Organic matter
:-.
-----C oxygen is measured in the bottle
oxygen is measured
in the bolt I e
Fig2.5
Principles of the BOD-analysis /12/. For ordinary municipal wastewaters, the dilution
with clean water is approx. 100 times. Originally, the BOD analysis was not developed
to be used for the design of treatment plants. Nevertheless, it is still extensively used
for this purpose, even if fractionated COD analyses begin to gather momentum in
more advanced designs, for example when using computer models.
Example 2.1
BOD (5 days) has been measured at 240 g 02/m 3 in raw wastewater. Estimate the value for BODoo.
Normally BOD/BODoo = 0.6-0.7
Here it is estimated that BOD/BODoo = 0.7
BODoo = BOD/0.7 = (240 g/m 3 )/0.7 = 340 g 02/m 3
(It is true that the calculation gives BOD= 342.8571429 g 0 2/m 3 , but as an estimate
has been asked for, 2 indicating numbers will suffice, that is, BOD = 340 g 02/m 3 ).
Modified BOD
If the oxidation of organic matter and ammonia is to be separated, a modified
BOD-analysis may be performed. A substance is added to the laboratory flask (for
example Thiourea) which impedes the ammonia oxidation but does not influence
the oxidation of organic matter. The analysis procedure itself is unchanged. The
analytical result then only covers the oxidation of organic matter.
The modified BOD-analysis is sometimes used for measurement on the effluent
from biological treatment plants.
Chemical oxygen demand (CODp and COD)
Different chemical oxidants are used: potassium permanganate and potassium
dichromate.
42