Sludge Dewatering

Mechanical removal of water from sludge to achieve 15-35% solids cake

Overview

Dewatering reduces sludge volume by 50-85% through mechanical separation. Common technologies include belt filter press, centrifuge, screw press, and filter press. Produces cake suitable for land application, composting, incineration, or landfilling. Polymer conditioning is typically required.

Theory and Equations

Basic Mode

$$m_{cake} = m_{feed} \times C_{feed} \times \frac{\eta}{C_{cake}}$$

$$Q_{cake} = \frac{m_{feed} \times C_{feed} \times \eta}{\rho \times C_{cake}}$$

Where:

• m = mass flow (kg/d)
• C = concentration (fraction)
• η = capture efficiency
• ρ = cake density (≈1000 kg/m³)

Advanced Mode

Solids Balance:

$$Q_{feed} \times X_{feed} = Q_{cake} \times X_{cake} + Q_{filtrate} \times X_{filtrate}$$

Capture Efficiency:

$$\eta = \frac{Q_{cake} \times X_{cake}}{Q_{feed} \times X_{feed}}$$

Polymer Dose Optimization:

$$\text{Dose} = f(\text{Zeta potential}, \text{CST}, \text{TTF})$$

Where CST = capillary suction time, TTF = time to filter

Model Parameters

Parameter	Belt Press	Centrifuge	Screw Press	Filter Press	Units
Feed Solids	4-8	2-6	2-6	15-25	%
Cake Solids	18-25	22-28	18-25	30-40	%
Capture	90-95	92-98	90-95	95-98	%
Polymer	3-10	2-8	3-8	1-5	kg/tonne DS
Capacity	200-500	50-200	100-250	Batch	kg DS/m·h

Design Criteria

Equipment	Hydraulic Loading	Solids Loading	Advantages
Belt Press	50-200 kg/m·h	200-600 kg/m·h	Low maintenance, simple
Centrifuge	-	1000-4000 kg/h	High capture, automated
Screw Press	-	100-500 kg/h	Low polymer, energy
Filter Press	Batch	Batch	High cake solids

Worked Example

Given: Centrifuge dewatering

• Feed: 100 m³/d at 3.5% solids
• Cake: 25% solids
• Capture: 95%

Calculate:

1. Feed solids: 100 × 0.035 × 1000 = 3,500 kg/d
2. Cake solids: 3,500 × 0.95 = 3,325 kg/d
3. Cake volume: 3,325 / (0.25 × 1000) = 13.3 m³/d
4. Volume reduction: (100 - 13.3) / 100 = 87%
5. Filtrate solids: 3,500 - 3,325 = 175 kg/d (5% loss)

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