wastewater treatment essay

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

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.

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