Introduction

Nitrogen is one of the most abundant elements. About 80 percent of the air we breath is nitrogen. It is found in the cells of all living things and is a major component of proteins. Inorganic nitrogen may exist in the free state as a gas (N₂), or as a nitrate (NO₃^-), nitrite (NO₂^-), or ammonia (NH₃⁺).

Nitrogen-containing compounds act as nutrients in streams in rivers. Nitrate reactions (NO₃^-) in fresh water can cause oxygen depletion. Thus, aquatic organisms depending on the supply of oxygen in the stream will die. The major routes of entry of nitrogen into bodies of water are municipal and industrial wastewater, septic tanks, and animal wastes. Bacteria in water quickly convert nitrites (NO₂^-) to nitrates (NO₃^-).

Nitrites can produce a serious condition in fish called "brown blood disease".

Nitrites also react directly with hemoglobin in human blood and other warm-blooded animals to produce methemoglobin. Methemoglobin destroys the ability of red blood cells to transport oxygen. This condition is especially serious in babies under three months of age. It causes a condition known as methemoglobinemia or "blue baby syndrome". Water with nitrite levels exceeding 1.0 mg/L should not be used for feeding babies.

What are Nitrates and Nitrites?

Nitrate (NO₃^-) and nitrite (NO₂^-) are naturally occurring inorganic ions that are part of the nitrogen cycle. Microbial action in soil or water decomposes wastes containing organic nitrogen into ammonia, which is then oxidized to nitrite and nitrate. Because nitrite is easily oxidized to nitrate, nitrate is the compound predominantly found in groundwater and surface waters. Contamination with nitrogen-containing fertilizers (e.g. potassium nitrate and ammonium nitrate), or animal or human organic wastes, can raise the concentration of nitrate in water. Nitrate-containing compounds in the soil are generally soluble and readily migrate with groundwater.

Water naturally contains less than 1 milligram of nitrate-nitrogen per liter and is not a major source of exposure. Higher levels indicate that the water has been contaminated. Common sources of nitrate contamination include fertilizers, animals wastes, septic tanks, municipal sewage treatment systems, and decaying plant debris.

The ability of nitrate to enter well water depends on the type of soil and bedrock present, and on the depth and construction of the well. State and federal laws set the maximum allowable level of nitrate-nitrogen in public drinking water at 10 mg/L (10 parts per million).

Nitrification

Nitrification is the conversion of ammonia (NH₃⁺) to nitrate (NO₃^-). How is this done? This is a two-step process that is done with oxygen and and two types of bacteria, Nitrosomonas (ammonia-oxidizers) and Nitrobacter (nitrite-oxidizers), known collectively as the nitrifiers.

Ammonia + Oxygen + Alkalinity + Nitrosomonas = Nitrite

Nitrite + Oxygen + Alkalinity + Nitrobacter = Nitrate

Nitrite (NO₂^-) is the unstable form of nitrogen and is easily converted because it does not wish to be in this form. The total conversion of ammonia to nitrate takes 4.6 parts oxygen and 7.1 parts alkalinity to convert 1 part ammonia.

Denitrification

Denitrification is the conversion of nitrate (NO₃^-) to nitrogen gas (N₂). How is this done? Heterotrophic bacteria (capable of utilizing only organic materials as a source of food) utilize the nitrate as an oxygen source under anoxic conditions to break down organic substances.

Nitrates + Organics + Heterotrophic bacteria = 
Nitrogen gas, Oxygen and Alkalinity

Total Kjeldahl Nitrogen

Now that you understand the different forms of nitrogen and terms that you will be dealing with, the next questions are what forms of nitrogen do you test for and what can you use to test for them?

Total Kjeldahl Nitrogen or TKN is defined as total organic nitrogen and ammonia nitrogen. Total Kjeldahl Nitrogen (TKN) is an involved test that many wastewater treatment facility labs are not equipped to perform. If you can't perform this test, you still need to monitor the nitrogen cycle at the plant. The ammonia values are approximately 60% of the TKN values, and the organic nitrogen is generally removed in the settled sludge. Also, TKN generally equals 15-20% of the Biochemical Oxygen Demand (BOD) of the raw sewage. The following tests are a must to monitor and control the nitrogen cycle: pH, alkalinity, ammonia, nitrite and nitrate.

Nitrification In the Treatment Plant

To establish a good handle on nitrogen in your wastewater treatment facility, you must develop a good sampling program that will give a complete profile of your system.

The first sampling point would be the raw influent or primary effluent if you have a primary clarifier. Typically, what is entering the facility will be high in alkalinity and ammonia with very little to no nitrite or nitrate. Why is this important? When you start converting ammonia to nitrate in the aeration tank many hydrogen ions will be released. When alkalinity drops below 50 mg/L your pH can drop dramatically. You should never allow the pH of the aeration tank to drop below 6.5. Biological activity will be inhibited and toxic ammonia can bleed right through your system.

The next sampling points to establish are in your aeration tank. The length of the tank will dictate how many sampling points are required. Generally you would use three locations: the front, middle and end of the aeration tank. If a suitable environment is maintained in the aeration tank most of the ammonia will be converted to nitrate by the time it leaves the tank.

The final sampling point will be the plant effluent prior to chlorination. There should never be less than 50 mg/L of alkalinity. The pH should never be out of the permitted range. Ammonia should have extremely low concentrations. Nitrite should be very low to non-detectable and the majority of the nitrogen will be i the nitrate form.

Throughout all of your testing the nitrite levels should be very low. Why bother testing for them then? High levels of nitrite in the system indicate there is, or about to be, a problem with the nitrification cycle. Nitrosomonas (ammonia-oxidizers) bacteria aer harder to kill than Nitrobacter (nitrite-oxidizers) bacteria. If the Nitrobacter bacteria are killed off, the Nitrosomonas bacteria will continue working on the ammonia and you will have a jammed cycle with h igh levels of nitrite. An effluent with high nitrite concentrations will be difficult to disinfect because of the tremendous chlorine demand it poses.

What type of problems might you encounter while performing nitrification? A decrease in the aeration tank pH due to insufficient alkalinity causing ammonia to bleed through the system wich will cause a decrease in the microbiological activity. An inability to completely nitrify due to a lack of dissolved oxygen; mixed liquor suspended solids, mean cell retention tim, and cold temperatures.

All these factors can inhibit the nitrification cycle. High ammonia discharges can affect your toxicity testing. High nitrite levels will cause a tremendous chlorine demand making disinfection difficult, jeopardizing your fecal coliform limits. Leaving sludge that is high in nitrate too long in the secondary clarifier can cause the sludge blanket to rise to the surface when the nitrogen gas is released. This will make quite a mess and will jeopardize your total suspended solids limits.

Why bother to nitrify at your wastewater treatment facility if there can be this many problems? Aside from permit limits, ammonia is toxic to fish and other aquatic life. Ammonia discharges also place a very high oxygen demand on the receiving streams.

Denitrification In the Treatment Plant

Now that we have converted all this ammonia to nitrate, how can we remove it from the system or more specifically perform denitrification?

An anoxic zone (oxygen deficient) will have to be established within the wastewater treatment facility. Regardless of where and how you do this, the principles will always be the same. The dissolved oxygen levels must be as close, without reaching, 0.0 mg/L as possible. A safe target point to avoid septicity while starting your zone would be 0.5 mg/L. A good operating point would be 0.2 mg/L. There must be a carbon source. Raw influent usually works fine but some plants have to supplement the carbon source by injecting methanol or ethanol. It takes about 2.0-2.5 parts methanol for every part nitrate that is denitrified. The MLSS concentration must be kept in balance with the food supply. In other words, the Food to Microorganism (F/M ratio) should be in the proper range (on the lower end) for the type of process you are operating. The pH of the anoxic zone should be close to neutral (7.0) and never drop below 6.5.

How will all this come together to work? Heterotrophic bacteria need a carbon source for food. They obtain their oxygen the easiest way possible using the following sequence: free and dissolved oxygen, nitrate, and then sulfate (SO₄). If your zone has no free or dissolved oxygen, the "bugs" will have to obtain their oxygen source by breaking down the nitrate that are returned to the anoxic zone in the activated sludge. As the "bugs" utilize the nitrate as an oxygen source to break down the carbon, their source of food, nitrogen gas will be released to the atmosphere.

Bugs + Carbon + Nitrate = 
Nitrogen Gas + Oxygen + 3.6 parts Alkalinity

How will you now when all the nitrate is used up? The next place the bugs will go for their oxygen source is the sulfate. As the sulfates are used up, sulfides will combine with hydrogen to form hydrogen sulfide and this stinks like rotten eggs. Why would you want to perform denitrification at your facility? The obvious reason would be total nitrogen limits in your discharge permit, others include alkalinity and oxygen recovery, the desire to produce a high stabilized effluent, and a reduction of problems with rising sludge in your clarifier.

Review

Nitrate (NO₃^-) and nitrite (NO₂^-) are naturally occurring inorganic ions that are part of the nitrogen cycle. Microbial action in soil or water decomposes wastes containing organic nitrogen into ammonia, which is then oxidized to nitrite and nitrate. Water naturally contains less than 1 milligram of nitrate-nitrogen per liter and is not a major source of exposure. Higher levels indicate that the water has been contaminated. Common sources of nitrate contamination include fertilizers, animals wastes, septic tanks, municipal sewage treatment systems, and decaying plant debris. State and federal laws set the maximum allowable level of nitrate-nitrogen in public drinking water at 10 mg/L (10 parts per million).

Nitrification is the conversion of ammonia (NH₃⁺) to nitrate (NO₃^-). Denitrification is the conversion of nitrate (NO₃^-) to nitrogen gas (N₂). Total Kjeldahl Nitrogen or TKN is defined as total organic nitrogen and ammonia nitrogen. If you can't perform this test, you still need to monitor the nitrogen cycle at the plant.

Sources

Understanding the Basic Principles of Nitrogen by Robert Scott