Author(s): L. Jonsson
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Keywords: Dissolved air flotation; Separation zone; Flow structure; Air bubbles; Dissolution
Abstract: Dissolved air flotation (DAF) is a separation method, based on the use of microscopic air bubbles, for suspended flocs generated in drinking water production and wastewater treatment. Studies have been performed on a pilot plant showing that basically two different flow structures can occur in the separation zone in the DAF tank. There are strong indications that the stratified flow condition, with an upper bubble-containing layer and a lower, more or less bubble-free layer is required for good separation efficiency. Data on the vertical extent of the upper layer in relation to the hydraulic load are presented and a simple model of the flow structure in this layer is proposed. Bubble behaviour is discussed in relation to this model and in order to counteract contradictory conclusions it is hypothesized that the dissolution of bubbles has to be considered. Theoretical and experimental evidence of the relevance of this latter mechanism are put forward, i.e. concerning time scales and scope for dissolution of bubbles in the tank.
Year: 2003