THE REGULATION OF THE SYNTHESIS OF THE DENITRIFYING
ENZYMES IN ACTIVATED SLUDGE
Thomas J. Simpkin, Research Assistant
William C. Boyle, Professor
Department of Civil and Environmental Engineering
University of Wisconsin—Madison
Madison, Wisconsin 53706
INTRODUCTION
The use of activated sludge for the treatment of municipal and industrial wastewaters dates to
the 1920s. Problems with activated sludge, however, are still common. These problems (such as bulking, foaming, rising sludge, process inefficiency, and process upset) are due to undesirable traits or
characteristics of an activated sludge. Trait regulation of an activated sludge is a function of the
biochemistry and the microbiology of the sludge. Thus, a more thorough understanding of the biochemistry and microbiology of activated sludge could lead to better control over the traits of the
sludge and thereby improve operation and design.
This paper presents a conceptual model of the biochemical and microbiological regulation of
activated sludge traits. Denitrification is used as an example of how the model would operate based
on biochemical and microbiological theory. To determine the actual configuration of the model for
activated sludge, an assay for the denitrifying enzymes was developed. This assay is described along
with results which give clues as to the importance of enzyme synthesis in regulating the denitrifying
enzymes. The results suggest that although enzyme synthesis is a factor in the regulation of the denitrifying enzymes, synthesis occurs at relatively low levels of nitrate and relatively high concentrations
of oxygen.
A CONCEPTUAL MODEL OF TRAIT REGULATION IN ACTIVATED SLUDGE
Properties or traits of activated sludge, like any biological community, are dependent on the
members of the community, the enzymes which are synthesized, and the enzymes which are activated.
Thus, the traits of an activated sludge are regulated at three levels; ecological selection, enzyme synthesis, and enzyme activity (as diagramed in Figure 1).
Ecological Selection
A large variety of organisms may enter the activated sludge system by a variety of means. In order
to become a significant part of the community (mixed liquor), the organisms must in some way out
compete other organisms for a limiting factor, thus creating an ecological niche where they can survive [1]. This is the first level of regulation, Ecological Selection. This level of regulation is responsible
for the "adaptive response" of a sludge acclimating to a new wastewater over a long period [2].
Organisms may bypass this regulation by entering the system in relatively large numbers. The mass
of organisms entering a system, however, must be high in relation to the mass of organisms which
are produced within the system to have a significant impact on the overall make up of the mixed
liquor. In other words, the biomass entering must be high relative to the organic strength of the waste
(assuming a constant yield factor).
The organisms surviving the first level of regulation have a set of genes which dictate the potential
traits the organism can express. The sum of the genes and their corresponding traits can be thought
of as the genotype of the sludge.
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