Return to Past Operator Notebook ListNITRIFICATION in POTABLE WATER TREATMENT
and DISTRIBUTION
Introduction:
As we learned in the Basics Section, nitrification can occur in almost any water. In potable water systems that have naturally-occurring ammonia in the raw water source, or a treated water to which we have added ammonia as part of the chloramine disinfection treatment process, nitrification is very possible.
"An estimated two-thirds of medium and large systems in the U.S. that chloraminate experience nitrification to some degree. One-third of these systems have nitrification occurring to the degree that it causes operational problems. The other one-third likely have nitrification occurring, but to a lesser degree that is not a major problem." (Reference July 1995 AWWA Research Foundation Order Number: 90669)
OUR MOTIVATION: Nitrification may result in significant microbiological growths, complete loss of disinfectant residual, and Total Coliform Rule failure.
General
My research of the literature has not revealed any nitrification
impacts in the water treatment plants themselves. Due to the short
detention times, etc., one would not expect to have occurrences
in the treatment facility itself. It may be possible in some cases,
such as: if the clearwell also functions as a large reservoir,
with a high water age during low flow events. As such most all
occurrences of nitrification occur in the water distribution system.
Nitrification in
the distribution system can have the following impacts:
1) A decrease in ammonia concentrations. The ammonia is converted
into nitrites, then into nitrates.
2) A subsequent increase in organic nitrogen concentrations will
occur.
3) An increase in nitrite and nitrate concentrations.
4) A further reduction in the chloramine residual will occur as
the remaining chloramine oxidizes the nitrite that is produced
during nitrification.
5) A decrease in pH may occur, if the alkalinity value is unable
to buffer the hydrogen ion release
6) As we stated in the Basics Section, nitrification requires
oxygen, so a reduction in the dissolved oxygen concentration will
take place, quite possible depleting the water of almost all of
the DO.
7) Most systems have reported a significant increase in heterotrophic
bacterial populations in the distribution piping areas where nitrification
is taking place.
NITRIFICATION FACTORS
The AWWA and others have set two categories of factors that cause
nitrification:
1) water quality and
2) distribution system conditions.
Water Quality Factors
The water quality factors that influence nitrification are a list
of those items required for optimal nitrification in the Basics
Section. They include the following:
Concentration of ammonia &
Chlorine to ammonia - nitrogen ratio
When practicing chloramination, we find that excess ammonia concentrations
allow nitrification to occur easily. In order to reduce nitrification
potentials adjust the chlorine to ammonia ratio to 5:1
pH
As we reviewed before, the optimal pH range for nitrifying bacteria
is about 7.0 to 8.5. Nitrification has been reported with pH values
ranging from 6.5 to 10.0
Temperature
Nitrification usually occurs when water temperatures are above
15°C. There have been reports of nitrification when the distribution
water temperatures were below 10°C.
Free chlorine residual
Established free chlorine residual (85% HOCl )
It appears that there are far fewer cases of nitrification occurring
in water systems that have low ammonia values, and an established
free chlorine residual (85% HOCl ) is reported.
Chloramine residual
Many studies suggest that by utilizing higher chloramine residual
levels we may be able to prevent nitrification from occurring.
According to one source (Geo. Clifford White), once nitrification
occurs, chloramine residual is quickly degraded, and very hard
to re-establish without first clearing the system of nitrification.
Concentration of organic compounds
Organic compounds can react with chloramine to form organic chloramines
resulting in poor residual disinfection.
Water Distribution
System Factors
This category includes those items found in the distribution system:
Water age (water detention time)
It takes time for the nitrifiers to reproduce and establish themselves
in an environment. Dead end mains, water stand pipes, low water
use in large mains, etc., all may contribute toward an adeqaute
detention time for nitrification.
Reservoir design and operation
Long detention times (high water age), incomplete reservoir turnover
allow for the nitrifiers to reproduce and establish themselves.
Sediment and tuberculation in piping
&
biofilm formation on pipe walls
The accumulation of sediments and the formation of biofilms on
the interior pipe walls, can provide protection for nitrifying
bacteria.
Monitoring Plan
The best defense is the establishment of an effective monitoring
plan.
First, it is recommended that a survey of the existing distribution
system be undertaken. This involves the identification of potential
or existing nitrification areas such as:
a) low water use areas
b) low chloramine or chlorine residuals points in the distribution
system
c) the concentration of free ammonia in remote areas of the distribution
system
d) and other such areas
Then, a monitoring and testing plan would be established to detect potential or existing conditions for corrective action:
Sample sites representative of nitrification
determination would be selected:
Raw source water
Finished water
Reservoirs
Dead-end mains and standpipes
Coliform monitoring stations
Analysis of these samples would include
some of the following:
Dissolved Oxygen
Temperature
pH
Alkalinity
Heterotrophic plate counts
Nitrogen series: free and total ammonia
nitrite
nitrate
Chlorine dose and residuals or
Chloramine residual
Total Organic Carbon (TOC)
Sampling frequency
AWWA and others recommend that it should be at least monthly,
and possibly weekly, during seasons when nitrification potential
is high.
Control Measures
Control measures basically involve corrective actions or removal
of nitrification factors from the system.
For the Water Quality
Category, this would include one or more of the following items:
a) The establishment of a free (HOCl 85%) or breakpoint chlorination
of specific problem areas or on a system-wide basis
b) Increasing the chloramine residual leaving the treatment plant
or at re-chlorination stations
c) Increasing the chlorine to ammonia-nitrogen ratio to minimize
the concentration of free ammonia leaving the plant
d) Removing organic compounds at the treatment plant
For the Water Distribution
System Category, this would include one or more of the following
items:
a) Implementing distribution cleaning measures such as flushing
and pigging to remove sediment and tubercules
b) Identify and eliminate dead-end mains
c) Reducing water age (detention time) in reservoirs and throughout
the distribution system
Breakpoint chlorination is probably the most effective control measure for nitrification once an episode is underway. While reducing the available amount of free ammonia, increasing chloramine residuals and reducing organic chloramines show promise for long-term system improvement.
GLOSSARY
autotrophic: usually applies to a microorganism which utilizes inorganic substances (materials) for growth and energy, such as algae and nitrifiers.
denitrification: reduction of nitrate (and/or nitrite) into nitrogen gas or ammonia; Nitrogen removal by anaerobic microbes that strip away the chemically bound oxygen from the nitrite and nitrate, which liberates the nitrogen as nitrogen gas into the atmosphere, (or ammonia).
methemoglobinemia: a disease in infants that is caused by high levels of nitrate in the water.
nitrification: the process of converting ammonia nitrogen into nitrite nitrogen then into nitrate nitrogen, causing the liberation of hydrogen ions H+ (acidity)
nitrosomonas bacteria: an autotrophic bacteria group that uses only ammonia for energy (breaks the nitrogen-hydrogen bond for the energy).
nitrobacter bacteria: an autotrophic bacteria group that uses only nitrite for energy (breaks the nitrogen-hydrogen bond for the energy).
oxidation: a chemical reaction characterized by oxygen uniting or combining with other elements; to remove hydrogen, as by the addition of oxygen; the action of increasing valence of an element.
reduction: chemical reaction characterized by the loss
of oxygen; the addition of hydrogen to a compound; also the lowering
of the chemical valence of a positive element in a compound.
REFERENCES
WPCF, MOP 11, Vol II, WPCF, 1990
George Tchobanoglous & Franklin Burton, Wastewater Engineering, 3rd Edition, Metcalf and Eddy, Inc
Texas Water Utilities, Manual of Wastewater Treatment 6th Edition, Texas Water Utilities, 1991
Kenneth A. MacKichen & Mark J. Hammer, Hydrology & Quality of Water Resources, Whiley, 1981
Walter Weber, Physicochemical Processes For Water Quality Control, Whiley, 1972
Mark J. Hammer & Warren Viessman, Jr., Water Supply and Pollution Control, 5th Edition , HarperCollins, 1993
July 1995 AWWA Research Foundation Order Number: 90669
Nitrification Occurrence and Control in Chloraminated Water
Systems
Prepared by Gregory J. Kirmeyer and Lee H. Odell, Economic and
Engineering Services, Inc.; Joe Jacangelo and Andrzej Wilczak,
Montgomery Watson, Inc.; and Roy Wolfe, Metropolitan Water District
of Southern California
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