High-Rate Digestion Control
II. Techniques for
Evaluating Acid-Base Equilibrium
FREDERICK G. POHLAND, Associate Professor
Civil Engineering Department
Georgia Institute of Technology
Atlanta, Georgia
INTRODUCTION
Considerable emphasis has been placed on the development of the anaerobic
digestion process in order to better apply it to the treatment of domestic and industrial waste organic materials. From this effort, various high-rate digestion
schemes have evolved which are frequently identified by special nomenclature
descriptive of a particular process modification. However, whatever the modification, the fundamental purpose of the process remains and with it the more subtle
biologically and chemically mediated reactions which determine its conversion
and stabilization efficiency.
The efficiency of conversion and stabilization is known to be dependent upon
certain environmental optima which may be established internally by processes
indigenous to the system or controlled by certain externally applied adjustments.
One important environmental requirement is an optimum pH. In an aqueous system, such as the anaerobic digestion process, pH implicitly defines acid-base conditions and all associated chemical equilibria. To explicitly describe all these
equilibria and their individual constituent species and interactions, in a system as
ill-defined as the anaerobic digestion process, is a tedious and almost insurmountable-task. However, it is believed that judicious choice of primary equilibria,
within the limits of intuition and conceptual knowledge, can serve as a rational
basis for further investigation and interpretation.
This presentation will identify these primary equilibria and their effect on the
pH of the anaerobic digestion process. It will also indicate the feasibility of developing appropriate process models from the associated equilibria expressions
which are capable of describing pH response during the several conditions of normal
and retarded digestion.
GENERAL PERSPECTIVE
In a previous paper (1), the origin and significance of some of the acidic and
basic constituents of the high-rate digestion process were considered using the
sequential two-phase acid-methane fermentation concept in which the waste organic material is converted first to volatile organic acids and then to methane. In
addition"? recognition of this internal production and/or the addition of certain
acids and bases, including the volatile organic acids, carbon dioxide, ammonia,
and lime, as well as their possible interactions, led to the identification of the
companion conjugate acid-base pairs considered responsible for regulating the pH
during normal and retarded digestion conditions. The relationships established between these acids and bases were described in terms of alkalinity expressions and
- 353 -