Measuring Ammonia with Online Analyzers
Treatment facilities use online ammonia analyzers to monitor and control treatment processes. Controlling ammonia levels can make treatment processes more reliable and cost effective. Currently, there are three major types of online ammonia analyzer technologies available to measure ammonia concentration in a treatment process stream.Ammonia is a chemical combination of elemental hydrogen (H) and nitrogen (N) occurring extensively in nature. The physical state of ammonia is dependent on temperature and pH, but pH normally is the determining factor. At a high pH, ammonia is expressed as NH3 and is referred to as “free ammonia.” In this state, ammonia is a colorless gas that is partially soluble in water.
At low pH or acidic conditions, ammonia becomes completely soluble in water and forms ammonium (NH4+) that is referred to as “ionized ammonia.” In addition, ammonia and ammonium also can be expressed as ammonia-nitrogen and ammonium-nitrogen, respectively, which is the quantity of elemental nitrogen present in the form of ammonia, expressed as NH3–N or ammonium, NH4+–N.
In high-nitrogen environments, free and ionized ammonia coexists and the quantities of each are summed to measure the concentration of total ammonia in mg/L. The general chemical behavior of free and ionized ammonia in water is described by the formula
NH3–N + H2O = NH4+–N + OH–
Ammonia-Nitrogen + Water = Ammonium-Nitrogen + Hydroxyl ion
Under conditions of low pH, the high concentration of hydrogen ions, H+, converts ammonia to ammonium as described by the following equation.
NH3–N + H+ Æ NH4+–N
Ammonia-Nitrogen + Hydrogen Ion Æ Ammonium-Nitrogen
Under conditions of high pH, ammonium is converted to ammonia by the following equation.
NH4+–N Æ NH3–N + H+
Ammonia-Nitrogen + Hydrogen Ion
Measuring Ammonia Levels
Treatment facilities use online ammonia analyzers to monitor and control treatment processes. Controlling ammonia levels can make treatment processes more reliable and cost effective.
Wastewater treatment plants use online ammonia analyzers to optimize activated sludge and biological nutrient removal (BNR) processes. For example, ammonia is a nutrient, byproduct or feed additive for all activated sludge wastewater treatment processes.
In addition, some advanced wastewater treatment plants use online ammonia analyzers to monitor nitrification to meet ammonia discharge limits. Finally, some water treatment plants use online ammonia analyzers when monitoring chloramination, a drinking water treatment process used to create a disinfectant residual.
Types of Online Ammonia Analyzers
Currently, there are three major types of online ammonia analyzer technologies available to measure ammonia concentration in a treatment process stream:
electrodes (ISE), and
• Ultraviolet (UV) absorbance or multiple wavelength UV absorbance spectrophotometers.
Each technology detects ammonia concentrations using different analytical methods. In addition, manufacturers of each technology utilize different methodologies for such functions as sample transport, sample conditioning, chemical addition, primary measurement and secondary signal conditioning and amplification.
All of these analyzers require the addition of chemical reagents to the sample. Therefore, each analyzer has a sample cell and requires 3 to 15 minutes to perform a complete sample analysis. Automatic calibration and cleaning cycles are available options with ammonia analyzers.
Calibration and cleaning cycles may take 15 to 45 minutes per cycle and occur between measurement cycles. The analyzer holds the output value from the last measurement cycle while performing the next measurement, calibration or cleaning. If the process ammonia concentration changes significantly during one of these cycles, the analyzer output will show that change in concentration after the next measurement cycle. Figure 1 shows an example of the analyzer output step change. In addition, each analyzer has an electronics module that controls sample processing and converts signals from the sample cell to an output signal.
Colorimetric Ammonia Analyzers
Online colorimetric ammonia analyzers use a colorimeter (a light intensity meter capable of measuring the intensity of light at a specific wavelength) to measure the color intensity of sample solutions. The ammonia analyzer colorimeter is set to measure light intensity at a wavelength within the range of 645–655 nm. The color is produced by the addition of reagents to the sample and its intensity is proportional to the free ammonia concentration in the sample. This method of measurement is based on the Standard Methods phenate method 4500-NH3 – F (APHA et al., 1998).
The ammonia colorimeter compares the color intensity of two wastewater samples. The first is a reference sample and is used as a basis for comparison with the second test sample. To produce the color, the free ammonia in the sample is first converted to monochloramine by the addition of hypochlorous acid.
NH3–N + HOCl Æ NH2Cl + H2O
Ammonia-Nitrogen + Hypochlorous Acid Æ Monochloramine + Water
The ammonia colorimeter first treats the sample with reagent (1) that acts as a buffer to adjust the pH to a value greater than 12. Raising the pH of the sample forces any ammonium ions to convert to free ammonia.
NH4+–N Æ NH3–N + H+
Ammonia-Nitrogen + Hydrogen Ion
After reagent (1) is added, reagent (2) is added to the reference sample as a color indicator, specific for monochloramine. When
reagent (2) combines with any monochloramine in the first sample, the solution turns green. The color intensity increases in direct proportion to the concentration of monochloramine. The colorimetric analyzer reads the color intensity. Since no hypochlorous acid was added to the reference sample, the color intensity is a measure of the amount of monochloramine initially present in the wastewater and also ensures that the colorimeter corrects for any other interference.
The colorimetric analyzer then takes a second sample solution and adds reagent (1), a buffer that converts ammonium ions to free ammonia. The analyzer then adds hypochlorous acid (HOCl) that converts any available free ammonia to monochloramine. Finally, the analyzer adds the monochloramine specific reagent, which turns the second sample solution green. At this time, the colorimetric analyzer reads the color intensity in the second sample solution that is a measure of the amount of monochloramine produced by the reaction of free ammonia in the sample with the hypochlorous acid.
Free ammonia concentration is calculated by subtracting the first sample solution’s reference monochloramine concentration from the second sample solution’s monochloramine concentration. Total ammonia concentration is calculated by adding the first sample solution’s reference monochloramine concentration to the second sample solution’s monochloramine concentration. Figure 2 illustrates a generic colorimetric ammonia analyzer and its basic components.
Ion-Selective Electrode (ISE) Ammonia Analyzer
Online ISE ammonia analyzers are probe-type analyzers that use an ammonia ISE and a reference electrode. This method of measurement is similar to Standard Methods ammonia—selective electrode reference 4500-NH3–D (APHA et al., 1998).
The ISE ammonia analyzer feeds sample through a flow cell or sample chamber. Sodium hydroxide (NaOH) is added to the sample to raise its pH to a value greater than 11, to convert all ammonia to free ammonia, NH3. (Note that the sample chamber is not pictured in Figure 3 due to variations in manufacturers’ designs.)
Any free ammonia released in the sample chamber from the reaction with the sodium hydroxide reagent permeates into the ISE ammonia analyzer membrane cap. The membrane cap’s internal solution of ammonium chloride (NH4Cl) reacts with the free ammonia and changes the pH of the membrane cap’s ammonium chloride solution.
The ISE analyzer probe measures the change in pH of the
membrane cap’s ammonium chloride solution that is proportional to the
amount of free ammonia concentration in the sample solution. The ISE ammonia
analyzer electronics module uses the change in pH to calculate the
concentration of free ammonia in the sample.
The ISE ammonia analyzer probe measures the change in pH of the membrane cap’s ammonium chloride solution (similar to a standard pH probe) using three sensors; a pH or measuring electrode sensor, a reference electrode sensor and a resistance temperature device or detector (RTD) sensor.
The pH or measuring electrode sensor consists of a thin glass membrane filled with a neutral buffer solution (i.e., a solution that has a pH of 7) that is immersed in the membrane cap’s ammonium chloride solution. The pH sensor’s thin glass membrane contains a silver wire coated with silver chloride that is suspended in the neutral buffer solution. When the sample solution from the ISE ammonia analyzer sample chamber releases free ammonia into the membrane cap’s ammonium chloride solution, hydrogen ions pass through the pH sensor’s thin glass membrane and cause the silver wire to conduct. Charged hydrogen ions flow through the wire to produce an output voltage in logarithmic proportion to the hydrogen ion concentration present in the membrane cap’s ammonium chloride solution.
The reference electrode sensor establishes a stable reference voltage output for the ISE ammonia analyzer’s electronics module. This reference electrode connects to a porous reference junction filled with an electrolyte solution (gel or liquid). The electrolyte solution contains a predetermined concentration of hydrogen ions that provides a stable reference voltage output to the electronics module.
The temperature sensor allows the electronics module to compensate for temperature changes in the sample solution. An RTD most often is used to measure temperature changes.
All of these ISE ammonia analyzer components—the membrane cap filled with NH4Cl solution, the pH sensor, the reference electrode sensor, the porous reference junction and the temperature (RTD) sensor—may be contained inside a single ammonia ISE probe. The ISE ammonia analyzer consists of the single ammonia ISE probe and an electronics module. The ammonia analyzer electronics module uses sensitive input electronics and a microprocessor to analyze all of the input signals from the sensors and calculate the free ammonia concentration. The ISE ammonia analyzer electronics module usually is remotely mounted and can be connected to a control and automation system. ISE ammonia analyzer components vary by manufacturer. Figure 3 illustrates a generic ISE ammonia analyzer and its basic components.
Ultraviolet Absorbance Spectrophotometer Ammonia Analyzer
Ultraviolet light absorbance spectrophotometers (UV light wavelength intensity meter) use an ultraviolet light source to measure the
absorbance and/or transmittance of UV light waves passing through a sample. The UV light absorbance ammonia analyzer is calibrated to measure the wavelength of UV light (within the range of 200–450 nm). The analyzer has a UV light source that is located on one side of the sample cell and the UV spectrophotometer is located on the opposite side (sometimes this sample cell is adjusted in length depending on the ability of the sample solution to absorb UV light). Some UV ammonia analyzers use multiple paths of UV light to adjust for turbidity or other interference.
In order to measure ammonia concentration, a sample is collected and a reagent is added to the sample that acts as a buffer by adjusting the pH of the sample to a value greater than 12. To measure ammonia in the sample, hypochlorite is added. The hypochlorite reacts with free ammonia in the sample to form monochloramine. As the UV light strikes the sample, some of the UV light is absorbed by the monochloramine concentration of the sample and the remaining UV light passes through. The UV spectrophotometer measures the UV light that passes through the sample. The ammonia analyzer measures the difference in the transmitted UV light versus the amount of UV light generated by the spectrophotometer. This difference in UV light is proportional to the amount of free ammonia in the sample. Figure 4 illustrates a generic ultraviolet absorbance spectrophotometer ammonia analyzer and its basic components.
Ammonia Analyzer Electronics Module
The electronics module portion of any ammonia analyzer processes signals from the analyzer’s sensors. The output signal from the electronics module then can be used to monitor or control the ammonia concentration in the process. The output signal may be available in a combination of analog (4–20 mA-dc) or digital (RS-232) formats. In addition, the electronics module also may control the analyzer’s sample collection (pumping and valves), sample conditioning (filtering and chemical reagents), self-cleaning and self-calibration systems.
Treatment Processes Affecting Analyzers
Samples that have the following interference may affect the ability of the ammonia analyzer to measure accurately. Table 1 lists these interferences.
Some analyzers have self-cleaning systems and may use one or all of the following items.
• Mechanical cleaners (circulating grit or brush)
• Pressurized air/fluid flow
• Backflushing with chemical solutions
Sample Conditioning Systems
In order to eliminate these interferences it may be necessary to condition the sample before analysis. Filtering and/or additional chemicals may be required to ensure accurate analysis of ammonia. Manufacturers provide either a specific sample conditioning system for their respective analyzer or a generic sample conditioning system. Sample conditioning systems include pumps to collect the sample, filters to remove objects that can interfere with the sample reading and reagents to prepare the sample for the sensor to detect a specific constituent.
Table 2 outlines typical applications for ammonia analyzers in water, wastewater and industrial treatment processes.
Selecting an accurate and reliable ammonia analyzer is a challenge. It is essential to take into consideration the different types of ammonia analyzer technologies, installation, maintenance, cost and the most important parameters to your specific needs to select the most suitable analyzer for your application.
Testing Ammonia Analyzers
The Instrumentation Testing Association tested eight online ammonia analyzers at the City of Houston Beltway Wastewater Treatment Plant over a three-month period. The ITA test was conducted in a wastewater treatment environment of an activated sludge aeration basin. Testing ammonia analyzers in wastewater involves more variables and provides a harsher environment than testing in water, thereby best representing a worst case scenario. Although these analyzers represent three different technologies (ion-selective electrode [ISE], colorimetric and UV absorbance), each instrument system is unique in its sampling, operation, maintenance and electronic analysis. Table 3 lists analyzer manufacturers and technologies evaluated at the City of Houston.
For a list of references, go to our website at www.waterinfocenter.com.
Copyright © Instrumentation Testing Association, 2001, reprinted with permission. Information contained in this article is excerpted from ITA’s Online Ammonia Analyzers for Water and Wastewater Treatment Applications A Performance Evaluation Report.