Oxygen supplementation by U-Tube to the Tombigbee River

Disolved oxygen (D.O.) in the Tombigbee River in Alabama, USA occasionally decreases below 4 mg/L and forces two paper mills to reduce their effluent discharge loads to the river. These mills have limited capacity to store their treated effluents on-site. Therefore if the duration of critical D.O. conditions in the river exceeds the length of time that storage capacity is exhausted, paper production capacity must be curtailed Since the combined capital costs of these two paper mills exceeds $2,000,000,000, the amortization exceeds $1,000,000 per day and it is prohibitively expensive to shut down paper production. permission was granted to these mills by the river regulatory agency to supplement the D.O. resource of the river during critical periods to offset the effluent BOD loading to the river (4.7 kg O-2/kg BOD5) This, in turn, negates the need to cut back on paper production. Two oxygen supplementation installations were made. Both use commercial oxygen, - instead of air - that is dissolved into the water by a U-Tube process. Each U-Tube forces the water down 175 feet (53.4 m) under a baffle and back to a river diffuser located 35 feet (10.7 m) below the river surface. Oxygen gas is injected at the U-Tube entrance and the bubbles are swept by the water down and back up through the U-Tube. This greatly increased hydrostatic pressure provides high oxygen absorption efficiency of the commercial oxygen while producing a D.O. concentration of 50 mg/l in the 28 oC discharge. The diffuser in the river facilitates very rapid mixing/dilution of the high D.O. discharge with the low D.O. river water. The D.O. several hundred feet downstream of the diffuser never exceeds 8.0 mg/l. One U-Tube installation adds 12,000 lb/day (5450 kg/day) of D.O. directly to the 35 MGD (1.53 m/sec) treated effluent from one mill. The second U-Tube installation 35 miles (58 km) downstream pumps 120 MGD (5.25 m(3)/sec) directly from the river though the U-Tube and adds 40,000 lb/day (18,180 kg/day) of D.O. before discharging through another diffuser located on the bottom of the river. Copyright (C) 1996 IAWQ.