There’s a lot of science, engineering, and careful maintenance happening inside and outside of North American landfills. This series of four blogs dive into strategic landfill system integration and the systems balancing act that owners, operators, and their teams manage to control high liquid conditions.
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It was once assumed that all (or most) landfill liquids drained to the leachate collection system at the bottom, but increasingly this assumption is not so valid. Rather, liquids accumulate in the waste matrix, posing challenges for landfill owners and operators.
These liquids impair gas extraction as they migrate into gas system extraction features. Wetter conditions in the waste mass reduce slope stability and increase leachate seeps through exposed sideslopes. These factors exacerbate a landfill operators’ ever-present battle with odor control.
High-liquid conditions have escalated at some sites due to recent increasing precipitation trends and the introduction of sludges and other special, wet wastes to offset declining MSW waste streams. Meanwhile, once in contact with waste, liquids become leachate, a costly byproduct to manage (collection, treatment, and disposal). Getting on top of these issues is a top priority for landfill designers, operators, and managers.
Landfill design in theory
Landfills are designed to collect and remove contaminated liquids (leachate) that make their way to the bottom. Designs include a protective liner and a blanket of permeable material such as gravel or sand on top of the liner. Perforated pipes embedded in this permeable blanket drain leachate to low points in each cell for removal, typically through submersible pumps. Or that’s how it works in theory, explains Eric Peterson of SCS Engineers.
“But in reality, the waste near the bottom becomes so dense as it decomposes and compresses naturally that it becomes less and less permeable. The density impedes liquids’ movement through the waste to the leachate collection layer. What liquid doesn’t reach the collection layer sits near the bottom of the landfill or becomes perched between layers of trash, especially ever-increasing quantities of plastic, which create impermeable zones—picture layers of plastic bags and sheeting material interspersed in the waste mass that create multiple zones of perched liquids.
Why else does liquid accumulate in landfills?
In addition to increased liquid levels tied to material density, decomposition, and other waste characteristics, there are contributing factors more related to system design than to the waste itself. One such issue ties to the placement of soil layers on top of the leachate drainage layer.
“Many landfill designers use soil intending to protect the drainage layer and the bottom liner system from damage caused by waste settlement. But that soil becomes compacted and is of low permeability, impeding leachate’s movement out of the waste and into that drainage layer,” explains Bob Dick of SCS Engineers.
He calls out another design-related issue: some operators delay installing the geomembrane final cap and are reluctant to install temporary intermediate exposed caps. These infrastructures are the best defense against infiltration of precipitation, one of the biggest thorns in an operator’s side as they work to stave off excess liquids. Management is becoming harder as precipitation rises, particularly in the Midwest, Northeast, and Mid-Atlantic states experiencing record-breaking rainfalls over the last few years.
The problems posed by liquids naturally occurring in waste and from storms call for robust designs to remove the liquids rather than allowing them to accumulate within the mass.
A few potential solutions
Install a geocomposite drainage net and aggregate of stone or sand and perforated piping at different levels within the waste mass.
This multi-component system collects, and gravity drains leachate to a sump location other than the bottom liner by giving liquids an exit ramp higher up in the landfill. Dick advises placing this system at multiple locations and heights throughout waste lifts, including adding mezzanine collection infrastructure (in the middle of the mass). The idea is for liquids to be captured at different depths, routed to intentional collection points, then removed to short circuit the otherwise tortuous journey for liquids draining through compacted, decomposed waste, daily cover soils, and intermediate covers to reach the bottom.
Aggregate and perforated piping can be of varied configurations; which one you choose depends on multiple factors, such as local availability of stone and aggregate, manpower resources to install and maintain the system, and especially depends on the planned location for the system. Some work well on exterior slopes with lesser volumes of waste and resulting liquids. Others are more suited for interior slopes where waste actively accumulates and generates gas, requiring special features to divert liquids from nearby gas wells. But all have the same overall design concept ─ the idea that liquids will move through the infrastructure on a preferred pathway to the bottom leachate collection layer or mezzanine collection layers.
Install a temporary exposed geomembrane cap and remove it when it’s time to resume waste placement.
Fifteen years ago, installing these temporary caps was uncommon. Operators waited until achieving final grade, then installed a final cap. But Dick says it can take decades to get to final grade. In the meantime, liquid infiltrates the interim soil cover and ultimately saturates waste.
“As we realize the consequences of high-moisture content and excess leachate, more operators are becoming open to investing in these interim caps to reduce the liquid in the waste mass,” he says.
Maintain proper slopes to avoid ponded water.
Rain and melting snow tend to form ponds of water that sit on the landfill’s top deck if it’s flat. With enough accumulation, the liquid drains into the waste. Peterson advises maintaining a deck with at least a 4% slope to facilitate the movement of rainwater and other precipitation off the waste. This is a typical design but is often not implemented. More careful surveying and grading of the waste is needed.
Proactively checking for potential issues
Liquids can accumulate in gas wells, so periodically measuring liquids levels in this infrastructure is a good practice.
“It gives insight into what’s happening with liquid stats and leachate collection system performance that, if impaired from clogging, would block gas collection and impede leachate removal. So, monitoring liquid in vertical wells is a good proactive measure, flagging when you have to work on your leachate system before there is an issue,” Peterson says.
Inspecting leachate drainage pipe cleanouts with cameras should be performed regularly. GIS software can visually display the data and works well to inform operators of their liquids situation.
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In Part II of this series, publishing tomorrow, our landfill engineers discuss the impact, mitigating, and controlling liquid-related issues to circumvent three specific problems: gas collection system inefficiencies, slope instability, and leachate seeps.