Too Much Air? Understanding the Critical Role of Aeration Systems
Excessive aeration in activated sludge and aerobic digestion processes can waste energy and impede treatment performance.
Wastewater operators today are saving energy, using fewer chemicals and improving control of their activated sludge process by optimizing their aeration systems, including aerobic digestion.
Aeration zones are critical in the activated sludge process, which includes secondary treatment and nitrogen and phosphorus removal. The question is: Can you over-aerate that process? Many operators would say, “No, the more the better!” But the real answer is: “Yes.”
Too much is not good
Over-aeration wastes energy and can negatively affect process performance. Most wastewater treatment plants operate their aeration zones and aerobic digesters at 1-3 mg/L dissolved oxygen (DO). Anything more may waste DO and energy. Even aerating at 1 or 2 mg/L over the DO setpoint can be extremely wasteful.
Over-aeration can also cause operational problems. Operators love a mixed liquor that settles well. This happens when the microorganisms in the aeration tank excrete a sticky film around their cells as their food gets depleted. Aeration keeps the bugs in suspension, allowing them to collide and ultimately stick together, forming a floc. The floc exhibits a snowball effect: By the time it reaches the secondary clarifier, it is denser than water and settles. Over-aeration can break this floc apart, causing pin floc or small, dispersed floc that does not settle well.
Anoxic zones used for denitrification can also turn partially aerobic, or oxic, due to the high DO in the internal (nitrate) recycle flow. In a biological nutrient removal facility, internal recycle flow can be 400 percent or more of the influent flow. This recycle flow pulls liquid that is high in nitrates from the end of the aerobic zone, then discharges it back to the head of the anoxic zone, where there should be very little or no DO.