NITRIFICATION: the Basics

Introduction:
Operators in potable water and wastewater treatment plants should know and understand nitrification and denitrification so that they are best able to recognize, anticipate, and subsequently control these processes - whether they are intentional or not.

the NITROGEN CYCLE

The three main "fertilizer" ingredients for plants are nitrogen, phosphorous, and potassium. We will consider here, the effect of nitrogen on water quality and the disinfection process, in both water and wastewater treatment.

The nitrogen cycle diagram best illustrates the relationships that exist among the various forms of nitrogenous compounds and the changes that occur in nature. The atmosphere serves as the reservoir for the nitrogen from which nitrogen is constantly removed by the action of lightning and certain nitrogen-fixing bacteria and algae. As you can see from the nitrogen cycle figure, nitrogen is converted into plant protein organic nitrogen. This means that nitrogen is one of the many elements that make up protein. From here it can be converted into animal protein organic nitrogen. Animals give off waste material, urine and fecal matter, both of which contain organic nitrogen forms. For example, fish release ammonia from their gills. Plant tissues are the only source of nitrogen for animals.

 

"The COMPOUNDS, CONDITIONS, and PLAYERS INVOLVED"

AMMONIA
There are two forms of ammonia, unionized (NH3) and ionized (NH4+). The unionized form is the only toxic form. An equilibrium exists in water between the quantity of unionized (NH3) and ionized (NH4+) ammonia, with the percentage of the toxic form increasing with increasing temperature and pH values.

For example, at 25 degrees C, in fresh water, less than 0.1% of the total ammonia is in the toxic form, but at pH 8.5 NH3 accounts for more than 10% - an increase of over 100 times. Ammonia is less toxic at lower pH levels. The only way to know how much ammonia is in the toxic unionized form is by determining the pH and temperature and then calculating the percentage of ammonia in the unionized toxic form. (There are tables created for this purpose also.) The long-term safe concentration of ammonia varies with different animal species, but the value of 0.02 mg/L of NH3 is most frequently referenced.

What are we measuring and reporting?
Ammonia contains atoms of nitrogen (N) and hydrogen (H). Some analysis may report Total Ammonia, as described above, but other analysis refer to the quantity of nitrogen in the ammonia molecule only. In this case the analysis units are presented as total ammonia nitrogen (TAN), which is the sum of NH3 -N + NH4 -N. Results reported in the two forms are not the same. It is necessary to know the percentage of nitrogen in the ammonia molecule to compare the two units. The atomic weight of nitrogen is 14 and that of hydrogen is 1; thus the molecular weight of NH3 is 14 + 3 = 17. Nitrogen is 14/17 or 82% of the weight of the ammonia molecule. The difference is much more important for nitrite and nitrate. Therefore, one (1) mg/L of NH3 is the same thing as 0.82 ppm of NH3 -N. Obviously, we need to know in what "units" we are reporting, so that we can compare values, etc.

NITRITE
Nitrite may be produced from ammonia as well as from nitrate. Excessive nitrite (NO2 -) is toxic to aquatic organisms, particularly in fresh water (it is much less toxic in sea water). The long-term safe level of NO2 - varies with different species; 0.1 & 0.2 mg/L are considered safe limits in soft and hard water, respectively.

For some analysis, the the results may be reported in the units of NO2 - or NO2 -N. The atomic weight of oxygen is 16; thus the molecular weight of NO2 is 14 + 32 = 46. Nitrogen is 14/46 or 30% of the weight of the nitrite molecule. Therefore, an analysis value of 1 mg/L of NO2 is the same thing as 0.3 ppm of
NO2 -N.

NITRATE
Nitrate is the end result of the process of nitrification. Nitrate concentrations above 100 mg/L have been shown to be nontoxic to many species.

OXYGEN
They are strict aerobes.

ALKALINITY
Alkalinity is a measure of the buffering capacity of the water. The release of hydrogen ions (H+) may lower the pH of lightly buffered water.

Nitrification bacteria are very "fragile" organisms. They are the "trout" of the bacterial groups. Water with low alkalinity may in fact allow for the nitrifiers to create a toxic environment for themselves, and limit their growth. For this reason, it is beneficial to maintain an alkalinity level above 100 mg/L. Alkalinity is NOT to be used interchangeably with the term "basic," as that is the term used to describe the opposite condition of "acidic." (Please see our Operator Notebook summary on this topic if you need more information.) Alkalinity should be at least eight times the concentration of ammonia and ideally over 100 mg/L. This value may be higher for raw wastewaters with higher influent ammonia concentrations than the "normal." The theoretical reaction shows that between 7.07 and 7.14 mg of alkalinity (as CaCO3) is consumed for every mg of ammonia oxidized. Rule of thumb is 10 to 1. The result is an overall reduction in alkalinity and a possible drop in pH if the alkalinity is low to start with.

pH
The optimal pH for nitrification is 8.0; with nitrification limited below pH 6.0.

HARDNESS
This is a measurement of the concentration of calcium and magnesium in the water.

TEMPERATURE
Increasing temperature can increase the rate of nitrification and nitrifier growth, and decreasing the temperature will decrease the rate of nitrification and nitrifier growth. Optimum temperature is about 25° C. Nitrification is inhibited at 10°C or less. ( Nitrobactor , which converts nitrite into nitrate, has been shown to be the first one to slow in growth rate when the temperature drops.) Please see the temperature versus MCRT graph in the wastewater section for a visual representation of temperature impacts.

NITRIFICATION

Bacteria and chemical actions convert these two compound groups (plant and animal protein groups) into ammonia (NH3+ or NH4+ ) .

Nitrification is a two step process:

First Step: The nitrosomonas group of bacteria, known as nitrite formers, convert ammonia under aerobic conditions to nitrites and derive energy from the oxidation:

Another way to balance this equation is:
NH4+ + 1 1/2 O2 ----> NO2- + 2H+ + H2O
(In any case, for the chemists in our group, 66 kcal of energy are liberated per gram atom of ammonia oxidized.)

The following microbes are also known to convert ammonia into nitrites: Nitrosospira, Nitrosouva, Nitrosolobus, Streptomyces, and Nocardia (our favorite activated sludge foam causing microbe).
NOTE: these organisms are chemo-autotrophs, growing with ammonia as the energy substrate and CO2 as the main carbon source. Generally, the optimum growth temperature is 30 degrees C and the optimum pH is between 7.5 and 8.0.

Second Step: Then the "nitrite nitrogen" is oxidized by the Nitrobacter group of nitrifying bacteria, also known as the nitrate formers, into the "nitrate nitrogen" form:

Another way to balance this equation:
NO2- + 1/2 O2 -----> NO3-.
( 18 kcal of energy is liberated per gram atom of nitrite oxidized.)

Nitrospina, and Nitrococcus are also known to convert nitrite into nitrate.
They are mesophiles, with a temperature optimum of 28oC and a pH range of 5.8-8.5, with an optimum pH range between 7.6 and 7.8.

The nitrates that are formed may now serve as fertilizer for most plants. It requires about 4.5 parts of oxygen to convert one (1) part of "ammonia nitrogen" into "nitrate nitrogen".

For a "basic summary" then, nitrification means the conversion of ammonia into nitrites; then the nitrites into nitrates.

If we have "nitrification", can we have the opposite reaction? YES!

DENITRIFICATION

In denitrification, nitrates are reduced to nitrites, and then the reduction of nitrites to nitrogen gas occurs. (Reduction is the opposite of oxidation.) Reduction of nitrites is carried all the way to ammonia by a few bacteria organisms, but most of them carry the reduction to the end product of nitrogen gas, which escapes into the atmosphere. The bacteria utilize the nitrites and nitrates as the oxygen source in their life processes. This process is carried out under anoxic or near anaerobic conditions.

This chemical reaction is shown as:


Many different groups of anaerobic and facultative bacteria can denitrify. Pseudomonas and Bacillus are aerobic denitrifiers.

(Later we will discuss why we would WANT to denitrify or NOT to denitrify.)

 

CHLORINE BREAKPOINT & AMMONIA REACTIONS

Chlorine breakpoint and reactions with ammonia: the "number one impact" in water and wastewater treatment! Why?

In our water treatment plant we may wish to chloraminate, (mix ammonia and chlorine together). WE MUST get the ratio of chlorine to ammonia correct!

If our wastewater treatment activated sludge plant is being operated at a "nitrifying" MCRT value; (or our "other nitrifying plant facilities" are at an organic loading rate and hydraulic loading rate for a biological process that has a sufficient organic material reduction that forces nitrification to completely occur), then we are not going to impact our disinfection by chlorination process. This is because the nitrate form does not react with chlorine. But if we only "partially nitrify", when we chlorinate with an "ammonia nitrogen" and "nitrite nitrogen" species in the disinfection process we will have a significant impact on our chlorine demand levels! Let us review what happens to create this situation.

In the addition of chlorine to water, whether it is water or wastewater treatment, the reactions will be the same: chlorine forms hypochlorous acid very quickly:
Cl2 + H2O = HOCl + Cl--

When the hypochlorous acid comes into contact with ammonia, we have the hypochlorous and hypochlorite ion reacting with the ammonia. This yields a monochloramine, dichloramine, and/or trichloramine species....depending on the chlorine to ammonia ratio, pH of the water, and the water temperature. The three possible chloramines, also termed/described as "combined chlorine" are formed as follows:

(Trichloramine, also called nitrogen trichloride, can only be formed in a low pH situation, not normally experienced in the plant processes, so we can almost disregard it for operational impacts.)

Typically, most our chlorine residual analyzers have probes that read only the monochloramine form, as it is the predominate form.

In water treatment, the ratio of chlorine to ammonia is critical. We will discuss in the water impact section.

In wastewater, all of this leads to being caught in the "transition zone".....between having a lot of ammonia in the wastewater being disinfected, or having a completely nitrified effluent for disinfection. A small amount of ammonia, even one mg/L breaks through, then our chlorine residual automatic feed system senses the decreasing chlorine residual, and the controllers ramp up, to feed up to 10 mg/L dosage for each 1 mg/L of ammonia, to make up for it. Nitrite is a very unstable form, and the nitrobacters convert it to nitrate very quickly. It is almost impossible to get a lab value on the nitrite level! So, if we test for ammonia, and find ammonia in a detectable concentration, it is safe to say that the nitrite form is transitioning also.

(10 MGD)(8.34 lb/gal)(10 mg/L chlorine dose) = 834 pounds of ADDITIONAL chlorine, added to the present feed rate.....NOW, make it a 2 mg/L ammonia concentration, and we get to add 1,668 lb/day chlorine feed rate! At this rate, we can see how easy it is to create a chlorine feed rate that is two to three times our "normal" rate!

At very high doses of chlorine, with small amounts of ammonia, oxidation - reduction reactions occur that convert ammonia into elementary nitrogen.

2Cl2 + 3H2O + 2NH3 -------> 6HCl + 3H2O + N2

According to this reaction, 1 mg/L of ammonia consumes 7.6 mg/L of chlorine. A review of the literature and process data suggests that it may vary from 8 to 10 mg/L of chlorine for every
1 mg/L of ammonia consumed by the chlorine! This is costly, in BOTH water and wastewater treatment!

Again, for "positive enforcement": nitrate is non-reactive with the chlorine residual in the water, and nitrite is reactive as shown by the reaction:
NO2--- + HOCl-- ---------> NO3 + H+ + Cl--

In the following wastewater treatment discussion, we will move back and forth between the well-known "Breakpoint Chlorination Curve," Figure 1 here, and the "Ammonia Impact Upon Chlorination," trend/graph, Figure 2 in the Wastewater Treatment section. This is certainly not meant to confuse anyone! This exercise is necessary to: a) fully understand the breakpoint chlorination curve, b) interpret trended information regarding chlorine applications in disinfection pertaining to ammonia impacts, and c) apply all of this information in our disinfection operations.

"The Breakpoint Chlorination Curve"
We start with the "Breakpoint Chlorination Curve," Figure 1. At ‘Segment 1’, we start to add chlorine. We keep adding chlorine, but there is no creation of a chlorine residual, as there is a "chlorine demand", which consumes the chlorine we are adding.

Figure1

When we have added sufficient chlorine to equal this chlorine demand, we are now able to start creating a chlorine residual. This is at the beginning of ‘Segment 2’ on Figure 1. As we add more and more chlorine we see an increase in the combined chlorine residual. The chlorine is combined with ammonia and other organic molecules that contain amine (nitrogen) based compounds as part of their composition. (In lay terms, this means that an ammonia-like molecule is attached to a larger carbon-chain molecule.)

As we continue to add chlorine, we finally run out of ammonia, and organic compounds that can accept chlorine. We then begin to further "fill up" the available receptor sites on the nitrogen atom, until the nitrogen atom is liberated. This is labeled ‘BREAKPOINT’ on Figure 1.

The addition of any more chlorine results in the formation of a "free chlorine" residual. As we increase the chlorine feed rate further, we create an even greater "free chlorine" residual, as illustrated in ‘Segment 4’, on Figure 1. We will discuss chloramination in the Water Treatment Impacts section.

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