Environment & Humans Revision Structured Questions

1. (a) Before 1977, many of the rivers in Singapore were polluted with waste materials discharged from factories. The rivers were so polluted that the number of fishes decreased drastically. Since then, efforts have been ramped up to reduce pollution in the rivers. Sewage treatment plants were built so that raw sewage will not be discharged into the river.

Explain how sewage treatment uses biotechnology to improve water quality.

Explain how this treatment would affect the number of fishes in the river.

(b) Human beings are continually using and depleting the Earth’s natural resources. This
upsets the balance of nature and causes lasting damage to the environment. Human activities have led to deforestation and over-fishing.
Suggest ways that humans can conserve the forests and fishing grounds.

2.  The figure below shows the results of an investigation on the effect of sewage discharged in a

kuo chuan.PNG

(a) (i) From which point X, Y or Z, is the sewage being discharged into the river?

(ii) Explain your answer.

(b) what process could have taken place after 30m downstream?

(c) Based on the evidence shown in Fig. 10.1, state and explain what could have happened to the number of algae at the following distances

(i)  15 m to 35 m from downstream

(ii) after 35 m downstream

(d) State and explain changes in the oxygen level in the river.

(i) downstream from point Y to 15 m.

(ii) downstream from 20 m to 30 m.

(iii) downstream from 30 m to 60 m.



1. (a) Sewage is channelled into an aeration tank where aerobic microorganisms like bacteria and fungi digest organic pollutants into harmless soluble substances;

The digested produced are absorbed and used by bacteria for their growth;

Sewage is aerated by bubbling air through it to ensure sufficient oxygen for aerobic respiration;

Sludge removed from the settling tanks is treated by anaerobic digesters. Methane and carbon dioxide gas that are produced in
this process are used to generate electricity;

The number of fishes in the river would increase;

The decrease in concentration of organic pollutants and bacteria will cause oxygen levels to increase and hence providing sufficient oxygen for the fishes to take in to perform cellular respiration

(b) Prevent tree-felling by enacting laws to protect remaining forests;

These trees are cut down selectively and at a regulated rate;

Young trees are not felled and new seedlings are planted to replace trees that were cut down for timber;

Perform reforestation;

Check trees regularly and control the number of insects and diseases that harm them;

Research are carried out to improve the quality of forest trees and make the forest more productive;

Banning the use of drift nets which indiscriminately trap all forms of marine life;

Using nets with certain mesh size so that young or immature fish are not caught;

Regulating the entry of ships into fishing grounds;

Limiting period of fishing in fishing grounds;

Banning the harvesting of fishing of endangered species;

Raising endangered species of fish in hatcheries and releasing them into the fishing grounds where the fish populations are decreasing;

2. (a) (i) point Y

(ii) Sewage contains organic matter. When it is discharged at point Y, there is a sudden increase in organic matter in the river.

(b) eutrophication

(c) (i) The number of algae increases tremendously. Nitrates and phosphates are good sources of nutrients for the growth of algae. As a result, they grow and multiply rapidly.

(ii) The number of algae decreases rapidly. This could be due to insufficient sunlight reaching the submerged algae. The sunlight is blocked by the thick layer of algae growing near the water surface.

(d) (i)  Oxygen level starts to decrease when sewage is discharged from point Y. The decrease is caused by the rapid growth of bacteria that uses up the oxygen in the river.

(ii) Oxygen level increases may be due to the rapid growth of algae that releases large amounts of oxygen when they carry out photosynthesis

(iii)  Large concentration of algae have built up and deplete oxygen through respiration. Oxygen is also used up when the death of algal cells and other
marine life becomes increasingly significant.





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