Leachate Flow Control

John A. Banks and David H. Penoyer

Operating a bioreacting landfill involves a number of non-traditional landfilling practices, including the introduction of additional moisture into the waste, typically by recirculating the site's leachate. However, because the amount of leachate varies and some state agencies might be less accepting of this practice, other types of liquids also may be used.

To create a bioreactor landfill, additional liquids must be added to establish a moisture content between 40 and 70 percent by weight (vs. typical municipal solid waste with a moisture content ranging between 5 and 20 percent by weight).

Recirculating leachate to add moisture to the waste has several advantages over using other types of liquids, including:

How Much Leachate is Needed?

Using a landfill in Florida as a case study, SCS Engineers of Tampa, Fla., simulated the effects of leachate recirculation flow rates needed to operate a bioreactor landfill. The computer simulation was run for a two-year period to test the affects of leachate recirculation on alternative leachate collection and removal system (LCRS) designs.

Because Florida has an annual average rainfall of about 50 inches per year, the normal moisture content of the waste typically is at the high-end of the scale — approximately 20 percent by weight. Recognizing that some of the liquids introduced into the bioreactor landfill would not be absorbed and would need to be recycled again, engineers conducted simulations of high-volume leachate recirculation over two-years.

Using a 32-acre bioreactor landfill cell as a model, engineers assumed the bioreaction process would begin when the waste depth reaches 20 feet, which corresponds to having approximately 1 million cubic yards of waste in place. Using an in-place density of 1,200 pounds per cubic yard would be equivalent to having 600,000 tons of in-place waste. To reach the final moisture content of 45 percent, engineers calculated that the final mass of liquids in the waste must be 390,000 tons. Therefore, another 65 million gallons of liquid needed to be added.

Rainwater would provide some of that liquid, but not all the rain would be absorbed due to evaporation and transpiration. Using the U.S. Army Corps of Engineers' Hydrologic Evaluation of Landfill Performance (HELP) model, engineers determined that 18 inches of rainfall, or approximately 16 million gallons, would percolate to the waste mass per year. Leachate recirculation would have to provide another 49 million gallons of moisture.

Under the Earth

As part of the landfill design, the LCRS required a lateral drainage layer, collection pipes, pumping facilities and storage/treatment/disposal facilities.

In their study, engineers ran the HELP model for a two-year simulation period testing various recirculation rates. The first year determined whether the required amount of moisture addition was being achieved. The second year tested the effect of high-recirculation rates/water addition on the performance of the LCRS.

The HELP model was used to determine configurations of the lateral drainage layer and the spacing between the collection pipes to prevent mounding on the liner. Typically, the mounding (or head) should not exceed the thickness of the lateral drainage layer and be no more than 12 inches, as specified by most regulations. Other requirements would depend on the liner system components and local regulations.

The HELP model also was used to calculate the height of head over the liner based on the hydraulic conductivity and thickness of the lateral drainage layer, the slope of the lateral drainage layer, and the length of the slope of the lateral drainage layer to the collection pipes.

To minimize the amount of fill material used to construct the subgrade of the liner system and to minimize the loss of landfill airspace, engineers tried to keep the slope of the lateral drainage layer as small as possible. However, regulations generally require a minimum 2 percent slope because construction irregularities might cause a lesser slope to be ineffective.

The slope distance and spacing between collection pipes is determined by many issues, including construction costs, pumping configurations, long-term maintenance costs and site geometry. Pipe spacing usually is limited by the requirements for the maximum head allowed on the liner, 400 feet between collection pipes being the maximum. For this study, engineers used 200 feet between the pipes, or a slope length of 100 feet.

Although the HELP model assumes natural materials will be used for the lateral drainage layer, geosynthetic drainage nets (geonets) often are used for this layer to create high hydraulic conductivities. To determine the flow characteristics with the geonets, engineers used the manufacturers' data for transmissivity (index values), performed unit conversions and applied factors of safety to determine the input values to the model.

Soil profiles and loading pressure will affect the geonet's performance. Consequently, using laboratory index tests based on conditions that are similar to the actual field conditions assist in calculating flow values that are closer to the actual performance.

Additionally, other factors affect the flow through geonets, such as intrusion, biological clogging, chemical clogging and long-term creep. These factors were taken into consideration when index values were converted to the appropriate units to account for the expected reduction in hydraulic carrying capacity of the geonet.

Changing the Layers

Engineers used the model to test three alternative lateral drainage layer designs:

To obtain an adequate level of additional water in the first year, the recirculation rate was set at 95 percent in the model. This resulted in an average recirculation rate of 69,000 gallons per day, (gpd) 3,600 gpd of leachate being removed and 40 million gallons of liquid added to the landfill in the first year. All three lateral drainage layer designs met the design requirements for head on the liner for the first year.

Extending the 95 percent recirculation rate to the second year caused significant increases in the recirculation quantities, as leachate began to accumulate in the cell. The rate of recirculation reached 500,000 gpd and created about 250,000 gpd of leachate for disposal. This simulated the potential impact to the LCRS from high recirculation rates for an extended time or from uneven distribution of recirculated liquids.

Under the severe conditions, in the second year, the head on the liner exceeded the design requirements for two of the drainage layer designs: The head on the single 250-mil biplanar geonet was calculated at 22.5 inches; the head on the double 250-mil biplanar geonet reached 8 inches. However, the head on the third design was measured at 0.2 inches, indicating that the single 300-mil triplanar geonet would continue to perform under severe conditions.

Watch the Time

Based on the simulations, engineers demonstrated that most liner designs using a geosynthetic drainage net can accommodate high recirculation rates for a limited time. However, maintaining the leachate recirculation rates necessary for waste biodegradation can lead to design exceedances in the LCRS, if continued for extended periods, if channeling or if uneven distribution of the recirculated liquid occurs.

Consequently, when using an existing LCRS in a bioreactor landfill, operators should consider how long to continue recirculation. Additionally, when designing a LCRS for a bioreactor landfill, or if large amounts of moisture are anticipated, designers should:

John A. Banks is a project manager for SCS Engineers in Tampa, Fla. David H. Penoyer is a project engineer for the company.

This feature was based on a paper presented at Waste Tech 2001. For more information on leachate management, visit www.wasteage.com.