Page 731 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

Level of organization Examples 22.2.1.4 Photosynthesis
Cells Bacteria The empirical cell formula for an algae biomass is
Algae C 7.6 H 8.1 O 2.5 N. An associated photosynthesis reaction is
Fungi
Protozoa (Benefield and Randall, 1980, p. 326)
Organelles Nuclei
7:6CO 2 þ 2:5H 2 O þ NH 3 ! C 7:6 H 8:1 O 2:5 N þ 7:6O 2
Mitochondria
Ribosomes (22:4)
Flagella
The reaction is proportional to the radiant energy and varies
Supramolecular systems Membranes
Enzymes diurnally. In monitoring dissolved oxygen (DO) concentra-
tions over 24 h cycles, Kartchner et al. (1969) found an
Macromolecules Nucleic acids approximate sinusoidal variation during daylight hours. The
Proteins oxygen produced has been considered a source for the aerobic
Polysaccharides
Lipids treatment of wastewaters, such as in ponds. At the same time,
the algae biomass ‘‘respires’’ and has an oxygen demand,
Monomers or building blocks Nucleotides
Amino acids which reduces the DO concentration after sunset with a steady
Sugars decline until sunrise. The DO concentrations may reach levels
Fatty acids in the range of 30–40 mg=L in some stagnant waters and as
3–
Inorganic molecules CO , NH 3 , H 2 O, PO 4 xx high as 16 mg=L in fast-moving mountain streams. At night the
2
stagnant waters are likely to go ‘‘anoxic.’’ Observations in a
FIGURE 22.2 Biosynthesis—from inorganic molecules to cells. waste-stabilization pond at Logan, Utah (Hendricks and Pote,
(Adapted from Prescott, L.M. et al., Microbiology, 6th edn., 1974) showed that while the biomass was highly concentrated
McGraw-Hill, Dubuque, IA, 2005, p. 200.) throughout the summer, the pond water turned almost clear in
the fall, with the only explanation being that the algae die and
sink to the bottom with subsequent anaerobic decay.
‘‘spontaneous’’ but depends on an input of energy. The ATP
yield of the catabolism pathway provides the energy for the 22.2.1.5 Energy Principles
cell synthesis, in anabolism, i.e., the ATP-to-ADP conversion, Thermodynamic principles apply to biological reactions, as
as well as for cell maintenance. for any chemical reaction, and provide another tool for analy-
Biosynthetic pathways are divergent, as contrasted to con- sis and description. The basic concepts are seen in texts on
vergent catabolic pathways. For example, the compound physical chemistry (see, for example, Daniels and Alberty,
‘‘aspartate’’ is a precursor to the synthesized products, lysine, 1955) or in texts on aqueous chemistry (see, for example,
methionine, threonine, and isoleucine. The precursors of bio- Sawyer and McCarty, 1967). The few lines are not intended to
logical macromolecules in all cells are saccharides, fatty acids, replace a course on chemical thermodynamics, but to indicate
amino acids, purines, and pyrimidines (Rawn, 1989, p. 244). some of the ideas that are relevant to biological treatment. If
The microorganism begins with simple precursors and thermodynamics is familiar, then the outline here merely
constructs evermore complex molecules until new organelles highlights a few principles that are common in biological
and cells arise, illustrated in Figure 22.2 (Prescott et al., 2005, treatment.
p. 200). Most of the ATP available for biosynthesis is applied Free energy is a fundamental parameter and is usually meas-
in protein synthesis. Proteins, of whatever composition, are ured by changes from one state to another. Any compound has a
made of only 20 amino acids, joined by peptide bonds; free energy of formation, DG f ,. The free energy of a reaction,
different proteins have different amino acid sequences (see DG R , is the sum of the free energies of formation of product
Appendix 22.A). compounds times the respective moles minus the free energies
of formation of reactant compounds times the respective moles.
22.2.1.3 Cell Division If the reaction occurs under ‘‘standard’’ conditions, which is the
The sequence of events from the formation of a new cell common reference, the values can be calculated from handbook
through the next division is called the ‘‘cell cycle.’’ For data and the reaction energy is designated, DG R 8.If it occurs
example, a new E. coli cell grows in length with constant under other conditions, such as the physiological concentrations
diameter and at some threshold mass (after perhaps 10 min) it of a cell, the free energy of a reaction can be calculated by the
will trigger its DNA replication and then at another threshold familiar relation, DG ¼ DG8þRTlnQ,where Q represents con-
it will divide length-wise by binary fission. Each daughter cell centration ratios with stoichiometric numbers as exponents. The
receives at least one copy of the genetic material. The time free energy is a parameter invented to account for the two
sequence for the cell cycle is about 40 min for DNA replica- disparate aspects of any process: energy change as given by
tion and 20 min for cell division with a new cell created. After the enthalpy change, DH, and the entropy change, DS.For a
the separation of chromosomes a cross-wall or septum forms reaction, DG ¼ DH TDS. In a process, defined here as a
between the old cell and the new cell (abstracted from Prescott ‘‘state’’ change, the DG wants to follow a negative gradient,
et al., 2005, pp. 280–281). and if such a path is available, the process is spontaneous.