Leachate management at unlined landfills
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by Matt Evans
There are two by-products of landfill activities that must be managed: leachate and landfill gas. Leachate is precipitation and waste moisture that accumulates within a landfill. Landfill gas is a by-product of anaerobic decomposition of the landfill waste. The focus of this article is on the management of leachate; however, management of leachate and landfill gas frequently go hand-in-hand because the two can often be managed within a single system, as described in the case study presented at the end of this article.
History and background
The current state-of-the-art for landfill design is to include the installation of a liner and leachate collection system at the base of new landfills. The purpose of these systems is to prevent the flow of leachate into underlying soils and groundwater, and minimize the pooling of leachate at the base of a landfill. Solid waste rules typically limit the level of leachate head at the landfill base at 30 cm. Many older landfills were constructed without liners and leachate collection systems, and subsequently have leachate migrating into underlying soils and groundwater, or accumulating at the base of the landfill as a result of underlying impermeable soils. Regulatory agencies have applied the 30 cm rule to require leachate extraction at these older landfills.
 The track mounted drill rig at the Sioux Falls Regional Sanitary Landfill |
Both developed and developing countries have unlined landfills. Although they are being phased out in many parts of the world, there are still many active unlined landfills. Even in areas where unlined landfills have been phased out, there are many closed unlined landfills without leachate collection or management. Unmanaged leachate within a landfill can create a number of problems, including slope instability, groundwater contamination, and slope seeps which then drain into surrounding surface water.
Leachate management issues frequently provide opportunities for installation of a leachate extraction system. Conditions that preclude the installation of such a system include: limited disposal options for the extracted leachate, limited concern for the unmanaged leachate within the landfill, permeable underlying geology, waste type, daily soil cover practices, and cost.
Geological characteristics at a site have an important effect on an unlined landfill. For example, the underlying soil type may have a low permeability (such as clay), causing leachate to pond on top of the underlying soils. In areas with highly permeable soil types (such as sand), contamination of underlying groundwater and soils is a concern.
Leachate extraction system design considerations
There are three primary ways to design a leachate collection system for an unlined landfill. These include installation of vertical collection wells, horizontal collectors, and perimeter collection well or French drain systems.
n Vertical wells: In this type of installation, vertical wells are drilled into a landfill and pumps are fitted within the wells. These pump the leachate collected within the wells out into a surrounding leachate lateral system. The case study presented below describes a vertical well system design.
n Horizontal collectors: Horizontal collectors would be directionally drilled along the estimated base of a landfill or open-trenched within the waste. The theory is that the leachate will drain down through the landfill and into the underlying horizontal collectors. Horizontal collectors have had mixed success because the exact bottom of the landfill is often unknown in unlined landfill situations, and there is no way to place a gravel or sand drainage medium around the collector.
n Perimeter collection wells or French drains: In areas where the landfill is draining into a highly permeable soil type (such as sand) vertical collection wells may be drilled around the perimeter of a landfill. In this case, the leachate within the landfill has drained out of the base of the landfill and into the underlying groundwater aquifer. The perimeter collection wells intercept the leachate-impacted groundwater as it migrates off-site. In low permeability soils, a French drain can be trenched around the perimeter to intercept perched groundwater. A slurry wall can be installed in conjunction with this drain to optimize groundwater collection.
In all three of these designs, the leachate is most often transported to a main leachate header system that surrounds the landfill. The leachate is then transported to a leachate storage tank, pond, or sewer line for treatment and/or disposal. There are a number of ways to complete the laterals and header design, and much of the layout and design of this portion of the leachate collection system depends on the topography of the landfill and surrounding area, as well as the future expansion plans of the landfill. In addition to the layout of the header and lateral system, there are a number of other design considerations that need to be reviewed, including pump type (i.e. electric or air-driven), and the well/collector material type (i.e. HDPE or PVC).
Benefits of effective leachate management
The concerns associated with leachate accumulation in an unlined landfill can be reduced by installing a leachate extraction system. As leachate is extracted from the landfill it is important to note that a secondary by-product of the leachate extraction will likely be increased landfill gas production. The increase in landfill gas production is due to the increased rate of waste degradation now that the waste material is no longer saturated.
The increase in landfill gas may have economic benefits due to the increased viability of landfill-gas-to-energy projects and carbon offset trading. However, it is important to note that the increased landfill gas production can cause odour problems and landfill gas migration problems that were otherwise not a concern. Because of these potential negative aspects of the increased landfill gas production that accompany the leachate removal, it is good practice to include a plan to control landfill gas in conjunction with the leachate extraction system.
Case study – Sioux Falls Regional Sanitary Landfill
Over the past three years, R. W. Beck has worked with the City of Sioux Falls to design a dual-phase leachate and landfill gas extraction system in the unlined 100 acre (40.5 ha) portion of the City of Sioux Falls Landfill, located approximately 6 miles (9.7 km) west of Sioux Falls, South Dakota. The system consists of 42 (of approximately 100 eventual) vertical leachate and landfill gas extraction wells, connecting laterals and headers, leachate drains, and lift stations to dewater the landfill. Historically high leachate levels (up to 60 feet (18 metres) above the base of the landfill) need to be lowered in order to satisfy regulatory closure requirements and allow landfill gas collection and recovery. R. W. Beck designed a unique extraction system utilizing a single well casing and drain system, combining both gas and liquids recovery. Air-driven leachate pumps were installed in each well, discharging leachate to the perimeter landfill gas header pipe where the leachate and condensate are separated out at six different lift stations.
Well installation and construction
On May 24, 2005, Terra Engineering and Construction Corporation of Madison, Wisconsin, began the installation of the dual-phase wells. The wells were drilled with a track-mounted drill rig. The rig drilled a 36-inch (91 cm) borehole to depths of over 100 feet (30 metres) below the landfill surface.
A 6-inch (15 cm) diameter CPVC Sch. 80 well screen and casing were installed into the completed borehole. A gravel pack was placed into the borehole to one foot (30 cm) above the top of the well screen. On top of the gravel pack, the annular borehole space was filled with general fill (which is a low-permeable clay material at the landfill) up to two feet (60 cm) from the surface. The final two feet were filled with bentonite. Figure 1 is a schematic representation of the dual-phase well design used as part of the 2005 phase of the construction.
 figure 1. Schematic of the dual-phase well design |
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Well head design
The well head was designed to collect both the leachate and landfill gas from the well and distribute it into a single lateral pipe.
The original design was to enclose the wells in a manhole to reduce concerns about freezing – winter temperatures in the area can drop below 0 degrees Fahrenheit (-18C). However, after installation of the wells it was determined that a manhole around the well head would be problematic for two reasons: the manhole would create a confined space, and when the landfill settles the manholes would settle, causing the top of the well heads to push through the top of the manhole.
Without the manhole around the well heads, the pumps are more susceptible to freezing. Freezing can be prevented if a low flow of leachate through the pump is maintained. Even if the pumps do freeze during portions of the winter, they will restart when the weather warms. The well head design was modified in 2007 partly to reduce freezing concerns by reducing the length of exposed leachate hose.
Extraction system piping network
After the leachate and landfill gas are collected at the well head, the leachate and landfill gas are distributed off the landfill in a series of lateral and header pipes to a perimeter manifold pipe. The system consists of ten branches of laterals and headers that connect into the perimeter manifold pipe. The following subsections describe the network of piping. Figure 2 presents the layout of the lateral and header system for the wells installed as part of the first phase of the system in 2005.
 figure 2. Lateral and header system layout for wells installed in the first phase |
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Lateral and header systems
Shallow trenches for the HDPE lateral and header pipe were dug into the landfill surface using a backhoe. Trenches located on the crown of the landfill maintained a 4% minimum slope to counter future settlement. Trenches located on the side-slopes of the landfill maintained a 5% minimum slope. Survey elevations were collected at 25-foot (7.5 metre) intervals during the trenching excavations as part of the field quality analysis/quality control (QA/QC). Maintaining the minimum slope requirements on the crown of the landfill is essential in order to drain the leachate, as well as maintain effective landfill gas collection. If leachate and condensate fill up the collection pipe due to insufficient slope, the flow of gas through the pipe will be reduced, or stopped.
Perimeter manifold
The leachate and landfill gas header pipes connect into a 12-inch (30 cm) HDPE leachate and landfill gas manifold pipe. Once in the manifold pipe, the leachate drains by gravity to one of the six lift stations. At the lift stations, leachate is separated from the landfill gas. The lift stations are discussed in more detail below. The locations of the perimeter manifold pipe layout and lift stations are shown in Figure 2.
 Perimeter manifold installation |
The manifold was sloped at a minimum of 1% when the gas flow was concurrent with the flow of the leachate and at 3% minimum slope when the gas flow was counter to the flow of the leachate. The 3% slope is necessary to prevent the leachate from forming waves when flowing against the flow of landfill gas. This wave action within the leachate could disrupt the flow of landfill gas.
Lift stations
The lift stations are constructed of a single (continuous) section of 30-inch (76 cm) diameter HDPE pipe. A water seal of 5 feet (1.5 metres) is maintained within the lift station, in order to prevent atmospheric air within the lift stations from being pulled into the manifold pipe when a flare system or landfill gas utilization system is operating. A vacuum of up to 60 inches (1.5 metres) of water column can be applied to the manifold pipe in order to draw landfill gas to the landfill gas utilization system or flare system. Other design considerations with flare include cleanouts for access into the pipe system, and valves for isolating the lift station without taking the entire system off-line. Figure 3 is a schematic representation of a lift station.
 figure 3. Schematic of a lift station |
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Success so far
The leachate extraction system began operation on 10 October 2005. To date, over 2,000,000 gallons of leachate have been pumped out of the landfill. The system has also been successful in collecting and flaring landfill gas, which it is presently doing at a rate of approximately 1000 cubic feet (28 m3) per minute. The City is also pursuing a landfill-gas-to-energy project.
Matt Evans is a registered professional engineer with a Bachelor of Science degree in Civil Engineering from Washington University in St. Louis, Missouri. The majority of his engineering consulting work has been in the area of solid waste management, focusing primarily in the development, permitting, design, and construction of landfills.
e-mail: mevans@rwbeck.com
