What Is
Nitrification?
Nitrification is a microbial process that converts ammonia and
similar nitrogen compounds into nitrite (NO2–) and then
nitrate (NO3–). Nitrification can occur in water
systems that use chloramine for their residual disinfectant.
The problem is greatest when temperatures are warm and water
usage is low. For example, a number of water systems in Texas saw
their first episodes of nitrification during the rainy summer of
2007.
How Can It Be
Prevented?
Simply stated, the key points to preventing nitrification
are:
Also, monitor for nitrification
regularly. The longer you wait to act, the harder it can be to
bring nitrification under control.
Optimize Your
Chloramination Process
Before you calculate how much ammonia to add in chloramination,
be sure you know whether your treated water already contains some
FAA (free available ammonia). If it does, take this concentration
FAA into account when you consider how much ammonia to add to
balance the chlorine.
Remember: One key to preventing nitrification
is to ensure that the biofilm has no ammonia to work with. So keep
FAA levels low in your post-chloramination water:
- In many areas, this means you should make sure FAA leaving the
plant is less than 0.1 mg/L—preferably, less than 0.05
mg/L.
- In some areas, particularly in East Texas, you can allow FAA
levels of up to 0.2 mg/L due to competition in the water for the
ammonium ion.
- Systems with pH levels in higher ranges (> 9.0) will be
less susceptible to nitrification and might be able to safely
operate with higher FAA levels.
- These are guidelines. No two water systems are alike. Know
your water and make adjustments accordingly. If you need help,
consider our direct assistance to water
systems.
Reduce Water Age
Disinfectant levels fall when water stands still in your system.
If usage falls, flush your mains to keep new water moving into your
system.
When usage is low for extended periods, consider whether to
reduce the holding volume of water in your reservoirs to ensure low
residence times.
Do Preventive
Maintenance
Be sure to include measures to reduce biofilms and tuberculation
in your regularly scheduled maintenance:
- Some systems have found that a hard flush once a year helps to
keep nitrification in check.
- Especially if your system has cast iron pipes, mechanical
pigging can remove deposits and tuberculation where nitrifying
bacteria can set up shop.
- When usage is low, consider rotating reservoirs out of service
to clean their interiors vigorously.
- Some systems find it necessary to periodically switch to
chlorine as disinfectant as a part of their preventive maintenance
routine. For more about switching to chlorine see “How can I stop nitrification once it has
begun?”
Replace Aging
Infrastructure
Corroding pipes and equipment provide plenty of crevices for
nitrifying bacteria to escape your residual disinfectant. If you
find your system having to do excessive maintenance to keep your
infrastructure clean, consider replacing the problematic components
with newer, less corrodible equipment.
How Can I Detect
Nitrification?
Nitrification will usually show up first in areas where
residence time (or “water age”) is highest—for
example, dead-end mains, storage tanks, and areas where pressure
planes overlap. Watch these areas especially carefully.
To be sure nitrification is not happening, watch these
indicators well enough to know your system's baseline for each:
- Residual disinfectant. Low levels of residual
disinfectant can allow bacteria in the biofilm to multiply. When
residual disinfectant drops below your normal baseline,
nitrification may soon follow. So monitoring and mapping levels of
residual disinfectant proves to be a quick and inexpensive tool you
can use to pinpoint affected areas and focus your mitigation
efforts.
- Nitrite and nitrate levels in distribution.
Not all systems routinely monitor for these contaminants in
distribution. But one variety of nitrifying bacteria oxidize
ammonia to produce nitrite, and another will oxidize nitrite to
produce nitrate. By monitoring nitrite and nitrate levels, you can
know when action is needed.
- FAA levels. FAA is food to nitrifying
bacteria. So if FAA levels are low in at least part of your
distribution system, nitrification could be the cause. Look more
closely at the data from any location that shows FAA levels below
those of either the water leaving your treatment plant or other
locations in your distribution system.
Frequently, but not always, systems that have nitrification
occurring also have coliform-positive test results, increases in
heterotrophic plate counts (HPC), or both.
If you see symptoms of nitrification, first check to be sure
they aren’t masking other problems. For example, check the
nitrogen balance in your system. With all values in mg/L, the
nitrogen balance of your system is:
FAA (as N) + NO2– (as N) + NO3– (as N) +
(0.27 × NH2Cl) = Nitrogen
balance
This number will fluctuate somewhat under normal
operating conditions. If you find that certain areas in your
distribution system have high nitrate + nitrite levels and
an increased nitrogen balance, then you should check for and rule
out cross connections, line breaks, and other sources of
contamination before you determine that nitrification is the
culprit.
Finally, if you are sure an event has begun, act quickly to bring nitrification under control.
How Can I Stop
Nitrification Once It Has Begun?
The key to stopping nitrification is to starve the nitrifying
bacteria of nitrogen. The most effective way to do this is to
switch disinfectant from chloramine to free chlorine. When you take
this step, keep these key points in mind:
- 30 days before you switch, you must notify
our Public Drinking Water Staff by letter or e-mail of this
planned change in treatment method. Include this information in
your notice to us:
- PWS ID and name
- PWS contact name, title, and phone
- Estimated start and end date
- PWS ID and names of customer systems
- Reason for change in treatment (routine preventive maintenance;
corrective maintenance due to nitrification)
- Coordinate this treatment change with appropriate adjustments
in your disinfection byproduct (DBP) sampling. In other words, your
DBP sampling should accurately represent standard operating
procedure.
- As a courtesy, notify your customers before the change occurs.
Be sure to mention these points:
- A temporary change has been made to the treatment process to
improve the quality of water being served to our customers.
- Some taste and odor changes may briefly occur, but there are no
associated health risks.
- The name and phone number of the person customers can contact
at your water system if they have any questions.
- It is fairly common for customers to notice (and complain
about) changes in odor whenever you change disinfectants. Some
customers will notice the different taste of free chlorine; others
may notice when you switch back and the chloramine wavefront hits
the chlorinated water in the distribution system. To minimize
noticeable changes in odor, increase flushing of your distribution
lines following each change. To determine where odors are most
likely to be noticeable, monitor your distribution for both free
and total chlorine until levels stabilize.
Where Can I Get More Help?
Our Financial, Managerial, and Technical (FMT) Assistance
Program helps public water systems stay informed of the best ways
to deal with today’s problems. There is no cost to water
systems for the training itself, although each system is
responsible for having the appropriate materials and equipment on
hand.
A new round of this directed assistance will be available this
September. To participate, contact our
Utilities and Districts staff and ask about the FMT Assistance
Program.
Where Can I Learn More?
For advice about process controls, changing disinfectants, and
other operational matters, contact our Public
Drinking Water staff. These helpful resources are available
online:
