Sludge separation: dissolved air flotation (DAF)

Purpose

• Removing floating substances

• Separation of suspended solids

Principle

Dissolved air flotation allows the flotation of suspended solids or oil on water by using fine air bubbles. As the name implies, these air bubbles are produced by dissolving air in water (under pressure). This water is subsequently mixed with the wastewater under atmospheric conditions. The dissolved air escapes and produces fine air bubbles which adhere to the sludge flocks. A scraper bridge removes the floating sludge.

Scheme

Applications

Flotation can be used as a stand-alone purification system, as a pretreatment step, as a sludge clarification step after a physicochemical treatment or as an effluent polishing step after a clarifier. A DAF is typically significantly more compact than a conventional clarifier and produces a sludge fraction with a higher dry-matter content (2 - 4 %). The disadvantages of a flotation compared to a clarifier are the higher energy consumption and the higher unit cost.

Example of realisation