*** Compilado por RESOL Engenharia LTDA ***
March 1996 TRANSPORTATION, WATER AND URBAN DEVELOPMENT DEPARTMENT, WORLD BANK Urban No. UE-12
SANITARY LANDFILL DESIGN AND SITING CRITERIA
For two decades, solid waste components in World Bank projects have focused on collection of solid wastes, with equipment provided to upgrade operations at existing open dumps. In the past several years, the private sector is increasingly being involved in the collection of solid waste and World Bank projects are beginning to place greater priority on implementation of new sanitary landfills. The following guidance provides an examination of some of the issues which need to be addressed in landfill siting and design.
Sanitary landfill is the most cost-effective system of solid waste disposal for most urban areas in developing countries. Composting of solid waste costs 2–3 times more than sanitary landfill, and incineration costs 5–10 times more.
A sanitary landfill is a contained and engineered bioreactor and attenuation structure, designed to encourage anaerobic biodegradation and consolidation of compacted refuse materials within confining layers of compacted soil. At a proper sanitary landfill, there are no nuisance impacts of constant burning, smoke, flies, windblown litter, and unsightly rubbish heaps.
Refuse in a proper sanitary landfill is not directly exposed to rainfall, surface runoff or groundwater. Leachate generation is derived only from a limited quantity of infiltration which reaches the waste deposit and captures the byproducts of waste biodegradation. While little leachate is generated in a sanitary landfill compared to an open dump, leachate concentrations are much higher—organics are higher by a factor of more than 10—and thus needs to be properly treated.
Sanitary landfills located in arid areas, where there is minimal potential for leachate generation, may have more relaxed design requirements than those located in wet areas. Similarly, sanitary landfills located on coastal lands underlain by naturally saline and unpotable groundwater may have more relaxed design requirements than those in inland areas overlying potentially usable groundwater regimes. In these areas of lower impact potential, impermeable lining of the landfill may be unnecessary. Instead, measures to enhance natural attenuation by soil's adsorption, precipitation, filtration, and ion exchange capacities need to be considered.
Sanitary landfill design needs to provide for daily cover of fresh refuse, route runoff away from waste cells, incorporate mitigative measures to manage leachate and gas produced within the landfill cells, provide for a final soil and vegetative cover, and establish an environmental monitoring system of upgradient and downgradient groundwater monitoring wells and surface water sampling locations. Typically the daily cover material is soil; however, tarps or inert materials (i.e., construction debris or compost residuals) could be used.
Since the sanitary landfill is the most important control node of the refuse collection system, a gate-house for record-keeping operations and a weighbridge are recommended. A weighbridge generally costs no more to purchase than one refuse collection truck, and assures the productivity of the entire collection fleet.
Minimizing Leachate Generation. A sanitary landfill is a step by step construction activity involving daily layering, compacting, and soil covering of refuse into cells. The space wherein the refuse would be placed should not be subject to seasonally high groundwater levels or to periodic flooding. The site preparation and landfilling operation must be designed to minimize contact of surface runoff and percolating rainwater with the refuse. This requires diversion of upgradient surface drainage away from the landfill operational area, sloping of the cells to avoid ponding of waters on top of them, and compaction of refuse and daily soil cover as each cell is being constructed so that infiltration potential is minimized.
Soil used for lining, interim cover, and final cover should be wetted with water (usually from a water tank truck) to reach optimum moisture (about 50%) so that good compaction can be obtained. Final soil cover needs to be sloped (2–3%) to avoid ponding of waters on top of the refuse filled area and to minimize infiltration. Grass is planted in the final soil cover to limit erosion.
Leachate Management. At sites where potentially usable groundwater exists in unconfined layers, any rain and surface runoff waters which percolate through the refuse and become contaminated leachate need to be collected. The leachate collection system typically consists of a network of perforated plastic pipe within a gravel bed which is placed over the landfill liner. The gravel bed may be protected from clogging by a geotextile liner above. Leachate pipes need to be selected to withstand the compression forces of the waste deposit and equipment operating at the landfill. The landfill liner and the leachate collection network need to be properly sloped (about 2% slope) to enable gravity flow of contaminated waters to treatment ponds.
For sites in developing countries, the landfill liner would typically consist of relatively impermeable clay soil placed in thin layers (of about 25 cm depth each) at optimum moisture content and compacted with a roller. Good compaction of the base material is essential, to avoid uneven settling of the overlying leachate collection pipe network. Sites with only a clay soil liner may require the leachate collection pipes to be placed at closer intervals than sites with additional plastic liners, to optimize the capture of most leachate.
At large landfills receiving municipal refuse for major metropolitan areas, where hazardous waste quantities could be received in significant quantities, additional impermeable plastic liners may be necessary to protect sensitive groundwater resources.
In countries with low levels of precipitation, a leachate drainage pipe network under the refuse might not be needed and leachate may be collected by providing a dike at the toe of the site and a gravel trench with a leachate interception pipe. In these dry countries, treatment may need to be only a storage/evaporation pond.
Leachate treatment in developing countries is typically accomplished through a series of lagoons. The lagoons are commonly designed to encourage a first phase of anaerobic decomposition, followed by facultative or aerobic stablization. To the extent possible, full evaporation in a final storage lagoon is desirable so that no discharge of treated effluent is necessary. If full evaporation is not possible, recirculation of treated effluent back to the landfill (on the completed areas of fill), irrigation of the buffer zone of trees and bushes, discharge to a sewage treatment plant, or tanker haul to a sewage treatment plant is recommended. Discharge to a surface water is acceptable only if the effluent is treated and/or diluted to a level wherein there would be no significant adverse impact on the water quality requirements of the receiving water.
Gas Management. In addition to leachate management, landfill gas management is a critical component of sanitary landfill design. During peak periods of anaerobic decomposition, the landfill gas reaches methane concentrations of about 50%. Minimum requirements are that the landfill gases be properly ventilated to avoid build-up to potentially explosive levels and migration laterally from the site.
Flaring of landfill gases reduces greenhouse gas emissions and needs to be considered for large sites serving metropolitan areas. Gas recovery for energy conversion is not typically considered economically viable (i.e., unless the purchase price established by the electrical company which controls the electricity grid is at least 0.06 $US/kwh), but should be examined.
If there is no plan to flare or recover the gas, the gas ventilation may be designed as follows: (1) during site preparation the landfill side slopes are lined with impermeable clay to curtail lateral migration of the gases and then lined with coarse rock or gravel to allow gases to escape to the atmosphere; and (2) rock-filled, wire mesh wrapped, vertical wells of about 1 m diameter (spaced about every 0.1 ha).
If flaring or recovery are anticipated, gas vents may be implemented as follows: (1) during landfilling, using perforated plastic pipe (about 15 cm diameter) packed in gravel, with capping of this pipe and the gravel with a larger closed pipe just below the ground surface; or (2) upon completion of a specific area of the landfill, by drilling a borehole and installing a perforated pipe within gravel packing.
Stability. Side slopes of the landfill should not be more than 2.5:1, otherwise erosion and loss of soil cover could occur. It is important that soil cover exist even on the side slopes of the landfill cells, as well as on the surface. Without good soil cover, air will be able to enter freely and underground fires will persist. When there are underground fires, cavities within the solid waste will develop and the surface of the landfill may collapse. Serious accidents have been known to occur in such circumstances.
Composting. In cities where compost is a marketable product and the market is willing and able to cover the additional cost (i.e., above the cost of sanitary landfill) of producing compost, composting should be conducted at the sanitary landfill site. If composting shares the landfill site, the investment for access, fencing, gate control, water supply, electricity, and mobile equipment can be shared—thus lowering the production cost of compost to potentially affordable levels. For example, the wheeled loader used for excavating soil cover for the landfill can be connected to a windrow turning machine to conduct the open air composting operation.
In most cities, only 10–30% of the incoming trucks to the sanitary landfill are from sources which have only wastes with good quality for composting, such as wastes from purely residential neighborhoods, and wastes from special generators such as markets, restaurants, slaughterhouses, and food processing plants. If the compost operation shares the same facility as the sanitary landfill, this allows the gate keeper to divert only the loads with appropriate quality of wastes to the composting operation. Rejects and residuals from the composting process can be readily discharged to the sanitary landfill. If the market for composting is only seasonal, the operation can be conducted on a seasonal basis. And, finally, if rainy season conditions inhibit the ease of composting, the operation can be conducted during dry seasons only. Location of composting at the sanitary landfill thus optimizes quality control of the product, flexibility for the operation, and cost minimization.
Construction Phasing. Construction of a sanitary landfill occurs in zones, over the life of the site. Typically each zone provides a capacity for about 3 years of solid waste. At the start of construction, the access road, entrance gate, weighbridge, fencing, water supply and Zone I refuse cell area are constructed. Refuse cell development always begin from the lowest elevation of each zone. Leachate treatment facilities to handle flows generated at the peak period over the life of the site are constructed from the onset, usually at the lowest elevation of the site so that leachates from all areas can flow to them by gravity. As the capacity of the Zone I refuse cell area nears its complete utilization, the Zone II refuse cell area needs to be prepared (i.e., with base grading and compaction, lining, leachate collection networks, gas ventilation systems, etc). And so on, over the life of the site, until each Zone of the landfill is completed.
For landfill gas recovery (for purposes of energy production), the complete encapsulation of the waste deposit would be necessary, so that gas does not escape and air does not enter and thus dilute the gas composition or inhibit gas formation. Gas is recovered actively, through vacuum pumping to preserve the methane content. Encapsulation requires impermeable soils and/or liners at the base, sites, and top of the waste deposit. If recovery of landfill biogas is an objective and the climate is warm and wet, each Zone of the landfill should have a life of only 3 years because the period of peak gas generation occurs rapidly.
Siting. Each sanitary landfill is uniquely designed to conform to the soil, geologic, topographic, and water resource conditions of the site. To minimize the costs of operating a sanitary landfill, the first and most critical step is proper siting in a location which enables economic operations and cost-effective environmental protection. Also, proper siting is essential to minimizing the cost of refuse collection. Site selection criteria are provided below as guidance (see box).
A proposed landfill site can be selected even though it does not meet each of the screening criteria. Engineering design can mitigate inadequate site conditions—but at a cost. When selecting a site which does not meet all of the screening criteria, possible engineering solutions which would bring the site into conformance with the intent of the unmet criteria shall be incorporated in the design.
World Bank Environmental Requirements. Solid waste components which involve new sanitary landfills are usually given an "A" ranking for environmental priority. With this ranking, the following activities are typically required to be accomplished prior to project appraisal: (1) documentation that site selection has been conducted to address the type of siting criteria listed below; (2) an environmental report to describe the site selected, outline potential environmental impacts of sanitary landfill at the site, and propose mitigative measures; (3) public education and local consultations with residents in the vicinity of the proposed sanitary landfill, including an open forum where all interested parties have an opportunity to express their opinions concerning site selection; and (4) conceptual design and budgetary costing of the sanitary landfill, including mitigative measures identified in the environmental report and responsive to the local consultations, and compensatory measures to benefit the host community may also be considered. If the proposed sanitary landfill is a substantial part of total project costs, the "A" ranking may also require that: (1) all design and tender document development be completed by the time of appraisal; and (2) the local environmental agency provide a letter of environmental permission allowing sanitary landfill construction to proceed.
Private Sector Involvement. Sanitary landfill is a public good, because benefits of environmentally sound disposal are derived collectively. Nevertheless,
private sector involvement, if properly arranged, can increase the likelihood that landfill design and operation specifications will be followed. Ideally, the landfill would be built, operated, and owned by the a private company under a 10–15 year concession agreement, with a guaranteed minimum of waste quantity delivered (or paid for) by its municipal clients. Under this type of arrangement, the private company should also provide closure and post-closure guarantees. If government decides to build and own the facility, a private company could provide operation under a 5–8 year contract or franchise agreement.
SANITARY LANDFILL SITING CRITERIA
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