By: Tony Palmer, Maureen Ross, P.E. and Stephen G. Nutt, P.E.
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+
Ammonium-Nitrogen Æ
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:
• Colorimetric,
• Ion-selective
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+
Ammonium-Nitrogen Æ
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.
Applications
Table 2 outlines typical applications for ammonia analyzers
in water, wastewater and industrial treatment processes.
Analyzer Selection
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.
About The Author: Tony Palmer is the executive director for the Instrumentation Testing Association, Henderson, Nev. For more information about ITA and its testing contact, 702-568-1445. Maureen Ross, P.E., is involved with technical programs for the Instrumentation Testi
