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A
Presentationon
BOD, COD, TOC
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Water quality parameters
Indicators of organic contamination :
Dissolved Oxygen (DO)
Biological Oxygen Demand (BOD)
Chemical Oxygen Demand (COD)
Total Organic Carbon (TOC)
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Oxygen Demand
Oxygen is demanded in waste water forthe oxidation of both inorganic as wellas organic matter.
Oxygen is essential for the livelihood ofmicro organisms.
Measured in several ways BOD - Biochemical Oxygen Demand
COD - Chemical Oxygen Demand
TOC Total Organic Carbon
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Dissolved Oxygen (DO)
Dissolved Oxygen (DO) The amount of freeoxygen dissolved in water, expressed in
milligrams per liter, parts per million, or
percent of saturation.
Oxygen is essential for maintaining
ecosystem in a balanced state.
Solubility of DO in waste water is only
95% of that in Distilled water.
Indicates of freshness of sewage.
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Dissolved Oxygen is necessary for the
life of aquatic organisms.
For prevention of offensive odors.
The ideal dissolved oxygen level for fish isbetween 7 and 9 milligrams per liter
(mg/L).
DO depends upon factors solubility of
oxygen, temperature, partial pressure of
oxygen & purity of water
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Dissolved Oxygen levels (DO)
Animals and plants grow and
reproduce unimpaired when :
DO levels > 5 mg/L.
Living organisms become stressed
at DO levels 3 ~ 5 mg/L.
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Dissolved Oxygen levels (DO)
At levels under 3 mg/L, a conditionknown as hypoxia prevails: many species
move elsewhere and non -mobile species may
die.
Anoxia, occurs when the water
becomes totally depleted of oxygen (less than
0.5 mg/L; anaerobic conditions)
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Biochemical Oxygen Demand (BOD)
Defined as the oxygen required by micro
organism for biological decomposition of organic
matter under aerobic conditions at standard
temperature.
The measurement of BOD is used to
determine the level of organic pollution of a
sewage .
The greater the BOD, the greater the degree
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BOD: A Bioassay
Briefly, the BOD test employs abacterial seed to catalyze theoxidation of 300 mL of full-
strength or diluted wastewater.The strength of the un-dilutedwastewater is then determinedfrom the dilution factor and the
difference between the initial D.O.and the final D.O. BOD
Bottle
BOD DO DOt i f
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BOD with dilution
ti f
s
b
BOD =DO - DO
V
V
WhereBODt = biochemical oxygen demand at t days, [mg/L]DOi = initial dissolved oxygen in the sample bottle,
[mg/L]DOf = final dissolved oxygen in the sample bottle,[mg/L]Vb = bottle volume, usually 300 or 250 mL, [mL]V
s
= sample volume, [mL]
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BOD - Oxygen Consumption
CBOD
NBOD
y
or
BOD(mg/L)
Time
L=oxidizable carbonaceous material remaining to be oxidized
BOD y L Lt t o t
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NBODNitrogenous BOD (NBOD)
NH O NO H O H Nitrosomonas
3 2 2 215 .
NO O NONitrobacter2 2 31
2
Like CBOD, the NBOD can be modeled as a simple 1st orderdecay:
dL
dt k L
N
N
N
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BOD Modeling
"L" is modeled as a simple 1st order decay: dL
dtk L 1
L L eok t
1Which leads to:
We get: BOD y L et t ok t
( )1 1
BOD y L Lt t o t And combining with:
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BOD - loss of biodegradable organicmatter (oxygen demand)
Lo
Lt
LorBO
Dremaining
Time
Lo-Lt = BODt
BOD
Bottle
BOD
Bottle
BOD
BottleBOD
Bottle
BOD
Bottle
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Limitations
1. BOD test measures only biodegradable organics.
2. Long period of time is required to obtain
results.
3. High concentration of active bacteria is
required.
4. Pretreatment is needed if the samples containstoxic wastes.
5. The effect of nitrifying bacteria should be
reduced by pretreatment.
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Chemical Oxygen Demand
A chemical measure of the amount
of oxygen required to oxidize organic
substances in water or wastewater. A strong
oxidizing agent (+acid and heat) are used to
oxidize all the carbon compounds in the
waste water sample.
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COD: A chemical test
The chemical oxygen
demand (COD) of a waste is
measured in terms of the
amount of potassium
dichromate (K2Cr2O7)
reduced by the sample
during 2 hr of reflux in amedium of boiling, 50%
H2SO4 and in the presence of
a Ag2SO4 catalyst.
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CODThe stoichiometry of the reaction between dichromate
and organic matter is:
CaHbOc + Cr2O7-2 + H+ Cr+3 + CO2 + H2O
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BOD v/s COD
COD test is faster than BOD analysis: usedfor quick assessment of wastewaterstrength and treatment performance
Like the BOD, it does not measure oxidantdemand due to nitrogeneous species
It does not distinguish between
biodegradable and non-biodegradableorganic matter. As a result COD's arealways higher than BOD's.
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TOC
Total organic carbon analysis is a determination oforganic carbon in a sample regardless of its
oxidation state or biodegradability.
Applicable to small concentration of organic
matter. It can be performed very rapidly & its becoming
more popular
The oxidation process is based on the following
stoichiometry:
C H N O ab d
O aCOb
H Oc
Na b c d ( )
4 2 2 22 2 2 2
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Test:
1. Measured with TOC Analyzer.
2. Injecting of sample into high temperature
furnace.
3. Oxidation of organic carbon into CO2 in
the presence of catalyst.
4. CO2
measured by Infrared Analyzer.
5. To eliminate errors due to inorganic
carbon, sample is acidified & aerated.
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TOC - Pyrolysis Instrument
CO2 Detector Recorder
Syringe
O2
Condensor
Furnace
Sample Inlet
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TOC - UV/per sulfate Instrument
CO2 Detector Recorder
Syringe
O2
Condensor
Sample
Inlet
Persulfate
Solution
UV Reactor
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TOC - The CO2 Detector
DemodulatorAmplifier
Sensing
Cell
Sample
Reference
In Out
Chopper
IR
Source
A non-dispersive infra-red analyzer (NDIR)
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Results:
Typical values for Domestic waste:
BOD - 250-350 mg/l
COD - 200-500 mg/lTOC - 100-300 mg/l
BOD5/TOC ratio= 1 to 1.6COD/BOD5 ratio= 1.25 to 2.5
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