University of Chicago
Type of paper: Thesis/Dissertation Chapter
Measuring the BOD and COD Value, and Calculation of the BOD/COD Ratio
Before discussing the results of the BOD and COD tests carried out, it is vital to know the different between both tests. The BOD is the measure of the dissolved oxygen which is done with a DO meter. Dissolved oxygen (DO) is the amount of oxygen present in the water sample. Whereas BOD is a calculation and it is the amount of oxygen that would be consumed in 5days by the bacteria. However in this experiment, BOD is the difference of the dissolved oxygen after 7 days.
COD on the other hand measures the microorganisms’ chemical oxidation instead of the biochemical oxidation like BOD. So this is the reason for the tenuous relationship between COD and BOD. These tests do not have a direct relationship but one can be obtained empirically by performing the test a number of times. Keeping this in mind we can now analyze the results obtained in this lab which are 136. 2 mg/L and 365. 2 mg/L for the BOD7 and COD values respectively.
The BOD/COD ratio is 0. 373. Knowing that the BOD/COD wastewater ratio should be approximately 0. , the percent yield obtained is 75% so there are definitely different sources of errors that affected the ratio which will be mentioned and elaborated on in this discussion. Additionally, this ratio makes it possible to presume that the tested sample is from industrial wastewater. This wastewater can be cleaned by making it undergo chemical treatment. This ratio can be categorized in the acceptable zone hence primary treatment would be the best way of treating this water sample. One of the conditions for the BOD test is that it has to be incubated in the dark to avoid photosynthesis.
In other words, to avoid algae growth; this will consume the oxygen in the water and as a result increase the BOD values. This test must also be carried out at constant temperature, at 20? C being that this test consists of microorganisms whose activity has a direct relationship with temperature increase. This part of the lab was taken care of by the lab technician. Obtaining accurate results, in this lab, is difficult because many factors affect the readings in this test however they were minimized as much as possible.
Two of these factors are the interferences. They can be categorized in two main parts; things that consume the oxygen hence contribute to the BOD and things that affect the growth of the microorganisms such as toxic materials. Two very important interferences that will contribute to the BOD are Nitrification and Carbonaceous BOD. Nitrification is a nitrifying bacteria which will oxidize the nitrogen, consume oxygen and in turn, increase the BOD value beyond what is desired for the Organic curve. This is called the NBOD.
So to prevent this, inhibitors were added to prevent the growth of the nitrifying bacteria. This is another reason why the BOD test is stopped after 5 days because the nitrifying bacteria are really slow and start consuming the oxygen from 5 days until day 8. See Graph 1 in Appendix. Although in this lab, the BOD7 test was done instead of the BOD5 the inhibitors would have prevent the growth of the nitrifying bacteria. The second interference is the inorganic reduction agent such as the first ion which reacts with oxygen and falsely increases the measured BOD.
In general, the test for these high concentrations of ions are done separately and deducted from the BOD measurements. In this case, however this isn’t done. Knowing that this synthetic wastewater should have a ratio of about 0. 5, another source of error is that once the sample is obtained the test must be done immediately if not the sample must be kept in the fridge at 4? C for a maximum of 6 hours if not the test can no longer be performed. However, in our lab, that part was left to our lab technician.
Another common source of error is to avoid air bubbles inside the BOD bottles. So it is important to fill the bottles to the top making sure that there is no air bubble. A way of avoiding air bubbles is filling the bottles to the side. If there are air bubbles in the bottle, leave it open for 5 minutes to get rid of the air bubbles if it didn’t work hit the bottle were the bubbles are. Shaking the bottle is not an effective way. To prevent air from coming in, the BOD bottles are designed in a way that water can be added at the top of the lid.
Another major source of error for both the BOD and COD values must have been while pipetting the samples. Additionally, different teams calculated the different standards hence this add a major uncertainty to the absorbance. The seed is a group of microorganisms. Usually the seed is added in case the sample does not have enough microorganisms or organic bacteria. However, in wastewater there are more than enough bacteria so there is no need for a seed. In this lab, since the water sample was synthetic wastewater, a seed had to be added.