Wastewater Nitrification: How it works
Nitrification is the sequential conversion of ammonia to nitrite and ultimately nitrate:
- NH4+ + 1.5 O2 → NO2– + H2O + 2H+
Ammonia in wastewater could originate from a variety of sources, including:
- Proteins (meat and blood), urea, amino acid products, casein, corrosion inhibitors, process chemicals and raw materials or cleaning chemicals containing quaternary ammonium compounds.
In an activated sludge system or other biological treatment system, Nitrification under aerobic conditions.
- Nitrification is a bio-chemical reaction that occurs inside bacteria.
- Two species of bacteria are involved in the process – Nitrosomonas and Nitrobacter.
- These bacteria are collectively known as nitrifiers and are autotrophic, i.e. they get their carbon source from inorganic carbon (carbonates, bicarbonates) or carbon dioxide.
In nitrifying activated sludge process only 3-10% of bacteria is autotrophic (nitrifiers).
- Nitrifiers possess cytomembranes, which are extensions of the cell membrane away from the cell wall and toward the cytoplasm.
- These are the active sites for oxidation of ammonium and nitrite ions.
- It is on the cytomembranes of Nitrosomonas and Nitrobacter, where ammonium ions and nitrite ions, respectively, come in contact with enzymes that add oxygen to each ion.
A healthy and stable population of nitrifiers (Nitrosomonas and Nitrobacter) will not exist without the following conditions:
- Oxygen: Nitrifiers are obligate aerobes, i.e. they require free molecular oxygen and are killed off by anaerobic conditions. Maximum nitrification occurs at a D.O. (Dissolved Oxygen) level of 3.0 mg/l. Significant nitrification occurs at a D.O. level of 2.0 to 2.9 mg/l. Nitrification ceases at D.O. levels of <0.5 mg/l. Approximately 4.6 kg of oxygen are required for every kg of ammonium ions oxidised to nitrate (This compares with a requirement of 1 kg of oxygen to oxidize 1 kg of carbonaceous B.O.D.). An absence of oxygen for <4 hours does not adversely affect nitrifiers when oxygen is restored. To ensure effective nitrification always maintain a D.O. level of > 1.5 mg/l.
- Temperature: Nitrification is temperature sensitive. The optimum temperature for nitrification is generally considered to be 30°C.
>45°C Nitrification ceases
28-32°C Optimal temperature range
16°C Approx. 50% of nitrification rate at 30°C
10°C Significant reduction in nitrification rate – 20% of rate at 30°C
<5°C Nitrification ceases
- Alkalinity and pH: Alkalinity is lost in an activated sludge process during nitrification. Nitrifiers use alkalinity as a carbon source, i.e., they use an inorganic form of carbon. Hydrogen ions (H+) are produced when ammonium ions are oxidised to nitrite: NH4+ + 1.5O2 → 2H+ + NO2– + 2H2O. Nitrous acid (HNO2) is also produced during the oxidation of ammonium ions. This destroys alkalinity: H+ + NO2–→ HNO2. 7.14 mg of alkalinity as CaCO3 are destroyed for every mg of ammonium ions oxidised. If the pH drops below 6.7, there is a significant decrease in nitrification. Therefore, it is important to maintain an adequate alkalinity in the aeration tank to provide pH stability and also to provide inorganic carbon for nitrifiers. After complete nitrification, a residual alkalinity of 50 mg/l in the aeration tank is desirable. If this alkalinity is not present, then alkalinity should be added to the aeration tank. The optimal pH range for nitrification is 7.2 to 8.0. A substantial reduction in nitrification activity occurs at pH levels below 6.7.
- High Mean Cell Residence Time (Sludge Age) or low F: M.: Mean Cell Residence Time (MCRT) is the average number of days that micro-organisms are kept in the activated sludge process before they are wasted from the system. A high MCRT is required to increase the number of nitrifying bacteria in the activated sludge process. The necessary MCRT or F: M values are temperature dependent. Nitrifier activity and reproduction are decreased during cold temperatures. Therefore, in winter, an increase in the quantity of nitrifiers (MLVSS) or an increase in MCRT is often required to maintain effective nitrification. Reducing the wasting rate (WAS rate) will increase the MCRT.
- Inhibition/Toxicity: Inhibition is temporary short-term or long-term loss of enzymatic activity. Toxicity is permanent loss of enzymatic activity or irreversible damage to cellular structure. Small increases in inhibitory wastes can cause a dramatic reduction in nitrification. Nitrifiers grow slowly and only account for a small portion of the bacterial assemblage in an aeration system. Nitrifiers are excellent indicators of toxic shock in an effluent treatment plant significant loss of nitrification will occur before loss in efficiency of carbonaceous BOD removal. Nitrifying bacteria are also inhibited by relatively low concentrations of free ammonia (10 mg/l for Nitrosomonas; 0.1 mg/l for Nitrobacter) and free nitrous acid (1.0 mg/l for both Nitrosomonas and Nitrobacter). Free ammonia (NH3) is produced from ammonium ions under a high pH in the aeration tank. Free nitrous acid (NHO2) is produced from nitrite ions under a low pH in the aeration tank. This type of inhibition is known as substrate inhibition. Substrate inhibition usually occurs at a concentration of 400-500 mg/l ammonium ions or when ammonium ions are converted to nitrite ions at a faster rate than nitrite ions are converted to nitrate ions.
- BOD: Soluble and simplistic forms of cBOD can inhibit the activity of nitrifying bacteria. They are able to enter the cells of nitrifying bacteria and inactivate their enzyme systems. This form of cBOD must be degraded significantly or completely by organotrophs in order for nitrifying bacteria to oxidize ammonium and nitrite ions. Nitrifiers are dependent on organotrophs to reduce cBOD to relatively low concentrations (<40-50 mg/l). Excess BOD can cause a significant oxygen demand, which may cause a drop in D.O. that adversely affects nitrifying bacteria. Fluctuations in BOD loading may lead to intermittent nitrification.