The Moccasin Bend WWTP uses pure oxygen produced by a cryogenic oxygen plant for activated sludge treatment of the influent waste stream, which consists of municipal and industrial wastes. The waste stream contains a high proportion of high strength industrial waste, which results in a typical COD loading of about 2000 mg/L, which is approximately four times the strength of normal municipal waste. For oxygen transfer the plant employs conventional surface aerators which splash the wastewater up into the oxygen rich atmosphere in the headspace of the enclosed cells. An extension of the surface aerator shaft operates a submerged impeller that serves to mix the basin contents and distribute the oxygen throughout the basin. Each cell is 60’ x 60’ x 17’ SWD and they are arranged in trains of eight cells in series. Four trains of eight cells each are employed, for a total of 32 cells. The surface aerators/mixers are powered by 125 or 100 horsepower motors, with the 125 hp motors installed on the first couple of basins in a train. Although the overall oxygen utilization through a train of eight cells is good, the operator elected to investigate alternative oxygen transfer systems due to the following concerns with the surface aerator system: · High Energy Consumption · Precise level control requirements in order to prevent high torque and consequent overloading of the gear boxes and motors · Inability to control oxygen feed to each individual cell as needed · The surface aerators are reaching the end of their serviceable life and are becoming prohibitively expensive to maintain
The alternative oxygen transfer system that was tested consisted of a Chemineer high efficiency up-pumping mixer for basin mixing, oxygen distribution and solids suspension. The oxygen was injected into the test cell using a Mazzei Injector Corp. 12” Model 12094 injector and four Model #60 nozzles which were submerged in the basin. The #60 nozzles were installed on a manifold positioned in the basin to discharge the oxygen/water mixture from the injector underneath the Chemineer up-pumping mixer. The model 12094 injector was powered by a 30 HP Gorman Rupp pump. The layout and articulation of the oxygen transfer system are detailed in the following pictures.
The oxygen injection and basin mixing system was evaluated for mixing uniformity of basin contents, oxygen utilization, energy efficiency and dissolved oxygen (DO). Four oxygen feed rates were evaluated, 53 SCFM, 91 SCFM, 98.5 SCFM and 106 SCFM. Table 1 and Chart 1 summarize the test results. 1. Mixing Uniformity was determined from the Total Suspended Solids (TSS) in grab samples pulled from various points in the basin. The uniformity ranged from 88-92%. 2. Oxygen utilization was determined from the mass of oxygen applied relative to the mass of oxygen in the off-gas using the following basic formula: (Off-gas Volume x O2 Concentration) / (Feed-gas Volume x O2 Concentration) Oxygen utilization ranged from 54% to 79%. 3. Energy efficiency was calculated from the mass of oxygen transferred into the wastewater divided by the total brake horsepower for operation of the Chemineer mixer and the injector system. The energy efficiency ranged from 6.3-9.9 pounds of oxygen per brake horsepower applied. 4. Dissolved oxygen concentrations greater than 0.2 mg/l were very difficult to maintain due to the high strength of the influent waste and that the test cell was the first one in the train and was also the receiving cell for the return activated sludge (RAS). On one occasion, the influent loading was dramatically reduced resulting in a DO of 4.7 mg/L.
Case study - Pure oxygen injection pilot test