SECOND U.S. CONFERENCE ON MUNICIPAL SOLID WASTE MANAGEMENT ALTERNATIVE CAPPING SYSTEM FOR LANDFILLS OFFER A UNIQUE OPPORTUNITY

Transcription

1 SECOND U.S. CONFERENCE ON MUNICIPAL SOLID WASTE MANAGEMENT ALTERNATIVE CAPPING SYSTEM FOR LANDFILLS REUSE OF DISCARDED CARPETS OFFER A UNIQUE OPPORTUNITY N. C. Vasuki, P.E., DEE Chief Executive Officer Delaware Solid Waste Authority Arlington, Virginia June 1992

2 ALTERNATIVE CAPPING SYSTEM FOR LANDFILLS REUSE OF DISCARDED CARPETS OFFER A UNIQUE OPPORTUNITY INTRODUCTION The solid waste management system of the United States relies on sanitary landfills as the mainstay for disposal. Despite the efforts to recycle and reuse materials, nearly 1.3 billion tons of raw solid waste and 0.2 billion tons of ash residues from waste to energy conversion systems will be deposited in the landfills over the next decade. This is primarily due to landfills continuing to offer the lowest disposal cost among the various short term solid waste management options available to communities. More recycling and reuse of materials, even on an expedited basis, will not result in a significant diversion of solid wastes away from the landfills. What may, in fact, happen is that solid waste residues from recycling and reuse systems will increase because of inherent process inefficiencies. While on a local level there may be an apparent diversion from the landfill as a result of implementing a recycling program, on a national basis compensating increases will be noticed in locations where recycling or reuse plants are developed. Continued population growth and an improving national economy will also contribute to more solid waste production. Such increase in waste generation may not be linear because of improvements in technology. Many smaller and substandard landfills with low capacities are being closed, and many new, larger ones are emerging instead, -1-

3 despite increasingly stringent state and federal regulatory requirements. Finding new sites and developing modern landfills will certainly become increasingly difficult as new national standards under subtitle ltd" of the Resource Recovery and Conservation Act (RCRA) are implemented on a nationwide basis. The phrase "sanitary landfilltt'" was, perhaps, first used in the early Thirties to compare an organized fill against an open dump. Over the last six decades, "sanitary landfill" design has evolved gradually, and the open dump is no longer acceptable to society. Ground water contamination from the old open dumps has resulted in greater awareness of leachate generation in the sanitary landfills, particularly in the non-arid areas. The need for leachate containment was generally recognized in the Seventies and is now well accepted as a minimum standard. New landfill designs incorporate natural and/or flexible membrane liners (FML) for collecting and containing leachate. Leachate containment design leads to leachate removal and treatment systems. In order to minimize the volume and the cost of leachate treatment, methods of reducing the infiltration of precipitation become important. While the landfill cell is active, infiltration can be controlled to a limited extent by the slope of the working faces, the daily cover, the intermediate cover and the compaction density. LANDFILL CAP When the landfill has reached its full capacity, good engineering practice requires a stable and impermeable cap to enhance runoff and reduce infiltration of precipitation. While the goal of the cap design is to minimize moisture entry, it must -2-

4 simultaneously allow decomposition gases to exit, prevent erosion of topsoil, and be flexible enough to adjust to the differential settlement of the underlying strata of wastes. LANDFILL CAP REOUIREMENTS Most states have regulations which specify the minimum requirements or standards for final cap design. The standards vary from state to state. The recent landfill regulations published by USEPA under subtitle D of RCRA establish the minimum national standards. The State of Delaware published its first comprehensive landfill regulations in 1974"', which required all landfills to meet the following criteria for cap design. "(b) Final Cover Final cover of suitable material, compacted to a minimum thickness to two feet, shall be placed over all completed areas or cells by the end of the working day on which the area or cell was completed. Areas receiving final cover shall be seeded to provide vegetation to stabilize the cover and prevent soil erosion and dust." It8. Completed sections. The permittee shall inspect the entire surface of completed sections of the landfill on a quarterly basis following placement of the final two feet of cover material. ~ l l cracked, eroded, and uneven areas shall be promptly repaired. In 1990'3', the State modified the regulations, and the cap design requirements changed as follows: -3-

5 2. Composition of the capping system The capping system shall consist of at least the following components: a. A final grading layer on the waste, consisting of at least six (6) inches of soil, to attain the final slope and provide a stable base for subsequent system components. Daily and intermediate cover may be used for this purpose. b. An impermeable layer, consisting of at least: (1) a 20 mil geomembrane underlain by a geotextile, or (2) 24 inches of clay at a hydraulic conductivity of 1 x cm/sec or depth of equivalent material having a hydraulic conductivity less than 1 x cm/sec, such depth to be determined based on the hydraulic conductivity of 24 inches of clay at a hydraulic conductivity of 1 x cmlsec. Alternative materials may be used for the impermeable layer with prior written approval of the Department. c. A final cover consisting of: (1) Eighteen (18) inches of soil to provide rooting depth and moisture for plant growth, and -4-

6 (2) (3) The 3. Final slopes Six (6) inches of topsoil or other material approved by the Department to support the proposed vegetation; or A suitable layer of alternative material or combination thereof to assure adequate rooting and moisture retention to support the proposed vegetation. permittee shall propose a suitable vegetation dependent upon the quality and characteristics of the topsoil and compatible with the intended final use of the facility. Maintenance schedules and application rates for fertilizer and mulch shall also be submitted for approval. a. The grades of the final slope shall be constructed in accordance with the following minimum standards: (1) The final grade of the top slope, after allowing for settlement and subsidence, shall be designed to promote run-off; (2) The final grades of the side slopes shall be, at a maximum, three horizontal to one vertical (3: 1). b. The top and side slopes shall be maintained to prevent erosion of the capping system and to insure complete vegetation cover." -5-

7 Conventional cap designs have the following problems: 1. Clay caps require good moisture control during placement to obtain the necessary compaction density in order to achieve the design permeability standards. If field quality control is inadequate, the cap will develop cracks which later become conduits for moisture movement. 2. Clay caps are difficult to repair if, as a result of differential settlement, the cap shears at several locations. 3. Heavy downpours during construction and prior to the establishment of a healthy and thick grass cover, results in severe erosion. 4. Placement of a soil layer over the FML for a landfill cap poses problems of slippage, especially if the side slopes are very steep. 5. Holding soil layers over the FML, especially on the side slopes and along the edge of the top surface, is difficult. Geotech fabrics or nets may become necessary to provide the friction needed to retain the soil cover. 6. FML tends to fail when substantial differential settlements, such as sink holes, develop. 7. Repairing caps becomes progressively more difficult and time consuming when the cap is a composite layer. Considering the above, the Delaware Solid Waste Authority staff looked for innovative solutions. Synthetic yarn carpetsc4) (SYC) which have become the mainstay of floor coverings in modern buildings and homes, appear to offer an intriguing possibility. -6-

8 The continuing sales, use, and frequent replacement of SYC's testify to the wide acceptance of such materials. Ever since E.I. dupont de Nemours (DuPont) introduced the nylon yarn in 1940, the yarn's marvelous properties have been utilized for an extraordinary range of uses. SYC became one of the largest segments of the market for nylon and other synthetic yarns. The DuPont Company's estimates indicate the North American sales of SYC to be 1.2 billion square yards annually, enough to cover 2,000 square feet living area in 5.4 million homes. DuPont further indicates that nearly 75% of the annual sales are for replacement markets. This confirms the availability of a steady supply of used carpets in every community which, unless incinerated, will end up in landfills. Most used carpets are discarded in landfills where they are not exposed to sunlight or an aerobic environment. Because landfills have anaerobic conditions, organic material is slowly degraded by anaerobic bacteria when there is moisture and nutrients available to sustain them. The current trend is to maintain the landfills in as dry a condition as possible. Some new landfills are designed to recirculate leachate to enhance biodegradation of many organic materials in municipal solid wastes. Leachate recirculation improves the availability of moisture and nutrients necessary to sustain healthy anaerobic biota. SYC remains unaffected even in such landfills because yarn manufacturers are constantly working to improve yarn stability. LANDFILL COSTS The construction cost of a modern landfill cell varies -7-

9 according to the State regulatory requirements. When double composite liner systems (i.e. clay and FML) are specified, the cost of construction could be as high as $650,000 per acre ($1.61 million per hectare). In comparison, a recent bid on a 24 acre (9.6 hectare) double FML cell'5' indicates the cost at $250,000 per acre (or $617,500 per hectare). A 24-acre (9.7 hectare) cell would, therefore, need between $6 million and $15.6 million capital investment. Assuming that such a cell has a 672,000 ton (612,200 tonnes) useful disposal capacity, the capital cost per ton would vary between $8.9 to $23.2 per ton (or $9.5 to $25.5/tonne). In terms of volumetric measurements, such a cell would have a capacity 3 to landfill 1.47 million yd. (1.12 meters3) at a capital cost varying between $ to $10.61 yd. ($5.36 to $13.93 per meter3). The daily cover requirements of 6" (15.2 cm) of soil would reduce the usable space by 232,250 yds. 3 (177,672 meters3). Most state regulations set also set operating criteria for landfills. Many of the criteria have evolved through practice. There is no real rational design or even empirical design basis for such regulations. The requirement in Delaware (similar to many other states) includes 6 inches (15.2 cms.) daily cover and 12 inches (30.5 cms.) intermediate cover, if the working phase of the landfill is left unused for a week or more at a time. In Delaware, final capping of the landfill costs about $26.00 per yard' ($23.8/m2) if clay is used, whereas the construction of the base of the landfill costs'5' only about $5.17 per yard' ($ per m). -8-

10 LANDFILL CAPACITY CONSERVATION: Each year, new regulatory requirements and inflation result in increased costs for construction and operation of landfills. The future cost of capping is also similarly affected. In view of this situation, DSWA strives to conserve landfill volume space by looking for innovation. The use of chemical foam for daily cover provides some relief, and for a 24-acre (9.7 hectare) landfill cell, it would extend the capacity by as much as 10%. That is, instead of disposing 672,000 tons (611,200 tonnes), the same landfill sans soil daily cover would store 739,200 tons (672,000 tonnes). Some foam formulations can only be used when there is no rain. On the east coast and also the northwest coast of the U.S.A., full potential capacity conservation by using such foams does not appear feasible. Other chemical foam systems do provide better performance during rainy weather; however, they are more expensive. Therefore, the selection of the foam system depends both on the cost of alternate daily cover and the cost of foam, and the need for capacity conservation. REUSE OF DISCARDED SYC: A DuPont Company scientist approached DSWA about testing the use of discarded carpets as daily cover at its Southern Solid Waste Management Center in April The landfill manager, with the consent of Delaware Department of Natural Resources and Environmental Control (DNREC), agreed to test pieces of carpets as intermediate cover instead. That preliminary test indicated the potential for reuse of discarded carpets, and it appeared feasible for final cover, also. Again, with the consent of Delaware DNREC, -9-

11 an attempt was made to cap the top of a Test Cell at the DSWA Central Solid Waste Management Center. The Test Cells are special, one-acre cells, designed to compare the performance of a landfill with leachate recirculation against a landfill without leachate recirculation. The installation of the cap is indicated schematically in Figure I. The DuPont Company supplied the carpets, the Hypalon FML, and a "green astroturf" type carpet to cap the area in order to improve the aesthetics. This installation effort indicates a capping cost of about $9.00 per yard' ($8.25 per m'), and, therefore, a potential savings of $17.00 per yard' ($15.60 per m') in addition to conserving landfill volume. The SYC cap on Test Cell #2 shows good promise. Based on this, pilot effort, the Authority has sought permission from the State DNREC to cap a 22-acre (8.89 hectare) landfill cell starting in August Personnel involved in solid waste regulations have indicated willingness to approve the alternate cap if some of their technical concerns are satisfied. TRIAL CAP RESULTS The use of SYC cap over test cell #2 showed that SYC sheds precipitation very well. The use of a Hypalon FML improved this important aspect of the cap design. However, the rapid runoff has a tendency to erode the side slopes and therefore the same cap system should be employed on the side slopes. As an alternate, the runoff could be channeled through appropriate pipes. The use of a green astroturf layer over the SYC improved the appearance of the cap. The astroturf showed a tendency to crack, when walked over, under extremely cold conditions. The simple tie-down anchoring -10-

12 I P I ARTIFICIAL CAP

13 system performed reasonably well although the carpet layer showed a tendency to lift during heavy winds. The placement of the layers of SYC did not create any hardships. The layers can be easily lifted for repair of the landfill surface and placed right back on the surface. CONCLUSIONS 1. Landfill caps can be constructed at a lower cost. 2. The SYC caps allow easier access to repair and regrade landfill surfaces when they develop differential settlements. 3. There is a real saving in landfill volume capacity if SYC is used for daily cover (left in place), intermediate cover, and capping system. 4. Discarded SYC could save a useful purpose instead of just occupying space within the landfill. 5. Regulatory agencies should be persuaded to allow an alternative cap system (which is a radical departure from the conventional system) to save landfill volume and improve performance. RESEARCH NEEDS 1. New anchors, preferably made out of post consumer plastics (mixed plastics), must be developed to hold down SYC during heavy winds. Anchors must be designed to allow removal of SYC layers for repair or regrading of landfill surfaces. 2. Weathering characteristics of the top layer of SYC (astroturf) need further examination. -12-

14 3. A method of cementing an FML to "astroturf" would save installation time and reduce the lifting problems in high winds. 4. Although SYC is generally fire retardant, its ability to survive a landfill fire when used for daily cover or intermediate needs further examination. REFERENCES 1. Salvato J.A., Wilke W.G., and Mead B.E.; "Sanitary Landfill - Leaching prevention and control", Journal WPCF, Vol. 43, No. 10, Oct Dept. of Natural Resources and Environmental Control, State of Delaware, Solid Waste Regulations Dept. of Natural Resources and Environmental Control, State of Delaware, Solid Waste Regulations Vasuki N.C., Kumar V., and Vinod Y. ; "Plush, Pile, or Shag? - Reuse of Carpets on Landfills: An Intriguing Possibility. M.S.W. Management, Vol. 2, No. 2, March/April Central Solid Waste Management Center, Cell D Project, Delaware Solid Waste Authority, April