Filtering Water To Prevent Pollution
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Ander, Perry Robeson High
Students will see the techniques that are used to filter our water.
Students will gain an idea as to various pollutants which can
contaminate our water and an appreciation of the need to keep our
water supplies clean. If supplies are available students can
filter water at their desks.
1. A large "filtering tube" (This is a large test tube which is open
at the top and narrows down into an opening at the bottom. If
not obtainable a long necked glass funnel is sufficient.)
2. A large glass fish tank or jar
3. A bag of charcoal pebbles, sand, and gravel obtainable at a retail
tropical fish store.
4. Micropore filter paper
5. Petri dishes
6. Chlorine (Obtainable at a retail outlet store that sells swimming
7. A strainer (found in the kitchen) or screening
For this lesson students should be issued a "lesson outline". This
outline, numbered Roman numeral I-VI would enable the students to answer
questions as they are presented during the lesson. For example numeral
I would require students to define pollution; numeral II would ask
students to list three ways in which water is polluted; III would
require the students to list four ways in which water is purified; IV
would ask students to write what part of the water purification was
represented by pouring water through a household strainer; V would ask
students to write what occurred when the filtered water from the
strainer was allowed to settle in a test tube and what part of the
filtration process was represented here; VI might ask students to write
what occurred when water was filtered further from the test tube through
a filtration funnel lined with layers of charcoal, sand and gravel. Of
course the answers to these questions would become clear as the lesson
was presented. Also the outline can be varied to the teacher's tastes.
The teacher should begin the lesson by asking the student to define
pollution. As suitable answers are generated Roman numeral I can be
filled in by class. Next teacher could generate from the class what
they believe pollutes our water. As answers are generated Roman numeral
II can be filled in. This point should be emphasized by "polluting" a
fish tank or vessel which has been filled with clean water. Pollutants
can be represented by such household products as car motor oil, food
dyes, scraps of paper, soil, leaves, clay balls, or coffee grounds.
These should be added as students express their ideas as to what
pollutes their water. A dramatic effect should be achieved as students
see their water "polluted" before their eyes.
Now the teacher should try to elicit an answer to the question:
"List four ways by which polluted water is made pure." At this point
students could be asked to assemble a "puzzle" which the teacher has
passed out to each student. Similar to the Tic-Tac-Toe quiz show on
morning T.V. this puzzle, when assembled, spells out the techniques of
water filtration; namely screening of polluted water, sedimentation of
polluted water, filtration of the water, and chemical treatment of the
water. This can be done by drawing a jigsaw puzzle of perhaps 5 pieces
on a single piece of unlined paper and simply writing the steps of the
puzzle on the drawn pieces. The puzzle can easily be reproduced, cut
out, and handed to pupils. The teacher can now elicit a response when
the question "What is the first step of water purification" is asked.
By reading from their assembled puzzle students can readily answer:
"Screening." At this point the teacher can readily filter some of the
polluted tank water through a household strainer. Perhaps students
can do this at their desks with strainers brought from home. Their
outline should be filled out too, i.e., what did straining the polluted
water remove from the water? (large particles). Next the teacher can
ask for the second step of the purification process. Once again by
referring to their puzzle students can readily reply that it is
"sedimentation." Now, the water that has been screened can be spilled
into a beaker of perhaps 250ml. or a test tube, and allowed to settle.
More impurities should settle out. Simultaneously students should
further fill out their outline describing what they see.
Again, by reading the puzzle, students can reply that
"filtration" is the third step of water purification. At this point
the teacher should refer to the filtering funnel which has been
layered from top to bottom in the order: one layer of pebbles, one
layer of sand, one layer of charcoal. A long stemmed funnel lined with
filter paper should be placed below the layered filtering funnel to
further filter the water. Of course students are writing what they
see on the outline.
Original polluted tank water should be quite a bit cleaner by now.
But the demonstration can be further continued handing students a petri
dish and three small slices of boiled potato to use as a growth media
within the petri dish. Onto one slice a drop of the polluted water
from the tank should be placed; onto a second slice a drop of the
filtered water from the filtering funnel should be placed; onto the
third potato slice a drop of the filtered water and a drop of chlorine
mixed with this filtered water should be placed. Cover the dish and
allow for bacterial growth for a few days. Students can now complete
their class outline sheet by noting on which portion of the potato
did bacteria grow. That is, did the filtered water show any
difference in bacterial growth than the polluted water? Did the
filtered water plus the chlorine show differences in bacterial growth
than the other two drops? The completed outline could now be handed
to the teacher as the pupil's progress.
It can be noted that a long stem clear glass funnel can be
layered with the materials mentioned and used as a "filtering funnel."