15.7.3 second source of oxygen in water is

15.7.3 Importance of Dissolved Oxygen (D.O) in water

            The concentration of dissolved
oxygen in water is of vital importance for the support of aquatic life.. Lower
the concentration of dissolved oxygen, more the water is polluted. Dissolved oxygen of a sample of water
is defined as the mass of oxygen
dissolved in unit volume of water. It is usually expressed in parts per
million (ppm). The optimum usage value of D.O for domestic use is 4-6 ppm which
is able to maintain aquatic life.

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Oxygen reaches water through two sources. The first is that oxygen
dissolves at the surface of the water from the atmosphere. Still water takes up
oxygen slowly whereas running water takes it up more rapidly. The second source
of oxygen in water is from photosynthesis. When many aquatic green plants are
present, the water often becomes supersaturated with oxygen during daylight. But
after dark, photosynthesis stops but the plants continue to use and actually
reduce the amount of dissolved oxygen. Therefore, during a 24 hour period, same
water samples have a considerable range of dissolved oxygen levels.

          Water bodies receive excess nutrients
from biodegradable detergents, nitrogen and phosphate fertilisers applied to
agricultural fields and sewage plant discharge etc. This nutrient stimulate
excessive plant growth (algae, periphyton,plant weeds)often called an algal
bloom, reduces dissolved oxygen in water. Water with low concentration of
dissolved oxygen is called hypoxic. This leads to anaerobic conditions that
inhibit the growth of other living organisms in water body.

       
This process in which nutrients enriched water bodies support a dense
population which kills animal life by depriving it of oxygen and results in
subsequent loss of biodiversity is known as Eutrophication 

     

                                                                    Fig 15.4 Eutrophication

To
measure the quality of waste water, two types of methods are used in
environmental chemistry

            i) Biochemical oxygen Demand

            ii) Chemical oxygen Demand

i) Biochemical oxygen Demand (BOD):

            The polluted water may contain large
amounts of organic compounds. Some of these can be oxidised by dissolved oxygen
in the presence of microorganism. Organic compounds present in the polluted
water serve as a food for bacteria..Hence the amount of dissolved oxygen
utilised by these bacteria is called BOD.
Biochemical Oxygen Demand (BOD) of a sample of water is defined as

            The amount of dissolved oxygen required for the oxidation of organic
matter by aquatic microorganisms under aerobic conditions at 20oC
for a period of 5 days. Its value is expressed in ppm.            BOD is used as a measure of degree
of water quality. Clean water would have BOD value less than 5 ppm whereas highly
polluted water has BOD value of 17 ppm or more. To measure BOD, the sample of
water is first saturated with oxygen. It is then incubated at 293 K for five
days. This allows microorganisms in the water sample to oxidise pollutants. The
remaining amount of dissolved oxygen is determined on the basis of difference
(DO1 – DO5) between amounts of dissolved oxygen on the
fifth day (DO5) and on the first day (DO1).The amount of
dissolved oxygen used by bacteria for degradation of organic material of waste
sample can be determined. This is known as BOD

(ii) Chemical Oxygen Demand (COD)

         BOD measurement takes 5 days so
another parameter called the Chemical Oxygen Demand (COD) is measured. Chemical oxygen demand (COD) is
defined as

            The
amount of oxygen required by the organic matter in a sample of water for its
oxidation by a strong oxidising agent like K2Cr2O7 in
acid medium in a period of 2 hrs.

            In COD determination, the water
sample is treated with a known quantity of an oxidising agent like K2Cr2O7
 in acidic medium. This reagent
oxidises most of the polluting substances including those which are resistant
to microbial oxidation. The remaining K2Cr2O7 is
determined by back titration with a suitable reducing agent like ferrous
ammonium sulphate (Mohrs salt). From the concentration of K2Cr2O7
consumed, the amount of oxygen used in the oxidation may be calculated using
the following chemical equation

K2Cr2O7(aq)  + 4 H2SO­4  ?  K2
SO­4 (aq)  + Cr2 (SO­4)3(aq)
+ 4 H2O + 3 (O) aq

    The results are expressed in terms of
amount of oxygen in ppm that would require to oxidise the contaminants. This is
called COD. Thus, generally, the COD value is higher that BOD for the same
liquid waste water. The oxygen equivalent of K2Cr2O7   consumed
is taken as a measure of the COD.

                                                  
1ml of 1N   K2Cr2O7 =  0.008 g oxygen.

The
results are expressed in terms of amount of oxygen in ppm that would be
required to oxidise the contaminants. This is called COD. Thus generally, the
COD value is higher than BOD for the same liquid waste water.