Q.5 What is rain water harvesting? Discuss
its advantages and limitations. (AKTU. - 2009 - 10)
Ans. Rainwater
harvesting is a technique of water conservation by increasing the recharge of
ground water through capturing and storing rain water. This can be done by
constructing special structures such as dug wells, percolation pits, lagoons,
check dams, tanks, etc. Rainwater harvesting is not only useful for
low-rainfall regions but also for rainfall-rich areas. The objectives of
rainwater harvesting are as follows:
· Rainwater harvesting fulfils the demand of
water in domestic, industrial, and agricultural sectors.
· It helps to raise the water table by
recharging ground water.
· It helps in minimizing groundwater
pollution.
· The proper harvesting of rain water reduces
soil erosion, flooding, and run off.
· It reduces the incidence of floods.
· Rain water when absorbed by the land
becomes the main source of raising the water table and stores more ground
water.
Advantages: -
· Rainwater harvesting promotes self-sufficiency and
fosters an appreciation for water as a resource.
· It
also promotes water conservation.
· It
conserves energy as the energy input needed to operate a centralized water
system is bypassed.
· Rainwater
is one of the purest sources of water available.
· Rain
water is soft.
Limitation: -
· Water
is not potable
· Water
can not be immediately used pumping of water required.
· Initial
investment for tank required.
· This
requires considerable floor area.
Q.6 What do you mean by self purification
of rivers? Explain the various natural mechanisms involved in this process. (AKTU. - 2007-08, IInd Sem.)
Ans. Self Purification Of Rivers: -
When wastewater is discharged into a
river/stream, the receiving water gets polluted due to the waste products
present in the effluents. But the conditions do not remain the same for ever,
because the natural forces of purifications go on acting upon the pollution
elements and bring back the stream water into its original state. This process
is known as Natural self-purification phenomenon.
Natural Mechanisms: -
Various natural mechanisms
involved in self-purification of rivers are:
(i)
Physical Forces: -
These include dilution and dispersion,
sedimentation and sunlight.
(a)
Dilution and Dispersion: -
When wastewaters (or sewage) are
discharged into river/stream, the suspended and dissolved solids get rapidly
dispersed and diluted into the large volume of receiving water. As a result,
the concentration of organic matter reduces and thus the nuisance potential of
wastewaters (or sewage) is also reduced.
(b)
Sedimentation: -
The heavier suspended solids
settle down at the point of outfall of the wastewaters (or sewage), and thus
the portion of river water ahead becomes free of them.
(c)
Sunlight: -
It is effective in purification
through its sterilizing effect on certain bacteria; through its bleaching
effect; and through photosynthesis by chlorophyll bearing organisms (such as
algae), who take energy from sun and convert it into food for other forms of life,
thus absorbing carbon dioxide and releasing oxygen in the process. Oxygen
produced is used in the oxidation of unstable organic matter.
(ii)
Biochemical Forces: -
These are chemical forces aided
by biological forces. These are oxidation and reduction:
(a)
Oxidation: -
The oxidation of organic matter
present in wastewaters (or sewage) effluents starts as soon as the effluent
outfalls into the river water containing dissolved oxygen. The bacteria and
other micro organisms act upon the organic matter and convert it into simple
chemical substances and mineral matter. The deficiency of oxygen so created,
will be filled up by the atmospheric oxygen, and the oxygen produced by the
chlorophyll bearing organisms (such as algae). This is the most important action
responsible for carrying out the self-purification of river/streams. It will
continue till all of the organic matter has been completely oxidised.
(b)
Reduction: -
It takes place as a result of
hydrolysis of organic matter settled at the bottom either chemically or
biologically under anaerobic conditions. The complex organic matter will get
mineralized and gasified.
The receiving water thus becomes
free of the pollution and will return to its normal condition with a certain
distance or time.
