HEAVY METAL REMOVAL FROM SLUDGES USING
COMBINED BIOLOGICAL/CHEMICAL TREATMENT
Thomas D. Hayes, Research Associate
William J. Jewell, Associate Professor
Randolph M. Kabrick, Research Specialist
Department of Agricultural Engineering
Cornell University
Ithaca, New York 14853
INTRODUCTION
Sludge disposal for many cities in the United States has become a problem of increasing
dimensions. The U.S. now generates about 4.8 billion dry kg per year of sewage solids
which should be disposed of or utilized in a manner posing the least amount of threat to
the environment. At present, most of this sludge is disposed of by means of landfill, ocean
dumping and incineration; relatively small amounts are applied to agricultural lands. In
light of certain emerging environmental, energy and economic constraints, a shift toward
a greater reliance on land application for sewage sludge disposal appears likely.
Land application, however, is not without its own particular set of problems. The prospect
of applying increased amounts of sludge to the land has called considerable public attention
to the potential effects of this practice on the public health. In particular, the presence of
cadmium and other heavy metals in land-applied sewage sludges has become a subject of
increasing concern among scientists and the general public, alike. Within the decade, evidence
has been found indicating that cadmium in sludge-amended soils can be accumulated and
magnified in the food chain [1-4]. Moreover, cadmium has been reported to accumulate
in the human kidney to life-time levels not far below concentrations that would be expected
to produce damage to this organ [5]. In response to these findings, governmental guidelines that attempt to establish "safe" limits for sludge cadmium loadings to cropland
could in effect require many sanitary districts to use greater land areas for the disposal of
high-cadmium sludges. The logistical and economic difficulties that would result from
sharp reductions in sludge loadings to land are incalculable; clearly, the problem of limiting
cadmium discharges to the environment emphasizes the need for a new generation of treatment technologies capable of removing cadmium and other potentially hazardous heavy
metals from municipal sludges. This preliminary study was undertaken to determine the
potential of using combined biological-chemical treatment to solubilize cadmium in
sewage sludge and, hence, to convert it to a form that could be removed with the water
fraction upon dewatering.
BACKGROUND
The removal of heavy metals from municipal sewage sludges has seldom been emphasized
in the past as a necessary treatment step. Consequently, heavy metals recovery from
municipal sludges is a relatively new concept as reflected by its neglect in the literature.
Even today, in light of recent food-chain cadmium revelations, there is still room for debate
over the methods that should be used to limit sludge cadmium loadings to croplands.
Nevertheless, research on sludge metal stripping as a treatment step preceeding land application could provide alternatives for municipal sludge management that would be difficult
to ignore.
Heavy metals are common components of sewage sludges because they can be removed
from dilute wastewaters, passing through a municipal treatment plant with a high degree of
efficiency. Sources contributing metal loadings to municipal sewers are numerous and
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