Modeling, simulation and operational parameters of dissolved air flotation

This paper presents the mathematical formulation of a model for bubble-flee agglomeration in the contact zone of dissolved air flotation (DAF). It also contains rising velocity formulae for a bubble-hoc agglomerate in the separation zone of DAF. The bubble-flee agglomeration model is based on the population balance of air bubbles, particles, and bubble-particle agglomerates, and the rate of bubble-flee 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 (A(sc)) is derived as an important static parameter determining DAF performance, and should be greater than 1. The traditional DAF parameter, the air-solid ratio (A(s)), 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-flee collision rate, the DAF model simulation revealed, the kinetic parameter (bubble volume concentration times flee diameter, phi(b)d) Should be greater than 6.0 x 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 (much less than 100 mg/L), phi(b)d instead of the air-solid ratio is a dominant operational parameter in ensuring an acceptable bubble-flee collision frequency. The bubble volume concentration required for treating a low concentration suspension can be reduced with the increase of flee size by pre-flocculation.