The International Information Center for Geotechnical Engineers

Biodegradation in Municipal Solid Waste landfills

A Case Study Performance of North American Bioreactor Landfills - Leachate Hydrology and waste settlement (Bareither et al., 2010a)


The most common strategy to accelerate decomposition is to stimulate microbial activity by adding moisture to the waste via recirculation of leachate and addition of supplemental liquids, a practice that is becoming more common in North America (Pohland 1975; Barlaz et al. 1990; Pacey et al. 1999; Reinhart et al. 2002; Mehta et al. 2002; Warith 2002; Benson et al. 2007; Bareither et al. 2008a.An assessment of state-of-the-practice at five full-scale landfills operating as bioreactors is studied (Table 3). This study focuses on effectiveness of liners and leachate collection systems, leachate generation rates, leachate recirculation practices and rates, effectiveness in moistening the waste, and settlement of the waste over time. Residential and commercial nonhazardous solid wastes are the dominant waste streams received at each landfill.


Table 3. General characteristics of landfills selected for study



Reason for Bioreactor Operation

The reason for implementing a bioreactor varied between landfills.  At the two northeastern landfills Landfills L and G, state regulations require the operator to have the capacity to treat all leachate on-site. At these landfills, recirculation is the least expensive leachate management alternative and reducing treatment costs was a significant factor in the decision to recirculate leachate. Landfill G was also equally interested in airspace recovery and enhanced gas generation. Landfills M and D were interested in maximizing waste decomposition to reduce the long-term risk and associated costs for managing and treating leachate and gas. However, leachate disposal was an advantage at Landfills M and D too. At both landfills, off-site treatment required trucking leachate 40 to 60 km. Thus, significant cost and emissions savings were achieved at Landfills M and D by supplanting off-site treatment with recirculation. The motivation at Landfill Y was to monitor and document bioreactor performance in a full-scale landfill operation. Two modest-size cells were filled and covered rapidly so that bioreactor performance could be monitored and documented. Landfill Y also valued the cost savings accrued via leachate recirculation in lieu of leachate treatment.



Settlement data amenable to analysis were available for Landfills L, M, and Y. A baseline survey of the cover elevation at Landfill Y was conducted several months following waste placement. At each landfill, settlement was not monitored during the first month following waste placement, and initial compression was essentially complete prior to monitoring. Thus, the analysis presented herein corresponds to longer-term secondary compression due to mechanical and biological mechanisms. 


Fig 15. Settlement curves for: a) NE, W, and pilot project conventional and recirculation areas at Landfill Y;   b) conventional, recirculation, and Trench 5 areas at Landfill M


Settlement curves strain versus log time for the full-scale and pilot cells at Landfill Y are shown in Fig. 15(a). A change in the rate of compression occurs around 500 days for the pilot and full-scale bioreactor cells full-scale NE and W cells, bioreactor pilot cell. This change appears to correspond to the onset of active biodegradation, which increases the rate of settlement . This transition is associated with the onset of appreciable methane production at Landfill Y, which began 1–2 years following waste placement in both the pilot cell and the full-scale cells. A less distinct break is evident around 500 days in the settlement curve for the conventional pilot cell. This more subtle break is consistent with the modest methane production from the conventional pilot cell, which was around 20% of methane production from the bioreactor pilot cell and nearly ceased three years after waste filling.


Settlement curves for Landfill M are shown in Fig. 15(b). No distinct break was evident in the curves for Landfill M, and the settlement data were for conditions greater than 1,000 days after filling was complete (both conventional and bioreactor cells).Thus, active biodegradation was assumed to be occurring when the settlement data were collected at Landfill M. Both cells were actively producing methane during the period when the settlement data were collected.


The rate of settlement at landfills L, M, and Y was quantified using the secondary compression ratio Cα, which is commonly used in practice and describes the rate of change in strain relative to the logarithm of time (Sowers 1973; Bjarngard and Edgers 1990).Linear regression was used to determine Cα. For Landfill Y, Cα was computed for mechanical and biodegradation-induced compression, with the two mechanisms distinguished by the slopes before and after 500 days elapsed time. On average, the Cα are larger for both mechanical and biodegradation-induced compression in the bioreactor landfills compared to the conventional landfills 1.3 times for mechanical, 1.6 times for biodegradation.


Fig. 16 Relationship between biodegradation-induced secondary compression ratio and dose for recirculation in horizontal trenches


Figure 16 shows Cα for biodegradation-induced settlement as a function of the average dose for Landfills L, M, and Y in this study. the trend indicates that Cα increases as the recirculation dosage increases. Thus, greater settlement rates may be expected at landfills where recirculation is conducted more aggressively.


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