High-Rate Digestion Control
I.   FUNDAMENTAL CONCEPTS OF ACID-BASE EQUILIBRIUM
F. G. POHLAND, Assistant Professor of Civil Engineering
Georgia Institute of Technology
Atlanta, Georgia
R. J. ENGSTROM, Technical Development Supervisor
Armour Industrial Nitrogen Division
Atlanta, Georgia
INTRODUCTION
Successful operation of high-rate digestion is dependent upon a basic understanding of the environmental factors affecting its control. The identification of
the various equilibria which are established during digestion, and their influence
on each other with respect to environmental response is of prime importance in
explicating the difficulties which are encountered periodically during conventional
operation. Acid-base equilibrium is one very important factor and is responsible
for maintaining a buffered environment. An introduction to the origin, relationships, and significance of the acidic and basic constituents of digestion with
special emphasis on periods of retarded digestion is presented.
ORIGIN OF ACIDIC AND BASIC CONSTITUENTS OF DIGESTION
As organic material is decomposed and transformed during digestion, several
intermediates and end-products tend to accumulate and predominate. Hydrolysis
of fats and oils coupled with fermentation of carbohydrates and proteins gives rise
to the intermediate organic fatty acids. Acetic, propionic and possibly butyric
acids are the most abundant. The relative concentrations of each are dependent
upon digestion conditions (1). The organic acids are subsequently fermented to
methane and carbon dioxide usually by the beta oxidation mechanism described by
McCarty (2).
Decomposition of proteinaceous material will also give rise to the production
of ammonia as hydrolysis and deamination of the constituent amino acids progresses. The ammonia will tend to ionize in solution and participate in the equilibrium established between itself and the acid products of digestion either as an
acid-salt or as bicarbonate alkalinity.
This reaction will hold also for the normally present salts of Ca++, Mg++, K+,
and Na+ as well as for any alkalinity added for neutralizing purposes. The acid
salts formed by the reaction of the organic acids with the alkalinity are decomposed during methane fermentation to methane and carbon dioxide with the release of alkalinity to the system.
It is evident, therefore, that the bicarbonate alkalinity and the organic acids
comprise a significant portion of the acidic and basic constituents which may accumulate during anaerobic digestion.
RELATIONSHIP BETWEEN ACIDIC AND BASIC CONSTITUENTS OF DIGESTION
The relationship between the acidic and basic constituents of the process can
be approached from a consideration of the origin and reaction of alkalinity with
organic acids.    As indicated previously, alkalinity derives primarily from its pro-
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