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6.4 Sound technical options

In Planning a landfill it is useful to think about the facility in terms of four key phases from initial concept to final closure. These phases are:

1. Siting
2. Design
3. Construction, operation, and environmental monitoring
4. Closure and post-closure.

The key considerations for sound management of the facility are listed in the adjoining box. Attention to these issues is important for achieving safe and effective MSWM at the landfill. Such sound management can be enhanced by the judicious use of resources at each phase of landfill development.

Siting

Siting can be one of the most difficult processes in the landfill process. The main considerations are:

• capacity
• public involvement in the siting process
• hydro-geology/cover material
• access
• proximity to airports

Capacity. In the siting process, the available land area is a key consideration. In order to minimize the transaction costs associated with design, permitting, siting, and closure and post-closure requirements, it is desirable to have a facility that will operate for at least two to three years. In practice, many short-term facilities turn into long-term facilities, so it is important that all aspects of the siting process be observed even when planning a short-term controlled dump. Ideally, a site should be sought with sufficient capacity for 10 - 20 years of operation, particularly in the case of sanitary landfills.

Higher environmental standards are increasing the construction, operation, and closure costs of landfills. In combination with a landfill capacity crisis in some countries, these factors are leading to the construction of regional landfills that can respond to environmental concerns in a cost-effective way. Such regional landfills serve a larger region than would normally be served by a municipal landfill. These considerations apply in developing countries as well as industrialized ones. The construction and use of transfer stations will reduce the higher transport costs brought about by the use of more distant, regional landfills.

Public involvement. Public opposition can be strong and protracted. The planner must be prepared to involve potentially affected communities in the siting process. He or she must establish a dialog and working relationship with representatives from the candidate communities and address their concerns in the design and implementation of the landfill plan.

Hydro-geology. It is desirable to take advantage of the geology of a site. In particular, the types of soil and rock underlying the landfill and the thickness of each soil layer can restrict the migration of the leachate toward groundwater and reduce the concentration of contaminants. For example, clay soils significantly slow the migration of leachate and can reduce the concentration of heavy metal contaminants. A bedding of igneous rock also serves to contain leachate. Sand, on the other hand, will do little to slow leachate migration and has little capacity to remove contaminants.

Cover material. The availability of cover material is also an important consideration in the siting decision. As discussed in the section on landfill operations below, the compacted MSW must be covered by 15-30 centimeters of soil at the end of each day's operations. This creates a large demand for cover material and can lead to prohibitive costs if this soil has to be trucked over distances far from the landfill site. Characteristics of suitable cover material are summarized in the section on cell operations below.

Unmanaged leachate is a serious problem at dumps throughout the world. The leachate from this dump in Accra flows directly into a canal. (credit: Raymond Asmani-Boateng)

This uncontrolled dump is situated immediately next to Laguna Lake, in the Philippines, which planner are hoping to use as part of Manila's drinking water supply. (credit: Antonio Fernandez)

Access. To contain hauling costs and discourage the use of illegal dump sites, it is important that a landfill be located reasonably close to the area it is designed to serve. At the same time, siting a landfill too close to a populated area will expose local residents to the environmental and health threats that landfills may pose. Since urban areas are growing rapidly, especially in developing countries, an area near the edge of existing settlements will very likely be too close to populated areas in the foreseeable future. The ideal location would therefore be sufficiently far from the city to allow for future population growth, but close enough to be reasonably accessible.

In many cases, the use of transfer stations within a city can facilitate the siting of a landfill at a greater distance from population centers. The Collection and transfer part of the Sound Practices section addresses the issue of transfer stations in further detail. The roads that provide access to a landfill must be adequate to handle the types and quantity of vehicles that will be used. Planning a landfill requires evaluation of existing and new roads, and must include provision for maintenance of the roads needed to reach the site.

Accessibility notwithstanding, the geological considerations described above are paramount in ensuring the environmental soundness of a landfill. Even when it is necessary to site a landfill far from a city, the expense that results from additional road construction and increased hauling costs may well be less than the cost of remediating a contaminated drinking water supply.

Proximity to airports. A landfill should not be sited closer than two kilometers from the nearest airport. Birds converging at the landfill may pose a problem for aircraft and the landfill may pose other problems for airport operations if the facilities are located too close to each other. The required separation may be larger depending on the size and type of the landfill and of the airport.

Other considerations. A landfill that is sited in a gently sloping area will facilitate leachate collection. Landfills should not be sited: (a) in very windy areas, where it would be difficult to control wastes; (b) near existing gas, sewer, water, or electrical lines, unless the landfill definitely does not present a problem to such infrastructure; or (c) near residential areas, churches, schools, etc.

Sound practice involves the evaluation of several candidate sites and the selection of the final site from a prioritized list of such sites. The accompanying table presents a simpli-fied ranking scheme to aid the site selection process.

In estimating the capacity and NIMBY factors the planner should consider projected population growth and future trends in land development in the area. It is not uncommon for an initially isolated site covering an extensive area to find itself constrained by burgeoning urban development, confronted by hostile residential neighbors, and restricted from expanding to its planned capacity. Land use restrictions and zoning by the city planning department can help to minimize such conflicts. Municipal authorities should integrate these commitments into the initial landfill site plan wherever possible.

An environmental risk assessment (EnRA) should be made once the final site is selected. This assessment should analyze rigorously both environmental and human health aspects of the landfill proposal. The actual content of such an EnRA will depend on local regulations. However, at minimum, the EnRA should detail the risks to ground and surface water from the facility, the risks from gaseous emissions, the impact of vermin, traffic, litter, and noise on the area, and the potential for recovery of the land after the site has completed the post-closure period.

These general guidelines apply equally to controlled dump and sanitary landfill facilities. In the case where an existing open dump is to be upgraded to a controlled dump or sanitary landfill, the siting decision has already been made. In this case the planner applies the above procedure to the areas for proposed expansion of the existing site.

The reader may find several references that provide greater detail on the landfill (and other MSWM facility) siting process. Among these, The Solid Waste Handbook (McGraw-Hill) and the classic textbook by Tchobanoglous et al. contain understandable, general guidelines.

Design

The design of a landfill will significantly affect its safety, cost, and effectiveness over the lifetime of the facility. Key items requiring attention in the design stage are listed in the adjoining box.

These considerations may have different implications for controlled dumps and sanitary landfill facilities. The facility should be designed to operate effectively given the mix of capital, labor, and expertise available to its owners. Thus, labor-intensive controlled dumps should be designed where capital is severely limited, labor is available at low cost, and there is a shortage of expertise and infrastructure to service a highly mechanized facility. Such conditions prevail in the cities of many developing countries. However, an appropriately designed facility can still avail such cities of safe and dependable landfilling.

Capacity
In the case of the controlled dump, planned capacity may not be protected by zoning and land use restriction guarantees provided by municipal authorities. Therefore, the landfill planner must use the designated site strategically to minimize the risk of future incursion by municipal development and maximize the total area available for landfilling over the lifetime of the facility. Strategies may include acquisition of property surrounding the landfill site or working inward from the border deemed most susceptible to urban growth.

For the sanitary landfill, capacity is often granted by regulatory permits on an incremental basis. Thus, rather than declaring the capacity of the entire site at the initial design stage, the planner develops it in two- to five-year increments once the overall site meets the required zoning and land use requirements. This practice allows the landfill owner to selectively develop the property with the flexibility to convert it to more lucrative alternative land uses that may develop over time. This is particularly applicable to private landfill owners. In the case of municipal facility owners, incremental development may also be advisable for the same reasons. However, municipal planners must also ensure that appropriate sites with the required capacity are always available for future needs.

Public/private ownership and operation
In most developing countries landfills are owned and operated by local governments or other public agencies. This is often a function of the substantial capital investment required to construct such facilities. Where proven expertise is available in the private sector, municipal planners should explore the option of privatizing landfill operations on a contractual basis. This option should be weighed carefully as it involves issues of cost recovery and the payment of fees for tipping privileges at the landfill. There are several references on the privatization of municipal services in the literature. The municipal planner should read these with due attention to their feasibility for the local situation.

Monitoring and control of leachate
Leachate management is a key factor in safe landfill design and operation. The natural decomposition of MSW, in combination with rain infiltration into the site, causes potentially toxic contaminants to flow toward the bottom of the landfill. The wetter the climate, the greater the potential risks of groundwater and surface water contamination from the landfill. As explained above, the geology of a site can exacerbate or reduce the amount of leachate that enters the environment.

A variety of wastes can contribute contaminants to landfill leachate. Paint pigments and household batteries can release heavy metal contaminants. Household hazardous waste (e.g., paint products, garden pesticides, automotive products) and hazardous wastes from commercial and industrial generators can release organic chemical contaminants.

A sanitary landfill contains engineering features to prevent the release of hazardous substances to the environment. Natural or synthetic materials are often used to line the bottom and sides of landfills to prevent the migration of leachate into nearby groundwater and surface water. Many landfills use liners constructed from two feet or more of compacted clay. Other liners consist of thin sheets of plastic made from a variety of synthetic materials. Any type of liner, however, could fail to contain leachate. Natural and synthetic liners can crack, particularly if improperly installed, or can lose strength over time. In an effort to improve the containment of leachate, more than one liner or a mix of natural and synthetic liners, called a composite liner, is a recommended alternative.

To minimize the production of leachate, covering material should be applied after each dayÕs MSW is spread. When a landfill is closed, a final cover is applied. These covers are discussed below.

Leachate collection and treatment. Leachate retained by the liner will accumulate and possibly leak through the liner unless it is removed by a leachate collection system. Leachate collection systems are installed above the liner and usually consist of a perforated piping system which collects and carries the leachate to a storage tank. Periodically, leachate must be removed from the storage tank and treated or disposed of. The most common leachate management methods are: discharge to a wastewater treatment plant, on-site treatment followed by discharge to sewerage or surface water (depending on the quality of the treated leachate), and recirculation back into the landfill. All of these options generally require a pumping system. They require considerable maintenance due to the corrosive nature of the leachate.

Leachate recirculation. Recirculating leachate over waste in landfills has been shown to increase the quantity (by nearly a factor of 10) and quality of methane gas for recovery as well as possibly reduce the concentration of contaminants in leachate and enhance the settling of the waste. There is now considerable experience with such bioreactor landfills in the US, Europe, and Brazil. In a country like Brazil, with a warm climate and highly organic MSW, landfill cells stabilize within three to five years. Landfill gas recovery for energy is feasible, and the cells can be excavated after five years and the space reused. The humus-like material that is excavated (about 65%) can be recycled as compost, and the remaining material used for daily or final cover of the next cells.

As a substitute for a leachate treatment system, leachate reinjection may be particularly appropriate for areas with low rainfall, since leachate production in those areas is more dependent on the humidity of the waste than on the infiltration of rain. In some instances, this technology can be more cost-effective than other leachate treatment systems.

Leachate recirculation is a relatively new technique of leachate management. As a management technique its goal is to stabilize the waste through accelerated and controlled decomposition, accompanied by methane recovery. This is in contrast to standard landfill technology, which focuses more on isolation.

Possible drawbacks of leachate recirculation include clogging of leachate collection systems, increasing the release of leachate to the environment (even though the main effect is the opposite), and increasing odor problems. After weighing these advantages and disadvantages, federal regulators in the US decided to allow leachate recirculation only at landfills that have a composite liner and a leachate collection system that meets specified performance requirements. In developing countries, however, the trade-offs may be different.

Inexpensive methods of leachate control. The state-of-the-art liners and leachate collection systems described above are too expensive for many developing countries. There are three practices that are much cheaper and that may be practical in some situations.

1. As described in the description of transfer stations (see Collection and transfer in the Sound Practices section), MSW in an area with low rainfall can be partially dried at transfer stations prior to landfilling. This will reduce the leachate produced at the landfill itself.
2. For areas where pre-drying is impractical or where the soil conducts water very well (and where leachate runoff would therefore be a major problem), it may be necessary to site a landfill in an area with a steeper grade than would otherwise be chosen. In conjunction with a well-distributed leachate collection system, this can reduce the dangers of water table contamination. The actual grade required would depend on the conductivity of the soil, the stability of the graded surface, and other site- specific engineering considerations. The use of steeper grading, along with the denser placement of leachate collection pipes, will add to the cost of a landfill. However, these changes may be far less expensive than importing, transporting, and applying clay or synthetic liners.
3. To avoid the initial expense and ongoing maintenance requirements of pumping leachate, leachate can be collected in a concrete-lined holding pond, constructed downhill from the landfill. The best option in many cases is then to allow the leachate to evaporate as much as possible. The handling of the leachate residue must be considered in weighing this option. There is experience with this in the treatment of wastewater that may be adapted to leachate ponds in developing countries. In arid areas, the pond, or a set of cascading ponds, should be designed to maximize the surface area of the collected leachate, in order to maximize the rate of evaporation. In order to avoid the risk of overflow into surface water supplies, such ponds must be large enough to handle all of the expected leachate and rainfall. During rainy times, the holding pond solution may not work as well or at all, since evaporation may not counteract the combination of leachate and rainfall. Even in this circumstance, however, the leachate from an overflowing pond will be diluted by rainfall.

At controlled dumps monitoring operations may involve the scheduled withdrawal of samples to test for indicator contaminants such as bacteria, heavy metal ions, and toxic organic acids. The EnRA should establish the background concentration of these substances as a basis for evaluating performance standards for the landfill.

Monitoring operations at sanitary landfills may involve computerized statistical sampling and automatic reporting of results at the regulatory agency. Such systems are costly and require skilled personnel to use correctly and to maintain. The choice of system is dependent on the capital and human resources available to the landfill owner/operator and the requirements of local regulations.

Monitoring and control of landfill gas
Landfill gas is primarily a mixture of methane and carbon dioxide produced by the decomposition of organic matter in the MSW. Landfill gas is highly flammable and poses a risk of explosion if not properly managed. Gas management is generally required at sanitary landfills. At controlled dumps, there should at least be monitoring to determine if dangerous amounts of gas are being released. A low-cost design to handle landfill gas may consist of buried vertical perforated pipes, using the natural pressure of the gas to collect and vent or flare it at the surface. This is called a passive collection system. More costly active collection systems utilize a buried network of pipes and pumping to trap the gas. The gas is then processed and used for process heat or electricity generation. Because landfill gas is potentially explosive, pressurized collection is risky and expensive.

A poorly managed leachate pond at a Thai dump (credit: Chris Furedy)

A gravity-fed leachate plant downhill from Tiger's Cave dump in Guangzhou. The plant has had numerous technical problems. (credit: Chris Furedy)

The accompanying box lists the conditions under which it may be economically feasible to recover methane from a landfill. Generally, all of these conditions must be met. From an environmental perspective, collecting and using methane is ideal because it captures the energy value in the gas and prevents the release of a greenhouse gas to the atmosphere. However, in many situations, not all of the conditions stated above are fulfilled, and landfill gas recovery is not economical.

The technical feasibility of recovering economical quantities of methane gas depends on several factors. Perhaps the most important factor is the composition of the MSW. The production of methane gas depends on a relatively high percentage of organic MSW as well as proper nutrients, bacteria, pH, and a high moisture content. The size of the landfill must be large enough and contain enough MSW to produce economically recoverable quantities of methane. Generally, landfills that have a capacity of at least one million tons should produce enough methane to support recovery operations. The age of the landfill is also important because it can take anywhere from several months to a few years after the disposal of MSW before sufficient methane is produced. Early methane production can be enhanced by using uncompacted waste as the first layer of a landfill, thus allowing it to compost more quickly.

The engineering components of a landfill may also increase the quantity of methane gas that can be recovered. Landfill liners help keep methane from escaping from the landfill and help maintain the anaerobic conditions necessary for methane production. Similarly, a daily cover that keeps methane from escaping and inhibits the introduction of air into the landfill can increase the rate of methane production. Other factors, such as the geology of the landfill site, also play an important role. However, landfills that do not have engineered liners or covers and landfills sited in porous soils can still produce significant quantities of methane gas.

This plant is part of the landfill gas capture system in Izmir, Turkey. The gas will be used to generate electricity. (credit: Chris Furedy)

Access and tipping area
Fencing should be designed to restrict unauthorized access to the landfill and to keep out vermin and stray animals. Ideally, a vegetative hedge should be planted along the perimeter of the site (the plant Jatropha curcas L. is a good candidate for tropical countries) followed by a buffer zone several feet wide. The fence may be erected within the hedge. If possible the perimeter of the facility should be patrolled to minimize vandalism.

A staffed gate should be the point of entry to the facility for vehicles and any waste pickers. Separate provisions must be made for access to emergency vehicles and equipment. Ideally, the gate should be equipped with scales for the weighing of vehicles as they enter and exit the facility. This provides a record of the tonnage of material entering the facility. Weighscales are an essential element of a sound municipal MSWM system, as they provide critical information for planning purposes and for operational management of collection vehicles.

Weighbridge at a new landfill in Bandung. Accurate assessment of the quantity of waste dumped is critical in MSWM planning. (credit: Chris Furedy)

In the case of the controlled dump, the designer may forego mechanical scales when the total expected gate receipt of waste is less than 200 tons per day. Manual scales or calibrated tables of weight-to-volume for delivery vehicles may be used to provide estimated gate receipts in these cases.

Ideally, the tipping area should be at the working cell. This entails the design of access roads to these locations within the site. This may not be possible at controlled dumps and smaller sanitary landfills, either due to a lack of resources for road construction or the lack of maneuvering room for the delivery vehicles. In such cases tipping may occur close to the gate area and the load transported to the working cell by smaller vehicles or muscle-powered carts.

Pre-processing and waste picker policy
The landfill is the least efficient alternative for materials recovery operations since the waste is usually well mixed by the time it arrives there and the cost of transporting the material to the landfill has already been incurred. Nevertheless, where composting is desirable at the landfill and/or waste picking activity is permitted, sorting of the waste should occur close to the gate or tipping area rather than at the working cell. This reduces the risks to sorters from the equipment and activity at the active cell. It also reduces the amount of material that has to be transported to the cell. Such activities reduce the volume of material to be landfilled and prolong the life of the facility.

Where waste picking is the established policy, the process can be most efficiently managed by the awarding of permits for the recovery of materials or for the conduction of on-site composting operations. Such permits would facilitate more ready control of waste picker activity and their accountability to the facility owner. The issue of waste pickers is discussed in greater detail below.

Where mechanical equipment, such as compactors and bulldozers, is used in landfill operations, waste picking activity should be restricted to separate resource recovery facilities or prohibited altogether from the site. Waste picking policy should be established during the design phase of the facility so that alternatives may be provided for the displaced workers and operating procedures set for safe landfill operations.

Operations and safety manuals
These manuals should be prepared or acquired during the design phase of the landfill. This permits their content to be specifically tailored to the processes for which the facility is designed. Several handbooks and manuals on landfill operations are available and the planner should tailor these to the specific needs of the site. Clear operating procedures and well-trained workers are vital to safe and effective landfill operations.

Closure/post-closure plans
Closure and post-closure plans are required in the permitting process of most sanitary landfills. Their essential elements are:

• plans for the sealing and application of final cover (including vegetation) to the site;
• plans for long-term leachate and gas management system monitoring;
• plans for long-term ground and surface water monitoring;
• financial assurance guarantees to the local or state government; and
• land use restrictions for the site

In the case of controlled dumps in most developing countries, closure and post-closure plans are not prepared. Facilities are not required to make financial allowances for these activities. However, the provision of ongoing monitoring and control of the facility after its useful life is an unavoidable reality of landfill management. It is necessary to monitor these facilities to ensure their continued safety to the health of surrounding communities and the environment for periods that may exceed 30 years after their closure. Cost recovery programs may be instituted during the operation of the facility to provide funding for these activities. Alternatively, taxes or other revenue-raising options may be employed to secure the required funds.

Community relations
This is vital if the landfill is to win community support over its lifetime. The designer should establish a program for ongoing dialog with the community. This should be based on transparency in landfill operations and procedures and a commitment to addressing community concerns. Some facilities offer give-backs to their host community. These include free street paving to compensate for the heavy vehicles in transit to the landfill. Another option is the waiving of tipping fees to residents of the host community. Community relations begins with the first consideration of the landfill site, and the formal program should be incorporated into the design of the landfill management plan.

Leaflets like this one are very useful in educating the public about MSWM issues. (credit: Bangkok Municipal Administration)

Construction, operation, environmental monitoring, and special wastes

Construction
The amount and type of construction depends on the class of landfill, on physical conditions at the site, and on local regulatory requirements. Construction issues include:

• construction of access roads
• erection of fences, gates, and the tipping area
• site preparation for the diversion of precipitation and the control of runoff
• installation of the leachate and gas monitoring systems
• installation of the leachate and gas treatment systems
• construction of administrative offices, physical plant, and other buildings at the facility
• preparation of the general working area, including:
  • land clearing, grading, and excavation
  • compaction of the base and application of the liner
  • installation of the leachate collection system.

Operation
It is useful to define some common terms when discussing landfill operations.

A cell is the basic unit by which a landfill is developed. It is the general area where incoming waste is tipped, spread, compacted and covered. A basic cell consists of a base of synthetic or soil liner, compacted MSW and a compacted cover layer of soil. Cells range in height from two to three meters. The landfill is filled with waste following an ordered cell by cell sequence. Special cells in the landfill may be reserved for materials such as medical waste or construction and demolition debris. The dimensions of a cell depend on the type of equipment being used, the planned use of the cell and the dimensions of the overall site.

A lift is the completed layer of compacted waste in the cell. MSW is laid down in rows 25 to 75 centimeters thick. This layer is then compacted either by multiple passes of a manual roller or a mechanical compactor. A second layer of waste is then spread upon the first and subsequently compacted. The process is continued until the desired height of the lift is attained. As noted above, this ranges from two to three meters. The completed lift is covered with a layer of soil that is 15 to 30 centimeters thick when compacted. This completed lift and cover material constitute a cell. Compaction increases the capacity of the landfill. Mechanical compactors achieve densities of 500-900 kilograms per cubic meter. Manual rollers, likely to be found at controlled dumps, will achieve significantly less than this. The planner should obtain information on the compaction capabilities of any equipment that is under consideration.

The working face is the length and width of the row in which the waste is being deposited. The width should be twice the width of the spreader (or twice the width of the row of rakes in manual spreading). The length should be adjusted so that the lift will be at its maximum height at the end of the day's operations.

Some landfills use multiple lifts. Thus, when one cell is completed, another is placed above it. This second level of cells is not begun until the first level is filled with cells at the same level. It is customary to stack cells into a final pyramidal structure. The final cell in the stack is covered with soil that is 60 centimeters thick when compacted. When stacked cells are employed, the final slope should be chosen to facilitate the runoff of precipitation while ensuring the stability of the structure.

In addition to the area method of landfilling described above, two other means of laying down waste are also employed: the ramp and trench methods. Both the area and ramp methods are good for sites where the terrain (hilly or rocky) is unsuitable for excavation of trenches (the area method where no excavation is possible, the ramp method where only some is possible). Both methods require cover material to be trucked in. The trench method is used where excavation is possible. In this method the excavated soil may be used as cover material.

Operations at the landfill may be classified into three main areas:

• gate operations
• cell operations
• administration

Gate operations include weighing and recording gate receipts, tipping (if direct access to the working cell is not available), pre-processing (including materials recovery and composting if practiced at the site), and the transportation of the tipped waste to the lift site at the working cell.

Cell operations include:
tipping: either of waste directly transported by the haul vehicle or of residues preprocessed at the gate area and transported to the cell.

spreading and compacting: This may be done manually by workers with rakes and weighted rollers. Mechanical spreaders and compactors are also available. These achieve much higher compaction than manual means. However this equipment is costly and requires specialized maintenance. For many cities in developing countries, manual spreading and compacting is a sound practice. The reader is directed to the waste management literature for further details on spreading and compacting operations and equipment.

daily cover: It is important to cover new MSW at the end of the day with a layer of soil or compost. This can sometimes be done by applying a 15-30 centimeter layer of soil that was excavated earlier or by using fine construction and demolition residue handled elsewhere at the site. This daily cover is used to control disease vectors such as rats and insects, reduce blowing of waste and odors, and slow down the infiltration of rainwater. It also results in more gas generation, which may or may not be desirable. This practice also makes waste picking much more difficult. In Europe, both dried sewage sludge and low-grade compost are used for daily cover. A mulch made of pulped paper is also sometimes sprayed on. In some cases, a removable tarp can be used.

The ratings of some soils commonly used as cover material are summarized in the following table.

The most basic controlled dump site should still have a site office for checking vehicles and their loads. (credit: Chris Furedy)

Administration includes site permits and regulatory compliance, worker training and safety, public relations, permitting franchisees, record keeping, accounting, and all other clerical duties associated with the operation of an industrial facility. The success of the landfill depends on a strong administration that is committed to adequate financing of the facility, transparency in its operations, and accessibility of information to the public. These factors promote good technical and safety performance of the landfill. Numerous texts provide details on all aspects of landfill operations. Topics range from the selection and use of equipment to the establishment of tipping fees for cost recovery. The reader is advised to consult these in planning for landfill operations.

Environmental monitoring
Monitoring of leachate and gas migration from the landfill is necessary to determine their impacts on surface and ground water and air quality, respectively.

The landfill operator should monitor for the presence of heavy metals in potentially affected surface and ground waters. If these are unacceptably above the background concentrations (determined by the environmental risk assessment) landfill leachate could be the source of the problem. At controlled dumps simple withdrawal from test wells dug at 5, 10, and 15 meters from the site along the path of groundwater flow can give a profile of the groundwater contamination and the extent of its attenuation by natural processes.

The landfill operator should also monitor for landfill gas on the site. If dangerous concentrations are present, this gas must be collected and either vented or flared, if it is not to be used. At controlled dumps, test vents may be sunk at the North, East, South, and West corners of the facility in the buffer zone. These can serve as a low-cost test for the migration of landfill gas off the site.

In the case of sanitary landfills, the permitting process generally specifies a more detailed monitoring program. Where such provisions are absent, the facility owner should install a reliable system based on available resources. This will permit the operator to react to problems as they occur and reduce the risk of injury or damage from escaping gas or leachate contamination.

Special wastes
Special wastes include medical waste, household hazardous wastes, tires, used oil, construction and demolition debris, human excreta, and sewage sludge. The management of these wastes is covered in the next section of this Source Book. Their coverage here addresses sound technical options for their disposal at the landfill. In all cases the first option would be to redirect the waste to the appropriate facility. Some sanitary landfills are prohibited from accepting several of these components as a matter of regulatory policy. However, for many other landfills, particularly controlled dumps in developing countries, the facility is the final and only legitimate destination for these special wastes.

Medical waste: The first option would be to redirect medical waste to an appropriate incinerator or other treatment facility. If not possible, these wastes should be buried in a separate cell at the landfill and covered with lime. Special care should given to the management of leachate from these cells.

Household hazardous waste (HHW): Special handling of HHW is only required if it is delivered to the landfill as a separate load from regular MSW. In such cases, management at the landfill begins with documentation of the shipmentÕs source, contents, and transporter. In other cases HHW mixed with the regular MSW stream is processed according to routine operations. Ideally, the landfill should have a posted policy of not accepting special loads of HHW. However this may not always be possible. For HHW consignments, such as deliveries from HHW collection programs, the first option is to redirect the consignment to an appropriate hazardous waste treatment, storage, or disposal facility. If no such facility is available, as is the case in many developing countries, then separate collection of HHW is not sound MSWM policy. These components are likely to pose less risk to health and the environment if they remain diluted in the general MSW stream. The landfill operator has the option of storing the waste until an appropriate hazardous waste treatment, storage, or disposal facility becomes available or mixing and diluting it with incoming MSW for disposal. Depending on the exact nature of the waste, the regulatory environment, and the outcome of consultation with municipal and state authorities and community groups, the operator must choose the lesser of these two evils. Under no conditions should chemically incompatible materials be stored together. If no other options are available, solvents and other potentially explosive materials should be destroyed by burning at the site. If the landfill operator is uncertain about the chemistry of a HHW consignment, he/she should immediately seek professional advice.

Tires: Tires should not be disposed of with regular MSW. They do not remain compacted and adversely affect the structural stability of the completed cell. Open storage poses a fire hazard. It also poses a health hazard as a breeding ground for mosquitoes. If possible, the tires should be shredded to reduce their volume and used creatively around the site. Some such uses include terracing for erosion control and padding and bumpers on gates and heavy machinery. The unused material should be disposed of in a separate section of landfill.

Used oil: Used oil should be rejected by the landfill operator. It is unsafe to keep for long- term storage and is highly polluting of ground and surface water if it leaks in the landfill.

Construction and demolition debris: This is largely inert material. Where possible it should be handled at a separate cell in the landfill, both to facilitate possible future recovery and because such debris does not require the same leachate management efforts as other wastes. Laborers may recover masonry blocks, wood, and metal for resale, although this should not be done at the working lift. Where gravel and stone bits are present in sufficient quantity they may also be used as cover material at the landfill. However, special processing to separate this material involves costly screening machinery which may be beyond the means of most controlled dumps.

Excreta and sewage sludge: Ideally these wastes should be dried before delivery to the landfill. They should be handled by specially trained and equipped workers. At the landfill they are mixed and disposed of with the regular MSW. Where facilities are available, composting is a resource recovery option for these wastes.

Closure and post-closure

In a regulated system, closure and post- closure (CPC) plans are required to obtain the landfill operating permit. Thus CPC plans are prepared during the design of the landfill. Since financial assurance is a requirement of such plans, provisions should be made at that time for financing these inevitable operations. CPC planning is new to many developing countries. However, as discussed in the section on landfill design, closure and post-closure are important phases in the management of a landfill. They should be explicitly included in its planning. This applies equally to controlled dumps and sanitary landfills. It also applies to planning for the financing of these operations and to the identification of the party responsible for their implementation. In the case of a publicly owned facility, the agency responsible for operating the landfill would be a prime candidate for CPC operations. In the case of a privately owned facility, CPC operations are the owner's responsibility.

CPC operations include the following: sealing and application of final cover: A final impervious layer of clay and/or synthetic material is used to cap the landfill after operations have ceased. A layer of soil is applied above this cap and vegetation is planted to control erosion and improve the aesthetics of the site. The final cover should be constructed to minimize the incursion of precipitation into the landfill by maximizing its runoff and diversion from the site. At controlled dumps where clay or synthetic liners are not an option, the landfill may be capped with soil compacted by multiple passes of the weighted roller or mechanical compactor. In this case, grading and drainage are important to divert precipitation. Vegetative cover and other techniques to control erosion are also important low-cost needs.

This once-objectionable dump site in New Delhi has been planted with trees and shrubs and now provides a breathing space in the growing city. (credit: Chris Furedy)

leachate and gas management and monitoring: Leachate and gas will continue to be produced long after daily operations have ceased at the landfill. Thus, the management systems must be maintained for the period specified in the regulations or until monitoring tests of the leachate and gas emissions indicate that their quantity and/or composition have declined to acceptable levels. This long-term responsibility can seem like an undue burden to the facility operator. However, these programs can significantly reduce the risks of environmental and health damage from uncontrolled leachate and gas migration. Where they prevent the need for costly remedial actions, the savings from post-closure environmental controls can be quite large. This is because landfill remediation is costly. Even at controlled dumps, every effort should be made to maintain the basic systems discussed earlier in the design and the lift operation sections of this text. These systems also contribute to long-term risk reduction and savings from avoided remediation.

ground and surface water monitoring: These programs are generally required at sanitary landfills during the post-closure period. They are generally not required at controlled dumps. In either case they can provide early warning of potential contamination due to leachate from the closed landfill. This allows for timely interventions.

financial assurance guarantees: These are required for privately owned sanitary landfills in OECD countries. For municipally owned sanitary landfills and for controlled dumps, it is still sound practice for these provisions to be made explicitly at the time that the landfill is designed. This forces the owner to allocate funds for CPC activities, either through the generation of income from tipping fees, from the sale of bonds, or by appropriations from public funds. Municipal taxes (either direct or indirect) may be the source of such funds. However, in many developing countries such funds are not retained by the municipality and must be procured from the treasury of the central government. Without firm institutional commitments the supply of these funds can be unreliable. This can subsequently jeopardize the entire CPC program.

land use restrictions for the site: land at the landfill will continue to subside for several years after closure. Additionally, sites without long-term gas management will continue to pose a threat of fires and explosion. While the site is under CPC management it should be restricted from other land uses. When the CPC period is over, tests should be conducted to ensure that the site is structurally stable and safe from chemical hazards. The outcome of these tests should determine the future uses of the site. In areas of high population density and limited open land, former landfill sites are frequently used for secondary, usually recreational, purposes. In some cases, these sites may be paved and used for residential, commercial, or industrial development.

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