Optimizing Enhanced Biological Phosphorus Removal in Oxidation Ditches


  • Alyssa Mayer, Dan Miklos, Joe Rohrbacher PE., Katya Bilyk PE, Sarah Galst PE - Hazen and Sawyer

There are many oxidation ditch installations at water resource recovery facilities throughout the United States. Mechanical aerators typically provide both aeration and mixing, while inducing a relatively high velocity within the ditch. The mixed liquor suspended solids (MLSS) typically travels the entire loop within 15 minutes, but makes hundreds of revolutions before exiting the ditch. Unlike a conventional activated sludge configuration equipped with diffused aeration, the mechanical aerators in an oxidation ditch supply the required oxygen to a very small footprint, rather than uniformly throughout. As a result, relatively high dissolved oxygen (DO) conditions exist near the aerators but quickly return to anoxic conditions downstream. Aerator speeds are adjusted to control the oxygen-deficient zones within the ditch which are characterized as “aerated-anaerobic” or “aerated-anoxic” zones. These conditions promote nitrogen removal through simultaneous nitrification and denitrification (SND) and can also promote the growth of polyphosphate-accumulating organisms, if properly controlled.

The oxidation ditch configuration, while well-suited for carbon and nitrogen removal, poses some additional challenges when attempting to implement enhanced biological phosphorus removal (EBPR). These systems can be effectively designed, modified or optimized to meet low effluent total phosphorus (TP) limits, but there are several factors that must be addressed, including:

(1) Appropriate anaerobic zone sizing (high F:M ratio to facilitate phosphorus release) (2) Sufficient aeration capacity to maintain aerobic conditions for complete phosphorus uptake (3) Aeration control to maintain appropriate DO conditions under varying loading conditions (4) Internal recycle and return activated sludge (RAS) management to prevent DO and/or nitrate “poisoning” of anaerobic zone

Several case studies will be presented to demonstrate unique and cost effective approaches that have been implemented to optimize EBPR at several different types of oxidation ditch plants.

For more information, please contact the author at amayer@hazenandsawyer.com.

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