The purpose of a hazardous waste landfill is to isolate wastes which are not yet recoverable. To insure proper hazardous waste confinement, state, federal, and local regulations maintain standards for the design, construction, and operational stages of all land application sites.
The law considers hazardous waste to be a solid waste or a combination of solid wastes, which because of its quantity, concentration, or physical, chemical or infectious characteristics may :
A. pose a threat to human health or the environment when improperly managed.
B. cause, or significantly contribute to an increase in mortalitity or an increase in serious irreversible, or incapacitating reversible illness.
To minimize the potential of adverse environmental effects from wastes deposited at hazardous waste landfill sites, the EPA has produced specific regulations regarding the characteristics of wastes suitable for landfilling.
A. Noncontainerized hazardous wastes containing free liquids, whether or not absorbents have been added.
B. Containers holding free liquids unless all freestanding liquid has been removed by decanting or other methods or has been mixed with absorbent or solidified so that freestanding liquid is no longer observed.
Geologically speaking, hazardous wastes are deposited below grade in an existing natural or excavated depression. Landfilling then proceeds above grade be mounding the soil and waste creating more disposal capacity. Compatible wastes are disposed of together. Waste separation is accomplished using individual control cells within the landfill.
Control cell sizes vary depending on the rate of waste inflow and the characteristics of the waste being disposed of. The general range for the cell height is from 16 to 20 ft. Two methods of depositing hazardous waste into the control cell includes application of hazardous soils and containerized raw wastes. Bulk hazardous waste in soils is deposited and then compacted. Containers of solidified hazardous wastes are placed upright in the cell leaving sufficient space imbetween for the application and compaction of compatible bulk hazardous wastes. Cover soils is placed over all applied wastes at the end of the working day to impede rainfall infiltration and to control odor and airborne emissions.
(figure 1) Dual liner landfill system. Drawing by Glenn Newsome
Regulations require that all hazardous waste landfill sites provide a system which will control and collect leachate. Prior to waste application, a double liner system (figure1) is placed on the bottom and the side slopes of the land fill. The liner system catches leachate generated by rain infiltration and deposited wastes and feeds it to the collection system using gravity. The bottom of the landfill is sloped to facilitate the removal of leachate. Leachate that accumulates in the bottom of the land fill is guided by perforated plastic collection pipes(figure 2 ) to one or more centralized collection sumps. From here, the leachate can be pumped out for proper treatment and disposal.
(figure 2) Installation of leachate collection pipes. Photo by Brent Showalter
EPA regulations require that the bottom liner be a layer of recompacted clay (figure 3) with a minimum thickness of 3 ft and a permeability of no more than 1 * 10-7 cm/s. The upper liner must be an FML (flexible membrane liner). The number of collection pipes required is dictated by how many will prevent the leachate from exceeding 1 ft in depth above the bottom liner. The placement of a low permeability cap (figure 4) over the completed portion of the landfill also prevents leachate generation. The caps require a permeability less than or equal to the liner on the bottom of the landfill.
(figure3) Compacted clay after force testing. Photo by Glenn Newsome
Surface water originating off site is often directed away or around the site using pipes or ditches designed to handle at least a 24 hour, 25 year storm. Runoff from exposed excavation areas is directed to siltation basins and then discharged off site. Runoff from the active landfill area is directed to holding sumps where it is sampled for contamination. If contaminated, the water must be treated before discharge off site. Runoff from completed landfill surfaces is treated the same way, but on a larger scale than active landfill area runoff. All on site runoff collection and holding facilities must be designed to handle runoff volumes resulting from a 24 hour, 25 year storm.
(figure 4) Drawing by Glenn Newsome
Monitoring for leachate emissions in the vadose zone, ground water, and surface water is required by regulation and permit conditions. In the monitoring of ground water, monitoring wells are installed to a depth of around 10 ft below the historic water table depth. The position of the monitoring well is determined after analysis of the maximum probable extent of leachate contaminant migration.
Send comments or suggestions to:
Student Authors: Glenn Newsome and Brent Showalter
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
