Liner systems

FIG. 25-76 Schematic of an FML plus compacted-soil double-liner system for a landfill, (Drawing not to scale.) (U.S EFA, EFA/530/SW-S5-012 Washington, DC., 19S5 EiomEieerrmn, H M., Standard Handbook of Hazardous Waste Treatment and Disposal, AtcGiutu-H) //, 19SS. )... 

The U.S. EPA guidance6 discusses three types of liners FMLs, compacted clay liners (CCLs), and composite liner systems (an FML overlying a compacted low-permeability soil layer). Material specifications in the guidance for FMLs and CCLs are briefly reviewed below, along with regulations regarding all three liner systems. 

The minimum thickness specification for an FML top liner covered with a layer of soil is 0.75 mm for an FML without a soil cover layer, the specification is 1.14 mm. An FML in a composite bottom liner system must be at least 0.75 mm thick. Even though these FML thicknesses meet U.S. EPA specifications, 0.75mm is not a suitable thickness for all FML materials. In fact, most FML materials installed at landfills are in the range of 1.50-2.50 mm in thickness. Other key factors affecting the selection of FML materials include chemical compatibility with waste leachate, aging and durability characteristics, stress and strain characteristics, ease of installation, and water vapor/ chemical permeation. 

U.S. EPA s rationale for the requirement of composite bottom liner option in the final doubleliner rule is based on the relative permeability of the two liner systems.13 The results of numerical simulations performed by U.S. EPA,10 which compared the performance of a composite bottom liner to that of a compacted soil bottom liner under various top liner leakage scenarios, showed that liquids passing through defects in the top FML enter the secondary LCRS above the bottom liners. The hydraulic conductivities of bottom liner systems greatly affect the amount of liquids detected, collected, and removed by the secondary LCRS. 

U.S. EPA also determined the total quantity of liquids entering the two bottom liner systems over a 10-year time span with a constant top liner leak rate of 50 gallons/acre/day. A composite bottom liner with an intact FML accumulates around 70 gal/acre, primarily through water vapor transmission. Even with a 10-ft tear, which would constitute a worst-case leakage scenario, a composite liner system will allow 47,000-50,000 gallons/acre to enter that bottom liner over a 10-year time span. Compacted soil liners meeting the 10-7 cm/s permeability standard will allow significant quantities of liquids into the bottom liner, and potentially out of the unit over time, on the order of hundreds of thousands of gal/acre.5... 

The numerical results indicate superior performance of composite liner systems over CCLs in preventing hazardous constituent migration out of the unit and maximizing leachate collection and removal. Consequently, owners of new units subject to the double-liner requirement of FISWA are now installing composite bottom liners or double composite liner systems.3... 

U.S. EPA bases its 1 gallon/acre/day leak detection sensitivity on the results of calculations that show that, theoretically, an LDS overlying a composite bottom liner with an intact FML component can detect, collect, and remove liquids from a top liner leak rate <1 gallon/acre/day. This performance standard, therefore, can be met with designs that include a composite bottom liner. Based on numerical studies, one cannot meet the leak detection sensitivity with a compacted soil bottom liner, even one with a hydraulic conductivity of 10-7 cm/s. Therefore, the emphasis of this standard is on selecting an appropriate bottom liner system. 

Basically, U.S. EPA regulations and guidance16 require that the final cover be no more permeable than the liner system. In addition, the cover must be designed to function with minimum maintenance, and to accommodate settlement and subsidence of the underlying waste. The regulations do not specify any design criteria for liner materials to meet the performance standard for permeability. 

The guidance16 recommends a three-layer cap design consisting of a vegetative top cover, a middle drainage layer, and a composite liner system composed of an FML over compacted low-permeability soil. The final cover is to be placed over each cell as it is completed. 

Regulatory requirements for hazardous waste landfill double-liner systems are given in 40 CFR 264.3 The minimum liner system design standard generally considered to meet these requirements includes, from top to bottom4 ... 

Final cover systems are another important component of waste containment systems used at landfills. While liner systems are installed beneath the waste, final cover (or closure) systems are installed over the completed solid waste mass. For hazardous waste landfills, 40 CFR 264 requires that the landfill be closed with a final cover system that meets certain performance criteria, most notably, that they have a permeability less than or equal to the permeability of any bottom liner system or natural subsoils present. U.S. EPA guidance documents517 recommend that final cover systems for hazardous waste landfills consist of at least the following, from top to bottom ... 

After a landfill site has been chosen and a basin has been excavated, the basin is lined with one or more layers of water-retaining material (liners) that form a leachate bathtub. The contained leachate is pumped out through a network of pipes and collector layers. Liners may be constructed of synthetic polymer sheets or of clay. U.S. EPA s MTG3A7 relies on a composite liner that utilizes the advantages obtained from combining both liner systems. 

Clay liners, synthetic liners, or combinations of both are required in landfills. Figure 26.1 depicted the synthetic/composite double-liner system that appears in U.S. EPA s MTG. As explained... 

FIGURE 26.16 Profile of the MTG double-liner system. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)... 

Double-liner systems are more prone to defects in the structural details (anchorage, access ramps, collection standpipes, and penetrations) than single-liner systems. 

The drainage system for removing leachate or other aggressive liquids from landfills, surface impoundments, and waste piles is critically important. Even if a liner has no leaks, the phenomenon of molecular diffusion will allow some of the organics from the liquids ponded on top of the liner system to leach through the FML and the clay. The timely collection and removal of that leachate is at the heart of this section. 

An alternate system, one based on gravity, requires penetration of both the FML and clay components of the secondary composite liner system. It also requires a monitoring and collection manhole on the opposite side of the landfill cell. The manhole and connecting pipe, however, become an underground storage tank that needs its own secondary containment and LDSs. 

The hot-feed rubber extruder is usually characterised by a relatively large screw depth and a relatively short L/D ratio of the barrel of 3 to 8 1 with the greatest number of machines having a ratio of 4 1. The barrel comprises usually a cast iron outer with either a traditional replaceable nitride liner, or, in the case of one manufacturer, of a single piece construction with an integral cast liner which has a surface hardness of Rockwell C60-62 and a hardness depth of 1.5 mm. The functional life for the bimetallic barrels is longer than for conventional nitride liner systems. 

Sulfur Impregnation (14). Stronger and more durable products such as Portland cement concrete, ceramic tile and even wood and paper products have been shown to be feasible. In addition, sulfur as an impregnant for fabric liner systems would have application in ponds, holding tanks, etc. 

A double or triple composite liner system provides maximum protection. 

Finally, if high-level waste is considered to be hazardous waste under RCRA, requirements on construction, operation, and closure of a disposal facility, including the provision of a liner system, leachate collection and removal system, and leak detection system (see Section 4.2.2), would need to be addressed. Such requirements are impractical at a geologic repository for disposal of high-level waste... 

Concrete and fiberglass vaults are often used, although they can be subject to environmentally induced cracks. Soil and clay liners are not allowed. Flexible liner systems have been developed that may be a cost-effective and environmentally sound alternative. State-of-the-art liner technology has overcome many of the previous problems with seams, low mechanical strength, and chemical resistance. 

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