Sludge Thickening

Increase sludge solids concentration to reduce volume for downstream processing

Overview

Sludge thickening increases solids concentration from 0.5-4% to 4-10%, reducing volume by 50-90%. Common methods include gravity thickening, dissolved air flotation (DAF), gravity belt thickening, and centrifuge thickening. Benefits include reduced tankage, pumping, and chemical costs for downstream processes.

Theory and Equations

Basic Mode

$$C_{out} = C_{in} \times \text{Concentration Factor}$$

$$Q_{out} = \frac{Q_{in} \times C_{in}}{C_{out}}$$

Typical concentration factors:

• Gravity thickening (WAS): 2-3× (2% → 4-6%)
• DAF (WAS): 3-5× (1% → 3-5%)
• Gravity belt: 4-6× (0.5% → 2-3%)
• Centrifuge: 5-8× (1% → 5-8%)

Advanced Mode

Solids Flux for Gravity Thickening:

$$G = v_s(X) \times X + v_h \times X$$

$$v_s(X) = v_0 \times e^{-k \times X}$$

Where:

• G = solids flux (kg/m²·h)
• v_s = settling velocity (m/h)
• v_h = hydraulic velocity (m/h)
• X = solids concentration (kg/m³)

DAF Float Rate:

$$F = \frac{Q \times (X_{in} - X_{out}) \times \eta}{A}$$

Where:

• F = float solids loading (kg/m²·h)
• η = capture efficiency (0.90-0.98)
• A = tank surface area (m²)

Model Parameters

Parameter	Gravity	DAF	Centrifuge	Units
Feed Concentration	0.5-3.0	0.5-1.5	0.5-2.0	%
Product Concentration	3-6	3-5	5-8	%
Solids Loading	30-60	40-80	100-300	kg/m²·d
Capture Efficiency	85-95	90-98	90-95	%
Polymer Dose	0-3	2-8	3-10	kg/tonne DS

Design Criteria

Type	Loading	Area per Unit	Depth	Typical Use
Gravity	30-60 kg/m²·d	50-300 m²	3-4 m	Primary, mixed
DAF	40-100 kg/m²·d	10-100 m²	2-3 m	WAS
Gravity Belt	-	-	-	Pre-thickening
Centrifuge	-	-	-	All sludge types

Worked Example

Given: Gravity thickening of WAS

• Flow: 100 m³/d at 1.5% solids
• Target: 4.5% solids
• Capture: 92%

Calculate:

1. Solids mass in: 100 × 0.015 × 1000 = 1,500 kg/d
2. Solids captured: 1,500 × 0.92 = 1,380 kg/d
3. Thickened volume: 1,380 / (0.045 × 1000) = 30.7 m³/d
4. Volume reduction: (100 - 30.7) / 100 = 69%

References

1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Edition
2. WEF (2018). Design of Water Resource Recovery Facilities, MOP 8

Change Log

Date	Version	Changes
2025-12-30	1.0	Initial documentation