Tackling the Landfill Gas and Leachate Conundrum

Landfills are designed to enable liquids to flow to the leachate collection system at the bottom, but inevitably some of it won’t make it there; decomposing waste eventually becomes so dense that some liquids can’t penetrate it and become trapped between layers of trash and soil. These perched liquids can migrate into the gas collection system, impeding gas extraction. Tackling this leachate and gas conundrum calls for orchestration of a complex balancing act. 

Better pumping systems, new drainage features, and new well designs are part of the strategy. But even with these tools, keeping liquids/leachate moving along its intended course while maximizing gas collection is a continuous battle, increasingly so as operators take in more wet waste.

Pete Carrico, an environmental scientist at SCS Engineers, explains the challenge:  

“When you drill an extraction well and insert perforated piping and surrounding gravel, the liquids pass through the gravel (stone or rock) into the piping, and the well fills with fluid, impeding flow of gas that ordinarily would travel upward. Even once the well is complete, liquids keep draining into the gravel pack, so it’s an ongoing challenge.” 

Consequences could extend to impact on other systems, even slopes. Enough gas pressure could open cracks, and ultimately contribute to slope instability, in turn triggering other issues. So, removing liquids from extraction wells is crucial on many fronts. 

“The main goals are to improve gas flow and, at the same time, control odors, emissions, and heat from landfill (liquids hold heat more effectively than gas). If the gas is used for energy production, we improve that too. The more gas you can get the better,” Carrico says.    

A common approach is to install pneumatic pumps in wells to draw down liquids that block the slot lines in the pipes intended to collect gas.  

However, this dewatering process has caveats. Well-derived liquids tend to be higher in certain undesirable constituents than landfill leachate and can cause problems at publicly owned treatment works (POTWs) where they are sent, comingled with landfill leachate. Additionally, dewatering can be costly. Pumps require ongoing maintenance. And it takes a lot of skill to run them, as there are nuances, namely around how fast the pumps cycle.   

If they run too fast, they can draw in solid particles, so the idea is to pump at a rate to mitigate clogging of the slot lines in the pipes and see that the perforated piping can go back to doing its job of collecting gas.   

One emerging trend, designed as an alternative to pumping, is to build a stone structure at the bottom of the landfill before placing waste, rather than drill a well into the mass once it accumulates. As waste accumulates, this porous column (sometimes called a gabion well) serves as a conduit from the top down to the leachate system at the bottom.  

Matt Stutz, an environmental engineer at Weaver Consulting, explains how it works and benefits:  

“This bottom-up feature allows for liquids to pass directly down to the leachate system and gas to come up as it’s intended to, preventing liquid from accumulating in wells and facilitating gas extraction. And gas can be collected much earlier –soon after waste placement. These wells have been constructed at a few sites and are looking promising,” he says.  

Carrico’s team has also seen encouraging results with this system and says the gas flow can be several times, to 10 times, greater than with traditional wells drilled down into waste that has been placed in cells.  

Similar to this vertical well concept, some collectors are designed to allow both liquid and gas to travel along their appropriate paths. Slope collectors in particular are gaining traction. They are placed on interior slopes as waste is being filled. Liquids drain down to the leachate collection system and gas exits from another part of the collector. It’s the same concept as the vertical feature, with the only difference being it runs on a diagonal and is lighter so it can be built on a slope. 

For now, pumping wells remains the most common method to stave off liquid-related gas problems, but operators who go this course have to stay on top of their game.  

Operators should have a maintenance program where they routinely pull and clean pumps.  They should also conduct regular surveys of a well’s regulator counters to determine if pumps are cycling efficiently, advises Edwin Calvache, an environmental engineer at TetraTech.  

“But what’s most important is to measure water levels to determine if and how much liquid is blocking the pipes.  Without this information, you do not know how efficient your gas collection is. You look at two measurements: depth to bottom and depth to water, which indicate your water column,” he says. 

Calvache advises that good standard practice is to have more than 50% of the perforated pipe open to maximize landfill gas collection. 

Operators do need to think about potential issues with meeting POTW’s requirements.  Stutz has seen facilities turn down comingled liquids where the well-derived fraction was likely the problem. There is one recourse; setting up separate force mains off the landfill: one for well liquids and one for leachate. They feed into separate tanks so the well liquids can be tested and treated before comingling, if needed. 

It’s not common, but this is an emerging design trend adopted by a few operators who had issues with their POTW, Stutz says. 

He has this last advice: Know that liquid in a gas well is to be expected. 

“Any time you drill into a media (including waste) that has high moisture content and place a perforated pipe surrounded with gravel, it is anticipated that you will collect some liquids. 

It’s not bad. It’s just one of many conditions that must be addressed. To reduce the effectiveness of a well to a single parameter is not appropriate. You must do a full evaluation to determine if and how liquid levels may be impacting gas collection.”