RECOVERING AIRSPACE FROM MUNICIPAL SOLID WASTE LANDFILL CELLS:

The Cedar Hills Regional Landfill Example

Presented at the:

SWANA’s 21st Annual Pacific Northwest Regional & Canadian Symposium April 5 - 7, 2006, Richmond, BC, Canada

By: Victor O. Okereke, Ph.D., P.E., DEE (1) (Board Certified Environmental Engineer) Carla Talich, P.E. (2)

Michael McEwen (3)

(1)(3) King County Solid Waste Division, 201 South Jackson Street, Suite 702 Seattle, WA, 98104, Telephone (206) 296 – 4411 (2) URS Corporation, 1501 4th Avenue, Suite 1400, Seattle, WA 98101-1616

ABSTRACT The development of new landfills has been increasingly difficult since the late 1980's due primarily to stringent environmental regulations, extensive permitting requirements and siting constraints. Siting difficulties are due mostly to the unwillingness of most urban and suburban communities to accept new landfills in preferred locations. In order to keep disposal costs stable despite these difficulties, Landfill Managers, Owners, Operators and Engineers developed alternative approaches that enable the life of existing facilities to be prolonged. This paper describes landfill operations management practices, settlement monitoring, settlement modeling, recent approaches evaluated for the optimization of landfill airspace and the impacts of landfill life optimization on refuse disposal economics in general. 1. INTRODUCTION The practice of landfill airspace conservation has been evolving over the past thirty years. Implementation of stringent landfill regulations in the early 1980's precipitated the closure of several small landfills and a shortage of landfill capacity. This situation led to the development of numerous regional, larger and privately - owned landfills in the 1990s, and stability in landfill capacity. Beginning in the late 1990's, discussions about municipal landfill capacity in the solid waste management sector has evolved into a debate about whether there is indeed a continuing shortage or excess of landfill capacity. Tierney (1996) argued that landfill space shortage was a myth perpetuated by the anti-recycling sector. In a response to Teirney, Ruston (1996) outlined the economic nature of landfill airspace,

1

0

10

20

30

40

50

60

70

80

1980 1985 1990 1995 2000 200 5

Y ear

Ave

rage

Lan

dfill

Tip

Fee

($/T

on)

N o rthea s t M id -A tlantic S o uth M id w e st S o uth C e ntra lW es t C entra l W est U S A ve rag e

Figure 1. Average Disposal Price in the USA Source: Repa, NSWMA Research Bulletin 05 - 3 (2005)

$ 1 4 4

$ 9 6

$ 8 0

$ 4 8

$ 3 6$ 4 2

$ 0

$ 2 0

$ 4 0

$ 6 0

$ 8 0

$ 1 0 0

$ 1 2 0

$ 1 4 0

$ 1 6 0

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0

T o n s p e r D a y (T p d )

$/To

n

Figure 2. Economies of Scale at Municipal Landfills Adapted from: US Environmental Protection Agency (1997)

describing it as a commodity that is priced according to supply and demand. He noted that the growth of recycling is predominant in highly populated regions of the USA where landfills are expensive relative to the national average. As shown in Figure 1, landfill disposal costs remain high in the densely populated Northeast and Mid-Atlantic and lowest in the West Central and

2

South Central regions, the lowest populated areas. Ruston (1996) and Repa (2002) indicate that disposal costs are moderate on the West Coast where the resale cost and revenues for recyclable materials remain high. The average national tipping fees have increased only by 7% over the past ten (10) years. Prior to 1998 tipping fees increased at about 7% a year. The change in the national average tipping fee from 1985 to 1998 during the landfill "boom years" was $23.61 or about 300%. These costs represent the "spot market " price for disposal. Other tipping fees exist (e.g., waste accepted under long-term contract) and may be higher or lower than the spot market price (Repa, opt. cit.). According to a survey completed by NSWMA (2004), there is direct correlation between available landfill capacity by region and population, and tipping fees. On a national level, there is twenty (20) years of disposal capacity. However, regional or state-by-state, Alaska, Connecticut, Delaware, North Carolina, New Hampshire, and Rhode Island have less than five (5) years of available disposal capacity. The western and pacific states have greater capacity while the northeastern states have the least capacity. Based on the foregoing information, it appears that there is no shortage of landfill disposal capacity on a national level, while there are shortages in certain States and excess capacity in some. Landfill airspace conservation is an important approach for keeping tipping fees stable in those States with rapidly declining landfill capacity. The ability of landfill operators to optimize airspace in existing landfills through a variety of operational and design innovations that include increased compaction and the use of alternative daily cover materials has been credited for keeping tipping fees stable (Bailey 2005). Generally, larger regional landfills, most of which are owned and operated by the private sector, benefit from the economies of scale (Figure 2) more than smaller-to-moderately sized landfills that are mostly publicly held. However, these regional landfills require higher capital investment, larger land spaces, favorable environmental conditions and sustainable political will to enable their construction. Therefore, as existing landfills near capacity, and since local governments are not prepared or willing to invest the required tax dollars necessary to build and operate new regional landfills that will be competitive with the privately owned facilities, alternative solid waste management plans must be made. The most common solid waste management alternatives utilized by local governments include; waste export to one or more regional landfills, modification of operating practices to extend the life of the existing landfills, full or partial privatization of the local solid waste disposal sector and public-private sector partnerships. This paper describes approaches used in conserving and recovering landfill airspace and the associated costs and benefits. Discussed in some detail are operations management practices, settlement monitoring and modeling, and methods used to evaluate the impact of landfill airspace recovery at the Cedar Hills Regional Landfill and the impact of landfill life optimization on refuse disposal economics in general. 2. BACKGROUND 2.2 Overview Detailed description of conventional landfill airspace conservation and recovery methods has been published in several solid waste management publications. According to descriptions

3

provided by Bolton (2000), Bader (2003) and SWANA (2005), landfill airspace and recovery methods can be divided into two primary approaches; passive approaches, include the use of less soil for daily cover or alternative daily cover materials, recovery and reuse of daily and secondary cover, recovery and reuse of materials used to build landfill tipping pads, increasing moisture content, utilization of airspace gained from self weight-induced landfill settlement or from settlement-induced by surcharging with soil. Active approaches include, increased compaction efforts, and optimization of daily cell geometry to enable less soil usage. Other active methods include, landfill reclamation (USEPA, 1997), waste processing (Browning and Wayne, 1990), and the construction of a new refuse cell below groundwater elevation (O'Connell, 2002). This latter method increased landfill capacity by 60% at the Southeastern Public Service Authority landfill in Chesapeake, Virginia and costs only an additional $ 300,000 over conventionally designed landfills. The Regional Municipality of Niagara (2003) indicates that shredding and baling would conserve 35% and 50% more airspace respectively than conventional landfill operating practices. According to Wayne (op. Cit.), waste processing by milling or baling produce airspace savings of 21% and 30% respectively, over non-processed wastes. Duffy (2003) noted that increased compaction effort can reduce waste volume by 50%, especially when enabled by Global Positioning System (GPS)-guided compaction equipment. . Additional supportive evidence about the economic benefit of landfill airspace optimization is documented by Dayton (2004) for the Hilo Landfill in east Hawaii, where airspace conservation, resulted directly in avoided costs totaling $1.46 million and $12 to $14 million respectively in alternative disposal facility planning and early closure costs. 2.2. Landfill Settlement and Airspace Recovery Estimates of future settlement are necessary for effective planning of the utilization of recoverable landfill airspace from affected landfill areas. The mechanics of settlement and methods for estimating landfill settlement are described by Sowers (1973), Yen and Scanlon (1975), Rao et al (1977), Edil et al (1990), Manassero (1996), Yee (1999), Bowders et al (2000), and, Leonard and Floom (2003). Generally, there are two major types of landfill settlement, primary (due to self-weight) and secondary compressions or settlements. Detailed discussion of the associated mechanisms is outside the scope of this paper. Estimating methods are difficult and unreliable if not supported by long-term settlement monitoring to verify calculated results. In general, two approaches are used to estimate landfill settlement. One approach is based on curve fitting analysis with logarithmic, power and hyperbolic functions using monitored data, while the second method utilizes published data to estimate settlement rates for equivalent landfills. According to Sowers (1973), total landfill settlement varies from 5% to 30% of the total height. ). Future and final settlement may take up to 30 years while 90% of settlement occurs within the first five years of landfill completion Rao et al (1977). The initial primary settlement should be completed within one to two months after refuse placement and may be up to 10% to 30% of the total refuse depth while the secondary or long-term settlement will not exceed 75% of total settlement (Yee, 1999). According to Chapman and Yates (2002),

4

conventional MSWLFs settle about 2% to 5% within two years and up to 15% after ten years. Leonard and Floom (2003), indicate that 70 to 75% of landfill settlement is due to biodegradation. 2.3. Landfill Airspace Economics Several solid waste management economists and analysts have proposed alternative approaches for the economic valuation of landfill airspace. Rao (1977) used a life cycle cost approach while Harkins and Cohen (1996) provided examples of incremental cost basis for landfill cost analysis. Mackey (2002) used a Market-Based Tip Fee (MBTF) approach to evaluate economic benefit of additional airspace created from the optimization of leachate collection systems. In the “Mackey method”, all costs incurred from the creation of additional airspace must be recovered from the additional disposal capacity. If the Owners Tip Fee is greater than the MBTF, then the Airspace is over-valued. However, if the Tip Fee is less than the MBTF, the airspace is under-valued and could be increased. Bolton (2004) used a Total Cost approach to perform landfill airspace economic analysis. In this latter approach, landfill airspace is valuated as a function of capital and operating costs versus value of airspace added. A combination of these methods is used in this paper. 2.4. Cedar Hills Regional Landflll – Case Study The Cedar Hills Regional Landfill (CHRLF or “Facility”) is one of the largest landfills in the western USA and is operated by the King County Solid Waste Division (SWD). Mixed municipal solid waste (MMSW) has been accepted at variable rates since the early 1960's and today the facility receives 2,500 to 2,800 tons of MMSW per day. The facility has an estimated airspace capacity of about sixty six (66) million cubic yards and current aerial survey and engineering evaluation indicates that the landfill will reach capacity in about ten (10) years. The SWD has selected Waste Export to a regional landfill or landfills as the long-term waste disposal strategy after CHRLF reaches capacity. To enable the completion of a Waste Export plan, the SWD has evaluated several alternative approaches intended to extend the life of the facility since, as documented in (KCSWD, 2006), disposal at the facility has been determined to be the lowest cost disposal option for ratepayers in the CHRLF service area. In addition, and according to this 2006 report, extension of the facility’s life allows more time to make decisions about ownership and operation of an intermodal facility, contracting for disposal services, and improvements to the solid waste transport system in preparation for waste export. The following sections describe landfill airspace conservation and recovery practices used at the Cedar Hills Regional Landfill. 3. LANDFILL AIRSPACE CONSERVATION METHODS A combination of passive and active methods is used to conserve and recover airspace at CHRLF.

5

3.1. Materials Recovery and Reuse Program Starting in 2003 and continuing daily since 2005, daily cover and crushed rock or other materials have been stripped from the landfill surface and tipping floor as illustrated in Figure 3. Intermediate cover is also stripped before placement of final cover. The cover materials are reused in the subsequent daily cell while reusable tipping floor materials are recovered to the extent possible after processing in a screening plant. A rented screening plant was used intermittently at the inception of the program. The rate and consistency of the program has been steady since 2005 with the purchase of a portable screening plant.

Figure 3. Materials Recovery for a Typical Daily Cell 3.2. Alternative Daily Cover (ADC) Use Program Since November 2005, ADC materials have been used to augment soil use. The ADC materials are reusable Fabrene® high-density polyethylene tarps. Currently, a single panel measuring 40- feet wide and 100-feet long is used daily. Daily cells at the CHRLF average about 23,000 square feet in surface area. The Tarps are deployed and retrieved daily with a Tarpomatic® machine. The beneficial impact of the Materials Recovery and Reuse, and the ADC programs is demonstrated with the estimated landfill operating factors shown in Table 1. Between 2000 and 2005 average daily soil use was 635 cubic yards compared to 506 and 384 cubic yards respectively during selected periods of the materials recovery and reuse, and a combination of materials recovery and ADC programs implementation. This translates into approximately 20.3% and 39.5% reduction in soil usage respectively, or in terms of landfill capacity, average of 47,000 and 92,000 cubic yards conserved annually.

6

Table 1. Landfill Airspace Utilization Factors

Year Soil Usage

(Cy/day) Soil/Tonnage Ratio (cy/ton)

Soil/Airspace Ratio (cy/cy)

2000 770 0.29 0.18 2001 610 0.24 0.16 2002 609 0.24 0.16 2003 608 0.24 0.16

2003 (1) 506 0.20 0.13 2004 607 0.24 0.16 2005 606 0.24 0.16

2006 (2) 384 0.14 0.08 Note :

1. Weighted average values over 77 days of dedicated rock and material recycling 2. Average values over 30 days of material recovery and ADCM use

3.3. Refuse Compaction Sensitivity analysis completed by this author to determine the relative impact of daily soil cover reduction and higher compaction density on landfill life indicate that average percent landfill life gained per unit increase in density (or reduction in soil use) ranged from 1 % to 11% for soil use reduction and 8% to 60% for increased density. These results demonstrate the superior impact of higher refuse compaction density on landfill life. Historically, airspace utilization (AU) density for the CHRLF has been estimated from topographic data acquired from annual aerial photogrammetric and ground topographic surveys. Ground surveys are completed quarterly each year and used to verify the photogrammetric information. The AU density (annually) has varied from 1,100 to 1,340 Ib/Cy, but in the past five years has been consistently estimated to be about 1,340 Ib/Cy. Future landfill capacity projections are based on average density values of 1,300 Ib/Cy and 1,400 Ib/Cy. To enable the attainment of the higher value, the King County Solid Waste Division (KCSWD) purchased and deployed a 125,000 Ib BomaG® compactor in November 2005, which is now utilized in combination with existing 80,000 Ib to 90,000 Ib compactors. In addition, a monthly refuse compaction-monitoring program was established in 2005 to evaluate the progress of this effort. The impact of the new compactor on AU density will be evaluated after enough time has elapsed since its deployment.

7

4. RECOVERED LANDFILL AIRSPACE AND BENEFIT ANALYSIS Two alternative approaches were evaluated for recovering airspace within the CHRLF. These methods include recovering airspace in refuse cells that have been permanently closed, and those that have been temporarily closed. Permanently closed cells have been evaluated for observed differential settlement in order to determine the available airspace that may be recoverable. This information is also valuable in estimating projected settlement in areas that have yet to be final-closed. 4.1. Airspace Recovery in Closed Landfill Cells Recoverable airspace in closed landfill areas is estimated by comparing the landfill surface at closure to the current surface. Settlement within refuse areas has been observed over time by the utilization of settlement monuments located throughout the landfill. The locations of these settlement monuments are presented in Figure 4. Four closed refuse cells, the Main Hill (closed in 1986), Central Pit (closed in 1988), Areas 2/3 (closed in 1991), Area 4 (closed in 1999), as shown in Figure 4 were evaluated to determine the potential for airspace recovery. Differential settlement is estimated by developing and comparing cross sections through selected locations. Three data sources were used to generate a cross section illustrating the observed differential settlement within the landfill cell. The related cross sectional area is shown in Figure 5. The historic bottom grades of the Main Hill, Central Pit, and Areas 2/3 and 4 are based on pre-1960’s topography, referenced from the USGS 1949 Quadrangle for Maple Valley. The upper-most line shows the final grades extracted from the landfill refuse filling plans. Estimated settled topography is based on July 25, 2005 survey data. Figure 5 shows that existing settlement in the associated closed landfill cells have reached 15 feet in several locations. The recovery of this airspace may be implemented by developing about one or two new refuse lifts on top of the settled lifts. Available and added capacity is estimated to range between 2.9 to 3.3 million tons or about two to three years of added landfill life. The recovered airspace is about 23% of the original airspace capacity of approximately 13 million tons. This option will require the removal or destruction in-place, of the existing flexible membrane in the cover system, while the recovered soil will be reused as daily cover. Engineering, permitting, construction and operating costs are the primary cost elements considered for the related feasibility analysis. Observed settlement in closed areas varies, but averages 10 to 15 feet on the top sections of the landfill cells. Sideslopes were also observed to settle an average of 10 to 15 feet vertically, with a distance perpendicular to the slope of about 7 to 10 feet. Landfill sideslopes with refuse placed at 3 horizontal to 1 vertical (3:1), have been observed to settle to an average grade of 4:1. Total measured settlement over the past twelve years is the range of 4% to 7% of the total cell height.

8

Figure 4. Settlement Monitoring Locations

9

4.2. Airspace Recovery in Unclosed and Future Undeveloped Landfill Cells The second approach to airspace recovery is to recapture airspace created by differential settlement within refuse areas that have not received final cover. This approach includes filling the refuse area nearly to the top of the permitted elevation, or to the top of the second to last lift. Once refuse and cover soil have been placed to this predetermined elevation, filling stops and the area is temporarily closed using a soil cover system on the top deck. The sideslopes of the refuse area are closed using a geomembrane cover system in order to provide additional environmental controls of leachate, stormwater, and landfill gas. After a specified period, the top of the refuse area is filled to its final grades to utilize the portion the original airspace recovered through differential settlement. Planning for the recovery and utilization of existing airspace created from past differential settlement involves a direct analysis of costs versus benefits. The recovery of potential airspace that would be created by future differential settlement, an unknown quantity requires the prediction of future landfill settlement and the associated refuse capacity. The determination of future landfill settlement was evaluated by calibrating a previously determined power function (Okereke et al, 2004) and a variation of the equation proposed by Rao et al (1977), with settlement data collected over the past twelve years from closed landfill cells. One result of the calibration effort is shown in Figure 6. Future settlement values were estimated with Model A based on superior performance (as measured by the Mean Average Error) during calibration.

Figure 5. Observed Settlement in Closed Landfill Cells

10

The settlement modeling equations have the following forms: Model A (Rao, 1977): Total Settlement, ΔS = CpHwLog (Po+Dp/Po) + CsHwLog (T2/T1) Where: ΔS = Total landfill settlement, feet (ft); Cp = Primary compression ratio (typically, 0.1 to 0.4 for municipal solid waste landfills); Hw = Thickness of Waste, ft; Po = Original stress, pounds per square feet (Psf); Dp = Increase in stress, Psf; Cs = Secondary compression ratio (0.02 to 0.07 for MSWLFs); T2 = Final time over which settlement was evaluated, days; T1 = Start of secondary settlement, days. Model B (Okereke et al, 2004): Total Settlement, ΔS = AWt(T)b

Where: A,b = Calibration parameters; Wt = Initial waste thickness (ft) ; T = cumulative time since waste final placement in days.

0

1

2

3

4

5

6

7

8

9

0 1000 2000 3000 4000 5000C um ulative D ays

Cum

ulat

ive

Settl

emen

t, ft

Cum ulative S ettlm entP redic ted M odel AP redic ted M odel B

Figure 6. Calibration of Settlement Data for a Single Location Area 5 first began receiving refuse in 1999. It was scheduled for final closure in 2006; however, it is currently planned to receive an interim final cover with final closure to follow in approximately 8 to 10 years. Side slope areas are scheduled to receive interim final cover, which includes flexible membrane liner while the top deck will receive soil cover. In 6 to 9 years additional waste will be placed on the top deck to recover the airspace induced by settlement. Area 6 began receiving refuse in 2005, while Area 7 has not yet been developed. Long-term planning indicates that Area 7 will be completed in 8 to 9 years. Once this is complete, the top

11

of Area 5 will be re-opened, filled with refuse and final-closed with a geomembrane cover system. Areas 6 and 7 will follow in a similar fashion. Based on the estimated life of cells 5,6 and 7, the total predicted settlements are 11,9 and 2.5 feet respectively. An average of 8 feet is used to develop the future landfill settled surface. This forecasted surface is presented in Figure 7. Grades for the bottom of the refuse area are based on the constructed or expected bottom-lining system. Projected landfill airspace and capacity added for this alternative ranges from 1.1 to 1.7 million tons or one to one and one half years of added life. This translates to a recovery of about 10% of the planned capacity of about 14 million tons for these cells.

Figure 7. Predicted Settlement Profile in Current and Future Cells

4.2. Economic Analysis The potential benefit of landfill airspace recovery for the two approaches evaluated in this paper was estimated by comparing the present values (PV) of the avoided life cycle costs (PVLCC),

12

and the incremental avoided cost for variable waste export scenarios. The life cycle costs were computed with the Net PV's of the estimated capital, operating, ancillary costs and the offsetting revenue from the additional airspace over selected periods. A 5% discount and 3% inflation rates respectively were used. Avoided cost (or benefit) is the difference between the total cost for waste export and the net cost or revenue of utilizing the recovered airspace. The results are presented in Table 2. The pertinent equations are as follows: PV = F/ (1+i) n N PVLCC = ∑ Costn * 1+r/(1+i)n-2006

2006 Where: F = future costs, n = year in which cost occurs, i = discount rate, Costn = Cost in year n r = inflation rate , PVLCC = Present value of avoided life cycle costs.

Table 2. Development Alternatives Avoided Costs (Savings)

Scenario Description Total Estimated

Incremental Capital Costs

Lifecycle Net Present Value

of Avoided Costs Per Ton

Lifecycle Net Present Value of Avoided

Costs Recovery of airspace in open and future cells (5, 6, and 7)

Operate through 2016

$8,000,000 $0.48 $14,000,000

Recovery of existing airspace in closed cells (2/3, 4/ and Central Pit)

Operate through 2018

$60,000,000 $0.55 $16,000,000

Recovery of future airspace in present cells 2, 3, 4, and Central Pit and future cells 5, 6, and 7

Operate through 2019

$68,000,000 $1.03 $30,000,000

Note: Lifecycle analysis is through 2028, the duration of the Interlocal Agreements. Revised from: KCSWD (2006) The results in Table 2 represent the benefit attained if full export of waste is delayed until full utilization of the recovered landfill airspace is completed. Figure 8 shows the results of incrementally exporting refuse (as opposed to full export) to another remotely located regional landfill or landfills and the resultant extension of the CHRLF life. The results in Table 2 indicate that the capital cost of airspace recovery from the closed landfill cells is significantly higher than for the open or future cells. Since some of the closed landfill cells were developed under different regulatory guidelines, the degree of disparity in capital costs may be unique to the facility, but not typical. The higher costs are significantly influenced by costs associated with potential bottom liner system augmentation and retrofitting existing environmental control

13

facilities. The current average cost of disposal at the facility is $ 32.86/ton, which is below the 2005 average values of $34.29/ton (national) and $ 37.74/ton (western states).

Costs of DisposalPartial Early Export - 200,000 tons at 2010

{cash flows}

Increase over baseline = $21 - 25m [~$.71/ton]

20152006 2010

{32.86}

{46.25}

$

2016

Disposal operations savingsLRF cost reduction per year Higher export costs

2028

Figure 8. Effect of Partial Early Exports Source: KCSWD (2006)

The impact of partial early waste export on landfill life was also evaluated. This analysis (KCSWD, 2006) assumed that 200,000 tons of waste (20% of the current waste stream) would be exported beginning in 2010 while CHRLF continued to operate. Figure 8 shows that partial early waste export will increase the disposal cost by 29% to $ 46.25/ton while the life of the facility would be extended about one year (2015 to 2016). The larger bars on the figure represent disposal costs while the bar between 2015 to 2016 represents the avoided costs of full waste export or benefits of partial early waste export. The KCSWD report indicates that this would have two important impacts. First, the per ton contribution to the landfill reserve account will be lowered beginning in 2006. Second, and more importantly, delaying export of the remaining 80% of the waste export stream would mean lower disposal costs to ratepayers for one additional year. 5. SUMMARY AND CONCLUSIONS The shortage of landfill airspace that was common in the 1990's has been mitigated by landfill airspace optimization in existing landfills. The conservation of airspace in existing landfills appear to be the primary reason for the stability of disposal costs in the USA over the past ten years and not necessarily owed to overbuilt capacity in regional landfills. The cost of refuse disposal is dependent on a variety of factors including the economics of landfill airspace supply and demand, demography and regional location of the disposal facility. Some highly populated regions in the USA still have shortage of landfill airspace capacity while some have excess capacity.

14

Landfill airspace conservation practices of, daily cover and materials recovery, and alternative daily cover use resulted in a reduction in soils use in the range of 20% to 40% at the Cedar Hills Regional Landfill. The utilization of landfill airspace recovered from differential settlement in closed and open landfill cells would add significant capacity to existing landfills. The analysis completed for the current study indicates that 23% and 10% respectively of the existing airspace in the closed and open/future landfill cells are recoverable from passive differential settlement mechanisms. Measured landfill settlement in closed landfill areas at the CHRLF is the range of 4% to 7% of the total landfill cell over the past twelve years. Based on existing literature values, this represents about 50% of total potential settlement expected at a MSLWF and indicates that additional settlement and recoverable airspace is possible at this facility in the future. Settlement monitoring provides useful data for the projection of future recoverable airspace. When adequately calibrated, current settlement models are excellent tools for estimating landfill settlement rates. Although results summarized in this paper show that capital costs associated with the recovery of airspace from closed landfill cells are significantly higher than from open and future cells, this observation is not expected to be common if the affected past and future cells are developed under similar regulatory guidelines. In addition, this higher differential in capital costs is mitigated by the higher recoverable airspace in the closed cells as measured by the Present Value of the life cycle costs. Measured in total dollars, the full utilization of the existing and projected recoverable airspace at the CHRLF will result in avoided early waste export costs in the range of $20 to $25 million dollars. Finally, the results presented are consistent with those reported for other landfills. The utilization of additional landfill airspace conserved through modified operational methods or recovered from settled landfill cells, results in millions of dollars in avoided costs for building new disposal facilities and /or planning for, alternative waste disposal facilities or waste export to a remote regional landfill or landfills. 6. ACKOWLEDGEMENTS We thank the King County Solid Waste Division, and in particular Theresa Jennings and Kevin Kiernan for supporting the effort involved in the development of this paper. Special thanks to Emily Robbins for final editing and to, Tom Karston, Jamey Barker, Shirley Jurgensen, Theresa Koppang and Dean Voelker for their helpful comments.

15

7. REFERENCES 3Di West Geoterra Mapping Group (January 2000-July 2005). Aerial Topography. Bader, C.D. (Sept., 2003). Conserving Landfill Airspace. MSW Management. Bailey, J. (2005). Rumors of a Shortage of Dump Space were greatly exaggerated. August 12. New York Times. Bolton, N. (2000). Landfill Airspace and Waste Density: The Big Picture. MSW Management Bolton, N. (2004). Alternative Daily Cover. Finding the Bottom Line. MSW Management, May/June, pp.52-60 Bowders, J., Bouazza, M, Loehr, E., and Russell, M, (2000). Settlement of Municipal Landfills. In: 4th Kansai Int'l Geotechnical Forum. IIAS, Kyoto, Japan. Browning, J.S., and Wayne, H.M.(March, 1990). Landfill Cap Stability. Journal of Resource Mgt. and Technology, Vol. 18. No.2, pp. 21 -31. CH2M HILL (April, 2000). Cedar Hills Regional Landfill Area 5 Phase II Development Record Drawings. King County Department of Natural Resources, Solid Waste Division. Chapman, C., and Yates, A. (Sept./Oct. 2002). Bioreactor Landfills: An Idea Whose Time Has Come. MSW Management. Duffy, D.P. (2003). Effective Landfill Airspace Management. MSW Management. Pp. 30-42 Edil, T.B., Ranguettle, V.J. and Wuellner, W.W. (1990). Settlement of Municipal Refuse. Geotechnics of Waste Fills -Theory and Practice. ASTM STP 1070:223 - 239. Harking, D.A., and Cohen, A.S. (1996). The Use of Incremental Cost Analysis in the New York Solid Waste Management System. SWANA's 34th WASTECON Proceedings. HDR Engineering, Inc. (May, 2002). Cedar Hills Regional Landfill Area 6 Phase I Development. May 2002. King County Department of Natural Resources, Solid Waste Division. HDR Engineering, Inc. (September, 2003). Cedar Hills Regional Landfill Area 6 Phase II Development. King County Department of Natural Resources, Solid Waste Division. KCSWD (2006). Preliminary Transfer and Waste Export Facility Recommendations and Estimated Systems Costs, Rate Impacts and Financial Policy Assumptions. 4th Milestone Report. Leonard, M.L. and Floom, K.J. (2003). Estimating Method and Use of Landfill Settlement. In: Proceedings. -SWANA's Wastecon. Silver Springs, MD. Pp. 163-177.

16

Mackey, R.E. (2002). Leachate Collection System Layout based on the Value of Landfill Airspace and Geosynthetic Testing. Proc. of 7th Annual SWANA Landfill Symposium. Manassero, M., Van Impe, W.F.and Bouazza, A.(1996). Waste Disposal and Containment. In. Proceedings of Second International Congress in Environmental Geotechnics, Osaka, Vol. 3. Pp. 193 - 242. National Solid Waste Management Association, NSWMA (2004). Municipal Solid Wastes. 2004 Research Bulletin O'Connell, K.A. (2002). Digging for Gold. Waste Age. Okereke, V.O., Horton, B. and Shuri, F.(2004). Design Challenges and Approaches for the Horizontal and Vertical Expansion of the Cedar Hills Regional Landfill. In. Proceedings of SWANA's 7th Annual Landfill- Symposium. Louisville, Kentucky. Rao, S.K., Moulton, L.K. and Seals, R.K. (1977). Settlement of Refuse Landfills. Proc. ASCE Conf. on Geotechnical Practice for Disposal of Solid Waste Materials. Regional Municipality of Niagara. (2003). Long Term Disposal Planning Study, Task D - Technical Brief. Repa, E.W. (2005). National Solid Waste Management Association (NSWMA) 2005 Tip Fee Survey. NSWMA Research Bulletin 05-3. Ruston, J. F. (1996). Commentary on Anti-Recycling Myths. Environmental Defense Fund. Washington D.C. Schultz, L.I., and Weber, S.F. (2003). Guide to Computing and Reporting the Life Cycle Cost of Environmental Management Projects. National Inst. of Standard and Technology (NIST). Segal, G. F. and Moore, A.T. (2000). Privatizing Landfills: Market Solutions for Solid -Waste Disposal. Policy Study No. 267. The Reason Foundation Sowers, G.F. (1973). Landfill Stabilization for Structural Purposes. The 8th Int. Conference on Soil Mechanics and Foundation Engineering, Moscow, USSR, 207 -210. SWANA. (Sept. 2005). Landfill Airspace Utilization: Measurement and Management. SWANA Research Foundation. Final Draft. Tierney, J. (1996). Recycling is Garbage. New York Times. URS Corporation. Cedar Hills Regional Landfill Plan of Operations (Check Draft). January 2006. King County Department of Natural Resources, Solid Waste Division.

17

USEPA. (July 1997). Landfill Reclamation. Solid Waste and Emergency Response, 5306W. EPA - 530-F-97-001. USGS Quadrangle (1949). Maple Valley, Washington. Waste Management System. In: SWANA's 34th WASTECON Proceedings. Yee, K. (1999). Upgrading of Existing Landfills by Dynamic Consolidation. A Geotechnical Aspect. Master Builders Journal. Yen, B.C. and Scanlon, B.(1975). Sanitary Landfill Settlement Rates. ASCE. J. of Geotech. Engineering, 101(5). 475 - 487.

18