Effects of Waste Placement Practices on the Engineering Response of Municipal Solid Waste

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1 Effects of Waste Placement Practices on the Engineering Response of Municipal Solid Waste Abstract Laboratory and field testing was conducted to assess the effects of moisture addition on waste placement practices and to further develop an understanding of the physical and engineering properties of municipal solid waste (MSW) as a function of placement conditions. It was determined that the addition of moisture prior to compaction of waste resulted in significant environmental and financial benefits including: 1) increased landfill capacity up to 60 percent of original capacity; 2) extension of the life (i.e., years in operation) of a landfill; 3) improved waste degradation and methane production; and 4) increased net revenue up to 25 percent. Introduction Landfilling is the most common method of management for municipal solid waste (MSW) in the U.S. Waste disposal rates have remained consistent while permitting and construction of new landfill sites have decreased due to scarcity of suitable sites, enhanced regulatory requirements, and citizen opposition to siting resulting in the need for optimizing waste placement procedures and maximizing landfill capacity. The optimization of waste placement procedures and the physical and engineering properties of MSW are still not well documented or understood. The physical and engineering properties of waste can be difficult to determine and standardize due to the heterogeneous nature of the waste and variability in waste composition between landfills. Gaining a better understanding of the physical and engineering properties of MSW is required to improve waste placement efficiency and to prolong the life of landfills. In addition, an improved understanding of waste properties would allow for safer and more environmentally efficient facilities, both during operation and post-closure of a landfill. Furthermore, waste placement procedures and compaction techniques have remained consistent over the past two decades even though significant changes have occurred in compaction equipment. Optimizing waste compaction at landfills would increase waste placement efficiency and inherently streamline landfill operations and result in long-term environmental and financial benefits. 1

2 The purpose of the investigation was to expand upon the knowledge of physical and engineering properties of MSW and most importantly, improve waste placement efficiency using cost-effective and readily-implementable operational procedures. The field testing implemented in this investigation was the first documented comprehensive full-scale compaction study of waste in the United States. New knowledge was generated in the investigation for study of landfills and the data obtained during this investigation serves as a critical resource for future research. Test Program An extensive laboratory and field test program was conducted at Santa Maria Regional Landfill (SMRL) in Santa Maria, CA. In particular, meso- (i.e., intermediate) and full-scale test programs were implemented to determine the effects of systematic moisture addition on placement efficiency and asplaced density of MSW. Moisture addition (Figure 1) was incorporated into the daily waste placement operations to assess representative landfill conditions. Two 16 x 46 m test plots were constructed for the meso-scale compaction tests. Approximately 890 kn (100 tons) of residential MSW (RMSW) was placed into a test plot and compacted at target moisture contents of 55 (baseline as-received), 65, 80, 95, and 110%. The full-scale compaction investigation was conducted in a similar manner to the meso-scale investigation. However, the compaction tests were conducted on the active face of the landfill and were representative of the entire incoming daily waste stream (i.e., residential, commercial, and self-delivered wastes). A daily average of 2940 kn (330 tons) of MSW was placed and compacted at target moisture contents of 45 (baseline as-received), 65, 85, and 105% moisture contents. In addition to field tests, supplemental laboratory experiments were conducted on waste sampled from the landfill and on waste manufactured in the laboratory. Specific gravity (G s ), particle size distribution, moisture content, and organic content of MSW were measured and evaluated in order to characterize the waste at SMRL and expand upon the available data on the physical properties of MSW. Testing was performed using the American Society for Testing and Materials procedures commonly applied to soils in geotechnical engineering applications. Geotechnical engineering provided a framework for testing and analysis of wastes and modifications were made to the testing and analysis procedures to account for the unique properties of wastes including heterogeneity and time-dependent behavior. Field 2

3 testing and monitoring also was conducted to determine the field behavior of variably placed MSW. In particular, compression characteristics and temperature of wastes were measured in the field. A total of 8 magnet extensometer settlement arrays (Figure 2) were installed at SMRL with magnetic extensometer rings deployed at waste lift interfaces in order to monitor the compression response of individual fresh and old waste lifts and obtain a better understanding of the compressive behavior of waste. The settlement of the individual waste lifts was monitored for approximately 1 year. Results Compaction curves generated for the meso- and full-scale field studies were bell shaped and similar to soil compaction curves (Figure 3). The maximum dry unit weight ( ) and operational unit weight ( ) for the meso-scale compaction study were 8.5 and 13.3 kn/m 3 with corresponding optimum moisture contents of and, 78.5 and 79.5%, respectively. The maximum dry and operational unit weights for the full-scale test were 7.0 and 9.8 kn/m 3, respectively corresponding to optimum moisture contents of and, 76 and 75.5%, respectively. Moisture addition prior to compaction yielded beneficial waste placement results. An operational waste placement factor (OWPF) was defined as the quotient of waste unit weight for modified moisture conditions and waste unit weight for baseline moisture conditions. The OWPF represents a multiplier as to how much more waste can fit into a given volume of landfill space. OWPF values were determined to be 1, 1.33, 1.66, 1.37, and 0.83 for RMSW (meso-scale) compacted at target moisture contents of 55, 65, 85, 90, and 110%, respectively. OWPFs for the full-scale study were calculated to be 1, 1.28, 1.55, and 0.80 for target moisture contents of 45, 65, 85, and 105%, respectively. The specific gravity of MSW was determined to be significantly lower than that of soil. In addition, the specific gravity was determined to vary due to mechanical (i.e., compaction and compression) and biochemical (i.e., biodegradation) processes. A summary of the specific gravity test results is provided in Table 1. The particle size distribution of old MSW was comparable to a well-graded coarse-grained soil. The average baseline moisture content of incoming MSW at SMRL was 42.7% (dry-weight basis). The average moisture content of residential MSW, commercial MSW, and self-delivered MSW were 3

4 determined to be 57.7, 46.3, and 12.0%, respectively. The organic content of fresh and old MSW was determined to be 77.2 and 23.5%, respectively. In general, settlement data obtained in this investigation represented the secondary compression stages (i.e., mechanical creep and biochemical compression) of the wastes. On average, individual waste lifts (i.e., layers of waste placed at a landfill) settled 2.5% of the original lift thickness over the one-year duration of field monitoring. In addition, both fresh and old waste exhibited recompression behavior, caused by loading and unloading processes, which has not been documented up to this point. Temperature at SMRL overall increased over time due to heat generation in the waste mass. The temperature increased on average 3 to 6 C between the initial and final day of measurements for wastes that were 0.3 to 9 years old, respectively. Significance of Investigation Obtaining a better understanding of the physical and mechanical behavior of waste as a function of placement conditions is vital for improving landfill efficiency and optimizing airspace utilization. The laboratory and field test program presented herein provided new and extensive data that can be used for improving waste management practices and landfill operations. The significance of this investigation is summarized by the following conclusions: 1. G s of MSW had not been thoroughly evaluated up to this point. Based on the results of this investigation, the recommended specific gravity values to be used in analyses for fresh uncompacted, fresh compacted, and old wastes are 1.1, 1.2, and 2.2, respectively. Also, a linear relationship was developed between G s and degree of degradation of wastes, which can be used to estimate G s for variably degraded wastes. 2. Variation of MSW moisture content was established in an extensive database, which can be adapted for investigation of hydraulic and mechanical response of landfills with similar operational constraints. 3. Better understanding of the compressibility behavior of waste can result in more accurate settlement predictions, which are critical for landfill storage capacity calculations and for determination of the remaining life expectancy of a landfill. Recompression of waste needs to be considered for accurate 4

5 predictions of airspace availability at landfills and effective use of surcharge loading to maximize waste placement at a given landfill site. Findings from this investigation provide a conceptual framework for quantifying timing for surcharge loading to obtain desired amounts of waste settlement. 4. Systematic moisture addition to MSW prior to compaction resulted in optimal waste placement efficiency, maximum as-placed density of the waste, and significant environmental and financial benefits. Waste compacted at or near optimum moisture conditions increases landfill capacity up to 60% and in turn extends the life of the landfill minimizing the need for vertical expansion, additional siting/permitting, and/or diversion of waste to another landfill site. In addition, there is potential for significant net revenue gains when waste is compacted at or near optimum moisture conditions. At optimum moisture conditions, the net revenue due to combined waste placement procedures (i.e., moisture addition prior to compaction) and settlement over a 5-year period was estimated to be $5.6 million for a MSW landfill of similar size to SMRL. This represents a 25% increase in revenues on an annual basis. Furthermore, systematic moisture addition prior to compaction of waste may be financially and environmentally more beneficial than a bioreactor landfill system. A bioreactor landfill application is an alternative landfill management approach in which moisture is added to the waste mass at a later date than placement to enhance biodegradation of the wastes. However, the moisture addition typically occurs after waste height reaches a permanent elevation and with construction of extensive moisture delivery networks. A comparison between the costs and revenue associated with systematic moisture addition at SMRL and a typical bioreactor is provided in Table 2. The moisture addition at time of waste compaction scheme developed in this investigation has already been adapted by the Santa Maria Regional Landfill and is currently being used to obtain improved waste placement efficiency at the facility. 5

6 Figure 1. Systematic Moisture Addition Prior to Compaction of Waste Figure 2. Schematic of a Typical Settlement Array Inside a Borehole at SMRL 6

7 Figure 3. Compaction Curves from Meso- (left) and Full-Scale (right) Compaction Studies Table 1. Summary of Specific Gravity Experiments Waste Type Manufactured MSW Fresh MSW Old MSW Soil Particle Size or Moisture Condition Coarse Medium Fine As-Received Optimum Uncompacted (Typical) Compacted

8 Table 2. Financial Summary and Comparison Santa Maria Regional Landfill Financial Bioreactor Parameters Optimum Wet of Optimum Landfill a ( = 45%) ( = 76%) ( = 105%) Cell Footprint (ha b ) Total Weight of Waste x x x10 (kn) 5.5 x10 6 Systematic Moisture Addition Costs ($/ha) 0 14,000 26,100 0 Leachate Treatment c Costs ($/ha) 115, , , ,000 Total Additional Costs $0 $1,059,525 $1,237,050 $4,207,500 Systematic Moisture Induced Revenue ($/ha) Settlement Induced Revenue ($/5-year period) Gas Recovery Revenue ($/ha) Total Additional Revenue 0 561, , ,433,100 1,707,500 1,053, ,600 $0 $6,640,600 -$480,250 $7,395,000 Net Revenue $0 $5,581,100 -$1,737,300 $3,187,500 a Data used for the financial analysis of a bioreactor landfill was obtained from existing literature b One hectare (ha) is equivalent to approximately 2.5 acres c Leachate treatment costs for SMRL were provided by landfill operator 8