Advances in Water Treatment and Enviromental Management

152 WATER TREATMENTFig. 10: Specific Mass-Transfer Coefficient k L• a/U SLfor DifferentVoid Fractions ßThis result leads to the conclusion that the operation conditions with a turbulenceproducing grid have no significant influence with low air flow rates but reduces themass-transfer with increasing Q a .The relation between the mass-transfer coefficient and the air flow rates are plotted inFig. 9b. The significant greater k-values for the lowest water flow rate Q w and higherair input rates Q a (increasing ß) may be explicable with the accumulation of bubblesin the upper part of the bubble column with a longer residence time and therefor abetter efficiency of the oxygen. The calculation of k related to the spatial mean velocityof the gas phase USG results in decreasing values with increasing air flow rates(comparison of the maximum and minimum values:Q a -factor 10; k-factor 5).These results are supported by calculation of the maximum efficiency of oxygentransfer:where ξ 0 is the volumetric oxygen concentration of air input in the bubble columnand ξ E0 is that of the air output at the top of the column, see table 2.The better oxygen transfer from bubbles to water at the lowest air input Q a (η ~ 6-8 %) is caused by the single rising bubbles with diameters d B =3- 5 mm. Bubbles ofthis diameter range enhance oxygen transfer by bubble deformation and bubblesurface oszillations [15]. The movement of the bubble surface results in the continousproduction of of great local concentration differences ∆C=(Cs-CL) in the interfacialarea of the bubbles. This leads to an increasing mass-transfer corresponding toequation (2).An increasing air flow rate Q a produces bubbles with larger diameters and bubbleagglomeration by coalescence. This effect leads to reduction of the interfacial areaand to a adequate loss in oxygen entry [15].(7)