Page 770 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactors 725

described extended aeration and aerated lagoon flow 14. Metcalf and Eddy. In 1972, Metcalf & Eddy (1972)
schematics. The paper recognized the need to break incorporated the reactor theory approach, which, in
with the past, i.e., addition to the journal literature, accelerated the
assimilation of science. The reactor theory culmin-
The lack of understanding of the fundamental microbiol- ated a 20 year quest for a rational basis for acti-
ogy and the total dependence of engineers on the empirical vated-sludge design theory. The 1972 book
design criteria are the causes of the problems concerning continued the classic series started by Leonard Met-
extended aeration systems.
calf and Harrison P. Eddy with their three-volume
and set, American Sewerage Practice, published in
1914, 1915, and 1916 with a textbook version
‘‘....no longer can the design of biological waste treat-
in 1930.
ment systems be based on crude empirical relationships.’’
15. Nutrients. Continuing with the story, during the
1970s, new demands were placed on the activated
10. Michaelis–Menten verification. In 1968, Pearson
sludge, i.e., to nitrify, denitrify, and remove phos-
(1968) reported that after 8 years of laboratory
phorus as related to evolving standards for ambient
research dealing with the growth kinetics of mixed
waters. Activated-sludge processes were developed
cultures of anaerobic organisms over a wide range of
to deal with these new exigencies, resulting in a
growth rates, the results indicated that the Michaelis–
complex mix of associated technologies.
Menten model fit the experimental data better than
16. IWA model. To integrate the various facets of acti-
any other expression. In other words, although Pear-
vated-sludge theory, the development of a compre-
son knew of the Michaelis–Menten=Monod model
hensive computer model was started in 1983 by an
since the work of Andrew Gram, he let further
IWA Task Group, chaired by Professor Mogens
experimental results confirm its validity before com-
Henze of the Technical University of Denmark
mitting.
(Henze, 1987). The model accommodated complex-
11. Materials-balance model. In 1968, Pearson (1968)
ity, including process variations and nitrification, and
described the materials-balance approach to
was dynamic in that input variation could be
reactor analysis, incorporating Michaelis–Menten
included. The model development has continued
kinetics. He proposed the model as applicable to
with updates being provided by periodic publications
either aerobic or anaerobic systems. His paper
by the Workgroup.
delineated substrate-balance, cell-balance, and
oxygen-balance equations and established the basis
23.2.1.4 Modern History
for the modern approach to activated sludge. Pear-
The ‘‘modern’’ history of water pollution control is considered
son’s paper reflected a chemical engineering influ-
here to have begun about 1950. Educational institutions, con-
ence which had been creeping into sanitary
sulting engineering firms, equipment manufacturers, govern-
engineering education since the early 1960s (Pearson
mental agencies, and professional organizations have had
required his doctoral students to minor in chemical
roles. A fervor in the field coincided essentially with the
engineering).
environmental movement; solving the problem of pollution
12. Pearson’s critique of the state of the art. Pearson in
was a focal point of the movement, and with it wastewater
his 1968 paper analyzed the state of the art of know-
treatment.
ledge, pointing out that most of the progress in
Within the broad field of wastewater treatment, activated
wastewater treatment technology was the result of
sludge was and is the ‘‘workhorse’’ of treatment processes.
field experiments, done largely by trial and error.
Issues have related to how to make the process work more
Further, he asserted that this past research did not
effectively, more reliably, more cheaply, etc. This has
accrue benefits because of the lack of an adequate
involved improved understanding, better designs, operating
theoretical basis upon which to design meaningful
guidelines, training operators, process modeling, etc.
experiments and to obtain interpretable data. Much
To give a sense of one of the key persons who helped
of the research, he stated, was undertaken within an
shape the foundation of our modern theory and knowledge of
empirical framework of analysis directed to specific
biological treatment, Box 23.1 reviews briefly, the career of
system characteristics of interest or to practical prob-
Professor Erman Pearson. It is written in the first person since
lems to be resolved.
it is from personal interactions to a large extent.
13. Lawrence and McCarty. In 1970, Lawrence and
McCarty (1970) published a paper that established
23.2.2 ACTIVATED-SLUDGE REACTOR ANALYSIS
the concept of sludge age which became the major
parameter for operation. They also formalized As noted, modern reactor theory is based on the mass balance
reactor theory as the theoretical basis for activated- of substrate and cells, combined with Monod kinetics. The
sludge design. The paper became the defining mod- empirical parameters, substrate utilization rate, U, and sludge
ern characterization of activated sludge. age, u c , may be derived from reactor theory.