Biological Nitrogen Removal Processes: Technologies Comparison Guide

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Biological Nitrogen Removal Processes: Technologies Comparison Guide

16 September, 2025

Biological nitrogen removal is very important in cleaning wastewater. Too much nitrogen in wastewater can hurt rivers and lakes. Experts think the world makes over 300 billion tonnes of wastewater each year. Nitrogen in this water is almost 14% of what the planet can handle. Nitrification-denitrification and the A2O system help cut down nitrogen pollution. These methods remove nitrogen well and do not cost too much. They are also simple to use. Better treatment can make the environment safer. It also helps give cleaner water to people.

Biological Nitrogen Removal Processes

Nitrification and Denitrification

Nitrification and denitrification are two main ways to remove nitrogen from wastewater. Nitrification changes ammonia into nitrite, then into nitrate. Denitrification turns nitrate into nitrogen gas, which leaves the water. These steps use special bacteria and enzymes. The table below lists the important bacteria and enzymes:

Process	Key Microbial Pathways and Enzymes
Nitrification	Ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). AOB uses ammonia monooxygenase and hydroxylamine oxidoreductase. NOB uses nitrite oxidoreductase.
Denitrification	Denitrifying bacteria use nitrate as an electron acceptor and organic matter as an electron donor. Enzymes include nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase.

Nitrification and denitrification lower nitrogen in wastewater. These methods are used in big treatment plants. The table below shows how well they work:

Study	Nitrogen Removal Efficiency	Conditions
Full-Scale Domestic WWTP	~50%	Biological process with activated sludge and trickling filter
Municipal WWTP (SF1)	48%	Longer SRT (> 60 d) with low COD:N ratio
Municipal WWTP (SF2)	61%	Normal operation with high COD:N ratio

Anaerobic/Anoxic/Aerobic (AAO) Process

The AAO process is the most common way to remove nitrogen in big wastewater plants. It has three main stages: anaerobic, anoxic, and aerobic. Each stage helps remove nitrogen and other pollutants. The table below explains each stage:

Stage	Description	Contribution to Nitrogen Removal
Anaerobic	Wastewater enters this zone. BOD is partly removed and nitrogen compounds change to N2.	Initial nitrogen removal through denitrification.
Anoxic	Denitrifying bacteria reduce NO2- and NO3- to N2 using carbon from organic matter.	Further reduction of nitrogen compounds to nitrogen gas (N2).
Aerobic	Nitrification occurs, converting NH4-N to NO2- and NO3-. Microorganisms absorb phosphorus.	Final conversion of ammonia to nitrate, helping nitrogen removal.

Research shows AAO is a standard way to remove nutrients. It is used in many big treatment plants. AAO removes organic substances and nutrients together. Studies show AAO helps improve nitrogen removal in city wastewater plants.

Deammonification

Deammonification is a new way to remove nitrogen. It saves energy and costs less than older methods. This process has two steps: partial nitrification and anaerobic ammonium oxidation (anammox). The table below lists the main bacteria groups:

Mechanism of Deammonification	Microbial Groups Involved
Partial Nitrification	Aerobic Ammonia Oxidizing Bacteria (AOB)
Anaerobic Ammonium Oxidation	Anaerobic Ammonium Oxidizing Bacteria (AnAOB)
	Nitrite Oxidizing Bacteria (NOB)

Deammonification works well for wastewater with lots of NH4+-N.
Important bacteria groups are AnAOB and NOB.
Anammox changes NH4+-N to N2 using NO2−-N.

Deammonification uses less electricity than older methods. Older ways need 35 kWh/(P.E.∙a), but deammonification needs only 21.5 kWh/(P.E.∙a). Plants can save 13.5 kWh/(P.E.∙a), which is 405 MWh/a for a plant with 30,000 P.E. Energy recovery is also better with deammonification.

Simultaneous Nitrification-Denitrification (SND)

SND is a way to remove nitrogen by doing nitrification and denitrification at the same time in one tank. This process uses biofilm and suspended biomass. Some microorganisms use NH4+-N as an electron donor and NO2− N as an electron receptor, making N2. The table below shows the usual conditions:

Parameter	Value/Condition
Nitrogen Removal Efficiency	80 to 96 percent
COD:N Ratio	At least 10.0
Optimum DO Concentration	0.3 to 0.7 mg/L
Cycle Operations	Aerobic (4.0–7.0 h), Anaerobic (3.0-4.0 h), Settling (0.5–1.0 h)

SND improves nitrogen removal by combining both steps.
Special denitrification organisms are in biofilm and suspended biomass.
SND is used in moving bed biofilm reactors (mbbr) for better pollutant removal.

Partial Nitritation-Anammox (PNA)

PNA is a biological way to remove nitrogen using partial nitritation and anammox. This process is used in city and factory wastewater treatment. PNA removes nutrients well but has trouble removing carbon and nitrogen at the same time. The table below lists the main good and bad points:

Advantages	Limitations
95% reduction in biodegradable carbon	Low temperatures
60% decrease in oxygen demand	Low ammonium concentrations
80% reduction in sludge production	Need to suppress nitrite-oxidizing bacteria (NOB) activity

PNA lowers oxygen use and sludge production.
It does not work well at low temperatures or low ammonium levels.
Operators must control NOB activity for best results.
PNA is often used in mbbr systems and five-stage bardenpho designs.

Enhanced Biological Nutrient Removal (EBNR)

EBNR is an advanced way to remove nitrogen. It also helps remove phosphorus from wastewater. EBNR uses nitrification and denitrification for nitrogen removal. It uses phosphorus-accumulating organisms (PAOs) to remove phosphorus. The table below shows the main features:

Feature	Description
Nitrogen Removal	Uses nitrification (ammonia to nitrate) and denitrification (nitrate to nitrogen gas).
Phosphorus Removal	PAOs cycle through anaerobic and aerobic conditions to remove phosphorus biologically.

EBNR helps stop harmful algal blooms and makes water cleaner. It is used in moving bed biofilm reactors and other biological ways to treat nitrogen. EBNR supports nitrogen reduction and better pollutant removal.

Nitrogen Removal Techniques Overview

Biological vs. Chemical Methods

Biological nitrogen removal uses tiny living things called microorganisms. These microorganisms change nitrogen compounds into safe nitrogen gas. They live in special tanks at wastewater treatment plants. The bacteria break down ammonia and nitrate in steps. This way does not need many chemicals. It also makes less waste.

Chemical methods use things like chlorine, lime, or ferric chloride. These chemicals mix with nitrogen compounds and make solids. Workers take these solids out of the water. Chemical methods can remove over 95% of nitrogen. But they make extra sludge that must be thrown away. Buying and using chemicals costs more money.

The table below shows how biological and chemical methods are different:

Method Type	Removal Efficiency	Operational Costs
Biological Methods	Higher efficiency	More cost-effective
Chemical Methods	>95% efficiency	Higher due to sludge disposal and chemical purchases

Biological methods usually cost less to use. They work well for many kinds of wastewater. Chemical methods work fast, but they need more money for chemicals and waste.

Tip: Most treatment plants use biological methods first. They use chemical methods only when they need extra nitrogen removal.

Physicochemical Alternatives

Physicochemical methods use both physical and chemical steps. Some examples are ion exchange, air stripping, and membrane filtration. Ion exchange swaps nitrogen ions with other ions in a special resin. Air stripping blows air through water to take out ammonia as a gas. Membrane filtration uses filters to separate nitrogen compounds.

Physicochemical methods can remove nitrogen quickly. They are good for small or special treatment systems. But these methods cost more and use more energy. They also need skilled workers to run the machines.

Operators use physicochemical methods as a backup or for extra cleaning after biological treatment. These methods help meet strict water rules when biological or chemical methods are not enough.

Why Nitrogen Removal Matters

Environmental Impact

Too much nitrogen in water can hurt nature. When nitrogen gets into rivers or lakes, it can cause eutrophication. This means too many plants and algae start to grow. Fish and other animals may die because there is not enough oxygen.

Too much nitrogen is bad for ecosystems. It causes eutrophication and can be toxic to living things. Eutrophication makes oxygen drop, which is dangerous for animals that need air in water.

When people add extra nutrients, it causes lots of phytoplankton to grow. This makes the water cloudy and lowers its quality. These blooms use up oxygen, making places where fish cannot live.

Low-oxygen events from eutrophication are a big problem for fishing. They hurt both freshwater and ocean areas all over the world.

Taking out nitrogen from water helps fish, plants, and people. Nitrification and denitrification are important steps for this. These steps lower the chance of bad algae blooms and keep water safe.

Regulatory Drivers

Many countries have rules to keep nitrogen low in water. These rules help make sure water is clean and safe. The table below lists some main standards:

Region	Regulatory Standard	Specific Limits
United States	Laws depend on the water and how sensitive it is to eutrophication	BOD5 and TSS: max 30 mg/l with 85% reduction; COD not standardized.
	Stricter rules for water that can get too many nutrients	TP must be 0.1 mg/l, which needs expensive technology.
European Union	The UWWTD (Wastewater Directive) is the main law	Sets treatment levels and how much pollution must be removed to protect water life.
China	Rules say NH4+–N can be up to 8.0 mg/l (5.0 mg/l in winter)	Focuses on fixing low C/N ratio in city wastewater for good denitrification.

Treatment plants must follow these rules. Operators use nitrification and denitrification to remove nitrogen. Meeting these rules stops pollution and keeps people healthy.

Water Quality Benefits

Taking out nitrogen makes water better for people and nature. When plants remove nitrogen, there are fewer algae blooms and dead zones. This keeps rivers and lakes healthy.

Removing nitrogen is needed to make water cleaner. It helps control how much nitrogen goes into lakes downstream. This stops eutrophication, which can cause harmful algae and hurt water life. Using less fertilizer and better treatment are good ways to help.

Clean water is good for swimming, fishing, and drinking. Nitrogen removal also helps cities meet tough water rules. Using nitrification and denitrification in treatment plants protects nature and helps everyone.

Technology Comparison Criteria

Picking the best nitrogen removal process depends on a few key things. These things help workers and engineers choose what works for their plant.

Efficiency

Efficiency shows how much nitrogen a process can remove. High efficiency means most nitrogen is taken out. This makes water safer for nature. Some systems, like partial nitritation-anammox, work well with less energy. Operators want methods that keep working even if nitrogen levels change.

Cost

Cost means the money to build and run the system. Some new ways, like electrodialysis, cost less to start than old ones. They also cost less to use than nitrification/denitrification and anammox. When picking a process, workers must think about cost and how much nitrogen they need to remove.

Electrodialysis costs less to build and run.
Old methods may need more money for chemicals and power.
Plants must look at both short-term and long-term costs.

Operational Complexity

Operational complexity means how hard it is to run the system. Many things can make these systems tough to manage:

Microbial interactions can change how well removal works.
Things like temperature can affect the process.
Advanced math models help but make more work for staff.

Simple systems are easier to use and fix. Complex ones may need more training and careful watching.

Environmental Sustainability

Environmental sustainability looks at how the process affects the earth. Some old ways use lots of energy and make greenhouse gases. Newer ways, like anaerobic ammonium oxidation, use less energy and make less waste. Aerobic granular sludge helps by making sludge easier to deal with.

Constructed wetlands are a green choice. They use plants and bacteria to remove nitrogen. This lowers energy use and greenhouse gases. Some advanced systems, like sequencing batch reactors, may make more gases. Others, like the A2O process, keep gases low. Picking the right process helps protect nature and saves resources.

Biological Nitrogen Removal: Technology Comparison

Strengths and Limitations

Biological nutrient removal processes have both good and bad points. Many big wastewater plants use these systems because they remove nitrogen well. There are two main designs: single anoxic zone and two anoxic zone systems. The table below shows how each one works:

Process Type	Average Effluent Total Nitrogen Concentration (mg-N/L)	Performance Stability
Single Anoxic Zone	6 to 8	Stable and Predictable
Two Anoxic Zones	About 2	Higher Reliability

Strengths:
- Two anoxic zone systems take out more nitrogen than single zone systems.
- These systems often work better because they let nitrification and denitrification happen at the same time.
- They are more reliable than systems that only remove BOD5 and TSS.
Limitations:
- Single anoxic zone systems cannot get nitrate-nitrogen very low.
- Both systems can work differently if workers change how they run things or if the oxygen system has problems.

Many mbbr systems use these designs to help remove more nitrogen. Operators in big plants often pick two anoxic zones for better results.

Note: The seasons can change how well these systems work. Cold weather slows down the microbes, so nitrification and denitrification happen slower. Plants may need to change the temperature or oxygen in winter to keep removal rates high.

Suitability for Wastewater Types

Different biological nutrient removal processes work best with certain kinds of wastewater. The table below shows how two common processes do with different COD/N ratios:

BNR Process	COD/N Ratio Range	Optimal Conditions	Performance Characteristics
O/A	Low (60–110 mg/L)	Best for low organic content	Higher total nitrogen removal rate
A/O	High (200 mg/L)	Good for high influent organic load	Effective under specific conditions

O/A processes are best for wastewater with little organic matter. They remove more nitrogen in these cases.
A/O processes work better when there is more organic matter coming in. They do well under these conditions.

Many big wastewater plants use mbbr technology to treat different types of water. Mbbr systems can work with both city and factory wastewater. Operators can change the process to match the COD/N ratio and get the removal they want.

Tip: Plants should test the water before picking a process. Choosing the right process for the wastewater type helps keep nitrogen removal high.

Scalability

Scalability means how well a process works when going from a small lab to a big plant. Studies on the Feammox-NDFO process show that knowing about the microbes and how they change is very important. Operators must watch things like pH, temperature, and where the carbon and nitrogen come from. These things affect how well nitrogen is removed.

Researchers have tested biological nutrient removal in labs and in real plants. They found some processes work well in small tests but have problems in big plants. For example, mbbr systems can be used in big plants if workers control the right things. Big wastewater plants often use mbbr because it can handle changes in flow and load.

Lab studies help find the best ways to remove nitrogen.
Field tests show how the process works with real wastewater.
Operators must change the process as they scale up to keep nitrification and denitrification working well.

Plants that want to try new biological nutrient removal processes should start with small pilot tests. These tests help find problems before using the process in big plants.

Biological Nutrient Removal in Practice

Municipal Wastewater Applications

Cities use different ways to clean water before it goes back to nature. Workers pick a method based on how much nitrogen they must remove. The table below shows some common ways and their results:

Technology	Reported TN Concentration (mg/L)	Notes
Activated Sludge Process	10 – 15	Commonly used, but may struggle with low TN concentrations.
Biological Aerated Filters	10 – 15	Effective but may require additional processes for ultra-low TN.
Anammox Processes	< 1	Highly efficient for low TN concentrations, often combined with other methods.
Limited Denitrification Technology	< 1	Aims to reduce TN from 15-25 mg/L to ultra-low levels, combining biological and physical-chemical methods.

Workers in city plants often use activated sludge or aerated filters. If they need to remove more nitrogen, they add anammox or limited denitrification. These choices help cities follow strict water rules.

Industrial Wastewater Applications

Factories have special problems with their water. Their water can have lots of ammonia or even toxic chemicals. Biological processes can fix these problems well. They use two-sludge systems and change the temperature to get better results. The table below shows some important facts:

Parameter	Value
Ammonium concentration	50004+ L−1
Average nitrification rates	0.11 to 0.18 4+ −1 −1
Maximum nitrification rate	0.10, 0.21, 0.37 at 15°C, 20°C, 25°C
Maximum denitrification rate	0.64 (ethanol), 0.11 (methanol)
Treatment duration	450 days
System type	Two-sludge system

Biological methods can treat toxic things like cyanide.
Autotrophic denitrification uses different electron donors and makes less sludge.
These ways cost less and do not make harmful waste.

Decentralized and Small-Scale Systems

Small towns and faraway places need special ways to remove nitrogen. Workers must think about the reactor type, the bacteria, and the cost. The table below lists key points:

Consideration	Description
Reactor Design	Choices include sequencing batch reactors, continuous flow, and fixed-film reactors. Design affects efficiency.
Microbial Community Management	Operators must grow and keep the right bacteria for nitrification.
Process Control Parameters	They must watch dissolved oxygen, pH, and temperature.
Economic Constraints	High costs can make advanced systems hard to use in small places.

Note: Small systems need careful planning. Workers often use simple reactors and focus on keeping the right bacteria. Cost and easy use are very important for these systems.

Choosing the Right Nitrogen Removal Technique

Decision Factors

Picking the best way to remove nitrogen depends on many things. Every treatment plant is different. Workers need to think about these main points:

Type of Wastewater: Some plants clean city water. Others treat water from factories. The amount of ammonia and other stuff changes what process works best.
Regulatory Limits: Local laws say how much nitrogen can be in the water. Plants must follow these rules to keep nature safe.
Available Space: Some systems need big tanks or more machines. Small plants may need smaller setups.
Budget: It costs money to build and run these systems. Plants must choose something that works and fits their budget.
Staff Skills: Some ways need workers who know how to run and fix the system.

Tip: Plants that know their water and goals can pick the best process for them.

Matching Technology to Needs

Workers can choose the right technology by looking at what their plant needs. The table below shows which technologies fit different needs:

Technology	Best For	Key Points
Anammox	Plants with high ammonia and low carbon	Saves carbon and energy, but faces challenges in mainstream use
UCT-MBR System	Plants needing high nitrogen removal	Can improve removal by changing process setup and balancing electron flows
Single-Sludge Plan	Simple operation, less equipment	Good for small plants, easy to manage
Double-Sludge Plan	Plants needing more control	Balances electron donors and acceptors, boosts efficiency

Workers can help nitrogen removal by changing how they balance electron donors and acceptors. Some systems, like UCT-MBR, work better without adding more carbon. This saves energy and money.

Plants that try different plans and check results can find the best and cheapest way for their needs.

Each biological nitrogen removal process works best in different ways. The table below compares how A2/O and DOD systems work in warm and cold seasons: