Measurements of N_js and Power Requirements in Unbaffled Bioslurry Reactors

Abstract

The remediation of urban/industrial polluted lands is a topic of crucial importance nowadays. Bioremediation techniques are widely employed to remove organic pollutants from contaminated soils because of their simplicity and cheapness. The bioslurry reactors mechanically agitated by stirrers represent one of the most promising bioremediation techniques. In the present work an unbaffled stirred vessel filled with solids and water is experimentally investigated from a fluid dynamic point of view. Air presence within the tank is guaranteed by the central vortex formation (typical occurrence concerning stirred vessels unprovided with baffles) instead of an intrinsically more expensive insufflation device. Experiments were carried out aiming at assessing the minimum impeller speeds at which all solid particles get suspended (Njs) as well as the relevant power requirements (Pjs). Njs was assessed by adopting the well known “one second criterion” by Zwietering (1958). Power measurements at complete suspension conditions (Pjs) were carried out by assessing the torque transmitted by the impeller to the tank with the apparatus described in Grisafi et al. (1998). The dependence of both parameters on system geometrical configurations (impeller diameter, impeller clearance and impeller type) were investigated. Results were compared with relevant literature ones obtained for the case of the more common baffled systems suggesting that from a fluid dynamic point of view, biodegradation operations can be conveniently conducted in unbaffled stirred vessels. Finally, the Pjs values collected show that a tank stirred by a six-bladed Rushton turbine with a diameter equal to one third of tank diameter and a clearance equal to one third of tank diameter is the most economical configuration among the ones investigated in the present paper.