Chemistry Reference
In-Depth Information
low-molecular-weight (LMW) compounds. BOM is of significance in water treat-
ment works since it serves as an energy and carbon source for bacteria, thereby
promoting bacterial re-growth. Some LMW biodegradable compounds can be
directly used for metabolism, whereas high-molecular-weight (HMW) ones have
first to undergo enzymatic attack.
Humic substances found in water account for about 50-75% of the DOC [6].
Although humic substances are generally regarded as resistant to biodegradation,
some studies reveal the contrary. For example, it was strikingly noted that humic
substances accounted for a large proportion of the BOM in a stream [7]. In relatively
recent studies, a small fraction of humic substances found in rivers and lakes was
also shown to contribute to BOM [8]. Obviously, the timescale has to be considered
in order to have an accurate idea about biodegradability of humic substances [6].
Bearing in mind the fact that BOM is an acronym and not a parameter, two
different parameters came into use, namely:
Biodegradable Dissolved Organic Carbon (BDOC)
.
Assimilable Organic Carbon (AOC).
.
Biodegradable Dissolved Organic Carbon (BDOC) As the name implies, the BDOC
test aims at the determination of the DOC fraction that is bioavailable for bacteria.
BDOC constitutes the total amount of DOC that is removed from water by het-
erotrophic microorganisms due to both cell synthesis and mineralization.
According to the original BDOC procedure, the water sample is filter sterilized and
then reinoculated with the same sample that was filtered through a 2-
m pore size
filter for removal of particles and protozoa [9]. Then, the sample is incubated in the
dark at 20
m
C. In the BDOC test, the DOC remaining in solution is measured with
respect to time. The incubation period lasts up to 30 days until the achievement of
a plateau in DOC. Then, the BDOC value is calculated from the difference between
the initial and final DOC.
The procedure in BDOC determination is somewhat similar to the BOD
procedure, but the BDOC test takes much longer than the BOD test. However, the
BOD test cannot be conducted at the very low organic carbon levels found in
drinking water supplies, and the BDOC tests provides other advantages. In the
BOD test oxygen-consuming compounds such as ammonia may interfere with
the oxygen demand determination. On the other hand, the BDOC test truly
reflects the biodegradability of organic matter, since it is based on the direct
measurement of organic carbon.
Depending on the rate of biodegradation, the DOC in a water sample may be
divided into fractions, namely fast-BDOC, slow-BDOC and non-BDOC or NBDOC,
representing the truly nonbiodegradable portion [10]. The fast-BDOC or the readily
biodegradable fraction in BDOC can be directly used by bacteria. The slowly bio-
degradable fractions in BDOC undergo hydrolysis at variable rates [11]. Usually,
extending the duration of the BDOC test to 30 days allows adequate estimation of
different fractions.
Assimilable Organic Carbon (AOC) This parameter expresses the fraction of DOC
that can easily be assimilated into biomass during bacterial growth. Thus, unlike
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