Title: BOD:COD Ratio of Natural and Synthetic Chemicals
Aims: To show the necessity of making dilutions for samples of unknown BOD..
To show that organic chemicals can exert a BOD and also have bactericidal and toxic
effects at high concentrations.
To calculate the BOD:COD ratio of various samples.
Introduction: BOD determination is an empirical test in which standardised
laboratory procedures are used to determine the relative oxygen requirements of
wastewater effluents and polluted waters. The test measures the oxygen required for
the biochemical degradation of organic material. It may also measure the oxygen used
to oxidise reduced forms of nitrogen unless an inhibitor prevents their oxidation. The
BOD was originally developed to stimulate conditions that occurred following the
discharge of an organic effluent into a river. The test now has wider applications and
in this practical the BOD’s of various organic chemicals will be determined
Organic chemicals when in concentrated form can exert a considerable oxygen
demand. For this reason dilutions of the samples need to be carried out. If no dilution
of the samples took place then all the dissolved oxygen could be used up during the
five day incubation period. The samples can be diluted in standard dilution water.
This dilution water contains essential nutrients for microbial activity which are
necessary for the breakdown of the sample.
It is necessary to have a population of microorganisms present in the sample that are
capable of oxidising the biodegradable matter present. Adding a population of
microorganisms to the dilution water can seed samples that do not contain a sufficient
microbial population. The seed to be used in this practical will be obtained from a
local sewage treatment plant. In this practical the BOD of coke will be determined.
The expected BOD of coke lies between 60,000 - 90,000. The sample will need to be
diluted before incubation.
A COD determination will also be carried out on the coke sample. The COD test
measures the oxygen equivalent of that portion of the organic matter in a sample that
is susceptible to oxidation by a strong chemical oxidant. The COD test is linked to
the BOD test, but its major advantage is that it gives more rapid results. Because
nearly all organics are oxidised in the COD test and only some are decomposed during
the BOD test, COD values are usually greater than BOD values
Materials:
Winkler bottles, glass stoppered
Various pipetts
Volumetrics,4
Dissolved Oxygen meter
Dilution water
Seed material
Method:
Dilution water preparation - as per manual
Seeding of dilution water - as per manual
Sample dilution and preparation
estimated BOD of coke sample 60,000 - 90,000
sample must be diluted to give BOD of between 2 - 8 mg/l D.O
therefore dilutions carried out 1/100 1/10.000
RESULTS:
1/20.000
BOD RESULTS:
Sample used - coke
Dilution
DO o
DO 5
Difference
BOD 5 mg/l d.o
Blank
8.7
7.9
0.8
0.8
1/100
8.9
0.0
8.9
0.0
1/10.000
8.9
2.4
6.5
57,000
1/20.000
8.8
4.3
4.5
74,000
Calculation:
BOD = (DO.o - DO. 5 sample) - (DO.o - DO. 5 blank) * Dilution factor
BOD =
(8.8 - 4.5)
(8.7 - 7.9)
* 20,000 = 74,000
Average BOD = 65,000 mg/l
COD RESULTS:
Dilutions
COD mg/l
1/250
118,000
1/500
124,000
1/1000
117,000
Average COD = 119,667 mg/l
BOD:COD Ratio = 1:1.82
CLASS RESULTS: BODS, BLANKS & EXPECTED RANGES
SAMPLE
BOD mg/l D.O
BLANK BOD mg/l
RANGES
MILK
116,000
0.8
40,000 - 120,000
PLAX
897,000
---
>600,000
COKE
--------
5.3
60,000 - 90,000
PLAX
170,000
2.7
>600,000
ANTIFREEZE
220,000
0.0
0.5 - 2,000,000
COKE
---------
1.8
60,000 - 90,000
ETHANOL
---------
1.2
0.6 - 1,500,000
ETHANOL
1,600,000
1.8
0.6 - 1,500,000
TOOTHPASTE
51,000
1.2
10,000 - 100,000
ANTIFREEZE
---------
2.0
0.5 - 2,000,000
FORMALDEHYD
255,000
3.3
1.5 - 2,000,000
38,000
2.7
60,000 - 90,000
E
COKE
CLASS RESULTS FOR BOD:COD RATIO
SAMPLE
BOD
COD
BOD:COD RATIO
Coke
65,000
119,667
1:1.82
Milk
116,000
212,250
1:1.83
Plax
1,500,000
295,000
-------
Coke
---------
973,333
-------
Plax
170,000
340,000
1:2
Antifreeze
1,340,000
1,132,500
1:.84
Coke
----------
101,250
------
Ethanol
----------
---------
------
Milk
71,500
151,000
1:2.11
Coke
54,000
112,000
1:2.07
Ethanol
1,600,000
2,240,000
1:1.4
Toothpaste
51,000
18,750
1:.36
Antifreeze
--------
216,000
------
Formaldehyde
255,000
579,200
1:2.27
Discussion: In this practical the BOD of a coke sample was determined. It was
thought that the BOD of the coke sample was somewhere between 60,000 to 90,000
mg/l. Therefore before incubation dilutions needed to be carried out on the sample.
The coke sample was diluted sufficiently so as to have a BOD of between 2 - 8 mg/l
dissolved oxygen. Three different dilutions of the coke sample were carried out, a
1/100, 1/10,000 and a 1/20,000. The samples were diluted in seeded standard dilution
water and the initial dissolved oxygen levels of each were recorded. The samples
were then incubated at 20 c for five days. A blank comprising of the seed and the
dilution water was also incubated under the same conditions as the samples. After the
incubation period the dissolved oxygen levels of the samples and the blank were
measured. The average BOD for the coke sample was determined to be 65,000 mg/l
D.O. The blank had a D.O uptake of 0.8 mg/l which would indicate that the dilution
water did not exert an excessive oxygen demand during the incubation period. It
would also indicate that the correct amount of seed was added to the dilution water.
All of the dissolved oxygen was used up in the 1/100 diluted sample because the
dilution was too small. The small dilution resulted in too much organic matter being
present in the sample for the microoganisms to break down aerobically. The 1/10,00
and the 1/20,000 dilutions did give a BOD and they also fulfilled the criteria necessary
when conducting a BOD test using seeded dilution water which states that at least 2
mg/l of D.O must be used up and there must be at least 1 mg/l D.O remaining after
incubation
A COD determination of the coke sample was also carried out. Dilutions of the
sample also needed to be carried out for this test. High range Hack tubes which
measure up to 1500 mg/l were used. The dilutions used were 1/250, 1/500 and
1/1000. The viles were allowed to reflux for two hours and after cooling thier
absorbance was measured using a Hack meter. The COD readings were multiplied by
their dilution factor and the average COD of the coke sample was found to be 119,667
mg/l. When this value is compared to the BOD for coke a ratio of 1:1.82 is found.
The COD value is greater than that of the BOD because the oxygen in the COD
sample is subjected to more severe oxidising conditions.
In the class results some interesting points can be noted. Firstly the oxygen uptake in
the blanks ranged from 0.8 mg/l O2 to 5.3 mg/l O2. When seeded dilution water is
used, as was the case in this practical the oxygen uptake should be between 0.6 - 1.0
mg/l O2. The high oxygen uptake in some of the blanks might be indicating that there
was organic matter present in the dilution water which exerted an excessive oxygen
demand. It may also be a sign that there was dirty or contaminated glassware used.
However because of the wide variation in the results for the blanks it is more likely
that too much seed was added by some of the groups. If the seed concentration is too
high, it can exert a significant oxygen demand. One of the groups had no oxygen
uptake in their blank. This result is very unlikely and it is possible that no seed was
added to the dilution water. A coke sample was analysed by four groups. The
expected BOD for coke is between 60,000- 90,000. Although the BOD of the sample
worked well for group one, (65,000) two of the other groups were unable to get a
BOD, while the fourth group got a BOD of 38,000 (average). None of these three
groups fulfilled the criteria for BOD analysis, i.e 2 mg/l used up and 1 mg/l remaining.
In one of the groups a 1/60,000 dilution of the coke sample was carried out but still all
the oxygen was used up during incubation. For this to be the case the BOD of the
coke sample would need to be in excess of 250,000 mg/l. This is not correct, it is
likely a mistake was made when making up the dilutions. For the COD an average
value of 119,667 mg/l was found. Two of the other groups had CODs in this range
but one group had a COD of 973,333 mg/l. Once again this result is too high.
Two plax samples were also examined. The BODs of the two groups varied, one got
a BOD of 170,000 mg/l (average), while the other got 879,000 mg/l (average). The
anticipated BOD of plax was greater than 600,000, so from these results it would
appear that the results of the second group are more accurate. The second group got a
COD of 295,000 mg/l which was far lower than their reported BOD of 1,500,000
mg/l. This group didn’t average their BOD results so this is why the result is greater
than the one stated above (879,000). The first group got a COD of 340,000, which
gave a BOD:COD ratio of 1:2.
Two groups tested an ethanol sample. The BOD range for ethanol was expected to be
between 600,000 - 1,500,000. One of the groups did not get any BOD or COD result,
while the other group got a BOD of 1,600,000, and a COD of 2,240,000. This gave a
BOD:COD ratio of 1:1.4. Two groups also examined a milk sample and their results
were very similar. One group examined a toothpaste sample. They got a BOD of
51,000 mg/l (average), and their COD result was18,750 mg/l. This gave a BOD:COD
ratio of 1:.36. The expected BOD for formaldehyde was 1,500,000 - 2,000,00 mg/l.
The group that analysed formaldehyde got a BOD of 255,000 mg/l. This result would
appear to be out by a factor of ten. There was probably an error in the dilutions to
blame for this. The COD for formaldehyde was found to be 579,000 mg/l. This result
gave a BOD:COD ratio of 1:2.27. Two ethanol samples were also examined. One
group failed to get a BOD or a COD. This may be because the initial D.O of the
samples before incubation was low, about 7.5 mg/l. The second group did get a BOD
and a COD result. The initial D.O of the samples was higher, about 8.7 mg/l, so this
probably helped to get a BOD. The BOD:COD ratio was found to be 1:1.4. From an
examination of the class results it can be seen that there was no toxic or bactericidal
effects shown in the samples. This is probably because the dilutions used were too
great to show any effects. For example it known that ethanol in high concentrations is
bactericidal, but in the practical the lowest dilution used was a 1/10,000. This dilution
was not low enough to show the bactericidal effect. From the results it can also be
seen that reproducibility of BODs is very difficult. Some of the results were
misreported. Some groups did not average their BOD and COD results before they
calculated their BOD:COD ratios. This may have been a deliberate attempt to obtain
a BOD:COD ratio in the region of 1:2. Some groups were under the impression that
their ratio should have been 1:2, whereas in reality this ratio applies to sewage
Conclusion: In this practical it was shown that it is necessary to make dilutions of
some samples when determining BOD. It was also shown that many chemicals are
biodegradable and can exert a high BOD. It can also be seen that reproducibility of
BOD results is difficult. BOD:COD ratios were also determined for most of the
samples.