Energy recovery is included in the example, although existing and proposed regulations do not require it. High efficiency in landfill gas recovery is assumed for the analysis.
Both the energy requirements and the air emissions for MSW collection and landfill operation depend most strongly on the efficiency of truck use. Overall energy requirements for land filling are low; therefore, when gas recovery is included, the strategy is a net energy producer of about 2 million Btu per ton of MSW over 20 years.
The data base also allows landfilling to be integrated with any other selection of MSW strategies. Because all integrated strategies include landfilling, Exhibit II and the computerized data base provide the calculations that have already been performed for all the integrated strategies listed in Table 1.1 in "Introduction."
Emissions
Collection and Processing Equipment
No data (on a per ton of MSW transported) were found for the actual emissions generated by collection programs. Accordingly, information from a local community was used, and emissions were estimated on the basis of the fuel used.
No data were found on actual emissions during the construction and operation of landfills, including emissions from heavy equipment used for landfill compaction and operations and releases from MSW as it is compacted. Nor were data found on particulates and dust that may result from placing daily cover on landfills.
Table 6.5
ENERGY AND EMISSIONS FOR STRATEGY 1: LANDFILL WITH GAS RECOVERY
Total Collection Processes Disposal
Landfill space (assuming a depth
of 50 ft), 10(-5) acres 2.00 2.00
Solid waste (lb) 2000 2000
Energy Required (million Btu) 0.081 0.079 0.002
Energy Produced (million Btu) 2.20 0.00 2.20
Net Energy (million Btu) 2.12 -0.079 2.20
Air Emissions
Particulates (lb) 0.02 0.02
Carbon Monoxide (lb) 0.79 0.79
Hydrocarbons (lb) 0.08 0.08
Nitrogen oxides (lb) 0.32 0.32 NA
Carbon dioxide (lb) 225 225
Carbon dioxide-combustion (lb) 212 212
Water (lb) 188 188
Methane (lb) 14.34 14.34
NMOC (lb) 0.75 0.75
Dioxin/furan {10(-6)lb}(a)
SO(2) {10(-3)lb}
HCI {10(-3)lb}
Antimony {10(-6)lb}
Arsenic {10(-6)lb}
Cadmium {10(-6)lb}
Chromium {10(-6)lb}
Lead {11(-6)lb}
Mercury {10(-6)lb}
Nickel {10(-6)lb}
Zinc {10(-6)lb}
Total Heavy Metals {10(-6)lb} NA NA
Effluent
Leachate (gal) 80 80
Leachate (lb) 667 667
Chloride (lb) 1.13 1.13
Sodium (lb) 0.73 0.73
Potassium (b) 0.60 0.60
COD (lb) 0.16 0.16
Arsenic {10(-3)lb} 86 86
Cadmium {10(-3)lb} 3 3
Chromium {10(-3)lb} 163 163
Copper {10(-3)lb} 43 43
Nickel {10(-3)lb} 108 108
Lead {10(-3)lb} 48 48
Mercury {10(-3)lb} 6 6
Zinc {10(-3)lb} NA NA
Total Heavy Metals {10(-3)lb} 457 457
AOX (lb) 1.08 1.08
(a) This is total dioxin/furan as specified by EPA in CFR, 1991a
Click here for table in WK1 format.
Landfill Air Emissions
No data were found on actual emissions from spraying leachate at the working face of the landfill, or from aeration in leachate treatment or sewage treatment plants.
No data were found on changes in the composition of trace organic components in landfill gas over long periods.
Several sources stated or implied that dioxins have been measured in the emissions from combusting landfill gas. However, none of these sources provided quantitative data on those emissions.
Landfill Water Emissions
No data were found to document changes in composition of leachate over 20 years or longer for use in estimating whether metal and organic concentrations decline or remain roughly steady. Comparisons of leachate during a landfill's acidic stage and during its methane-generating stage were found, but none of these data covered long periods. EPA data from the early 1970s analyzed leachate from the landfill types that were common at that time (Bogner, 1992). Those data might be useful for long-term comparisons.
Models exist to help predict the amount of leachate that would penetrate the bottom liner of a landfill, but few data were found. No data were found on the amounts and composition of leachate from shredfill or balefill operations.
Long-term studies of leachate composition may eventually reflect the changing composition of the waste stream. The recent significant reductions of mercury in alkaline cells and the popularity of zinc-air cells as replacements for mercury batteries in hearing aids are examples of technological changes that will influence waste stream composition. Reduction of metals in inks is another example (Usherson, 1992). New laws in Europe and California also require elimination of lead from the 2 billion wine bottle caps produced each year that are made of lead (Andre and Karpel, 1991).
Ash Monofill Water Emissions
The amounts of metals and organic materials entering the ground below ash monofill liners have not been widely studied. Therefore, those estimates are based on fewer data than any other estimates presented in this section. The assumptions on which the estimates are based are discussed below, along with indication of gaps in the data.
Data are available on the composition of leachate from a closed monofill over 4 years, but not for the 20-year time frame of interest here. The data show that highly soluble materials- potassium, sodium, and chloride-appear in roughly the same concentrations each year (Roffman, 1992). By extrapolation, it is assumed that the leaching of those ions is at steady state, and that the leachate does not become saturated with them. However, all the heavy metals that were detected during 4 years of monitoring decreased sharply during the study period; therefore, it was assumed for the analysis reported here that the low levels noted in year 4 will be the maximum concentration for the next 16 years. That assumption is believed to be conservative.
It has been assumed that the depth of the monofill is the same as that for a regular MSW landfill. Very few design data on existing ash monofills were found.
The difference in volume between a raw MSW landfill and an ash monofill is known. To estimate the surface area on which rain will fall, it is necessary to assume a depth for the ash in the monofill. It has been assumed that the depths for both types of landfills are equal.
Data on MSW landfills provide estimates of the amount of rainfall on the closed, capped landfill surface that enters the landfill. The fraction of rainfall that is collected as leachate on the liner and the fraction that leaks into the ground below have also been estimated. Similar data for ash monofills were not found, and thus the proportions reported for raw MSW landfills were used for ash monofills as well. However, if the ash in the monofill hardens, as is frequently reported, it would be unreasonable to assume that rates of infiltration or of percolation to the bottom of the monofill were the same as those for MSW landfills. Because data on the amount of leachate that escapes MSW landfills were applied to ash monofills in this analysis, the estimates of leachate escaping to the ground in this report are likely to be overstated, and the estimates of the amounts of metal that are released in leachate may be too large as well.
Energy
Few data were found on the energy requirements for collecting and landfilling MSW; those data that do exist are based on truck capacity rather than on the actual tonnage collected. Nor were actual energy data (on a per ton of MSW basis) found for ongoing landfill construction, filling, compacting, and covering.
NOTES:
(1) A landfill 120 feet deep that occupies only 0.15% of U.S. continental landspace could accommodate the MSW created over the next 1,000 years at current generation rates (Wiseman, 1991). By way of comparison, the United States loses 1.8 billion tons of cropland topsoil each year from erosion into the Gulf of Mexico and into the oceans (Council on Environmental Quality, 1990).
(2)Thornloe notes that an optimal separation for biodegradation is not a normal goal of curbside collection or MSW separation programs (Thornloe, 1991).
(3) Using a 20-year period underestimates gas production and may underestimate landfill gas recovery, but available published data are insufficient to permit extrapolation beyond that time period.
(4) That estimate is based on a value of 909 Btu per cubic foot (LHV) for methane (Kumar, 1987), or 500 Btu per standard cubic foot of landfill gas, and 1-1.8 standard cubic feet of methane per dry pound of MSW (Augenstein and Pacey, 1991). A typical heat content of MSW is 4,500 Btu per pound (EPRI, 1989).
(5)That is, landfill gas release is uncontrolled by comparison with energy and emissions releases in a combustor, which vary in a predictable fashion with the chosen operating conditions.
(6)That estimate is based on the following assumptions: (1) methane generation of 1-1.8 standard cubic feet per dry pound of MSW, (2) 30% moisture, and (3) 85% capture.
7. MATERIALS COLLECTION, SEPARATION AND RECYCLING
-OR-