Particles from a post denitrifying Kaldnes Moving Bed Process were studied using an optical borescope, a visualisation technique that provides opportunities for both qualitative and quantitative studies of the separation mechanism in Dissolved Air Flotation (DAF). Rise rates for particle/bubble aggregates were estimated showing great variability. Two groups of aggregates were distinguished; relatively small flocs (<100 microm) with single bubbles attached rising comparatively slowly and large flocs (>100 microm) with several bubbles attached rising very fast. The high rise rates for large aggregates are discussed, possibly explained and suggested as the reason for the effective separation of large particles noticed in previous studies. Removal efficiency of different size categories of particles in DAF were investigated on the basis of particle size analysis indicating increased separation efficiency with increasing particle size.
A Discfilter with 10 and 18 microm filter openings, respectively, was placed in parallel to a flotation plant for separation of biological flocs from a post-denitrifying Kaldnes Moving Bed Process, the last treatment step at the municipal wastewater treatment plant at Sjöunda, Malmö, Sweden. The effluent concentrations from the 10 and 18 microm filter were 2-5 and 2-8 mg SS L(-1), respectively, which is comparable to, or better than, the flotation plant. Comparison with experiences from activated sludge plants shows that the Discfilter works especially well after the Kaldnes process. Particle size distribution (PSD) studies show that particles larger than the filter openings of 10 and 18 microm are separated with approximately 90% efficiency, whereas most of the smaller particles pass the filter. This fact indicates that the major particle separation mechanism is physical blocking. These findings point to the possibility of improving the prediction of the separation efficiency by combining measurements of turbidity and suspended solids with particle size analysis.
This study presents practical implications for particle separation in Dissolved Air Flotation (DAF). The objectives were to localise where particles are separated from the water phase and to determine what particles, in terms of size, are removed by the DAF-process. Both pilot- and full-scale plants were investigated. Particle sizes were analysed with a light-blocking particle counter and an optical borescope was used for visualisation of particle-bubble aggregates. It was found that particles are preferably separated upstream in the process, i.e. close to the contact zone. Furthermore, separation efficiency for particles increased with increasing particle size.
The removal mechanism in dissolved air flotation (DAF) was studied on the basis of particle size data obtained with a light-blocking particle counter. Mass-and size measurements of particulate matter, i.e. biological floc from the Kaldnes Moving Bed™ Process, were performed at different locations in the cross section of a DAF-pilot plant located at Sjölunda wastewater treatment plant in Malmö, Sweden. The particle size distribution (PSD) showed that approximately 97 % of the particles in the influent were smaller than 40 µm. However, 75 % of the mass was theoretically allocated to the remaining 3 %. The results indicate that particles, especially large particles, are separated close to the contact zone. This was strengthened by measurements of suspended solids. Measured and theoretically estimated values of SS were compared. Furthermore, measurements of rise velocities of aggregates from the full-scale plant were considerably lower than surface loading velocities according to experience.
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