where:

• $HL$ = hydraulic loading (m³/m²·h)
• $Q$ = feed flow rate (m³/h)
• $W$ = belt width (m)
• Typical range: 15-40 m³/m²·h

Solids Loading:

$$SL = \frac{Q \times TSS}{W \times L_{drain}}$$

Typical range: 100-250 kg/m²·h

Thickening Factor:

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

Typical TF: 4-6 for WAS, 3-5 for mixed sludge

Polymer Dose:

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

Where:

• $k_{poly}$ = 0.003-0.007 kg/kg DS
• Typical: 3-7 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 cake DS	$C_t$	% DS	3.5-6.5	5.0	Desired thickened concentration
SVI	-	mL/g	80-250	150	Sludge volume index
Polymer dose	$k_{poly}$	kg/tDS	3-8	5	Specific polymer demand
Belt width	$W$	m	0.5-3.0	1.5	Standard widths: 0.5, 1.0, 1.5, 2.0, 3.0m
Belt speed	$V_b$	m/min	0.5-3.0	1.5	Adjustable via VFD
Drainage length	$L_d$	m	3-6	4.5	Active gravity drainage zone
Mesh opening	-	mm	0.3-0.8	0.5	Belt screen opening

Output Parameters

Parameter	Unit	Description
Effective area	m²	Belt area (W × L)
Thickened sludge rate	m³/d	Cake discharge volume
Filtrate rate	m³/d	Drainage return to plant
Solids capture	%	Percentage to cake
Polymer consumption	kg/d	Daily polymer usage
Power consumption	kW	Belt drive and polymer system
Wash water usage	m³/d	Belt cleaning water

Configuration Guide

Step-by-Step Setup

1. Select Belt Width

   Standard sizes:

   • 0.5m: Small plants (<100 m³/d)
   • 1.0m: 100-300 m³/d
   • 1.5m: 300-600 m³/d
   • 2.0m: 600-1000 m³/d
   • 3.0m: 1000-2000 m³/d

2. Determine Belt Length

   • Total length: 8-12m (includes return, tensioning)
   • Active drainage zone: 3-6m
   • Flocculation/distribution zone: 1-2m
   • Cake discharge zone: 0.5-1m

3. Calculate Required Area

   Method 1: Hydraulic Loading

   For Q = 400 m³/d = 16.7 m³/h
   Use HL = 25 m³/m²·h
   A = 16.7/25 = 0.67 m²
   
   For 1.5m width:
   L_required = 0.67/1.5 = 0.45m → Use 4.5m drainage length
   Total belt area = 1.5 × 4.5 = 6.75 m²

   Method 2: Solids Loading

   Solids = 400 × 10 kg/m³ = 4,000 kg/d = 167 kg/h
   Use SL = 150 kg/m²·h
   A = 167/150 = 1.11 m²
   L_required = 1.11/1.5 = 0.74m → Use 4.5m (conservative)

4. Set Belt Speed

   • Low solids (0.5-0.8%): 2-3 m/min (shorter drainage time acceptable)
   • Medium solids (0.8-1.2%): 1.5-2 m/min
   • High solids (1.2-2.0%): 0.5-1.5 m/min (longer drainage time)

   Drainage time check:

   For Vb = 1.5 m/min, Ld = 4.5m:
   td = 4.5/1.5 = 3 minutes ✓ (typical 2-5 min)

5. Configure Polymer System

   • Inline static mixer (10-15 seconds contact)
   • Polymer type: Cationic, high MW (8-12M Da)
   • Makeup: 0.2-0.5% active solution
   • Dose: 5 kg/tonne DS (adjust via jar test)

6. Design Plows

   • Number: 3-6 plows along drainage length
   • Spacing: Every 0.5-1.0m
   • Purpose: Turn cake, release trapped water, prevent channeling
   • Type: Fixed angle or oscillating

7. Configure Wash System

   • High-pressure sprays: 3-6 bar
   • Location: Underside of return belt
   • Flow: 1-3 m³/h per meter width
   • Filtrate recycle or clean water

Typical Design Values

Sludge Type	Feed DS	Polymer	Cake DS	HL	SL	Capture	Belt Speed
Extended aer. WAS	0.6-0.8%	6-8 kg/t	4.0-5.0%	20-30 m³/m²·h	100-180 kg/m²·h	90-94%	2-3 m/min
Standard WAS	0.8-1.2%	4-6 kg/t	4.5-5.5%	22-35 m³/m²·h	120-220 kg/m²·h	92-96%	1.5-2.5 m/min
Mixed sludge	1.2-2.0%	3-5 kg/t	5.0-6.5%	25-40 m³/m²·h	150-250 kg/m²·h	93-97%	1-2 m/min

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	4.0-4.5	82-86%
Standard WAS	120-180	0.8-1.2	4.5-5.0	78-84%
Standard WAS	80-120	1.0-1.5	5.0-5.5	75-82%
Mixed sludge	100-150	1.5-2.0	5.5-6.5	70-77%

Solids Capture

• Typical: 92-96%
• With optimized polymer: 94-97%
• Poor performance: <90% (indicates issues)

Filtrate Quality

• TSS: 150-400 mg/L (acceptable for return to plant)
• Higher than rotary drum or DAF
• Biological sludge: 200-350 mg/L
• Mixed sludge: 150-300 mg/L

Troubleshooting

Issue	Cause	Solution
Low cake concentration	Polymer underdose	Increase dose, check polymer quality
	High belt speed (short drainage)	Reduce speed, increase retention
	Belt blinding	Improve wash system, check pressure
	Inadequate plowing	Adjust plow frequency/angle
High filtrate TSS	Polymer underdose	Increase dose gradually
	Poor flocculation	Check mixer, increase contact time
	Belt opening too large	Replace with finer mesh
	Polymer overdose	Reduce dose (deflocculation)
Belt tracking issues	Misalignment	Adjust tracking rollers
	Uneven cake loading	Improve feed distribution
	Worn tracking rollers	Replace rollers
Belt blinding	Inadequate wash pressure	Increase pressure, clean nozzles
	Greasy sludge	Hot water wash, improve screening
	Fine particles	Accept, increase wash frequency
Cake sticking to belt	Excessive polymer	Reduce dose
	Poor wash system	Improve spray coverage/pressure
	Belt material incompatible	Replace belt (polyester vs. polypropylene)
Variable performance	Feed flow fluctuations	Flow equalization, auto polymer control
	Temperature changes	Adjust polymer seasonally
Excessive wash water	Overcompensating for blinding	Optimize polymer first, then wash
Belt slipping	Low tension	Adjust tension, check drive
	Worn drive roller	Replace roller, check coating

Operation & Maintenance

Daily Checks

• Visual inspection of cake formation
• Monitor filtrate clarity
• Verify belt tracking (centered)
• Check wash water pressure
• Observe cake discharge
• Review polymer dosing

Weekly Maintenance

• Clean wash spray nozzles
• Check belt condition (wear, damage)
• Verify polymer makeup concentration
• Inspect plows (wear, alignment)
• Measure cake and filtrate DS
• Check belt tension

Monthly Tasks

• Full belt inspection (tears, blinding)
• Measure filtrate TSS (lab test)
• Lubricate drive bearings
• Check tracking roller alignment
• Inspect polymer static mixer
• Review performance trends

Annual Tasks

• Replace belt (typical life: 1-3 years)
• Overhaul drive system
• Replace worn plows
• Full wash system inspection
• Check structural frame
• Review performance vs. design

Advanced Features

Auto-Polymer Control

Filtrate turbidity-based:

• Online turbidity sensor
• PID control adjusts polymer dose
• Target: <400 NTU
• Reduces polymer use by 10-20%

Belt load-based:

• Measure belt sag (mat thickness)
• Increase polymer if mat too thin
• Reduce if mat too thick (over-conditioning)

Variable Speed Drive

Adjust belt speed based on:

• Feed solids concentration
• Cake dryness target
• Solids loading (peaks)

Plow Optimization

• Adjustable plow angles (30-60°)
• Frequency control (every 0.5-1.0m)
• Oscillating plows for better cake turning

Wash Water Recycling

• Collect wash filtrate
• Settle fines (small clarifier)
• Recycle supernatant for wash water
• Reduces fresh water consumption by 70-80%

Cost Comparison

Capital Cost

• 1.0m width: £100-150k
• 1.5m width: £150-225k
• 2.0m width: £200-300k

Operating Cost (per 1000 kg DS/d)

Item	Cost
Polymer (5 kg/t @ £2,000/t)	£10/d = £3,650/year
Power (2-3 kW × 24h @ £0.15/kWh)	£7-11/d = £2,800/year
Wash water (2 m³/h × 24h @ £1/m³)	£48/d = £17,500/year
Belt replacement (every 2 years)	£10,000/year amortized
Maintenance labor	1-2 hours/week
Total	~£34,000/year

Note: Wash water cost is significant. Recycling reduces to ~£3,500/year.

vs. Rotary Drum:

• Similar polymer usage
• Slightly lower capital cost
• Higher wash water usage
• Simpler maintenance

vs. DAF:

• Lower power (no compressor)
• Similar polymer usage
• Higher wash water requirement
• Lower capital cost

Worked Example

Given

• Standard WAS: 350 m³/d @ 1.0% DS (10 kg/m³)
• SVI: 130 mL/g
• Target cake: 5.0% DS
• Polymer: 5 kg/tonne DS

Solution

Step 1: Calculate solids load

Solids = 350 × 10 = 3,500 kg/d = 146 kg/h

Step 2: Select belt size

Use SL = 150 kg/m²·h
A_required = 146/150 = 0.97 m²

Use 1.5m width, 4.5m drainage length:
A = 1.5 × 4.5 = 6.75 m² ✓ (provides capacity margin)

Actual SL = 146/6.75 = 21.6 kg/m²·h (very conservative)

Step 3: Verify hydraulic loading

Q = 350 m³/d = 14.6 m³/h
HL = 14.6/6.75 = 2.16 m³/m²·h ✓ (within 15-40 range)

Step 4: Set belt speed

For 1.0% feed: Use 1.8 m/min
Drainage time = 4.5/1.8 = 2.5 minutes ✓

Step 5: Cake production

Solids to cake: 3,500 kg/d × 0.94 capture = 3,290 kg/d
Cake at 5.0% DS (50 kg/m³):
Q_cake = 3,290/50 = 65.8 m³/d
Volume reduction = (350-65.8)/350 = 81.2%

Step 6: Filtrate

Filtrate = 350 - 65.8 = 284.2 m³/d
Solids in filtrate = 3,500 × 0.06 = 210 kg/d
TSS_filtrate = 210,000/284.2 = 739 mg/L ✓ (acceptable)

Step 7: Polymer usage

Daily polymer: 3.5 tonnes × 5 kg/t = 17.5 kg/d
Annual: 6.4 tonnes/year
Cost @ £2,000/tonne: £12,800/year

Step 8: Operating cost

Polymer: £12,800/year
Power (2.5 kW × 24 × 365 × £0.15/kWh): £3,285/year
Wash water (2 m³/h × 24 × 365 × £1/m³): £17,520/year (or £3,500 if recycled)
Belt replacement: £10,000/2 = £5,000/year

Total with wash recycle: £24,585/year
Total without recycle: £38,605/year

Related Units

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

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed.
2. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
3. Ashbrook. Gravity Belt Thickener Design Manual.
4. HUBER Technology. Gravity Belt Thickener ROTAMAT® Ro9, Technical Guide.
5. CIWEM (2013). Sewage Sludge Treatment and Disposal, Manual of Practice.