The Urban Water System

water challenge

Introduction

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.

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