Biochemical oxygen demand (BOD)
The biological oxygen demand (BOD) is also
called biochemical oxygen demand. The BOD refers to the amount of oxygen
required for the biotic degradation of organic matter in bodies of water.
The BOD is a
pollution parameter mainly to assess the quality of effluent or wastewater.
Untreated wastewater has usually a high oxygen demand. Industrial wastewater
tends to a higher chemical oxygen demand.
COD caused by dissolved chemicals or pollutants from washing processes which adds
more pollutants in the water. Both BOD or COD requires a lot of dissolved
oxygen from the water. The significance of BOD is very clear. If oxygen is used
from organic impurities, it can’t be used by other creatures living in the
water.
Drinking water
is tested as well to check, if there is any organic matter present. To evaluate
drinking water, the TOC - Total Organic Carbon is measured, instead of the
biochemical oxygen demand. This TOC method is faster and can be done online,
whereas the method to measure the biochemical oxygen demand takes 5 days to get
results.
The biochemical
decomposition of organic substrates is carried out by microorganism. For this
work the bacteria need energy. Aerobic bacteria, that do this work, need
dissolved oxygen to produce energy. This oxygen is consumed in this process and
the amount of dissolved oxygen in the water gets less. If there is a lot of
organic material present in the water, the oxygen demand is correspondingly
high to carry out the decomposition. This oxygen is then lacking the plants and
animals that also live in this water.
A low
biochemical oxygen demand ensures that the dissolved oxygen is not only
consumed by organisms from the wastewater. But that there is enough oxygen left
for fish and plants.
To perform BOD
analysis there are some standards are set.
like to store
sample for 5 days at 20*C.
There are two
methods to measure the BOD level. Both methods are empirical tests.
Method I: A sample of the water is kept at a constant
temperature of 20°C in the dark. After a period of five days, the oxygen
content is measured. In comparison to the original value, the oxygen
consumption during the measurement period indicates the oxygen demand in the
water.
Method II: If a very high BOD is to be expected or if other toxic
or inhibitory substances are present in the water, the sample can be diluted at
the beginning. In this way it can be prevented that too little oxygen is
present to break down the organic substances.
After the 5th
day the remaining dissolved oxygen in the water sample is measured. With this
oxygen the BOD level can be calculated.
Drinking water
should have after 5 days a BOD of well below 1 mg/l. Acceptable wastewater from
a sewage treatment plant should have a BOD of around 20 mg/l.
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