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.