Environmental Engineering Reference
In-Depth Information
of O 2 . Converting this number of moles into mass of O 2 in mg, this equals to
(N V=4Þ321000 or N V 8; 000, where 32 is the molecular weight of O 2 .
(b) Note the stoichiometric mole ratio is 1:4 (O 2 : S 2 O 3 2 ) and the volume of water sample
withdrawn from 300-mL BOD bottle in the standard method is 200 mL
0 : 025mol S 2 O 3 2
L
1mol O 2
4mol S 2 O 3 2
32 1000mgO 2
1mol O 2
200mL ¼1:0 mgO 2
1
1mL
L
(c) In general, oxidizing agents liberate iodine (I 2 ) from iodides (I ), thereby consuming
more Na 2 S 2 O 3 or overestimate DO (positive interference). Some reducing agents will reduce
iodine to iodide, resulting in negative interference. This explains why the standard methods
have established several modified Winker methods, including the use of NaN 3 , KMnO 4 , and
alum flocculation for
interfering NO 2 ,Fe , and suspended solids,
the removal of
respectively.
2. Biochemical Oxygen Demand (BOD)
BOD refers to the amount of oxygen that would be consumed if all the biodegradable
organics in one liter of water were oxidized by aerobic bacteria. The BOD test is
based on the determination of DO. For the standard 5-day BOD 5 test, 300-mL
BOD bottles (tapered water seal stopper) are used, saturated with DO to about
9 mg/L, and the initial DO is measured (DO 0 ). The parallel samples are incubated
at 20 C in the dark for 5 days, and the DO is measured again (DO 5 ). The BOD 5 is
calculated as:
BOD 5 ðmg
=
LÞ¼DO 0 DO 5
ð6
:
14Þ
The above direct method is for samples with BOD 5 of less than 7 mg/L. However,
most wastewater samples exceed this value, and serial dilution is needed. The usable
dilutions must have initial DO 7-9 mg/L and final values of
1 mg/L, and a
minimum depletion between initial and final DO of 2.0 mg/L is required to
accurately calculate the BOD. It is also critical to assure the presence of essential
nutrients (Ca, Mg, Fe salts, and phosphate buffer), absence of toxic materials, and
inoculation with multi-micro organism seed.
>
3. Chemical Oxygen Demand (COD)
A BOD test provides the closest measure of the processes actually occurring in the
natural water system. However, this test is very time-consuming (5 days) and
involves many uncertain factors such as the origin, concentration, pollutants, the
number, and viability of active microorganisms present to affect the oxidation of all
pollutants. The COD test is used as a measure of the oxygen equivalent of the
organic matter content of a sample that is susceptible to oxidation by a strong
chemical oxidant as opposed to biological oxidation in the BOD test.
In a COD test, results can be obtained in 2 h or less. Also the method is simple
and inexpensive. When wastewater contains only readily oxidizable organic matter
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