ADV measurements in a flotation tank with bubble containing water

Dissolved Air Flotation (DAF) is a method for separating suspended, microscopic particles in water and wastewater treatment. The basic idea is to generate huge amounts of microscopic bubbles, which should attach to the particles whereby less dense aggregates are obtained, rising to the water surface. The flow structure in a flotation tank is important for the separation efficiency. ADV measurements have been performed in a DAF pilot plant for different hydraulic loads and dispersion flows. The bubbles (volumetric fraction < 3-4 ‰ ) constitute a difficult environment for the ADV. Tests were performed to evaluate the ADV application. Direct measurements showed that the acoustic wave velocity was not affected. Velocity directions seemed to be fairly accurate, whereas magnitudes were underestimated. The measurement point seemed to be located very close to the ADV transmitter. Multiscattering of the acoustic waves was important. Mapping of the flow structures showed that three states existed – a low dispersion flow (< 4 %) giving a large rotational flow in the tank, higher dispersion flow and limited hydraulic load (< about 16 m/h) caused a stratified flow situation beneficial for the separation and finally a large hydraulic load (> 15-20 m/h) caused the stratified structure to break down with detrimental effect on the separation.