Environmental Engineering Reference
In-Depth Information
and is free from toxins, the results of a COD test provide a good estimate of the
BOD. One disadvantage of the method is that dichromate can oxidize materials that
would not ordinarily be oxidized in nature. Therefore, the COD test is unable to
differentiate between biologically oxidizable and biologically inert organic matter.
The COD test can also generate a large volume of liquid hazardous waste (acid,
chromium, silver, and mercury).
During COD measurement, the water sample is refluxed with excess of
potassium dichromate (K
2
Cr
2
O
7
) in concentrated sulfuric acid for 2 h. The
reaction involved in the usual case, where organic nitrogen is all in a reduced state
(oxidation number of 3), may be represented in a general way as follows:
C
n
H
a
O
b
N
c
þdCr
2
O
2
7
þð8dþcÞH
þ
! nCO
2
þðaþ8d3cÞ=
2H
2
O
þcNH
4
þ2dCr
3þ
ð6
:
15Þ
where d ¼ 2n
2. The reaction requires strong acidic conditions
and elevated temperature. As shown in Eq. 6.15, organic matter is converted to CO
2
and water, organic nitrogen in a reduced state will be converted to NH
4
, whereas
higher oxidation states will be converted to nitrates. After dichromate digestion is
complete, the excess of dichromate is titrated with ferrous ammonium sulfate
(FeðNH
4
Þ
2
ðSO
4
Þ
2
):
6Fe
2þ
þCr
2
O
2
7
þ14H
þ
! 6Fe
3þ
þ2Cr
3þ
þ7H
2
O
=
3þa
=
6b
=
3c
=
ð6
:
16Þ
The indicator used for the above equation is a chelating agent 1,10-phenanthroline
(ferroin). When all Cr
2
O
2
7
is reduced, ferrous ions (Fe
2þ
) react with ferroin to form
a red-colored complex:
Fe
2þ
þ3C
12
H
8
N
2
! FefC
12
H
8
N
2
g
3
ðredÞ
17Þ
Note that Cr
2
O
2
7
has a yellow to orange brown color depending on the con-
centration, and Cr
3þ
has a blue to green color. So the color of the solution during the
titration starts with an orange brown, and then a sharp change from blue-green to
reddish brown, which corresponds to the color of Cr
2
O
2
7
,Cr
3þ
, and FefC
12
H
8
N
2
g
3
,
respectively. Calculation of COD is made using the following formula:
ð6
:
N ðV
0
VÞ8000
V
s
COD ðmg
=
LÞ¼
ð6
:
18Þ
where N is the normality of Fe(NH
4
)
2
(SO
4
)
2
, V
s
is the water sample volume (mL),
and V
o
and V are the volumes (mL) of Fe(NH
4
)
2
(SO
4
)
2
titrated for the blank and the
sample, respectively. Note that the same conversion factor of 8,000 is shown above
as previously described in Eq. 6.13 for DO. In Eq. 6.16, the net electron loss per mol
Fe
2þ
is 1 mol.
6.3.5 Oil and Grease in Water and Wastewater
The term ''oil'' represents a wide variety of substances from low to high molecular
weight hydrocarbons found in petroleum to light gasoline, heavy fuel, and
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