Advances in Water Treatment and Enviromental Management

146 WATER TREATMENT2.4 Experimental FacilityThe measurements were conducted in a closed-circuit system with a volume V of 2m 3 of water (see Fig. 3). The flow is driven by two pumps with variable discharges Q wup to 0.070 m 3 /s, which is equivalent to a Reynolds number of 2.0.10 5 . The workingcross-section, made of plexiglass, measured 2 m high with a diameter of 0.44 m. Auniform velocity distribution was obtained by an arrangement of screens in a diffuser.Freestream turbulence was found to be lower than 5% (= type T0). A variation inturbulence intensity could be achieved by grids at the entrance of the cross-section.The size of the selected grid mesh is M=5 cm and the quadraticsl bars are b=1,5 cmper side (=grid type T1).Gas (Air) is injected through 25 nozzles, positioned at each cross of the turbulencegrid (see Fig. 4). The gas flow rate in each nozzle can be controlled individually. Themass-transfer measurements have been carried out in co-current flow with anaverage volumetric void fraction,ß, in the range of 0 < ß < 10%.The continous measurements of dissolved oxygen in the fluid were performed bymeans of membrane electrodes in accordance to the ASCE-Standards [1].Determination due to this dynamical method is permissible because of the relativeshort length of the column and the small fineness ratio. The assumption of completemixing is then justified. The air was analyzed before and after passing the bubblecolumn in an oxygen detector. Because of this method it is possible to carry out amass-balance of the system.Fig. 3: Cross-Section of the Bubble Column