Page 732 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactions and Kinetics 687

This means that the enthalpy (defined as the internal energy plus mostly since the 1950s; texts from that period through the
PV energy) wants to go from higher to lower values. The entropy recent decade have summarized available knowledge and
wants to increase, which results in a more negative value of DG. were cited extensively. Of the reputed 1000 papers on activated
For example, a gas confined to a bottle will diffuse once the cap sludge to the early 1970s, several have been ‘‘landmark,’’ a few
is off; its entropy is increasing giving more disorder. To get the of which have been identified and, in some cases, cited.
gas back in the bottle (more order) requires energy. Essentially
all water treatment involves trying to get more order from
disorder. In biological treatment where we want to remove a 22.3.1 COMPOSITION OF SUBSTRATES
dissolved substrate from water the cells have more order. Also,
Formulae for substrate and cells are required as the first step
we can settle the cells and further process them (such as vacuum
in constructing a balanced chemical equation. Categories of
filtration), creating still more order.
substrates include organic compounds such as glucose,
The foregoing summary is intended to introduce a few of the
alcohols, proteins, etc; and inorganic compounds such as
thermodynamic terms that are mentioned in this text. The review
ammonia (occurring as NH 3 or NH 4 , depending on pH), nitrates,
þ
is not intended to explain a topic that warrants a focused study.
and sulfates.
The idea of treating microbial cell synthesis as a chemical
reaction and relating the free-energy of reaction to the yield of
22.3.1.1 Domestic Wastewater and Organic
cells and applied to biological treatment was reviewed during the
Compounds
early to mid 1960s by Servizi and Bogan (1963, p. 64) and
McCarty (1964) and are what might be termed, ‘‘classic’’ papers Formulae for representative organic substances of interest in
on the topic. wastewater treatment are given in Table 22.4. Domestic was-
tewater is first, and was derived from laboratory analysis, as
was the formula for bacterial cells. A general formula for
22.3 BIOLOGICAL TREATMENT OVERVIEW
carbohydrates is given as CH 2 O, which has a multitude of
The two aspects of biological treatment are (1) the reaction, variations, a few of which are shown, e.g., glucose, sucrose,
and (2) kinetics. The reaction is best understood as a balanced and lactose. Other substances listed are indicative of the
chemical equation and the kinetics is usually expressed by the variety found in raw wastewaters or may occur at some
classic Monod equation. In addition to understanding prin- stage of treatment. Urea, in the bottom row, is a source of
ciples, certain reference material is useful, including chemical ammonia and comes from the breakdown of protein to amino
formulae on substrates and cells; and coefficients such as cell acids, which breaks down further to give urea as a product,
yield, and Monod. The material reviewed has been building and is a physiological process (Rawn, 1989, p. 468).

TABLE 22.4
Formulae for Sampling of Organic Compounds
Substance Composition Formula Reference
Domestic wastewater Fats, protein, sugars C 10 H 19 O 3 N Orhon and Artan (1994, p. 95)
Bacteria cells Protein, fats, lipids, etc. C 5 H 7 O 2 N Porges et al. (1956, p. 43)
Carbohydrates Cellulose, starch, sugars CH 2 O Eckenfelder and Weston (1956, p. 21)
Glucose C 6 H 12 O 6 Eckenfelder and Weston (1956, p. 21)
Sucrose C 12 H 22 O 11 Eckenfelder and Weston (1956, p. 21)
Lactose C 12 H 22 O 11 H 2 O Eckenfelder and Weston (1956, p. 21)
Protein Amino acids, C 16 H 24 O 5 N 4 Orhon and Artan (1994, p. 95)
nitrogenous organics
Grease Fats and oils C 8 H 16 O Orhon and Artan (1994, p. 95)
Methyl alcohol CH 3 OH Eckenfelder and Weston (1956, p. 21);
Orhon and Artan (1994, p. 95)
Ethyl alcohol CH 3 CH 2 OH Eckenfelder and Weston (1956, p. 21);
Orhon and Artan (1994, p. 95)
Acetic acid CH 3 COOH Eckenfelder and Weston (1956, p. 21);
Orhon and Artan (1994, p. 95)
Propionic acid CH 3 CH 2 COOH Orhon and Artan (1994, p. 95)
Urea H 2 NCONH 2 Stryer (1981, p. 33)
Sources: Orhon, D. and Artan, N., Modeling of Activated Sludge Systems, Technomic Publishing Co., Lancaster, PA, 1994;
Eckenfelder, W.W. Jr. and Weston, R.F., in McCabe, J. and Eckenfelder, W.W. Jr., (Eds.), Biological Treatment
of Sewage and Industrial Wastes, Volume 1, Aerobic Oxidation, Reinhold Publishing, New York, 1956.