Water Quality and Pollution

• The fertilizers are the rich source of nitrogen and phosphorous used for the growth of plants.
•  When the fertilizers are introduced in the pond, these nutrients increase the fertility of the pond. 
• The phytoplanktons are the autotrophic organisms in the aquatic environment that utilize these nutrients and increase in the number. 

Water-soluble pollutants are detergents, fertilizers, oils from factories and industries, ammonical nitrogen etc. Whereas oxides of nitrogen, carbon, and sodium are air pollutants that cause gaseous pollution. 

Public hygiene may be described as the activity which is associated with the population or the public to maintain cleanliness and good health. The drinking water should be kept clean which prevents the transmission of the water-borne diseases in the population. This is considered as a part of public hygiene. 

Potable water is water that is safe enough to be consumed by humans or used with low risk of immediate or long-term harm.

Excess fertilizer runoff from the Mississippi River watershed enters the Gulf of Mexico, causing massive algal blooms. When these blooms die and decompose, they consume the oxygen in the water, creating a hypoxic 'dead zone'.

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.

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.

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.

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

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 maximum limit of nitrate in drinking water is 50 ppm

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. 

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.