David H. Canup

Discarded tires, long considered a landfill pest, may actually be a blessing in disguise for some landfill operators. Across the country, old tires are being processed into chips and used in leachate collection systems as an alternative to gravel or sand.

Currently, these often under-recycled waste products, which cannot be landfilled in their original form, pose a problem for many states. For example, Texas has an estimated 80 million recycled tires sitting in storage. Only 15 percent of the state's discarded tires are recycled, according to the Texas Natural Resource Conservation Commission.

The North Texas Municipal Water District (NTMWD), which provides regional water, wastewater and solid waste services to cities north and east of Dallas, turned its attention toward this tire supply when it began renovating and recycling a 30-year-old, 140-acre landfill northeast of Dallas.

After reviewing numerous options for leachate collection, including all gravel and sand alternatives, and visiting a Florida landfill where recycled tires were successfully used, the NTMWD decided to pursue the recycled-tire option.

According to NTMWD officials, used tires present many benefits in landfill construction, including:

* Ease of leachate flow. Recycled tires have larger pore spaces than gravel or sand and do not have fine minerals that restrict flow through the collection system and prevent complete removal of landfill liquids.

* Ease of transportation. The light-weight tire chips are easier to transport and maneuver than gravel or sand. One cubic yard of tire chips weighs approximately 900 pounds; a cubic yard of gravel weighs more than 3,000 pounds.

* Readily available supply. To construct the first eight-acre section of the landfill, NTMWD used 1.2 million tires, or almost 11,000 tons. Over the next decade, 10 more sectors will be constructed. This project will help diminish Texas' oversupply of used tires.

* Durability. The tire chips are expected to last the life of the landfill, according to NTMWD.

* Cost-effectiveness. By using the tires, NTMWD saved approximately $400,000 in the first sector. The total cost of using tires on both the landfill bottom and side slopes was $812,765. In contrast, using gravel on the bottom and sand on the side slopes would have cost $1,256,025; gravel on the bottom and tires on the landfill's side slopes would have cost $1,103,725; and sand on both the bottom and side slopes would have cost $1,138,060. (For other landfills, cost savings figures may fluctuate project-by-project, depending on the tire supply and transportation costs.)

To construct the landfill (see diagram), NTMWD excavated the site down to an 800- to 900-foot-thick layer of Austin Chalk limestone. To address flood concerns, NTMWD built a 500-year levee using approximately 170,000 cubic yards of clay which came from the site. Currently, the Environmental Protection Agency requires a 100-year levee.

Next, the NTMWD constructed a compacted clay liner in six-inch layers over the bottom of the landfill, until a minimum thickness of three feet was reached. Each layer of clay was tested to a permeability of 1 x 10 -7 centimeters per second or less.

Also, fabric panels were stitched together to cover the clay liner's 297,000-square-foot surface. This cover prevents the tire chips from coming in contact with the clay liner.

Finally, a two-foot layer of tire chips was spread over the fabric, providing a porous collection medium to transfer any liquids to a sump built into the lowest point of the first sector. From the sump, a pump removes the leachate and then a pipeline transports the liquids to a wastewater treatment plant. In addition, NTMWD installed 21 ground monitor wells and eight permanent gas monitor wells around the site.

"This landfill now offers several protective barriers to separate the solid waste from the environment," said Carl Riehn, NTMWD executive director. "We're not only using recycled products to construct the landfill, but we're also recycling the landfill itself."

The NTMWD landfill reportedly is the first in Texas to use this type of leachate collection system. Across the state, other landfill operators are following the district's lead and applying for permits for similar systems. Several other states, including Florida, Minnesota and California, also are implementing this collection alternative, according to NTMWD.