What are Nitrates and Nitrites?

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.

The plant maintained a nitrifying bacteria population in the cooling tower for pH control and to help meet nitrite discharge limitations associated with the cooling tower blowdown. The plant maintains the nitrifying bacteria population  Purposely maintaining a nitrifying bacteria population in the cooling tower is a very unique process and requires a delicately balanced microbiological control program. If too much biocide is fed, it kills off the nitrifying bacteria and leads to increased acid feed costs, exorbitant biocide costs, and, potentially, discharge violations.

When reviewing the makeup water chemistry, note that these water characteristics lead to a moderate to highly corrosive environment, especially as measured by the Larson-Skold index (explained below). The characteristics of this makeup water dictate close control of the cooling water treatment corrosion inhibitor program in order to meet specified performance targets. A short description of nitrification, denitrification, and the Larson-Skold index follow.

Nitrification. Nitrification is the two-step biological conversion process by which ammonium (NH₄ ⁺) or ammonia (NH₃) is oxidized into nitrite (NO₂ ^-) and then further oxidized into nitrate (NO₃ ^-). In the first step, ammonia-oxidizing bacteria known as Nitrosomonas convert ammonia and ammonium to nitrite. Next, nitrite-oxidizing bacteria called Nitrobacter complete the conversion of nitrite (NO₂ ^-) into nitrate (NO₃ ^-) These bacteria, known as “nitrifiers,” are strict “aerobes,” meaning they must have oxygen to perform their work, which is possible in a cooling tower environment. The reactions are generally coupled and proceed rapidly to the nitrate form; therefore, nitrite levels at any given time are usually low. This is important to the power plant, which has a nitrite discharge limitation due to aquatic toxicity concerns.

The nitrification process also produces nitric acid as part of the bacteria’s metabolic processes. The plant maintains its cooling tower circulating water pH in the control range of 6.5 to 7.0 by utilizing the nitric acid produced to reduce scaling potential and to optimize the oxidant use. This also avoids the use of sulfuric acid feed, which minimizes potential exposure to hazardous acid and reduces the cost associated with acid purchase. Nitrosomonas and Nitrobacter are able to effectively nitrify with a pH of 6.5 to 7.0; however, the nitrification process stops at a pH below 6.0.

Denitrification. Denitrification is the biological reduction of nitrate (NO₃ ^-) into nitrite (NO₂ ^-) and eventually nitrogen gas (N₂) by facultative heterotrophic bacteria under anaerobic conditions. Though these bacteria may prefer to use oxygen in their metabolic processes, under anaerobic conditions, they will extract oxygen from the nitrate, which reduces it to nitrite. Heterotrophic bacteria need a carbon source as food to live, and facultative bacteria can get their oxygen by taking dissolved oxygen out of the water or, as discussed previously, by taking it off of nitrate molecules.

Denitrification and associated anaerobic environments can also lead to elevated nitrite levels. Elevated nitrite levels are undesirable at this location due to environmental discharge limitations and aquatic toxicity concerns. Biofouling and accumulated biomass can produce and foster anaerobic reducing environments and thus should be properly controlled.

Larson-Skold Index. The Larson-Skold index describes the corrosivity of water relative to mild steel. The index is based upon evaluation of in-situ corrosion of mild steel lines exposed to Great Lakes waters. The index is the ratio of equivalent parts per million (epm) of sulfate (SO₄ ^2-) and chloride (Cl^-) to the epm of alkalinity in the form of bicarbonate plus carbonate (total alkalinity):

Larson-Skold index = (epm Cl^- + epm SO₄ ^2-)/(epm HCO₃ ^- + epm CO₃ ^2-)

The Larson-Skold index has been correlated to observed corrosion rates, and to the type of attack observed in a study of Great Lakes waters, and to makeup water sources of similar composition to Great Lakes waters. The index is useful as a predictive tool in evaluating the potential corrosivity of a particular makeup water source as well as the anticipated corrosivity of that water when cycled and treated in a cooling tower system. The index actually correlates the pitting corrosion potential associated with corrosive chloride and sulfate anions versus the amount of natural buffering provided by the total alkalinity in the water.

The Larson-Skold index might be interpreted by the following guidelines:

• Index <0.8: Chlorides and sulfate probably will not interfere with natural film formation.
• Index >0.8 and <1.2: Chlorides and sulfates may interfere with natural film formation. Higher-than-desired corrosion rates might be anticipated.
• Index >1.2: The tendency toward high corrosion rates of a local type should be expected as the index increases.

Water analysis and the equation above indicate that the makeup water for this power plant has a Larson-Skold value of 2.3, which means it is fairly corrosive. Within the circulating cooling tower water, the value jumps to 24.5 due primarily to the formation of nitric acid from the nitrification process. The nitric acid dramatically reduces the natural buffering capabilities of the water typically associated with bicarbonate alkalinity and puts a much greater demand on the corrosion inhibitor portion of the treatment program.

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