Waste Management Institute
Sewage Treatment Tour
- Adapted from Critical Issue:
Water. You Can Make a Difference!, Water Resources Education
Kit, Grades 4-6, by Cornell Cooperative Extension of Nassau County,
NY, and Water Wise: Lessons in Water Resources, Cornell Cooperative
Goes To School
Cornell Waste Management Institute
- GRADE LEVELS: 9-12
- SUBJECT AREAS:
How do we clean up our wastewater?
To understand how our wastewater is treated and to make a connection
between what is put down the drain and where it goes.
- - paper
- pen or pencil
- set up a tour of a wastewater treatment plant
leachate, permeability, aquifer
The following information gives teachers sufficient background
knowledge to inform students about modern wastewater treatment.
It is up to the discretion of the teacher to cover as much on
this topic as he/she wants. Class ability and time available
will be the deciding factors. The important concept is not the
water treatment process itself, but rather the fact that most
wastewater needs to get treated before it can be recycled or
reused and that this treatment results in a sludge which must
be disposed of.
- In rural areas, wastewater exits the
house and enters cesspools or septic tanks. These are tanks in
the ground where sewage is partially purified through decomposition
by bacteria. The water slowly percolates back to the groundwater
supply which can then be pumped to the surface as it is needed.
- In more densely developed areas, sewers
are used. These are underground pipes connected to a wastewater
treatment facility. Once in the treatment plant, wastewater goes
through a series of actions which will help to clean up the water.
A wastewater treatment plant's basic function is to quicken the
natural processes by which water purifies itself.
- Wastewater treatment is vital to the
purification of our water and the health of the population. At
present, two basic stages exist in the treatment of wastes: primary
- In the primary stage of treatment,
solids are allowed to settle and are removed from the water.
Here's how it works: as sewage enters a plant for treatment,
it flows through a screen. The screen removes large floating
objects such as rags and sticks that may clog pumps and small
pipes. After the sewage has been screened it passes into what
is called a grit chamber where sand, grit, cinders and small
stones are allowed to settle to the bottom.
- The unwanted grit or gravel from this
process is usually disposed of by filling land near a treatment
plant. With screening completed and the grit removed, the sewage
still contains dissolved organic matter along with suspended
solids. In a sedimentation tank the suspended solids will gradually
sink to the bottom, forming sludge. Then, the sludge is mechanically
removed from sedimentation tanks.
- The secondary stage of treatment removes
up to 90% of the organic matter by making use of the bacteria
in it. Two techniques are used in this stage: trickling filters
and the activated sludge process.
- A trickling filter is a bed of stones
from 3-6 feet deep through which sewage passes. Bacteria gather
and multiply on these stones until they can consume most of the
organic matter in the sewage. The cleaner water trickles out
through pipes in the bottom of the filter, then flows to another
sedimentation tank to remove the bacteria. To complete the process,
the water gets chlorinated for disinfection purposes.
- The other technique which is being
used more today is the activated sludge process. After the sewage
leaves the settling tank in the primary stage, it is pumped to
an aeration tank where it is mixed with air and sludge loaded
with bacteria. It is allowed to remain here for several hours.
During this time, the bacteria break down the organic matter.
The sludge can be reused by returning it to the aeration tank
and mixing it with new sewage and an ample amount of air.
- Meanwhile, the sewage flows from the
aeration tank to another sedimentation tank to remove the bacteria.
The final step, as with the first technique, is the addition
- In some cases, tertiary treatment also
is used. In this final step, chemical treatment is used to remove
specific compounds such as phosphates. This allows the water
to be in better condition before it is put back into the water
- As our water supply demands are increasing,
tertiary treatment allows us to use wastewater to recharge the
groundwater supply. However, most wastewater plants do not employ
this procedure due to cost and feasibility.
- 1. Take a tour of a wastewater treatment
plant, following the path of the water as it gets treated. Find
out how many gallons of wastewater are treated per day, and how
many people the plant serves.
- 2. Discuss questions such as the following:
- - What steps does the water go through
in wastewater treatment?
- What is the role of microorganisms?
- Where does the water go after leaving the wastewater treatment
plant? How often is it tested, and for what pollutants?
- How much sludge is produced? How is it treated or disposed
- Students may wish to study and contrast
a leach field with a sewage treatment plant.
- If sludge from the wastewater treatment
plant is composted or spread on land, the class might also want
to visit the site of these operations.
Back to top