Wednesday, 17 September 2014



1.     The Hydrologic or Water cycle is probably the most important of all the natural cycles in the biosphere.

2.     More than 97% of water in the biosphere is found in the oceans, the remaining 3% is found on the continents and in the atmosphere.

3.     But more than 70% of this latter portion is locked in glaciers and icecaps.

4.   The water on which human depends so heavily – lakes, streams and ground water – accounts for less than 1% of the total supply and it is this water , which is currently being used and reused in many parts of the world.

5.  The Hydrologic cycle of the biosphere depends on the reciprocity of evaporation and precipitation.

6.     Liquid water on the earth goes into the atmosphere as vapour by evaporation and transpiration of plants.

7.     The vapour is returned to earth as rain or snow. Figure illustrates the complete hydrologic cycle.

8.     Most evaporation occurs over the oceans but some oceans lose more water by evaporation than they gain by precipitation.

9.   The difference is made up by runoff and seepage from the continents, over which there is more precipitation than evaporation.

10.    The continent lose more than 50% of the precipitation through evaporation and the remainder is temporarily stored in lakes and rivers or as ground water which is later discharged into oceans.

11.    The global cycle can be summerised as shown in table below :

Whole Earth
Gain by inflow

12.  Each year, an estimated 4,23,000 km3 of water is evaporated, and the same quantity is precipitated over the whole surface of the earth. 

13.  The amount of water, which is temporarily stored and is later discharged into the oceans is 37,000 km3 and this is the amount which is potentially available for human needs.

Reference Books :
Environmental Pollution Control Engineering, C.S. Rao .



1.     Sulphur, like nitrogen, is a basic constituent of proteins in plants and animals.

2.   It is found in the biosphere in a wide variety of forms. Sulphur dioxide (SO2) and Hydrogen Sulphide (H2S) are the important gaseous forms, and the sulphate ion (SO42-) is the common form found in water and soil. The distribution of sulphur in the environment ia as shown in figure.

3.  Sulphate ion is reduced after being absorbed from the soil by plants and bacteria, and ultimately incorporated as the sulphydryl group(-SH) in proteins.

4.     Some sulphates are reduced under anaerobic conditions directly to sulphides, including H2S, or to elemental sulphur by a class of bacteria known as Desulfovibrio bacteria, found largely at ocean bottom.

5.    The hydrogen sulphide thus produced escapes as a gas into the atmosphere and replenishes the sulphur lost by precipitation.

6.     In the presence of oxygen, H2S is rapidly oxidised to sulphates by bacteria of genus Thiobacillus.

7.  Even in the absence of oxygen, several types of bacteria such as Chlorobacteriaceae and Thiorhodaceae oxidise H2S to elemental sulphur.

8.   Atmosphere receives sulphur through bacterial emission (H2S), fossil fuel burning (SO2), wind-blown sea salts(SO42-) and volcanic emissions (H2S, SO2, SO42- ).

9.      Most of the sulphur in the form of SO2 or H2S is converted to sulphur trioxide SO3 , which dissolves in water droplets to form sulphuric acid. The sulphates and the acid then precipitate with rain.

10.  The sulphur cycle is overloaded due to burning of fossil fuels at an ever increasing rate. As a result, the SO2 emitted into the atmosphere this way constitutes a significant fraction of total global sulphur transport. This increased amount of sulphur is changed mostly to the form of sulphuric acid  in rain water causing adverse ecological effects.

Reference Books :
Environmental Pollution Control Engineering, C.S. Rao .



1.  Nitrogen in its gaseous form constitutes 79 percent of the atmosphere. However, it cannot be used directly by most forms of life. 

2.    It must be first “ fixed ” before it can be utilised by plants and animals. By fixation, nitrogen is converted into it’s chemical compounds, largely nitrates (NO3) and ammonia (NH3). 

3.  The fixation of nitrogen takes place through both physicochemical and biological means although the latter is by far the much bigger contributor. 

4.  The biological fixation is limited to a few , but abundant organisms like the free living bacteria AZETOBACTER and CLOSTRIDIUM, nodule bacteria on leguminious plants like Rhizobium and some Blue-Green Algae. 

5.  These are the keys to the movement of nitrogen from the atmospheric reservoir into the cycle as shown in figure.

6.    The nitrates are assimilated to form amino acids, urea and other organic residues in the Producer,  Consumer & Decomposer cycles. 

7.  The amino acids and urea are then converted to ammonia through a process called “Ammonification”. 

8.   To complete the  cycle, Denitrifying bacteria convert the ammonia into nitrites, then into nitrates, and then back into gaseous nitrogen. 

9.   In this way, under normal circumstances, the total amount of Nitrogen Fixed equals the total amount returned to the atmosphere as gas.

10.  Man has interfered with this natural cycle by industrially fixing nitrogen. 

11.  This includes production of Nitrogen Fertilisers and oxidation of nitrogen during Fossil Fuel combustion. 

12.  Most of the excess nitrogen is carried off into rivers and lakes and ultimately reaches the Ocean. 

13.  This increased runoff has greatly increased the productivity in many aquatic environments and has contributed to the process of EUTROPHICATION.

Reference Books :
Environmental Pollution Control Engineering, C.S. Rao .



1.     The Biosphere contains a complex mixture of carbon compounds in a dynamic equilibrium of Formation, Transformation And Decomposition. 

2.     The producers, through the process of Photosynthesis, reduce the carbon dioxide from the atmosphere to organic carbon.

3.     This then passes through consumers and decomposers, then usually re-enters the atmosphere through respiration and decomposition.

4.     Additional return from producers and consumers occur through the non - biological process of combustion.

5.     Even though the amount of CO2 in the atmosphere is of major concern, in fact, the atmosphere reservoir for carbon is the smallest and the oceans hold the largest amount, serving as a vast “sink” for CO2 .

6.     Apart from the daily production and consumption of carbon, the earth has significant reserves of bound carbon in the form of inorganic deposits such as limestone and organic fossil fuel deposits consisting of mainly coal and petroleum.

7.     Due to the combustion of fossil fuels, weathering and dissolution of carbonate rocks, and volcanic activity, some of the bound carbon returns to the atmospheric aquatic reservoir as carbon dioxide or carbonic acid.

Typically reservoirs for carbon (expressed in billion tonnes)2 are :
  •  Oceans – 40,000
  •  Fossil Fuels, Rocks and Minerals – 5,000-10,000
  • Vegetation and Soil – 2,000
  • Atmosphere – 750
Thus, the oceans store more than 50 times as much as the atmosphere. Human activity releases roughly 7.0 billion tonnes of carbon (in the form of CO2) into the atmosphere every year. This is a small amount compared to that held by the atmosphere, and an even smaller figure compared with that held in the oceans. Out of the 7.0 billion tonnes, only 3.0 billion tonnes accumulate in the atmosphere and the rest is taken up by the Oceans and the Terrestrial plants. The exact mechanism by which the sea water interacts with the air above it to remove CO2 is not clearly understood but the Oceanic reservoir tends to regulate the atmospheric CO2 concentration.

Even though the net amount of 3.0 billion tonnes added to the atmosphere each year is a tiny fraction of the total held by the atmosphere, it assumes significance because the natural processes and the environment maintain a dynamic equilibrium whereas the human activity puts an additional burden on nature, thereby disrupting the delicate balance. Any global event that alters the exchange of CO2 between the atmosphere and the ocean can significantly affect the concentration of CO2 in the atmosphere.

Studies have shown that plants tends to grow faster in a CO2 enriched atmosphere, but this benefit is offset by denudation of forests by man thereby decreasing nature’s ability to remove the excess CO2 from the atmosphere. As a result, a detectable increase in the concentration of atmospheric CO2 has been observed.

Reference Books :
Environmental Pollution Control Engineering, C.S. Rao .