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The Sanitary Landfill 345
until most of the readily degradable compounds have decomposed and the waste temperature grad-
ually decreases during the final phase of maturation of the remaining organic matter.
Due to the substantial amounts of heat generated in an aerobic bioreactor, large quantities of
leachate can evaporate. In a study of two bioreactor landfills, leachate volume was reduced by 86
and 50% (Hudgins and Green, 1999). Changing the rate of air and liquid injection will alter the tem-
perature of the waste pile. To ensure against possible waste combustion, the waste mass is wetted
adequately and air injection is uniform throughout the waste mass.
Aerobic bioreactor landfills require much greater inputs compared with their anaerobic coun-
terparts. According to Weathers et al. (2001), the additional power required to inject air into an aer-
obic bioreactor was 12 times higher than the power required to extract landfill gas from an
anaerobic bioreactor. However, postclosure costs should be reduced substantially due to reductions
in landfill gas generation and cover settlement (Campman and Yates, 2002).
Because of the higher reaction rates, aerobic biodegradation is a more rapid process than anaer-
obic biodegradation. Consequently, aerobic landfills have the potential to achieve waste stabilization
in 2 or 4 years as opposed to decades or longer for conventional landfills. The rapid rate of waste sta-
bilization in aerobic landfills also offers the potential for ‘mining’ of the landfill waste (see below).
Aerobic bioreactors have many of the same benefits as anaerobic bioreactors, only these benefits
are achieved more quickly. Since aerobic bioreactors do not produce significant quantities of methane,
there is little potential to sell landfill gas for energy (U.S. EPA, 2000). The following benefits have
been observed at aerobic bioreactor landfills (Hudgins and Green, 1999; Campman and Yates, 2002):
● More rapid waste and leachate stabilization
● Increased rate of landfill settlement
● Reduction of methane generation by 50 to 90%
● Capability of reducing leachate volumes by up to 100% due to evaporation
● Potential for landfill mining
● Reduction of environmental liabilities

10.7.3 ANAEROBIC VS. AEROBIC
Recall from the discussion of conventional sanitary landfills that MSW deposited in a landfill tends
to undergo five phases of decomposition, starting with a brief aerobic phase, through two anaero-
bic phases followed by a methane generation phase and finally maturation. Aerobic bioreactor land-
fills attempt to sustain the Phase I activity over a longer period of time than that occurring in a
conventional MSWLF. In contrast, anaerobic bioreactor landfills attempt to reduce significantly the
time involved for Phase IV activities (methane generation) to possibly 5 to 10 years (a 75% reduc-
tion), with 5 to 7 years for Phase IV considered optimum (U.S. EPA, 2000).

10.7.4 HYBRID BIOREACTORS (SEQUENTIAL AEROBIC–ANAEROBIC)
The hybrid bioreactor landfill accelerates waste degradation by employing a sequential
aerobic–anaerobic treatment to rapidly degrade organics in the upper lifts of the landfill and collect
gas from lower sections. Operation as a hybrid results in the earlier onset of methanogenesis com-
pared with aerobic landfills (U.S. EPA, 2003).

10.7.5 PRACTICAL OPERATIONAL ISSUES
10.7.5.1 Waste Preprocessing

Wastes may be placed directly into a bioreactor landfill after which they are compacted by heavy
machinery, or wastes can be shredded prior to placement. The goal for such preprocessing is to
achieve optimum exposure of waste material to the bioreaction process. There have been some con-
cerns (U.S. EPA, 2000) regarding the absence of decomposition in bioreactor landfills when MSW
is placed inside plastic bags, which may or may not be broken open during conventional compaction