NITRIFICATION: in WASTEWATER TREATMENT

Of principle concern to all of us are the impacts of nitrification and denitrification in our facilities, whether intentional or not. There are situations for us to desire these conditions (we need to remove the nitrogen from our plant effluent for receiving water quality issues), and other times to avoid them (downstream process impacts, when nitrogen removal is not required). There exists the potential for a Nitrogen Oxygen Demand (NOD) in our effluents by the fact that the nitrogen conversion from ammonia to nitrate requires an oxygen uptake from the receiving waters to occur. The nitrification reaction in the receiving waters relies on the water’s temperature, dissolved oxygen concentration, microorganisms present, etc. Attached nitrifying populations are more prevalent than suspended populations in the receiving waters. In fact, deep, slow-moving rivers are mostly absent from nitrification effects.

How does nitrification play a part in wastewater treatment? How may we use it to our benefit? If we wish to produce a nitrified effluent to improve water quality in our receiving waters, etc., then we need to discuss the design and operational aspects to do so.

FACILITY DESIGN

General:
Nitrogen Species in Municipal Wastewater
A review of the literature written on water quality, suggests that a "normal" domestic wastewater consists of a total nitrogen value in raw wastewater averaging 15 - 50 mg/L. 60% of the nitrogen content being bound in the complex organic matter, 40% of the nitrogen found in the ammonia form and <1% is in the nitrite or nitrate form.. Each person contributes 8 to 12 pounds of nitrogen per year to the wastewater treatment stream in the forms of food processing, urine, and feces.

BOD:TKN Ratio
This ratio is calculated from TKN and BOD measurements on the nitrification step influent. The higher the BOD portion the lower the nitrifier fraction. A BOD:TKN ratio of 2.5 or less should show good nitrification in the treatment system.

Normal Nitrogen Removal Process
During the normal treatment of domestic wastewater, secondary treatment may remove up to 30% of the total nitrogen coming into the plant.

F/M Ratio and MCRT
Generally speaking, to insure complete nitrification (all ammonia and nitrite is converted to nitrate), a higher MCRT of 10 days or more is usually required (see MCRT vs/ temperature graph). Lower water temperature may increase the detention time required to make the process complete all reactions. Generally speaking, a F/M of <0.3 will allow for nitrification. The process is more efficient if the soluble carbonaceous BOD removal is almost complete.

For a "suspended growth" (activated sludge) type of system, we may elect to design larger aeration basins, with additional aeration capacity, to reduce both the CBOD and the ammonia in the one set of aeration basins as shown in this diagram:


With the CBOD reduction to less than 10 to 15 mg/L, (some references say 20 mg/L), then the nitrification can occur in the same aeration basins. It is hard for CBOD to break through in this type of process (like the other nitrifying processes), due to the very low Food/Microorganism ratios. The F/M ratios are not really useful for daily operational use in most nitrifying activated sludge plants with their high MCRTs. Activated sludge plants operated in the nitrification mode, are usually easier to operate, as you have fewer variables to track and manipulate, such as maybe a critical F/M ratio to keep from nitrifying.

Oxidation ditches are racetrack shaped reactors in which rotating brushes both aerate and move the mixed liquor at several points in the loop. The amount of oxygen supplied is determined by the level of the brush rotor in the mixed liquor, and the velocity of the flow remains constant. The region just downstream of the brush aerators is fully aerobic which is where nitrification occurs. As the mixed liquor travels around the ditch, the nitrification reactions take place in which oxygen is consumed. Subsequently, an anaerobic zone is formed near the upstream area of the brushes. (See Operator’s Notebook, "Selectors" for the details.)

We can also place our primary effluent into roughing trickling filters for the majority of CBOD reduction, followed by aeration basins for nitrification, and then the secondary clarifiers for solids separation:

Another option is to design a set of trickling filters placed in series operation, so that the ever decreasing CBOD wastewater is able to be nitrified in the last trickling filter. Another trickling filter option is to include recirculation from the filter underdrain system back over the tower media for increasing the mass (numbers) of nitrifying bacteria.


Our last design option may include a set of rotating biological contactors (RBCs), placed in series operation, so that the ever decreasing CBOD wastewater is able to be nitrified in the last train of RBCs. At low hydraulic load ratings, and soluble CBOD loadings of less than 15 mg/L:

Next, let us establish a few facts for the operation of our facility that will be nitrifying:

As stated previously, nitrification requires (stoichiometrically) about 4.57 parts of oxygen to convert one (1) part of ammonia into nitrate. This reaction for the chemists in our group is as follows:
2NH4HCO3 + 4O2 + Ca(HCO3)2 = Ca(NO3)2 + 4CO2 + 6H2O
Conversely then, nitrification is inhibited when the dissolved oxygen (DO) is less than 1.5 mg/L (Less than 1 mg/L has an even larger impact on inhibiting nitrification). We will need to maintain about 3 mg/L dissolved oxygen in the wastewater nitrifying process for optimum performance.

Nitrifiers have longer lives and reproduce much more slowly (nitrobacter is the slowest to reproduce), and therefore require longer Mean Cell Residence Times (MCRTs ) in our activated sludge plants in order to select/enhance this form of treatment. Biofilters and Rotating Biological Contactors (RBCs) will therefore require more surface area and media to accomplish this. For example, the MCRT needed to provide a nitrified effluent as related to temperature (EPA data) would graph as follows:

Due to temperature impacts you may find a lower value for the summer, ie: 10 day MCRT and a higher value for winter, ie: 14 to 16 day MCRT. If you lose your nitrifiers during the cold times (ie: high flow rates due to I & I event) , you may not be able to get the population back until warmer water temperatures return. The best conditions for nitrifiers are a neutral to slightly alkaline pH, plenty of dissolved oxygen, an adequate supply of carbon dioxide, and warmer temperatures.

Nitrifiers will be among the first microorganisms that toxins will affect. Toxins can have an inhibiting affect, as in a normal CBOD reduction, and loss/partial nitrification situation developing. A nitrified effluent produces a "swimming pool" clear water when the process is dialed in to the best solids settling characteristics and the secondary clarification process.

Each mg/L of ammonia nitrogen converted into nitrate will consume about 7 mg/L of alkalinity (buffering capacity ). This is due to the hydrogen ions being liberated from the ammonia as an acid. This is usually not much of a problem for most domestic wastewater’s.

Nitrification costs more in both tankage (and/or media) and also in operational costs due to the energy required to place the required oxygen into the water. (Remember: it requires about
4.5 mg/L DO to oxidize the ammonia into the nitrate form.) In biofilters, more trickling filter recirculation is required to accomplish this task.

There are other significant impacts. Some of these are:
The BOD test measures two things: the carbonaceous BOD (CBOD) and can also measure the nitrogenous oxygen demand of the sample being tested. The raw wastewater and primary effluents when tested, contain very few nitrifying bacteria, and the test measures only the CBOD. If the sample is a secondary treatment process or final effluent sample, and nitrifiers are present, then we normally conduct an "inhibited BOD test". This means that we place a nitrification inhibiting substance into the samples, (HACH #2533) to eliminate the additional measurement of the nitrification reaction. A fully nitrified effluent contains only nitrates....there are few to little ammonia and nitrite forms in it. As such, it does not impact the test.

REMEMBER also: Ammonia, in the un-ionized form is toxic to fish and other water organisms, and may not be desirable to place into the receiving waters. We may need therefore to remove the ammonia from the water, or at least discharge a "nitrified effluent" which is not toxic. Often a cause for failing bioassays!

Many of us love to say that "Nitrification is something we either do or do not do!" Most of our problems occur when our facility’s go into and out of nitrification....the "partial nitrification" status! The downstream process is most always the disinfection process, and can be greatly affected by partial nitrification!

As we have stated in the previous sections on nitrification, the idea of nitrification as a process is to eliminate nitrogen from the water being treated. What happens if we do not eliminate ALL of the ammonia and some starts to break through? What happens if we WANT to stay out of nitrification, and we start converting ammonia into nitrate?

"Ammonia Impact Upon Chlorination," trend/graph, Figure 2.
This trend screen is an actual screen from our wastewater treatment facility. (In order to show it here, we have taken the necessary liberty to enhance the actual data plot.) This screen covers a 24 hour period of time, starting at the left side of the page. Starting then at the left, as it shows at ‘Segment A’, we are in full nitrification. In ‘Segment A’ the treatment plant flow rate, in MGD units, is increasing from the night’s low flows, (the black line). The actual chlorination basin influent chlorine residual, in mg/L units, (dark blue line) is shown along with the chlorination basin effluent chlorine residual (the light blue) , in mg/L units. (Note that it is slightly lower than the initial value due to a "latent chlorine demand"). The chlorine in the chlorine contact basin has been analyzed and is described as a "free chlorine residual."

Moving to the right, note that as time progresses we start to show an increase in ammonia, starting at ‘Segment B’. The ammonia is "breaking through" the aeration basins upstream. The ratio of chlorine to ammonia at this point is quite high. Therefore any ammonia introduced at this point will result in the chlorine oxidizing all of the ammonia. The consumption of the chlorine in this process of oxidizing ammonia, results in a reduction of the free chlorine residual when compared to the set-point. The instrumentation therefore increases the quantity of chlorine it is feeding to compensate. As more and more chlorine is consumed the instrumentation loop fights to maintain the chlorination dosage set point. This instrumentation increase of the chlorine feed rate is shown on the graph, by the green line, in pounds per day units (signal is from the chlorinator).
In ‘Segment C’, we see that when the ammonia has increased to a level capable of supporting a combined chlorine residual, there is a corresponding decrease in the chlorine feed rate. Note: in this particular case the combined chlorine residual is higher than that of the free chlorine residual when we're fully nitrifying. How can this be?
Please refer now to the ‘breakpoint curve, Figure 1. Find the area of the plot to the right of the breakpoint, marked ‘Y’. This is the approximate area of the free chlorine residual. When the combined chlorine residual is established, it moved back to the area marked ‘X’.
Also, please note the offset in time between the decrease/increase curve of the chlorine basin influent and the corresponding decrease/increase curves of the chlorine basin effluent trends.
As the ammonia level decreases at the end of 'Segment C' it finally reaches a level where once again the ratio of chlorine to ammonia creates another chlorine demand situation. In 'Segment D', the instrumentation increases the chlorine feed once again, as shown by the chlorine feed rate in pounds per day (the green line) to compensate for this consumption of chlorine. The greater the demand, the greater the drop in the chlorine concentration, and the greater the corresponding increase in the chlorine feed rate to offset it.
In 'Segment E' we have returned to a fully nitrified effluent and have established a free chlorine residual once again.
As shown on this trend screen there are two periods of time where the chlorine residual has dropped significantly...... once coming out of the nitrified situation and then once when returning to the fully nitrified condition. If we can tighten up the control loop to respond faster to this consumption of chlorine, we may lessen the magnitude of the "chlorine residual drop." Also, depending on such factors as the pH and temperature of the water being treated, the chlorine concentration and species that the chlorine is in, solids concentration (turbidity), the number of microbes in the water, the contact detention time, etc., we may not be able to meet the disinfection standards during these periods of time.
Online and ""kinda real-time" ammonia, nitrite, and nitrate analysis equipment is quite expensive. Most ammonia analyzers, with their REQUIRED micro-filtration systems, run around $35,000 to $40,000. While expensive, one must be very careful when analyzing the disinfection trend screens without this equipment. While full plant data suggests that the breakthrough of ammonia over powers the effects of a breakthrough and presence of nitrites, (which also consume chlorine), we must sample and analyze the wastewater to properly identify the species of nitrogen we are dealing with, and its source.
Obviously, the principal operational considerations in all of this are the disinfection impacts. As the number, and duration, of nitrification impacts increase, we expect to experience an increase in the number of disinfection impacts.
NOTES and THOUGHTS: If you are experiencing disinfection impacts such as these, do NOT jump to conclusions! Another factor to consider, which may contribute to disinfection impacts and trend screens which will resemble this one, includes denitrification effects originating in secondary clarifiers. In addition, it is quite easy to create conditions in nitrifying facilities which will enable us to experience BOTH nitrification and denitrification impacts at the same time. (One easy case: unequal solids and hydraulic detention times in secondary clarifiers).
Also, remember that high ammonia content return streams such as biosolids dewatering, and industrial discharges will provide variances in the ammonia content entering your facility.

Biosolids Utilization
Biosolids are generally applied to agricultural fields in "agronomic rates." That means, if there are no other limiting chemical compounds or elements such as copper, etc., the biosolids will be applied in tons per acre for the crop that is to be planted. The rate of application will match the nitrogen uptake rate of the crop, and therefore, result in no additional nitrates entering the groundwater. (Ammonia is stored in the soil by being attached to negatively-charged clay particles by electrostatic interactions. For comparison, nitrate has the same electrostatic charge as the clays, and is repulsed by them. It is then easily washed (leached) from the soil by irrigation and rainfall activity.
The conversion of ammonia into nitrate can be very rapid, if all of the optimal conditions are available. Some studies have shown that 50% of applied ammonia fertilizer can be converted into nitrate within 30 days after application.

DENITRIFICATION:
For complete nitrogen removal we usually find the denitrification reaction controlled in a separate treatment process. First we reduce the carbonaceous BOD to a low (< 15 mg/L ) value. Then, ammonia is converted into nitrites and nitrates. The nitrites and nitrates are then placed into an oxygen-free condition, where the bacteria utilize the nitrites and nitrates as the oxygen source in their life processes. This strips the oxygen off of the nitrogen atom, which combines with another nitrogen atom, and leaves the solution as nitrogen gas into the atmosphere.

Oxygen
Denitrification takes place under anoxic conditions, where no free oxygen is available. This usually happens in the secondary clarifier or in a dedicated anoxic tank.

pH
The optimum pH for denitrification is 7.0 to 7.5. The denitrification process produces bicarbonate. In theory, about 3.57 mg CaCO3/mg N2 gas is realized.

Carbon Source
Denitrifying bacteria require carbon for cellular growth. If outside sources of carbon are required acetate, raw wastewater, methanol, or other sources of carbon are added.

Sedimentation
Generally speaking, the higher MCRT processes required for full nitrification produce pin floc, and ashing. In addition, the solids should spend no more than 35 minutes in a secondary sedimentation basis, or denitrification may occur. Prolonged detention of activated sludge in secondary (final) sedimentation tasks allows formation of sufficient nitrogen gas to "float the sludge" in block-like clumps to the surface, if nitrates are present in adequate amounts for this to occur. This is often referred to as the "rising sludge" problem.

Anoxic Selector for Denitrification
Some wastewater treatment facilities will have a tank for denitrification and subsequent nitrogen gas stripping.

In all of the denitrification processes, a carbon source MUST be present in order for the cells to reproduce. In many cases this source is acetate, methanol, or even a small portion of raw wastewater (in the case of an oxidation ditch, etc)

There are three principle methods to accomplish this:
a) Propriety processes such as Bardenpho where they alternate anoxic and aerobic cells to force these reactions; or the A/O process where two or more non-aerated cells in series are placed prior to the aeration cells. (In the A/O process, denitrification takes place in the second anoxic cell.)

b) Post-denitrification where the aeration basin is followed by an anoxic reactor. The anoxic reactor is usually starving for a carbon source for cellular formation, and a carbon source such as sugar, or methanol is fed.

c) Pre-denitrification consists of an anoxic denitrification reactor followed by a fully aerated, combined CBOD/nitrification basin

OXIDATION DITCH
How would you accomplish this in an oxidation ditch?

A denitrification zone (an anoxic selector) can be made ahead of the anaerobic zone by bringing a small portion of the raw sewage influent (as a carbon source) to this selector (zone). Nitrates are then converted into nitrogen gas, and released into the atmosphere. The mixed liquor then either flows into an anaerobic selector, or contacts another brush aerator so that the organic nitrogen produced by the denitrifying bacteria is oxidized. The raw wastewater flow rate, organic loading, number of aerators, size of the ditches for detention time, and the number of anoxic selectors required are obvious design parameters.

Oxidation ditches which are also designed to denitrify can remove up to 97% of the total ammonia nitrogen from the water through their process of nitrification and denitrification. Since oxidation ditches are also one of the easiest wastewater processes to operate and maintain, if it were not for the large amount of land required for this process, far more of them would probably be constructed!

GLOSSARY

autotrophic: usually applies to a microorganism which utilizes inorganic substances (materials) for growth and energy, such as algae and nitrifiers.

denitrification: reduction of nitrate (and/or nitrite) into nitrogen gas or ammonia; Nitrogen removal by anaerobic microbes that strip away the chemically bound oxygen from the nitrite and nitrate, which liberates the nitrogen as nitrogen gas into the atmosphere, (or ammonia).

methemoglobinemia: a disease in infants that is caused by high levels of nitrate in the water.

nitrification: the process of converting ammonia nitrogen into nitrite nitrogen then into nitrate nitrogen, causing the liberation of hydrogen ions H+ (acidity)

nitrosomonas bacteria: an autotrophic bacteria group that uses only ammonia for energy (breaks the nitrogen-hydrogen bond for the energy).

nitrobacter bacteria: an autotrophic bacteria group that uses only nitrite for energy (breaks the nitrogen-hydrogen bond for the energy).

oxidation: a chemical reaction characterized by oxygen uniting or combining with other elements; to remove hydrogen, as by the addition of oxygen; the action of increasing valence of an element.

reduction: chemical reaction characterized by the loss of oxygen; the addition of hydrogen to a compound; also the lowering of the chemical valence of a positive element in a compound.

REFERENCES

WPCF, MOP 11, Vol II, WPCF, 1990

George Tchobanoglous & Franklin Burton, Wastewater Engineering, 3rd Edition, Metcalf and Eddy, Inc

Texas Water Utilities, Manual of Wastewater Treatment 6th Edition, Texas Water Utilities, 1991

Kenneth A. MacKichen & Mark J. Hammer, Hydrology & Quality of Water Resources, Whiley, 1981

Walter Weber, Physicochemical Processes For Water Quality Control, Whiley, 1972

Mark J. Hammer & Warren Viessman, Jr., Water Supply and Pollution Control, 5th Edition , HarperCollins, 1993

July 1995 AWWA Research Foundation Order Number: 90669

Nitrification Occurrence and Control in Chloraminated Water Systems
Prepared by Gregory J. Kirmeyer and Lee H. Odell, Economic and Engineering Services, Inc.; Joe Jacangelo and Andrzej Wilczak, Montgomery Watson, Inc.; and Roy Wolfe, Metropolitan Water District of Southern California

 

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