22    DETAILED ENGINEERING DESIGN AND FULL-SCALE
TESTING FOR TREATMENT OF AN EP-TOXIC
SLUDGE
Robert Chiesa, Group Leader
Ajit K. Chowdhury, Senior Chemical Engineer
James E. Morse, Senior Engrg. Technical
RMT, Inc.
Madison, Wisconsin 53708-8923
Thomas R. Wirth, Senior Environ. Engineer
General Motors Corporation - Central Foundry Division
Defiance, Ohio 43512-0070
INTRODUCTION
General Motors Corporation-Central Foundry Division (GMC-CFD) operates a ferrous foundry in
Defiance, Ohio. Particulate emissions from the cupola operations are collected by wet cap and venturi
scrubber emission control systems. The wastewater from the emission control systems is conveyed to a
settling basin, and the effluent is reused in the plant. The accumulated sludge in the settling basin has
been periodically removed by hydraulic dredging, and disposed in an on-site, state-approved, solid
waste landfill.
Previous testing of the wet cap and scrubber emissions (at point of origin) verified that they were
nonhazardous. However, in the process of complying with NPDES permit requirements, several
sludge samples from different parts of the basin were collected and analyzed. Slightly more than half
of these samples released lead in the EP Toxicity test above the hazardous waste criterion of 5.0
mg/L.
Bench-scale treatability tests were conducted with different chemical additives to evaluate various
treatment possibilities for rendering the sludge non-EP Toxic. Because hydraulic dredging would
likely be used to remove the sludge from the basin, these bench-scale tests were conducted over a range
of solids contents. Results indicated that the EP Toxicity lead concentration could be reduced to less
than 2 mg/L using a phosphoric acid (H3P04) dosage of 2.06 gallons H3P04 per ton of dry sludge
solids. The bench-scale tests also showed that both the pH and the residual dissolved phosphate
concentration of the wet sludge after treatment were good indicators of treatment effectiveness.
A process assessment was performed to evaluate several treatment options and develop a conceptual design for the selected treatment system, based on technical, economic, and regulatory considerations. Based on the bench-scale treatability testing and process assessment, the selected treatment
process included hydraulically dredging the basin sludge (approximately 400,000 cubic yards) to an
existing on-site landfill designed to handle dredged slurries, with in-line chemical treatment in the
slurry pipeline to render the solids non-EP Toxic. The treated solids would settle in the landfill, and
the supernatant would be returned back to the settling basin. Because the settling basin was a critical
part of the foundry's wastewater treatment system, the dredging and treatment were required to occur
while continuously operating the settling basin. Detailed engineering was performed, the system was
constructed, and field trials using full-scale equipment were run to verify the ability of the treatment
system to render the solids non-EP Toxic. Based on the field trial results, modifications were made to
the treatment system, and the basin was remediated by dredging and treating the sludge.
This paper focuses on the detailed engineering design and full-scale field trial phases of the project.
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
187