Wastewater Treatment Essay, Research Paper

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.

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.