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The Sanitary Landfill 343
10.7 THE BIOREACTOR LANDFILL
When MSW is deposited in a conventional Subtitle D landfill, certain events such as partial waste
decomposition, gas production, leachate generation, and stabilization inevitably occur. Recent
investigations (Rathje and Murphy, 1992) involving core sampling of sanitary landfills have
revealed that wastes do not degrade significantly even after many decades, resulting in labels such
as ‘dry tombs’ for these systems.
An innovative approach to MSW disposal, which actually encourages rapid MSW decomposi-
tion and speeds stabilization, is the bioreactor landfill. The enhanced waste degradation and stabi-
lization carried out by the indigenous microbial populations within the waste is accomplished
through the addition of liquid (typically leachate) and air. The enhanced microbiological processes
within a bioreactor can transform and stabilize the decomposable organic waste within 5 to 10 years
of implementation, compared with many decades for conventional Subtitle D landfills where wastes
are essentially sealed off from air and moisture.
To date, there is still disagreement among scientists and engineers as to the precise definition
of a bioreactor landfill. The Solid Waste Association of North America (SWANA, 2001) has defined
a bioreactor landfill as:
any permitted Subtitle D landfill or landfill cell, subject to New Source Performance Standards/Emissions
Guidelines, where liquid or air, in addition to leachate and landfill gas condensate, is injected in a con-
trolled fashion into the waste mass in order to accelerate or enhance biostabilization of the waste.

Bioreactor landfill technology has been in use for over a century. The concept originates from
the systematic treatment of urban wastewater that began in the late 1800s. Bioreactor landfills can be
conceptualized as an extension of anaerobic and aerobic digestion at wastewater treatment plants.
Three general types of bioreactor landfill configurations are currently in use (U.S. EPA, 2003)
and are outlined below.

10.7.1 ANAEROBIC BIOREACTORS
Landfill degradation of MSW frequently is rate-limited by insufficient moisture (Campman and
Yates, 2002). The average landfilled MSW has a moisture content from 15 to 40%, depending on
the composition of the wastes, season of the year, and weather conditions (Emcon Associates, 1980;
Tchobanoglous et al., 1993; Kiely, 1997). However, maximum methane production in landfills
occurs at a moisture content of 60 to 80% wet weight (Farquhar and Rovers, 1973), suggesting that
most landfills are well below the optimum moisture content for methane production.
In an anaerobic bioreactor landfill, moisture is added to the waste mass uniformly in the form
of recirculated leachate, local water, or other sources to obtain optimal moisture levels. Liquid is
injected into the waste via horizontal trenches, vertical wells, surface infiltration ponds, spraying,
and prewetting of waste (Figure 10.31). Methods of liquid addition are addressed in detail by
Reinhart and Townsend (1998). Biodegradation occurs under anaerobic conditions and produces
landfill gas, primarily methane and carbon dioxide, in approximately equal proportions.
Anaerobic bioreactor landfills require careful monitoring at startup. If the waste is wetted too
rapidly, a buildup of volatile organic acids might lower leachate pH, inhibiting the methane-pro-
ducing bacterial population and reducing biodegradation rate. Optimal conditions for methanogenic
bacteria include a pH near the neutral point. Leachate parameters such as pH, volatile organic acids,
and alkalinity and gas parameters such as methane content are direct indicators of the activity of the
methanogenic bacterial population. A high-volatile organic acids to alkalinity ratio ( 0.25) indi-
cates that the leachate might possess a low buffering capacity and conditions could inhibit methane
generation (Campman and Yates, 2002).
When the methane content of the landfill gas exceeds approximately 40%, the methanogenic
bacterial populations are considered established. A decrease in the methane gas content below 40%