Environmental Engineering Reference
In-Depth Information
bioreactors. This bioreactor landfill contains about 70,000 tonnes of waste in a cell of area
2.85 hectares (7 acres). Leachate, storm water, and air were injected into vertical risers with
force main and header from storage tank that were retro-fitted for the closed landfill. The
operation started in June 2000 and approximately 3,785 m 3 (one million gallons) of leachate
has been re-circulated (USEPA 2007). Two other aerobic landfills are in operation in Georgia
(the Columbia County landfill) and in Atlanta (a privately operated bioreactor). For each
aerobic landfill system, the air injection comprises of air compressor and piping connected to
vertical air injection wells. Leachate collected in holding tanks at each site was pumped back
into each aerobic system through a leachate recirculation system installed on top of the
intermediate cap. The air injection rates were 56 m 3 /min and 100 m 3 /min for the Columbia
County Landfill and Atlanta Landfill, respectively (Hudgings and Harper, 1999).
Monthly topographic surveys of Williamson County Landfill bioreactor surface were
performed to detect settlement across the site. An initial survey of the landfill bioreactor
surface was conducted in January 2000, several months prior to the start-up of the landfill
bioreactor as a baseline. Results of settlement as of April 2005 show a 0.53-meter to 1.2-
meter drop in the surface elevations since the landfill bioreactor operations began. The
comparisons of the April 2005 elevations with the original survey in January 2000 show a
5.1% to 10.7% decrease in waste height over a 59-month period of operation (USEPA 2007).
The study conducted in Georgia is interesting since it was conducted after landfills went
through anaerobic process first. It was observed that the average settlement was 4.5% and the
greatest settlement was 9% at Columbia County Landfill and 10% at Atlanta Landfill. The
methane generation was reduced to 50% for the Columbia County Landfill and 50 to 90% for
the Atlanta Landfill after aeration started. The operation of each system was also a dynamic
process. It was observed in some areas that the decay process would revert back to anaerobic
once temperatures decreased and that the air would make its way to other nearby areas,
thereby repeating the process. Also, if too much leachate was applied in the aerobic areas,
there would be an insufficient amount of oxygen for the bacteria and reverted the decay
process back to anaerobic. Conversely, if too little leachate was applied, waste mass
temperatures would tend to rise into the thermophillic range (40 to 70 0 C). In several cases, it
was also observed that air could provide a cooling effect on the waste mass temperatures
above 60 0 C or increase temperatures that were below 20 0 C, provided sufficient moisture was
applied (Hudgins, 1998; Hudgins and Harper, 1999).
Partially degraded waste has produced difficulties in post mining attempts. Therefore,
before moving to the mining step, it is essential to ensure that most of the organic waste is
decomposed before mining commences.
3. Biocell stage 3: Mining for recovery of useful/recyclable products
Mining will ensure recovery of space and various products (Murphy, 1993; Zee et al.,
2003). Recycling a biocell involves a series of activities that includes excavation of cell,
separating, sorting, and processing of recyclables. Separation of waste based on size can be
performed through the use of various types of screens (trammel, vibrating, rotating, and disc).
Once recovered, the recyclables can be crushed, bailed or shredded for the convenience of
Usually, the excavated waste is stockpiled and fed to a shaker with sieves to separate the
leftover organic waste. The organics that passes through 1 cm - 5 cm sieve size can be used
as compost in agricultural applications or as refuse derived fuel (RDF). The collected non-
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