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The Role of Iron in Anaerobic Digestion
JOHN T. PFEFFER, Assistant Professor of Civil Engineering
University of Kansas
Lawrence, Kansas
JAMES E. WHITE, Public Health Engineer
Iowa State Department of Health
Des Moines, Iowa
INTRODUCTION
The past decade has produced innumerable studies of the anaerobic digestion
process. A substantial number of these studies have been conducted in laboratories
using raw sewage sludge or synthetic substrate. The use of the synthetic substrate
was particularly applicable to the study of the mechanism of anaerobic digestion.
This enabled tne investigator to add a known entity to the system. With this
chemically defined substrate, the effect of the process on the substrate was more
easily ascertained.
In order to use a synthetic substrate, it was first necessary to determine the
nutritional and environmental requirements of the microorganisms employed in the
process. This has been a significant problem in the laboratory studies using the
synthetic substrate. An apparent optimum environment has been well established
by many workers. Also the major and micro nutrients necessary for bacterial
metabolism have been well defined.
However, there appears to be a deficiency of some material in the synthetic
substrates that have been used in the past. This deficiency inhibited the growth
of the methane producing microorganisms. Addition of either sludge or supernatant from a digester would stimulate the growth of the methane bacteria and
proper digestion could be established. As soon as this additive was washed-out,
the inhibition of the methane bacteria would again begin. Speece and McCarty
(1) found that the addition of iron salts to the synthetic substrate produced the
same effect as did the sludge or supernatant. As a result of their report, a study
was undertaken to determine the role of iron in anaerobic digesters using a synthetic substrate.
PREVIOUS WORK
In the past, most laboratory studies have employed a raw sewage sludge as
the substrate. These studies have been quite successful. It was possible to obtain optimum growth of both groups of microorganisms as long as the environment was optimum. However several workers have attempted to use nutritionally
balanced synthetic substrates. Success with this type of substrate was not easily
attained. Apparently there was some critical material that was not present in the
substrates used.
Culp (2) was not able to obtain satisfactory digestion of dilute organic wastes
when employing Metrecal or acetic acid as a synthetic substrate. With an initial
seed of digested sludge, satisfactory digestion was obtained until the original seed
was purged from the system. The gas production decreased and the volatile acids
began increasing. It was possible to re-establish good digestion by adding more
digested sludge or dried supernatant solids.
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