Page 376 - Materials Chemistry, Second Edition

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The Sanitary Landfill 347
Aerobic bioreactor landfills rely on high temperatures as well as the addition of oxygen to sus-
tain the bioreaction. For such operations, careful regulation of moisture and oxygen levels enable
control of the waste mass temperature as well as fire potential.

10.7.6 OTHER CONSIDERATIONS
10.7.6.1 Leachate Strength Reduction

Bioreactor landfills decrease the strength of landfill leachate more rapidly than conventional
Subtitle D landfills. Chemical oxygen demand (COD) is one common indicator of leachate strength.
Reinhart and Townsend (1998) summarized measurements of COD half-lives for conventional and
bioreactor landfills. The time it takes for COD to be reduced by 50% (i.e., the half-life) is about ten
times faster in a bioreactor landfill than in a conventional landfill. Data up to this point, however,
are limited (Campman and Yates, 2002).

10.7.6.2 Waste Mass Shear Strength
A dry waste mass may be quite strong as evidenced by modern landfills having waste heights of up
to 90 m (approx. 300 ft) and slopes steeper than 3:1. However, the addition of water adds weight
but not shear strength, which affects traditional landfill design factors such as the waste mass geom-
etry. Some geometries used for dry landfills may not work with bioreactor landfills because of dif-
ferences in the shear strength of the waste and elastic displacement caused by water addition
(Campman and Yates, 2002).

10.7.6.3 Waste Settlement
Accelerating MSW degradation can reduce the need for new landfills by conserving volume.
Conventional landfill settlement is typically around 10% of landfill height and generally occurs over
a number of years as the waste decomposes (Koerner and Daniel, 1997). Settlement of the waste
mass in a bioreactor landfill can be significant over time, involving 10 to 25% of the landfill height.
Aerobic bioreactors might achieve this settlement within 2 to 4 years, while anaerobic bioreactors
might require 5 to 10 years (Campman and Yates, 2002). Pilot-scale landfill cells in Sonoma County
and Mountain View, California, experienced settlement by as much as 20 and 14%, respectively, in
leachate recirculation cells and approximately 8 to 10%, respectively, in the conventional dry cells
(Reinhart and Townsend, 1997). Waste settlement varies greatly and is dependent on type of waste,
amount of cover, and compaction. Settlement also will not be consistent across the landfill surface.
Gas collection and other internal landfill systems (such as leachate collection and recirculation)
must be able to shift with this settlement (U.S. EPA, 2000).
Increased rates of settlement before closure will permit additional MSW to be placed in the
landfill before a cap is put in place. Such additional waste placement can therefore reduce the need
for new landfills. These benefits can be realized only when waste decomposes prior to closure.
Landfill operators may choose to delay closure in order to take advantage of increased space cre-
ated by additional waste settlement.

10.7.6.4 Metals
The long-term fate of metals in bioreactor systems is generally unknown. Since heavy metals tend to
concentrate during wastewater biosolids treatment, similar effects could be anticipated in bioreactor
landfills during waste decomposition, and such changes in heavy metal concentrations could be seen
in leachate quality. Issues regarding the behavior of metals in the landfill environment include:
● Microorganisms may concentrate metals
● pH and sulfides in the landfill may affect metal mobilization
● Potential for remobilization of metals if landfill conditions become anaerobic