CTB3365x – Introduction to Water Treatment
W5a2 – Nitrogen removal
Merle de Kreuk
If you already watched the movie about the nitrogen cycle,
you understand that with introduction of the Haber-Bosch
process, the natural Nitrogen cycle has been disturbed. This
has caused nitrate accumulation in drinking water resources,
eutrophication and acidification. Therefore, nitrogen removal
has been introduced to sewage treatment processes.
Today you will learn several configurations for nitrogen
removal. Furthermore, you will be able to understand how the
sludge load is coupled to the nitrogen removal efficiency and
finally, a little bit more is explained about Anammox, the
shortcut of the Nitrogen cycle.
So, we will focus on the anoxic and aerobic tank of the sewage
treatment plant.
The first mechanism by which nitrogen is removed in
wastewater treatment is bacterial growth. Nitrogen is
incorporated in cell material, as it is part of proteins,
aminoacids, DNA, certain lipids etc. You might recognize that
nitrogen is part of the overall molecular formula of biomass
generally expressed by this equation: C5 H7 O2 N.
The bacteria can take this nitrogen needed for growth from
ammonium in the wastewater. Calculating with the mole mass
shows that 14 grams of nitrogen is needed for the growth of
113 grams of biomass. If you know this value, you can
calculate whether or not biomass growth alone is enough to
remove the nitrogen from your influent.
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Let’s consider the average BOD and ammonium concentration
in Dutch influent and a sludge growth Yield of 0.6 kg VSS/kg
BOD converted. It can be calculated that per liter influent 108
mg VSS is produced, containing about 13 mg N. Would this be
enough to get to the effluent demands of 10 mg total N/L,
coming from 40 mg/L in the influent?
Exactly, no it wouldn’t, and that is why other processes are
needed: namely nitrification followed by denitrification. We
will start with nitrification, the aerobic conversion from
ammonia to nitrate.
Nitrification is a two-step oxidation process performed by two
different species: the ammonium oxidation to nitrite is done
by Nitrosomonas species, while the oxidation of nitrite into
nitrateis performed by Nitrobacter species. These organisms
are Chemo-Litho-autotrophic organisms, so can you tell what
their carbon source is? And their electron donor? Indeed, CO2
is the Carbon source, ammonium or nitrite the electron donor
and since the organisms are fully aerobic, oxygen serves as
electron acceptor.
From the overall equation, you can see that for every
ammonium oxidized 2 oxygen are needed. This means that
each gram of ammonium-N converted, uses 4.57 g of oxygen.
Since the catabolism generates energy for the anabolism, part
of the ammonium is not oxidized but incorporated in cell mass.
This leads to the net value of 4.2 grams of oxygen needed per
gram ammonium‐N removed.
In this overview you see the differences between the
heterotrophic and autotrophic organisms used in sewage
plants. In particular, the different growth rates of the two
organisms and their oxygen demands determine their
competition in treatment plants. A high sludge loading rate
(Bx), means that the food to mass ratio of the system is high
and thus a lot of biomass can be produced. This will lead to a
low sludge retention time as more sludge must be wasted to
keep the concentration in the aeration basin constant. This is a
good situation for heterotrophic organisms, since with high
maximum growth rates and high affinity for oxygen,
heterotrophic organisms can survive at relatively short sludge
ages, high loading rates (Bx), and low oxygen concentrations.
On the other hand, the autotrophic nitrifiers are slow growing
organisms. This means that to achieve nitrification, the solid
retention time, or sludge age should be long enough to keep
them in the system. A general rule is that the sludge age needs
to be at least longer than 2.5 days to guarantee nitrification.
This is obtained at sludge loading rates below 0.15 kgBOD/kg
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biomass/day. Also the oxygen concentration needs to be
relatively high during nitrification as maximum nitrification
rates have been observed at DO concentration of 3 to 4 mg/l.
With nitrate formation, we are only half way through the
nitrogen removal process. In the past some treatment plants
were limited only on ammonia concentration in the effluent so
nitrification was the only goal. New limits focus on limiting the
total nitrogen in the plant effluent, thus the nitrate must also
be removed through denitrification where nitrate is converted
to di-nitrogen gas.
Denitrification is performed by heterotrophic organisms, which
are able to use nitrite or nitrate as electron acceptor in the
absence of oxygen. It is important to remember that these
heterotrophic organisms need an organic carbon source, which
can be the BOD from the sewage, but also other organic
compounds, such as methanol.
The equations show the conversion of these organics with
nitrate as electron acceptor. You should notice that this
reaction produces hydroxides, which is useful since it
compensates the proton production during nitrification,
limiting the pH change in the buffered sewage.
There are two main configurations in which nitrification and
denitrification can take place: pre- and post‐denitrification.
First I will show you the post-denitrification. In this
configuration the sewage is first fed to an aeration tank, in
which the aerobic biological processes occur. We know that if
the sludge retention time is long enough nitrification will occur
and ammonium will be oxidized to nitrate. Next, the sludge
water mixture will flow to an unaerated tank. The water
contains nitrate, so this is an anoxic tank and denitrification
could take place. Only, one ingredient for the denitrification is
missing. Since the BOD already has been (partly) oxidized in
the first step, denitrification will not happen anymore. An
additional C-source and electron donor is needed in this
reactor for the denitrifying bacteria. This could be solved by
diverting some influent to the anoxic tank. However, as the
influent also contains ammonium, bypassing the aerobic tank
will lead to ammonium being discharged with the effluent.
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An alternative option to avoid this is the dosage of an external
C-source in the anoxic tank, such as methanol, but this adds
operational complexity and is expensive. A solution to avoid
these drawbacks is to put the anoxic tank before the aeration
tank in a setup known as pre‐denitrification. In this
configuration the BOD containing sewage goes directly to the
anoxic tank to act as the C‐source and electron donor for the
denitrifying bacteria. However, as you might have noticed, the
difficulty with this setup is that the influent sewage contains
ammonium that needs to first be aerated for nitrification
before, it can be denitrified. Part of the nitrate from the
aeration tank will therefore be recycled to the anoxic tank with
the return sludge from the final clarifier. However, this flow is
typically far too small to significantly reduce the nitrate
concentration in the effluent. As a solution, we introduce an
additional recycle from the aeration tank to the anoxic tank.
Now it is possible to denitrify in the anoxic tank, since both
BOD from the influent and nitrate from the recycle are
present. In the aerobic tank, remaining BOD will be further
oxidized by oxygen and influent ammonium that passed the
anoxic tank will be nitrified to nitrate. Because not all the
water can be recycled, some nitrate will always make its way
to the effluent in a pre-denitrification setup which limits the
ultimate nitrogen removal capacity of this configuration.
Can you make a quick and dirty estimation of the ratio
influent/return flow if the influent contains 40 mg
ammonium/L and effluent nitrate demands are below 10
mg/L? Indeed, the recycle needs to be around 3 times the
influent flow. Of course, we are ignoring some parameters in
this estimation: Some nitrate is also returned with the return
sludge flow, so this can be subtracted from the internal nitrate
return flow. There will also be some nitrogen removal by
growth that is not incorporated in this balance. An internal
recycle ratio of 3 to 4 is typical for activated sludge systems.
Lower ratios can be used if influent total nitrogen
concentrations are low. Higher recycle ratios are generally
avoided, since the additional effect on the nitrate
concentration in the effluent is minimal and too much oxygen
from the aeration tank will be introduced in the anoxic tank.
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A special case of nitrification/denitrification can be found in
the ultra-low loaded Carrousel® system developed in the
Netherlands. This system is based on general oxidation ditch
technology and is designed as a race track, with quite high
water flows. Influent can pass a separate anoxic zone first, but
can also be introduced in the aerated zone of a circuit system.
By measuring the actual oxygen concentration at different
points in the reactor, the aeration can be precisely controlled.
In that way aerated and anoxic zones can be alternated,
leading to a good nitrogen removal by nitrification and
denitrification in the different zones of the track. The recycle
rate in such system is very high, since only a small part leaves
the reactor every lap and thus hydraulic residence times are
relatively long. Effluent total nitrogen concentrations well
below 10 mg/L are reached with these systems.
So, conventional N-removal needs lots of oxygen in the first
phase and an electron donor in its second phase. In the late
90’s, a shortcut in the nitrogen cycle was discovered:
Anammox (ANaerobic AMMonium OXidation) . The Anammox
bacteria can use ammonium and nitrite to produce di-nitrogen
gas directly. Main advantages of this process are that only half
of the ammonium has to be converted to nitrite. Therefore, it
only consumes 38% of the oxygen compared to full
nitrification.
This leads to large energy savings. Furthermore, no carbon
source is needed as in conventional denitrification, because
these organisms are autotrophs. Since they have their
optimum growth rate at elevated temperatures, the Anammox
process is very suitable for the nitrogen removal from rejection
water of sludge digestion. Anammox bacteria have a very low
growth rate, with a doubling time ranging from 9 days to two
weeks and therefore sludge retention in Anammox systems is
very important.
This can be done in a two-step or one step Anammox reactor.
In a two-step reactor the water is first partially nitrified in a
Sharon reactor into a mixture of nitrite and ammonium, after
which it is fed to the anoxic Anammox reactor.
One step Anammox reactors come in many configurations with
names, as Demon, Oland and Anammox. All make use of the
growth of Nitrosomonas species in combination with
Anammox. Preferably they both grow in a granule or biofilm,
in which the outer layer of oxygen consuming nitrosomonas
shield the Anammox that are inhibited by oxygen.
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The first application of Anammox was built in 2002 at a sludge
treatment plant in Rotterdam, The Netherlands. This plant was
already equipped with partial nitritation, to treat the rejection
water from the sludge digester. A one to one mixture of
ammonium and nitrite is fed to the reactor and converted by
the Anammox bacteria. With an influent concentration of 1 g
nitrogen/L, a conversion rate of 5 kg N per cubic meter reactor
per day is reached.
A new development that is being studied is the application of
Anammox in the main stream of a wastewater treatment plant
at colder temperatures,
but more about that in the Master-track Watermanagment,
which can be followed on campus as well as online.
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