This paper questions the basic practice of using large unaerated zones and large mixed liquor recycle flows to achieve TN and TP removal. It shows that the use of chemicals and different process approaches may be simpler, cheaper and, maybe, even better.
Most people in our profession seem to believe the only way to design nutrient removal wastewater treatment plants (wwtp) is to use some form of Modified Ludzack-Ettinger (MLE) or Bardenpho process. These processes are very well proven. They are able to meet moderately low TN and TP standards without chemical additions or the extra sludge that goes with such additions.
Recently new, stricter standards have emerged on the East Coast of the United States known as Enhanced Nutrient Removal (ENR), and defined as meeting an annual average TN of 3.0 mg/L and an annual average TP of less than 0.3 mg/L. These new standards and other recent activities have resulted in a re-evaluation of biological nutrient removal (BNR) designs. This paper is intended to present some of this re-evaluation and to critically analyze these questions:
- Why choose processes with systems that use high levels of energy for mixing unaerated zones and for MLSS recirculation?
- How valid is the “extra sludge” argument?
- Is using up to four times as much reactor tankage really worth it?
- Are purchased chemicals really more expensive than the costs associated with traditional processes?