Paper Chromatographic Method for
Volatile Acids HI — Toxicity of
Propionic Acid
A. M. BUSWELL, Consultant
University of Florida
Gainesville,   Florida
G. B. MORGAN, Assistant Research Professor
Department of Civil Engineering
University of Florida
Gainesville, Florida
In the various studies of the fermentation which occurs during the biological stabilization of organic matter, especially as it occurs in the digestion of sewage sludge, the following substances have been observed to varv
with the conditions and completeness of the process. These substances may
be divided into three classes; namely, gases, substances dissolved in the
liquor, and the biota.
The gases qualitatively consist of methane, carbon dioxide and smaller
amounts of hydrogen. Under adverse conditions, the gases may consist of
hydrogen and carbon dioxide but little or no methane.
Quantitatively, the yield of gas amounts to about 10 cf per lb volatile
matter under good conditions and the gas is composed of about 65 per cent
methane and 35 per cent CO2 (excluding N2). In extremely poor conditions,
the amount of gas decreases to less than one cf per lb organic matter, composed of approximately 2/3 CO2 and 1/3 hydrogen.
Of the substances in solution, considerable data are available on the
volatile acids, alkalinity, ammonia, and pH.
Qualitatively, the volatile acids have been considered to consist mainly
of acetic. Recent research, including the material presented herewith, reveals the importance of the other acids. Quantitatively, the volatile acid
content remains low and stable during good digestion but increases rapidly as
an indication of the introduction of inhibiting factors.
Alkalinity, depending as it does largely on CaC03 or Ca(HC03)2 may
remain constant at a value determined by the character of the local water
supply or, as is usually the case in domestic wastes, the alkalinity may increase due to ammonalysis of nitrogenous matter. If however the production
of methane is inhibited, the volatile acids accumulate until they exceed the
alkalinity. By this time gas production including the H2-CO2 phase has
practically ceased and in most cases cannot be reestablished.
The pH, as is evident from the above discussion, does not drop until the
volatile acids have accumulated to a value exceeding the various buffers including the volume or dilution effect. When this occurs all fermentation is
arrested.
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