30    BIOAUGMENTATION OF STRESSED ANAEROBIC FILTERS
WITH METHANOGENIC ENRICHMENT CULTURES
Nancy Lynch, Graduate Research Assistant
Department of Civil and Environmental Engineering
Lacy Daniels, Associate Professor
Department of Microbiology
Gene F. Parkin, Associate Professor
Department of Civil and Environmental Engineering
The University of Iowa
Iowa City, Iowa 52242
INTRODUCTION
Although anaerobic processes for the treatment of wastes have been used for many years and are
applied throughout the world, the notion persists that they are unstable and difficult to operate
effectively. In particular, it is considered that a system which develops problems embarks upon a
slippery slope from which it is difficult to retrieve stability.
Historically, the initial remedy for a troubled digester consisted of pH maintenance, generally with
the application of additional alkalinity. In many cases, this is an effective solution to the problem and
it has been shown that anaerobic systems can operate with high levels of volatile acids as long as
alkalinity is sufficient to protect the narrow pH requirements of the methanogenic bacteria.1
Recently, advances in understanding the microbiology and biochemistry of the anaerobic breakdown of complex organic materials have suggested new methods for controlling system operation and
improving recovery from shocks. An overall scheme of the bacterial degradation process consists of
three main parts: 1) hydrolysis and fermentation of complex organic molecules into acetate, H2, and
organic acids with more than 2 carbons; 2) conversion of long-chain acids into acetate, H2, and C02;
and 3) methane production from the cleavage of acetate and the reduction of C02 with hydrogen. In
natural systems, the relative amounts of bacteria that carry out the biochemical reactions are determined by a complex set of variables including food availability, thermodynamics, product inhibition,
temperature, and the presence of toxic materials. The flow of energy through such a scheme is
depicted in Figure 1.
Molecular hydrogen appears as a product in the first two sets of reactions, and is consumed by
methanogenesis in the third. The concentration of hydrogen in an anaerobic system is considered by
recent investigations to be the most important parameter in the efficient operation of the process.3>4'5
Thermodynamics for the interacting series of processes results in a cascading group of reactions in
which each stage is closely dependent upon the previous and, in some cases, the following set of
reactions.
Syntrophic bacteria, also referred to as acetogenic bacteria, responsible for the conversion of long-
chain organic acids to acetate, are the most sensitive to hydrogen concentration (Table I). Under
standard conditions, the free energy change for the reactions they use to produce energy are unfavorable. It is only when the partial pressure of hydrogen is reduced to levels of 10-3 to IO-5 atmospheres
that the reactions will take place. Because of this, the syntrophs must exist in an obligate symbiotic
relationship with an effective hydrogen consumer, and are difficult to maintain in a pure culture. For
many years, the syntroph-methanogen consortium was considered to be a single bacterium, Methano-
bacillus omelianskii.
It can be seen from Table I that the reactions of the heterotrophic bacteria are favorable under both
standard conditions and those of reduced hydrogen partial pressure. It is useful, however, to consider
the acetate-producing reaction separately from the others because it provides substrate directly for the
third group of bacteria, the methanogens. The other reactions represent production of substrate for
the syntrophs. Under conditions of very low hydrogen concentration, the production of acetate is
more favorable relative to the production of the longer chain acids. At standard conditions, however,
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