Environmental Chemistry: Water Pollutants and Biological Oxygen Demand (For CBSE, ICSE, IAS, NET, NRA 2022)

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Water Pollutants

Domestic and Municipal Pollutants

The sewage contains garbage, soaps, detergents, waste food and human excreta and is the single largest sources of water pollution. Pathogenic (disease causing) microorganisms (bacteria, fungi, protozoa, algae) enter the water system through sewage making it infected. Typhoid, cholera, gastroenteritis and dysentery are commonly caused by drinking infected water. They cause a number of diseases, such as, polio, viral hepatitis and may be cancer which are resistant to like the organic matter are oxygen demanding substances.

Industrial Pollutants

Many industries are located near rivers or freshwater streams. These are responsible for discharging their untreated effluents into rivers like highly toxic heavy metals such as chromium, arsenic, lead, mercury, etc. along with hazardous organic and inorganic wastes (e. g. acids, alkalis, cyanides, chlorides, etc.) . River Ganges receives wastes from textile, sugar, paper and pulp mills, tanneries, rubber and pesticide industries.

Agricultural Waste

Manure, fertilizers, pesticides, wastes form farms, slaughterhouse, poultry farms, salts and silt are drained as run-off from agricultural lands. The water body receiving large quantities of fertilizers (phosphates and nitrates or manures becomes rich in nutrients which leads to eutrophication and consequent depletion of dissolved oxygen. Consumption of water rich in nitrates is bad for human health especially for small children.

Physical Pollutants

Physical pollutants can be of different types. Some of them are discussed below:

Radioactive Wastes

Radionuclides found in water are radium and potassium-40. These isotopes originate from natural sources due to leaching from minerals. Water bodies are also polluted by accidental leakage of waste material from uranium and thorium mines, nuclear power plants. They cause tumors and cancer.

Thermal Sources

Various industries, nuclear power plants and thermal plants require water for cooling and the resultant hot water is often discharged into rivers or lakes. This results in thermal pollution and leads to the imbalance in the ecology of the water body.

Sediments

Soil particles carried to streams, lakes or oceans form the sediments. The sediment become polluting due to their large amount. Soil erosion defined as the soil carried by flood water from crop land, is responsible for sedimentation. The sediments may damage the water body by introducing a large amount of nutrient matter.

Eutrophication

  • The word eutrophication is derived from the Greek word which means well-nourished as (EU: true, Trophos: feeding) .
  • Eutrophication of a water body results due to the release of large amount of nutrients by the action of aerobic bacteria on organic wastes entering a water body naturally or by human activity.

Biological Oxygen Demand (BOD)

There are many organic compounds or waste present in a water body. The microorganisms present in the system act upon this waste for their own consumption and growth. In the process the metabolic activity requires oxygen which is met by the dissolved oxygen present in water. It is this amount of oxygen which is defined as biological oxygen demand (BOD) .

Water Pollution – Some Control Measures

The treatment of this wastewater is carried out in the following three stages:

Primary Treatment

When the wastewater is to be dumped off into a river or flowing steam, the treatment is carried out by sedimentation, coagulation and filtration. This is known as primary treatment.

The following steps are performed to do primary treatment of water:

  • Sedimentation: This step is carried out in large tanks specially built for this purpose in sewage treatment plant. The polluted water is allowed to settle so that silt, clay and other matter settle to be bottom and water is slowly allowed to move out. Fine particles do not settle and are thus required to be removed in the next step.
  • Coagulation: Fine particles and colloidal suspension are combined into large particles by a process called coagulation. This step is carried out by the addition of special chemicals called coagulants (flocculants) such as potash alum. The large particles either settle to the bottom or are moved in the next step.
  • Filtration: Suspended particles, flocculants, bacteria and other organisms are filtered by passing the water through a bed of sand or finely divided coal or through some fibrous materials. The total impurities collected in these steps are called sludge. It is used as a valuable fertilizer. On composting (i.e.. the action of anaerobic bacteria) , it releases sludge gas. It consists mainly of methane gas which is used for cooking purposes.

Secondary or Biological Treatment

The water after primary treatment is not fit for drinking purposes and has to undergo further treatment. This is done through secondary or biological treatment. A commonly used method is to allow polluted water to spread over a large bed of stones and gravel so that the growth of different microorganisms needing nutrients and oxygen is encouraged. This is called secondary treatment of water.

It involves the following processes:

Softening: By this treatment undesirable cations of calcium and magnesium are removed from hard waters. Either water is treated with lime and soda ash to precipitate ions as carbonates or it is passed through cation exchangers. This makes water soft.

Aeration: In this process, soft water is exposed to air by forcing air through it to add oxygen to water. This encourages bacterial decomposition of organic matter into harmless products such as carbon dioxide and water. The addition of oxygen reduces carbon dioxide. Sulphide etc. The water is as yet not fit for drinking purposes. The pathogenic and other microorganisms need to be killed.

Tertiary Treatment

The tertiary treatment is actually disinfecting water. Chlorine is the most commonly used disinfectant used for killing bacteria. However, chlorine also reacts with traces of organic matter present in water and forms undesirable chlorinated hydrocarbons. It is therefore desirable to reduce the organic matter in water before passing chlorine gas.

Tertiary Treatment

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