HEAVY METALS INTERACTIONS IN THE ANAEROBIC
DIGESTION SYSTEM
James W. Patterson, Professor
Pritzker Department of Environmental Engineering
Illinois Institute of Technology
Chicago, Illinois 60616
Shi-Sheng Hao, Research Chemist
Peer Consultants, Inc.
Rockville, Maryland 20852
INTRODUCTION
Historically, there has been widespread concern relative to the inflow of heavy metals into
publicly owned treatment works (POTW). The initial basis for this concern was the possibility of toxic impacts on the biological processes of the POTW. This concern has now been
largely alleviated as a result of the establishment of local pretreatment ordinances regulating
industrial dischargers. However, other concerns remain, as a result of an evolving awareness of
the water quality implication of discharge of large volumes of POTW effluents incorporating
trace quantities of heavy metals [ 1 ], as well as the implication of large scale accumulation of
these heavy metals into POTW sludges.
The presence of certain of these metals in municipal sludges will adversely affect sludge
management options available to the municipal authority. As examples, high metal levels can
result in air pollution, ash disposal and mechanical operating problems for sludge incineration.
The presence of the single metal, cadmium, at excessive levels can, under proposed federal regulations, prevent the disposal of a sludge into land [2]. Currently, the approximately 18,000
U.S. POTW generate 5 million tons (dry wt) per year of sludge. About 25% of this sludge is
landfilled, 25% is spread on land surfaces (mainly agricultural land), 15% is disposed of in the
oceans, and 35% is incinerated [3].
Numerous reports on the toxic effects of heavy metals on the anaerobic digestion system
have appeared in the literature. Relatively little is known, however, about the physical-chemical
interactions within the process between the digesting sludge mass and the influent metals
associated with primary and excess secondary sludges. There is little information available on
the speciation or redistribution of metals within the anaerobic digestion process. There is a
need for improved understanding of the various physical-chemical phenomena controlling heavy
metal interactions in the anaerobic digestion system, in order to explain and predict the dynamics of metals transport through and distribution within the anaerobic digestion system. This
predictive capability can guide the design and operation of plants to meet required metal
levels in sludges, and can also define maximum POTW influent metal levels which can be tolerated if discharge requirements are to be met.
EXPERIMENTAL METHODS
This study was conducted to study metal dynamics in the anaerobic digestion system. It
was desired to have the experimental conditions simulate modern full-scale digester operation as
closely as possible. To achieve this, two 14-liter digesters, each connected as shown in Figure
1, were operated on a 14-day detention time at 35 C. The digesters were manually shaken
twice each day, to provide mixing of the digester contents.
The digesters were fed with secondary sewage sludge from the west-southwest plant of the
Metropolitan Sanitary District of Greater Chicago (MSDGC). A 1-liter volume of anaerobically
digested sludge was withdrawn daily from each digester for analysis, before feeding an equal
volume of secondary sewage sludge. The aliquots of digester contents withdrawn daily were
subject to frequent routine analyses, for process control. Analyses performed included pH,
alkalinity, volatile acids, chemical oxygen demand (COD), total and soluble sulfides, soluble
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