Denitrification and Nitrification
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Sewage is made
up of carbonaceous waste and a small amount of nitrogen
waste. Nitrogen is a found in proteins and in ammonia, an
important compound in sewage. In domestic sewage the
ammonia could make up about a half of the total BOD.
Urine is the major source of ammonia in domestic sewage.
Ammonia needs a lot of oxygen and certain bacteria to
decompose it to nitrate. This process is called
nitrification. There are a large number of bacteria that
carry out carbonaceous oxidation but nitrifying bacteria
require special attention. This is because the numbers of
nitrifying bacteria are low, reproduction is slow, oxygen
requirements are high and reaction with ammonia is slow
and reduced further in cold weather. Nitrate is a
compound containing nitrogen and oxygen. Complete
nitrification occurs when all ammonia has been converted
to nitrate. The nitrate is then broken down via a process
called denitrification. Denitrifcation occurs under
anoxic conditions, when there is no dissolved oxygen in
the sewage, and the bacteria utilise the oxygen in the
nitrate and release nitrogen gas, as bubbles. If these
bubbles attach themselves to sludge it may be lifted and
float to the surface. Denitrification can be a nuisance
if it occurs in the wrong place. Anoxic zones or tanks
are sometimes created to ensure denitrification occurs
where it can be properly controlled. Anoxic conditions
can occur in final settling tanks. This can cause rising
sludge when the liberated nitrogen gas buoys up the
sludge and lifts it to the
surface resulting in high SS in the effluent. Nitrate
compounds behave as nutrients when discharged to rivers
and can encourage the growth of algae. The EA sometimes
requires nitrate to be removed from sewage effluent and
the consent may place limits on nitrate compounds. When
this is the case, nitrification and denitrification will
form part of the treatment process.Fig1.
Nitirification and Denitrification.
Reproduced
with kind permission from Anglian Water Services Ltd
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