Growth Factors Impacting Nitrification
Growth Factors Impacting Nitrification
Nitrification is a critical step in many wastewater treatment systems and becoming more important as regulations tighten regarding nutrients in industrial effluents. Nitrification is the biological oxidation of ammonia, first to nitrite and then to nitrate. It is carried out in activated sludge systems by two unique types of bacteria – Nitrobacter and Nitrosomonas. These bacteria are much more sensitive to environmental conditions than their heartier carbonaceous cousins.
The efficiency and effectiveness of an activated sludge system’s nitrification process depend on several factors or growth pressures. Most of these growth pressures are similar to those for carbonaceous BOD conversion, but often the acceptable ranges are much tighter. Below each of the critical areas affecting nitrification are discussed.
Ammonia Concentration: Without sufficient residual ammonia, nitrification cannot be supported. It is somewhat counter-intuitive, but some systems lose nitrification when the influent ammonia drops below a given amount. Since the first path of ammonia removal from the wastewater is via nutrient uptake by heterotrophic bacteria, the ratio of carbon-based material (BOD) to Total Kjeldahl Nitrogen (TKN) is a primary determinant of the degree of nitrification that can be expected. Also, as the BOD/TKN ratio increases, the fraction of nitrifying organisms decreases.
pH: Nitrification is a very pH dependent process. Whereas, carbonaceous bacteria function quite well throughout the range of 6.0 – 9.0, nitrifiers prefer a much tighter pH range, typically 6.8 – 8.2.
Alkalinity: In addition to the pH requirement, nitrification requires that attention is paid to the alkalinity available. Each mg/l of ammonia that is oxidized (converted to nitrate) requires 7.15 mg/l alkalinity. Typically, systems are controlled to a residual alkalinity of 50 – 100 mg/l alkalinity, as CaCO3.
Dissolved Oxygen: Nitrification is an oxidative process and both Nitrobacter and Nitrosomonas as strict aerobes. Nitrification requires 4.33 mg/l of oxygen per mg/l of NH4+-N. Dissolved oxygen residuals in the aeration tank of a nitrifying system must be maintained at residual DO levels of 1.0 – 4.0 mg/l to ensure adequate oxygen availability.
Detention Time: The time required for nitrification is directly proportional to the amount of nitrifiers present. Because the rate of oxidation of ammonia is essentially linear, short-circuiting must be prevented. The minimum aeration basin detention time is ~4 hours at 22 ° to 24 °C.
MCRT, Sludge Age, and f:m ratio: When reviewing the performance of the activated sludge process for selection of an optimum F/M ratio and/or MCRT, the requirements for nitrification must be taken into account. Because nitrifiers reproduce much more slowly than the heterotrophs, it is usually necessary to operate at higher MCRTs (Sludge Age) and lower f:m ratios when nitrification is a goal.
Temperature: Nitrifying bacteria are mesophilic, with optimum temperature being around 85 F (30 C). Below 85 F, the rate of nitrification rapidly decreases until it stops completely below 50 F (8 C). Although nitrification can be achieved at elevated temperatures as high as 110 F (43 C), the rate of ammonia removal is inhibited.
Nutrients: Obviously, nitrogen is not an issue here, as nitrification will not occur until the carbonaceous demand for ammonia is met. However, nitrifiers still need orthophosphate and this is an often-overlooked parameter when troubleshooting nitrification issues.
Toxicity and Inhibition: Nitrifying bacteria are much more susceptible to toxicity and inhibition than heterotrophic bacteria. Both Nitrosomonas and Nitrobacter are inhibited by unionized ammonia (NH3), which is present at elevated pH values. Since Nitrosomonas are more sensitive than Nitrobacter, the result may be a high level of NO2– in the final effluent. There are many other compounds that can exert inhibition on nitrifiers, such as thiourea, cyanide, phenol, anilines, and heavy metals (silver, copper, nickel, chromium, mercury, and zinc.
For additional information:
- Bitton, Gabriel, Wastewater Microbiology, Wiley-Liss, New York, 1994
- Eckenfelder, W. W. and Musterman, J. L., Activated Sludge Treatment of Industrial Wastewater, Technomic Publishing Company, Lancaster, 1995
- Eckenfelder, W.W., Ford, D. L., and Englande, A. J., Industrial Water Quality, 4thEdition, WEF Press, McGraw Hill, New York, 2009
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