The DAF contact zone offers the opportunity for flocculated water to mix with the air-laden recycle flow for the first time. The flocs and the bubbles attach in this region. This zone is considered by the authors to be of the utmost importance for DAF performance. Nevertheless, hydrodynamics in the contact zone and the mechanism of bubble floc attachment have been poorly understood. This lack of understanding means that the hydrodynamic conditions for optimum solids capture within the contact zone, which are governed by the geometry and flow, are not known.
First, the dimensionless parameters which represent the hydrodynamic conditions in the contact zone were identified by literature review and using the Buckingham π theorem. Those parameters are Reynolds (Re) No., Froude (Fr) No., Weber (We) No., Local Weber (We) No., and Bubble Capacity (Bc) No.. Second, the particle removal rate was investigated by changing each value of the dimensionless parameters by changing the entrance pipe diameter for Re No., Fr No., We No., and Local We No., and by changing the pressure of saturator and recycle ratio for Bc.
Particle removal rate, when gravity dominates and the flow is semi-turbulent (Fr No. < 0.2, Re No. < 8000), is higher than when inertia dominates and the flow is turbulent. For the We No., higher removal rates were obtained in the range of the We No. < 40. Relative to the Local We No., smaller bubbles which are influenced more by surface tension than inertia, demonstrated a higher particle removal rate than larger bubbles. For the Bc number, the optimal range is from 0.3 to 0.5.
