A dissolved air flotation (DAF) system uses pressurized water containing what form of air to aid flotation?

• A. Dissolved air
• B. Entrained air
• C. Dissolved and entrained air
• D. Air injected

Answer: (C)

The main idea is that flotation in a DAF system relies on microbubbles that attach to suspended solids so they become buoyant and rise to the surface. The feed water is pressurized to saturate it with air, so when it is released to atmospheric pressure inside the flotation tank, most of the dissolved air comes out of solution as tiny bubbles. Those microbubbles do the lifting. In practice, there are also small air bubbles that are already present in the water—entrained air—due to mixing and previous piping effects. This entrained air adds to the bubble population and improves flotation efficiency. So, the water in a DAF system contains both dissolved air (which forms microbubbles upon pressure release) and entrained air (existing tiny bubbles), making the combination the best description of the air form aiding flotation.