Effect of High Concentrations of
Individual Volatile Acids on
Anaerobic Treatment
PERRY L. McCARTY, Associate Professor of Civil Engineering
Stanford University
Stanford,   California
MARC H. BROSSEAU, Engineer
Lalande and Valois
Montreal, Canada
Acetic, propionic, and butyric acids are the major volatile acids formed as
intermediates in the anaerobic treatment of complex organic materials. The majority of the methane formed in this process results from the decomposition of these
acids by a group of bacteria called "the methane formers" (1). A sudden increase
in volatile acid concentration indicates the methane formers are not removing the
acids as quickly as they are formed and this is one of the first indications of unbalanced digestion. Although it is generally agreed high volatile acids and unbalanced digestion go together, it is still actively debated whether a high volatile
acid concentration is the result of unbalance or tne cause.
Although the volatile acids serve as the major source of "food" for the methane bacteria, Buswell and coworkers (2) in 1936 suggested that a high concentration of volatile acids was inhibitory to the digestion process even though a neutral
pH be maintained. McCarty and McKinney (3) recently have shown, however,
that acetic acid, the most prevalent acid formed in methane fermentation (1),
was not toxic to the methane bacteria even at concentrations of 10, 000 mg/l or
more. Since then, Buswell and Morgan (4) indicated that perhaps some volatile
acids are more inhibitory to the methane bacteria than others. They proposed that
propionic acid is detrimental to digestion, based partly on the reports of Wachsman
and Storck (5) on the lysis of protoplasts by propionic acid. They also indicated
that propionic acid was found to be present in several digesters which had failed.
Buswell and Morgan (4) thus suggest high volatile acids may be toxic to the
"methane formers" and hence the cause of stuck digesters. " However, there is
considerable evidence to indicate the volatile acids are not the cause but the result of unbalanced conditions and as such are not harmful to the methane organisms. In this case, proper digester control would require that the initial cause of
the upset be found and corrected. Thus, proper digestion could not be obtained
simply by reducing the volatile acid concentration. This study was instigated to
provide more evidence to prove or disprove these theories. Here the effect of high
concentration of the volatile acids, acetic, propionic, and butyric, both individually and in combination,  on the digestion of sewage sludge was investigated.
EXPERIMENTAL PROCEDURE
In this study laboratory digesters were operated to simulate field conditions of
operation. The digesters were initially seeded with digested sludge from a municipal treatment plant.    Additions of raw sludge and removals of digested sludge
- 283 -