MSW Management | Use of Geosynthetics in Bioreactor Landfills

One of the big surprises of landfill operations is the fact that very little waste deposited in a standard solid waste landfill actually decomposes in a reasonable amount of time.

By Daniel P. Duffy

The standard landfill is designed to keep out moisture, entombing solid waste and isolating it from the environment. In fact, another name for the standard landfill is the dry-tomb landfill. So it is not unusual to bore into the heart of a landfill (either dug as part of a landfill gas [LFG] system installation or performed as part of a site investigation) and recover readable newspapers that are several decades old.

A bioreactor landfill, on the other hand, accepts additional moisture as a means to accelerate decomposition of the deposited solid waste’s organic components. Air can also be injected or sucked through a permeable cover by applying a vacuum to the waste mass to facilitate aerobic decomposition. A bioreactor landfill can mitigate costs, shorten the time needed to process solid waste, and ease the typical ecological damage to the land being used. By accelerating the decomposition process, a bioreactor landfill extends its operational lifetime without incurring capital costs associated with additional lined disposal cells. Bioreactor landfills can also be prone to excess leachate, or LFG, however, and the heat of decomposition actually might cause fires despite the additional moisture. The proper use of geosynthetics in this application can minimize the potential for these problems.

Standard Design, Construction, Operations
The standard dry-tomb landfill design is based on the requirements of the Resource Conservation and Recovery Act (RCRA) Subtitle D.

The bottom-liner system used in a Subtitle D landfill can consist of either natural soils with sufficiently low permeability, compacted soil liners, or composite liners made from both compacted soils and geomembranes. New landfill disposal units typically are required to have composite liner systems that meet the requirements of Subtitle D, detailed as follows, for the landfill cover system and bottom liner:

A leachate collection system is constructed on top of this liner system in accordance with further Subtitle D requirements:

This leachate collection system is designed so leachate-head buildup on the liner system does not exceed 12 in. Depending on the operational condition of the disposal cell (closed, receiving waste, or recently opened) and the anticipated annual precipitation, the floor of the disposal cell can be sloped and leachate collection pipelines properly spaced to minimize leachate-head buildup. This is a critical concern for bioreactor landfills since they require more moisture (from either recirculated leachate or piped water) to accelerate the decomposition process.

For an RCRA Subtitle D landfill, the design of the final cover system must meet or exceed the abilities of the bottom liner or natural soils underlying the landfill. The following minimum standards are used:

Bioreactor landfills are constructed in much the same way as standard Subtitle D landfills. Both types, for example, require secure liners and final cover systems. Despite the addition of liquids to accelerate the decomposition process, bioreactor landfills are required to maintain a maximum 12-in. leachate head on the liner. However, modifications to the standard design are necessary to accommodate the unique operational aspects of bioreactor landfills.

The bioreactor landfill concept uses controlled amounts of liquid and recirculated leachate to provide a superior growth environment for microbes responsible for the accelerated decomposition of solid waste components. Liquid is introduced by several means: spraying onto the exposed waste surface, shallow injection points, saturation field tiles, or deep disposal wells. In addition, the waste itself can be processed and is often shredded prior to disposal, as smaller particle sizes are more prone to bacteria formation and subsequent decomposition.

Typical municipal solid waste is mostly organic, as shown in the following table from EPA Solid Waste Fact Book:

Theoretically all of the MSW stream should be reducible by accelerated decomposition except for nonorganic elements, such as plastic, metals, and glass. These categories account for almost one-third of the total wastestream by weight. The remainder, more than two-thirds of the total, could conceivably be greatly reduced or even eliminated through accelerated decomposition. It must be understood, however, that in practice some organics will only partially decompose, leaving a humic residue.

MSW arrives at the landfill with a density of approximately 0.20–0.35 ton/yd.³ (roughly 15–25 lb./ft.³). Typical site compaction efforts at a standard Subtitle D landfill can reduce the volume of waste deposited on the working face by approximately 50%, resulting in a compacted, in-place density of 0.40–0.70 ton/yd.³ For every ton of waste removed from the landfill by accelerated decomposition, 1.40–2.50 yd.³ of airspace is preserved. If the incoming waste is shredded prior to disposal, however, compaction is often not performed or does not result in significant in-place volume reduction. Since shredding does not affect the weight of the various waste components, accelerated decomposition results in proportional volume reduction of the waste.

Other important physical characteristics of waste include porosity, field capacity, and saturated hydraulic conductivity. Porosity is defined as the volume of potential liquid storage at saturation expressed as a fraction of the total volume. MSW has a typical porosity of 0.67 (approximately 67% of its total volume is empty space capable of containing liquids; the remaining 33% is solid material). Field capacity is defined as the liquid volume retained by the waste even after prolonged gravity drainage. Waste has a field capacity of approximately 29% (after extended periods of free drainage, 29% of its volume will still be liquid; of the remaining 71%, 33% will still be solid material, and 38% will be air voids previously occupied by liquid). The saturated hydraulic conductivity of solid waste is approximately 1.0 x 10^-3 cm/sec. (equivalent to many sands and gravels). These characteristics govern the amount of liquids introduced into the waste mass as well as their rate of inflow.

Aerobic/Anaerobic Conditions in Bioreactors
Bioreactor landfills may utilize either aerobic or anaerobic processes. Both result in accelerated decomposition of the waste and faster stabilization of the landfill. In a standard Subtitle D landfill, stabilization is achieved in approximately 15–80 years. A landfill utilizing anaerobic microorganisms, however, can stabilize a landfill within five to 10 years, and a landfill utilizing aerobic microorganisms can stabilize a landfill in only a few years.

Furthermore, the anaerobic process will generate much greater amounts of LFG (namely, methane) than the aerobic process, but it is relatively slow compared to the aerobic method. This can be either good or bad, depending on whether the LFG is treated as a potential source of energy or as a dangerous nuisance. Anaerobic decomposition occurs naturally in a landfill as a secondary stage of waste stabilization. As aerobic decomposition processes utilize available oxygen within the landfill, conditions become favorable for the propagation of anaerobic microorganisms. Typically this remains the permanent condition of a standard landfill equipped with a low-permeability final cover system that precludes air infiltration into the waste mass.

Working at a much faster rate than the anaerobic process, the aerobic process utilizes microorganisms that require oxygen for their cellular metabolism. Energy is produced as the microorganisms break down organic molecules in a process that consumes oxygen and produces carbon dioxide. Because the aerobic process does not generate as much methane, aerobic microorganisms can grow more quickly than anaerobic microorganisms, resulting in faster waste decomposition.

Aerobic decomposition generates relatively large amounts of metabolic heat energy, however. This can require the addition of significant amounts of water to prevent the waste mass from becoming flammable even at significant waste depths. Landfill fires—resulting from the heat from aerobic decomposition, lightning strikes on exposed waste, or ignition of explosive methane—are extremely difficult to extinguish. The closest analogy would be to peat fires or underground coal-mine fires that have been known to burn for years or even decades. Aside from excavating the waste near the fire (an expensive and potentially dangerous process), the only feasible way to contain or extinguish landfill fires is by deep injection of a nonvolatile gas or liquid to displace the oxygen feeding the fire.

Even if excessive decomposition heat does not result in waste ignition, the heat can melt or deform plastic components of the landfill management systems. These include geosynthetics of all types (especially HDPE liners), leachate collection pipes, water and leachate force mains, LFG extraction wells, and LFG header pipes. Controlled aerobic decomposition has been successfully utilized, but it remains a potentially dangerous operation requiring strict oversight and extensive management.

Bioreactor Landfill Concerns: Excess Leachate
Leachate-head buildup on the landfill’s bottom liner is a potential regulatory violation of RCRA Subtitle D. Given the heterogeneous nature of waste and its potential for wide variance in its constituents, bioreactor landfill operations are more art than science. While the quantity of in-place waste can be determined with relative certainty—by frequent surveys of the current working face to determine in-place volumes relative to the bottom-liner elevations or the previous survey, and by cross-referencing these volumes with truck-scale reports of incoming waste tonnage during the period between surveys—quality is a little harder to determine. Gross averages of waste constituents may be used as a planning benchmark, but the actual waste characteristics may vary daily, monthly, or seasonally. For example, cold-weather months will see less organics in the wastestream since lawn clippings and fallen leaves will not be present.

Either way, the amount of liquid introduced into the landfill must steer between too much liquid resulting in excessive leachate and too little resulting in lack of accelerated decomposition. The right amount of water or recirculated leachate must be determined by trial and error. And since the process of decomposition changes the characteristics of the waste, the "right amount" of liquid will also change over time. Therefore the leachat management system of a bioreactor landfill must be overdesigned to accommodate leachate far in excess of what would be expected for a standard landfill.

The Hydraulic Evaluation of Landfill Performance (HELP) model, a software package developed by the Vicksburg, MS, office of the United States Army Corps of Engineers, is the primary tool for predicting leachate-formation rates. In addition to leachate formed by precipitation (rainfall and melted snow), the HELP model allows for leachate recirculation and additional liquid inflows. In a standard landfill, peak leachate formation typically occurs early in the operational lifetime of a disposal cell. This is due to the lack of waste capable of retaining liquids up to 29% of its volume due to its inherent field capacity. Once a final cap and cover system is in place, preventing the percolation of precipitation into the waste mass, leachate formation typically drops to insignificant levels. For bioreactor landfills the opposite is the case, however. Once enough liquid has been introduced into the landfill and has achieved field capacity, the rate of leachate production will be roughly equal to the rate of liquid introduction. Furthermore, as decomposition progresses, there will be less and less waste volume. Conservatively assuming that the decomposed waste has a field capacity similar to that of recently deposited waste, the result is another source of increased leachate formation—leachate previously held in place by the waste’s field capacity.

The best leachate system for a bioreactor landfill incorporates a geocomposite drainage blanket into its collection layer. A geocomposite consists of a factory-bonded nonwoven geotextile cushion, a geonet drainage layer, and a nonwoven geotextile filter layer. A typical geocomposite drainage blanket will have a hydraulic conductivity of 2.37 cm/sec. (equivalent to a transmissivity of 0.00012 m² /sec. at a flow gradient of 1.0). This is in-plane flow through a blanket having a thickness of 0.508 cm, equivalent to 0.2 in. As with the typical granular soil drainage blanket, the hydraulic conductivity of the geocomposite can be increased with increased slope grade. Note that the ratio of hydraulic conductivity between geocomposites and granular soils (1 x 10^-3 cm/sec.) is approximately 2,370:1. The amount of leachate typically transmitted by a geocomposite over a unit width of 30 cm would be 36.12 cm³/sec. An equivalent unit width of granular soil in a 12-in.-thick layer would transmit 0.9 cm³/sec. The geocomposite, despite its thinness, still has a superior ratio of flow rate compared to a standard granular soil layer of approximately 40:1.

Standard landfill operations typically use geosynthetics as alternative daily cover (ADC). Geosynthetic ADC is relatively easy to install and a proven cost-saver compared to the standard 6-in. layer of soil spread across the working face at the end of the day. In order to function as a barrier against odors, dust, vectors, blowing debris, and percolation, lightweight impermeable geosynthetics are required. If left in place, however, these layers can create a series of impermeable layers that interfere with the introduction of moisture into a bioreactor landfill. Furthermore, they can create perched pockets of accumulated leachate whose only migratory path is horizontally out the sideslope of the landfill. This can be remedied by simply track walking the ADC sheets prior to starting disposal operations the next day. This creates perforations and tears in the sheets that allow leachate to percolate toward the leachate collection and removal system.

The standard 6-in. layer of soil spread out over the working face creates relatively low-permeable strata within the landfill. While not impermeable like a geosynthetic or a tarp, this cover soil has a hydraulic permeability several orders of magnitude less than the underlying or overlying waste. Even stripping this soil layer at the start of the working day (an expensive and time-consuming operation) will not significantly mitigate this problem as significant soil is left in place. Furthermore, the stripped soil almost always has embedded waste, making it useless for further daily cover operations. This soil has to be deposited someplace in the landfill, making another low-permeability layer.

Bioreactor Concerns: Excess Landfill Gas
While geosynthetics are more at risk from excess decomposition heat than they are a potential solution, geosynthetics can be effectively utilized to deal with the excess LFG generated by anaerobic processes. These processes take place in the absence of oxygen and result in carbon dioxide and methane. The amount is predictable in aggregate but not in individual areas due to the heterogeneous nature of waste.

Here again, geocomposites make for a simple solution to a problem of excess landfill byproducts. Geocomposites provide a high-capacity gas-venting layer. Typically the geocomposite venting layer is installed immediately below the low-permeability, compacted soil cover layer. The LFG enters the geocomposite layer and flows in plane through the geocomposite under the cover soil until it meets a leachate extraction well or collection pipe. The actual gaseous flow rate depends on the driving pressure head generated by the gas and by the slope of the final cover system.

However, most bioreactor landfills dispense with a final cover system altogether until the deposited waste has achieved complete stabilization. As this can take as long as 10 years, depending on the process used to accelerate decomposition, a fully developed disposal cell may remain without a final cover system for this entire period. Given that final stabilization might result in a significant reduction of in-place waste volume and associated settlement of waste grades, the stage might be set for another round of waste disposal and processing on top of the now-stabilized waste.

Conclusion
Far from being unnecessary or detrimental to the operation of a bioreactor landfill, geosynthetics in general and geocomposites in particular are a positive component of this kind of facility. Modifications to the standard landfill design can allow for bioreactor operations while meeting the construction and operational standards of Subtitle D.

Daniel P. Duffy, P.E., is a professional environmental engineer in Cincinnati, OH.