EFFECTS AND FATE OF SELECTED HEAVY METALS
IN ANAEROBIC DIGESTION
Thomas D. Hayes, Graduate Student
Thomas L. Theis, Assistant Professor
Department of Civil Engineering
University of Notre Dame
Notre Dame, Indiana 46556
INTRODUCTION
In recent years a great deal of interest has been generated over the distribution and
effects of influent wastewater heavy metals in conventional wastewater treatment.   Of
the various process components of conventional sewage treatment, anaerobic digestion
seems to be particularly vulnerable to excessively high heavy metal loadings to the
treatment plant [ 1 ].   The widespread use of anaerobic digestion in the treatment of
sewage sludges has underscored the importance of understanding the various control
problems stemming from excessively high heavy metal concentrations in treatment plant
sludges fed to anaerobic digesters.   This study was undertaken to observe the effects of
heavy metals on bench-scale anaerobic digesters and to determine the distribution of
these metals among various forms in the digester relevant to digester performance.
PREVIOUS WORK
Early investigations conducted in the 1920s and 1930s produced three important
observations on the effects of cations on anaerobic processes:   toxicity, stimulation and
antagonism.   In general, it was found that all cations could produce toxicity in any organism at some level but the relative toxicity of the cations varied when toxicities were
compared on a weight-weight basis.   Rudolphs and Zeller [2] investigated the effects
of 1000 mg/1 of various sulfates on the anaerobic digestion of sewage sludge and found
heavy metals more toxic than the light metal ions.   Early investigations also showed
that the toxicity of a cation varied in the presence of other cations in the medium.   This
ability of one cation to decrease the toxicity of another was termed "antagonism" [3].
Stimulatory effects of cations at concentrations below those where toxicity was produced were also recognized by these early investigators.   Hotchkiss [4], for example,
found that very low concentrations of extremely toxic cations, such as mercury and lead,
could produce a stimulatory effect in anaerobic digestion.
More recently, Barth, Ettenger, Stones, Salotto and McDermott [ 1 ] have summarized
the results of research conducted over a ten-year period on the effects of continuous
and pulse feeding of heavy metals, including zinc, chromium (VI), nickel and copper, on
anaerobic digesters but only in limited terms of pH and gas production.   Mass balances
conducted by these researchers showed that a significant amount of heavy metals was
removed from the bulk solution in anaerobic digestion, but no effort was made to investigate removal mechanisms or the ultimate chemical forms assumed by these metals.
Significantly, it was found that the anaerobic digesters receiving wasted secondary sludges
were far more sensitive to elevated heavy metal concentrations in the treatment plant
influent than were the activated sludge systems.   This was primarily due to the fact that
the heavy metals were removed from solution in secondary treatment and were concentrated in the aerobic sludges.   Barth et al. found that such removals (as high as 50 to
60%) resulted in high levels of heavy metals in digester feed sludges which were 6 to 37
times the metal concentrations of the treatment plant influent [ 1 ].   Other researchers
have noted the same phenomenon with mercury, cadmium, lead, manganese and iron
[5-9).   Data collated from various sources in the literature indicate a general relationship
between influent sewage heavy metal concentrations and metal concentrations found in
the combined sludges of municipal treatment plants as shown in Figure 1 for chromium,
copper, nickel and zinc [7, 10, 11].
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