Civil Engineering Reference
In-Depth Information
variability of its properties over time (often related to seasonal cycles of biological
activity), humic species typically dominate the NOM on a mass basis, contributing
from
50 percent to
90 percent of the DOC in most natural waters.
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Most dissolved humic substances are thought to have molecular weights of a few
hundred to a few thousand atomic mass units.
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Humic molecules contain aromatic,
carbonyl, carboxyl, methoxyl, and aliphatic units, with the phenolic and carboxylic
functional groups providing most of the protonation and metal complexation sites. As
opposed to synthetic polymers and many biological polymers (e.g., proteins), humic
molecules are not composed of unique, highly reproducible monomeric building
blocks.
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Rather, a group of similar building blocks is probably present in many humic
molecules, but the sequence and frequency of occurrence of the building blocks, and
the exact structure of the regions between adjacent building blocks, is probably dif-
ferent in every humic molecule.
Previous investigations of NOM from a wide variety of sources have led to some
generalizations about the characteristics of NOM molecules in different environments.
For instance, environments in which water is exposed to mineral surfaces that complex
and adsorb NOM contain low concentrations of dissolved NOM, especially humic
substances. NOM in lakes and reservoirs of moderate to high trophic status is often
dominated by material generated in the water body (autochthonous material), whereas
low-order rivers and streams usually carry more NOM that is generated exterior to the
water body (allochthonous NOM). Allochthonous NOM has large C / N ratios (near
100:1), is highly colored, and has significant aromatic carbon content, whereas au-
tochthonous NOM has lower C / N ratios (near 10:1), is almost colorless, and has low
aromatic carbon content.
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NOM Impact on Water Treatment
For the drinking water industry, the major goal of NOM characterization is to under-
stand and predict the reactivity of NOM or its fractions in specific treatment processes.
Any water treatment plant is likely to have specific compliance issues and / or opera-
tional problems that require attention and for which certain types of NOM character-
ization are useful. For example, if biological activity in the distribution system is a
major issue, then attention should focus on chemical classes contributing to the bio-
degradable compounds in solution (e.g., proteins, carbohydrates, amino acids) and less
attention can be paid to humic species (unless they have been altered by ozonation).
Alternatively, if the concentration of disinfection by-products is the major concern, the
humic part of NOM should receive special attention, and losses of carbohydrates and
proteins during the NOM characterization will not affect the results dramatically.
Table 3-6 provides a general evaluation of water quality issues associated with
NOM, and the fractions of the NOM that are most likely to be relevant for each issue.
Table 3-7 provides general information relating different NOM fractions to the for-
mation of some important disinfection by-products.
DISINFECTION BY-PRODUCTS
What Are They and Why Are They Important?
In the past three decades, the use of chlorine as both a drinking water disinfectant and
as an industrial chemical has moved from savior to suspect. Its incorporation in chem-
