Leachate Recycle Process Offers Pros And Cons

G. Fred Lee, Anne Jones-Lee

Alternate disposal options are a thriving topic in the 90s and federal regulations have brought one disposal concern into the limelight - leachate.

Recycling leachate in municipal solid waste (MSW) landfills can provide a means of disposal, enhance the rate of landfill stabilization (fermentation leading to gas) and can save money when leachate recycling is not available at a publicly owned treatment works. Leachate recycling also has been spurred by problems with dry-tomb landfilling, which can postpone, rather than prevent, groundwater pollution from landfill leachate.

Although leachate recycling is gaining recognition, the merits of recycling MSW leachate are controversial. In the mid-1980s, some regulatory agencies encouraged leachate recycling in MSW landfills, while others prohibited it. Critics said that because it increases the hydraulic loading to the landfill, it can increase the rate of groundwater pollution.

While leachate recycling can be conducted in single-composite-lined landfills that conform to Sub-title D requirements, groundwater monitoring programs for lined landfills have little chance of detecting pollution if it leaks through a composite liner. Because the initial leakage from an FML-lined landfill, such as a Subtitle D landfill, comes through holes, rips, tears and areas of deterioration in the FML, this type of leakage produces finger-plumes of leachate-contaminated groundwater that can go undetected between the vertical monitoring wells.

A double-composite lined landfill, in which the lower composite liner is a full-landfill area pan lysimeter leak detection system for the upper composite liner, can help prevent groundwater pollution. This system allows operators to detect leakage of the upper composite liner before the groundwater is polluted. When leachate is found in the lysimeter system, recycling must stop and the waste exhumed if the leakage through the upper composite can't be stopped.

Plastic bags can prevent recycled leachate from interacting with the MSW. Since landfill gas production rates depend on the waste's moisture content, waste that is not exposed to the recycled leachate will produce landfill gas at a slower rate than fully-exposed wastes. Shredding waste before burial can help shorten the duration of landfill gas production and decrease the time for landfill stabilization.

While leachate recycle can reduce the strength of some MSW leachate, it still can pollute large amounts of groundwater. Leachate recycling is best used during the early stage in a fermentation/ leaching, wet-cell approach to MSW management. In that approach, the leachate recycle-accelerated stabilization period is followed by a period of washing the fermented MSW residues with clean water. During this time, residues that would leak out of the landfill and pollute groundwaters are deliberately leached out of the wastes in a controlled manner. Without the clean-water leaching step and a leak detection system, leachate recycle will not protect groundwater resources. At this time, however, Subtitle D regulations prohibit adding clean-water to leach removable components of waste, which is an integral part of the wet-cell approach.

Leachate recycle offers several pros and cons, all of which should be carefully weighed.