Discussion Cod Updated

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.