To Nitrify or Denitrify?

A training session on nitrogen removal turns into a troubleshooting venture that helps a plant team correct operating issues and achieve permit compliance.
To Nitrify or Denitrify?
FIGURE 1: A properly operating anoxic basin is gently blended with no aeration.

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The Lab Detective was asked to provide some informal training about nitrogen removal in wastewater treatment plants, including a discussion of the various flow schemes used to remove nitrogen from the waste stream and return it to the atmosphere.

Always one to oblige, the detective launched into a lengthy description of the various forms of nitrogen in raw wastewater, how it got there, and where it goes. "Nitrogen enters the waste stream mostly from human beings in the form of urea," he explained.

"This organic form of nitrogen experiences some changes through the wastewater collection system, since it is combined with water to help carry it to the treatment plant. As the mixture of urine, fecal material, food waste, paper products and other flushable items make their way through the pipes, the organic nitrogen in the urine converts to ammonia and ammonium, the amount of each depending on the water temperature, pH and time in the collection system."

An operator asked, "So, which is it — ammonia or ammonium?"

The detective thought for a moment before replying, "I'm glad you asked, James. Great question! If the influent wastewater temperature stays between 15 to 30 degrees C and the pH is between 6.5 to 8.0, then ammonium ions dominate and we are really seeing ammonium at the headworks."

James replied, "OK, so what does all that mean?"

"Well," replied the detective, "it means the form of nitrogen expressed as ammonium ion is present in the liquid as a part of the liquid, a charged particle containing one part of nitrogen and four parts of hydrogen. It's expressed as NH4+; the plus sign designates it as a positively charged ion. Some folks call it ionized nitrogen. When nitrogen is in the un-ionized form, we call it ammonia, or NH3, and it exists as a dissolved gas in the water, like oxygen."

The "light bulb" came on above James' head and he replied, "Oh, like when I started up my aquarium and most of the fish I put in died. The salesperson at the fish store said they probably died from too much ammonia, a result of their own waste overwhelming the tank, now I get it!"

Nitrification

The detective continued, "Once the ammonium reaches the wastewater treatment plant, it continues to change. When oxygen is used to treat the wastewater entering the plant, some bacteria begin to use the ammonium as a source of energy. We call these specialized bacteria nitrifiers, and they use the energy gained from splitting the NH4+ along with calcium and other components in the wastewater to continue producing more nitrifying bacteria. Since these bacteria are normally strict aerobes, they do this process called nitrification in aeration tanks, or in tanks that contain enough dissolved oxygen.

"Nitrifying bacteria are pretty specialized. Several species live in our treatment plants, and this culture grows when the mixed liquor suspended solids (MLSS) environment is favorable to them. One species, nitrosomonas, can use oxygen during the oxidation of ammonium to a compound called nitrite. The second species, nitrobacter, carry on the oxidation of the nitrite to a fully oxidized form of nitrogen called nitrate."

The detective reminded the operators that this occurs under aerobic conditions with a favorable liquid temperature, slightly basic pH, and a good supply of alkalinity present as a carbon source for the nitrifying bacteria.

Denitrification

During the description of denitrification, the detective explained, "A very large percentage of the bacteria present in activated sludge systems are facultative anaerobes, which means they can survive and reproduce in environments that contain free oxygen or alternate forms of oxygen, like the one found in the nitrate compound (NO3), sulfate molecules (SO4) or even molecules of carbon dioxide (CO2). Denitrification takes place best in a tank considered anoxic — not oxic, or free of dissolved oxygen." (Figure 1)

The operators seemed to understand. One said, "So now that we have a better grasp of the theory of nitrogen removal, tell us how this is supposed to work in our little treatment plant. It seems that our plant is built backwards, if the process works the way you say it does."

"Great observation John!" the detective exclaimed. "The facility you operate is known as a Modified Ludzack-Ettinger (MLE) process." (Figure 2)

The MLE process

The detective went a little further. "Does anyone recall the environmental conditions required in the description of nitrification?" A young operator named Jared offered a correct list of the items needed for successful nitrification.

"Great job Jared! That's correct!" the detective said. Using the plant's flow pattern description shown in Figure 2, he then asked, "If it takes all those necessary conditions to nitrify, then what is really happening to the ammonium that entered the plant during its time in the anoxic zone?"

James replied, "I guess, since it takes all that oxygen and stuff to nitrify, then probably nothing is happening to the ammonium in the anoxic tank, right?"

"Yes, correct James!" said the detective. "Theoretically, the ammonium is staying intact through the denitrification (anoxic) zone, then converting to nitrite and nitrate in the aeration tank (oxic zone)."

James added, "It's like putting the cart before the horse, so to speak, isn't it?"

The detective laughed. "Yes, it kind of is, James."

During the training session, the detective asked that samples be brought in from various places around the treatment plant. He asked for screened influent, anoxic basin effluent, oxic basin effluent and clarifier effluent liquid samples. In the small lab at the plant, he performed a few process control tests with the samples, which included ammonia, nitrite and nitrate, alkalinity and pH, and orthophosphate.

The Lab Detective was stunned. The results did not reflect or confirm the material he described in his lecture on the MLE Process. Repeat analysis and accuracy checks with known standards only perplexed him further, since the QA/QC tests were all in line. The results (see Table 1) indicated that nitrification was occurring in the anoxic zone!

The detective used these results in a talk about using the data and applying it to the actual operation, turning the discussion into a makeshift troubleshooting session. He explained his hypothesis of nitrification occurring in the anoxic basin, looking for feedback from the operators.

Finally, James spoke up. "You know, it does seem to be quite aerated at the point where the return activated sludge (RAS) enters the anoxic basin," he said. "I have seen some dissolved oxygen present in that area, but I figured the meter must be incorrect, since there shouldn't be DO at that point. Sometimes I get a reading over 1.0 mg/L there."

Out to the plant

The detective calibrated his portable meters and lab equipment, did standard checks, and headed out to the treatment plant with the operators. Donning nitrile gloves, he began taking readings. The results confirmed the hypothesis conceived in the lab/classroom: nitrification was indeed occurring in the anoxic basin! Results of field testing showed that there was indeed dissolved oxygen in the anoxic basin in sufficient quantities to allow nitrification to occur and to greatly interfere with denitrification. Readings taken included:

Dissolved oxygen (DO), mg/LOxidation/reduction potential (ORP), millivoltspHTemperature, degrees C

The operators were particularly interested in the DO and ORP readings. Jared asked why the ORP was so low in the influent structure, yet the DO was over 1.0 mg/L.

The detective explained, "Remember that DO and ORP are not the same parameter. DO is a measurement of the amount of oxygen in solution, whereas ORP measures the liquid's ability to be either more oxidative (positive mV readings), or more reductive (negative mV readings). When the ORP reads zero, we have a balance of oxidizing and reducing chemical constituents."

Jared asked, "Isn't that like pH then? A 7.0 pH is a balance between acids and bases, right?"

"Yes," the detective confirmed. "Here in the influent wastewater, we have dissolved oxygen, an oxidizing agent, present from all the splashing and air entrainment. However, there are more reducing agents present than oxidizing agents, since the ORP reflects this. The most common reducing agent we encounter in influent wastewater is ammonia." Even in the afternoon sunshine, that proverbial "light bulb" lit brightly over Jared's head.

What next?

For this facility to properly remove the nitrogen from the wastewater and return it to the atmosphere, the anoxic basin needed to be optimized, and further readings indicated some level of DO throughout the basin. The Lab Detective worked with the operators to develop an action plan, including where and when to collect samples and take in-plant readings.

During the next week, the operators re-plumbed the RAS pump discharge and anoxic recycle discharge to be below the anoxic tank surface, eliminating the excessive splashing. Mixing of the anoxic basin contents was optimized by de-ragging the mixing pumps and redirecting the discharge flow to provide more thorough tank blending.

Re-piping of the influent flow stream to be in direct contact with the RAS and anoxic recycle flow helped the denitrification greatly. Since the piping at the facility was PVC, the modifications were relatively easy.

DO and ORP values decreased by the next day or so, and effluent nitrate results declined several days after that, achieving discharge permit compliance within two weeks. Since the Lab Detective's informal training session and visit, the plant remains in compliance with its total nitrogen parameters, and the operators have been able to use the knowledge from this experience at other local treatment plants, enabling them to achieve compliance, too.

ABout the author

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.



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