Stimulation of Anaerobic Degradation
by Organic Matter
JOHN T. NOVAK, Assistant Professor
Civil Engineering Department
University of Missouri-Columbia
Columbia, Missouri
MADHUGIRI S. RAMESH, Environmental Engineer
Woodward-Envicon, Inc.
Clifton, New Jersey
INTRODUCTION
Anaerobic digestion is usually thought of as being a multi-step process in which
complex organics are degraded to short chain fatty acids by facultative organisms and then
degraded primarily to carbon dioxide and methane by methanogenic bacteria. The methane
fermentation step has been observed to be the slowest or rate limiting step in this sequence
(1) and thus kinetic values describing this step would be useful in digester design.
Subsequently, enrichment cultures of anaerobic organisms have been subjected to pure
feeds of short chain fatty acid, and their carbon removal rates and their energy utilization
efficiency or organisms yield determined (2,3,4).
While such studies have provided valuable information which can be incorporated into
the design of these bacterial mediated treatment units (5), certain field units have been
shown to be more efficient in removing wastes than any of the laboratory units receiving
pure volatile acid feeds. It appears that under certain undefined conditions, the anaerobic
bacteria can degrade organics at rates faster than that observed in laboratory units which
are fed a sole carbon source.
For example, Torpey (6) was able to successfully digest concentrated primary and
activated sludge at detention times as low as 2.6 days. However, Lawrence and McCarty (2)
found that detention times of 3 days or greater would be necessary to prevent washout of
methogenic bacteria from occurring when the sole carbon source was a single volatile acid.
Attempts to stimulate the biological activity of anaerobic digestion units have centered
around providing some unknown growth factor, usually a heavy metal or vitamins. Others
have advocated pre-packaged enzymes, assuming that these will have the same effect as
maintaining a high active organism population. Some success has been achieved with these
attempts, chiefly the delineation of required heavy metals by Speece and McCarty (7).
Other attempts have provided erratic results, at times providing surges in gas production
but generally failing to generate a consistently higher rate. One method of stimulation
which has been successful although it has not found widespread use in the field has been to
return a portion of the dry digested sludge material. McCarty and Vath (8) found acetic and
butyric acid degradation rates could be enhanced by the addition of dried supernatent
solids. The stimulatory matter in these solids was apparently of organic origin since
additions of the ash of this material caused no increase in gas production.
It appears that many laboratory studies of biological waste treatment processes are
conducted using pure substrates so that data analysis can be simplified. This
oversimplification may yield incorrect results due to the ability of microorganisms to
directly assimilate organic intermediates into the cellular structure thereby saving energy.
This would suggest that currently used design criteria based on pure substrate removal data
may be incorrect and that optimization of energy utilization may result in more efficient
operation.
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