Greenhouse Gas Emissions from Solid Waste Management     

The decomposition of organic waste in solid waste landfills produces “landfill gas” which is 53% methane and 47% carbon dioxide.  The rate at which this gas is produced depends on the moisture content of the waste and the size of the landfill.  The amount of landfill gas produced depends on the amount of the “waste in place”.  The Workbook, Chapter 5 assumes that landfill gas will be produced for thirty years after landfilling.  However, since emissions are produced according to first-order kinetics, approximately 80% of the gas is produced in the first fifteen years.  The methane emissions from solid waste landfills were 46,459 MTCE in 1990, and 40,305 MTCE in 1996, with the decrease resulting from an increased efficiency of methane capture.  Carbon sequestration more than offset these emissions, with 110,438 MTCE sequestered in 1990 and 127,039 tons sequestered in 1996.

Because carbon dioxide is consumed to produce the organic component of waste placed in landfills, the carbon dioxide component of landfill gas, the Workbook does not include the carbon dioxide in landfill gas in the net emissions calculation.  Following similar reasoning, the methane in landfill gas that is collected and combusted, either by flaring (primarily for odor control) or for power production is not included in the net emissions total. 

 The methane emissions that are included in the total net emissions were estimated from data kindly supplied by a consultant to the Rhode Island Resource Recovery Corporation (RIRRC) – John Murphy of the DuFresne-Henry Corporation.  The amount of landfill gas generated at the Central Landfill in Johnston, RI, and the estimated percentage of collection is reported in the Landfill Gas table.  Projections of methane production to 2026 also were reported for “phases” of the Central Landfill that are expected to receive waste through 2012.  Since 1985, the Central Landfill has received approximately 96% of the municipal waste produced in Rhode Island (perhaps slightly less in the 1980’s) – the remainder currently goes to municipal landfills in Bristol, Charlestown and Tiverton.  The Central Landfill also receives variable amounts of commercial solid waste (depending on the tip fee, as compared to fees for out-of-state disposal) and small amounts of sewage sludge.  The amounts of these wastes disposed at the Central Landfill are reported in Appendix B: Table 10 (data supplied by Dante Ionata from RIRRC).

Prior to the mid-1980’s, a number of landfills were active in Rhode Island. Since the Workbook assumes that methane is produced for thirty years after waste is landfilled, it would have been desirable to estimate the emissions from waste placed in these landfills from 1960 and thereafter (for the 1990 totals) and from 1966 and thereafter for the 1996 totals.  Data for all known landfills in Rhode Island were obtained from the Solid Waste Division at RIDEM, and are reported in Appendix B: Table 3.  The amount of waste contained in these landfills is reported for only 32 of the 96 landfills that have been identified.  Unfortunately, the amount of waste disposed in these landfills as a function of time is unknown as is the nature of the waste.  Certain industrial wastes (e.g. demolition debris) are expected to produce methane slowly (wood) or not at all (cement). 

 The amount of waste in these 32 landfills is substantial, approximately 22 million tons.  However, since waste decomposes by a first-order processes, for the 21 of these landfills that closed prior to the mid-1980’s, the amount of methane being produced in the 1990’s would be expected to be relatively insignificant – 80% or more of the decomposition of municipal waste buried there would already occurred by 1990.  These 21 landfills account for 18 million of the 22 million tons.  Further, the largest of these landfills, the Cranston Sanitary (Capuono) landfill that contained four of these 18 million tons is now collecting methane emissions (estimated by Chris Walusiak from RIDEM to be producing landfill gas at a rate of 150 – 200 SCFM, which is approximately 2% of the rate at the Central Landfill).  Therefore, while we know that some methane emissions were occurring from closed landfills in 1990 and 1996, we believe that these emissions were insignificant relative to those from the Central Landfill, and we have not attempted to estimate their magnitude. 

 The methane emissions that are included in the total emissions estimates are reported in Appendix B: Table 5.  The emissions from the Central Landfill were adjusted upward to account for the 4% of municipal waste that is disposed of in other municipal landfills.  Since landfill gas is not collected in these landfills, this is a significant contribution to the total.

 For comparison to the measured amounts of landfill gas produced at the Central Landfill, we used the Workbook’s default method of calculation, which estimates the “waste in place”, based on a per-capita waste generation estimate.  “Waste in place” is defined as all waste that was landfilled in the thirty-year period before the inventory year, calculated as a function of population and per capita waste generation.  The workbook provides a factor to determine the waste in place based on the current population and the average annual national population growth for the 30 previous years.  Of course, the population for Rhode Island did not increase at the national rate for the 30 years prior to either inventory year.  Instead, we used the population data obtained every 10 years by the census bureau, and their Intercensal Estimates of the Resident Population reports.  Following the Workbook methodology, the total waste in place was divided between the waste that was put in small landfills, and in large landfills.  The workbook provides default values for the fraction of waste in large landfills verses small landfills based on the geographic region.  In the northeast region, 89% of the waste is assumed to be placed in large landfills and 11% is placed in small landfills.  (The rate of methane production is greater in small landfills then in large landfills.)  An emissions factor, the amount of methane emitted per ton of waste in place, was provided by the workbook.    The total value was then adjusted for oxidation that takes place in the soil over landfills. Details of these calculations, and the data used are located in Appendix B (Tables 1, 2, and 4).  This method estimates methane emissions approximately 30% smaller than the amount actually measured at the Central Landfill.  It is worth noting that the estimate produced by following the Workbook methodology falls well outside of the 20% error the Workbook claims.  The results of the Workbook method are reported here for comparison, but were not used in the total emissions calculations.

 A significant amount of carbon is sequestered through the landfilling of waste.  The Workbook considers only the amount of carbon that would not be expected to decompose in a 30-year period as sequestered, and estimates that this is 0.18 tons of carbon for each ton of waste landfilled.  A conservative estimate of the amount of waste landfilled in the inventory years assumes that all commercial waste generated in the state was disposed at the Central landfill (although we know that some unknown amount of demolition debris was disposed elsewhere).   

There was an overall increase in the amount of methane emitted from landfills from 1990 to 1996, due to an increase in the waste in place over this period.  These emissions were very substantially less than the amount sequestered – in 1996, sequestration was more than three times the methane emission rates.