Surface Aerator Interactions

Adequate mixing is essential for oxygen transfer and solids suspension in activated sludge systems. A case history is presented including: (1) selection of mechanical surface aerators based on single tank tests; (2) clean water testing of multiple machines in a large basin; (3) reporting of inadequate mixing and oxygen transfer under operating conditions attributed to aerator interaction; and (4) actions taken to improve performance. Supplemental velocity studies of another basin using floating surface aerators at spacings of 65 and 130 ft (19.8 and 39.6 m) and an operating depth of 10 to 15 ft (3.0 to 4.6 m) are reported. The study concludes: (1) a horizontal velocity of 0.5 fps (0.15 m/s) is required to maintain solids suspension up to 5,000 mg/L; (2) 100-hp (75-kw) mechanical aerators should have mimimum spacing of 70 ft (20 m); and (3) supplemental bottom mixing is required at basin depths greater than 15 ft (approx. 5 m).