Alaska Landfill Covered

Joel J. Grunwaldt, Wilford P. Kryger and Kelly S. Merrill

With Alaska's frigid climate and extreme seismic activity, successful landfill planning, design and management is a daunting task. However, year after year, the 275-acre Anchorage Regional Landfill (ARL) is recognized for its environmentally sound and economical waste disposal methods.

Most recently the ARL, which serves more than 250,000 people and receives about 990 tons of solid waste per day (tpd) - more than half of the state's waste - received the 1999 grand prize in the operations and management category from the American Academy of Environmental Engineers, Annapolis, Md.

Since early 1994, ARL's operation and management has been a joint effort between the Municipality of Anchorage (MOA) Solid Waste Services (SWS) department and CH2M Hill, Anchorage, which designed cells 4/5 and 6, two leachate treatment lagoons and the bird control system.

Waste is collected from the 2,000-square-mile municipality, which includes several communities and two military bases in an area reaching 20 miles north and 60 miles south of the landfill.

The landfill opened in November 1987 with the 25-acre cell 1, designed for a two- to three-year operational life. The cell is enclosed on three sides and open to the west to allow for future cell tie-in. Approximately 1.7 million cubic yards of material was excavated because of the terrain to accommodate the 120-foot-deep cell. The cell's bottom is approximately 40 feet above the groundwater table, and the cell is lined with a single, 80-mile high-density polyethylene (HDPE) liner.

Throughout the cell, there is 6 inches of sand and a nonwoven geotextile fabric beneath the liner. Another layer of geotextile fabric and 2 feet of granular drainage material protects the liner on the cell's bottom. The 2.5 to 1 sloping sides are protected with a layer of geotextile fabric and a 1-foot layer of washed 2-inch minus round rock. Additionally, there is a leachate collection piping system within the protective granular drainage layer on the cell's bottom.

The 15-acre cell 2 was tied into the west side of cell 1 in August 1990. It was designed and constructed similarly to cell 1, except that the protective geotextile underneath the liner on the base of the cell was replaced by a geosynthetic clay liner consisting of bentonite clay between two layers of geotextile fabric. The 7-acre cell 3, constructed in 1992, was designed similarly to cell 2.

A comprehensive development plan for the ARL was written in 1993, after several years of operating data were available from cells 1 through 3. The 23-acre cell 4/5 was designed in 1994 and construction was completed in fall 1995. Cells 41/45 was the first cell to be designed to meet the Washington D.C.-based U.S. Environmental Protection Agency's (EPA) Subtitle D requirements, implemented in October 1993.

The cell shape was changed from the original 1993 development plan to satisfy the slope stability requirements of Subtitle D in the highly seismic Anchorage area. A buttress berm was constructed to enhance the stability of cells 1 though 5, and cells 4 and 5 were combined as cell 41/45 to provide a more stable fill sequence.

Additionally, a comprehensive, site-specific seismic risk assessment of the ARL area was completed to determine appropriate earthquake design criteria. Based on this risk assessment, the ARL's Subtitle D design earthquake has a maximum peak ground acceleration of 0.54g with a Moment Magnitude of 9.5. While this design earthquake applies to the final landfill configuration, lower acceleration values were associated with temporary waste configurations based on achieving the same degree of risk specified by Subtitle D.

Other changes in cell 41/45 included revisions to the lining system, to take advantage of evolving geosynthetic technology and to enhance stability. Cell 4/5 was the first cell in which Geosynthetic Clay Liner (GCL) was installed below the HDPE liner on both the base and the side slopes. To provide stability on the side slopes, a heavy, woven, reinforcing geotextile was placed above the HDPE liner to support the 2-foot-thick layer of granular drainage material.

Crushed 3/8-inch minus sand and gravel was used as the granular drainage material above the composite liner system rather than the well-rounded material that had been used in previous cells. The well-rounded material was used to protect the liner from puncture by angular, fractured particles. The decision to use crushed material was based on a detailed testing program that confirmed that the crushed material would not damage the liner.

Because of the cell 4/5 changes, revisions to the original landfill development plan were necessary before designing the next unit, the 12-acre cell 6. In 1997, CH2M Hill was hired to evaluate cell layout and sequencing in the remaining undeveloped area of the ARL. Subsequently, cell 6's design and construction was completed in August 1999.

Features of the ARL include:

* Landfill gas control: A porous venting layer was constructed along the side slopes of each landfill cell to allow passive gas venting and eventual controlled flaring or energy recovery. A layer of washed sand and gravel was laid between the lining containment system and the garbage fill along the cell side slopes, allowing the gases to vent. When the final grade is reached, an impermeable cap will be installed to prevent precipitation from entering. The cap also will trap landfill gas that will be collected in peripheral manifolds for controlled flaring. If the landfill produces sufficient quantities of gas, it will be recovered for alternate energy.

The landfill cells have thermistor strings and gas monitoring probes installed at various depths within the porous venting passages on the side slopes and base. This equipment provides data on the internal landfill temperature, gas generation rates and concentrations. Currently, very little information exists on landfill gas concentrations in cold climates.

Because of southcentral Alaska's cooler climate and low annual precipitation (16 inches per year), relatively small quantities of landfill gas are generated. In summer 1998, New Source Performance Standard (NSPS) Tier II testing was performed to determine the site-specific, non-methane organic compound emission potential. The testing showed emission rate potentials of 32.8 mg per year for 1998 and 42.1 mg per year for the five-year period ending in 2002, which are below the NSPS regulatory threshold values of 50 mg per year.

* Groundwater monitoring: SWS has developed an extensive groundwater quality monitoring program for the ARL as well as for all three of the MOA's closed landfills. Three upgradient and four downgradient wells near the landfilled area at the ARL are monitored semi-annually. The groundwater monitoring system quantifies and characterizes surface water and groundwater entering the site from upgradient areas and exiting the site at downgradient locations. Surface water monitoring samples are taken from a spring site downstream.

* Leachate collection and treatment: Each landfill cell includes a leachate containment and collection system. Leachate is aerated in two lined lagoons to lower biological oxygen demand (BOD) before trucking the leachate to the municipal wastewater treatment plant. Because each operating cell is constructed like a lined bowl, the leachate collection system is designed to collect the precipitation that falls on the cells to prevent contaminated water from entering the groundwater system. Landfill leachate samples are taken monthly from the leachate lagoon.

The system consists of a series of lateral 6- to 8-inch diameter perforated HDPE pipes embedded in the protective granular drainage layer above the HDPE liner. The laterals are tied into 8- to 10-inch main header pipes, which lead to the low point of a particular cell. Toe berms were constructed across the open sides of the cells to provide temporary containment within the lined cell and to prevent stormwater from entering it.

In conjunction with cell 4/5 development in 1995, CH2M Hill designed a leachate lagoon system to store and pre-treat leachate. The two lined lagoons were designed to meet treatment limits for BOD loading, maximum daily flow and total suspended solids. The lagoons' capacity is 3.1 million gallons, which provides adequate volumes for leachate pre-treatment, emergency storage and peak storm equalization. Leachate accumulation in the landfill cells is kept below the 1-foot head limitation required by Subtitle D regulations.

The cell 6 project included an evaluation of leachate treatment and storage needs for all existing and future cells. In the primary lagoon, the submerged coarse bubble diffused aeration system installed in 1995 was kept intact. Floating aerators were installed in the secondary lagoon to achieve the desired BOD loading reduction associated with the full pre-treatment volume required for all existing cells. Heaters also were installed in the secondary lagoon to help prevent winter freezing problems. The floating aerators' performance will be compared to that of the submerged coarse bubble diffused aeration system in the other lagoon. The results will help determine the most effective aeration type to be used in later expansions.

* Bird control system: The ARL is located 2 miles from Bryant Army Air Field, the center of all Army aviation in the Anchorage area. The Army allows no more than 25 birds on the landfill's working face and no more than 50 birds within its line of sight. The innovative ARL bird control system, consisting of ropes and netting over each cell, has drawn inquiries from across the United States, Scandinavia, Australia and New Zealand, and has been cited as a successful bird control system in an EPA manual.

The large net, made of heavy-gauge nylon with openings 10 feet to 20 feet wide, is suspended between 71/416-inch Kevlar ropes attached to poles around the cell perimeter or to temporary towers within the landfill area.

With the cell 6 design, the bird control system evolved to include a central anchored tower with Kevlar ropes extending radially to support posts spaced at 40 feet on center along the landfill's perimeter. Rather than constructing separate bird control nets over each cell, the system for cell 6 will provide effective bird control for cells 6 through 10 simply by adding new ropes to the central tower and new perimeter support posts.

* Cold climate operations: Solid waste is landfilled year-round, with winter temperatures often well below 0 degrees Fahrenheit. Cover material trucks have been modified with an exhaust-through bed system to prevent soil from freezing to the truck bed, and collection vehicles also have been modified to operate in cold temperatures. Limited winter daylight is a challenge, and lights are placed at the working face.

* Litter control: The ARL is near the mouth of a canyon and frequently experiences winds up to 60 miles per hour. Specially strengthened litter-control fencing to minimize waste blown into the surrounding forested areas surrounds the landfill's active areas. Twenty-foot-high litter-control netting also has been installed behind the cell fencing.

* Weather monitoring: A weather station is located on-site to collect specific data for environmental modeling and to aid future designs.

* Transfer stations: Two transfer stations are located at the ARL to receive solid waste from cars and pickup trucks. A third transfer station is located in the Girdwood community, 50 miles south.

* Household hazardous waste collection facility: This government-owned, contractor-operated, 6,000-square-foot facility provides a convenient and environmentally sound way for Anchorage homeowners to dispose of hazardous wastes. The waste is segregated, classified, manifested, packed and shipped to an approved hazardous waste treatment and disposal site. Anchorage businesses generating less than 220 pounds per day also can use the facility.

* Recycling: Several materials, including waste oil, appliances and lead-acid batteries are collected for processing and recycling. Bins are located at the ARL public drop-off area and at transfer stations. The gases and scrap metal collected are made available to a recycler.

Successfully operating the Anchorage Regional Landfill (ARL) under challenging circumstances has been recognized nationally several times. Most recently, the ARL received the grand prize in the operations and management category of the 1999 American Academy of Environmental Engineers, Annapolis, Md., Excellence in Environmental Engineering competition. In 1998, ARL won both the Solid Waste Association of North America (SWANA), Silver Spring Md., Gold Award for Landfill Excellence and the American Public Works Association's Project of the Year for the Alaska Chapter.

Anchorage Solid Waste Services (SWS) and the ARL also provide the community with educational activities. SWS conducts numerous landfill tours each year for local schools and visiting officials. The field training exercise for the SWANA Manager of Landfill Operations course has been conducted four times at the ARL. Because of well-designed and visual screening, the landfill essentially is invisible to the public, despite being adjacent to the only highway accessing Anchorage from the north.