Water Pollution and Its Control Questions and Answers

Answer: B

The nutrient enrichment of a lake is called eutrophication.

When nitrogen and phosphorus from fertilisers and sewage enter a lake, they feed a rapid growth of algae and water plants. As the algae die and decay, oxygen in the water is used up, and fish and other animals may die. Natural eutrophication is slow; human activity speeds it up.

Biomagnification is the build-up of toxins up the food chain, water treatment is the purification of water, and a sanitary landfill is a method of waste disposal.

Answer: C

Knock-knee is caused by excess fluoride (printed 'Floride').

Long-term intake of drinking water with fluoride above about 1.5 mg per litre causes fluorosis. Besides mottled teeth, skeletal fluorosis makes bones dense but brittle and deforms the limbs; knock-knee (genu valgum) is a classic sign in endemic areas of India.

Lead mainly damages the nervous system and blood, mercury causes Minamata disease, and DDT accumulates in fat and affects reproduction in birds; none causes knock-knee.

Answer: A

Minamata disease is caused by compounds of Mercury.

It was identified in 1956 at Minamata in Japan, where a chemical factory discharged methylmercury into the bay. The methylmercury accumulated in fish and shellfish, and people who ate them suffered damage to the nervous system: numbness, loss of coordination, impaired vision and hearing, and in severe cases death. Children were born with the disease after exposure in the womb.

Cadmium causes itai-itai disease, a different Japanese outbreak. Cobalt is not linked to this disease, and fertilizers are not compounds of a single element; their run-off is associated with problems such as eutrophication and nitrate in drinking water.

Answer: C

The key accepts both statements, so the answer is Both 1 and 2.

The Pacific island nation of Palau passed a law in 2018, in force from January 2020, banning sunscreens containing chemicals toxic to coral reefs, and is recognised as the first country to do so (1).

Common sunscreen chemicals such as oxybenzone and octinoxate work by absorbing ultraviolet light, and studies have linked them to coral bleaching and damage to young corals, which is why such bans name them (2). So 'Only 1', 'Only 2' and 'Neither' each reject a statement the key accepts.

Answer: D

All four statements are accepted: All four.

Microplastics are conventionally defined as plastic particles smaller than 5 mm. Most plastics are polymers whose backbone chains are made of carbon and hydrogen. Because their surfaces are hydrophobic, microplastics adsorb persistent organic pollutants such as PCBs and DDT from water, concentrating them and potentially making the particles more toxic when eaten by organisms.

The recognised routes of human exposure include ingestion, inhalation of airborne particles, and skin contact, which the key treats as absorption through the skin, so the fourth statement is counted as correct.

Answer: C

E. coli is used as the indicator because it is a common inhabitant of the human intestine.

An indicator organism signals faecal contamination. E. coli lives in large numbers in the intestines of humans and warm-blooded animals and is shed in faeces, but it does not normally multiply in clean water. Finding it in a drinking water sample therefore shows that faecal matter, and possibly the pathogens that travel with it, has entered the water.

Most strains are not pathogens, so it is not used because it is an enteropathogen or an opportunistic pathogen. It is easy to culture, which is convenient, but ease of detection alone would not make an organism an indicator of faecal pollution.

Answer: D

The five-day BOD test is run at 20°C.

The standard BOD₅ test incubates the sample in the dark at 20°C for five days and measures the dissolved oxygen consumed by microbes oxidising the organic matter. Fixing the temperature matters because microbial rates roughly double for every 10°C rise, so results at different temperatures cannot be compared.

Indian practice also uses a three-day BOD at 27°C, but that is not the five-day test. At 0°C, 5°C or 15°C the microbes would work too slowly to give the standard figure.

Answer: C

A BOD value indicates the high organic matter content of sewage.

Biochemical oxygen demand is the amount of dissolved oxygen that microorganisms consume while breaking down the biodegradable organic matter in a sample, usually over five days at 20°C. The more organic matter, the more oxygen the microbes use, so a high BOD means heavily polluted, organic-rich water. Raw domestic sewage typically has a BOD of 200 to 300 mg/L.

The microbial load is counted directly by plate counts or MPN tests. Low oxygen content is measured as dissolved oxygen (DO), a different parameter. The test does depend on aerobic microbes, which may even be seeded into the bottle, but BOD measures the food they consume, not their presence.

Answer: A

The MPN test is carried out for Drinking water.

The Most Probable Number test is the multiple-tube fermentation method for estimating coliform bacteria. Measured volumes of the sample go into a series of lactose broth tubes; the pattern of tubes producing gas is read against statistical tables to give the most probable number of coliforms per 100 ml. Coliforms indicate faecal contamination, so the test is the standard check on the bacteriological safety of drinking water.

The same method can be applied to waste water and to dairy products, but the question takes its best-known use, the potability test. Air quality is assessed with samplers and settle plates, not with MPN tubes.

Answer: C

Namami Gange was launched in 2014.

The Namami Gange Programme was approved by the Union Government in June 2014 as a flagship integrated mission for the conservation of the Ganga, covering sewage treatment, river-front development, river-surface cleaning, afforestation and biodiversity, and is run by the National Mission for Clean Ganga.

2011 and 2012 predate it, and 2002 is unrelated; the earlier Ganga Action Plan dates from 1985.