Lay Leachate Lay

Fred Doran

Landfill owners must manage their leachate, no matter how high the price. For some, like those at Minnesota's Crow Wing County landfill, hauling it to a wastewater treatment plant is costly. By recirculating leachate back into the landfill, solid waste environmental regulations and save money. While the technology is experimental, so far it's been successful for Crow Wing, reducing disposal costs.

The Crow Wing County Municipal Solid Waste landfill near Brainerd, Minn., accepts about 35,000 tons per year (tpy) of municipal solid waste.

Three cells on 12.8 acres offer slightly less than 1 million cubic yards of disposal space. Currently, cell one is closed and cell two is open. Cell three is scheduled to open in 2000, and final closure is scheduled for 2005.

At first, Crow Wing hauled its leachate off-site. This was complemented in 1996 with a land application system in which leachate is applied to the final cover of an adjacent closed landfill. In 1997, the county implemented a recirculation system to cut costs further.

Called a "bioreactor" landfill, Crow Wing applies the leachate back into the open landfill, accelerating waste biodegradation.

This biodegradation depends on several factors. However, research indicates that moisture content is the most important variable because it facilitates nutrient transport throughout the waste. Recirculation provides a means of increasing the moisture content, which provides nutrients to the bacteria in the waste more quickly and helps to degrade it.

This is an alternative to the "drytomb" approach, where moisture is prevented from infiltrating the landfill, and the closed landfill must be monitored for decades after closure to prevent leachate leakage and landfill gas (LFG) impacts.

Recirculation in Action There are several ways to reapply leachate - spray application, infiltration ponds or cells, vertical injection wells and horizontal injection trenches.

Crow Wing's system uses horizontal injection trenches to recirculate raw and pretreated leachate, and, in the future, to collect LFG. By constructing a portion of the entire LFG collection system with the recirculation system, the county has saved approximately $120,000.

The county uses leachate recirculation as a backup to land application, which accounts for 1.5 million gallons of the 2.5 million gallons of leachate generated yearly.

First, leachate is collected through a piping system at the landfill base. It exits through a double-containment gravity flow pipeline and goes to a pump station.

From there, it is lifted to three aerated pretreatment ponds. The first pond provides primary treatment; the second is for long-term storage, settling and treatment; and the third provides final secondary treatment prior to disposal. Then, leachate is land-applied, recirculated or hauled to a wastewater treatment plant 150 miles away.

To recirculate the leachate, a network of perforated pipes, or laterals, are connected to a forcemain leading from a vault station and controlled by a valve to direct raw leachate from the pump station to the primary pond for treatment or back to the landfill. An inlet valve also allows the operator to recirculate pretreated leachate.

Leachate is injected into the landfill through a series of 11 recirculation laterals that run east and west within the landfill's three cells.

Constructed of high density polyethylene (HDPE), the 4-inch diameter laterals are spaced every 50 feet. They are not perforated within 50 feet of the landfill's east and west waste limits to reduce potential leachate seeps along the sideslopes.

Perforated laterals are centered in a 2-foot by 2-foot trench back-filled with the remnants of about 10,000 of the landfill's shredded waste tires. Using tires instead of aggregate saved $8,000 in construction costs. The trench is covered with a geotextile filter and intermediate cover.

Each recirculation lateral is dosed, or loaded, at 25 gallons per day per foot of trench. When one lateral reaches capacity, the next is opened, rotating until all laterals have been opened. Flow velocity does not exceed 100 gallons per minute. The landfill recirculates approximately 7,000 gallons of leachate per day when the recirculation system is operating, approximately three months each year.

The county monitors the system regularly, considering the leachate quality and quantity, leachate head levels on the landfill base, waste settlement, LFG methane concentrations and operational parameters. The operation recirculated more than 400,000 gallons of leachate in 1998, with no off-site hauling needed.

Recirculation helped the county save $145,000 in 1997 and 1998. This includes the avoided cost for the LFG collection system construction and the elimination of off-site hauling. These savings account for 50 percent of the recirculation system's construction and engineering costs.

In addition, the landfill is prepared for the increase of 1 million gallons of leachate that is expected when cell three opens. Cells two and three will be open simultaneously for about one year.

Because this technology is fairly recent, future experience will determine its full impact on landfill operation, cost savings and post-closure requirements. With a bioreactor landfill, post-closure care may require only minimal attention to leachate and LFG management.

While landfills with a high disposal rate, low precipitation and uniform leachate distribution can manage all of their leachate through recirculation, most landfills still need to dispose of a final part of the leachate elsewhere.

Leachate recirculation in a landfill can provide:

* Increased waste stabilization rate and reduced long-term pollution potential. When waste stabilization occurs at an operating landfill, pollution is less likely. This is because control systems such as liners, collection sumps and landfill gas (LFG) extraction are newer, and because operations and monitoring personnel are on-site.

* Leachate volume storage and volume reduction. Waste acts like a sponge. Recirculation allows waste to reach field capacity, or maximum moisture saturation, more quickly, providing leachate storage and reducing the volume generated for off-site disposal.

* Additional leachate treatment. A test recirculation project showed reduced chemical oxygen demand (COD) and bio-chemical oxygen demand (BOD) concentrations of more than 95 percent in reactors with and without recirculation. These measure the level of organics contained in the leachate. The reduction within the recirculation reactor, however, occurred in half the time.

* Ability to distribute additional nutrient and pH buffering. Leachate recirculation helps stabilize pH and add nutrients, ensuring the continued biodegradation process.

* Increased LFG generation. At two test cells operated by the Delaware Solid Waste Authority (DSWA) in Sussex County, the cell with leachate recirculation produced 10 times as much LFG as the dry cell. In another study, researchers determined that 30 percent of the LFG production resulted from leachate treatment within the bioreactor test cell.

* Accelerated landfill settlement before final closure. According to research, biodegradation of solid waste leads to increased settlement in a landfill of up to 25 percent of the waste thickness. Recirculation maximizes the benefit of settlement during landfill operation or early post-closure, allowing for more waste volume during operation and minimized cover maintenance during the post-closure period.

* Reduced leachate treatment costs. The DSWA saved more than $150,000 in 1994 by recirculating leachate rather than hauling it to a treatment facility. Some facilities have identified annual savings of $96,000 to more than $700,000 in reduced leachate treatment costs.

Leachate recirculation also results in concerns that should be addressed through design and operation including:

* Maintenance of a near neutral pH. An overloaded system with limited buffer capacity can halt the stabilization process. This means that if leachate with a low pH is being recirculated, and the landfill needs increased pH, biodegradation will be stalled.

* Increased LFG production. While this may be beneficial, landfill gas (LFG) generation can lead to odor or migration concerns. A "dry-tomb" landfill has the same methane generation potential. However, recirculation increases the generation rate and production occurs over a shorter time period.

* Decreased LFG collection efficiency. Too much water in the waste will make it more difficult to collect LFG. To avoid this, put the gas line above the recirculation line in the trench, spacing them at least 10 feet apart.

* Increased potential for leachate seeps. Recirculation increases the potential for leachate seeps from intermediate or final cover slopes. Landfills with low permeability daily cover and high slopes (less than 3-to-1) are particularly susceptible. Seeps also are possible if recirculation systems are placed over well-compacted, former haul roads.