A Dynamic Model of the Anaerobic
Digestion Process
JOHN F. ANDREWS, Professor
Environmental Systems Engineering
Clemson University
Clemson, South Carolina
INTRODUCTION
Most Mathematical models currently in use for the description of biological
processes are steady state models and therefore cannot be used to predict process
performance during start-up operations or under transient conditions resulting from
changes in process loading. Dynamic models can be used to make these predictions and the importance of such models is realized when one considers, for example, the problems of starting up anaerobic digesters, the frequency of digester
failure, and the effects of bulking on the activated sludge process. A failing digester or bulking activated sludge unit is definitely not at steady state ! Dynamic
models are also useful in obtaining a quantitative measure of process stability
which can be of considerable importance in selecting a process for a specific application. Most sanitary engineers will accept the statement that the trickling
filter process is more stable than the activated sludge process but would be hard
pressed to express this quantitatively.
Although the anaerobic digestion process has many advantages such as a low
production of waste sludge, low power requirements for operation, and production
of a useful product, methane, it has a poor record with respect to process stability as evidenced by the many reports of "sour" or failing digesters. This has led
to the increased use of more expensive processes such as aerobic digestion and wet
combustion. Many studies have been made to determine the causes of digester
failure and techniques for the prevention of failure. Prominent among the many
causes of failure mentioned in the literature are the effects of high volatile acid
concentration and low pH on the methane bacteria. Although most workers agree
that a pH less than 6. 5 is detrimental to the process there have been considerable
differences of opinion concerning the effect of high concentrations of volatile
acids. One group, as exemplified by the work of McCarty (1), feels that volatile
acids are inhibitory to the methane bacteria only in an indirect way through a reduction in pH and that this inhibition can be relieved by maintaining the pH near
neutrality. Another group, as exemplified by the work of Buswell (2), maintains
that volatile acid concentrations above 2, 000 to 3, 000 mg/1 are inhibitory ir-
regardless of pH and that this inhibition can be relieved by reducing the organic
load or diluting the reactor contents. The model that will be presented here resolves this conflict by considering the unionized fraction of the volatile acids as
the inhibiting agent. Since the concentration of unionized acid is a function of
pH and total acid concentration both are therefore of importance. The model also
shows that inhibition can be relieved by maintaining the pH near neutrality or
reducing the organic loading, or diluting the reactor contents.
Most current steady state models of the anaerobic digestion process are based
on the relationship between limiting substrate concentration and growth rate as
proposed by Monod (3). James (4) nas pointed out that in biological processes a
wide variety of substances may act as limiting substrates. Stewart (51 and Agardy,
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