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 donít know much about this subject so Iím 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 natureís 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