Scale-up rules for flocculation

Abstract

We present here a study of three baffled tanks, 0.15 m, 0.20 m and 0.30 m in inner diameter, agitated by Rushton turbine, with reference to scaling up the flocculation kinetics. A clay slurry was used as a model wastewater. The experiments were carried out at mixing intensity 40 W/m³ and clay concentration 0.58 g/L. The model wastewater was flocculated with the Sokoflok 16A organic flocculent (solution 0.1 wt%) in the range of dimensionless flocculent dosage D_F^⁎ from 0.448 to 5.241 mg/g. The flocs that formed were separated by means of sedimentation. The operational conditions were designed according to following proposed scale-up rules: i) P/V = const., ii) t_F^⁎ = N·t_F = const., iii) D_F^⁎ = const. and iv) u_sed = const. Applying the proposed scale-up rules, the flocculation efficiency was found to be practically the same irrespective of vessel size and flocculation dosage. For the given flocculated system and process conditions, the maximum degree of turbidity removal Z_e^⁎_max = 96.1 ± 0.6% was found for dimensionless flocculation time 1916 ± 5% and dimensionless flocculent dosage D_F^⁎ = 3.128 mg/g ± 8%, regardless of vessel size. The approach to scale-up proposed by Camp (1955) is discussed and criticized in the light of present-day mixing theory, and an explanation is offered of the shortcomings of this approach, and why it gives incorrect scale-up results.