Phosphorus Removal Via Oxidation Ditches

In an earlier post I mentioned changing the aeration setup at your facility to remove phosphorus. In this post, I will use the example of an oxidation ditch to expand further upon alternative aeration techniques.

An oxidation ditch (OD) is a variation of the extended aeration process; the wastewater typically flows along a circular or oval path. Mechanical aerators (horizontal axis type) provide oxygen and mixing action to move the MLSS along the pathway. To use a mechanical aerator to remove phosphorus biologically, you must control the aeration while keeping the velocity of the wastewater above approximately 1 ft/sec.

In some oxidation ditches the brushes or aerators are mounted horizontally in the channel that makes up the OD. By adjusting the run times as well as the speed and depth of the brushes, you can control the amount of aeration supplied to the bacteria and the velocity.

The influent flow and the return activated sludge (RAS) flow normally enter the ditch between brushes. By shutting down the brush before the influent and RAS addition point, you can create an anoxic or anaerobic zone, which is required for biological phosphorous removal.

Use this section of the oxidation ditch as a fermentation zone (low or no dissolved oxygen (DO)) for the production of volatile fatty acids (VFA) and phosphorus accumulating organisms (PAOs). The less DO present, the quicker and more efficient the reproduction and fermentation process will occur. In this low DO area there will also be a small release of phosphorus that accompanies the fermentation process.

To ensure that the velocity is kept above the recommended 1 ft/sec measurement, a mechanical mixer that does not add dissolved oxygen and only moves the wastewater through this section of the treatment may be added. An underwater propeller system for water movement could be used for this purpose. The detention time is half an hour to an hour in the low DO section of the ditch.

As the flow of wastewater moves from the fermentation section to the aerated section, the VFA are produced and stored in the bacterial cell and are now ready for use by the bacteria (PAOs).

The PAOs use the carbon in the biochemical oxygen demand (BOD) in the wastewater to produce more cells and use the VFA as an extra energy source during reproduction. As the VFA are oxidized for energy, polyphosphate bonds form in cells for storage of phosphorus.

The soluble orthophosphate is removed from solution in a luxury phosphorus uptake where the bacterial cells take in more phosphorus than they can use. It is then incorporated into polyphosphates within the bacterial cells.

To remove the polyphosphate the cells containing it must be removed from the treatment system through the wasting of sludge. This process only removes the material from the water; it still has to be properly disposed of or reused.

The city of London, Kentucky uses a modification of this type of treatment. Their influent phosphorus concentrations are between 8 and 10 mg/l and the effluent concentration is about .1 mg/l. They do not use any flocculants or coagulants to enhance phosphorus removal. It is strictly accomplished through aeration manipulation.

The removal of phosphorus and other nutrients is one of the most important jobs of a wastewater treatment plant. The physical modifications to the treatment plant can be complicated and expensive, but the changes in treatment aeration may be accomplished by understanding how and why the bacteria work the way they do. Additional information on phosphorus removal can be found in our wastewater treatment certification manual.

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