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Nitrogen is one of the principal elements of biological life. Due to its abundant presence in the environment, a significant amount of nitrogen in different forms such as Ammonia , Organic Nitrogen, Nitrate and Nitrite Nitrogen enters the water cycle. These forms of Nitrogen are present in both municipal and industrial wastewater. Presence of these species not only affects the water quality but also pose critical risks to the ecosystem as a whole. There has been a gamut of technologies available however stringent discharge norms laid for Nitrogen species, has prompted search for better methods.

In this article we will delve into the different types of Nitrogen species, their sources, their characteristics and impact on the environment , and a summary of available technologies with focus on biological Nitrogen removal processes such as Activated Sludge, SBR, MBBR, IFAS and MBR.

Nitrogen Species in Wastewater

Though Nitrogen is present in the environment both as molecular nitrogen gas (N2) , in wastewater, it is mainly associated with various organic and inorganic forms as mentioned below.

Forms of Nitrogen

Organic Nitrogen

In generated all nitrogen associated with organic compounds is termed as organic Nitrogen (Org.N). This Organic Nitrogen can be present as various chemicals such as proteins, amino acids, amines, amides and many more. In wastewater, the organic nitrogen can be present as dissolved in water or as particulate organic nitrogen associated with solids.

Inorganic Nitrogen

In generated all nitrogen associated with organic compounds is termed as organic Nitrogen (Org.N). This Organic Nitrogen can be present as various chemicals such as proteins, amino acids, amines, amides and many more. In wastewater, the organic nitrogen can be present as dissolved in water or as particulate organic nitrogen associated with solids.

Nitrogen compounds associated with inorganic chemicals are mainly present as:  

  • Free Ammonia 
  • Hydrolyzed Ammonia 
  • Nitrite Nitrogen 
  • Nitrate Nitrogen

Source of Nitrogen in Wastewater

Various nitrogen species can enter the water cycle from numerous sources due to both domestic and industrial activities. Some of the common sources are:

Municipal Wastewater

In Municipal wastewater, one of the principal sources of Nitrogen pollution is Urea through urinal discharge. This urea quickly dissociates in wastewater and becomes Ammonia or Ammoniacal Nitrogen.Apart from urinal discharge, fecal discharge consists   of organic nitrogen bound to various proteins, lipids, amino acids etc. This organic nitrogen can be present in both particulate as well as dissolved form.

Industrial Discharge

Agriculture industry utilizes a large amount of ammonium nitrogen based fertilizers such as ammonium nitrate and ammonium sulfate which can be washed out into water bodies with ran off.

Agricultural run-off

A wide range of industries such as petrochemicals, pharmaceuticals, textile, dairies, food processing, synthetic chemicals producing and other utilize many organic and inorganic These nitrogen containing substances are then finally discharged into the process effluents generated by these industries. Both organic and inorganic forms of Nitrogen are present in quite high concentrations in these effluents. Many of these pollutants are extremely toxic and harmful to ecological life adversely.

Stormwater Runoff

Ammonia can be present in urban and industrial stormwater runoff due to contamination from road surfaces, parking lots, and industrial facilities.


Landfill leachate, the liquid that drains from landfills, can contain ammonia due to the decomposition of organic matter and the presence of ammonia-based products in the waste.


Fish farms and aquaculture operations often use ammonia-containing compounds to control pH levels in water. This can result in ammonia-rich effluent if not managed properly.

Measurement of Nitrogen Compounds in Aquatic Environment

Ammonia Nitrogen

In Wastewater, ammonia may be present as:

  • Free Ammonia (NH3) or unionized Ammonia
  • Ionized Ammonia (NH4.N) or Ammoniacal Nitrogen

Based on the PH and temperature of the wastewater, Ammonia Nitrogen is speciated between free Ammonia and Ammoniacal Nitrogen.

Organic Nitrogen

In wastewater analysis, organic nitrogen is measured as organically bound nitrogen in its trivalent state. This trivalent state includes natural material such as proteins, peptides, nucleic acid, urea and some synthetic organic materials such quaternary ammonium compounds nitrogen containing pesticides and polymers. Analytically, Organic Nitrogen and Ammonia Nitrogen are measured as Total Kjeldahl Nitrogen (TKN).

Total Nitrogen

Total nitrogen is the measurement of all forms of element Nitrogen present in the wastewater. It includes both organic and inorganic nitrogen, including cyanide.

Nitrate Nitrogen

Nitrate (NO3.N) and Nitrite (NO2.N) Nitrogen are also measured in the wastewater analysis as both of these forms have specific impact on the environment.

Impact of Nitrogen on Aquatic Environment

Ammonia is a colorless, odorless gas that is naturally present in the environment. It is also a major waste product of fish and other aquatic animals. Ammonia can be toxic to aquatic life at high concentrations, and it can have a number of harmful effects on the aquatic environment.

Direct toxicity to aquatic animals

Ammonia can be directly toxic to aquatic animals, even at relatively low concentrations. It can damage the gills and skin of fish, making it difficult for them to breathe and regulate their body temperature. Ammonia can also damage the internal organs of fish, leading to death.

Nutrient enrichment

Ammonia is a nutrient that can contribute to eutrophication, a condition in which excessive nutrients in a water body lead to the growth of algae and other aquatic plants. Eutrophication can lead to a number of problems, including fish kills, decreased water quality, and the loss of biodiversity.

Decreased dissolved oxygen

Ammonia can also decrease the dissolved oxygen levels in water. Dissolved oxygen is essential for aquatic animals to breathe, so a decrease in dissolved oxygen can lead to fish kills and other problems.

Disruption of the nitrogen cycle

Ammonia can disrupt the nitrogen cycle, which is the process by which nitrogen is converted between different forms in the environment.This can have a number of negative consequences for the aquatic environment.The impact of ammonia on the aquatic environment can vary depending on a number of factors, including the concentration of ammonia, the pH of the water, the temperature of the water, and the presence of other pollutants.

Fig 1 : Excessive Algal growth due to discharge of Nitrogen

Treatment Technologies for Removal Ammonia

There are a number of treatment technologies for ammonia removal from wastewater. The most common technologies include:

Physical methods

These methods use physical processes to remove ammonia from wastewater. Examples of physical methods include:

  • Air stripping: This method uses air to volatilize ammonia from wastewater.
  • Membrane separation: This method uses membranes to separate ammonia from wastewater.
  • Ion exchange: This method uses resins to exchange ammonia ions for other ions.

Chemical methods

These methods use chemicals to react with ammonia and remove it from wastewater. Examples of chemical methods include:

  • Oxidation: This method uses chemicals to oxidize ammonia into less harmful substances.
  • Neutralization: This method uses chemicals to neutralize ammonia, making it less alkaline.
  • Adsorption: This method uses materials to adsorb ammonia molecules onto their surface.

Biological methods

These methods use microorganisms to break down ammonia into less harmful substances. Examples of biological methods include:

  • Nitrification: This process uses bacteria to convert ammonia into nitrite and then nitrate.
  • Denitrification: This process uses bacteria to convert nitrate into nitrogen gas.

 The choice of treatment technology depends on a number of factors, including the concentration of ammonia in the wastewater, the desired level of removal, the cost of the treatment, and the environmental impact.

Here is a table summarizing the pros and cons of each treatment technology:

Table 1:

Physical treatment methods
Simple, effective, and relatively inexpensive
Not always effective for high concentrations of ammonia
Chemical treatment methods
Effective for high concentrations of ammonia
Can produce harmful byproducts
Biological treatment methods
Effective for low to moderate concentrations of ammonia
Can be slow and require a long retention time

Table 2:

Air stripping
Simple, relatively inexpensive
Low removal efficiency
Membrane separation
High removal efficiency
Ion exchange
High removal efficiency
Requires regeneration chemicals
Effective for high concentrations of ammonia
Can produce harmful byproducts
Effective for high concentrations of ammonia
Can produce salt byproducts
Simple, relatively inexpensive
Low removal efficiency
Biological process, no chemicals required
Slow process
Biological process, no chemicals required
Requires anoxic conditions

Biological Ammonia Removal: A Sustainable Process

Biological ammonia removal is a sustainable process for removing ammonia from wastewater. It uses bacteria to convert ammonia into less harmful nitrogenous compounds, such as nitrate or nitrogen gas. This process is more sustainable than chemical methods of ammonia removal, as it does not require the use of harsh chemicals or energy-intensive processes.

There are two main types of biological ammonia removal processes: nitrification and denitrification.


Nitrification is the process of converting ammonia into nitrate. This process is carried out by bacteria that use oxygen as an electron acceptor. This is the first step in the biological removal of ammonia. It is carried out by two types of bacteria, ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). The AOB converts ammonia to nitrite, and the NOB converts nitrite to nitrate. This process requires oxygen, so it is carried out in an aerobic environment.


Denitrification is the process of converting nitrate into nitrogen gas. This process is carried out by bacteria that do not require oxygen.This is the second step in the biological removal of ammonia. It is carried out by a group of bacteria called denitrifiers. Denitrifiers use nitrate as an electron acceptor, and they reduce it to nitrogen gas (N2). This process does not require oxygen, so it can be carried out in an anaerobic environment.

Nitrification Process Description

Nitrification is a two-step process that converts ammonia (NH3) to nitrate (NO3-). The first step is the oxidation of ammonia to nitrite (NO2-) by ammonia-oxidizing bacteria (AOB). The second step is the oxidation of nitrite to nitrate by nitrite-oxidizing bacteria (NOB).

Chemical reactions

The chemical reactions for the two steps are as follows:

Step 1:

NH3 + 1.5O2 → NO2- + H2O + 2H+

Step 2:

NO2- + 0.5O2 → NO3-

Chemical equations

The overall chemical equation for nitrification is:

2NH3 + 3O2 → 2NO3- + 2H2O

Types of microorganisms involved and their characteristics

The two types of microorganisms involved in nitrification are Ammonia Oxidizing Bacteria(AOB) and Nitrite Oxidizing Bacteria (NOB).

AOB: AOB are autotrophic bacteria that use ammonia as their source of energy and carbon dioxide as their source of carbon. AOB are found in a variety of environments, including soil, water, and wastewater treatment systems. They are typically aerobic bacteria, meaning that they require oxygen to survive. 

NOB: NOB are also autotrophic bacteria, but they use nitrite as their source of energy. NOB are also aerobic bacteria, but they are more sensitive to oxygen than AOB.

Fig 3 : Nitrosomonas Bacteria
Fig 4 : Nitrobacter
Fig 5 : Colonies of nitrifying bacteria, Blue colonies are Ammonia Oxidizing and Red colonies are Nitrite Oxidizing bacteria

Amount of oxygen and alkalinity requirement

Nitrification is an aerobic process, so it requires oxygen. The amount of oxygen required depends on the concentration of ammonia in the environment. For example, a concentration of 1 mg/L of ammonia would require about 4.76 mg/L of oxygen.

Nitrification also requires alkalinity. Alkalinity is a measure of the ability of a solution to neutralize acids. The amount of alkalinity required depends on the concentration of ammonia and the pH of the environment. For example, a pH of 7 and a concentration of 1 mg/L of ammonia would require about 7.46 mg/L of alkalinity.

Denitrification process and its chemical reaction

Denitrification is the opposite of nitrification. It is the process by which nitrate is reduced to nitrogen gas (N2). Denitrification is carried out by a group of bacteria called denitrifying bacteria.

The chemical reaction for denitrification is:

NO3- → N2 + 2O2 + 2H2O

A short complete description of nitrification and denitrification

Nitrification is a biological process involving two steps that convert ammonium into nitrate via nitrite. This is performed by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), under aerobic conditions. It requires oxygen and can lead to a decrease in pH.

Denitrification, on the other hand, is an anoxic process that reduces nitrate to nitrogen gas, ultimately releasing nitrogen back into the atmosphere. Denitrifying bacteria facilitate this process. Both nitrification and denitrification play vital roles in the nitrogen cycle, helping to maintain the balance of nitrogen in ecosystems and prevent the accumulation of excessive nitrogen compounds.

Factors Affecting the Nitrification Process


  • Biomass Activity: The level of biological activity within the biomass affects the efficiency of the nitrification process.
  • Stability: The stability of the biomass population influences the consistency of nitrification performance.
  • Retention within Reactor: How well the biomass is retained within the reactor impacts its ability to carry out nitrification.

Wastewater Matrix

  • Presence of Organic Pollutants and their Structure: The types and structures of organic pollutants in the wastewater can impact nitrification rates.
  • COD:N Ratio: The ratio of Carbonaceous Oxygen Demand (COD) to Nitrogen content affects the availability of nutrients for nitrifying microorganisms.
  • Inhibitors: Certain substances present in wastewater can inhibit nitrification by suppressing microbial activity.
  • Salinity and Calcium Ions (Ca²⁺): Elevated salinity and calcium ion levels can hinder nitrification.
  • Heavy Metals: The presence of heavy metals in the wastewater can be toxic to nitrifying microorganisms.

Process Parameters

  • Food-to-Mass Ratio (kg N/kg MLSS/day): The ratio of nitrogen-containing compounds to mixed liquor suspended solids affects the growth of nitrifying bacteria.
  • C/N Ratio: The ratio of Carbon to Nitrogen influences the nutrient balance for nitrification.
  • pH: The acidity or alkalinity of the environment impacts the activity of nitrifying bacteria.
  • Temperature: The temperature of the system affects the rate of nitrification, with higher temperatures generally promoting faster activity.
  • Dissolved Oxygen (DO) Levels: The concentration of dissolved oxygen in the water affects the aerobic nitrification process.

These factors collectively play a crucial role in determining the effectiveness and efficiency of the nitrification process in wastewater treatment systems.


Activated Sludge Process and its Variants for Nitrification and Denitrification

The activated sludge process is a biological wastewater treatment process that uses a consortium of microorganisms to remove organic matter and nutrients from wastewater. The process consists of two main stages:

Aeration tank

This is where the microorganisms are mixed with the wastewater and oxygen is added. The microorganisms use the oxygen to break down the organic matter in the wastewater.

Sedimentation tank

This is where the microorganisms are separated from the wastewater. The clarified wastewater is then discharged to a receiving body of water.

The activated sludge process can be modified to include nitrification and denitrification. Nitrification is the process of converting ammonia to nitrate, while denitrification is the process of converting nitrate to nitrogen gas.

Here are some of the variants of the activated sludge process for nitrification and denitrification: 

Conventional activated sludge process: 

This is the most common variant. It consists of two separate tanks, one for aeration and one for sedimentation.

Sequential batch reactor (SBR): 

This is a variation of the activated sludge process in which the aeration and sedimentation stages are carried out in the same tank in a sequential manner.

Fig : Activated Sludge Process Variants for Nitrification and Denitrification

Cons of Activated Sludge Process for Nitrification

  • Operational Challenges: Maintaining the delicate balance of microbial populations for efficient nitrification can be challenging, especially during sudden load variations.
  • Energy Intensive: The process requires aeration to supply oxygen for microbial growth, leading to energy consumption and associated costs.
  • Sludge Production: The process generates excess sludge that needs proper management, including disposal or further treatment.
  • Risk of Oxygen Depletion: Inadequate aeration or process control can lead to oxygen depletion in the treatment tanks, affecting treatment efficiency. 

Upsets in Nitrifying Plants and their Prevention

Nitrifying bacteria, the principal organisms responsible for the conversion of Ammonia nitrogen to Nitrate and Nitrite, have specific attributes which make them highly susceptible to attract and process disturbance. 

Specific Attributes of Nitrifying bacteria:

  • Slow growth and regeneration time
  • Lower cell yield (3% of oxidized – N)
  • Weak flocculation ability
  • Highly sensitive to certain chemicals
Fig 5 : Nitrosomonas
Fig 6 : Nitrobacter

Due to these specific attributes,  maintaining the required level of Nitrification in any suspended growth based Activated sludge processes is an extremely tedious and complex task for any wastewater treatment operator. 

Some of the types of upsets and remedial action to correct them are summarized in the table below:

Table 1 : Types of Upsets in Biological Nitrification Process

Types of upsers

Biomass wash-out

Disturbed process

Inhibited process


biomass does't settle, can't be separated and leaves the reactor

Lower degree of nitrification,slower process

Total inhibittion

Bioactivity in the reactor

Unchainged, but lower efficiency due to lower MLVSS in the reactor

Lower, but able to recovery after elimination of the causes

No bioactivity

Possible reasons

- biomass morphology:

bulking or small flocs

- high salinity

- deficite in nutrients

- overloaded separation

- Waste water matrix: structure and inorganic pollutants

- Fluctuations: quality and quantity of pollutants 

- Remarkable changes of process para-meter: pH, temperature

- Plant operators

Table 2 : Remedial Action for Nitrification Process restoration

Types of upset

Possible measure

Weakly flocculating, slowly setting biomass, wash-out

- Addition of biodegradable organic carban source

- Immobilisation on carrier material

Periodical inhibition by reversible inhibitors

- Temporary addition of powdered activated carbon

- Addition of nitrifying biomass from external plants

Continuous inhibition by reversible acting inhibitors

- Identification and source treatment of the inhibitory effluent stream

- Adaptation of nitrifying biomass

- Semi-continuous addition of separately generated nitrifying biomass

Irreversible inhibition by strong inhibitors

- New startup with external biomass

- Identification and source treatment of inhibitory effuent streams

Measures for Emergency Cases

In case of emergency cases, it is very much crucial to collect the information about the nitrification process upsets events which includes complete detailed analysis of each stream contributing to the wastewater flow to check all parameters for plant operations such as DO levels, pH, alkalinity, salinit, change in chemical composition. 

Some of the immediate steps required to be taken are:

  • Use of buffer tanks to store toxic streams 
  • Dilution of inlet wastewater to reduce bulk toxicity 
  • Addition of external biomass from similar Nitrifying wastewater treatment plants
  • Addition of attached growth MBBR/IFAS media in the reactor to retain and immobilize the poorly flocculating Nitrifying biomass.

Levapor Carriers based MBBR and IFAS : An Ideal Process for Stable Nitrification

The major issues related to washout, periodic and continuous inhibition of Nitrification Process can be corrected effectively with the addition of advanced PU foam based Levapor MBBR and IFAS media impregnated with activated carbon.

Salient Features of Levapor MBBR and IFAS media

  • Reticulated technical grade PU foam matrix
  • Very high surface area and high inner porosity 
  • Impregnation with activated carbon 
  • High adsorption capacity 

Levapor MBBR and IFAS media made of PU foam reticulated matrix impregnated with activated carbon provides very high inner porosity for efficient retention of nitrifying biomass.

The distinct advantages offered due to above features for ammonia removal processes are:

  • Prevention against toxic shock loads
  • Retention of higher amount of line, active Nitrifying biomass
  • Stable and higher process Kinetics
  • Higher removal efficiencies
  • Smaller footprint of biological reactors

Application of Levapor MBBR/IFAS Media for Nitrification

Levapor MBBR and IFAS media have been applied for following applications for nitrification of municipal wastewater as well as industrial wastewater.

  • Nitrification of agro chemicals wastewater
  • BNR for municipal wastewater
  • Upgradation of existing activated sludge process for Nitrification
  • Removal of ammonia from fish hatcheries and aquaculture system in RAS
Author Bio

Amit Christian is a MSc graduate in Environment Science from Middlesex University, London, UK. He has been active in the field of water and wastewater treatment since 1998. He specializes in design, engineering, and management of various biological wastewater treatments such as Activated Sludge Process (ASP), Sequencing Batch Reactor (SBR), Moving Bed Bio Reactor (MBBR), Integrated Fixed Film Activated Sludge (IFAS). He has helped various Industrial and Municipal clients in troubleshooting , optimizing their biological wastewater treatment processes to achieve latest Stringent norms for Ammonia Removal.

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