Turn on the tap and out comes water. Unused water goes down the drain. In this activity, students gain an understanding of where the water comes from, how it gets into the tap, and where it goes when it flows down the drain.
Students will need to be able to access dictionaries or books that explain the New Words in order to gain a more complete understanding of a typical urban water system, or cycle, as we call it here.
Three steps occur in the drinking water treatment plant. These include chlorination, flocculation, and sedimentation/filtration. The chlorination step is necessary to kill any bacteria in the water, particularly those responsible for cholera and typhoid fever. In the next step, chemical coagulants, such as polyhydroxyaluminumchloride, are added to the water. These chemicals cause dirt particles to “stick together” as larger, heavier “floc particles” that fall to the bottom of settling tanks where they can be removed. The very fine particles that remain after this step are removed from the water by sand filtration. This step is very similar to Activity 4 Purifying Water included in the Grade 7 Unit on Pure Substances and Mixtures. The filters have light, black pebbles on top, with sand and gravel arranged in layers, with the largest particle sizes on top. As the water passes through, fine particles are removed. The filtered water is then treated with sulphur dioxide to remove all but a trace of chlorine. In some urban water systems, fluoride is added for the benefit of your teeth.
The sewage treatment plant carries out a number of processes to remove waste from the water before returning it to the environment. In this plant, raw sewage is passed through coarse screens to remove large material. Then a settling tank is used to precipitate most of the inorganic matter from the water. Any remaining organic matter is then removed through additional settling and aeration tanks. In the aeration process, micro-organisms, aided by air pumped through the water in the tanks, oxidize the organic matter. The sludge left after these processes is precipitated in a final settling tank and the water is chlorinated and returned to lakes or streams. The waste sludge from all the steps is collected in digesters where anaerobic bacteria further oxidize it. The gas given off in this process is either burned off or used to produce heat. The non-organic sludge that remains is piped away from the plant for disposal.
What You Need
- Student Work Sheets
- Round tipped pencils
What to Do
- Teacher gives a simple demonstration by turning on a water tap, fountain, or uses a poster or photograph of a fountain, hose, fire hydrant, or other source of water in the average urban community.
- Students suggest the sources of water that are available to supply the water that comes out of taps in their own community (if they know), or in other communities in Canada or worldwide.
- Teacher points out that it is often safe to drink the water from streams and lakes in wilderness areas. In more settled regions, however, it is not wise to drink the water. Students are asked to suggest reasons why this is the case.
- Students are given the instruction and recording sheet. They are to assemble an urban water system. There will be two different parts to this system: providing domestic and other types of water, and providing a sewer system to remove used water and waste products. Students are to take a series of individual components mixed up on the sheet, decide the order in which these components should be assembled, do the assembling, and then briefly explain how they made their decisions. Each component will consist of a drawing of the particular component and its name. In addition, short descriptions of the purpose and/or characteristics will need to be matched up with each of the components to complete the activity.
- Discussion of how the water system created in step 4 helps:
- a) protect human beings using the water,
- b) protects the environment.
- As an extension, and supplement, to the activities in the Grade 7 unit on Pure Substances and Mixtures, students may be asked to research the various steps included at the start and the end of an urban water system: what is done, what materials/processes are used, and why each step is important in treating water for human consumption or for returning water to the natural environment