Page 80 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Water Contaminants 35

range of substantive content that may be extracted from the According to Randtke (1988, p. 40), organic contaminants
published material on a given contaminant. The organic car- in water may be grouped into three classes:
bon story (Box 2.A.1) is more extensive than most due to the
health significance of carbonaceous DBPs in drinking water, 1. Natural organic matter (NOM): Humic substances,
which started to be understood only since the mid-1970s. In microbial exudates, animal wastes, and products of
the United States, the subsequent regulations from EPA pro- degraded tissue
vided the mandates for the ensuing attention to DBPs. The 2. Synthetic organic chemicals (SOCs): Pesticides,
review here is brief. volatile organic chemicals (VOCs), and other chem-
icals produced commercially or as waste products of
manufacturing
3. Chemical by-products and additives: Substances that
BOX 2.A.1 HUMIFICATION
enter or are formed during treatment or in the distri-
Organic matter is comprised of humic and fulvic acids, bution system
indicating that their composition is of the same func-
tional groups that make up lignins and, to a lesser NOM is the source of color—a traditional contaminant, and is
extent, other plant polymers; they have more carboxylic a precursor of DBPs, which are possible carcinogens, an issue
acid functional groups, however, and they are surface since the mid-1970s. In addition, the residual NOM after
active. The components of humus consist of plant poly- treatment may serve as substrate for bacterial growths such
mer segments that have been oxidized to carboxylic as in the distribution system. In the United States, the issues of
acid groups at one or more ends of the segments. In DBPs and SOCs and possible health effects led to the 1974
the case of lignin polymers, the unaltered segments are Safe Drinking Water Act. Enforcement of the Act, by regula-
more hydrophobic than the carboxylic groups. A mol- tions, was stimulated by the capabilities to measure chemical
ecule that has both a hydrophobic (nonpolar) part and a concentrations in micrograms (mg) per liter through develop-
hydrophilic (polar) part is called an amphiphile. ments in instrumental analyses methods. In the case of organic
Humification is a process by which biomass consist- compounds, such instrumental methods were exemplified by
ing of dead plant and animal remains is converted to advances in gas chromatography and mass spectrography and
humis; this is one of the basic steps of the carbon cycle. associated lower costs (see Box 2.1).
The organic compounds that make up plant and animal
tissue are thermodynamically unstable in the oxidizing
2.A.1 CATEGORIES OF ORGANICS IN WATER
atmosphere of the earth’s surface. The tissue is thus
converted back to carbon dioxide and water that are The organics found in waters are characterized according to
catalyzed by enzymes from organisms. Some of the various schemes, depending upon the water and the purpose
tissue is, however, only partially oxidized, which is of the characterization. The characterization evolves as new
the source of the organic compounds that accumulate problems become known, and is a function of the purposes of
as humus. the waters being considered. Wastewaters, for example, have
Vascular plants (those with water- and food-conduct- been characterized traditionally in terms of BOD and SS. The
ing tissues) are the dominant group in most terrestrial BOD measurement is done by a 5 day BOD test and is
environments. The tissues of these plants consist of dependent upon having a properly seeded test bottle. The
three groups of polymers: (1) cellulose, (2) hemicellu- test measures the organic carbon that is degraded in 5 days
lose, and (3) lignin. Lesser quantities of aliphatic poly- by the microbe’s species in the seed. The test does not meas-
esters, starches, proteins, phenolic macromolecular ure TOC, but for a given wastewater, BOD may be propor-
species, and lipids are present also. tional to TOC. Only a portion of the organic carbon is
The degradation of plant polymers involves depoly- biodegradable, and the test measures the portion that is bio-
merization and oxidation reactions that are catalyzed by degradable within a 5 day period. The test actually calculates
enzymes. Polysaccharide polymers such as cellulose BOD based on the measured concentration of dissolved oxy-
and hemicellulose usually undergo hydrolytic depoly- gen at the beginning and end of the test under incubation at
merization reactions, whereas lignin is degraded mainly 208C for 5 days, taking into account the dilution of the sample
by oxidation. Lipids undergo hydrolysis and oxidation. with ‘‘BOD’’ water. The BOD water contains a prescribed
The products from lignin and lipid degradation are, in mixture of nutrients, as specified in Standard Methods, and is
general, oxidized fragments in which much of the saturated with oxygen. This test has its origin most probably
chemical structure of the original polymer is preserved. about 1900.
In addition to amphiphiles produced by degradation, For natural waters that are sources of municipal water
some of the lignin in wood is present as amphiphilic supplies, organic carbon was not of great concern until recent
lignin–carbohydrate complexes. These complexes have years, i.e., beginning about 1973. NOM occurs in surface
number average molecular weights on the order of waters in concentrations mostly in the range 3–6mg=L, and
6000–8000 Da. as noted, is comprised mostly of humic and fulvic acids.
These acids are products of decaying vegetative organic