The correct answer is Biological Oxygen Demand.

• Biochemical oxygen demand (BOD), is the amount of dissolved oxygen used by microorganisms in the biological process of metabolizing organic matter in water. 
• Water pollution by organic wastes is measured in terms of Biochemical Oxygen Demand (BOD).
• BOD is the amount of dissolved oxygen needed by bacteria in decomposing the organic wastes present in water.
• The higher value of BOD indicates low dissolved oxygen content of water and high water pollution.

Key Points

• It is used in sewage treatment.
• Methods used in measuring BOD.
  • Dilution method
  • Manometric method
  • Biosensor
  • Fluorescent
  • Polarographic method
  • Software sensor
  • Real-time BOD monitoring

Explanation:

Dissolved Oxygen:

• The Winkler Method is a technique used to measure dissolved oxygen in freshwater systems.
• Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in the water. The amount of dissolved oxygen in a stream or lake can tell us a lot about its water quality. 
• For the survival of fish in a river stream, the minimum dissolved oxygen is prescribed 4 mg/l.
• The main compounds for conducting this test are Azide Iodide, Starch solution, and Sodium thiosulphate.

Important PointsNatural factor affecting Dissolved Oxygen:

• Aquatic life: Animals living in water use up dissolved oxygen. Bacteria take up oxygen as they decompose materials. Dissolved oxygen levels drop in a water body that contains a lot of dead, decomposing material.
• Elevation: Since streams get much of their oxygen from the atmosphere, streams at higher elevations will generally have less oxygen.
• Salinity (saltiness): Salty water holds less oxygen than freshwater.
• Temperature: The Solubility of oxygen decreases as temperature increases. Cold water holds more dissolved oxygen than warm water.
• Turbulence: More turbulence creates more opportunities for oxygen to enter streams.
• Aquatic Vegetation: Aquatic vegetation and algae directly release oxygen into the water during photosynthesis (during the day). At night, plants actually use oxygen for their metabolism.

Thus, the dissolved oxygen concentration decreases with the increase in the temperature of the water.

Explanation:

Turbidity:

• Turbidity is the measure of the relative clarity of a liquid.
• It is an optical characteristic of water and is a measurement of the amount of light that is scattered by material in the water when a light is shined through the water sample.
• The higher the intensity of scattered light, the higher the turbidity.
• The WHO (World Health Organization), establishes that the turbidity of drinking water shouldn't be more than 5 NTU, and should ideally be below 1 NTU.
• According to the Central Ground Water Board, BIS (IS 10500 and revised module IS 10500:2012)             
• Acceptable limit = 1 NTU
• Permissible limit = 5 NTU

Important Points

The following table shows drinking water standards for different minerals present in water:

Water pollution:

Water pollution, the release of substances into subsurface groundwater or into lakes, streams, rivers, estuaries, and oceans to the point where the substances interfere with beneficial use of the water or with the natural functioning of ecosystems. 

Key PointsE. Coli:

• E. coli (Escherichia coli), is a type of bacteria that normally lives in your intestines.
• It’s also found in the gut of some animals.
• It naturally occurs in the intestines of human beings and animals.
• Some versions of E. coli make humans sick by making a toxin called Shiga.
• This toxin damages the lining of the intestine.
• Coliform bacteria are found in human faeces.
• E.coli is the most common indicator of water pollution.
• So if E.coli are detected in drinking water it indicates a serious health risk and that water should not be used for drinking.
• The presence of such bacteria in the surface water is a common indicator of faecal contamination.  

• The sanitation program test detects the water for clearing contamination of faeces.
• They are commonly found in sewage and if E.coli are detected in water then it indicates faecal contamination.

Hence, the common indicator of water pollution is E. Coli.

Additional InformationEntamoeba Histolytica:

• Entamoeba histolytica is a protozoan parasite in the large intestine of humans which causes amoebiasis (amoebic dysentery).
• Although commonly found in water it is not a common indicator of water pollution.

Eichhornia Crassipes:          

• It is the scientific name of water hyacinth.
• Water hyacinth is a free-floating perennial aquatic plant, native to tropical and sub-tropical South America.
• It is an invasive species in India.
• Water hyacinth proliferation can block waterways, limiting boat traffic, swimming and fishing.
• The weed prevents sunlight and oxygen from reaching the water column and submerged plants.
• By crowding out native aquatic plants, dramatically reduces biological diversity in aquatic ecosystems.
• However, it is not an indicator of water pollution.

Lemna Paucicostata:

• Lemna is a genus of free-floating aquatic plants referred to by the common name "duckweeds."
• They are morphologically divergent members of the arum family Araceae.
• They are not indicators of water pollution.

• The hardness of water is that characteristic that prevents the formation of sufficient leather or foam.
• It occurs due to the presence of carbonates, bicarbonates, sulphates, chlorides of calcium and magnesium(Ca and Mg).
• The hardness of water is due to the presence of dissolved minerals of Ca2+, Mg2+, Al3+, Iron and other heavy elements. Specifically due to Calcium Sulphate, Calcium Bicarbonate, Calcium chloride, Magnesium Sulphate, Magnesium Bicarbonate and Magnesium chloride.​

Key Points 

Temporary hardness (Carbonate hardness):

• Temporary hardness is caused by the presence of dissolved bicarbonates of Calcium (Ca(HCO3)2 ), Magnesium (Mg(HCO3)2 ), Iron and other heavy elements.
• It can be eliminated by mere boiling.

Ca(HCO3)2 → CaCO3↓ + H2O + CO2 ↑

Mg(HCO3)2 → MgCO3 + H2O + CO2 ↑

MgCO3 → Mg(OH)2↓ + CO2 ↑

Permanent Hardness (non-carbonate hardness):

• Permanent Hardness is due to the presence of dissolved chlorides and sulphates of Calcium, Magnesium, Iron and other heavy elements.
• It cannot be eliminated by boiling.

Hence, we can conclude that Magnesium sulfate is responsible for the permanent hardness of the water.

Additional Information

Degree of Hardness:

• The degree of hardness is the extent of hardness.
• It is expressed as weight in milligrams of CaCO3 equivalent to all hardness causing substances in one million milligrams of water. That is parts per million (ppm). Or it can be expressed as weight in milligrams of CaCO3 equivalent to all hardness causing substances in one litre of water.​

CaCO3 as reference for Hardness:

• CaCO3 is stable, non-hygroscopic and is obtained in pure form. Therefore a standard hard water solution can be prepared by dissolving accurately weighed CaCO3 in dilute HCl and make up to a known volume.
• CaCO3 is insoluble in water. Therefore it can be easily precipitated in water treatments.
• Molecular weight of CaCO3 is 100, so mathematical calculations are easy

Key Points

Hardness:

• Hardness in water is determined by the Versonate Method.
• It is also known as EDTA (Ethylene diamine tetracetate acid) as EDTA is added to the wine red coloured solution (mixture water sample and Eriochrome Black T) to estimate hardness.

Chlorine:

• Chlorine in water can be detected by three tests: i) Starch iodide test, ii) Ortho-toluidine test and iii) Ortho- toluidine arsenate test.
• The first two test determines total form of chlorine in water and the later detects only free form of chlorine in water sample.

Dissolved Oxygen:

• Dissolved oxygen in a water sample is determined by Wrinkle’s Method.
• Main compounds for conducting this test are Azide Iodide, Starch solution and Sodium thiosulphate.

Chloride:

• Chloride in water is determined by Mohr’s Test.
• It involves addition of potassium chromate and silver nitrate sequentially to the water sample.
• Excess of chlorides may lead to cardiac problems and kidney diseases.

​Thus, the correct match is A-2, B-3, C-1, D-4

Explanation:

Minamata disease:

• Minamata disease is sometimes referred to as Chisso-Minamata disease.
• The disease Minamata was first discovered in 1956 in Japan.
• It is a neurological syndrome caused by severe mercury poisoning.
• It is methylmercury (MeHg) poisoning that occurred in humans who ingested fish and shellfish contaminated by MeHg discharged in wastewater.
• Symptoms include ataxia, numbness in the hands and feet, general muscle weakness, narrowing of the field of vision, and damage to hearing and speech. 

Itai-Itai:

• Itai-Itai, which literally translates to ‘it hurts, it hurts’ is the name given to cadmium poisoning.
• This name was given to the disease by the locals of Toyama Prefecture of Japan in 1912.
• This disease was recognized as the Four Big Pollution Diseases of Japan.
• Cadmium is a naturally occurring toxic heavy metal.
• It is an extremely toxic industrial and environmental pollutant classified as a human carcinogen.
• It is used in the manufacturing of NiCd rechargeable batteries.

Thus, Minamata and Itai-Itai diseases caused by metallic water pollution are caused by Hg and Cd metals respectively.

Additional Information

Brief details about the diseases caused by environmental degradation: 

Explanation:

Water Quality Parameters:

Physical Parameter:

• Suspended Solids
• Turbidity
• Colour
• Taste and Odour
• Temperature

Chemical Parameter:

• Dissolved Solids
• Alkalinity
• Hardness
• pH
• Chemical Content

Biological Parameter:

• Organics
• Coliform Index
• MPN Index

Thus, taste and colour is the physical characteristic of water.

Concept:

• Impure water is water with some other compounds like chlorine, fluorine, sand, heavy metals, and other chemicals in it.

Explanation:

We can purify the water by both of these methods easily:

By adding alum:

• Alum is added to water to purify it.
• Alum is aluminium sulfate.
• It acts as a coagulant agent in water and attracts all small particles to make a large impurity particle.
• This impurity particle will reside down and after it, all the floating impurities will get filtered.
• So, it is a good method to purify water.

By boiling:

• Boiling of water is the major way to purify water.
• It is a simple method that can kill all the biological contaminants like parasites or microorganisms from the water.
• We have to boil the water at its highest boiling point for some time in this method.
• It is tried and tested method for all peoples from the centuries.

Thus, impure water can be made pure by adding alum or by boiling.

Additional InformationOther common methods of water purification are as follows:

​Filtration:

• It is an excellent method of water purification.
• In this method, you have to pass the water through a filter to remove all the dirt and dust particles from the water.

Distillation:

• It is a method of water purification by using a heat source to vaporize water.
• When the water becomes vapour and it separates all the contaminants from the liquid.

The correct answer is 1974.

• The water (Prevention and Control of Pollution) Act was enacted in 1974.

Key Points

• This act is also called the Water Act.
• This act provided for the establishment of the Central Pollution Control Board and separate State Pollution Control Boards.
• This act was last amended in 2003.

Additional Information

• The Air (Prevention and Control of Pollution) Act was enacted in 1981 for the betterment of air quality.
• The Environment Protection Act was enacted in 1986. 
• The separate Ministry of Environment and Forest was created in 1985.