The document discusses dissolved oxygen (DO), chemical oxygen demand (COD), and biological oxygen demand (BOD) as key indicators of water quality and pollution levels. It details methods for measuring these parameters, including the Winkler method for DO and the use of potassium dichromate for COD. Additionally, it outlines the significance of BOD in assessing organic pollution in water and provides standard values for evaluating water quality.

1. DEMAND
ANALYSIS
BY,
J.SAM

2. DISSOLVED OXYGEN (DO):
• Amount of oxygen dissolved (and hence available to sustain
marine life) in a body of water such as lake, river or stream .
• Dissolved oxygen is the indicator of water pollution.
• DO levels in natural and waste water depend on physical,
chemical and biological activities in water body.
• Amounts of oxygen that can be dissolved in water by
temperature.
• It is usually determined by Winkler’s method known as
iodometric method.
• Method was developed by L.W.Winkler , a hungarian
chemist in 1888 .

3. The Winkler method with azide modification:
 Alsterberg azide modification.
 Rideal stewart modification .
 Alum flocculation modification .
 Copper sulfate sulfanic acid flocculation
modification .
 Alkaline hypochlorite modification .

4. Principle :
 The basic principle of this method is that DO reacts
with manganese ions to form manganesedioxide
which further reacts with I¯ (Iodine ion) to liberate I2
in an amount chemically equivalent to the original
dissolved oxygen .
Mn⁺² + ½ O2 +2OH¯ → MnO2 ↓ +H2O
MnO2 +2I¯ + 4H⁺ → Mn⁺² + I2 + 2H2O
This liberated iodine is titrated with sodium thiosulphate
(Na2S2O3) .
I2 + 2S2O3²¯ → 2I¯ + S206²¯

5.  Manganese sulphate
 Alkali iodide – azide reagent
 Sulphuric acid
 Starch indicator
REAGENT :
 Standard sodium thiosulphate 0.025N

6. PROCEDURE :

7. CALCULATION :
If the whole content is used for titration,
DO mg/l = V₁ × N × 1000/V2 – V3
When only a part of the contents have been used ,
DO mg/l = V1 × N × 8 × 1000 / V4 (V2 – V3 )/V2
Where, V1= volume of titrant (sodium thiosulphate)
V2= Volume of sampling bottle
V3= Volume of MnSO4 and KI solutions added
V4= Volume of the part of the contents titrated
N= Normality of sodium thiosulphate (0.025N)

8. DO LEVEL :
Water quality DO LEVEL
Good quality(20°C sea level 8-9 ppm O2
Moderate pollution 4-5 ppm O2
Highly polluted <4.5 ppm O2

9. CHEMICAL OXYGEN DEMAND (COD ) :
• COD is an empiric laboratory assay which measured the
amount of organic matter contained in a water sample
with the help of chemical oxidizing agents such as
potassium dichromate and potassium permanganate .
 COD is a means of measuring all organic matter
(biodegradable and non- biodegradable ) .
 This test is useful in studying pollution strength of
domestic and wastewater .
 COD values are greater than BOD values .
 The open reflux method is suitable for a wide range
of wastes is discussed here .

10. PRINCIPLE :
 The organic matter gets oxidized
completely by K2CR2O7 and silver sulphate
catalyst in the presence of H2SO4 to
produce CO2 +H2O . The excess K2CR2O7
remaining after the reaction is titrated
with Fe(NH4)2 (SO4)2. The dichromate
consumed gives the O2 required for
oxidation of the organic matter .
3(CH2O) +16H⁺ + Cr2O7²¯ → 4Cr³⁺ + 3CO2 + 11H2O

11. REAGENTS :
Standard potassium dichromate 0.250N
Sulphuric acid reagent
Standard ferrous ammonium sulphate 0.1 N
Ferroin indicator
Mercuric sulphate
Potassium hydrogen phthalte (KHP) standard

12. Procedure :

13. Calculation :
COD mg/l = ( B – A ) × N × 1000 / ml sample
Where ,
A = volume of FAS used for sample ( in ml )
B = volume of FAS used to blank ( in ml )
N = normality of FAS

14. COD LEVEL :
0 – 5 mg / l indicates very clean stream
Chemical oxygen demand , ( 5 days at -20°C ) max :
Parameter standards
COD
Inland surface water : 250 mg/l
Marine coastal area : 250 mg/l

15. BIOLOGICAL OXYGEN DEMAND (BOD) :
 BOD is the amount of dissolved oxygen required to
breakdown the organic material of the sample by
aerobic biological microorganisms at certain
temperature over a specific period of time .
 This parameter is determined with the consumption
of oxygen by microorganisms during 5 days of
incubation at 20°C .
{CH2O} + O2 → CO2 + H2O

16.  BOD indicates a amount of pollution of water bodies.
 Lower BOD indicates the water is a good quality for
aquatic life or little aerobic acitivity whereas the
higher BOD indicates the water is highly polluted.
 When BOD is higher , then DO becomes lower .
 so, aquatic organisms are unable to survive at
normal rate , it becomes stressed , asphyxiate and
may die .

17. Determination of BOD :
 The most common and popular method for
determination of biological oxygen demand is
standard method which is recognized by U.S . EPA .
 It is not accurate quantitative test , although it
is widely used as an indicates the amount of
pollution .

18. Procedure :
• Take 2 samples of water .
• Record the DO level (ppm) of one immediately using
the method described in the dissolved oxygen test .
• Place the second water sample in an incubator in
complete darkness at 20°C for 5 days . If you don’t have
an incubator, wrap the water sample bottle in aluminium
foil or black electrical tape and store in a dark place at
room temperature (20°C or 68°F) .
• After 5 days, take another dissolved oxygen reading
(ppm) using the dissolved oxygen test kit .
• Subtract the day 5 reading from the day 1 reading to
determine the BOD result in ppm.

19. CALCULATION :
 The difference between the two DO levels
represents the amount of oxygen required for the
decomposition of any organic material in the
sample .
BOD mg/l = D₁ - D₂ / V
Where, D₁ = initial DO
D₂ = final DO
V = volume of sample

20. BOD LEVEL :
BOD LEVEL (mg/l) Remarks
1mg/l very clean
2mg/l clean
3mg/l fairly
5mg/l doubtful
10mg/l contaminated
BOD of some collected samples from different sources:
• Normal water = 0-3
• River water = 5-20
• Sewerage water = 50-100
• Industrial water = 150-1000
• Land for irrigation = 100
• Marine coastal area = 100
• Inland surface water = 30
• Public sewers = 350

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