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Biodegradation in Municipal Solid Waste landfills

 

Bioreactor technology - Its significance

Environmental conditions in the landfill will have a significant impact on the rate of MSW decomposition. The factors that have most consistently been shown to affect the rate of refuse decomposition are the moisture content and pH, and it is generally accepted that refuse buried in arid climates decomposes more slowly than refuse buried in regions that receive greater than 50 to 100 cm of annual precipitation.

 

Landfill operation has evolved over the past several decades. Initially, landfills were operated to minimize water infiltration and therefore decomposition. With the advent of leachate collection and treatment, there has been increased interest in the operation of landfills to maximize waste decomposition and CH4 production. This is done by the recirculation of leachate and sometimes other liquids through the waste (Figure 13). Landfills operated to enhance decomposition are referred to as bioreactor landfills. The Solid Waste Association of North America (SWANA) has defined a bioreactor landfill as "any permitted Subtitle D landfill or landfill cell where liquid or air is injected in a controlled fashion into the waste mass in order to accelerate or enhance biostabilization of the waste." (U.S. EPA)

 

Bioreactor technology is a process based technology which involves physical, chemical and biological process with proper leachate management to recover bioenergy in the form of landfill gas and residue as manure.
Physical process - Phydical process involves, shredding of the waste to an uniform size, proper mixing of the waste etc.
Chemical process - Chemical process for enhancement of microbial growth involves leachate recirculation, pH adjustment, addition of buffers and nutrients etc.
Biological process - Bioreactor landfill operates under optimal anaerobic environmental conditions for enhancement of bio-degradation process.

 

Decomposition and biological stabilization of the waste in a bioreactor landfill can occur in a much shorter time frame than occurs in a traditional “dry tomb” landfill providing a potential decrease in long-term environmental risks and landfill operating and post-closure costs. As shown in Figure 14, the landfill settlement occurs at a much faster rate and as a result will stabilize earlier as compared to the one without recirculation. Potential advantages of bioreactors include:

  • Decomposition and biological stabilization in years vs. decades in “dry tombs”
  • Lower waste toxicity and mobility due to anaerobic conditions.
  • Reduced leachate disposal costs
  • A 15 to 30 percent gain in landfill space due to an increase in density of waste mass
  • Significantly increased landfill gas generation that, when captured, can be used for energy use onsite or sold
  • Reduced post-closure care.

 

Fig12                                           Fig13

Fig 13. An anaerobic bioreactor landfill                                                   Fig 14. Effect of leachate recirculation on                                                                                                                                                          settlement magnitude [Cell A];                                                                                                                                                                                  [Cell F = control]                                                                                                                                                                                       (El Fadel , 1999)                       

 

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