IRON CONDITIONING OF SLUDGE FROM FOUNDRY FURNACE
EMISSION CONTROL SYSTEMS FOR ELIMINATION OF EP TOXICITY
William A. Stephens, Manager of Engineering
Thomas R. Stolzenburg, Water Chemist
Residuals Management Technology, Inc.
Madison, Wisconsin
Robert R. Stanforth, Chemist
Kota Kinabolu, Sabah
East Malaysia
James E. Etzel, Professor
School of Civil Engineering
Purdue University
W. Lafayette, Indiana 47907
INTRODUCTION
In a study conducted by USEPA, approximately 40% of the sludges tested from foundry emission
control wastes were classified as "hazardous" using an EP Toxicity leaching test [1]. In this test,
lead and/or cadmium will typically leach at concentrations near or above USEPA's hazardous waste
limits. Disposal of these solids in a hazardous waste landfill can be more expensive than treating the
sludge to reduce lead and cadmium leaching. Various chemical additives have been used either on
the wastewater or on the sludge after settling to immobilize the lead and cadmium. Investigators at
Purdue University and Residuals Management Technology have experienced success with treatment
of the waste using iron. The results and discussion of some of the iron tests will be presented here.
LEACH TEST PROCEDURE
The data presented here are results from submitting drying bed sludge or settling basin sludge
from various foundries to an EP Toxicity leach test. The sludges originate from the air emission
control systems for the metal melting cupolas. The EP Toxicity leach test is a standard USEPA batch
leach test at pH = 5 using acetic acid. This test is required for classifying solid waste intended for
disposal in a landfill [2]. If leachable concentrations of the listed constituents exceed hazardous waste
limits, the solid is classified as hazardous. The limits established for lead and cadmium are 5 mg/1
and 1 mg/1 respectively. These concentrations are based on a 100 gram sample of wet sludge subjected
to 1,600 mL of pH 5 water for 24 hours with agitation.
RESULTS AND DISCUSSION
Alum (aluminum sulfate) is commonly used to coagulate and settle the solids in wastewater. Ferric
sulfate is equally effective. However, ferric sulfate is far more effective in reducing lead leaching in
the resulting settled sludge when subjected to an EP Toxicity test. Cadmium leaching is reduced only
as a result of dilution. Figure 1 shows a sludge that leaches lead and cadmium at levels exceeding the
hazardous waste limits. Treatment of the sludge with aluminum sulfate did not lower lead leaching
to acceptable levels while ferric sulfate did.
In the coagulation process, aluminum and ferric hydroxide colloids form. Research on the adsorption of cations to these surfaces indicates that ferric hydroxide adsorbs divalent cations far more
efficiently and at lower pH values than aluminum hydroxide (Figure 2). Cadmium is adsorbed to a
lesser extent than lead. These results suggest that adsorption of soluble lead cations to ferric hydroxide
colloids is an important mechanism by which lead is immobilized in the EP Toxicity leaching test.
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