AREA A/B ENGINEERING REPORT - Waste Management

A2.1.5 Development of a “Biofilter” in the Bottom-Most Refuse Layer
Geosyntec Consultants
In addition to the protection provided by proper landfill construction and operation, there are
several natural processes and barriers to contaminant release that further protect HHE over time.
Researchers that have operated laboratory or field-scale landfill lysimeter studies (e.g., Bookter
& Ham, 1982) have noted that BOD and COD concentrations in leachate collected from upper
waste layers of a landfill waste are invariably higher than those in leachate collected from lower
waste layers. There is an increasing body of evidence that the observed development of stable,
low BOD/COD ratios in leachate collected from the basal leachate collection systems of mature
landfills is caused by liquids in a landfill percolating down through the bottom-most layers of
refuse before emerging as leachate. Studies in Japan (Shimaoka, et al, 1993) and China
(Youcai, et al, 2002) have demonstrated the capacity of existing lower lifts of MSW in a
simulated landfill and aged-refuse-based biofilter systems, respectively, to rapidly accelerate
leachate purification and waste stabilization. In the latter study where mature refuse was
removed from a landfill two to ten years after closure and characterized, the waste material was
found to have become organically stabilized, with most organic material degraded to inorganic
substances. Over the long term, this byproduct of post-disposal degradation would continue to
mitigate the potential for impacts from MSW leachate even if engineered containment structures
were to degrade over time.
The above studies suggest that the bottom-most waste layers are well decomposed due to moist
or saturated conditions from leachate percolation from above and these degraded layers act as
a biofilter with a relatively inexhaustible attenuating capacity for consuming degradable
organics in leachate. A key component of these findings is that a landfill does not have to be
operated as a bioreactor to realize the benefits of developing a basal biofilter layer because,
except for the most arid sites, all landfills will accumulate moisture on their bottom liner and
remove it as leachate. This phenomenon is of great value in effectively evaluating long-term
leachate conditions because it allows BOD to be used as a primary measure of overall leachate
quality. According to the research, if an improving trend in BOD concentration in leachate can be
demonstrated and leachate continues to percolate through the bottommost layers of refuse, it is
reasonable to expect that the concentration of degradable leachate organics will continue to
decline or remain steady in line with that of BOD.
This attenuating condition is also conducive to the continued immobilization of heavy metals that
may be present (Bozkurt, et al., 1999 and 2000; Belevi & Baccini 1989). Building on this, the
U.K. Environment Agency (Robinson, et al, 2004) describes an approach to design a buffer layer
above the leachate collection system to neutralize and potentially remove heavy metals from the
leachate. The UKEA document, which cites research work by Van Zomeren, et al (2003) who
modeled the lifetime of a 3-ft thick contaminated soil buffer layer based on its density, (alkali)
neutralization capacity, and rate of infiltration to remain effective for up to 450 years, concluded
that “it is clear that the inclusion of a buffer layer shows great potential in moderating the leachate
quality in terms of pH and heavy metal concentration.”
MD10186.doc 112 29 March 2009