AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
Although leachate generation rates are anticipated to be very low after landfill closure, LCSs are
conservatively designed to rapidly convey large leachate flows to sumps (Rowe, 1998)
throughout the operating and PCC periods. Flow rates in LCSs are typically high enough to
function even after accumulation of leachate particles in the drainage layer and pipes. Cleanout
pipes that connect to the main LCS pipes in the landfill are incorporated into the design to flush
debris from the main LCS pipes, if necessary. In addition, the high design flow rates reduce the
potential for the LCS to have sustained periods of saturation, decreasing the potential for
development of biological and chemical clog material in the LCS.
The potential for LCS clogging in MSW landfills has been considered by a large number of
researchers (e.g., Bass, et al., 1983; Koerner & Koerner, 1989, 1990, 1991, and 1995; Rohde &
Gribb, 1990; Brune, et al., 1991; Craven, et al., 1999; Fleming, et al., 1999; Fleming & Rowe,
2004; Van Gulck & Rowe, 2004; Rowe and Van Gulck, 2004; Cooke et al., 2005; Rowe, 2005).
These studies indicate that clog material forms “by biologically induced processes that involve the
removal of some of the organic leachate constituents (as implied by the reduction in COD) and
precipitation of some inorganic leachate constituents (as implied by the reduction in calcium
concentration)” followed by “an accumulation of inorganic particles originally suspended in the
leachate” (Rowe, 2005). They also suggest that the potential for clogging depends on the amount
and composition of leachate and on the details of the design of the LCS.
Based on the above studies, for clogging to occur, the following two conditions generally need to
exist: (i) inadequate design of a sand or geotextile filter; and/or (ii) unexpectedly high rate of
relatively high strength leachate produced in the landfill that keeps the LCS near saturation. The
first condition is mitigated by properly designing the filter to resist physical clogging. A LCS
generally includes a geotextile filter that is wrapped around LCS pipe bedding gravel; it may
also include a sand or geotextile filter between the drainage layer and the overlying soil or
waste layer. Methods for design of filters are well established, and there is extensive information
in the technical literature on sand and geotextile filter design (e.g., Giroud, 1982 and 1996;
Lafleur, et al., 1989; Luettich, et al., 1992; Koerner, 1998). The second condition for clogging of
LCSs in landfills is primarily related to the rate of leachate generation. The lower the leachate
generation rate, the lower the potential for clogging (other factors being equal). Given the
significant decrease in leachate generation rates after landfill closure, the potential for biological
clogging of the LCS decreases after the landfill is closed with a final cover system. The second
condition can therefore be avoided through good landfill operation (to limit unnecessary leachate
generation) and prompt application (as required under Subtitle D) of a suitably designed cover
system after site closure, supplemented with knowledge of leachate compositional trends (as
described in detail in Appendix A).
MD10186.doc 129 29 March 2009