Dissolved Air Flotation and Microscreening for Particle Separation in Wastewater Treatment

Abstract: Particle separation is one of the fundamental processes in wastewater treatment and a good understanding of the basic separation mechanisms ? sedimentation, flotation and filtration ? is critical in the design of wastewater treatment systems. Upgrading of treatment plants for enhanced primary treatment and effluent polishing are current examples of applications where effective, high-rate separation methods are of interest. Dissolved air flotation (DAF) and microscreening can be used in both applications. The purpose of this work was to study the separation mechanisms in DAF and disc and drum filtration ? referred to as microscreening ? by means of literature studies, particle size analysis and particle visualisation. The processes were studied mainly in the context of applications within tertiary treatment. Furthermore, a novel application based on chemically enhanced primary treatment utilising microscreening for floc removal was tested. Separation efficiency was found to increase with increasing particle size in DAF. Furthermore, particles were preferably separated upstream, i.e. close to the contact zone. Aggregates showed a great variation in rise rates. Relatively small aggregates, with single bubbles attached, showed relatively low rise rates, while large aggregates with numerous bubbles attached showed very high rise rates. Large aggregates rising very quickly could be one explanation of the effective upstream removal of solids. Another explanation could be the bulk flow simply directing flocs to the surface. With particles being separated upstream the straightforward application of the theory of hydraulic surface loading is not necessarily completely valid. With respect to microscreening it was shown that physical blocking of particles was the main separation mechanism. Floc strength and particle size were identified as key parameters in understanding and design of microscreens. Although it is an accepted and known technology, the areas of application of microscreening can be increased. The suitability in floc removal after a post-denitrifying biofilm process, the Kaldnes Moving Bed? Process, was demonstrated. Furthermore, a novel application was introduced where microscreening was used in chemically enhanced primary treatment resulting in high removal rates of SS, BOD, COD and total phosphorus at comparatively high filtration rates.