Wastewater Treatment
The reason for me doing this report is because I could not attend class
enough
to grasp the concept of Wastewater Treatment. This report is an
overview of each
stage of the treatment of sewage. I have included a diagram
of a typical sewage
plant. A) Primary Treatment The wastewater that enters a
treatment plant
contains debris that might clog or damage the pumps and
machinery. The material
is removed by screens, and is burned or buried. The
wastewater then passes
through a comminutor (grinder), where all the organic
material such as leaves
are mushed smaller so that they can be removed later.
1) Grit Chamber Back in
the day, long narrow channel-shaped settling tanks,
known as grit chambers, were
used to remove all the inorganic substances like
sand, silt, gravel, and
cinders. These chambers were made to allow inorganic
particles 0.008 in. or
bigger to settle at the bottom while the smaller
particles and most of the
organic material that remain in suspension pass
through. Today, spiral-flow
aerated grit chambers with hopper bottoms, or
clarifiers with automatic scrapper
arms are used. The grit is removed and
disposed of as sanitary landfill. Grit
build up can reach from 3 to 8 cubic
feet per1 million gallons of wastewater. 2)
Sedimentation With the grit
removed, the wastewater goes into a sedimentation
tank, where the organic
materials removed. The method of sedimentation can
remove about 20 to 40
percent of the biochemical oxygen demand and 40 to 60
percent of the
suspended solids. The big boys in the industry use a chemical
process known
as coagulation and flocculation in the sedimentation tank. I
really don’t
know much about this subject so I’m going to move on. 3)
Flotation The
alternative to sedimentation is a treatment called flotation, in
which air is
forced into the wastewater under pressures of 25 to 50 lbs per sq.
in. The
wastewater, is compressed with air, is then released into an open tank
;
there the rising air bubbles cause the suspended solids to rise to the
surface,
where the are wisked away. Flotation can remove more than 75 percent
of the
suspended solids. 4) Digestion Digestion is a microbiological process
that
changes the chemically complex sludge to methane, carbon dioxide, and a
harmless
fertilizer. The reactions occur in a closed tank or digestor that is
oxygen
deficient. The transformation happens after a series of reactions.
First the
solid matter is made soluble by enzymes, then the substance is
fermented by a
group of acid-producing bacteria, reducing it to simple
organic acids such as
acetic acid. The organic acids are then resolved to
methane and carbon dioxide
by bacteria. The sludge that is to thick is heated
and added to the digester as
many times as possible, where it sits for 10 to
30 days and is decomposed.
Digestion reduces organic matter by 45 to 60
percent. 5) Drying The digested
sludge is place on sand beds for air drying.
Air drying needs dry, warm weather
for it to work. Some plants have shelters
over the sand beds. Dried sludge in
most cases is used as a fertilizer
because of the 2 percent nitrogen and 1
percent phosphorus content. B)
Secondary Treatment After removing 40 to 60
percent of the suspended solids
and 20 to 40 percent of the BOD5 in the primary
stage by physical resources,
the secondary treatment biologically reduces the
organic material that stayed
in the liquid stream. Secondary treatment contains
keeping and speeding up
nature’s process of waste disposal. Aerobic bacteria
in the oxygen change the
organic matter to stable forms such as CO2 , water,
nitrates, and phosphates.
The new organic material that is made is an indirect
result of biological
treatment processes, and is removed before the wastewater
is dumped into the
streams. 1) Trickling Filter In this process, a waste stream
is sent over a
bed or column of some type of porous medium. A sticky film of
microorganisms
coats the medium and acts as the removal agent. The organic
matter in the
waste stream is absorbed by the film and changed to carbon dioxide
and water.
If the trickling filter step comes before the sedimentation stage it
can
remove about 85 percent of the BOD entering the plant. 2) Activated
Sludge
This stage is an aerobic process that adds sticky sludge particles
that have
millions of of actively growing bacteria stuck together by a
gelatinous slime.
Organic matter is assimilated by the floc and changed
to aerobic output. The
reduction of BOD varies between 60 to 85 percent. 3)
Stabilization Pond or
Lagoon Another way of biological treatment is the
the stabilization pond or
lagoon. Facultative lagoons are the most common,
being 2 to 5 ft deep, with a
surface area of several acres. Anaerobic
conditions succeed in the bottom area,
where the solids are decomposed. The
area near the surface is aerobic, allowing
the oxidation of dissolved and
homogenous mixture of organic matter. A decrease
in BOD of 75 to 85 percent
can be accomplished. There are many other ways and
stages of wastewater
treatment but these are the basic processes. I learned alot
about wastewater
treatment and the it is a good thing we have it so the lakes
and streams are
no as dirty.
Bibliography
1) Waterlink Industries. 2000.
"Wastewater Treatment." Science (Refreshed
Daily): 6pp.Online. Internet.
May 01, 2000. Available www.waterlink.com 2)
Microsoft Encarta
Encyclopedia 99 (1999). [Cd Rom computer program]. Redmond,
WA: Microsoft
Corporation.