Dissolved Air Flotation (DAF): Removing Oil, Grease and Light Solids from Effluent

Conventional clarifiers rely on gravity, letting heavy particles sink to the bottom. But many industrial effluents contain material that is light, oily or fatty and simply will not settle. Fats from a dairy, oil from a food processing unit or fibres from a paper mill can float, drift and carry over into downstream treatment. Dissolved air flotation solves this by doing the opposite of settling: it lifts contaminants to the surface.

How DAF Works

The principle is elegant. A portion of treated effluent is saturated with air under pressure, typically in the range of four to six bar. When this pressurised water is released into the flotation tank at atmospheric pressure, the dissolved air comes out of solution as millions of tiny bubbles, usually tens of microns in size.

These microbubbles attach to suspended particles, oil droplets and flocs, making them buoyant. The particles rise to the surface and form a floating sludge layer, which is removed by a skimmer. Clarified water exits from the bottom of the tank.

The Role of Chemical Conditioning

DAF performance improves dramatically with proper chemical treatment. Coagulants destabilise fine particles and break oil emulsions. Flocculants, usually polymers, then bind them into larger flocs that capture bubbles more effectively. Many DAF units include a flocculation tube or tank ahead of the flotation zone for this purpose. Jar testing is the simplest way to choose the right chemicals and doses.

Key Components

• Saturation system: recycle pump, air compressor and saturation vessel
• Release valves or nozzles: where pressure drops and microbubbles form
• Flotation tank: with sufficient surface area for particles to rise
• Surface skimmer: removes the float sludge
• Bottom sludge removal: for any heavier solids that do settle

Where DAF Excels

Food and Beverage Processing

Effluent from dairies, meat processing, edible oil refineries and snack manufacturers often carries high fat, oil and grease. DAF removes a large portion of this load before biological treatment, protecting the biological system from fat accumulation.

Paper and Pulp

Fibre recovery and clarification of process water are classic DAF applications.

Automobile and Engineering Workshops

Oily wash-down water benefits from DAF, often combined with oil skimmers.

Pre-treatment for Membranes

DAF is sometimes used ahead of membrane systems for surface water containing algae, which float rather than settle.

Advantages Over Gravity Clarification

• Effective for light and oily material
• Smaller footprint for the same flow
• Float sludge is often thicker than settled sludge
• Fast start-up compared with large settling tanks

Limitations to Consider

• Requires power for the recycle pump and compressor
• Needs chemical dosing for good results
• Release valves must be maintained to avoid clogging
• Operators need training to manage float removal and chemical dosing

Design Considerations

Key design parameters include hydraulic loading rate, solids loading rate, recycle ratio and air-to-solids ratio. These depend on effluent characteristics, so a proper analysis, ideally including oil and grease, TSS and COD, should come before sizing. Variable flow should be buffered with an equalisation tank so the DAF receives a steady feed.

Fitting DAF into the ETP

DAF usually sits after screening and an oil and grease trap and before biological treatment. By taking out fats and suspended solids early, it reduces the organic load, prevents scum problems in aeration tanks and keeps the downstream process stable. For many food industry plants, a well-run DAF is the difference between an ETP that complies and one that constantly struggles.

Need help with your water or wastewater project? Aguapuro Equipments designs, supplies and services water treatment, effluent treatment and bottling solutions. Send us an enquiry or contact our team to discuss your requirements.