Page 766 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

23 Biological Reactors

The ‘‘idea’’ of a reactor is broad and may be defined as a follows abstracts key notions from the extensive lore and, at
volume of any size or shape in which a ‘‘change’’ occurs. In a the same time, uses the reactor theory to explain the activated-
biological reactor, microorganisms transform, or ‘‘change,’’ sludge process and its variations. Reactor theory also gives a
substrate to desired reaction products. rationale for empirical parameters that were evolving during
The key themes in reactor theory are stoichiometry, kinet- the 1950s.
ics, and mass balance. The stoichiometry and kinetics were
reviewed in Chapter 22; in this chapter, the mass balance
23.2.1 HISTORY
principle is added, which is the basis for modeling. Models
are reviewed for three reactor categories: suspended growth, The beginning was in 1914 when Ardern and Lockett
attached growth, and anaerobic. Practice in design and oper- published their paper, ‘‘Experiments on the oxidation of
ation are summarized for each reactor category. sewage. . . . ’’ Actually, however, they stood on the shoulders
of some of their predecessors. We can understand the devel-
23.1 BIOLOGICAL REACTOR SPECTRUM opment of the field by first looking at where it stood in 1914,
and then the early development of practice and finally the
The idea of a reactor is to provide a favorable environment for milestones that enumerate how we arrived at current practice
a substrate to be transformed by bacterial enzymes to benign and knowledge.
products. To accomplish this purpose a host of reactors have
evolved to fit different purposes and conditions. All have the 23.2.1.1 Beginnings
common themes: stoichiometry, kinetics, mass balance, The account given by Metcalf and Eddy (1930) gives insight
which provide a framework for analysis and understanding, into the evolution of the activated-sludge process. The
but are supplemented by empirical parameters. nuances in their writing add flavor.
To relate to the literature, Table 23.1 identifies some reactor
names. Table 23.1 indicates that a classification of reactors has The aeration of sewage in tanks, to hasten oxidation of organic
limited coherence; one may classify a plug-flow reactor, for matter, was investigated as early as 1882 by Dr. Angus Smith,
example, as a continuous-flow reactor, but as a concept it who reported on it to the Local Government Board. It was
applies to other kinds of reactors as well. Land application studied subsequently by a number of investigators and in 1910
and aqua-culture are included to indicate that ‘‘passive’’ kinds Black and Phelps reported that a considerable reduction in
of treatment are sometimes preferred. Some, e.g., granular putrescibles could be secured by forcing air into sewage in
activated carbon and ‘‘soil,’’ are included so that the extent of basins (p. 636).
the reactor concept is seen. As a note, the table is not intended
to read across; the columns are intended to add some logic Following this, experiments by Clark and Gage at the
(or coherence) in categorization, but not the rows. Lawrence Experiment Station, conducted during 1912 and
1913 on sewage in bottles and in tanks partially filled with
roofing slate spaced about 1 in. apart, showed that in aerated
23.2 ACTIVATED SLUDGE
sewage, growths of organisms could be cultivated which
Reactor theory, i.e., materials balance combined with kinetics, would greatly increase the degree of purification obtained.
is a major theme of most chapters. The theory was adapted to The results of the work at Lawrence were so striking that
activated sludge from chemical engineering in the 1950s (see, their knowledge led Fowler to suggest experiments along
for example, Gram, 1956), which provided a framework for similar lines at Manchester, England, where Ardern and
understanding. Lockett carried out valuable researches upon this subject.
If the activated-sludge practice had started along the lines During the course of their experiments Ardern and Lockett
of reactor theory, the present state of the art would have a found that the sludge played an important part in the results
simple rationale as indicated above. Practice developed obtained by aeration, as announced in their paper of May 3,
empirically, however, and the state of the art is compounded 1914, before the Manchester Section of the Society of Chem-
by the consequent lore which unfolded over the decades, and ical Industry. At the outset it was necessary to aerate the
inconsistent units. sewage samples continuously for 5 weeks before complete
Over the decades since 1914 when Ardern and Lockett nitrification was obtained. By repeatedly drawing off the
launched activated-sludge practice with their historic paper, a clarified sewage and adding fresh raw sewage to the old
proverbial thousand papers, c. 1970, had been presented or sludge left in the experimental tank, the time for oxidation,
published, along with a number of books. The review that however, was reduced to 24 h and eventually to a few hours

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