Question 1

What components make up a DAF unit?

Accepted Answer

A complete DAF unit comprises four main sub-systems: (1) the recycle pump, which takes a portion of clarified effluent and pressurises it for air dissolution; (2) the pressure vessel or saturator, typically operating at 4–6 bar, where air is forced into solution in the recycle stream; (3) the flotation tank, where the pressurised recycle is released through nozzles, generating micro-bubbles that attach to suspended solids and FOG causing them to float; and (4) the surface scraper mechanism — a motor-driven flight or rotating paddle system — that continuously removes the floating sludge blanket into a collection trough. A sludge collection hopper at the base handles any settled dense solids purged periodically. Chemical dosing provisions (coagulant and flocculant) are typically integrated upstream.

Question 2

How is a DAF unit sized?

Accepted Answer

DAF unit sizing is primarily governed by the hydraulic surface loading rate, typically 3–6 m³/m²/hr for industrial effluents — this determines the required flotation tank surface area. Peak flow rate (not daily average) is used for sizing to handle discharge surges during clean-in-place (CIP) or shift changeovers. The Air-to-Solids (A:S) ratio — the mass of dissolved air released per unit mass of suspended solids — is then optimised, typically in the range 0.005–0.06 g air/g solids, to ensure adequate bubble attachment. Recycle ratio (the fraction of clarified effluent recycled through the saturator) is typically 15–30% of inlet flow. Coagulant and flocculant dosing upstream is also sized as part of the overall DAF system design.

Question 3

What is the difference between full-flow and recycle pressurisation DAF?

Accepted Answer

In full-flow pressurisation DAF, the entire influent stream is pumped through the saturator under pressure and then released into the flotation tank. This maximises bubble-particle contact but requires a large, robust pressurisation system and can damage fragile flocs formed upstream. In recycle pressurisation DAF — the more common configuration for industrial ETP — only 15–30% of the clarified effluent is recycled and pressurised. The raw influent (with pre-formed flocs) enters the flotation tank gently and meets the micro-bubble-laden recycle stream at the inlet nozzle manifold. Recycle pressurisation is gentler on floc structure, more energy efficient, and is the standard choice for food, dairy, and paper industry applications where floc integrity is important for high FOG/fibre removal.

Question 4

What chemicals are dosed ahead of a DAF unit?

Accepted Answer

Two chemical treatment stages are typically employed upstream of a DAF unit. First, a coagulant — most commonly aluminium sulphate (alum), ferric chloride, or a polyaluminium chloride (PAC) — is dosed and rapidly mixed into the influent stream to destabilise colloidal particles and emulsified FOG by neutralising their surface charge. This is followed by a flocculant, usually an anionic or cationic polyelectrolyte, which bridges and aggregates the destabilised particles into larger, more buoyant floc agglomerates. A flocculation contact time of 3–8 minutes in a gentle-mix flocculation chamber before the DAF inlet is important to allow floc growth without shear. Correct chemical selection and dose are determined by jar testing on the actual effluent.

Question 5

DAF vs lamella clarifier for high-FOG wastewater — which should I choose?

Accepted Answer

For high-FOG wastewater — such as from food processing, dairy, or edible oil operations — DAF is strongly preferred over a lamella clarifier. Lamella clarifiers rely on gravity settling and are effective for dense particles but cannot remove emulsified or free-floating oils and grease, which have a density close to or lower than water. DAF micro-bubbles actively attach to FOG droplets and lift them to the surface regardless of their density, achieving FOG removal efficiencies of 85–95% or higher with appropriate chemical dosing. Lamella clarifiers are better suited to high-TSS, low-FOG streams such as cooling water blowdown, mining wastewater, or coagulated inorganic slurries. If the effluent has both high TSS and high FOG, a DAF unit with integrated coagulation/flocculation is typically the most efficient single-stage solution.

Hydraulic Loading Rate	3–6 m/hr (surface overflow rate)
Recycle Ratio	15–30% of inlet flow
Saturator Pressure	4–6 bar
Tank Construction	SS 304/316 or MS epoxy-coated
Surface Scraper	Motor-driven flight scraper, variable speed drive
Dissolved Air Release	Micro-bubble size 40–70 µm
FOG Removal Efficiency	>90% from dairy/food effluent with chemical dosing
Sludge Concentration	2–5% TS from thickening duty

DAF Unit (Dissolved Air Flotation)

About DAF Unit (Dissolved Air Flotation)

Technical Specifications

How a DAF Unit Works

Coagulation / Flocculation Pre-treatment

Recycle Pressurisation

Micro-Bubble Release

Float Formation and Rise

Float Scraping and Sludge Collection

Key Advantages

Industries & Use Cases

Supply & Support

Related Equipment & Solutions

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