McDonald Group International, Inc

Water, Wastewater, Environmental Engineering 



Problem Description

Many wastewater treatment plants operating in Florida, USA, were designed around a process called extended aeration., In this process, wastewater is aerated for a period of 24 hours . Nitrogen in the influent, in the form of ammonia and organic nitrogen, is often converted entirely to nitrate. (Other processes are considered further below)

The vast majority of these wastewater plants discharge their effluent to percolation ponds, where it seeps into the groundwater. Florida gets most of its drinking water from groundwater. Drinking water must not contain more than 10 mg/L nitrate . To prevent degradation of its water supply, Florida imposed a nitrate limitation of 12 mg/L on the effluent from wastewater plants whose effluent ends up in groundwater.

The problem is of course that the vast majority of the sewage plants in Florida are not designed to get rid of the nitrate they produce from the influent ammonia. Many are in constant or intermittent violation of the nitrate standard. Even more frustrating is that a number of plants that are supposedly designed to get rid of nitrate, don't.

Typical Solutions and Strategies

There are several ways to solve the problem:

  • Aerator cycling
  • Process modifications
  • Quick and Dirty strategies
  • Land Application System strategies
  • Groundwater monitoring strategy
  • Other methods



Aerator Cycling:

The most common approach used to control nitrate is to cycle the air supply to the plant on and off. When the air is shut off, certain micro-organisms in the sewage plant utilize the oxygen that is chemically bound in the nitrate, converting it nitrogen gas.

When Does Aerator Cycling Work? It can work when the sewage plant is not near or over capacity nor significantly underloaded. Usually cycle off times need to be at least 1/2 hour long, and the total off time must be at least 35 to 45% of the day.

When Does Aerator Cycling Not Work? It doesn't work when the influent BOD (actually COD) is low and the influent nitrogen (measured as TKN is high.

What's all this business about adding dogfood, molasses, sugar water and methanol? All of these additives increase BOD (and COD), which can create more favorable conditions for nitrate removal processes to work.

What we can do to help: Provide us your plant flows, influent TKN and BOD (preferably COD) test data, blower type and horsepower, and we can develop a cycling strategy to meet your requirements. We can advise if an additive is needed and how much: Here is an example output from our computers:

With our software (recently updated) we have the ability to dynamically model the rise and fall of dissolved oxygen, ammonia and nitrate in wastewater plants as the air supply is turned on and off

Process Modifications:

If cycling isn't appropriate, installing a tank designed to function without aeration (or modifying an existing tank) called an anoxic tank can work in many cases. Modifying an existing plant often involves little more than adding a mixer, a pump, and some minor piping changes.

When Does It Work? This can work when the anoxic tank is the first tank in the treatment process, and when it is 25 to no more than 40% of the total treatment volume.

When Doesn't It Work? This doesn't usually work when the anoxic tank is installed after the aeration tank. It is unfortunate that many wastewater plants are built this way. The reason they don't usually work is that all the COD is consumed in the first aeration tank, leaving no organic fuel for the denitrification reaction to occur in the anoxic tank. It also doesn't work when the plant is underloaded or if the BOD and TKN loads are exceptionally different.

What we can do to help? Give us your test data (COD and TKN influent) and a diagram of your plant and we'll be glad to see if a process modification is appropriate.


Quick and Dirty Strategies

If the plant has a surge tank: Shut the air off to the surge tank and repipe the clarifier return flow to the surge tank. This may help some by creating a small anoxic volume of sewage and sludge in the surge tank. However it may draw the liquid down in your settling tank during low flows, causing an upset, and it does reduce the surge flows.

If the plant is lightly loaded: Cut the air in the plant back as much as possible while still maintaining settleability of the mixed liquor. If possible, remove aeration tankage from usage to get the actual hydraulic retention time down to 18 hours. This will help prevent complete nitrification of the influent ammonia and will also make it easier to denitrify what nitrate is converted.

When nothing works and you are really desperate: There are some theoretical approaches to nitrate control using certain chemicals. This requires a chemical feed pump and special but relatively available liquid chemicals. Don't try this without consulting with FDEP. We can help with chemical selection and dosage calculation.


Land Application System Strategies

Many types of crops, hay especially, take up nitrogen. One solution therefore is to modify the operating permit so that nitrogen reduction is permitted to occur by natural uptake of planted crops, rather than by a limitation on the treatment plant itself. We can help with the nitrogen balance, design and permitting needed to allow this.

Drawbacks to this method are that a significant amount of land is needed and it may not be inexpensive to build.

In effluent disposal systems that stay wet, certain aquatic plants can be utilized to take up excess nitrogen. It may be possible to grow the necessary plants and modify the operating permit to allow sampling at the outfall of the pond.


Groundwater Monitoring Strategies

The objective of the effluent nitrate limitation standard is to prevent nitrate in the groundwater from exceeding 10 mg/L . Nitrate is reduced during land application by soil reactions and by dilution with groundwater. The mechanism of removal is not very predictable, but one strategy is suggested by permitting the sinking of a groundwater monitor well and demonstrating through subsequent tests that such removal occurs. Of course, the risk is run that the test reports might show otherwise.


Other Methods

Breakpoint Chlorination In certain kinds of facilities it may be possible to remove nitrogen before it turns to nitrate using breakpoint chlorination. For a typical influent nitrogen content of 30 mg/L (as ammonia and organic nitrogen), dosages of 150 to 200 mg/L chlorine can reduce the effluent nitrogen content to less than 10 mg/L. This is obviously expensive, and problems from either a depressed pH or too high a chlorine content can result

Nitrate Filtration There are commercially available filters which use methanol that can be effective at removing nitrate. The downside of these filters is that they are expensive and methanol is dangerous to handle.

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