Power requirements for complete suspension and aeration in an unbaffled bioslurry reactor

Abstract

Remediation of contaminated soils is spreading as a matter of crucial importance nowadays. Bioremediation via bioslurry reactors of sites polluted by recalcitrant pollutants has been proved to be a valuable option, although optimization is needed to reduce process costs. Free-surface unbaffled stirred tanks (with central air vortex) have been recently proposed as a promising alternative to the more common systems provided with baffles. In a bioslurry reactor solid-liquid interfacial area, oxygen supply, solid loading per reactor unit volume should be maximized, and, at the same time, operation costs have to be kept low. In this regard, the minimum impeller speeds for complete suspension Njs (suspension of all solid particles) and aeration Nca (air vortex ingested by the turbine and dispersed as bubbles in the system) represents a reasonable compromise between process yield and power requirements. To this purpose, a flat bottomed unbaffled tank with diameter T=0.19 m was investigated. The tank was filled with water up to a height H=T. It was stirred by a radial sixbladed Rushton turbines (RT) with diameter D=T/3 and H=T/3. Mono-dispersed particles with diameter dp=250-300μm and density p≈2500 kg/m3 were employed. Solid loadings B% ranging from 2.5% (weight of solid/weight of liquid) up to the very high 160% w/w were tested. The visual Zwietering criterion along with the aid of a digital camera was employed to evaluate Njs values. An acoustic criterion was adopted to assess Nca. A static frictionless granite turntable was employed to measure the impeller torque at Njs and Nca and to assess the relevant specific power requirements js and ca. Results show that the dependence of Njs and Nca on B% is much lower at low solids loading (B<30%), while a larger dependence was found at larger B% values (B>30%). The relevant specific powers per unit mass of solids (i.e. js and ca) were found to exhibit a minimum, at B≈20% for js and B≈60% for ca. On overall, data collected suggest that operating a radially stirred unbaffled bioslurry reactor loaded with a concentration B≈30% could be the best compromise to minimize the costs for achieving complete suspension and aeration conditions.