BIOREACTOR LANDFILLS increasingly are being viewed as viable waste disposal options because they help to speed biodegradation and increase the waste stabilization rate and waste disposal capacity. Bioreactors also can improve leachate quality, generate landfill gas (LFG) quicker and provide faster surface settlement. However, because the quantity of liquid needed to maximize biodegradation often is greater than the leachate quantity generated at landfills, how much extra liquids are needed?
To evaluate the relative benefit of adding liquid in quantities greater than leachate generated onsite, Houston-based Waste Management Inc. tested two Virginia landfills — the Maplewood Recycling and Waste Disposal Facility and the King George County Landfill and Recycling Center.
Maplewood and King George landfills were chosen because they receive similar municipal solid waste streams and are located in the same geographic area, with similar weather conditions.
Maplewood resides in Amelia County, Va., near Richmond. The landfill liner will cover about 404 acres upon completion. Construction of the landfill's first phase began in 1992. King George landfill in King George County, Va., is also near Richmond. The landfill liner will cover about 290 acres upon completion. The first phase of liner system construction began in 1996. Both landfills were constructed with double-liner geomembrane systems to protect against potential leachate leakage.
Both landfills have been designed with 10-acre bioreactor test areas and 10-acre control areas.
In designing the bioreactor program, operators initially set a goal of applying at least 4 million gallons per year of liquid at the Maplewood landfill, and at least 8 million gallons per year at King George. Operators also hoped to:
Provide several liquid delivery options;
Uniformly distribute liquid through the waste;
Minimize uncontrolled LFG releases; and
Monitor the bioreactors' performance.
King George receives nearly twice the amount of liquid as Maplewood. Additional liquid at King George comes from stormwater, non-hazardous liquids or other biota-rich liquid wastes. To meet the test's goals, a liquid application system was designed with a capacity of at least 8 million gallons so that liquids could easily be applied to the waste. The systems consist of a series of gravity draining trenches. This involves two steps to evaluate the system's “application capacity” — the amount of liquid that can be expected to flow by gravity from the trenches — and then the head on the liner resulting from adding liquid to the waste.
Application capacity depends on the waste's hydraulic properties, the trench's dimensions and liquid head within the trench. An engineering analysis estimated the time required for an individual trench to drain. The head on the liner system resulting from leachate recirculation was estimated using the EPA's Hydraulic Evaluation of Landfill Performance (HELP) model. The analyses indicated that for the conditions encountered at the site, the trenches should be a minimum of 3 feet wide, 6 feet deep and spaced approximately 100 feet apart to maintain the head on the liner system within regulatory limits.
Operators also investigated the potential for liquid addition to cause stability problems by considering the presence of a high phreatic surface within the landfill resulting from application and poor liquid drainage in the waste. Numerous circular and block failure surfaces within the waste mass and through the foundation soils under the landfills were evaluated. The safety factor for slope stability remained above the minimum value of 1.5, partially because the critical surface for slope stability was located outside the liquid application areas.
At both sites, trenches 12 feet deep and 6 feet wide were constructed, including a minimum 4-inch diameter perforated high-density polyethylene (HDPE) pipe backfilled with tire chips that are porous to allow leachate flow. Trenches were dug within the existing waste parallel to the existing ground contours and were spaced approximately 100 feet apart. Three trenches were installed at each site, with plans to install additional trenches by spring 2003.
To apply the liquids, a load of approximately 6,000 gallons of leachate or other liquid is transported in a tanker truck to the top of the landfill and then discharged into one or more of the trenches. An average of approximately 6,500 gallons of liquid is applied at Maplewood daily and approximately 19,000 gallons per day are applied in the King George trenches. The trenches have been fitted with a valve so that the discharge hose from the tanker truck can be connected to an individual trench. Liquid is allowed to drain from the tanker truck under gravity.
Generally, liquid is applied every three days. The trenches can drain on the days that liquid is not added to the landfill. Before additional liquid is applied, the liquid level in the trench is measured to ensure that it has properly drained.
Time for Tests
Because waste stabilization is the principal goal of the bioreactors, researchers measured the chemical and microbiological decomposition, physical settlement, leachate quantity, leachate quality, in-place waste density and air quality in response to liquid applications.
Operators expect that leachate quality will have higher concentrations of biochemical oxygen demand (BOD), nutrients, specific conductivity, etc., for several years and then exhibit low concentrations of these constituents. Leachate samples are obtained from sumps within the test and control areas, as well as at the leachate storage tanks onsite.
The LFG generation rates are expected to increase significantly during liquid application, and then decrease sooner than in non-bioreactor landfill areas. LFG composition is measured at the well heads in test and control areas by conducting a surface emissions scan in accordance with National Source Performance Standards (NSPS) requirements, and by collecting composite gas samples from the LFG collection system. The samples are laboratory tested for non-methane organic compounds (NMOCs) and volatile organic compounds (VOCs).
Changes in the biodegradable material's content will be indicated by changes in the content of cellulose, lignin, pH, moisture content and wet and dry density of the waste at different waste elevations. Each site has established a 100-foot grid from which to conduct surveys. The elevation of each point on the grid was measured before liquid application began, and landfill settlement will be measured over time. Chemical samples collected annually and evaluated through laboratory testing will help to provide additional information on waste degradation.
Over the four-year test, the landfills' owners hope to measure and identify:
Changes in leachate quality (annually);
The relationship between total quantity of leachate generated and liquid applied in the landfill phases;
The range of liquid application rates or quantities at various trenches, and potential issues from certain application rates;
Air quality permit compliance issues;
Performance of the trenches and appropriateness of trench spacing to uniformly distribute leachate through the waste;
Any seeps and whether they are attributable to operation of the liquid application system; and
The magnitude and settlement rate of the landfill surface in areas with and without liquid application.
Maplewood began adding liquids on Aug. 20, 2002, and King George began adding liquids on Nov. 1, 2002. Through the end of 2002, approximately 864,282 gallons (or 6,498 gallons per day) had been added at Maplewood and approximately 1.13 million gallons (or 18,558 gallons per day) had been added at King George.
Although the four-year test is not yet complete, operators believe when designing a bioreactor landfill:
The waste stabilization benefits will be relatively well-understood within the regulatory community. Regulators who were involved in the project permitting were familiar with bioreactor technology and supportive of the project.
Bioreactor or leachate recirculation operations should be phased-in at sites to allow operators to adjust to the specific operating system requirements. For example, attention is required each day to operation of the liquid application system. Large sites that recirculate all of the leachate generated onsite could require as much as one-half to one worker each day.
Attention should be given during system startup to ensure that problems are not encountered, such as leachate seeps or odor problems. Because waste composition varies by location, the performance of liquid application trenches should be determined by observation rather than by computation.
Operators considering bioreactors need a method to deliver leachate to the waste mass. At Maplewood and King George, leachate is trucked from storage tanks to the trenches. Both sites are constructing a pumping system, which would eliminate the need for trucks and reduce costs. However, an operator is necessary to run the system and ensure it is running properly.
Most importantly, the landfill owner believes cost and operational benefits can be substantial at landfills that either transport leachate from the site or for landfills that have a high cost of leachate treatment. The direct cost savings at these test bioreactors, for example, have paid for the entire capital cost of the program in the first year, as well as first-year operational costs. Additionally, indirect cost benefits are expected as decomposition-induced settlement provides future additional disposal capacity.
A version of this paper was presented by D.T. Mandeville of GeoSyntec Consultants, Columbia, Md.; J.W. Stenborg of Waste Management Inc. in Virginia; and E.P. Farrell of Virginia Department of Environmental Quality, Richmond, at the National Solid Wastes Management Association's Waste Tech conference.