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White Paper Fine-bubble diffusers, in particular, present major advantages in both capacity and power consumption. However, in order to achieve maximum ratio of oxygen transfer to energy consumed (SAE), a number of factors relating to the floor density and air flux of the diffusers must be evaluated. When full floor diffuser coverage systems are considered, diffuser floor density refers to the ratio of area covered by diffuser membranes to the total floor area. Diffuser flux, expressed as flow per membrane area, is directly related to the diffuser density if a fixed oxygen transfer rate requirement is considered. With extensive oxygen transfer test data at hand, oxygen transfer can be accurately predicted by taking into account the variables associated with the configuration of any full floor coverage diffuser installation. In summary, the driving forces behind oxygen transfer in fine-bubble systems in clean water can be attributed to the following: • Diffuser flux • Diffuser density and airflow uniformity • Depth Obtaining high transfer efficiency requires optimizing the bubble retention time, thereby increasing the total amount of oxygen transferred, per volume of air supplied to the system. Bubble retention time correlates to spiral flow effects, which are more or less present in any bottom diffuser installation and depend on the density and membrane flux of the installed system. The phenomenon of spiral flows is described in Figure 1 below. A densely installed diffuser system with low membrane flux mitigates secondary flow effects, allowing slower bubble rise within the reactor. Figure 1. Principle of spiral flows. Left: High membrane density fine-bubble diffuser system. Right: Low density-high flux installation, generating secondary spiral flows causing reduced bubble detention time and oxygen transfer rate. The system as a whole, including air blowers, responds positively to an increase in diffuser density, leading to lower flux and higher SOTE. In what can be described as a positive feedback response, high SOTE generates lower air flow, pressure, and power requirement. The depth of the aerobic reactor is a vital factor in assessing the oxygen transfer capacity of the aeration system as well as the OTE and the overall SAE. While increasing reactor depth increases the oxygen transfer rate or transfer efficiency SOTE of a bottom diffuser system, its relationship is sub-linear; i.e. SOTE increases slower than linearly versus diffuser submergence. At the same time, system pressure has a nearly linear relationship with diffuser submergence. The dynamics of these major factors in aeration system design should be taken into consideration during the engineering and design phase of an activated sludge facility. 26 wateronline.com ■ Oxygen Transfer In Oxidation Ditches The discussions above pertain mainly to conventional activated sludge processes, but the dynamics of air flux, diffuser density, and depth affecting performance are relevant to all variations of the activated sludge process. Oxidation ditches traditionally operate on the basis of supplying oxygen to the biological process in dedicated aeration zones, utilizing simultaneous mechanical generation of horizontal flow to ensure mixing of liquor and transfer of dissolved oxygen to non-aerated zones. Depending on the design of the ditch, non-aerated zones may also provide anoxic conditions for denitrifying bacteria. Surface Aerators Versus Fine-Bubble Diffused Aeration In Oxidation Ditches Similar to conventional activated sludge reactors, the oxygenating capabilities of mechanical surface aerators and their SAE should be considered when assessing the power requirements for an oxidation ditch application. In one common design, surface aerators rely on producing plumes of droplets above the liquid surface which create a large waterto-air surface area for oxygen transfer from the air to the liquid droplets. Oxygen transfer also occurs as air is drawn into the bulk liquid with the mechanical device. The mixing flow pattern in the reactor transfers oxygenated liquid from the surface. Inevitably, a gradient of dissolved oxygen levels forms, where the highest level of dissolved oxygen exists close to the surface and in the proximity of the aerating device. In the first generation oxidation ditches — many of which are still in operation today — the mixing requirements within an oxidation ditch were satisfied by the aeration device, as vertical or horizontal shaft aerators create a horizontal flow pattern. Installation of equipment above the liquid surface was seen as an attractive feature of surface aerators. There may be a number of limitations in such installations, including limited reach of oxygenated liquid in deep tanks, insufficient mixing of liquor, limited turndown capacity, low SAE, and generation of aerosol in the vicinity of the reactor, as well as frequent maintenance of rotating parts. The mixing capabilities of such equipment may also limit the horizontal transfer of dissolved oxygen along the lanes of the ditch. Table 2 summarizes features and advantages of surface aerators and diffused aeration. See Figure 2 on the next page for the working principles of vertical shaft mechanical aerators and fine bubble diffusers with low-speed horizontal mixers. Fine-bubble diffused aeration offers many advantages over mechanical surface aeration — as discussed previously — in oxidation ditches in particular. General benefits of diffused aeration in combination with submersible mixers include increased SAE (including mixer power), high oxygen transfer rates, independent mixing and aeration devices, completely submersed systems with low maintenance needs, and elimination of aerosol formation and associated odor reduction from rotating parts at the reactor liquid surface. With independent aeration and mixing devices, fine bubble Water Online The Magazine