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.
The benefits of implementing these cleaning recommendations for leachate collection pipes will help keep the pipes clean and fully functioning; helping landfill operators prevent the potentially serious complications of clogged pipes and immovable leachate.
By Dr. Ali Khatami, P.E., SCS Engineers
Some states require that leachate collection pipes be cleaned with high-pressure jets on a regular basis (for example, every ten years or even more frequently); however, the rules don’t clarify or set forth the specific conditions under which the jet cleaning should be performed. Some landfills have undertaken jet cleaning while the pipes are partially or fully submerged in leachate above the liner. Unfortunately, jetting under water may drastically reduce the effectiveness of the pressurized jet, resulting in a pipe that is not cleaned properly. This is even more important when the jetting is intended to remove biological growth on the pipe walls and in the perforation openings.
In addition, many states do not require videotaping the pipe after jetting. Videotaping is the best way to verify that the pipe was cleaned successfully. If the leachate collection pipes are not properly cleaned, then over a period of 20 years or so, they can be adversely impacted by severe biological growth and buildup in the pipe perforations to the point that liquid can no longer enter the pipe.
Another shortcoming is that the rules do not specifically require that the riser pipes (where the submersible pumps are located) be cleaned or videotaped. Therefore, due to the added costs, some landfill operators may not clean the riser pipes as part of the required cleaning events, or they may delay such cleanings for an extended period. This can prevent leachate from flowing into the riser with the direct and serious consequence that leachate cannot be removed from the sump.
Another issue to consider is that pressurized jet cleaning procedures do not necessarily push the solids that separate from the pipe wall out of the pipe inlet opening through which the cleaning nozzle entered the pipe. As a result, these solids flow out of the pipe and into the gravel bedding on the outside of the pipe, and can potentially clog the void within the gravel pack around the pipe or in the sump. Clogging the sump gravel can mean reduced flow capacity from the leachate collection pipe to the riser pipe and the submersible pumps.
To resolve these issues, SCS recommends the following:
The benefits of implementing these recommendations will help keep the pipes clean and fully functioning. These suggestions help prevent potentially serious complications that the landfill operator may have to address if the pipes are clogged and leachate cannot be removed.
Questions? Contact Ali Khatami, PhD, 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. Dr. Khatami has acquired extensive experience and knowledge in the areas of geology, hydrogeology, hydrology, hydraulics, construction methods, material science, construction quality assurance (CQA), and stability of earth systems. Dr. Khatami has applied this experience in the siting of numerous landfills and the remediation of hazardous waste contaminated sites.