PARTICLE SIZE ANALYSIS IN DISSOLVED AIR FLOTATION — SEPARATION OF PARTICLES FROM A BIOFILM PROCESS

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.