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White Paper by ISO:21630 (2007). Practically, a certain flow velocity should be maintained throughout an oxidation ditch. The relationship between the required thrust and horizontal flow velocity can be described by equation 3 (Uby, 2012): EQ 3: F~k*u2 Where: F = thrust, N k = reactor momentum loss factor, u = horizontal flow velocity, m/s (ft/s) For un-aerated ditches, a horizontal flow velocity of 1 ft/s (0.3 m/s) most often satisfies particle suspension and mixing of liquor. The loss factor k is a function of tank geometry and obstacles present in the reactor. For oxidation ditches with dedicated aeration zones, the values of both k and u in equation 3 need to be determined as functions of the aeration/mixing horizontal flow effects expanded on below. Using fine-bubble diffused aeration and low-speed mixers to generate a horizontal flow around an oxidation ditch presents additional consideration of the momentum losses generated by the vertical rise of bubbles across aerated zones. Proper placement of aeration and mixing equipment is required to ensure optimum performance with respect to mixing and aeration, otherwise spiral flow effects may develop, reducing transfer capacity and efficiency. Assessment of the ability to produce an even horizontal flow can be done by considering the modified Froude number for such systems. The modified Froude number (Uby, 2012) can be correlated to the total aeration loss factor, where a low loss factor for optimal performance is desired. Figure 2. Upper image: Vertical shaft surface aerator which oxygenates liquid by generation of plumes of droplets above the surface. Lower image: Diffused aeration in combination with submersible low-speed mixers, enabling an effective horizontal flow and enhancing the oxygenation of mixed liquor. diffused aeration systems not only provide the highest level of OTE, but also provide the capacity to efficiently address a wide range of operating conditions. Independent mixing and aeration offers another level of process control to minimize air flow requirement, adapting blower air flow or control valves against required and observed process parameters such as ammonia-nitrogen and dissolved oxygen concentrations. This level of operational control is restricted with a traditional single device serving both aeration and mixing demands. Table 2. Features of diffused aeration and surface aeration in oxidation ditches Feature Diffused aeration Mechanical surface aerators Potentially as high as 7-8 kg O2/kWh Limited to approximately 2 kg O2/kWh Installation Diffuser system with piping and blowers. Submersible mixers to generate horizontal flow. Single unit installation. May require additional stand-alone mixers and/ or supplemental aeration. Flexibility in engineering design and capacity Stand-alone units enable tailor-made solutions which meet oxygen transfer demands. Limited oxygen transfer variability. Number and size of units are the main flexibility factors. Installation cost Depends to a large extent on diffuser density and number of blowers and mixers required. Cost highly related to efficiency. Relatively low-cost with few additional components. May require additional concrete reinforcements. Very limited to none. Common issue due to droplet generation required for oxygen transfer. Process control capacity Highly flexible with independent blower air flow and/or valve control. Very limited. Maintenance Few mechanical parts which need maintenance. Frequent maintenance of rotating aerating parts. SAE Aerosols and icing in cold climates EQ 4: Fr = u2/gS · u/uq Where: Fr = Froude number, u = average cross-section horizontal flow velocity, ft/s (m/s) g = acceleration by gravity, 32 ft/s2 (9.8 m/s2) S = submergence of diffuser, ft (m) uq = air flow per aerated total aerated area, ft3/ft2/s (m3/m2/s) Figure 3 describes in qualitative terms the correlation between the Froude number and the aeration loss factor. For oxidation ditches, a Froude number value estimated using equation 3 should exceed 0.3 (an approximate critical Froude number) to eliminate spiral flows and ensure high oxygen transfer rates and efficient mixing of wastewater (Uby, 2012). Designing a robust oxidation ditch with stable performance requires a critical Froude number that strikes a proper balance between design depth (relating to S), diffuser flux (relating to uq), and mixer thrust (relating to u). The need to establish a critical Froude number relates to the point at which the horizontal thrust produced by submersible mixers breaks through the bubble curtain generated by the diffusers. While the aeration loss factor can be implicitly quantified in part by calculation of the Froude number, tank geometry, liquid viscosity, aeration grid layout, and mixer positioning also Combining Fine-Bubble Aeration With Horizontal Flow In order to mix liquor in a biological reactor, sufficient thrust must be generated to keep particles in suspension. The use of thrust as a standardized parameter to measure and compare mixer performance is described wateronline.com ■ Water Online The Magazine 27