Advances in Water Treatment and Enviromental Management

5. Waste Water Processing TechniquesGAS LIQUID MIXING AND MASS TRANSFER 1235.1 Submerged Turbine AeratorsThese are the water industries equivalent of the agitated mixing vessel. Variousdesigns are on the market, ranging from axial to radial flow turbines, with an airsparge beneath, usually aeration units are mounted within square or rectangulartanks, often with multiple units. Often the units are fitted with some form of baffling.The turbine performs multiple roles:– breakup and dispersion of sparged air– mixing of liquid in the tank– suspension of sludge, preventing settlementOne disadvantage of such systems is that additional equipment (i.e. a blower) isneeded to introduce the air. To overcome this, various designs of selfaspiratingturbines have been developed, e.g. Ref 3. However, these are generally less efficientat pumping the liquid.5.2 Surface AeratorsThese devices are similar to submerged turbines, but are positioned at or near thesurface. Air is drawn down from atmosphere and pumped into the tank by theagitator, Figure 8 (a). These can either be mounted on a fixed frame or float on thesurface. They have the advantage of lower capital and running cost than submergedturbines, but require more freeboard, due to the extensive surface disturbancescaused, and (if on a fixed frame) are less able to cope with fluctuating liquid levels.Additionally, the devices have less influence on the base of tanks, so suspensionperformance is reduced.As an alternative, “brush” aerators have been used, Figure 8 (b), i.e. impeller shaftmounted horizontally. In these designs mass transfer occurs through both liquid inair and air in liquid dispersion. They are often used in oxidation ditches, where alinear water flow is required, this being enhanced by the aerator.Finally plunging jet devices, Figure 8 (c), have been found to have great potential,Ref 4, but have yet to find their full commercial potential.5.3 Bubble DiffusersThese can be separated into two categories: coarse bubble diffusers, comprisingarrays of perforated sparge pipes, and fine bubble diffusers, comprising fine-pore,