Nitrogen & Phosphorus Chemistry

Nitrogen and phosphorus are essential nutrients, but excessive amounts can create water-quality problems. Wastewater operators monitor and control these nutrients because they affect biological treatment, dissolved oxygen demand, eutrophication, permit compliance, and process stability.

Operators should understand the major chemical forms of nitrogen and phosphorus, how treatment processes convert them, and how laboratory results relate to process performance.

Nitrogen in Water and Wastewater

Nitrogen can occur in several important forms:

• organic nitrogen;
• ammonia and ammonium;
• nitrite;
• nitrate;
• nitrogen gas.

Treatment processes can convert nitrogen from one form to another.

Organic Nitrogen

Organic nitrogen is nitrogen contained in organic compounds.

Sources include:

• proteins;
• urea;
• amino acids;
• biological cells;
• other organic material.

Ammonification

Ammonification is the biological conversion of organic nitrogen into ammonia or ammonium.

This occurs as microorganisms break down nitrogen-containing organic matter.

Ammonia and Ammonium

In water, reduced inorganic nitrogen exists mainly as:

• un-ionized ammonia, NH3;
• ammonium ion, NH4+.

The relative amount of each depends strongly on pH and temperature.

Ammonia-Ammonium Equilibrium

At lower pH, a larger fraction of reduced nitrogen is present as ammonium.

As pH increases, the fraction present as un-ionized ammonia increases.

Higher temperature also increases the fraction present as un-ionized ammonia.

Why Un-Ionized Ammonia Matters

Un-ionized ammonia is generally more toxic to aquatic organisms than ammonium.

This means the same total ammonia concentration can have different environmental significance depending on:

• pH;
• temperature.

Ammonia-Nitrogen

Laboratory results are often reported as ammonia-nitrogen or NH3-N.

This means the result is expressed as the mass of nitrogen contained in the ammonia and ammonium species, not the total molecular mass of NH3.

Nitrite

Nitrite, NO2-, is an intermediate form of nitrogen.

In a nitrifying biological process, ammonia is first oxidized toward nitrite before further oxidation to nitrate.

Nitrate

Nitrate, NO3-, is a highly oxidized form of nitrogen.

Nitrate is commonly produced during nitrification and can be removed biologically through denitrification.

Nitrogen Gas

Nitrogen gas, N2, is the final gaseous product of complete biological denitrification.

It can leave the liquid and return to the atmosphere.

Total Kjeldahl Nitrogen

Total Kjeldahl Nitrogen, or TKN, represents:

• organic nitrogen;
• ammonia/ammonium nitrogen.

TKN does not include nitrate or nitrite.

Total Nitrogen

A simplified total nitrogen relationship is:

Total Nitrogen = TKN + Nitrite-N + Nitrate-N

Depending on laboratory method and reporting convention, the exact reported components should always be confirmed.

Nitrogen Units

Nitrogen species are often reported as:

• mg/L NH3-N;
• mg/L NO2-N;
• mg/L NO3-N;
• mg/L total nitrogen.

The -N means the result is expressed as nitrogen mass.

Species Mass Versus Nitrogen Mass

Operators must distinguish between:

• mg/L nitrate as NO3;
• mg/L nitrate as N.

These are not numerically the same because nitrate contains both nitrogen and oxygen.

Nitrification

Nitrification is the biological oxidation of ammonia to nitrate.

It occurs in two general steps:

1. ammonia is oxidized toward nitrite;
2. nitrite is oxidized toward nitrate.

Nitrification Requirements

Successful nitrification requires:

• nitrifying microorganisms;
• dissolved oxygen;
• sufficient alkalinity;
• suitable pH;
• suitable temperature;
• adequate solids retention time.

Oxygen Demand of Nitrification

A commonly used operator approximation is:

About 4.6 lb O2 are required per lb of ammonia-nitrogen oxidized.

Alkalinity Consumption During Nitrification

A commonly used operator relationship is:

Approximately 7.14 mg/L of alkalinity as CaCO3 is consumed per 1 mg/L of ammonia-nitrogen nitrified.

Nitrification Example

If 15 mg/L of ammonia-nitrogen is nitrified:

15 × 7.14 = 107.1 mg/L alkalinity as CaCO3 consumed

This helps explain why pH can decline when influent alkalinity is insufficient.

Nitrification and pH

Nitrification can lower pH because it consumes alkalinity.

If pH becomes too low:

• nitrifying organisms can slow;
• ammonia removal can deteriorate;
• process stability can decrease.

Nitrite Accumulation

Normally, nitrite is further oxidized to nitrate.

Elevated nitrite may indicate an imbalance between the two nitrification steps.

Possible causes include:

• process inhibition;
• low dissolved oxygen;
• temperature change;
• toxic conditions;
• insufficient biological solids age.

Denitrification

Denitrification is the biological reduction of nitrate toward nitrogen gas.

It generally occurs under anoxic conditions.

Requirements for Denitrification

Denitrification generally requires:

• nitrate;
• low dissolved oxygen;
• active denitrifying microorganisms;
• a usable carbon source;
• suitable temperature and pH.

Carbon Source

Denitrifying microorganisms need an electron donor, commonly biodegradable organic carbon.

Possible carbon sources include:

• influent wastewater organics;
• internal process carbon;
• supplemental external carbon where designed.

DO and Denitrification

If dissolved oxygen is present at significant concentration, microorganisms generally use oxygen before nitrate.

Too much oxygen entering an anoxic zone can therefore reduce denitrification.

Alkalinity Recovery During Denitrification

Denitrification can recover part of the alkalinity consumed during nitrification.

This can help support:

• pH;
• buffering;
• overall biological stability.

Internal Nitrate Recycle

Biological nutrient-removal systems may recycle nitrate-containing mixed liquor from an aerobic zone to an anoxic zone.

This allows nitrate to contact:

• low-DO conditions;
• available carbon;
• denitrifying biomass.

Too Much Internal Recycle

Excessive recycle can carry unwanted dissolved oxygen into the anoxic zone.

This can reduce denitrification efficiency.

Denitrification in Clarifiers

If settled sludge contains nitrate and becomes anoxic, denitrification can occur in the sludge blanket.

Nitrogen gas bubbles can attach to solids and lift them toward the surface.

This can contribute to rising sludge.

Nitrogen Mass Loading

Nitrogen concentration alone does not describe the total nutrient load.

A mass-loading calculation must include flow.

A common wastewater relationship is:

Load, lb/day = Flow, MGD × Concentration, mg/L × 8.34

Ammonia Load Example

If plant flow is 2.0 MGD and influent ammonia-nitrogen is 25 mg/L:

Load = 2.0 × 25 × 8.34

Load = 417 lb/day NH3-N

If concentration remains the same but flow doubles, total ammonia load also doubles.

Phosphorus in Water and Wastewater

Phosphorus can occur in several forms, including:

• organic phosphorus;
• orthophosphate;
• condensed phosphate forms;
• particulate phosphorus.

Orthophosphate

Orthophosphate is a dissolved inorganic form of phosphorus that is readily available for biological use and chemical reactions.

It is often an important measurement in phosphorus-removal processes.

Organic Phosphorus

Organic phosphorus is phosphorus incorporated into:

• cells;
• organic matter;
• biological solids.

As organic matter decomposes, some organic phosphorus can be converted to orthophosphate.

Particulate Phosphorus

Phosphorus associated with suspended solids can be removed when those solids are removed.

Good solids separation therefore contributes to phosphorus removal.

Total Phosphorus

Total phosphorus includes phosphorus in dissolved and particulate forms measured by the specified analytical method.

Total phosphorus and orthophosphate are not the same measurement.

Phosphorus Units

Results may be reported as:

• mg/L P;
• mg/L PO4;
• mg/L orthophosphate as P.

Operators must pay close attention to the reporting basis.

Phosphate as P Versus Phosphate as PO4

A phosphorus result expressed as P represents only the phosphorus mass.

A result expressed as PO4 includes the phosphorus plus oxygen atoms in the phosphate ion.

The numerical values therefore differ.

Why Phosphorus Matters

Excess phosphorus can promote excessive plant and algae growth in receiving waters.

This can contribute to:

• eutrophication;
• oxygen depletion;
• water-quality impairment.

Biological Phosphorus Uptake

Microorganisms incorporate phosphorus into new cell mass.

Some phosphorus therefore leaves the treatment process when waste biological solids are removed.

Enhanced Biological Phosphorus Removal

Enhanced Biological Phosphorus Removal, or EBPR, uses specific biological conditions to encourage microorganisms to store more phosphorus than ordinary biomass growth alone would require.

Phosphorus-Accumulating Organisms

EBPR relies on microorganisms commonly called phosphorus-accumulating organisms, or PAOs.

These organisms can store phosphorus within their cells under the correct sequence of process conditions.

Anaerobic Zone in EBPR

In the anaerobic zone, PAOs can release phosphate while taking up readily biodegradable carbon compounds.

True anaerobic conditions are important because both dissolved oxygen and nitrate can interfere with the intended biochemical selection.

Aerobic Phosphorus Uptake

Under subsequent aerobic conditions, PAOs take up phosphate from the wastewater and store it within their cells.

Net phosphorus removal occurs when phosphorus-rich sludge is wasted from the system.

Importance of Sludge Wasting

If phosphorus-rich biomass is not removed from the system, the phosphorus has not truly left the treatment process.

Effective solids wasting is therefore part of biological phosphorus removal.

Nitrate Entering an EBPR Anaerobic Zone

Nitrate recycle into a zone intended to be anaerobic can interfere with EBPR because denitrifying organisms may consume carbon that PAOs need.

Volatile Fatty Acids

Readily biodegradable compounds, including volatile fatty acids, can support PAO metabolism in the anaerobic zone.

Insufficient suitable carbon can reduce EBPR performance.

Chemical Phosphorus Removal

Phosphorus can also be removed chemically by adding metal salts or other suitable treatment chemicals.

Common metal salts include compounds containing:

• aluminum;
• iron.

Chemical Precipitation

Metal ions react with phosphate to form less soluble compounds.

These precipitated solids must then be removed through processes such as:

• clarification;
• filtration;
• solids separation.

Chemical Dose and Phosphorus Removal

Increasing chemical dose may increase phosphorus removal up to practical limits, but overdosing can create problems such as:

• excess sludge production;
• chemical cost;
• pH reduction;
• alkalinity consumption;
• higher residual metals.

pH and Chemical Phosphorus Removal

Precipitation reactions depend on pH.

Operators should consider:

• chemical selected;
• influent phosphorus;
• pH;
• alkalinity;
• mixing;
• solids separation.

Mixing and Chemical Phosphorus Removal

Chemical feed must be mixed adequately so the reagent contacts phosphate throughout the process stream.

Poor mixing can cause:

• local overdosing;
• poor precipitation;
• inconsistent removal.

Chemical Sludge Production

Chemical phosphorus removal creates additional solids.

This can increase:

• clarifier solids loading;
• sludge handling;
• dewatering demand;
• disposal volume.

Biological Versus Chemical Phosphorus Removal

Biological phosphorus removal depends on microbial metabolism and sludge wasting.

Chemical phosphorus removal depends on precipitation and physical removal of the resulting solids.

Some facilities use both methods.

Phosphorus Release from Sludge

Phosphorus stored in biological solids can be released back into liquid under certain conditions.

This can occur during:

• anaerobic storage;
• digestion;
• solids handling;
• sludge breakdown.

Recycle Streams

Liquid returned from sludge-processing operations can contain elevated:

• ammonia;
• phosphorus.

Recycle streams can therefore increase nutrient loading to the main treatment process.

Sidestream Nutrient Load

A relatively small recycle flow can still carry a significant nutrient load if concentration is high.

Operators should evaluate both:

• concentration;
• flow.

Example of Phosphorus Load

If flow is 1.5 MGD and total phosphorus is 6 mg/L:

Load = 1.5 × 6 × 8.34

Load = 75.1 lb/day P

Nutrient Removal and Solids

Both nitrogen and phosphorus treatment can affect solids production and solids handling.

Examples include:

• biomass growth;
• chemical precipitates;
• phosphorus-rich waste sludge.

Nitrogen and Dissolved Oxygen

DO should be managed according to the biological zone.

• Aerobic zones need sufficient DO.
• Anoxic zones need low DO for denitrification.
• Anaerobic EBPR zones should avoid both DO and nitrate.

Nitrogen and Alkalinity

Nitrification consumes alkalinity.

Denitrification restores part of that alkalinity.

This interaction can affect overall process pH.

Phosphorus and pH

pH affects:

• phosphate chemical species;
• metal-phosphate precipitation;
• performance of chemical treatment.

Nutrient Sampling

Useful nutrient samples may include:

• influent;
• biological process zones;
• secondary effluent;
• final effluent;
• recycle streams.

Use the Correct Sample Location

A nutrient result should be interpreted according to where the sample was collected.

For example, nitrate concentration in an aerobic zone and nitrate concentration in an anoxic-zone effluent answer different process questions.

Interpret Nitrogen Species Together

Looking at only ammonia can miss important information.

A complete process review may include:

• ammonia;
• nitrite;
• nitrate;
• total nitrogen;
• DO;
• alkalinity;
• pH.

Example: High Effluent Ammonia, Low Nitrate

This pattern may indicate poor nitrification.

Possible causes include:

• low DO;
• low temperature;
• low pH;
• insufficient alkalinity;
• low solids retention time;
• toxicity.

Example: Low Ammonia, High Nitrate

This pattern may indicate successful nitrification but limited denitrification.

Operators may then review:

• anoxic-zone DO;
• carbon availability;
• internal recycle;
• anoxic detention time.

Example: Low Ammonia and Low Nitrate

If total nitrogen is also low, this may indicate effective nitrification and denitrification.

Other process data should still be reviewed before drawing conclusions.

Example: Poor Phosphorus Removal

Possible causes include:

• insufficient chemical dose;
• poor mixing;
• poor solids separation;
• inadequate biological phosphorus uptake;
• poor sludge wasting;
• phosphorus-rich recycle streams;
• incorrect analytical result.

Trend Nutrient Data

Trends can reveal gradual changes in:

• ammonia removal;
• nitrate production;
• denitrification;
• phosphorus removal;
• sidestream nutrient loading.

Do Not Adjust from One Result Alone

Unexpected nutrient results should be checked against:

• recent process trends;
• sample location;
• DO;
• pH;
• alkalinity;
• flow;
• chemical feed;
• laboratory QA/QC.

Common Nitrogen Chemistry Mistakes

• Confusing ammonia, nitrite, nitrate, and total nitrogen.
• Forgetting that TKN excludes nitrate and nitrite.
• Confusing nitrate as N with nitrate as NO3.
• Ignoring pH and temperature when evaluating un-ionized ammonia.
• Assuming nitrification only requires oxygen.
• Ignoring alkalinity consumption during nitrification.
• Allowing excessive DO into an anoxic zone.
• Ignoring carbon availability during denitrification.

Common Phosphorus Chemistry Mistakes

• Confusing total phosphorus with orthophosphate.
• Confusing phosphorus as P with phosphate as PO4.
• Assuming chemical addition alone guarantees phosphorus removal.
• Ignoring solids separation after precipitation.
• Ignoring sludge wasting in biological phosphorus removal.
• Ignoring nitrate entering an intended anaerobic EBPR zone.
• Ignoring nutrient-rich recycle streams.

A Practical Nitrogen Review Sequence

1. Review ammonia concentration.
2. Review nitrite and nitrate.
3. Review total nitrogen if available.
4. Confirm reporting units and whether results are expressed as N.
5. Review DO by process zone.
6. Review pH and alkalinity.
7. Review temperature.
8. Review solids retention time.
9. Review anoxic carbon availability and recycle rates.
10. Compare nutrient concentration with plant flow to evaluate mass loading.

A Practical Phosphorus Review Sequence

1. Review total phosphorus and orthophosphate.
2. Confirm whether results are reported as P or PO4.
3. Review biological process conditions.
4. Review chemical dose where chemical precipitation is used.
5. Review pH and mixing.
6. Review clarifier or filter solids separation.
7. Review waste-sludge removal.
8. Review phosphorus-rich recycle streams.
9. Calculate phosphorus loading when flow changes significantly.
10. Verify unusual laboratory results before major process changes.

What to Remember for the Exam

• Nitrogen occurs as organic nitrogen, ammonia/ammonium, nitrite, nitrate, and nitrogen gas.
• Ammonification converts organic nitrogen toward ammonia and ammonium.
• The balance between NH3 and NH4+ depends strongly on pH and temperature.
• Un-ionized ammonia generally becomes a larger fraction as pH and temperature increase.
• TKN includes organic nitrogen plus ammonia/ammonium nitrogen but excludes nitrate and nitrite.
• Total nitrogen can be approximated as TKN plus nitrite-N plus nitrate-N.
• Always distinguish results reported as nitrogen mass from results reported as the full chemical species.
• Nitrification biologically oxidizes ammonia toward nitrate.
• Nitrification requires oxygen, alkalinity, suitable pH, temperature, and adequate solids retention time.
• Approximately 4.6 lb of oxygen are required per lb of ammonia-nitrogen nitrified.
• Approximately 7.14 mg/L of alkalinity as CaCO3 are consumed per 1 mg/L of ammonia-nitrogen nitrified.
• Denitrification converts nitrate toward nitrogen gas under anoxic conditions.
• Denitrification requires low DO and an available carbon source.
• Excess DO entering an anoxic zone can reduce denitrification.
• Phosphorus occurs in organic, orthophosphate, condensed, and particulate forms.
• Total phosphorus and orthophosphate are different measurements.
• Phosphorus results may be reported as P or as PO4, which are not numerically equivalent.
• Biological phosphorus removal depends on microbial uptake and removal of phosphorus-rich sludge.
• Chemical phosphorus removal forms precipitated solids that must be physically removed.
• Nutrient mass loading depends on both concentration and flow.