Carbon, Nitrogen and Phosphorous Removal:
1. When examining carbon, nitrogen and phosphorous removal, there are
a number of different competing and separate metabolisms which may
interact.
2. For carbon metabolism, there are a number of options:
a. Anaerobic:
Conversion to intermediate volatile fatty acids (e.g. acetate,
propionate, butyrate etc)
Complete conversion to methane and carbon dioxide via the
volatile fatty acids route
(A range of compounds ranging from sugars to proteins to fats
can be broken down in this way)
b. Aerobic:
Carbon to carbon dioxide and water using oxygen as the
electron acceptor (aerobic)
(Both sugars and other carbon containing compounds such as
ethanol, acetate, lactate etc can be broken done in this way)
Carbon to carbon dioxide and water with a compound other than
oxygen being used as an electron acceptor
(Both sugars and other carbon containing compounds such as
ethanol, acetate, lactate etc can be broken done in this way)
3. For nitrogen metabolism, there are also a number of options:
a. Aerobic(Nitrification):
Ammonia may be converted to nitrite and to nitrate using oxygen as
the electron acceptor and carbon dioxide as the carbon source. In
this process, carbon dioxide is the carbon source and ammonia is
the energy source.
b. Anaerobic(Denitrification):
Nitrite and Nitrate may be converted to nitrogen gas. The carbon
and energy source for this process may be sugars, other organic
carbon such as ethanol, acetate and ethanol). Methanol is also
used.
In addition to these well established processes, there are at least
two other processes, the Anammox Process and the Sharon
Process
Anammox use CO2 as the carbon source and operates
anaerobically. The energy required for cell formation from Carbon
dioxide is obtained as a net NAD(H) production in the two linked
reactions (ammonia and nitrite forming nitrogen)
http://www.anammox.com/research.html
The Sharon process undertakes partial nitrification to nitrite follwed
by denitrification of nitrite. This reduces the oxygen required for the
process by eliminating the nitrite to nitrate conversion.
http://biomath.ugent.be/projects/infopage.php?SHARON
4. For phosphorous metabolism, the are also a number of options:
a. Anaerobic:
Acetate is used as a carbon source to from PHB
(polyhydroxybutyrate). The energy for this process comes from the
formation of ATP by the breakdown of polyphosphate. The
breakdown of polyphosphate leads to phosphate excretion by the
cell.
b. Aerobic:
PHB is broken down to acetate which is subsequently metabolized
to form carbon dioxide and water and this forms ATP. Hence PHB
is the carbon and energy source. In addition to growth, ATP is used
to reform Polyphosphate which in turn results in phosphate uptake
by the cell. Oxygen is used as the electron acceptor.
c. Anoxic:
The same process that occurs aerobically can be undertaken
anoxically, usually using nitrate as the electron acceptor. This
process is referred to as denitrifying phosphorous uptake.
5. In addition to the complex and interacting pathways, there are other
considerations when using mixed cultures:
a. CSTR’s are unable to allow the co-existence of more than one
organism except under exceptional circumstances. Consequently,
high density reactors (or at the very least CSTR with recycle) are
used.
b. For any organism, their maximum growth rate and their KS value
and their biomass from substrate yield will determine how much of
the relevant carbon and energy sources that any particular
organism may be able to use. Nitrifiers, for example, are much
slower growing and have a much lower yield than both heterotrophs
and denitrifiers.
6. As a consequence of the issues previously discussed, the minimum
requirement for a system to treat carbon, nitrogen and phosphorous
simultaneously is an aerobic reactor, an anaerobic reactor and a cell
recycle. This type of consideration is referred to as the “reactor design
and configuration option”
7. The other options are to use an SBR system which operates with cycle
which include anaerobic and aerobic periods and well as having cell
recycle through biomass settling during the cycle.
8. There is also the “floc based approach” whereby there is an attempt to
have a floc with an aerobic outer layer and an anaerobic inner core,
both of which can be present in an aerobic reactor.
9. Considering the “reactor design and configuration’ option, we then can
look at simple reactor configurations and then increase their complexity
to improve their ability to undertake simultaneous carbon, nitrogen and
phosphorous treatment:
a. CSTR – can achieve carbon to carbon dioxide conversion under
aerobic conditions. Nitrification cannot occur unless the residence
time of the reactor is very long and even then the performance is
poor. Denitrification and Phosphorous removal are not possible.
b. CSTR with recycle – can achieve both carbon to carbon dioxide
conversion as well as nitrification. Denitrification and Phosphorous
removal are not possible.
c. Aerobic Reactor and an Anoxic reactor. There are two options, the
aerobic reactor followed by the anoxic reactor or the anoxic reactor
followed by the aerobic reactor. For the first option, the first reactor
undertakes carbon conversion to carbon dioxide and ammonia
conversion to nitrate. The anoxic reactor then undertakes
denitrification. This process in anaerobic and requires an organic
carbon source. However, there is the possibility that the first reactor
has consumed most of the carbon source, in which case further
carbon addition would be necessary to the second reactor. If we
place the anoxic reactor first, then a liquid recycle stream from the
second aerobic reactor is necessary to return nitrate to the first
reactor for denitrification. In this case, there is no need to add any
carbon to the anoxic reactor since it is receiving the feed stream
directly. This demonstrates that the are more than one option in
many cases and each of the there has advantages and
disadvantages which have to evaluated and analysed.
Denitrification would only be possible using the anoxic reactor and
phosphorous treatment would be possible as long and the PHB
formation is possible anoxically and from a carbon source other
than acetate. This is unlikely since the acetate is the most common
source for PHB.
d. Finally we can consider a reactor configuration which has three
reactors (anaerobic followed by an anoxic reactor followed by an
aerobic reactor) with liquid recycle from the aerobic reactor to the
anoxic reactor using a clarifier. The anaerobic reactor is operated at
a retention time to favour the formation of volatile fatty acids and, in
particular, acetate. The first reactor also can undertake
phosphorous release as long as there are PHB containing
organisms in the reactor. Phosphorous release can also occur in
the anoxic reactor as long as there are PHB containing organisms
in the reactor. The liquid recycle stream form the aerobic reactor
contains nitrate to allow denitrification to occur in the second anoxic
reactor. The final aerobic reactor utiises the remaining carbon
source that has not been used to form acetate and in the
denitrification process. The converts the carbon fully and also
allows the formation of nitrate from the ammonia in the feed which
passes through the anaerobic reactor unchanged and also the
anoxic reactor (unless there is a nitrifying bacteria which can use
alternative electron acceptors to oxygen).In the aerobic reactor,
phosphorous in uptaken and remains inside the cell. The clarifier
can then be used to purge some biomass from the system. This
biomass contains the phosphorous removed in the process.
10. Finally, there are many more complicated configurations and each of
these aims to improve some or all of the deficiencies of the systems
above. They can, however, be analysed in the same way.