Civil Engineering Reference
In-Depth Information
Sea Water
Most Groundwaters
Surface Waters
Swamps
Effluent, Biological Treatment
Wastewaters
0.01
0.1
1
10
100
1000
10000
Total Organic Carbon - mg C/L
Fig. 3-1.
Ranges of TOC for various natural waters
out with numerous types of water samples indicate that the most sophisticated methods
usually allow the identification of only between 5 and 15 percent of the dissolved
organic compounds, all of which are generally identified at the nanogram and
microgram / L concentration levels. The bulk of the nonchromatographic DOC remains
largely uncharacterized.
Because NOM includes literally thousands of distinct chemical species, evaluation
of its properties based on a compilation of individual compounds is not realistic.
Rather, researchers have attempted to characterize NOM by grouping the NOM mol-
ecules into a limited set of categories (fractions). As a practical matter, the assignment
of molecules to particular categories is always operational, although the categorization
is often described in terms of fundamental or conceptual characteristics. An implicit
expectation of NOM fractionation is that fractions isolated from independent water
sources by the same set of procedures will have similar composition and properties,
though the concentration of a given NOM fraction in different sources may differ.
Numerous compounds can be present in the refractory, nonchromatographable frac-
tion. This poorly defined fraction is often referred to as ''humic or fulvic acids,'' the
distinction being an arbitrary molecular-weight cutoff of 50,000 (humic substance
50,000 daltons). Several models have been proposed for the structure of these humic
and fulvic acids. They generally describe the presence of highly condensed polyhy-
droxyaromatic cores.
10
Like snowflakes, NOM molecules are all unique while also sharing many common
properties. NOM characterization has been and remains a priority for the water treat-
ment industry, in part because such characterization holds the key to understanding,
predicting, and perhaps controlling NOM reactivity under water treatment conditions.
One common approach for characterizing NOM divides the mixture into hydro-
philic and hydrophobic fractions. The hydrophilic fraction includes carboxylic acids,
carbohydrates, amino acids and amino sugars, and proteins, while the hydrophobic
fraction includes so-called humic species. All these groups of compounds are likely
to be present in all natural waters, though their absolute and relative concentrations
are expected to vary from site to site. Despite the site specificity of NOM and some
