Nitrification is the biological oxidation of ammonia nitrogen to nitrate and accounts for most ammonia oxidation. However, carbonaceous bacteria utilize ammonia on the 5 mg/L ammonia for every 100 mg/L of carbon. Sources of ammonia can include; proteins, urea, amino acids, corrosion inhibitors, process chemicals and quaternary ammonia. Ammonia is oxidized by the nitrifying bacteria nitrosomonas and nitrobacter which account for about five to ten percent of an average microbial population. The nitrosomonas are the ammonia oxidizing bacteria (AOB) responsible for converting ammonia to nitrite while the nitrobacter are the nitrite oxidizing bacteria (NOB) responsible for converting nitrite to nitrate. The nitrosomonas bacteria are more sensitive than are the nitrobacter which can create nitrification issues. On the other hand, the nitrobacter generate new growth at a greater rate than do the nitrosomonas, which can lead to a buildup of nitrite as well as nitrous acid. Excessive concentrations of nitrous acid are poisonous to both AOB and NOB.
Nitrifiers contain membranes that allow for ammonia and nitrite ions contact with surface enzymes that add oxygen to each ion. Nitrifiers use carbonate or carbon dioxide as their food source and ammonia as their means of transferring energy.
For effective nitrification to occur, a number of key conditions must exist to help ensure treatment for ammonia, those being:
1. Process measurements: alkalinity, temperature, pH, dissolved oxygen (DO), biochemical oxygen demand – 5 day (BOD5)and oxygen reduction potential (ORP)
2. Process variables: detention time, nitrifier concentration and mean cell residence time (MCRT)
3. Lack of significant toxins or inhibitors such as (copper, chromium, nickel and zinc) and high concentrations of nitrous acid and free ammonia
Alkalinity
For nitrification to occur, among other things sufficient alkalinity must exist. The generally accepted concentration of alkalinity for nitrification is 7.14 ppm (expressed as CaCO3) per 1 ppm of ammonia nitrogen removed. Ammonification, conversion of organic to ammonia nitrogen occurs in the anaerobic environment found in the collection system, the same anaerobic environment that produces hydrogen sulfide. It is important to note that any organic nitrogen not converted to ammonia, will pass through the treatment system untreated.
When ammonia nitrogen is nitrified, oxidized to nitrate (NO3), the hydrogen portion is released thereby further acidifying the water leading to reduced pH and alkalinity. When nitrifying, a safe amount of alkalinity required to complete the process is based on influent ammonia and effluent ammonia concentrations and influent and effluent alkalinity. In short, the amount of alkalinity necessary to provide the expected degree of nitrification is calculated by subtracting the target effluent ammonia concentration from the influent concentration followed by multiplying the difference by 7.14 and then adding at least 50 mg/L to account for the required excess alkalinity to ensure completion of the nitrification process.
Calculation:
Influent Ammonia mg/L – Target Ammonia mg/L = Ammonia to be removed mg/L
Ammonia to be removed mg/LX 7.14 mg/L Alkalinity = Base Alkalinity Requirement mg/L
Base Alkalinity Requirement mg/L + 50 mg/L Residual Alkalinity = Total Alkalinity Required mg/L
Example:
Influent Ammonia (9.6 mg/L) – Target Ammonia (0.50 mg/L) = Ammonia (9.1 mg/L)
Ammonia (9.1 mg/L) X Alkalinity (7.14 mg/L) = Alkalinity (64.97 mg/L)
Alkalinity (64.97 mg/L) + Alkalinity (50 mg/L) = Alkalinity (114.97 mg/L)
Note: In this case, the alkalinity required should be rounded up to 120 or 130 mg/L
If the alkalinity concentration is not sufficient to support the desired degree of nitrification, the alkalinity concentration can be increased with caustic, lime, soda ash, sodium bicarbonate or other alkaline product. Often times, there is nitrification along with some degree of de-nitrification occurring simultaneously in the wastewater treatment process. Simultaneous nitrification and de-nitrification depends on dissolved oxygen (DO) concentrations that can be lower in some sections of the treatment system such as areas with less aeration such as bottom of tanks or near clogged diffusers. While this reaction will be discussed in greater length in another section, it is worth noting here that for every 1 ppm of nitrates de-nitrified about half, or 3.57 ppm, of alkalinity removed by the nitrification process is returned to the treatment process.
pH
An operational condition that is closely tied to alkalinity is pH. While pH and alkalinity are two separate matters, alkalinity provides the buffering capacity to limit the ability of the pH to change. Since the nitrification process produces an acid and therefore reduces the pH, beginning the nitrification process with higher pH concentrations if preferred over middle to lower concentrations. The pH range from 7.5 to 7.7 su is optimum while pH’s ranging from 6.0 to 9.0 su will often times be suitable for adequate treatment. Correction of low pH conditions are often times the same as that used to increase alkalinity; sodium hydroxide, lime, sodium bicarbonate, sodium bicarbonate or magnesium hydroxide.
Detention Time
Another required condition for substantial nitrification, as would be with nearly every biological reaction, is sufficient detention time. Under normal operating conditions that include water temperatures of about 16o to 21oC (60 to 70oF), the goal should be to provide at least a 4 hour hydraulic detention time. This hydraulic detention time is impacted by amount of nitrifiers, influent flow rates, side stream flow rates, return activated sludge (RAS) flow rates and dewatering etc. As the water temperature becomes colder, detention times necessary to provide the same level of treatment become longer due to subsequent reduced biological activity. As a general rule of thumb, biological activity halves with each 10o C drop in temperature.
Temperature
While temperature is one of the most significant variables regarding nitrification it is also a condition that we as operators usually have the least control over. For the most part, the best we can do is improve other operational conditions to offset some of the negative effects of temperatures. In most cases, temperature related issues are almost always closely related to low rather than high temperatures. Some exceptions to this are some industrial processes such as oil refining. While the ideal temperature range for nitrification is 30 to 36oC (86 to 97o F), process temperature ranging from 10 to 38oC (50 to 100oF) will provide adequate treatment in most cases, barring any other adverse operational conditions. Under more normal operating conditions, the nitrosomonas bacteria convert ammonia to nitrite much slower than nitrobacter converting to nitrate. However, as the temperature dips to below 15oC (59oF) the nitrosomonas bacteria nitrite production can outpace that of conversion to nitrate by nitrobacter leading to an over a 10 mg/L backlog of nitrites in the treatment system. As a side note: equalization tanks with a lot of exposure to cold ambient air temperatures can lead to severe reductions in process temperatures and nitrification. This condition may be improved by minimizing the time in the equalization tank. A similar action could be maintaining DO concentrations above 1 mg/L but not excessive which can help maintain higher temperatures by reducing introduction of cold air into the wastewater as well.
Sufficient nitrifier population
As a percentage of the entire biomass in the typical treatment system, the heterotrophs make up about 95% while autotrophs, the group that includes the nitrifiers, make up about 5%. This disproportionate population is the major reason why adequate nitrification can, at times, be difficult. This is also the basis for maintaining MLSS concentrations of 2,500 to 3,000 mg/L under normal conditions. Considering their relatively population, with a MLSS concentration of 2,500 the concentration of nitrifiers will only be about 125 mg/L. Considering that detail and their relative sensitivity, it is common for extended detention times to be needed for adequate treatment.
Sufficient dissolved oxygen
Nitrification requires about 4.5 ppm O2for every 1 part ammonium to be oxidized, roughly 3.5 mg/L for the ammonia oxidizing bacteria (AOB – nitrosomonas) to convert ammonia to nitrite and 1.0 mg/L for the nitrite oxidizing bacteria (NOB – nitrobacter) to convert from nitrite to nitrate. From an operations standpoint, the lowest DO concentration should be at least 1.0 mg/L while a more ideal concentration should be about 2.0 to 3.0 mg/L depending on other key process conditions mentioned earlier. Maintaining DO concentrationsmuch higher than 2.0 mg/L may not improve the process much, if at all, while increasing the power costs considerably.
Sufficient nutrients, orthophosphate in particular
Nitrification requires that biological inhibitors such as: cyanide, phenol, anilines, heavy metals (copper, nickel, chromium, mercury, zinc) not be present in greater than trace concentrations. While many of these are necessary for function of the process, concentrations in excess of trace amounts can be toxic. One of the first indicators of toxicity in the treatment system is lack of activity or death of the higher life forms in the biomass, as observed on slides under the microscope. Another signal that toxic conditions may exist is increased DO concentrations without any other process changes such as an increase in WAS rate or reduction in load to the treatment system.
Low BOD5 concentration
With such a disproportionate population of heterotrophs to autotrophs, on the order of about 95% to 5% respectively, high organic load conditions favor the heterotrophs ability to out compete the autotrophs for essential nutrients. As a result, decreasing loads leads to increases in the level of nitrification. This condition is the rationale for reducing the BOD5 concentration below 50 mg/L as soon in the process as possible to increase available tank volume where effective nitrification can occur. While nitrification will take place at higher BOD5 levels, this is about the point where nitrification becomes more efficient.
Mean Cell Residence Time (MCRT)
MCRT is an approximation of the average time that any particle of biomass remains in the treatment system. It considers the amount of solids inventory contained in the treatment system against any solids removed either purposely (waste activated sludge) as well as non-purposely in the final effluent. The length of the MCRT helps dictate the age and type of biology contained in the treatment system as well as the level of treatment that should be expected. The minimum expected MCRT is about 3 days at about 12oC or 54oF, with the more appropriate MCRT being about 7 days under the same set of environmental conditions. An MCRT much shorter than the minimum would likely result in a reduced level of treatment due to a young sludge age. At an MCRT much longer than 7 days, while the overall treatment may be good this condition may lead to fine over oxidized solids in the effluent as a result of endogenous decay.
Oxidation Reduction Potential (ORP)
Oxidation reduction potential is gaining popularity as a process control measurement, mostly due to its accuracy with low DO conditions. Inline ORP measurement devices seem to do better in the treatment environment than do DO meters. It is the measure of the oxidizing capacity of the mixed liquor in the treatment system. The general range for ORP for nitrification is +100 to +250 mV. These higher ORP readings are consistent with higher DO derived oxidation states.
Toxins and Inhibitors
Several elements and the concentration that they begin to inhibit effective nitrification are:
· Copper – 0.1 to 0.5 mg/L
· Cadmium – 5 to 9 mg/L
· Chromium – 0.25 mg/L
· Nickel – 0.25 to 0.50 mg/L
· Zinc – 0.01 to 1 mg/L
· Cyanide – 0.3 to 20 mg/L
· Lead – 0.5 to 1.7 mg/L
· Magnesium – 50 mg/L
· Mercury – 2 to 12.5 mg/L
· Silver – 0.25 mg/L
· Methanol – 60 mg/L
· Ethanol – 414 mg/L
· Nitrous Acid
· Free Ammonia (pH>7) – 20 mg/L
In more extreme cases of toxic inhibition, the DO concentration will rise significantly without any other process changes being made. This occurs to the reduced biological activity and sometimes reduced biological population.
Farming nitrifiers
As you undoubtedly aware, there are circumstances where nitrifiers either become less productive or are lost all together. In either case, the need to nitrify with likely require supplementation at some level. To grow large populations of nitrifiers you will need a tank where you can add the necessary chemicals; ammonia and caustic or some other alkaline compound. In this process, you will need to add enough ammonia to maintain an ammonia concentration of about 100 mg/L while adding enough caustic to maintain sufficient alkalinity for nitrification. Depending on the temperature, a robust population of nitrifiers can be developed over a couple of weeks.