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Consistent performance of DAF and the quality of DAF effluent—despite considerable variation in flocculation characteristics and flocculated water quality conditions—demonstrate the robust nature of this process. Good dissolved‐air flotation (DAF) and filtration performance were observed for all pretreatment flocculation conditions studied: flocculation times of 5 and 20 min, gate and propeller mixers, two and three stages of flocculation, and low (G of 30 s−1) and high (G of 70 s−1) mixing intensity. The research showed that DAF plants can be designed with flocculation times as low as 5 min and that DAF is a robust particle removal process—i.e., it can produce consistently good water quality for a variety of conditions. Coagulation chemistry (pH and coagulant dosage) has a stronger influence on DAF and filtration performance than physical flocculation design and operating parameters.

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Pretreatment considerations of coagulant selection, flocculation time and flocculation mixing intensity were studied for two different water supply types, aquatic humic and non-aquatic humic waters of low turbidity. Alum, ferric salts, and various polyaluminium chlorides (PACs) with different chemical properties were all effective in dissolved air flotation (DAF) when used under favorable conditions of dosage, pH, and flocculation time. A stoichiometry was found for the aluminium coagulants between coagulant dose and raw water DOC. Si and sulfate PACs were effective in treating cold waters with short flocculation times of 2.5 to 5 minutes. Flotation performance improved slightly with increasing flocculation mixing intensity. The results suggest that flocculation tanks be designed to produce strong, “pinpoint” size floc particles.

Fluid dynamics is believed to be of crucial importance for the removal mechanism in Dissolved Air Flotation. The paper is a presentation of experimental studies on velocity and air-content profiles in a DAF pilot plant. The correlation between air content and flow patterns is discussed. Two flow structures are detected. A stratified flow structure is defined by a horizontal transport of the water in a upper, less-dense layer, and a lower, downwards-vertical transport in the lower, dense layer. Between the layers a return flow is found. The short-circuiting flow is defined as an immediate downward deviation towards the outlet arrangement at the bottom of the tank, probably causing decreasing effluent quality. Density gradients, caused by differences in air content in the tank, are believed to generate a stratification of the water body, thus causing the water to be transported in layers. Measurements of air-content profiles correlate to the measured flow structures, also indicating a separation of the water body in two layers with a distinct boundary.