Coarse-bubble aeration

The air flow of the coarse bubble aeration system was then adjusted for the lower BOD of 30 mg/L that is assumed to occur at a depth of 73 m, with each of the 1,160 diffusers maintaining 2 mg/L in a 341 m area. An air flow rate of 29 scmh per diffuser (33,640 scmh total) was determined to be sufficient, assuming that the characteristic equations may be extrapolated to 73 m of depth. For this case, KlA, = 0.018 hr and KlAs = 0.003 hr During the high BOD period at 10 m depth, approximately 3.5 times as much air flow is needed to maintain aerobic conditions. 

A coarse bubble aeration system for McCook Reservoir requires only 1,160 diffusers, approximately one-third of the 3,438 diffusers needed for a fine bubble aeration system. However, significantly less air flow is needed for the fine bubble diffusers in comparison with the coarse bubble diffusers. At the more common depth of 10 m, the air flow required by the fine bubble diffusers was 39% of that required by the coarse bubble diffusers. These considerations and the mixing requirements of the reservoir are a part of the aeration system design. 

A direct comparison between the submerged MBR and external (sidestream) MBR has indicated that submerged MBR has an inherently lower fouling propensity as a result of the slug-flow hydrodynamic regime associated with the coarse bubble aeration [8]. 

Fouling control in submerged modules is achieved by coarse bubble aeration. (External) horizontal modules are operated in the side-stream mode. Most systems are relaxed periodically some can be back pulsed (at very low pressure). A schematic view is depicted in Figure 9.3. 

The effective saturation depth,, represents the depth of water under which the total pressure (hydrostatic plus atmospheric) would produce a saturation concentration equal to for water ia contact with air at 100% relative humidity. This can be calculated usiag the above equation, based on a spatial average value of T, measured by a clean water test. For design purposes,, can be estimated from clean water test results on similar systems, and it can range from 5 to 50% of tank Hquid depth. Effective depth values for coarse bubble diffused air, fine bubble diffused air, and low speed surface aerators are 26 to 34%, 21 to 44%, and 5 to 7%, of the Hquid depth, respectively. 

Increasing surfactant concentrations in the aeration cell has been found to decrease bubble diameter, bubble velocity, axial diffusion coefficient, but increase bubble s surface-to-volume ratio, and total bubble surface area in the system. The effect of a surface-active agent on the total surface area of the bubbles is also a function of its operating conditions. The surfactant s effect is pronounced in the case of a coarse gas diffuser where the chances of coalescence are great and the effectiveness of a surface-active solute in preventing coalescence increases with the length of its carbon chain. 

Flotation. After screening, washing, and dilution (flooding), the suspension pumped to primary flotation vessels comprises bitumen droplets, aerated bitumen globules, dispersed coarse and fine solids, and probably bubble-particle aggregates. This suspension is fed into a point somewhat higher than the center of the vessel as shown, for a continuous extraction circuit vessel, in Figure 21. The aim of the process here is to selectively separate the dispersed aerated bitumen droplets... 

---