Filtering Water To Prevent PollutionReturn to Biology Index
Ander, Perry Robeson High
723-1700 ext.113
Objectives: 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. Apparatus needed: 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 pool supplies) 7. A strainer (found in the kitchen) or screening Recommended Strategies: 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."