Better Daily Cover Material for Suppression of Leachate Formation

SEATTLE (Waste Advantage): Modern landfills exercise laser-like focus these days on suppressing gaseous odors and reducing the generation of leachate. Alternative daily cover materials (ADCMs) and earthen daily covers have different performance capabilities in this application. Plastic landfill cover, an ADCM classified by ASTM D6523 as a “Non-Reusable Geosynthetic”, has been found to provide a highly effective continuous water runoff barrier, a property also conducive to air pollution and odor control.

Comparison Study
A study performed by Mark W. Cadwallader, of Cadwallader Technical Services (CTS), tested and compared the fluid barrier performance of daily cover materials. A simulated daily cover test pad was constructed to determine the ability of different materials to shed moisture during simulated rain events.

Table 1 displays a summary of results from the test pad studies. The various daily covers tested differed in their time required to a simulated rain event to break through the cover and begin flowing into the collection trench. Once the cover reached a saturation point, break through occurred and a stabilized flow rate (measured in U.S. gallons per minute, gpm) of cover penetration began. This quantity was measured at the bottom of the collection trench. The total measured flow through the cover was calculated as a percent of total water flow (simulated rain) impacting the test pad.

Green Waste and Soil
A similar testing method was conducted for:
• Approximately 6 inches of loose green waste (pine mulch)
• Approximately 6 inches of a silty sand soil cover

The same scenario was applied and penetrating flows through the covers to the interceptor trench were collected and measured in comparison to the total flow through the flow meter—equal to the total simulated rainfall over the wetted area on the test pad. Both green waste and soil were dry, and no moisture testing was conducted, while the soil met Soil Classification description as a silty sand.

The green waste absorbed a significant amount of rainfall before reaching saturation and flowing through at effectively 100 percent penetration. The soil also absorbed a moderate amount of rainfall, but shed some as well, with a great amount of visible soil erosion in the water runoff. Non-Reusable Geosynthetic greatly exceeded the barrier properties of either the soil or the green waste, shedding most of the rainfall and absorbing only a small amount at the soil ballasted overlap.

Tarps and Foams
Geosynthetic tarps come in two classifications according to ASTM D6523 “Standard Guide for Evaluation and Selection of Alternative Daily Covers (ADCs) for Sanitary Landfills”—either reusable or non-reusable. The reusable tarps are thick mats that can be unrolled or pulled over a working face at the end of the workday and removed again before the start of the next day.

A non-reusable geosynthetic is simply buried in the working face and subsequently destroyed with the next day’s waste. An advantage of a non-reusable geosynthetic is that it continues with barrier benefits through to the following waste fill sequence. In other words, it provides a continuous extended barrier between waste and the environment. Reusable tarps provide good barrier action against fluid migration only while in place, but their time in actual coverage is limited to a partial day, being applied at day’s end and taken up from the working face before the start of operation the next day. Assuming there is a 12-hour workday, this would be 1/2 coverage through a 24 hour day, allowing the waste to go uncovered for the remaining 1/2 day. This means that the average reusable tarp provides exposure to precipitation for leachate generation with a penetration rate of 50 percent in the working face. In effect, reusable tarps offer less leachate penetration resistance than any of the other materials tested in this study.

Foams and slurries, while not tested in this study, can, for the most part, be expected to perform similarly to green waste but with a different degree of water absorption and penetration. In addition, these types of covers involve water mixing so you are inadvertently increasing leachate production. These are permeable materials and will logically not shed moisture as well.

Operational Benefits
Daily covers can make a significant difference to landfill operations, not only in air space savings and operation costs, but also in other less discussed topics such as leachate mitigation. Landfill gas and odor control, along with leachate volume reduction and management, are two such areas where the choice of daily cover materials can strongly impact a landfill’s operational benefits and future cost savings. Consider the water shedding properties of the current cover material you are using; there may be significant cost savings hidden under your covers. 

Courtesy: www.wasteadvantagemag.com