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8/2/2019 Bod Cod Toc_gaman Hitesh

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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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