The goal of nitrification is to achieve complete oxidation of the ammonia present in the wastewater. In a single-stage nitrifying process (Figure 1), the organic matter and ammonia are removed in a common tank. In a two-stage nitrifying process (Figure 2), the first stage removes most of the carbonaceous organic matter and the second stage oxidizes the ammonia. Typically, nitrification systems have longer SRTs and lower F:M ratios than systems designed for CBOD removal alone.

Figure 1: Single Stage Nitrification Process

Single-stage nitrification

Figure 2: Two Stage Nitrification Process

Two-stage nitrification

Nitrosomonas and Nitrobacter are the aerobic bacteria commonly referred to as “nitrifiers.” Because these are the only organisms involved in the nitrification process, the process, whether two-stage or single-stage, must be carefully controlled. The rate of nitrification depends greatly on such factors as temperature, pH, DO concentrations, and ammonia concentration. The key factors influencing the operation of the nitrification process are:

  • Dissolved oxygen;
  • Total Kjeldahl nitrogen (TKN) loading (see Figure 3);
  • Temperature;
  • Alkalinity and pH;
  • Mixed liquor suspended solids, mixed liquor volatile suspended solids, and nitrifier fraction;
  • Biochemical oxygen demand/TKN ratio;
  • Return activated sludge rate; and
  • Sludge wasting rate.

Because there are so many factors to consider, it may seem that the nitrification process is complex, but actually nitrification is only slightly more complex than common secondary activated sludge.

Figure 3: Different Forms of Nitrogen

All living organisms need carbon to build new cells. Nitrifying bacteria get carbon from dissolved carbonates, bicarbonates, and carbon dioxide in the wastewater, whereas more common heterotrophic bacteria obtain their carbon from organic matter. The nitrifiers are strict aerobes. This means they must have sufficient quantities of air or oxygen. Of the total oxygen demand exerted by the wastewater, a sizeable fraction often stems from the oxidation of ammonia to nitrate, as shown by Equation 1.

Equation 1: Nitrification Reactions

Nitrification reactions

The oxygen demand for complete nitrification is high. For most domestic wastewater, nitrification will increase the required oxygen supply and power requirements by 30 to 40% of that required for CBOD removal alone because complete nitrification requires from 4.3 to 4.6 lb oxygen/lb (Table 1) of ammonia nitrogen converted to nitrate, and wastewater typically contains 10 to 30 mg/L of reduced nitrogen.

Table 1: Oxygen Consumed During Nitrification

In addition to requiring oxygen, the nitrifiers consume bicarbonate alkalinity (HCO3-) and produce carbonic acid (H2CO3). Theoretically, 7.14 lb of alkalinity (measured as CaCO3) is consumed during the oxidation of 1 lb of ammonia (Table 1). This loss of alkalinity, if not otherwise compensated for, can cause a significant drop in pH, resulting in a decrease in the NR. The drop in alkalinity is proportionate to the oxidation of ammonia to nitrate (7.14 mg CaCO3/mg NH3-N converted to NO3-N). Typically, an operating pH level of 7.2 or higher is optimal in nitrification processes. Optimum growth of nitrifying bacteria has been observed in the pH range of 8 to 9, although other ranges have been reported. A substantial reduction in nitrification activity typically occurs at pH levels below 7, although nitrification can occur at a lower pH (less than 6.5) and above 9.

Nitrifiers are more sensitive to upset than most other microorganisms and require careful monitoring and process control. The nitrification process is typically the key to successful operation of the entire plant. When the nitrification process runs well, plant effluent is typically the best possible. Recycle flows from solids handling processes potentially may be large sources of more concentrated ammonia nitrogen. Nitrification systems typically are not operated at intermediate (40 to 80%) removals; stable operation is achieved when essentially complete nitrification (greater than 90%) occurs.

The loading on a nitrification system consists of ammonia plus organic nitrogen and it typically referred to as TKN. As complex organic nitrogen is broken down in the aeration basins, it is converted to ammonia. Treatment entails developing a sufficient supply of nitrifying bacteria, supplying those bacteria with their basic needs, maintaining a suitable environment for them, and ensuring they are not lost from the system. The ratio of nitrifying to carbonaceous microorganisms (the nitrifier fraction) will depend on the influent BOD:TKN ratio, mixed liquor pH and temperature, and, to some extent, the sludge-wasting schedule. Because temperature affects the growth rate of nitrifiers more than the growth rate of other microorganisms, the nitrifier fraction will tend to drop in cooler months and increase in warmer months.

Nitrifying organisms are present to some extent in all domestic wastewater. However, some wastewater is not nitrified in existing plants because the plants are designed for the higher growth rate of bacteria responsible for the carbonaceous removal. As the SRT is increased, nitrification typically takes place. A longer SRT provides for a larger inventory of nitrifiers, thereby preventing these organisms from being lost from the system when wasting occurs. The slower growth rate of nitrifiers makes them susceptible to upsets if there are quick increases in wasting rates.

Because of the longer SRT required for nitrification, the basin volumes for nitrifying systems are typically larger than those for carbonaceous removal alone. Some systems are designed to achieve nitrification in the second stage of a two-stage, activated sludge system and others are designed for nitrification of occur simultaneously with carbonaceous BOD removal. Excessive SRTs, however, can lead to settling problems or wasted air due to endogenous respiration.

For separate stage (two stage) processes, the BOD:TKN ratio is one of the two most important controls of the nitrifier fraction in the second stage. The other is the sludge-wasting schedule. The BOD:TKN ratio is calculated from TKN and BOD5 measurements of the nitrification influent. In a combined (single stage) process, the operator has little control over the BOD:TKN ratio.

The higher the BOD5:TKN ratio, the lower the nitrifier fraction. Looking at BOD5 as food for the common microorganisms and TKN (or ammonia) as food for the nitrifiers, when there is more BOD5 there will be more common microorganisms. When there is less BOD5 (and relatively more TKN), it is not possible to support as many of the common microorganisms, so the fraction of nitrifiers will be greater. It might seem like a good idea to have as many nitrifiers as possible, even a “pure culture” (100%). However, nitrifiers are tiny, poorly flocculating microorganisms and separating them from wastewater by sedimentation is difficult. The other microorganisms present in the system are essential because these non-nitrifying bacteria create a flocculent mixed liquor and, therefore, a clear nitrification effluent. This makes their presence desirable. On the other hand, the more nitrifiers (the higher the nitrifier fraction), the higher the NR. As a result, the operator of a separate stage process faces trade-offs as shown in Table 2.

Table 2: BOD/TKN Ratio

During the conversion of ammonia to nitrate, mineral acidity is produced. If insufficient alkalinity is present, the system’s pH will drop and nitrification may be inhibited. Approximately 7.1 mg alkalinity is required for each mg of ammonia nitrified. Almost half of the alkalinity, however, may be recovered through denitrification. To maintain the system’s pH, it is recommended that a residual alkalinity of 50 to 100 mg/L be provided. If insufficient alkalinity is available to meet the demand for nitrification plus the residual, supplemental alkalinity may have to be added through chemical addition or modifications made to the system to allow denitrification to take place.

In sewage nearly all the ammonia is derived from urine by the hydrolysis of urea, the daily production being about 6 g N per capita which would result in a concentration of 40 mg/L. if the daily discharge of sewage was 150 L per head. As the final oxidation product of nitrification is nitrate, the effluent from a nitrifying activated sludge plant may contain 30-50 mg/L N.

Oxidation of ammonia (nitrification) in the activated sludge process involves two types of bacteria both of which are autotrophic, i.e. they use dissolved carbon dioxide as their sole source of carbon for cell synthesis.

Nitrification results in a reduction in alkalinity as ammonia is oxidized to nitrous and then nitric acids.

Inhibitors which affect nitrification can be divided into two groups: (i) those which affect the growth rate of bacteria in general, such as sulfide (commonly found in septic sewage), cyanide, compounds of heavy metals, phenol, etc., and (ii) those which are specifically inhibitory to nitrifying bacteria, such as thiourea and allylthiourea and many other similar organic compounds which have carbon linked directly to both nitrogen and sulfur, e.g. thiocarbamates, …

It has been reported that the optimum temperature for the growth of Nitrosomonas is in the range of 30-36C with little if any growth below 5C; and that Nitrobacter have higher optimum temperatures (about 34-35C or as high as 42C).

Unless industrial effluents are present the pH value of sewage will reflect the hardness of the water supply. In hard water areas it is about 7.6 – 8.2 and in soft water areas it is 6.7 – 7.5. However, in soft water areas the pH value will decrease during treatment as CO2 is produced, ammonia is oxidized, and nitric acid is formed. In such circumstances the pH value may decrease to about 5.0 or lower, and thus nitrification will ultimately cease as the nitrifiers are inhibited.

A method, other than by chemical dosing, or preventing the pH value decreasing to the full extent that it might, is to use an ‘anoxic’ zone at the inlet of the aeration tank to reduce nitrous and nitric acids to nitrogen in the recycled sludge.

In order to confirm nitrification four tests should be conducted on two samples. The two sample locations are the influent to the biological reactor and the effluent from the secondary clarifier. The four tests to run on each sample are 1) alkalinity, 2) ammonia, 3) nitrate, and 4) pH. The expected results, when nitrifying, are shown in Figure 4. You can find more information on nitrification laboratory tests here.

Figure 4: Test and Expected Results When Nitrifying

Nitrification test results