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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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
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
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.
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