Fly Ash Stabilization of Sludge in a Landfill, 1994 Environmental Conference Proceedings

Lyle N. Tracy, Roy A. Koster, Robert Stalford, Scott R. Settlemire

Boise Cascade Corporation has operated the Farrington Mountain Landfill since 1976. Prior to 1992, the majority of waste landfilled consisted of effluent treatment plant (ETP) sludge which was dewatered by vacuum filters to a solids content of approximately 20 percent. This material was low in strength due to its high moisture content, and this low strength limited the slope and depth of waste that could be placed in the landfill. In general, the strength of sludge gradually increases in-place via consolidation if water expressed from the sludge is able to drain. In the southern portion of the landfill, identified as Phase II South, vacuum-filtered (VF) ETP sludge has not been able to drain adequately and thereby consolidate following placement. The presence of this sludge severely limits the long-term development options for the landfill.

To expand its landfill capacity, Boise desired to increase the permitted side slopes of the waste fill from the currently licensed 5H:1V (horizontal to vertical) to 3H:1V, and to increase the overall depth of the waste. To do this, portions of the VF sludge in Phase II South will be mixed with ash generated by the mill’s multifuel boiler to provide increased strength. The multifuel boiler ash exhibits pozzolanic characteristics and has been observed to solidify in the landfill when wetted with water or wet sludge. Based on the results of a geotechnical evaluation, expansion to 3H:1V sideslopes and increased final grades is possible, provided the existing VF sludge in the critical Phase II South area is stabilized to meet acceptable strength parameters. It was concluded that stabilization of the wet VF sludge by mixing with a minimum of 30 percent boiler ash by total weight (under laboratory conditions) will provide strength parameters necessary to meet or exceed minimum acceptable safety factors for stability required by the Maine Department of Environmental Protection. This paper addresses the geotechnical and stability evaluation performed to support the expansion.