Phosphorus Removal at Extended Aeration Plants

We just got our new KPDES permit, and now I have to meet a phosphorus limit. I operate an extended aeration plant for a small town. What are we supposed to do – build a new plant? We can’t afford that.

The above situation is one I have encountered with operators of extended aeration plants in the past. In these days of financial setbacks, we are all looking for ways to get more out of the things we already have – to get another year out of that truck or another couple of months out of that pump. On that note, it is important to consider phosphorus removal. If it’s not already a parameter on your KPDES permit, it may be very soon – at this point, you will have to meet a set permit limit.

Currently, most secondary wastewater treatment processes don’t remove more than 20 percent or so of the influent phosphorus. With changes in permitting, better removal percentages will be required to meet the KPDES limit.

To do phosphorus removal biologically, you have to optimize your treatment process for phosphorus removal. You do this by changing the way you aerate the wastewater. In an extended aeration facility, you need to produce an area with extremely low or nonexistent dissolved oxygen (anoxic or anaerobic areas). One method is to reduce the aeration to almost nil in the influent and return activated sludge (RAS) introduction section of the plant. These flows normally have low dissolved oxygen (DO) anyway.

Use this section of the aeration basin as a fermentation zone (low or no DO) for the production of volatile fatty acids (VFA) and phosphorus accumulating organisms (PAOs). The less DO, the quicker and more efficient the reproduction and fermentation process. In this low DO area, there will also be a small release of phosphorus that accompanies the fermentation process. To keep this area mixed and moving, some aeration is needed; not for the addition of oxygen, but for the mixing action given by the aeration itself. Reducing the volume of air added in this section will accomplish this, but watch out for a rise in the DO levels, and adjust the flow of air to counter the rise.

Another method for mixing is to insert a mechanical mixer that does not add DO and only moves the wastewater through this section of the treatment. 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 anoxic or anaerobic area.

As the flow of wastewater moves from the fermentation section to the aerated section, the VFA have been produced and stored in the bacterial cell and are ready for use by the bacteria (PAOs). The PAOs use the carbon in the biochemical oxygen demand (BOD) in the influent 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 orthophosphates in the wastewater are 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 cell.

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 changes to the aeration process in the plant will not cause an immediate change; it takes time for the process and the PAOs to establish themselves in high enough concentrations to remove enough phosphorus to make a difference. For more information on this process, view our training manual.

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