Page 729 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

684 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

BOX 22.3 ROLE OF LAW IN WATER Foods
TREATMENT
Hydrolysis Proteins Polysaccharides Lipids
Prior to 1965, the objectives of wastewater treatment
were (1) to maintain aerobic conditions, defined trad- Subunits Amino acids Mono and disaccharides Fatty acids and glycol
itionally as 2 mg dissolved oxygen per liter in the
receiving waters; (2) to not cause solids deposition; and Glycolysis
(3) to reduce concentrations of pathogens. If a down- ATP
stream water user was impacted by an upstream sewage
discharge, the injured party had only the courts and Pyruvate
‘‘common law’’ (i.e., cases not covered by statute) for
possible remedy. The process was long, arduous, and
Acetyl CoA
expensive, with outcome not certain. During the 1950s,
however, public perceptions of water quality issues
Citric
evolved and in 1965 the first federal law was enacted
acid
that had ‘‘teeth,’’ which was PL 89-234, the Water cycle
Quality Act of 1965. The legislation required a ‘‘para-
digm shift,’’ i.e., a change from the idea of ‘‘assimilative
capacity’’ to the requirement that states establish water- NADH
quality standards for interstate waters. The culmination
was the 1972 Clean Water Act, PL 92-500, which had a ATP
‘‘goal’’ of zero discharge of contaminants. The act Electron transport and oxidative phosphorylation +
included provisions for ‘‘effluent’’ standards, which O 2 NAD +
were easier to enforce than stream standards. The term CO 2
NH 3
‘‘contaminants’’ has been broadly interpreted and have H 2 O
included a host of chemicals, along with ‘‘nutrients,’’
Waste products
including nitrogen and phosphorous. The role of bio-
logical treatment has expanded commensurately. In
FIGURE 22.1 Catabolic biochemical pathways for ATP produc-
Europe, the European Union (EU) has developed tion. (Adapted from Rawn, J.D., Proteins, Energy, and Metabolism,
increasingly more stringent regulations for its member Neil Patterson Publishers, Burlington, NC, 1989, p. 243, Figure
countries. In some cases, other countries have looked to 10-11.)
the United States or the EU as possible models.

to end products. The first phase, to acetyl-CoA, is anaerobic
and produces two ATP molecules. The acetyl-CoA is formed
22.2.1 METABOLIC REACTIONS from pyruvate if an electron acceptor is present, e.g., oxygen
or nitrate (Orhon and Artan, 1994, p. 57). The acetyl-CoA
The term ‘‘substrate’’ is any substance that may be metabol-
then enters the citric acid cycle, which is aerobic. Water and
ized by an organism. Organic carbon is the substrate for a
carbon dioxide are end products, along with about 36 mol of
heterotrophic organism, also called a ‘‘chemoorganotroph.’’
ATP, produced by ‘‘oxidative phosphorylation.’’
An inorganic molecule, e.g., ammonia, may serve as the
Three points are relevant in further understanding: (1) if
substrate for an autotroph, also called a ‘‘chemolithotroph.’’
the starting point is glucose, the degradation is called
In metabolism, substrates provide energy for catabolism in
glycolysis and may occur via ‘‘Embden–Myerhoff’’ pathway;
which energy is stored in the compound, ATP. In anabolism,
(2) the reactions in the pathway approximate thermodynamic
the conversion of ATP to ADP provides the energy for cell
‘‘reversibility’’ which means that the ATP energy yield is
synthesis. It is not an overstatement to say that the reactions
near the theoretical maximum; and (3) the NADH, which
involved are immensely complex.
converts to NAD , functions to transport electrons along
þ
the pathway.
22.2.1.1 Catabolism
Catabolism is a sequence of reactions that yield energy, with 22.2.1.1.1 Electron Transfer (Abstracted from Orhon
each reaction facilitated by a specific enzyme. The particular and Artan (1994, pp. 44–64)
pathway depends upon the substrate and the microbe. The In catabolism, a chain sequence of electron transfers enables a
reaction sequence is ‘‘exothermic,’’ meaning that the standard stepwise release of energy. As is well known, an oxidized
free energy of reaction is negative, i.e., DG8 < 0, meaning that compound loses electrons. For an organic compound, its
the reaction is ‘‘spontaneous.’’ oxidation involves the removal of two electrons and two
Figure 22.1 depicts the phases of degradation of substrate, protons (called dehydrogenation). To illustrate, Equation
i.e., ‘‘food,’’ in three forms (proteins, polysaccharides, lipids) 22.1 depicts the oxidation of lactic acid to pyruvate,