Since the mid 1990s there has been a large increase in the hydraulic loading rates used to design dissolved air flotation (DAF) facilities for drinking water applications. High rate DAF processes are now available at loading rates of 20 to 40 m h−1. These high rate systems have a smaller plant footprint compared to conventional systems. The paper examines bubble and floc-bubble rise rates, and how the simple theory relating these rise rates to the hydraulic loading of the DAF separation zone is inadequate. It is shown for high rate systems that the flow through the separation zone is stratified and must be accounted for in relating separation zone efficiency to the hydraulic loading and bubble or floc-bubble rise rates. A conceptualized stratified flow model is used to explain why DAF tanks can be designed with hydraulic loadings >20 m h−1.

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