2. COD
The chemical oxygen demand (COD) test is commonly used
to indirectly measure the amount of organic and inorganic
compounds in water.
It is expressed in milligrams per liter (mg/L) or in parts per
million (ppm), which indicates the mass of oxygen
consumed per liter of solution.
Potassium dichromate (K2Cr2O7) has been shown to be the
most effective: it is relatively cheap, easy to purify, and is
able to nearly completely oxidize almost all organic
compounds.
The chemical oxygen demand test procedure is based on
the chemical decomposition of organic and inorganic
contaminants, dissolved or suspended in water.
3. There are many different COD tests. Perhaps, the most
common is the 4-hour COD.
The result of a chemical oxygen demand test indicates the
amount of water-dissolved oxygen (expressed as ppm or
milligrams/liter of water) consumed by the contaminants,
during two to four hours of decomposition from a solution
of boiling potassium dichromate.
The higher the chemical oxygen demand, the higher the
amount of pollution in the test sample. For the
contaminants that can be oxidized biologically, the
biological oxygen demand (BOD) method is used.
4. BOD
Biochemical oxygen demand or B.O.D is the amount of
dissolved oxygen needed by aerobic biological organisms in
a body of water to break down organic material present in
a given water sample at certain temperature over a specific
time period.
The BOD value is most commonly expressed in milligrams
of oxygen consumed per litre of sample during 5 days of
incubation at 20°C.
BOD can be used as a gauge of the effectiveness of
wastewater treatment plants.
A commonly recognized method for the measurement of
BOD.
5. Dilution method
To ensure that all other conditions are equal a very small
amount of micro-organism seed is added to each sample
being tested. This seed is typically generated by diluting
organisms with buffered dilution water.
The BOD test is carried out by diluting the sample with
oxygen saturated dilution water, injecting it with a fixed
aliquot of seed, measuring the dissolved oxygen (DO) and
then sealing the sample to prevent further oxygen dissolving
in.
The sample is kept at 20 °C in the dark to prevent
photosynthesis (and thereby the addition of oxygen) for five
days, and the dissolved oxygen is measured again. The
difference between the final DO and initial DO is the BOD.
6. THE BOD5 TEST
The so-called 5-day BOD measures the amount of
oxygen consumed by biochemical oxidation of waste
contaminants in a 5-day period.
The total amount of oxygen consumed when the
biochemical reaction is allowed to proceed to
completion is called the Ultimate BOD.
The Ultimate BOD is too time consuming, so the 5-day
BOD has almost universally been adopted as a measure
of relative pollution effect.
7. X DAVID HAYES · LA TROBE UNIVERSITY
These usually depend on whether a substrate (C, N, P)
for utilisation by wastewater microbial populations is
soluble or insoluble and determines the structure of
microbial communities (aerobic, anaerobic,
chemotrophs, heterotrophs, etc).
BOD was originally used in 19th Century Britian and is
often reported as the BOD5. The '5' was used as no
stream in Britian took more than 5 days to reach the
ocean. It is a reliable indicator of the degree of
purification wastewater has undergone.
Nevertheless, BOD/COD and BOD measured over longer
periods than 5 days provide a more accurate and precise
measure of the succes of the treatment process.
8. DISSOLVED OXYGEN (DO) AND WATER QUALITY
Dissolved oxygen (DO) refers to the
amount of oxygen dissolved in water.
This dissolved oxygen is not the same
as the oxygen in the water molecule.
Dissolved oxygen is present in all
rainwater and surface water supplies
due to contact with the atmosphere.
Oxygen enters the water as rooted
aquatic plants and algae undergo
photosynthesis, and as oxygen is
transferred across the air-water
interface.
9. … DISSOLVED OXYGEN (DO) AND WATER QUALITY …
A high DO level in a community water supply is good
because it makes drinking water taste better.
However, high DO levels speed up corrosion in water
pipes. For this reason, industries use water with the
least possible amount of dissolved oxygen.
Water used in very low pressure boilers have no more
than 2.0 ppm of DO, but most boiler plant operators try
to keep oxygen levels to 0.007 ppm or less
10. … DISSOLVED OXYGEN (DO) AND WATER QUALITY …
Oxygen comprises approximately 21% of the total gas in the
atmosphere; however, it is much less available in water.
The amount of oxygen water can hold depends upon:
temperature (more oxygen can be dissolved in colder
water),
salinity (more oxygen can be dissolved in water of lower
salinity), and
pressure (more oxygen can be dissolved in water at greater
pressure).
11. EXAMPLE…
For example, fish like trout are sensitive to low DO levels
(less than eight parts per million) and cannot survive in
warm, slow-moving streams or rivers. Decay of organic
material in water caused by either chemical processes or
microbial action on untreated sewage or dead vegetation
can severely reduce dissolved oxygen concentration. This is
the most common cause of fish kills, especially in summer
months when warm water holds less oxygen anyway.
Many lakes and ponds have
anoxic (oxygen deficient)
bottom layers in the summer
because of decomposition
processes depleting the oxygen.
The amount of dissolved oxygen
often determines the number
and types of organisms living in
that body of water.
12. DIFFERENCES BETWEEN BIOLOGICAL OXYGEN DEMAND
AND CHEMICAL OXYGEN DEMAND?
Biological Oxygen demand is the oxygen required to
oxidize only organic matter by micro organisms (as
actual process happening in treatment plants) while,
Chemical oxygen demand is the oxygen consumed to
oxidize all organic and inorganic matter and it employ
chemicals to do this process that why it is way more fast
then BOD experiment .
13. BOD VS. COD
BOD is similar in function to COD, in that both measure
the amount of organic compounds in water.
However, COD is less specific, since it measures
everything that can be chemically oxidized, rather than
just levels of biologically active organic matter.
The BOD test measures the oxygen demand of
biodegradable pollutants, whereas,
The COD test measures the oxygen demand of bio-
gradable pollutants plus the oxygen demand of non-
biodegradable oxidizable pollutants.
14. RELATIONSHIP BETWEEN THE COD AND BOD VALUES
IN WASTE WATER?
COD values are always higher than the BOD values.
Because COD includes both biodegradable and non-
biodegradable substances whereas BOD contains only
bio-degradable.
15. COD is based on chemical oxidation (previously with
dichromate). The test takes and few hours. Ammonia is
not oxidised.BOD is the biological oxidation of carbon
and ammonia and organic nitrogen species in the
wastewater.
High BOD/COD ratio indicates that toxicity is less. Low
ratio indicate that toxicity is more. BOD indicates that
wastewater has biodegradable content and so
difference means non-biodegradable or toxic
components are more
16. COD or Chemical Oxygen Demand is the total
measurement of all chemicals (organics & in-organics) in
the water / waste water;
BOD is a measure of, the amount of oxygen that require
for the bacteria to degrade the organic components
present in water / waste water.
17. Almost all treatment plants are required to measure one
of these three items as a measure of the pollution value
in the water.
COD or Chemical Oxygen Demand is the total
measurement of all chemicals in the water that can be
oxidized.
TOC or Total Organic Carbon is the measurement of
organic carbons.
BOD- Biochemical Oxygen Demand is supposed to
measure the amount of food (or organic carbons) that
bacteria can oxidize.
COD should always measure higher than TOC and then
BOD.
18. TURBIDITY
Turbidity appears as cloudiness or haziness of a fluid
caused by individual particles (suspended solids) that
are generally invisible to the naked eye, similar to smoke
in air.
It is a measure of the degree to which the water loses its
transparency due to the presence of suspended
particulates.
The more total suspended solids in the water, the
murkier it seems and the higher the turbidity.
Turbidity is considered as a good measure of the quality
of water.
19.
20.
21. Turbidity is measured in NTU:
Nephelometric Turbidity Units.
The instrument used for measuring it is called
nephelometer or turbidimeter, which measures the
intensity of light scattered at 90 degrees as a beam of
light passes through a water sample.
25. Total suspended solids is a water quality measurement
usually abbreviated TSS.
26. Filters for TSS measurements are typically composed of glass
fibres
TSS in mg/L can be calculated as:
[(dry weight of residue and filter - dry weight of filter)/
ml of sample] x 1,000,000
