A Volatile-Extractable Acid Method
FOR CONTROL OF ANAEROBIC DIGESTION PROCESSES
J. F. THOMAS, C. R. WHERRY AND E. A. PEARSON
Acting Assistant Professor of Sanitary Chemistry
Research Chemist
Associate Professor of Sanitary Engineering
University of California
Berkeley,  California
The mechanism of the methane fermentation has been studied extensively for over sixty years; however, the exact nature of complex
fermentation systems such as found in sludge and industrial waste digestion processes is still unresolved. It is well recognized that a variety
of organic acids are produced during anaerobic fermentation but little
is understood about the role of the individual acids. It is presumed that
least the simpler acids are precursers of methane and that acetic acid is
probably one of the immediate precursers.
An excess accumulation of organic acids in the fermenting mixture
indicates that acid formation is proceeding at a faster rate than the
methane fermentation. An excess accumulation of organic acids precedes, generally by several days, an unfavorable drop in the pH of the
digester which may result in the "stuck" digester. Adequate evidence
has not as yet been developed to indicate whether the "stuck" condition
results from an accumulation of certain organic acids, from the toxic
effect of a low pH, or from still other causes.
In spite of the limited knowledge regarding the mechanism of the
methane fermentation, one of the best single indices of digester operation has been the volatile acid determination. If the so-called volatile
acid concentration in the digesting mixture is low (less than 300 p.p.m.
as acetic acid) the methane fermentation is progressing satisfactorily;
if the volatile acid concentration is high (greater than 2000 p.p.m.),
the methane fermentation is out of balance and generally action must
be taken to prevent a stuck digester.
The direct steam distillation method (1) for the determination of
volatile acids and several modifications of it have been widely used as a
control test in digester operation despite the fact that the method is
subject to several inherent errors.
Because of the inadequacies of the distillation method, an ether
extraction procedure has been developed.   It should be noted that by
267