This paper presents the mathematical formulation of a model for bubble–floc agglomeration in the contact zone of dissolved air flotation (DAF). It also contains rising velocity formulae for a bubble–floc agglomerate in the separation zone of DAF. The bubble–floc agglomeration model is based on the population balance of air bubbles, particles, and bubble–particle agglomerates, and the rate of bubble–floc collision and attachment. The model has been designed for two cases: where particles are larger than bubbles and vice versa. The requirements of pre-treatment (coagulation and flocculation) and the amount of air as bubbles are discussed, with the support of a model simulation. The air supply–consumption ratio (Asc) is derived as an important static parameter determining DAF performance, and should be greater than 1. The traditional DAF parameter, the air–solid ratio (As), is proportional to the air supply–consumption ratio. The DAF model parameter evaluation suggested that it should be on the order of 0.01 or more. To ensure a sufficient bubble–floc collision rate, the DAF model simulation revealed, the kinetic parameter (bubble volume concentration times floc diameter, φbd) should be greater than 6.0 × 10−8 m. The performance of DAF in treating a high concentration suspension (>100 mg/L) is dependent mainly on the air–solid ratio (or the air supply–consumption ratio). For low concentration suspensions (≪100 mg/L), φbd instead of the air–solid ratio is a dominant operational parameter in ensuring an acceptable bubble–floc collision frequency. The bubble volume concentration required for treating a low concentration suspension can be reduced with the increase of floc size by pre-flocculation.

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