DISCUSSION
To measure oxygen demand, biochemical oxygen demand (BOD) relies on bacteria to
oxidize readily available organic matter during a five-day incubation period. COD
uses strong chemicals to oxidize organic matter. Generally, COD is preferred to BOD
for process control measurements because results are more reproducible and are
available in just two hours rather than five days. By the time you have the results from
a five day test, the plant conditions are no longer the same, so real time monitor and
control cannot be relied upon by the use of BOD. COD is a quick and easy
measurement to get a snap in time picture of what is going on in the system and with
trending, long term predictions can be made and monitor and control of the process at
the wastewater treatment plant can be optimized and controlled.
In this experiment we have used Potassium dichromate as an oxidizing agent under
acidic conditions. (Acidity is usually achieved by the addition of sulphuric acid.) Most
commonly, a 0.25 N solution of potassium dichromate is used for COD
determination. In the process of oxidizing the organic substances found in the water
sample, potassium dichromate is reduced (since in all redox reactions, one reagent is
oxidized and the other is reduced), forming Cr 3+. The amount of Cr 3+ is determined
after oxidization is complete, and is used as an indirect measure of the organic
contents of the water sample.
Because COD measures the oxygen demand of organic compounds in a sample of
water, it is important that no outside organic material be accidentally added to the
sample to be measured. To control for this, a so-called blank sample is required in the
determination of COD. A blank sample is created by adding all reagents (e.g. acid and
oxidizing agent) to a volume of distilled water. COD is measured for both the water
and blank samples, and the two are compared. The oxygen demand in the blank
sample is subtracted from the COD for the original sample to ensure a true
measurement of organic matter.
Based on the experiment result that we have got, there is a big difference between
experimental COD value with the theoretical value founds in the domestic waste
water. The theoretical value of COD for domestic waste water showed in the table 1.
Based on the COD value (5299.2 mg/L) by referring to the table we can see that our
COD value is out of range.
Source Normal COD range, mg/L
Plant influent 300 - 700
Primary effluent 200 - 400
Trickling filter effluent 45 - 130
Activated sludge effluent 30 - 70
Advanced waste treatment effluent
5 - 15
Table 1
In this experiment, the errors may conveniently divide into several types which are:
a. Errors in taking the waste water sample
The samples must representative with respect to both space and time, and the
particular characteristics to be analysed must not change during the time
between sampling and analysis. Unless the water stream is of a constant
homogenous composition and flow rate then the only way to ensure a sample
which truly representative, both spatially and temporally is to use sampling
technique on the whole stream. Problem occurs in obtaining a spatially
representative sample when two or more phases present for example
immiscible liquids and suspended solid
b. Errors in obtaining an aliquot of the sample
Having obtained a sample which is representative of water stream and having
preserved it successfully, it is necessary to obtain a representative aliquot of
the sample to be used in the actual chemical analysis. The problems arising
here are those of the presence of two or more phases. There is a problem when
conducting a sample which immiscible liquids are present. To overcome these
problems we should blending the sample effectively and care must be taken to
ensure the samples are not contaminated.
c. Errors in procedure
There are several errors in procedure when we conducting this experiment
which can affect our data recorded:
i. The volume of potassium dichromate solution used is insufficient due to the error when taking data at the measuring cylinder
ii. Sulphuric acid does not mixing properly with the waste water sample
iii. The mixture does not cooling properly after 1 hour boiling
To overcome these problems we must to:
i. Ensure all the apparatus are in the proper condition before conducting
the experiment
ii. Ensure the recorder’s eye must be perpendicular to the solution lines
iii. Ensure all the mixtures are mixing properly according to the guidelines
and the standard given.
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