Effects of Placement Conditions on Decomposition of Municipal Solid Wastes in Cold Regions

The effects of placement practices on decomposition of wastes were investigated at Anchorage Regional Landfill (Anchorage, Alaska) since 2002. Temperatures and gas concentrations of wastes placed at various seasons were monitored. Wastes were placed at sub-freezing temperatures during cold seasons. Waste temperatures generally increased upon placement. High variation was observed in waste temperatures near the surface whereas steady temperatures were obtained at depth. High maximum stable temperatures resulted from warm placement conditions. Steady temperatures between approximately –1 to +35°C were observed. The central portion of a frozen waste band (with a total initial thickness of 7 m at placement, currently between depths of approximately 8 m to 15 m) remains frozen 2 years after placement. Both the top and bottom regions of the frozen waste band have thawed. Heat Content (HC) varied between -8.2 (for 2-year-old waste at a depth of 11.9 m in frozen wastes) to +25.9°C-day/day (for 13-year-old waste at a depth of 32 m for waste placed in summer). The measured frost depths in waste ranged from 0.7 to 1.3 m and were less than that for native soil at the landfill site. Instantaneous thermal gradients ranged from -73 to +60°C/m. Gas concentrations were similar to air at the time of waste placement. Anaerobic decomposition conditions and onset of landfill gas production started within 3 to 4 years of placement for wastes placed during warm seasons. Virtually no decomposition or gas generation were observed in the frozen wastes. A 1-D numerical model was used to investigate distribution of temperatures for placement at varying temperatures and for varying lift thicknesses. It is recommended to minimize frozen lift thicknesses to obtain higher temperatures. Introduction Placement of municipal solid wastes (MSW) in landfills remains as the primary method of waste disposal in the United States and many parts of the world. Decomposition and degradation of wastes in MSW landfills cause generation of gas, leachate, and heat. Generation of these byproducts is closely interrelated. For example, gas production is significantly affected by temperature. Temperature affects reaction rates and microbial population balance for decomposition of organic constituents (Hartz et al. 1982). The level of decomposition increases with temperature up to limiting values. Mesophilic and thermophilic methanogenic bacteria are primarily responsible for gas production. Optimum temperature ranges for the growth of these bacteria are 35 to 45°C and 50 to 60°C for mesophilic and thermophilic groups, respectively (Cecchi et al. 1993). Optimum temperature ranges for maximum gas production from wastes range between 34 and 41°C based on laboratory studies (DeWalle 1978, Hartz et al. 1982, Mata-Alvarez and MartinezViturtia 1986). A temperature of 43°C was identified as the optimum temperature for gas production at a landfill in England (Rees 1980). Significantly reduced gas production rates are expected at temperatures approximately below 20°C and above 75°C (Tchobanoglous et al. 1993). Maximum reported waste temperatures generally varied from approximately 30 to 65°C and were observed within the middle one third depth to over one half depth of landfills with total waste heights of approximately 20 to 60 m (Yesiller et al. 2005). Climatic conditions, initial waste temperatures, and waste placement rates were determined to affect resulting waste temperatures (Hanson et al. 2005, Yesiller et al. 2005). Yesiller et al. (2005) presented a formulation for determination of heat generation in landfills as a function of climatic and operational conditions. Heat content (HC) of wastes was determined as the difference between measured waste mass temperatures and unheated baseline waste temperatures over time at equivalent depths. The peak HCs were directly correlated to waste placement rates and initial waste temperatures. The HCs were also affected by precipitation. The peak HC increased with precipitation up to a value 2.3 mm/day and then decreased with further increase in precipitation. Landfilling in cold regions presents specific operational challenges related to frozen material handling, freeze/thaw exposure to components of the waste containment system, and limited effectiveness for generation of landfill gas as an alternative energy source. The investigation described in this paper was conducted to evaluate the influence of a cold climate on the decomposition of municipal solid wastes. Field measurements were used to determine temperatures and gas composition at discrete locations throughout Anchorage Regional Landfill in Anchorage, Alaska. Measurements of temperature and gas composition were obtained over multiple years for evaluation of waste placement conditions and seasonal influence on measured parameters. Experimental Program Site Description Anchorage Regional Landfill (ARL) is a modern landfill with Subtitle D liner systems (barriers and leachate collection systems). ARL has a total of 8 cells and has been operational since 1992. ARL does not have an active gas collection system. Operational and climatic statistics for ARL are presented in Table 1. ARL has relied on deep excavations (on the order of 25 m) from a planning standpoint for development of cells. This has served two purposes; the first is maximization of airspace within the available footprint, the second relates to increased structural stability of the interim and completed landfill, particularly for seismic activity. Cell sizes were on the order of 6 to 8 ha due to airspace requirements, which yielded approximately 2.3 million m capacity. Cell construction has progressed on a 4 to 6 year schedule based on incoming acceptance rates. Cells were developed to provide adequate storage of waste for a filling period of 5 years at any given time. The average yearly intake typically fills two lifts over the average cell dimensions with average lift thicknesses of 3 to 4 m and waste unit weights of 5.2 kN/m. Site records indicate that placement varied on a semi-annual basis between higher elevation and lower elevation cells. Placement in higher elevation cells coincidentally tended to be concentrated during winter seasons. In all cases, wastes have been placed at lower elevation areas during significant wind events to shelter waste placement operations. The waste placement practices resulted in alternating winter/summer lifts in a vertical sequence in a given cell. Depending on the start date of placement of lifts, underlying lifts were often buried in a frozen or cooled state and insulated by the overlying lift. Table 1. Operational and Climate Statistics for ARL Parameter Value Design waste placement area (ha) 67 Design volume (m) 32,360,000 Average waste intake (t/year) 317,000 Waste placement rate (m/year) 4-7 Average waste column height (m) 26.5 Leachate pumped (m/m-year) 0.068 Gas produced (mgas/m waste/year) 0.80 Number of instrumented cells 4 Total temperature sensors 205 Total gas sensors 93 Climatic zone and description Cold temperate boreal zone, oceanic boreal climate Average daily high temperature (°C) 6.2 Average daily low temperature (°C) -1.5 Average daily temperature (°C) 2.3 Annual normal precipitation (mm) 408 Annual normal snowfall (mm) 1793 Mean annual earth temperature (°C) 5.4 estimated amount based on numerical simulation for medium-dry/dry conditions, gas not actually collected since gas collection system is not installed based on Landsberg et al. (1966) from NCDC (2004) based on reference measurements at site Field Measurements and Analysis Temperature and gas data were collected in multiple cells using horizontal and vertical arrays at ARL since 2002. Measurements were taken at discrete locations in various components of the landfill system including below liner, within liner, within waste mass, within interim cover, and within subgrade soils near the perimeter of the landfill. Sensors were placed in linear arrays in horizontal and vertical configurations. The arrays consisted of Type K thermocouple wire encased within flexible PVC tubing (for temperature measurements) and copper refrigeration tubing with gas sampling ports at the terminal end (for gas measurements). The length of horizontal sensor arrays was 206 m with 8 sensors on each array. The length of vertical sensor arrays ranged from 18 to 48 m and the number of sensors on each vertical array ranged from 4 to 21. Temperature measurements have been taken weekly and gas measurements have been taken monthly. Data analysis in this investigation included determination of spatial and temporal variation of temperatures and gas compositions within wastes. Effects of climatic and operational conditions were analyzed. Rates of temperature change were determined. Timing for onset of temperature increases and gas production were identified. Temperature envelopes depicting limiting maxima and minima with depth were developed for the waste mass. The envelopes were used to estimate heat content (HC) of the waste mass (Yesiller et al. 2005). Instantaneous vertical thermal gradients were calculated as the quotient of temperature difference between two adjacent sensors and vertical distance between the sensors for a single monitoring event. In addition, frost depths within the waste mass were determined as the 0oC isotherm using measured temperatures and also using the modified Berggren equation as described by Andersland and Ladanyi (1994). HC values were determined using the method that was developed specifically for analysis of wastes, whereas frost depths were determined using methods that were developed for analysis of soils. Numerical Modeling Finite element analysis was used to conduct a parametric study of the influence of waste filling practices on decomposition and resulting heat generation in wastes. A mesh containing 3-D quadrilateral brick elements was constructed for the model. ABAQUS FEA software (Version 6.5) was used in the analysis. The material properties used in the analysis are presented in Table 2. Simulations were conducted using transient thermal analysis with boundary conditions prescribed as a sinusoidal temperature function at the ground surface and a stable temperature at the far-field boundary at great depth. A temperature envelope generated using measured temperatures at depth at a reference control site was extrapolated to the surface to provide an idealized sinusoidal (seasonal) ground surface temperature function that had a mean value of 5.4oC and an amplitude of 9.9oC. The far-field boundary at depth was specified as a constant temperature of 5.4oC at a depth 75 m below the base of the waste mass. A step-function was used to model heat generation due to decomposition as a function of temperature. For temperatures equal to or below 0oC, zero heat generation was used and for temperatures above 0oC, a step function of heat generation rate with time was used. Heat generation rates were modeled as 2.5 W/m (first 120 days after waste placement) and 0.08 W/m (remaining time). Waste filling rates were selected in accordance with ranges consistent with site operation. Variable rates of filling and lift thicknesses (including variable thickness of frozen waste layers) were modeled to investigate the effects of filling sequences (rates respective to seasons) on resulting thermal conditions and decomposition potential for wastes. Table 2. Material Properties for FEA Modeling Parameter Waste Soil Unit weight (kN/m) 5.2 21.0 Thermal conductivity (W/mK) 0.23 2.35 Volumetric heat capacity (kJ/mK) 719 1783 Thermal diffusivity (m/s) 3.2 x 10 1.3 x 10 Latent Heat (kJ/kg) 32.1 1.7 Results Field Measurements Effects of overlying waste filling are demonstrated in Fig. 1 for two horizontal arrays. Wastes were placed above the 156 and 206 m sensors in summers of 2003 and 2004 for array Cell 4-5A. Sensors at 16-46 m were covered in fall 2003. Wastes were placed above the 106-206 m sensors in spring 2003 for array Cell 4-5B. Sensors at 156 and 206 m were damaged during waste placement, terminating their functionality. The shorter sensors were covered incrementally over the next year. Total depths of overlying wastes ranged from 0 to 9 m. Amplitudes of temperature fluctuations dampen upon placement of overlying wastes. Steady temperatures of approximately 19 to 21°C and 15 to 18°C were measured in Cell 4-5A and Cell 4-5B, respectively. Different responses were observed for placement during heating (spring and summer) and cooling (fall and winter) seasons. Relative to placement temperatures, final temperatures may be warmer or cooler due to placement season. Uncovered sensors continue to indicate significant seasonal temperature fluctuations including freezing during winter seasons. -15 -10 -5 0 5 10 15 20 25 30 35 Au g02 Fe b03 Au g03 Fe b04 Au g04 Fe b05 Au g05 Fe b06 Au g06 Date Te m pe ra tu re ( o C ) 16 m 26 m 36 m 46 m 56 m 106 m 156 m 206 m Cell 4-5A Cell 4-5B -15 -10 -5 0 5 10 15 20 25 30 35 Au g02 Fe b03 Au g03 Fe b04 Au g04 Fe b05 Au g05 Fe b06 Au g06 Date Te m pe ra tu re ( o C ) Figure 1. Temperature vs. time for horizontal arrays. Effects of waste age and initial placement temperature on resulting waste temperatures are presented in Fig. 2 for three vertical arrays. For all three arrays, wastes near the surface (beneath interim cover) underwent seasonal temperature variations whereas steady temperatures were reached at depth. Temperature variations in wastes with ages 3 to 18 years are presented in Fig. 2a. Maximum steady elevated temperatures of approximately 35°C were measured at a depth of 32 m for 13-year-old wastes that were placed in summer. Temperatures of both over and underlying wastes were less than the peak value. Temperature variations in wastes with ages 3 to 4 years are presented in Fig. 2b. Fluctuating, yet somewhat elevated temperatures were measured between 0 and 1.3 m. Elevated steady temperatures were measured below 2.3 m. Maximum steady temperatures (approximately 21°C) were measured at 8.3 m, whereas minimum steady temperatures (approximately 14°C) were measured at 18.3 m. The upper range of steady temperatures was observed for wastes placed during spring to summer 2002 whereas the lower range of steady temperatures was observed for wastes placed during winter to summer 2001. Results from an array containing significant fraction of waste placed in a frozen condition is presented in Fig. 2c. A frozen waste band with a thickness of 7 m was placed between November 2003 and January 2004. Wastes underlying the frozen band were placed in late summer 2002 (21 to 23 m) and late summer 2003 (16 to 20 m). A layer of waste was placed above the frozen wastes between April and July 2004 to a thickness of 8 m. The mid-region of the frozen waste band remains at subfreezing temperatures 2 years subsequent to placement at depths between 12.9 and 14.9 m (Fig. 2c). However, the top and bottom regions of the frozen waste band began to thaw within 2 months of placement. Stable temperature trends have developed ranging from -1 to 15°C. Even though the temperatures of the remaining frozen wastes appear to have stabilized at -1°C, eventual thawing is expected in the long term. In general, final waste temperatures increased with increasing placement temperature. However, the wastes placed near the vicinity of the frozen waste band (Fig. 2c) were affected by the presence of this cold mass and did not reach temperatures as high as comparable wastes in other cells (Figs. 2a and 2b). A comparison of temperature envelopes with depth for vertical arrays is presented in Fig. 3. Locations of bottom liners are marked with dashed lines in the plots. High seasonal temperature variations near the surface and steady temperatures at depth were observed. Such temperature envelopes in wastes typically display a bulb of elevated temperatures indicating high heat production towards the center of the landfill (Yesiller et al. 2005). This trend was observed in arrays Cell 1M and Cell 1A whereas distinctly cooler temperatures were associated with the envelope for frozen wastes in array Cell 6 displaying reverse concavity. The envelopes were used to predict HC with depth for a given location. HC determined from vertical arrays throughout ARL range from -8.2 (for 2-year-old waste at a depth of 11.9 m for the vertical array placed in frozen wastes) to +25.9°Cday/day (for 13-year-old waste at a depth of 32 m for a vertical array placed in summer). A negative HC indicates that the waste temperatures were below temperatures at equivalent depth for unheated conditions. -10 -5 0 5 10 15 20 25 30 35 40 Au g02 N ov -0 2 Fe b03 M ay -0 3 Au g03 N ov -0 3 Fe b04 M ay -0 4 Au g04 N ov -0 4 Fe b05 M ay -0 5 Au g05 N ov -0 5 Fe b06 M ay -0 6 Date Te m pe ra tu re ( o C ) 0 m 4 m 8 m 12 m 16 m 20 m 24 m 28 m 32 m 36 m 40 m 44 m 48 m