Because landfills often are located close to airports, to avoid interfering with air traffic, bird control is important. One control method is to constantly hide the waste under daily cover, but this taxes a landfill operator's budget. Plus, daily cover uses up to 20 percent of the nation's landfill volume. An alternative would be to place the waste in thick lifts to minimize the exposed waste. But this drastically reduces the opportunity to compact the waste and results in lower waste densities. However, biological pretreatment of municipal solid waste (MSW) provides a new way for controlling birds and actually increases waste densities.
Biological pretreatment of MSW was pioneered in the late 1970s in Germany. After 20 years, biological pretreatment efforts at 14 landfills are handling 900,000 tons per year, and 15 additional sites are designed or under construction.
These sites have shown that biological pretreatment: * Reduces the waste mass by about 15 percent due to degradation of organic components;
* Improves leachate quality; and
* Increases the waste density from 1,350 pounds per cubic yard (lbs/cy) up to 2,000 lbs/cy.
Biological pretreatment has produced another benefit: birds are not attracted to pretreated waste.
Based on aerobic decomposition of the organic components in the waste, biological pretreatment is similar to composting. Unlike composting however, MSW pretreatment requires a slightly different process due to the wide range of materials and large waste pieces.
The simplest and cheapest way to biologically pretreat MSW is to use the chimney-effect system. This is the archetype of all aerobic treatment methods in that it passively aerates the waste by using the natural tendency for warm air to rise.
To maintain airflow into the waste, the base of the stockpile should consist of a permeable material such as bulky waste, palettes or similar materials. Crushed tire parts also may be suitable. The airflow is driven by a temperature gradient between the outside and the inside of the stockpile.
Once the aerobic process has started, the temperature inside the stockpile rises to about 160 degrees. The warm air inside the stockpile rises and draws in colder air from the bottom and from outside. Perforated drainpipes are placed in the base layer and rise in the center to allow a better airflow and air distribution.
The time for pretreatment ranges from 9 to 18 months, depending on the degree of biological treatment desired. As an initial design value, a minimum area of 1 square yard per ton of MSW can be assumed. It is recommended that the bio-heap be covered with previously treated waste, fine material, wood chips or similar inert material to guard against odors, birds and wind. Drainage needs to be provided at the toe of the bio-heap, and sufficient moisture needs to be maintained within the bio-heap. Controlling the waste moisture content generally controls the temperature and rate of decomposition. Simple irrigation systems such as agricultural sprinklers are sufficient for this.
Germany's Wilhelmshaven landfill has successfully implemented the chimney-effect system. Originally, the pretreatment process was intended to be as simple as possible, with the major goals to save landfill airspace and to reduce leachate loading. The city of Wilhelmshaven estimates that the costs of the entire biotreatment efforts on the 250 ton per day landfill, including 70 tons per day of wastewater sludge, are less than 25 DM, or $13 per ton.
After 2 years of using the biological pretreatment process at Wilhelmshaven, an assessment was conducted. Large-scale density tests indicated that the average compacted density of the pretreated waste was about 2000 lbs/cy, while the average density for non-treated MSW was 1,350 lbs/cy. This saved about 40,000 cubic yards of landfill airspace.
Neglecting other effects such as reducing bird control costs, the calculation of the benefits depend mainly on the cost of airspace. Considering an average density of 1,350 lbs/cy for non-treated waste, 1 ton of waste requires a volume of 1.48 cubic yards (cy). With a weight loss of 15 percent by degradation and an increase in waste density to 2,000 lbs/cy, the same waste can be placed in just 0.85 cy after the biotreatment. This volume saving amounts to 0.63 cubic yards per ton (cy/t). With operating cost of $13 per ton, the break-even cost for airspace is about $20 per cy. And this does not include the post-closure benefits.
Despite the chimney-effect system's success, advanced biological pretreatment methods recently have developed to minimize treatment areas, gas extraction, leachate treatment and post-closure costs. The most recent system is the Biopuster, which was developed for soil remediation and onsite waste stabilization.
Basically, this system aerates the waste or soil instead of relying on passive aeration as in the chimney-effect system. Air is not continuously flushed into the waste, but pressure thrusts are produced in single impulses, which enable the air to diffuse at a high speed. Consequently, air supply to the waste is improved.
In the Biopuster system, air is supplied via injection pipes, which are connected to compressors. The moisture content in the air also can be changed to control waste moisture. To prevent uncontrolled emissions and odors, a gas extraction system must be installed, and the waste must be covered by an impermeable layer, such as a geomembrane. The collected gas then is directed to the biofilters.
This process improves aeration so that stockpile construction is higher than passively aerated bio-heaps. Thus, a smaller area is required compared to the chimney-effect system. And because temperature inside the heap may exceed 190 to 200 degrees, even less biodegradable materials can be decomposed.
While biological pretreatment has primarily occurred in Europe, the bird control benefit seems to be attracting North American landfills. For example, at the Anguilla landfill on St. Croix in the U.S. Virgin Islands, a pretreatment feasibility study is underway.
The landfill is located about 1 mile from the St.Croix airport but a runway extension is being planned, which would decrease the distance to 1,000 feet. So far, the bird population at the Anguilla landfill does not seem to be extraordinarily high and it is not threatening air safety. But this is due primarily to a copious and rapid placement of daily cover, which uses more soil than regulations require. Eventually, it is expected that both the cost and limited availability of soil on the island will force the landfill to change its methods.