Ammonia monitoring is widely used in:

• Aquaculture systems

• Wastewater treatment plants

• Environmental water monitoring

• Industrial process water

• Agriculture and nutrient management

Because ammonia is toxic to many aquatic organisms even at relatively low concentrations, continuous monitoring is essential for maintaining safe water conditions.

What Is Ammonia Nitrogen (NH₃–N)?

Ammonia in water exists in two chemical forms:

1. Un-ionized ammonia (NH₃)

2. Ionized ammonium (NH₄⁺)

These two forms exist in equilibrium and depend mainly on pH and temperature.

At higher pH and temperature, more ammonia exists as NH₃, which is the toxic form for fish and aquatic organisms.

When sensors report ammonia nitrogen (NH₃–N), they usually refer to the amount of nitrogen present in ammonia compounds, expressed in: mg/L NH₃–N

Main Types of Ammonia Sensors

Several technologies are used to measure ammonia in water. The most common ones are:

• Ion-selective electrodes (ISE)

• Optical ammonia sensors

• Gas diffusion sensors

• Colorimetric analyzers

Each technology has different strengths depending on the application.

1. Ion-Selective Electrode (ISE) Ammonia Sensors

The ion-selective electrode is one of the most common technologies for continuous ammonia monitoring.

Basic Principle

ISE sensors measure ammonia using a special membrane that selectively interacts with ammonium ions (NH₄⁺).

The sensor contains:

• A selective membrane

• An internal reference electrode

• An internal electrolyte solution

When the probe is placed in water:

1. Ammonium ions interact with the membrane.

2. This interaction creates a voltage difference between the sensing electrode and the reference electrode.

3. The voltage depends on the concentration of ammonium ions in the water.

4. The electronics convert the voltage into an ammonia concentration reading.

This relationship follows the Nernst equation, which links ion concentration to electrical potential.

Gas-Sensing ISE Variant

Many ammonia ISE sensors actually measure ammonia gas (NH₃) rather than ammonium directly.

These sensors include:

• A gas-permeable membrane

• An internal pH electrode

The measurement works like this:

1. Dissolved ammonia diffuses through the membrane.

2. Inside the sensor, ammonia reacts with water to form ammonium and hydroxide ions.

3. This changes the internal pH of the electrolyte solution.

4. The internal electrode measures this pH change.

5. The electronics convert this change into ammonia concentration.

This design improves selectivity and reduces interference.

2. Optical Ammonia Sensors

Optical sensors measure ammonia using fluorescent or color-changing chemical indicators.

These sensors contain a small optical sensing layer that reacts with ammonia molecules.

The process works like this:

1. A light source illuminates the sensing layer.

2. Ammonia interacts with the chemical dye.

3. The dye changes color or fluorescence intensity.

4. A photodetector measures the optical change.

5. The sensor converts this change into an ammonia concentration.

Advantages:

• No direct electrical contact with the sample

• Less drift compared to electrochemical sensors

• Good long-term stability

These sensors are increasingly used in environmental monitoring and aquaculture systems.

3. Gas Diffusion Ammonia Sensors

Some ammonia sensors use gas diffusion technology.

These systems work by converting ammonium ions into ammonia gas under controlled conditions.

The process works as follows:

1. The water sample is made more alkaline.

2. This converts ammonium (NH₄⁺) into ammonia gas (NH₃).

3. The ammonia gas diffuses through a gas-permeable membrane.

4. Inside the sensor, the gas is detected by either:

   • a pH electrode

   • a conductivity sensor

   • or an optical detector.

This method provides very accurate measurements but requires controlled chemistry inside the sensor.

4. Colorimetric Ammonia Analyzers

Some high-precision analyzers use chemical color reactions to measure ammonia.

The most common reaction is the indophenol blue method.

The process works like this:

1. Reagents are added to the water sample.

2. Ammonia reacts with chemicals to form a blue-colored compound.

3. The intensity of the color is measured with a photometer.

4. The absorbance of light corresponds to ammonia concentration.

These analyzers are very accurate but require:

• Chemical reagents

• Pumps and tubing

• Periodic maintenance

They are commonly used in laboratories and large wastewater treatment plants.

Factors That Affect Ammonia Measurements

Ammonia sensors must account for several environmental factors.

pH

Because ammonia and ammonium exist in equilibrium, pH strongly affects readings.

Higher pH shifts the balance toward toxic NH₃.

Temperature

Temperature influences:

• chemical equilibrium

• sensor response

• diffusion rates

Most sensors include automatic temperature compensation.

Interfering Ions

Ion-selective sensors can be affected by other ions such as:

• potassium

• sodium

High-quality sensors include membranes designed to reduce these effects.

Typical Ammonia Levels in Water Systems

Typical concentrations vary widely depending on the application.

In aquaculture, even 0.05–0.2 mg/L NH₃ (un-ionized ammonia) can already cause stress to fish.

Ammonia Monitoring in Water Treatment Systems

Ammonia is an important parameter because it indicates:

• organic waste accumulation

• protein breakdown

• microbial activity

• nitrification performance

In biological systems, ammonia is typically converted through the nitrogen cycle:

Ammonia → Nitrite → Nitrate

Monitoring ammonia helps operators detect:

• system overload

• biofilter failure

• oxygen limitations

• poor water circulation

Role of Ammonia Monitoring in Advanced Water Treatment

In advanced water treatment systems such as nanobubble oxygenation, ammonia monitoring can help evaluate biological performance.

Higher dissolved oxygen levels often improve:

• nitrification efficiency

• microbial oxidation processes

• biofilm stability

By monitoring ammonia alongside other parameters such as:

• dissolved oxygen (DO)

• oxidation-reduction potential (ORP)

• pH

• conductivity

operators gain a complete picture of water chemistry and treatment performance.