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Why is Nitrifier Growth so Slow?

Why is Nitrifier Growth so Slow?

Chemoautotrophic bacteria use CO2 as their carbon source and oxidation of non-organic material to generate cellular energy. The oxidation of inorganic material does not yield as much energy as the oxidation of organic carbon sources, as performed by heterotrophic bacteria, so nitrifiers have a very slow growth rate within the microbe community in wastewater plants.

Nitrifying bacteria are autotrophs, and use CO2 as the source of carbon for their cellular building material, so they do not contribute to the removal of BOD in the system. However, nitrifying bacteria are important to the system since decrease levels of ammonia to a concentration where heterotrophic bacteria are able to survive.

Although Nitrosomonas, Nitrobacter, and Nitrospira are commonly found in the soil and can easily wash into wastewater plants (WWTPs), many plants lose these bacteria because of environmental conditions, contamination with toxic compounds, low dissolved oxygen, or competition with other microorganisms.

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Seven Steps to Nitrification

Factors Affecting Performance of Nitrifying Bacteria

1.  Retention Time

Wastewater Retention Time_Nitrifier

Inside the basin, the nitrifier populations are low, about 10% of the total bacterial population.  Nitrifiers are slow growers, so if you waste heavily for a few days in a row or when temperatures are low, the entire population may be lost.

2.  Temperature

Wastewater Ammonia Control - Nitrifier Temperature

At temperatures less than 20°C (68°F), cellular metabolism slows down and fewer cells are nitrifying and dividing. Above 40°C (104°F), the proteins will be inactive and the cell membranes may break apart resulting in cell death.

3.  Dissolved Oxygen

Dissolved Oxygen and Ammonia in Wastewater

Heterotrophs have greater numbers than nitrifiers in wastewater and are more efficient at scavenging oxygen. In mixed populations with low D.O., the heterotrophs will be able to quickly use the available oxygen faster than the nitrifers since oxygen is also consumed in heterotrophic metabolism. It is a good idea to increase the D.O. levels with high incoming BOD or NH3.  Remember: D.O. levels increase with colder temperatures and decrease in warmer temperatures.

4.  Carbonate Alkalinity

Wastewater Ammonia - Carbonate Alkalinity - Nitrifiers

High rates of ammonia oxidation will acidify the environment and must be neutralized with additions of carbonate alkalinity (e.g., Na2CO3, CaCO3, or K2CO3) since 4 – 7 ppm alkalinity are used for every 1 ppm of ammonia. The biocatalyst in Nitrosomonas that carries out the first part of ammonia oxidation is inhibited (rendered inactive) by pH less than 7.

5.  pH

Ammonia & Nitrifiers pH

AOB only utilize ammonia not the ammonium ion. The levels of ammonia to ammonium vary depending on temperature and, more importantly, on pH. At lower pH values, most of the ammonia is in the ammonium form leaving you with a lot of nitrogen that the AOB cannot get rid of and conditions that would inactivate the ammonia-oxidizing biocatalyst. The ideal pH for ammonia oxidation is between pH 7-8.

6.  Floc Formation

Nitrifiers typically form aggregates of AOB and NOB along the edges of flocs made up of heterotrophic bacteria where the D.O. concentrations are high and they can still retain the protection of the biofilm. NH3 contains more energy per mol than NO2– so AOB may synthesize a capsule to encompass both cell types.

Ammonia Control and Floc Formation in Wastewater

7.  Toxicity
Heavy metals such as nickel, copper, zinc, cadmium and chromium can be toxic to nitrifiers. Exact levels of each are difficult to determine. Over chlorination can also cause toxicity. Also too high of ammonia (thousands of ppm) or a buildup of the intermediate nitrite can also cause toxicity. In short, nitrifiers are sensitive to toxicity, they are fickle and when temperatures get cold they are more subject to toxic effects of plant design, capacities, efficiency, and plant operation.