Fig. 1 shows a small sewage-treatment works beside a river containing fish and water plants. Waste water enters the works from nearby homes. The treated wat...
Question 1 Report
Fig. 1 shows a small sewage-treatment works beside a river containing fish and water plants. Waste water enters the works from nearby homes. The treated water is released through an outlet pipe downstream. Scientists found that the river water had fewer fish below the outlet during a period when a filter was damaged. They also recorded a strong smell and a large amount of decaying plant material on the river bed.
(a) Name the type of pollution caused when untreated sewage enters a river. [1]
(b) Describe two changes likely to occur in the river water below the outlet when sewage is not treated properly. [2]
(c) Explain why the number of fish may decrease when bacteria decompose sewage in the river. [3]
(d) Suggest one improvement to the treatment works that would reduce pollution of the river. [2]
Answer Details
(a) Untreated sewage entering a river causes water pollution. [1]
(b) Below the outlet, water may become more cloudy and have a lower dissolved oxygen concentration. Other acceptable changes include more bacteria or microorganisms, increased growth of algae or water plants, and a bad smell. Any two are required. [2]
(c) Bacteria feed on and decompose the organic material in sewage. They respire while doing this, using oxygen dissolved in the water. Too little oxygen may then remain for fish respiration, so fish numbers decrease. [3]
(d) The works could repair or replace the damaged filter. This prevents inadequately treated sewage being released. Other acceptable improvements include adding biological treatment tanks, removing solids before discharge, disinfecting waste water, or monitoring the outlet and stopping releases when treatment fails. [2]
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