The use of relatively simple but conceptually sound mathematical models is a powerful tool to identify and understand variables that affect the performance of a process. Such models have been used to better understand the many and complex dissolved air flotation (DAF) variables that are in play when water, flocs and air bubbles are mixed in the contact zone and then removed in the separation zone. The first purpose of this paper is to examine critically models that have been previously developed, primarily by the authors, for the contact and separation zones, and to summarize the insights gained from these efforts.

During the past 5–10 years, significant strides were made towards more efficient designs that allow reduced flocculation time and much higher hydraulic loading rates. These developments have also exposed some fundamental weaknesses in the traditional modelling approaches, as some systems are now working beyond the theoretical limits of feasibility. The second purpose of the paper, therefore, is to probe these weaknesses and to suggest some directions for future modelling efforts.