Gravity Thickener

Circular settling tank using gravity sedimentation and vertical picket rakes to concentrate sludge from 2-4% to 4-8% dry solids.

Overview

Gravity thickeners are the most economical method for thickening primary sludge and low-MCRT waste activated sludge (WAS). They operate on the principle of zone settling and compression, concentrating solids by allowing particles to settle while slowly raking settled sludge to a central underflow.

When to Use

• Primary sludge thickening (highly effective, 6-8% DS achievable)
• WAS from low F/M systems (MCRT <8 days)
• Combined primary + WAS (blended feed)
• Small to medium plants where footprint allows
• When polymer-free thickening is desired

Key Characteristics

• Simple, reliable operation
• Low operating cost (no polymer)
• Good for primary sludge (4-8% DS achievable)
• Poor for extended aeration WAS (2-3% DS only)
• Large footprint requirement
• Potential for septicity and odor

Operating Modes

Basic Mode

• Fixed solids flux rate based on sludge type
• Simple overflow rate criterion
• Default underflow concentration targets

Advanced Mode

• State point analysis using Kynch theory
• Dynamic flux curve based on SVI and sludge characteristics
• Compression zone modeling
• Real-time blanket level monitoring
• Optimized picket speed control

Mathematical Model

Governing Equation - Solids Flux Theory

The limiting solids flux determines thickener area:

$$G_L = V_s(X) \times X + V_u \times X$$

Where:

• $G_L$ = limiting solids flux (kg/m²·d)
• $V_s(X)$ = settling velocity as function of concentration (m/h)
• $X$ = solids concentration (kg/m³)
• $V_u$ = underflow velocity (m/h)

Vesilind Settling Velocity

$$V_s = V_0 \times e^{-kX}$$

Where:

• $V_0$ = maximum settling velocity (m/h), typically 5-15 m/h
• $k$ = settleability constant (m³/kg), related to SVI
• $X$ = solids concentration (kg/m³)

Relationship to SVI: $$k = \frac{0.35}{SVI}$$

Where SVI = Sludge Volume Index (mL/g)

State Point Analysis

The state point $(X_f, G_f)$ is where feed concentration meets the required flux:

$$A_{required} = \frac{Q \times X_f}{G_L}$$

Where:

• $A_{required}$ = thickener surface area (m²)
• $Q$ = feed flow rate (m³/d)
• $X_f$ = feed solids concentration (kg/m³)

Underflow Concentration

For a given underflow rate $Q_u$:

$$X_u = \frac{Q \times X_f}{Q_u}$$

Practical maximum $X_u$:

• Primary sludge: 60-80 kg/m³ (6-8% DS)
• Low MCRT WAS: 40-50 kg/m³ (4-5% DS)
• Extended aeration WAS: 20-30 kg/m³ (2-3% DS)

Solids Loading Rate

$$SLR = \frac{Q \times TSS_{in}}{A}$$

Where:

• $SLR$ = solids loading rate (kg/m²·d)
• Typical range: 30-60 kg/m²·d for primary, 15-30 kg/m²·d for WAS

Surface Overflow Rate

$$SOR = \frac{Q}{A}$$

Typical range: 15-30 m³/m²·d (low for WAS, high for primary)

Design Parameters

Input Parameters

Parameter	Symbol	Unit	Range	Default	Description
Sludge type	-	-	Primary/WAS/Mixed	Primary	Feed sludge characteristics
Feed concentration	$X_f$	% DS	0.5-4	2.0	Inlet solids concentration
Target underflow	$X_u$	% DS	3-8	5.0	Desired thickened concentration
SVI	-	mL/g	50-250	120	Sludge volume index
Solids flux limit	$G_L$	kg/m²·d	20-80	40	Design limiting flux
Picket speed	$\omega$	rpm	0.01-0.05	0.02	Rake rotation speed
Sidewater depth	$H$	m	2.5-4.5	3.5	Tank depth

Output Parameters

Parameter	Unit	Description
Required area	m²	Surface area for design flux
Tank diameter	m	Circular tank diameter
Underflow rate	m³/d	Thickened sludge withdrawal
Overflow rate	m³/d	Clarified liquid (return to plant)
Solids capture	%	Percentage of solids in underflow
Blanket depth	m	Settled sludge layer depth
Picket torque	kW	Rake drive power requirement

Configuration Guide

Step-by-Step Setup

1. Characterize Feed Sludge

   • Measure SVI (settling test)
   • Determine solids concentration
   • Identify sludge type (primary, WAS, mixed)
   • Calculate Vesilind parameters: $V_0$ and $k$

2. Determine Target Underflow Concentration

   • Primary sludge: 6-8% DS
   • WAS (low MCRT): 4-5% DS
   • WAS (extended aeration): 2-3% DS
   • Mixed: 4-6% DS

3. Calculate Required Area

   Method 1: Solids Flux (preferred)

   For primary sludge @ SVI=100:
   k = 0.35/100 = 0.0035 m³/kg
   V₀ = 10 m/h
   At X = 40 kg/m³: Vs = 10 × e^(-0.0035×40) = 7.4 m/h
   G = 7.4 × 40 = 296 kg/m²·d
   For Q=500 m³/d, Xf=20 kg/m³:
   A = (500 × 20)/296 = 33.8 m² → Use 40 m² (7.1m diameter)

   Method 2: Solids Loading Rate

   SLR = 50 kg/m²·d (primary sludge)
   Solids load = 500 × 20 = 10,000 kg/d
   A = 10,000/50 = 200 m²

4. Select Tank Dimensions

   • Diameter: $D = \sqrt{4A/\pi}$
   • Depth: 3-4m (2.5m minimum for blanket control)
   • Freeboard: 0.3-0.5m
   • Bottom slope: 1:12 to center (or flat bottom)

5. Design Picket Mechanism

   • Picket arms: 2-4 per radius
   • Picket height: 200-300mm
   • Picket spacing: 300-600mm vertically
   • Rotation: 0.01-0.05 rpm (1-3 hours per revolution)
   • Torque overload protection

6. Configure Underflow Control

   • Time-based withdrawal (intermittent or continuous)
   • Blanket level sensor (ultrasonic or conductivity)
   • Variable speed pumps
   • Target blanket depth: 40-60% of sidewater depth

Typical Design Values

Sludge Type	SLR (kg/m²·d)	SOR (m³/m²·d)	Underflow DS (%)	Depth (m)	Picket Speed (rpm)
Primary	40-60	20-30	6-8	3.0-3.5	0.02-0.03
Low MCRT WAS	20-40	12-20	4-5	3.5-4.0	0.01-0.02
Extended aeration WAS	10-25	8-15	2-3	3.5-4.5	0.01-0.02
Mixed (50/50)	30-50	15-25	4-6	3.0-4.0	0.02-0.03

Performance Criteria

Achievable Underflow Concentrations

Feed Sludge	SVI (mL/g)	Feed DS (%)	Underflow DS (%)	Volume Reduction (%)
Primary	50-100	2-4	6-8	70-85%
Primary	100-150	2-4	5-7	65-80%
WAS (MCRT 5-8d)	80-120	0.8-1.2	4-5	75-85%
WAS (MCRT 10-15d)	120-180	0.6-1.0	3-4	70-80%
WAS (extended aer.)	150-250	0.5-0.8	2-3	60-75%
Mixed	80-130	1.5-2.5	4-6	70-80%

Solids Capture

• Typical: 95-98%
• With good blanket control: >98%
• Poor settling sludge: 90-95%

Overflow Quality

• TSS: 100-500 mg/L (acceptable for return to head of plant)
• Higher during upset or blanket loss
• Biological sludge tends to have higher overflow TSS than primary

Troubleshooting

Issue	Cause	Solution
Low underflow concentration	Excessive withdrawal rate	Reduce underflow pumping, increase blanket level
	Poor sludge settleability (high SVI)	Reduce SRT, check for bulking, add polymer
	Inadequate retention time	Reduce feed rate, check for hydraulic overload
High overflow TSS	Hydraulic overload (high SOR)	Reduce feed, add capacity
	Blanket loss	Increase underflow rate, check picket operation
	Rising sludge (septicity)	Increase underflow frequency, reduce blanket depth
Picket torque alarm	Compacted blanket	Increase underflow withdrawal immediately
	Mechanical binding	Stop, inspect, clear obstruction
	Grit accumulation	Drain and clean tank
Odor issues	Septicity in blanket	Increase underflow frequency, reduce retention
	Dead zones	Check picket coverage, ensure slow rotation
	Anaerobic conditions	Aeration not recommended (disrupts settling)
Poor primary thickening	Grease/FOG coating solids	Pre-screen or skim, hot water washing
	Lightweight particles	Add coagulant or switch to DAF
Foaming	Biological activity in blanket	Increase withdrawal, spray water on surface
	Surfactants	Defoamer addition, reduce retention

Operation & Maintenance

Daily Checks

• Visual inspection of overflow clarity
• Monitor blanket level (ultrasonic or sludge judge)
• Verify picket rotation (2-3 hours per revolution)
• Check underflow concentration (jar test or online meter)
• Observe torque indicator
• Listen for unusual noises

Weekly Maintenance

• Clean overflow weir
• Check underflow pump operation
• Verify blanket level sensors
• Inspect feed distribution
• Review withdrawal schedule effectiveness

Monthly Tasks

• Measure SVI of feed and underflow
• Calibrate blanket level instruments
• Inspect picket mechanism (underwater camera or drain)
• Check for grit accumulation
• Review torque trends
• Verify overflow return TSS with lab test

Annual Tasks

• Drain and inspect tank interior
• Full picket inspection and repair
• Drive mechanism overhaul
• Repaint exposed surfaces
• Clean influent and effluent piping
• Review performance vs. design

Advanced Features

Dynamic Blanket Control

Level-based withdrawal: $$Q_u = K \times (h_{blanket} - h_{target})$$

Where:

• $Q_u$ = underflow rate (m³/h)
• $K$ = control gain
• $h_{blanket}$ = measured blanket level (m)
• $h_{target}$ = setpoint (typically 1.5-2.0m)

Polymer Addition (Hybrid Thickening)

Adding low-dose polymer (0.5-2 kg/tonne DS) can:

• Increase underflow concentration by 1-2%
• Improve capture rate to >98%
• Allow higher solids loading (+20-30%)

Cost-benefit: Polymer cost vs. downstream volume reduction

Temperature Effects

$$V_s(T) = V_s(20°C) \times 1.02^{(T-20)}$$

• Cold weather (5°C): 30% reduction in settling velocity
• Warm weather (25°C): 10% increase
• Design for worst case (winter conditions)

Flocculation Zone

Some designs include gentle mixing zone ahead of thickener:

• Low-intensity agitation (G = 20-50 s⁻¹)
• 10-20 minute retention
• Improves floc size and settling
• Useful for poor settling sludge

Worked Example

Given

• Primary sludge: 800 m³/d @ 3% DS
• SVI: 90 mL/g
• Target underflow: 6% DS
• Temperature: 15°C

Solution

Step 1: Calculate Vesilind parameters

k = 0.35/90 = 0.00389 m³/kg
V₀ = 12 m/h (typical for primary)

Step 2: Construct flux curve

At X = 30 kg/m³: Vs = 12 × e^(-0.00389×30) = 3.68 m/h → G = 110 kg/m²·d
At X = 40 kg/m³: Vs = 12 × e^(-0.00389×40) = 2.51 m/h → G = 100 kg/m²·d
At X = 50 kg/m³: Vs = 12 × e^(-0.00389×50) = 1.71 m/h → G = 86 kg/m²·d
At X = 60 kg/m³: Vs = 12 × e^(-0.00389×60) = 1.17 m/h → G = 70 kg/m²·d (underflow target)

Step 3: Determine limiting flux

Feed: Xf = 30 kg/m³
State point analysis → G_L ≈ 100 kg/m²·d (conservative)

Step 4: Calculate area

A = (Q × Xf)/G_L = (800 × 30)/100 = 240 m²
D = √(4×240/π) = 17.5m → Use 18m diameter (254 m²)

Step 5: Verify SLR and SOR

SLR = (800 × 30)/254 = 94.5 kg/m²·d ✗ Too high!
Revise to D = 20m (314 m²)
SLR = 76.4 kg/m²·d ✓ (within 40-60 range, borderline)
SOR = 800/314 = 2.55 m/d ✓

Step 6: Underflow rate

Solids balance: 800 × 30 = Qu × 60
Qu = 400 m³/d
Volume reduction = (800-400)/800 = 50%

Step 7: Picket design

Depth: 3.5m
Picket speed: 0.02 rpm
Torque (empirical): 0.5 kW for 20m diameter

Related Units

• DAF-Thickener - Alternative for high-SVI WAS
• Rotary-Drum-Thickener - Alternative mechanical thickening
• Gravity-Belt-Thickener - Alternative for WAS
• Anaerobic-Digester - Downstream stabilization
• Belt-Press - Downstream dewatering

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 17.
2. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1589-1610.
3. Dick, R.I. (1970). "Role of Activated Sludge Final Settling Tanks." J. San. Eng. Div., ASCE.
4. Vesilind, P.A. (1968). "Design of Prototype Thickeners from Batch Settling Tests." Water & Sewage Works.
5. Ekama, G.A. et al. (1997). "Secondary Settling Tanks: Theory, Design and Operation." Water Research.
6. CIWEM (2013). Sewage Sludge Treatment and Disposal, Manual of Practice.