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Particle removal by flotation and filtration: Pretreatment effects

TitleParticle removal by flotation and filtration: Pretreatment effects
Publication TypeJournal Article
Year of Publication1996
AuthorsValade M., Edzwald JK, Tobiason JE, Dahlquist J., Hedberg T, Amato T
JournalJournal American Water Works Association
Start Page35
Date Published12/1996

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.

Flocculation tanks prior to dissolved-air flotation (DAF) are typically designed with two stages and with total hydraulic detention times of 20-30 min.1-4 Effectively, flocculation tanks for DAF are designed in the same way as sedimentation in spite of fundamental differences between these two particle clarification processes. Sedimentation processes require large floc particles (100 mu m) with densities greater than water. Flotation does not require large floc particles. Floc particle densities less than water are required and are achieved by attachment of air bubbles to floc particles. Edzwald and co-workers3,5-7 have made the case on theoretical and experimental grounds that flocculation tanks ahead of DAF can be designed with much shorter detention times than those used in current practice.

Why reevaluate flocculation tank design, given that DAF plants with the current design (two stages with a total mean detention time of 20 min) seem to work successfully? There are several reasons for and advantages to improving flocculation tank design, all based on the concept of integrated plant design.3 -in simple terms, integrated plant design considers that what is done ahead of the flotation tank with coagulant addition and flocculation affects flotation performance, and in turn flotation tank performance affects filter design and performance. Some advantages, then, for improving and optimizing flocculation tank design are: ( 1 ) space and cost savings from smaller flocculation tanks, (2) better flotation performance, and (3) if tank design is integrated with filter design, better filter performance and lower filter costs.