High-Rate Digestion Control III —
Acid-Base Equilibrium and
Buffer Capacity
F. G. POHLAND, Associate Professor of Civil Engineering
Georgia Institute of Technology
Atlanta, Georgia
INTRODUCTION
The efficiency of conversion and stabilization of wastewater substrates during
anaerobic digestion is dependent upon microbial mediation of certain energy
yielding reactions necessary for growth and metabolism. The residual products of
these reactions directly influence the associated chemical and physical nature of
the system's aqueous environment. Because many of these products can be classified as either weak acids or weak bases, it is convenient to characterize the aqueous environment on the basis of acid-base equilibrium analysis using pH as the
master variable.
In this report, equilibrium expressions are developed which permit quantitative evaluation of primary acid-base equilibria contributing to the pH response
within the anaerobic environment. Confirmation for these expressions is provided by experimental results obtained during the anaerobic conversion of a complex organic substrate. These results are also used to describe the relative condition of the process and the concept of buffer capacity is introduced as a potentially valuable control parameter.
INITIAL CONSIDERATIONS
Techniques for the application of acid-base equilibrium for the analysis of
the aqueous environment of the anaerobic digestion process have been presented
previously (3,4). In addition, the major acidic and basic products of biochemcial
conversion of organic matter during the process have been identified and their
significance described within the context of the two-phase acid-methane fermentation concept.
Recognition that conversion is achieved as a result of certain mutually dependent oxidation-reduction reactions provides a convenient base for a simplified
description of the process as indicated in Table I. Although this description may
be somewhat hypothetical and exclusive of other possible reaction schemes, it
does possess a definite didactic value in accordance with present concepts. The
amino acid, cysteine, was abritrarily chosen to represent a possible but specific
combination of primary elements (CHONS) commonly present in organic wastewaters.
The reactions illustrate the sequential and eventual conversion of organic carbon to either carbon dioxide or methane with the associated release of the reduced
forms of nitrogen and sulfur. The oxidation-reduction sequence is mutually dependent; organic carbon is oxidized to carbon dioxide and/or intermediate products such as volatile organic acids (acetic acid) during the acid fermentation half-
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