Wastewater is usually very high in ammonia, a component of
urea, which occurs naturally in urine. Ammonia is generally removed from the wastewater
in the tertiary, or third, step of the sewage treatment process. (The primary stage
removes the sludge, or solid matter, and the secondary stage uses mechanical and
biological processes to break that sludge down.)
Tertiary
treatment for nitrogen usually occurs in two steps - first the ammonia is nitrified, or
converted into nitrates. This is an aerobic process which uses Nitrosomonas bacteria to
convert ammonia, NH3, into nitrite, NO2, and then that into nitrate, NO3. Then
denitrification is done under anaerobic conditions where other bacteria (there are many
species used) convert the nitrates into atmospheric nitrogen gas,
N2.
There are many case studies available, because no two
treatment plants do it exactly the same way. What I've given you is an overview of the
process. An important thing to keep in mind when thinking about wastewater treatment is
that most of the work is done by living organisms; a treatment plant is really a model
ecosystem. The first link below provides more technical information about the process,
and the other two links will lead you to actual case studies.
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