DAF Thickener (Dissolved Air Flotation)

Pressurized flotation system using micro-bubbles to float and concentrate low-density biological sludge from 0.5-1.5% to 4-6% dry solids.

Overview

DAF thickeners are highly effective for waste activated sludge (WAS), particularly from extended aeration systems with high SVI (150-300 mL/g). Micro-bubbles attach to sludge flocs, floating them to the surface where they are mechanically skimmed off as thickened float.

When to Use

Extended aeration WAS (MCRT >15 days, SVI >150)
Membrane bioreactor (MBR) sludge
Biological nutrient removal (BNR) sludge with denitrification
Industrial biological sludge (low-density, filamentous)
Combined WAS + primary (with polymer)
When footprint is limited

Key Characteristics

Excellent for high-SVI sludge (where gravity thickening fails)
Compact footprint (high surface loading rate)
Requires polymer addition
Higher operating cost than gravity thickening
Consistent underflow (float) concentration
Clarified subnatant quality excellent (<50 mg/L TSS)

Operating Modes

Basic Mode

Fixed air-to-solids ratio (A/S)
Simple polymer dose calculation
Default hydraulic loading rate

Advanced Mode

Dynamic A/S optimization based on feed characteristics
Real-time polymer dose adjustment
Pressure control for bubble size optimization
Recycle ratio modeling
Float solids prediction based on SVI and polymer dose

Mathematical Model

Governing Equation - DAF Loading

The key parameter is air-to-solids ratio (A/S):

$$\frac{A}{S} = \frac{1.3 \times S_a \times (f \times P - 1)}{S_s}$$

Where:

$A/S$ = air-to-solids ratio (kg air/kg solids)
$S_a$ = air solubility at 20°C and 1 atm = 19 mg/L
$f$ = saturation efficiency (0.5-0.9, typically 0.7)
$P$ = absolute pressure (bar), typically 4-6 bar gauge = 5-7 bar absolute
$S_s$ = solids concentration in DAF tank (kg/m³)

Typical A/S targets:

WAS: 0.02-0.04 kg air/kg solids
Primary + WAS: 0.01-0.03 kg air/kg solids

Recycle Rate

The recycle flow provides dissolved air:

$$Q_R = Q \times \frac{(A/S) \times S_s}{1.3 \times S_a \times (f \times P - 1)}$$

Where:

$Q_R$ = recycle flow rate (m³/h)
$Q$ = feed flow rate (m³/h)
Typical recycle ratio: 30-100% of feed flow

Surface Loading Rate

$$SLR_{DAF} = \frac{Q}{A_{DAF}}$$

Where:

$SLR_{DAF}$ = surface loading rate (m³/m²·h)
Typical range: 5-15 m/h (much higher than gravity thickeners)

Solids Loading Rate

$$SLR_{solids} = \frac{Q \times TSS}{A_{DAF}}$$

Typical range: 50-150 kg/m²·d

Float Concentration

$$C_{float} = \frac{Q \times TSS_{feed} \times \eta}{Q_{float}}$$

Where:

$C_{float}$ = float solids concentration (%)
$\eta$ = capture efficiency (0.95-0.98)
$Q_{float}$ = float withdrawal rate (m³/d)

Polymer Dose

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

Where:

$k_{poly}$ = specific polymer demand (kg/kg DS), typically 0.002-0.006
Typical: 2-6 kg polymer per tonne dry solids

Design Parameters

Input Parameters

Parameter	Symbol	Unit	Range	Default	Description
Feed concentration	$TSS$	mg/L	3,000-15,000	8,000	WAS solids concentration
SVI	-	mL/g	100-300	150	Sludge volume index
Target float DS	$C_f$	%	3-6	4.5	Desired thickened concentration
Operating pressure	$P$	bar(g)	4-6	5	Saturation pressure
Air-to-solids ratio	A/S	kg/kg	0.01-0.05	0.03	Design A/S ratio
Polymer dose	$k_{poly}$	kg/tDS	2-8	4	Specific polymer demand
Surface loading rate	SLR	m/h	5-12	8	Hydraulic loading
Recycle ratio	$R$	%	30-100	60	Percentage of feed flow

Output Parameters

Parameter	Unit	Description
Required area	m²	DAF tank surface area
Tank dimensions	m	Length × width × depth
Recycle flow	m³/h	Pressurized recycle rate
Float production	m³/d	Thickened sludge (float) volume
Subnatant flow	m³/d	Clarified liquid return
Solids capture	%	Percentage to float
Polymer consumption	kg/d	Daily polymer usage
Air compressor duty	kW	Compressor power requirement

Configuration Guide

Step-by-Step Setup

Characterize Feed Sludge
- Measure SVI (critical for DAF)
- Determine solids concentration
- Identify sludge type (WAS, MBR, industrial)
- Run jar test with polymer to estimate dose
Select Operating Pressure
- Standard: 5 bar gauge (6 bar absolute)
- Higher pressure (6 bar): Smaller bubble size, better for tough sludge
- Lower pressure (4 bar): Energy saving, adequate for easy-to-float sludge
Determine Air-to-Solids Ratio
- Extended aeration WAS (SVI 150-200): 0.03-0.04
- Standard WAS (SVI 100-150): 0.02-0.03
- Primary + WAS: 0.01-0.02

Calculate DAF Area

Method 1: Surface Loading Rate

For Q = 500 m³/d = 20.8 m³/h
Use SLR = 8 m/h
A = Q/SLR = 20.8/8 = 2.6 m²
Use 3m × 2m = 6 m² (conservative for peaking)

Method 2: Solids Loading

Solids load = 500 × 8 kg/m³ = 4,000 kg/d
SLR_solids = 100 kg/m²·d
A = 4,000/100 = 40 m²

Typically method 1 controls for DAF.

Calculate Recycle Rate

For A/S = 0.03, P = 6 bar abs, f = 0.7, Ss = 8 kg/m³
Air supplied per m³ recycle:
Ca = 1.3 × 19 × (0.7×6 - 1) = 77.9 mg/L = 0.078 kg/m³

Air required: 0.03 kg/kg × 8 kg/m³ = 0.24 kg/m³ feed
Recycle ratio: 0.24/0.078 = 3.1 m³ recycle / m³ feed

This is too high! Adjust:
- Increase pressure to 6 bar → Ca = 91 mg/L → R = 2.6 → Still high
- Reduce A/S to 0.02 → R = 1.7 → More reasonable
- Or use higher recycle pressure (7 bar)

Practical: Use R = 0.6 (60% recycle) with A/S = 0.025
QR = 500 × 0.6 = 300 m³/d = 12.5 m³/h

Size Polymer System
- Polymer type: Cationic, high molecular weight (8-12M Da)
- Dose: 4 kg/tonne DS
- For 4,000 kg DS/d: 16 kg/d polymer (active)
- Makeup system: 0.2-0.5% solution
- Aging: 30-60 minutes
Design Float Removal
- Beach scraper or flight scraper
- Scraper speed: 1-3 m/min
- Float depth: 100-300mm before removal
- Continuous or intermittent removal

Typical Design Values

Sludge Type	SVI	Feed DS	A/S	Polymer	Float DS	SLR	Capture
Extended aer. WAS	180-250	0.6-1.0%	0.03-0.04	5-8 kg/t	4-5%	6-10 m/h	95-98%
Standard WAS	100-150	0.8-1.5%	0.02-0.03	3-5 kg/t	4.5-5.5%	8-12 m/h	96-99%
MBR sludge	120-180	1.0-1.5%	0.025-0.035	4-6 kg/t	4-5%	7-11 m/h	95-98%
Primary + WAS	80-120	1.5-2.5%	0.015-0.025	2-4 kg/t	5-6%	9-13 m/h	97-99%

Performance Criteria

Achievable Float Concentrations

Feed Sludge	SVI (mL/g)	Feed DS (%)	Float DS (%)	Volume Reduction (%)
Extended aer. WAS	200-250	0.6-0.8	4.0-4.5	83-87%
Extended aer. WAS	150-200	0.8-1.0	4.5-5.0	80-85%
Standard WAS	100-150	1.0-1.5	5.0-5.5	75-82%
MBR sludge	120-180	1.0-1.5	4.5-5.0	75-82%
Primary + WAS	80-120	2.0-2.5	5.5-6.5	65-75%

Solids Capture

Typical: 95-98%
With optimized polymer: 97-99%
Poor performance: <90% (indicates issues)

Subnatant Quality

TSS: 20-100 mg/L (excellent, suitable for direct return)
Lower than gravity thickener overflow
Biological sludge: 30-50 mg/L typical
Primary + biological: 50-100 mg/L

Troubleshooting

Issue	Cause	Solution
Low float concentration	Excessive float withdrawal	Reduce float removal rate
	Insufficient polymer	Increase dose, check polymer quality
	Low A/S ratio	Increase recycle flow or pressure
	Poor polymer mixing	Check static mixer, increase contact time
High subnatant TSS	Polymer underdose	Increase polymer dose
	Hydraulic overload	Reduce feed rate, add capacity
	Poor air distribution	Check saturator, nozzle cleanliness
	Polymer overdose (deflocculation)	Reduce dose, check mixing intensity
Foaming	Excessive air	Reduce recycle or pressure
	Surfactants in feed	Defoamer spray, reduce air
	Poor float removal	Increase scraper speed/frequency
Low air dissolution	Saturator issues	Check saturation pressure, clean nozzles
	Compressor underperformance	Verify compressor output, check for leaks
	Temperature too high	Cool recycle, check heat exchangers
Brown/black subnatant	Sludge age too high (septicity)	Reduce SRT in aeration, increase withdrawal
	Poor float removal (re-sinking)	Increase scraper frequency
Scraper jamming	Float too thick	Increase withdrawal rate
	Rag/debris accumulation	Screen feed, manual cleaning
Variable performance	Feed flow variation	Flow equalization, automate polymer dosing
	Polymer quality	Check polymer batch, aging time

Operation & Maintenance

Daily Checks

Visual inspection of float formation
Monitor subnatant clarity
Verify recycle pressure (5-6 bar)
Check polymer dosing pump operation
Observe scraper movement
Review float withdrawal rate

Weekly Maintenance

Clean saturator nozzles
Check air compressor operation
Verify polymer makeup concentration
Inspect float beach/collection
Review polymer consumption vs. solids
Calibrate online instruments (pressure, flow)

Monthly Tasks

Drain and clean DAF tank
Inspect scraper mechanism
Check saturator internals
Calibrate polymer pump
Measure float and subnatant TSS (lab)
Review energy consumption (compressor)

Annual Tasks

Overhaul air compressor
Replace saturator nozzles
Full scraper and drive inspection
Clean/replace polymer static mixer
Pressure vessel inspection (saturator)
Review performance vs. design

Advanced Features

Auto-Polymer Dosing

Streaming current detector (SCD):

Measures zeta potential in real-time
Targets optimal polymer dose (neutral charge)
Can reduce polymer consumption by 10-30%

White Water Recycle

Using clarified subnatant as pressurized recycle:

Saves clean water
Improves consistency
Possible if subnatant TSS <50 mg/L

Variable Pressure Control

Adjust pressure based on feed characteristics:

High SVI: Increase to 6 bar for smaller bubbles
Low SVI: Reduce to 4 bar for energy saving

Coagulant Addition

Adding ferric or alum (0.5-2 kg/tonne DS):

Improves capture for difficult sludge
Reduces polymer consumption
Increases inorganic content of float

Energy Considerations

Power Consumption

Air compressor: $$P_{comp} = \frac{Q_R \times \Delta P}{\eta_{comp}}$$

Where:

$P_{comp}$ = compressor power (kW)
$Q_R$ = recycle flow (m³/h)
$\Delta P$ = pressure increase (bar)
$\eta_{comp}$ = compressor efficiency (0.6-0.7)

Typical:

0.5-1.5 kWh/m³ feed treated
For 500 m³/d: 10-30 kW average

Cost Comparison (Operating)

DAF vs. Gravity Thickening:

Item	Gravity Thickener	DAF Thickener
Power	1-2 kW (pickets)	10-30 kW (compressor)
Polymer	None	£50-150/d (for 4,000 kg DS/d)
O&M labor	Low	Moderate
Footprint	250 m²	6-20 m²
Capital cost	£200k	£500k-1M

When DAF makes sense:

High-SVI sludge (gravity fails)
Limited footprint
Excellent subnatant quality required
Willingness to pay for polymer/energy

Worked Example

Given

Extended aeration WAS: 400 m³/d @ 0.8% DS (8 kg/m³)
SVI: 200 mL/g
Target float: 4.5% DS
Operating pressure: 5 bar gauge (6 bar absolute)

Solution

Step 1: Select A/S ratio

High SVI → Use A/S = 0.03 kg air/kg solids

Step 2: Calculate recycle ratio

Sa = 19 mg/L, f = 0.7, P = 6 bar abs
Air dissolved per m³ recycle:
Ca = 1.3 × 19 × (0.7×6 - 1) = 77.9 mg/L = 0.078 kg/m³

Air needed: 0.03 kg/kg × 8 kg/m³ feed = 0.24 kg/m³ feed
Recycle ratio: R = 0.24/0.078 = 3.08

This is too high! Reduce A/S to 0.02:
Air needed: 0.02 × 8 = 0.16 kg/m³
R = 0.16/0.078 = 2.05 → Still high

Use higher pressure (7 bar abs):
Ca = 1.3 × 19 × (0.7×7 - 1) = 103 mg/L
R = 0.16/0.103 = 1.55

Or use R = 0.8 (80%) and accept A/S = 0.025

Use practical: R = 0.7 (70%), A/S = 0.027

QR = 400 × 0.7 = 280 m³/d = 11.7 m³/h

Step 3: Calculate DAF area

Use SLR = 8 m/h
Q_total = 400 + 280 = 680 m³/d = 28.3 m³/h (feed + recycle)
A = 28.3/8 = 3.5 m²

Use 4m × 2m = 8 m² (allows for peaking and redundancy)
Actual SLR = 28.3/8 = 3.5 m/h ✓ (conservative)

Step 4: Float production

Solids in: 400 × 8 = 3,200 kg/d
Capture: 96%
Solids to float: 3,200 × 0.96 = 3,072 kg/d
Float at 4.5% DS (45 kg/m³):
Q_float = 3,072/45 = 68 m³/d
Volume reduction: (400-68)/400 = 83%

Step 5: Polymer dose

Use 5 kg/tonne DS
Daily polymer: 3.2 × 5 = 16 kg/d
Annual polymer: 5.8 tonnes/year
Cost @ £2,000/tonne: £11,600/year

Step 6: Compressor sizing

QR = 280 m³/d = 11.7 m³/h
ΔP = 6 bar = 600 kPa
Power = (11.7 × 600)/(3600 × 0.65) = 3.0 kW
Use 5 kW motor (safety factor)
Energy: 3 kW × 24 h = 72 kWh/d

Gravity-Thickener - Alternative for low-SVI sludge
Rotary-Drum-Thickener - Alternative mechanical thickening
Gravity-Belt-Thickener - Alternative for WAS
Anaerobic-Digester - Downstream stabilization
Centrifuge - Downstream dewatering

References

WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 17.
Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1610-1625.
Edzwald, J.K. (2010). Dissolved Air Flotation and Me. Water Research, 44(7), 2077-2106.
Bratby, J. (2016). Coagulation and Flocculation in Water and Wastewater Treatment, 3rd Ed.
Haarhoff, J. & Edzwald, J.K. (2004). "Dissolved Air Flotation Modeling." J. Water Supply Research & Technology - AQUA.
CIWEM (2013). Sewage Sludge Treatment and Disposal, Manual of Practice.

Clarification

Disinfection

Filtration

Membranes

DAF Thickener (Dissolved Air Flotation) ​

Overview ​

When to Use ​

Key Characteristics ​

Operating Modes ​

Basic Mode ​

Advanced Mode ​

Mathematical Model ​

Governing Equation - DAF Loading ​

Recycle Rate ​

Surface Loading Rate ​

Solids Loading Rate ​

Float Concentration ​

Polymer Dose ​

Design Parameters ​

Input Parameters ​

Output Parameters ​

Configuration Guide ​

Step-by-Step Setup ​

Typical Design Values ​

Performance Criteria ​

Achievable Float Concentrations ​

Solids Capture ​

Subnatant Quality ​

Troubleshooting ​

Operation & Maintenance ​

Daily Checks ​

Weekly Maintenance ​

Monthly Tasks ​

Annual Tasks ​

Advanced Features ​

Auto-Polymer Dosing ​

White Water Recycle ​

Variable Pressure Control ​

Coagulant Addition ​

Energy Considerations ​

Power Consumption ​

Cost Comparison (Operating) ​

Worked Example ​

Given ​

Solution ​

Related Units ​

References ​

DAF Thickener (Dissolved Air Flotation)

Overview

When to Use

Key Characteristics

Operating Modes

Basic Mode

Advanced Mode

Mathematical Model

Governing Equation - DAF Loading

Recycle Rate

Surface Loading Rate

Solids Loading Rate

Float Concentration

Polymer Dose

Design Parameters

Input Parameters

Output Parameters

Configuration Guide

Step-by-Step Setup

Typical Design Values

Performance Criteria

Achievable Float Concentrations

Solids Capture

Subnatant Quality

Troubleshooting

Operation & Maintenance

Daily Checks

Weekly Maintenance

Monthly Tasks

Annual Tasks

Advanced Features

Auto-Polymer Dosing

White Water Recycle

Variable Pressure Control

Coagulant Addition

Energy Considerations

Power Consumption

Cost Comparison (Operating)

Worked Example

Given

Solution

Related Units

References