Heavy Metal Distribution in Anaerobically
Digested Sludges
MICHAEL S. GOULD, Graduate Assistant
EMIL J. GENETELLI, Professor
Department of Environmental Sciences
Rutgers University
New Brunswick, New Jersey 08903
INTRODUCTION
Heavy metals in sludges emanating from biological wastewater treatment systems have
become a topic of intense interest due to their potential as toxic agents in the environment.
These sludges, after anaerobic digestion, are commonly disposed of in landfills,
incinerators, oceans or on the soil. The chemical and physical characteristics of the sludges
are highly variable, but in general contain significant levels of heavy metals. Industries are
responsible for a portion of this problem, but data indicate that sludges from esentially
domestic wastewater sources can also contain substantial amounts of heavy metals (1,2,3).
Therefore complete elimination of heavy metals at the source is virtually impossible. It is
necessary to assume that heavy metals in sludge will persist and work should progress
towards limiting their detrimental effects in addition to removing them.
Just the presence of heavy metals in sludge does not mean that toxic effects will occur.
The concentration of heavy metal must reach a threshold level and be "available."
Availability depends on the chemical specie of the heavy metal present in the sludge.
Possible species are listed in Table I.
The nature of the heavy metals in sludge will affect their toxicity in the environment.
Dotson (4) states that only soluble trace elements are available to plants. It is known that
heavy metals are strongly partitioned into the solid phase (suspended solids) fraction of
sludges (5, 6). This could be the result of a variety of mechanisms.
OBJECTIVE
It was the objective of this work to determine if, within the solids fraction, particle size
affects heavy metal content. A surface effect may indicate what could be the major
mechanism of binding. Soluble metal concentrations indicate if solid phase precipitates
and/or complexation is involved.
MATERIALS AND METHODS
Samples of anaerobically digested sludge were composited from anaerobic digesters.
The sources of sludge was a municipal wastewater treatment plant in Keasby, New Jersey
which treates a wastewater that is approximately 80% domestic and 20% industrial. The
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