PILOT PERFORMANCE OF DEEP U-TUBES

U-Tube aeration has received considerable attention recently as part of a modification of the activated sludge process called “Deep Shaft” treatment. Presently there is only limited data on aeration characteristics of deep U-Tubes showing the effect of velocity, depth and gas/water injection ratios. This study determined the increase in dissolved oxygen, the oxygen absorption efficiency and the head loss across a 0.1 m diameter U-Tube for the following parameters: Velocity 0.9, 1.4, 1.8 m/s Depth 12, 30, 61, 103 m. Oxygen 4, 7 1/2, 25 percent O2/water ratio Air 5, 15, 25 percent air/water ratio Tap water was passed through the system for all of the above parametric conditions. In addition, 5 mg/1 of an anionic detergent was injected into the tap water to simulate domestic wastewater as recommended by the British. Also, in other tests, 100 mg/1 of Bentonite clay was injected into the tap water to simulate the colloids in domestic wastewater which provide nuclei for effervescence of supersaturated dissolved gases. Air injection at 25% air/water ratio indicated considerable improvement in Δ D. O. Using tap water a change from 30 to 61 m. depth caused the Δ D. O. to increase from 11 mg/1 to 17 mg/1. When pure oxygen was injected, however, at 25% oxygen/water ratio at 30 m. depth, the Δ D. O. was 68 mg/1 and increased to 95 mg/1 at 61 m. Negligible increases were noted in Δ D. O. at 103 m. depth. Over the velocity range investigated (0.9 to 1.8 m/s) there was no significant effect on Δ D.O. The maximum absorption efficiency for pure oxygen at 7.5% O2/water was observed to be 52% in the 103 meter deep U-Tube, demonstrating that the U-Tube requires capture and recycle of the off-gases if it is to be used in an efficient manner as the absorption system for an oxygen activated sludge system. The maximum oxygen transfer economy with air injection was 4 kg O2/kw-hr with 15% air/water injection ratio at 61 m. depth producing an effluent containing 21 mg/1 D.O. (Δ D. O. = 15 mg/1 220% saturation). The estimated total cost, amortization and operation, per ton of oxygen absorbed from air would be $23/ton of oxygen compared with $42/ton of oxygen using surface aerators which can produce only 2 to 4 mg/1 of D.O. in the discharge. The estimated total cost to absorb commercial oxygen at 90% absorption efficiency was $6.50/ton. Deep U-Tubes offer excellent potential as energy and cost efficient oxygen transfer systems. However, the optimal depth appears to be approximately 40 to 60 m. for air injection and 25 to 60 m. for oxygen.