Gas production during the active life of landfills is a well-known phenomenon, with many means to collect and dispose of landfill gas already developed and implemented in landfills across the world. What is less known in the industry is that concentration of landfill gas near the lining system can reach significant levels, causing high gas pressure developing in, and around, the leachate collection drainage layer. High-pressure gas can potentially fill voids within the drainage layer (geocomposite or sand), causing conditions impeding flow in the drainage layer, adversely affecting the free flow of leachate.
Leachate collection pipes encased in gravel are pervious media through which landfill gas can easily travel and high pressures transfer to the sump area. Such conditions can cause significant odors near the sump due to emissions of landfill gas through the drainage layer and the overlying sand layer on the side slope of the perimeter berm near the sump. In addition, high-pressure builds in the riser and cleanout pipes.
Consider an effective gas pressure removal system in the sump by installing vertical pipes on the riser pipes behind structures, on top of the perimeter berm, shown here. The vertical pipes are blind-flanged initially at cell construction completion. If the gas pressure build-up becomes significant, or odors are detected in the sump area, the landfill operator connects the vertical pipes to a vacuum source near the sump.
Using a connecting pipe to a vacuum source can also be used to discharge condensate from the gas collection and control system directly into the leachate collection riser pipe.
For a double lining system, with a riser pipe in the primary system and another in the secondary system, both risers will have vertical pipes on them, and both connected to the vacuum source.
However, condensate flowing down the connecting pipe from higher elevations toward the risers should not enter the secondary system. Block it by using a manifold, as shown in the image.
Operators may have a vertical pipe installed on the leachate collection pipe cleanout to apply vacuum directly to the leachate collection pipe.
Keeping gas pressure low in and around the leachate collection pipe promotes the free flow of leachate through the geocomposite or granular medium drainage layer to the leachate collection pipe, and improves leachate removal from the disposal cell.
Using gas removal piping at leachate sumps is highly recommended for warm or elevated temperature landfills where efficient leachate removal from the leachate collection system is another means for controlling landfill temperatures.
About the Author: Ali Khatami, Ph.D., PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.