Biological Oxygen Demand (BOD) refers to the amount of dissolved oxygen that aerobic organisms need in order to break down organic material in water over time. Its high value indicates the intense level of microbial pollution.

Maximum prescribed limit of Fe in drinking water is 50 ppm

Water pollution is the contamination of water bodies very often by human activities.

Pollutants which are slowly degradable remain in the environment in an unchanged form for many decades. For example, substances such as dichlorodiphenyltrichloroethane (DDT), plastic materials, heavy metals, many chemicals, and nuclear wastes etc., once released into the environment are difficult to remove. These pollutants cannot be degraded by natural processes and are harmful to living organisms. In the process of environmental pollution, pollutants originate from a source and get transported by air or water or are dumped into the soil by human beings.

All these industries cause water pollution by the water discharged into the rivers. The common industrial pollutants are sulphur, asbestos, poisonous solvents, poly-chlorinated biphenyl, lead, mercury, volatile organic chemicals, etc.

Water pollution is the contamination of water by substances harmful to life.

Pesticides and fertilizers containing chemicals causes water pollution. Pesticides contaminate water as the pesticides are toxic. Fertilizers contain nitrogen and phosphorous, high amount of which is injurious to the underwater organisms. Therefore, the answer is D.

A) Aldrin is a pesticide used to control soil insects such as termites, corn rootworm, wireworms, rice water weevil and grasshoppers.
B) The maximum permissible limits for nitrite and nitrate are 10 ppm and 50 ppm respectively.
C) The maximum permissible limit of fluoride is 2 ppm.

Pesticides are the chemical compounds that are used to kill pests, including insects, rodents, fungi, and weeds.

Water pollution caused due to the chemicals such as the chemical waste from the industries, chemicals used in agriculture such as pesticides, herbicides, etc. All these results in chemical water pollution.

These type of waste water causes the water pollution. It usually gives negative impact on our environment and not good for aquatic life.

There are various chemical pollutants which on mixing changes the colour, taste or other physical quality; changes the natural state of odourless, colourless form by bringing chemical changes and biological changes include an addition of pathogenic bacteria to the water by untreated sewage waste.

Standards of drinking water in Europe is decided by European Drinking water directive.

Clean water would have BOD value of less than 5 ppm whereas highly polluted water could have BOD value 17ppm or more.

Water is a polar solvent and benzene is non-polar in nature so, they are not soluble in each other.

The Food and Nutrition Board(FNB) recommends dietary copper intakes for adults is 1.5-3.0 mg as Cu is an essential macronutrient. 

 The prescribed upper limit concentration of lead in drinking water is about 50 ppb.

The maximum limit of nitrate in drinking water is 50 ppm

Gastrointestinal diseases refer to diseases involving the gastrointestinal tract, namely the esophagus, stomach, small intestine, large intestine and rectum, and the accessory organs of digestion, the liver, gallbladder, and pancreas.

E. coli (Escherichia coli) is the name of a germ, or bacterium,that lives in the digestive tracts of humans and animals. There are many types of E. coli, and most of them are harmless. But some can cause bloody diarrhea.

Enterococcus faecalis – formerly classified as part of the group D Streptococcus system – is a Gram-positive, commensal bacterium inhabiting the gastrointestinal tracts of humans and other mammals

Pollution is the release of harmful substances or energy into the environment by man in quantities that damage health and resources. The polluted air is undesirable for breathing, for the condition of buildings and monuments exposed to it or for animal and plant life. The polluted soil and land undesirable for raising food and fodder for animals or for recreation and aesthetic enjoyments.

Minamata disease, sometimes referred to as Chisso-Minamata disease, is a neurological syndrome caused by severe mercury poisoning. Symptoms include ataxia, numbness in the hands and feet, general muscle weakness, narrowing of the field of vision and damage to hearing and speech. Nearby anthropogenic sources, such as coal burning and mining of iron, can contaminate water sources with methylmercury, which is efficiently absorbed in the bodies of fish. Fish and shellfish concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organic compound of mercury. Minamata disease was first discovered in Minamata city in Kumamoto prefecture, Japan, in 1956. It was caused by the release of methylmercury in the industrial wastewater from the Chisso Corporation's chemical factory, which continued from 1932 to 1968. This highly toxic chemical bioaccumulated in shellfish and fish in Minamata Bay and the Shiranui Sea, which, when eaten by the local populace, resulted in mercury poisoning.

The rise in the average temperature of earth has been described as global warming which is mainly attributed to the increasing phenomenon of the greenhouse effect. It is believed that global warming can have several harmful effects on human health, both directly and indirectly. Since malaria is greatly influenced by climatic conditions because of its direct relationship with the mosquito population, it is widely assumed that its incidence is likely to increase in a future warmer world.

As global warming continues, malaria is set to spread in locations where previously it was limited, due to cooler climate. On the other hand, several theories have been put forward which are quite contrary to this prediction. There are multiple other factors which are accountable for the recent upsurge of malaria: for example drug resistance, mosquito control programs, public health facilities, and living standards. Due to the rising global temperature, more areas of the world will become favourable for the survival of tropical insects like mosquitoes. This could lead to the spread of these insects towards more northern and southern hemispheres and to higher altitudes. Some species of mosquito act as disease vectors and are responsible for the spread of illnesses like malaria, Dengue fever, and Yellow fever. Hence, global warming can increase the incidence of these diseases

Let us now study the causes of water pollution:

1. Industrial waste

2. Sewage and waste water

3. Mining activities

4. Marine dumping

5. Accidental Oil leakage

6. Burning of fossil fuels

7. Chemical fertilizers and pesticides

8. Leakage from sewer lines

9. Global warming

10. Radioactive waste

11. Urban development

12. Leakage from the landfills

13. Animal waste

14. Underground storage leakage

Fresh water is naturally occurring water on Earth's surface in ice sheets, ice caps, glaciers, icebergs, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams.

The definition of freshwater according to The United States Geological Survey is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt.

The water temperature should preferably standard temperature or room temperature for balanced ecology. Hence, it should be preferably less than 25 degree Celsius. 

Water pollution is the contamination of water bodies (e.g. lakes, rivers, oceans, aquifers and groundwater). This form of environmental degradation occurs when pollutants are directly or indirectly discharged into water bodies without adequate treatment to remove harmful compounds. Agricultural discharges, sewage and other wastes and industrial effluents all cause water pollution.

Explosive growth of plants and algae is a response to the addition of excess nutrients, mainly phosphates, which induces, the decaying of which consumes oxygen from the water causing water pollution. Phosphates are among the major nutrients that is required by plants for growth. 

Biochemical oxygen demand is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at $20^{\circ}C$ and is often used as a surrogate of the degree of organic pollution of water.

BOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organic compounds in water. However, COD is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biodegradable organic matter

In general, all common methods of sewage purification use sedimentation processes by which solids (sludge) sink to the bottom – in some cases this primary sludge is removed for further treatment.

After this comes the secondary or biological treatment of sewage purification, this involves biological processes and the use of microorganisms that “eats” away a lot of the hazardous contaminants. Use of oxygen by various implementation techniques is very important.

After the secondary stage of sewage purification comes final settlement of biological solids and then a tertiary treatment that removes even better the rest of the biological and organic materials in the water. 

Potassium permanganate is an inorganic chemical compound and it is a strong oxidizing agent. Potassium permanganate is used in water treatment to remove odor-causing compounds and is also used to remove iron and manganese in water.

Sodium carbonate, $Na _2CO _3$, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble. The water is softened because it no longer contains dissolved calcium ions and magnesium ions.

Factors responsible for the depletion of water table-

1. Polar stratospheric clouds (PSCs) are the medium on which reservoir chlorine compounds are converted into ozone-destroying chlorine radicals, they are essentially two types of  PSC known as type 1 and type 2.
2. Polychlorinated biphenyl (PCBs) is a group of organic compounds used in the manufacture of plastics, as lubricants, in transformers, adhesives and so forth. They are highly toxic to aquatic life and persist in the environment for long periods of time. They can accumulate in food chains and may produce harmful side-effects at high concentrations.
3. Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed (i.e demanded ) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period.
4. Chemical oxygen demand (COD) is defined as a  measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. Therefore, BOD is a measurement of water pollution.

Option (C) is correct.

In COD, the degradation is affected by an oxidizing agent, $K _2 Cr _2 O _7.$ COD  is a measure of amount of oxygen , required to degrade or breakdown the organic matter. 

Clean water has BOD which is less than 5 ppm. If it is equal to or more than 12 ppm, then the water is considered as polluted. So this indicates water be rich in dissolved oxygen, that helps the microorganisms to oxidize inorganic and organic materials. 

BOD refers to the dissolved oxygen that the microorganisms like bacteria need to oxidize, means to break down inorganic and organic matter in water. It is a measure of water quality. 

$BO{ D } _{ 5 }$ stands for 5-day Biochemical Oxygen Demand. The BOD test measures, oxygen is used as microbes breathe as they eat the contaminants  in wastewater.

$ Number \  of   \ milliequivalent $ = $\dfrac{weight \ in \ mg}{Equivalent\  weight}$