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CURSO INTERNACIONAL SOBRE DISEÑO Y

DISPOSICIÓN FINAL DE RESIDUOS SÓLIDOS

(RELLENOS SANITARIOS)

 

 

 

 

 

 

 

 

INTERPRETACIÓN DE LAS CARACTERÍSTICAS

DEL SITIO EN EL DISEÑO DE UN

RELLENO SANITARIO

 

 

 

 

 

Dr. Robert K. Ham

 

 

 

 

 

 

 

 

International Solid Waste Association

(ISWA)

 

 

 

 

 

 

 

PALACIO DE MINERÍA, MÉXICO D.F. 14-19 de Marzo de 1994

 

 

INTERPRETING SITE CHARACTERISTICS IN

LANDFILL DESIGN

INTRODUCTION

No single landfilling method is suited for all types of sites, and no single approach is exclusively optimal for any given site. Selection of landfill technology depends on the physical conditions of the site, the amount and types of solid waste to be accommodated, comparative costs of various options, and the physical and financial resources of the municipality. This document begins with a general description of basic landfill design choices, followed by the rather detailed design requirements, which must be considered in developing a good landfill. The two basic types of landfill methods are the trench (Figure 1) and the area (Figure 2) method. The trench method involves excavation of the site to obtain cover soil and to provide some of the space for the solid waste. It is best suited for sites characterized as follows:

· flat or gently rolling land surface

· low ground water table (at least 3 meters below ground surface for small landfills; at least 5 meters in general)

· soil layer or depth to bedrock at least as deep as groundwater

The area method involves minimal excavation of the site as cover is obtained elsewhere, often from a nearby hill. It is appropriate for most topographies and is the preferred choice for sites that receive large quantities of solid waste. A combination of the two methods is often used, especially, for large landfills extending more than perhaps 10 meters above the original ground elevation. In this case cover is obtained both from on-site excavaition and from off-site sources. The trench and area methods will be discussed in detail later.

CELL AND WORKING FACE DESIGN AND CONSTRUCTION

All true sanitary landfills consist of basic units, commonly termed "cells" (Figure 3). A cell is formed by spreading and compacting incoming solid waste in layers within a confined area. By the end of each working day, the compacted refuse is covered completely (including the working face) with a continuous layer of soil which is also compacted. The compacted waste and its daily, soil cover make up a "cell" (Figure 3). A series of adjoining cells at the same height constitute a "lift" (Figure 3). A completed fill consists of several vertical lifts, and may extend 30 meters or more above the initial ground surface.

The cells are designed based on the volume of compacted wastes requiring disposal. This in turn, depends on the density of the in-place solid waste. The field density of most compacted solid waste within the cell should be at least 595 kg/m3 (1000 lb/yd3). It should be considerably greater if sizable quantities of demolition rubble, glass, and well- compacted inorganic materials are present.

The working face is usually, the most obvious indication of good landfill operations. Unfortunately, the reverse is also true, as it is usually, also the most obvious indication of a

 

 

 

 

 

lack of good operations, which in turn can then be traced to a lack of professional ability or concern. There is no excuse for not confining incoming, waste to the working face, keeping, the working face as small as possible, and in general operating the working face properly. The working face is the area of the landfill where incoming solid waste is placed and compacted, so nearly all site activity is focused here. It is also the source of many of the environmental and aesthetic problems resulting from bad practice.

Waste is usually placed at the bottom of the working face. The exception to this practice is if road access makes it difficult to bring waste to the bottom, in which case waste can be placed at the top of the working face. A tracked crawler, dozer, or steel-wheeled compactor, then spreads the waste into layers 30 to 60 cm thick over the entire sloped working face and moves up and down the face several times to compact waste to 15 to 30 cm thick layers. Compaction studies suggest 3 to 5 passes are necessary to achieve goood compaction. Layers are constructed over each other until the end of the working day, when daily cover is placed and compacted to complete the daily cell.

The slope of the working face is a compromise between obtaining maximum compaction if it was nearly, horizontal, and minimizing daily cover requirements if it was nearly vertical. The best slope is no steeper than 311 (horizontal to vertical), and most operators prefer 4/1 or even 5/1 to give better equipment stability and good compaction. The width of the working face is that required to accommodate the number of vehicles placing the solid waste, at any time, allowing approximately 4 meters per vehicle. It is not necessary to have a very wide working face to accommodate the maximum number of vehicles expected at any time during the day; some waiting by, a few trucks during heavy periods is preferred over having a very wide working face, and the problem of maintaining it. All else being equal, the smaller the working face, the better the operation and the better the control of the waste. The height of the working face or the lift thickness is then whatever is necessary, within reason, to accept the waste and allow smooth operation of equipment. it generally ranges from 3 to 5 meters, with 4 to 5 meters preferred for large landfills receiving several hundreds of tons of waste per day. In practice, 3 meters is usually best.

It should be emphasized that there should be only, one working face receiving all of the waste. The only, exceptions would be if conditions are such that certain wastes must be placed at a different working face. Bad weather can require use of a second working if, for example, wastes not likely to blow can be placed at a working face open to the wind on windy days and light waste placed in a more sheltered working face. Another reason would be to make better use of equipment if, for example, non- compactable inorganic waste is placed separately so compaction equipment is used only on compactable wastes. A second working face should rarely, be used, and only, with clear justification, because in practice it is very difficult even for experienced operators to divide operations and work properly more than one working face.

Once the working face dimensions have been set, the height and width of the daily cells are also set. The remaining dimension, the length of the cell, is set by the amount of waste entering per day.

 

TYPES OF LANDFILLS

Trench Method

As implied, the trench method requires the excavation of trenches into which waste is disposed by, spreading and compaction (Figure l). The waste is deposited at the working face, compacted, and covered with the excavated soil. Excavated soil not used for daily, cover can be stockpiled for later use in upper lifts or for final cover, or may be used for berms to control surface water or visibility.

Determination of the depth of excavation is an important engineering decision. Clearly, with a deeper cut, more volume is available for solid waste and more soil is obtained for cover and other construction activities. On the other hand, a deep cut makes it more difficult to get waste and equipment to the working face, at least initially, and will place the waste closer to groundwater, increasing the potential for contamination. A deep cut also exposes more surrounding soil to potential gas migration and can make gas control more difficult. Finally, side slope stability can become more of a hazard with deep cuts.

Usually the depth of cut is limited by groundwater or bedrock. Even in landfills lined with relatively impermeable soil such as clay, or located in clay, it is common to leave a minimum of 3 meters of undisturbed soil above the groundwater to provide some protection against contamination and to certainly avoid placing waste directly in groundwater even at its seasonal or yearly highest elevation. Similarly, because bedrock is often fractured, providing no attenuation of contaminates in leachate, it is common to excavate no closer than 3 meters to bedrock. Another geological reason to limit the depth of cut is to place the waste in the most impermeable soils available. If a clay or silt soil is located over a sand or gravel layer, it is wise to not excavate into the more permeable soil because the soils will form a conduit for gas and leachate flow. If it is necessary to cut through such a permeable layer, it is necessary to excavate more than needed for the waste itself, backfilling with one or more meters of compacted clay at the bottom or side or both of the excavation to seal off the permeable layers.

If the depth of cut is not limited by geological features, it is necessary to make an engineering judgment decision by comparing estimated soil requirements to complete the landfill and the depth of cut to obtain the soil, considering access difficulties as the trench gets deeper. Other factors are the value of the land and the difficulty of obtaining new landfills, proximity to waste generators, and the surrounding land use. If the landfill area is valuable and it is difficult to obtain a new landfill, one will want to maximize the space available by cutting deeper, etc.

There is a special landfill concept that can arise when determining the depth of excavation for a trench landfill. If the cut extends into groundwater, below the water table, and the leachate is not allowed to build up in the landfill by pumping it out, the landfill is called an inward gradient site. The concept is to control the leachate level within the landfill so it is always less than that of the groundwater surrounding the site. Groundwater flow will be into the landfill instead of having leachate flow out of the landfill to contaminate the ground water. A leachate collection system is required (along with leachate treatment and controlled discharge as to a wastewater treatment plant, etc.), and if such a site is not in silt or clay soil, a liner of such soil will be necessary to limit the inward flow of groundwater. This design concept is to be used only after careful study and upon assurance of continued leachate control over many years.

Sidewall stability is a critical factor in trench design and is a function of the characteristic strength of the soil, depth of the trench, distance between trenches, and the slope of the sidewall. It is best to have a geotechnical engineer determine the sidewall slope to avoid slippage and the attendant hazard to workers, but in general the slope should be no steeper than 1/1 in clays and 2/1 (horizontal to vertical) in less stable soils. Other factors that may affect soil stability and permissible steepness of sidewall slope are weather, soil moisture content, erosion potential, and the length of tie the trench is to remain open.

The remaining dimension of the trench is length. Typically, this is a function of the volume desired, where the volume is such as to accommodate one to two years of solid waste per trench. In this way, most of the excavation for each trench can be timed to be performed during the months considered best for excavation (not wet or not freezing, for example) or when excavation equipment is more available on a regular basis. If the area is subject to seasonal windy conditions, waste can be placed at the lowest and most protected portions of the trench during that period, etc. Knowing the approximate volume of landfill space required per year, the depth of cut, and the width of the working face, the design engineer can adjust trench width and length to produce a reasonable shape within the overall dimensions of the site. It is common, but by no means necessary, for the length to be 5 to 10 times the width of the trench.

Since the amount of required cover material is a function of the width of trench, theoretically the trench should be as narrow as possible. However, because width must be adequate to permit dumping and accommodate the compaction equipment, practicality demands that the trench be sufficiently wide to accommodate the number and types of vehicles that use the fill. Because of the cost and difficulty of road access to the lower portions of the trench and in consideration of the cost of excavating deeper trenches to gain volume if the trench is narrow, it is common for the trench to be several times wider than the working face. In general, the width of a trench should be an even multiple of the width of the working face.

Alignment of the trenches relative to the prevailing wind exerts a significant influence on amount of blowing litter. The alignment most effective in terms of reducing the amount of blowing is one that is perpendicular to the prevailing wind.

To ensure drainage, the bottom of the trench should be sloped along its length. If the climate is wet, the first lift will involve bringing the waste to the top of the working face, and it is probably best to start landfilling at the higher end of the trench where it should be drier. This is especially true if reasonably impermeable cover soils promote runoff of clean water over the completed cells to the base of the trench, where it can be collected and pumped out to the low end. Water falling on the open working face will be absorbed by the waste. Any water that is collected at the bottom of the trench should be tested and pumped out of the trench to surface water if uncontaminated, or perhaps onto the working face, or it should be treated prior to discharge to surface water. Refuse should not be deposited into standing water. Surface water can be diverted from around the trench by constructing temporary berms on the sides of the excavation.

Depending upon the projected size of the fill, trench excavation may, be done either continuously at a rate adjusted to landfilling requirements, or periodically on a contract basis.

The completed trench landfill will typically have 1/2 to 1/3 of its refuse depth below the original ground surface with the remainder above the original surface. It must project above the surface to assure slopes to promote surface runoff of precipitation. Accordingly, the last phase of a trench landfill involves placing waste over portions of the previously filled trenches to bring the site to its final grade as shown in Figure 1. The designer must assure that sufficient cover soils are obtained from trench extraction, or from other sources, to complete final cover and berm requirements.

Area Method

The area method is most often used on gently sloping or hilly land. It can be adapted to quarries, strip mines, ravines, valleys, canyons, or other land depressions. Unlike the trench method, the area method involves no excavating to obtain space for placing the solid waste and so is an easy method to follow at least in concept (Figure 2). In the area method, a layer of waste is spread and compacted on the surface of the ground (on the inclined SIOPC). Consecutive lifts are then constructed until the desired height of the landfill is attained. Final cover soil is then spread and compacted over the entire mass of waste.

Cover soil is obtained from on or off the site, perhaps from construction activity nearby, etc. If the land is flat or rolling, the soil may be scraped up near the working face or transported from a borrow area on the site. If a canyon, gravel pit, or other form of depression is involved, the soil may be taken from its sides or from a borrow pit. If the soil available on site is not appropriate (gravel, for example) proper soils should be carried in from another area. The landfill designer must estimate all soil requirements for cover and berm construction, etc., and determine that sufficient soils are available to complete the landfill. A good designer will also match the soils available to uses, so the best soils are marked for each use (for example, sands and gravels for drainage, top soil and clay for final cover, etc.).

Width and length of the working face depend on characteristics of the terrain, volume of solid waste delivered daily, and the number of trucks that will unload at one time. The working face dimensions should be adequate to accommodate delivery trucks and the solid waste as received.

Ramp Method

The ramp or progressive slope method is a variation of the area method in which cover material is excavated directly in front of the working face and spread and compacted over the waste. As it does not involve the importation of cover, and as a portion of the waste is deposited below the original surface, the ramp method promotes greater efficiency in site utilization when only a single lift is constructed. It is only, applicable to one lift, however, and in practice it is difficult to provide a consistent excavation pattern which yields sufficient soil for final cover on the top and possibly one side, and daily cover on the working face. It is also difficult to control top grades to provide desired slopes with no low spots where surface water ponding can occur, and no steep slopes which can erode. This method is useful only when land is cheap and readily available. It also, obviously, cannot be used if a liner is to be placed for gas or leachate control.

LANDFILL DESIGN REQUIREMENTS

Cover Soil

Cover soil is used to confine the solid waste, improve the appearance of the landfill, control access to the waste (by humans but also by animals and insects), reduce odor, promote runoff of surface water, and to seal the waste from the open environment and support vegetation growth over geologic time. Clearly, the cover required to simply hide the waste is much less than that required to protect the waste over geologic time. Accordingly, there are typically three levels or types of cover used at a landfill: daily, intermediate, and final cover.

Daily cover is used on the working face to seal it until the next operating day. It may also be used on top and sides of the daily, cell if these areas are to be exposed less than perhaps 30 days. Since it is a temporary cover, daily cover is only a thin layer sufficient to improve the appearance of the landfill and control the waste to reduce odors and to slow down or discourage access. Depending on the smoothness of the compacted waste, 15 cm of compacted soil may be sufficient to hide the waste. Adequate cover in general, but daily, cover in particular, is an obvious indication of a well-run landfill. It reflects the level of competence and concern of the owner and operator, and greatly affects the morale of workers and acceptance by the public. It is critical to sustained acceptable landfill practice.

If cover is exposed to erosion or traffic, or is meant to protect the waste for more than a few weeks, additional soil should be used. This cover is called intermediate cover. A thickness of 30 cm is common. Such cover should be sloped to promote runoff. In dry areas or seasons, or in areas not subject to wind erosion, daily cover may provide adequate protection for longer periods; however, such is usually not the case and intermediate cover should be used on all waste except at the working face.

Daily or intermediate cover can use virtually any type of soil, although a silty sand or loam is often considered best. Clay or fine silts can be used, but can be difficult to spread and compact under wet or dry conditions, and access can become very difficult under wet conditions.

Final cover provides the top and sides of the landfill with a seal to protect the solid waste from the environment "forever", in other words over geologic time. Accordingly, it must be carefully designed and placed to minimize long-term problems and provide maximum protection. It can be a complex system of different soil layers, ranging in function to support vegetation, minimize erosion, promote surface water runoff, promote moisture or gas flow and retard moisture or gas flow. Some layers are there simply to protect other layers. As suggested from this list of sometimes contradictory functions, final cover must be designed based on climate, size of the landfill, surrounding land use, final use of the site, etc.

One of the most critical functions of final cover is to promote surface runoff and to retard downward flow of water into the solid waste where it becomes leachate. To minimize leachate formation, final cover is designed to minimize downward flow of precipitation.

Figure 4 shows nine different layers that can be considered for final cover, depending on the situation.

The most basic design of a final cover, however, only contains two layers: 1) the surface or vegetative support layer, and 2) the hydraulic barrier layer (Figure 5). It is advisable to use a thickness of at least 60 cm for the surface layer and 30 cm for the hydraulic barrier. This design would be acceptable in areas with high evaporation and low rainfall, (i.e., warm and dry,) and is depicted in Figure 5. In other climates where additional protection is needed as in humid areas, it may be necessary to include additional soil or additional layers. In particular, the hydraulic barrier in wet climates should be at least 60 cm thick.

In order to prevent the downward flow of water, the cover must be designed such that the major fraction of rainfall and melting snow become run-off. This can be accomplished by building a cover having a slope no less than 5 percent. This incline promotes the flow of water over the cover; however, this slope is not so steep as to promote erosion. Erosion is also reduced by establishing vegetation. Vegetation, in turn, promotes evapotranspiration (where moisture from the soil is released to the atmosphere through plant uptake and evaporation). Thus, slope and vegetation play an important role in the performance of the cover. If the grade is well controlled and if minimum settlement is expected, a slope as little

as perhaps 3% can be used, but such a flat surface provides a tendency for ponding of surface water. There will be a tendency to produce more leachate with a greater threat of ground water contamination.

The maximum slope to reduce erosion and long-term maintenance problems depends on rainfall amount and intensity. In general, a maximum of 4/1 (horizontal to vertical) has been found to be reasonable, with as steep as 3/1 for dry areas or areas with less potential erosion, or where upkeep is no problem. In areas subject to extensive erosion, a maximum slope of even 5/1 may he prescribed. In all cases, it may be necessary to place a series of ditches and berms to intercept surface water every 30 meters or so along the steeper slopes to reduce erosion problems.

 

In basic final cover design, the hydraulic barrier is below the surface or vegetative support layer. The hydraulic barrier is the component of the cover specifically designed to prevent the passage of liquids into the waste. In the event that the layer of top soil (vegetative support) does not have a sufficiently high permeability to allow lateral flow over the hydraulic barrier, infiltration can be substantially reduced by the incorporation of a lateral drainage layer above the hydraulic layer as shown in Figure 4. The introduction of the drainage layer into the design brings about additional degrees of safety, complexity, and cost. This is because the drainage layer must be accompanied by a filter zone. The filter zone consists of a layer of carefully selected soil. This layer, as its name implies, serves the purpose of preventing downward motion (filtering) of small soil particles from the vegetative layer into the drainage layer. These particles would eventually clog the drainage layer.

Finally, if brush and tree growth is promoted and burrowing animals are present, it would be necessary to include a biotic barrier. This barrier is generally located between the filter and drainage layers. The biotic layer is designed to prevent damage to the hydraulic barrier due to tree roots or animals.

The individual layers are, in order from top to bottom, as follows:

Surface (Vegetative) Layer. This layer is needed to protect the cover from erosion due to wind and water flow. This layer should be made up of nutritive and dense top soil in order to support plant growth. This material can be mixed with composted yard debris, sludge, or animal manures. A typical minimum thickness is 15 cm, but when used alone over the barrier layer, the thickness should be sufficient to protect the barrier layer from roots, drying and desiccation, freezing, most animal burrowing, etc., in which case local conditions must be taken into account. A thickness of 60 cm is usually adequate.

Filter Layer. When fine soils are placed above coarse soils, there is potential for the migration (piping) of the fine soils into the voids of the layer of coarse grains. This phenomenon results in the plugging of the coarse layer. Filter layers are used to remove fine particles from infiltration and to allow upward flow of landfill gases. Soil of appropriate particle size to prevent piping according to standard geotechnical criteria are used for this purpose, usually, 15 cm thick. In the event such soils are not available, geotextiles may be used. Unfortunately, geotextiles are expensive and so may be prohibitive for many landfills. In the absence of a filler layer, an extra thick drainage layer may be substituted so at least the lower portion of the drainage layer is hopefully, not plugged by fines.

Biotic Barrier. The integrity, of the hydraulic barrier must be maintained in the design of the final cover. Plants and animals can perforate the hydraulic barrier and thus ruin the design.

One method of controlling this potential problem is through frequent mowing and pruning of plants and through the use of rodenticides. Another method of control is through the installation of a biotic barrier. A biotic barrier consists of a layer of construction debris or crushed rock of such size to prevent or minimize the movement of plants and animals. Typically, a 1 m thick biotic barrier should be adequate.

Drainage Layer. The design of a final cover should, in most cases, incorporate the design of a drainage layer. The few exceptions would be in arid areas where precipitation is very low. The purpose of this layer is to intercept the downward flow of water and drain it laterally before it can penetrate the hydraulic barrier. As shown in Figure 4, the layer must slope downwards to collection points on the perimeter of the landfill. The layer should be made of porous material, such as sand. A typical thickness is 15 to 30 cm.

Hydraulic Barrier. This is the most important layer of a final cover. The main function of the hydraulic barrier is to prevent the movement of precipitation into the solid waste to become leachate. In industrialized countries, these barbers are made of fine-grained carefully compacted clay soil. If clay is not available, coarse soils can be mixed with other materials such as bentonite clay or fly-ash in order to attain the desired permeability. The success of the final coyer depends upon the maintenance of integrity of the hydraulic barrier.

The integrity of the hydraulic barrier can be destroyed by desiccation, freezing and thawing, animal burrows, worm holes, vegetation roots, and settlement or physical movement of the cover as the waste consolidates and decomposes. To reduce the impact of these problems, and to reduce the likelihood of the entire thickness of the barrier being affected, the barrier layer should be at least 30 cm thick in dry or non-critical climates, or 60 cm to 1 m elsewhere. A typical hydraulic conductivity requirement is less than 1 x 10-6 cm/sec as compacted in place, or less than 1 x 10-7 cm/sec., for wet climates or in areas where ground water quality is important. The hydraulic barrier is the most effective long- term means to control or limit leachate generation and hence groundwater contamination; hence, care should be taken to design, construct and maintain it properly.

A synthetic membrane can be used in place of soil as a hydraulic barrier. These materials may be prohibitively expensive for many applications, plus they are difficult to install and maintain. If a synthetic membrane is used, it must be properly protected from mechanical damage both during construction and operation by installing an adequate base layer under it and a protective layer such as sand on top. The base must be smooth, with no large or sharp objects. Frequently, to limit flow through the inevitable imperfections in the membrane or seams, a clay hydraulic barrier is also used under the membrane.

Foundation Layer. The foundation layer is designed to support the hydraulic barrier and to provide a smooth surface for it. It also supports the load from the cover. This layer is made of compacted coarse-grained soil placed on top of the uppermost waste lift. If gas control is of concern, this layer may include coarse soil or be over a layer of very coarse soil or construction debris in which is placed pipes or vents to allow gas collection and flow from the landfill. This layer forms part of a static or dynamic gas collection system.

One of the main concerns in the design of a final cover is subsidence or settlement due to consolidation and decomposition of the wastes. Thus, one of the most effective means of protecting the foundation layer, and therefore the final coyer, is by ensuring that the wastes are thoroughly compacted. Where good waste compaction is rendered, settlement may decrease the landfill's final depth by 15 percent. With poor compaction, settlement increases to perhaps 25 percent.

LOCATION OF THE LANDFILL WITHIN THE PROPERTY BOUNDARY

Locating the landfill within the property at first may seem to be simple. If land is scarce and the designer wishes to make maximum use of the land, it is easy to design the waste boundary, called the "footprint" of the landfill, to be as close to the property boundary as possible. A few square meters are reserved for the access road, a gate house and, whatever structures are needed for equipment, otherwise its all landfill. Unfortunately, however, this often leads to problems both during operation as well as after closure.

In general, a space of 20 to 30 meters is needed between the waste footprint and the property boundary. The area is used for access roads, which are needed around the entire landfill for maintenance of slopes and vegetation, both during operation and after closure. The roads also provide access to monitoring devices for ground water or gas, and for movement of waste or soil. Besides roads, space around the waste footprint is needed for excavation, surface water drainage berms and ditches, surface water holding ponds, fences, trees or other visibility screen plantings, cover storage piles, etc. Even more space is usually needed at the entrance area. The designer must plan for all of these site requirements by setting aside appropriate space and designing the roads, berms, etc. at the outset that will be needed both during operation and after closure. Additional space may be needed to separate the waste from particularly sensitive adjacent property uses, such as for residences or parks, for example.

Berms or soil stockpiles should be designed to limit visibility of the landfill. Line of sight drawings should be made to aid the design of fences, tree lines and berms or stockpiles to shield the operation from view. Fences can be as high as 4 to 5 meters, which is good to limit visibility, but if additional height is needed soil piles or berms should be built and vegetated, perhaps with a tree line or a fence on top to control visibility. Since berm slopes should be no steeper than 2/1 horizontal to vertical to promote vegetation and limit erosion, it is easy to have several meters of the area around the footprint taken up by such structures alone. Many designers prefer 3/1 slopes on the exterior side of berms depending on climate, etc.

The last decision regarding waste footprint location is depth and slope of the base of the landfill. The depth of excavation was considered previously for a trench landfill, but many of the same considerations hold for area or combination trench and area landfills as well.

Clearly, the depth of excavation is limited by the location of groundwater or bedrock, or permeable layers of soil, and the guidelines given for a trench landfill apply to all designs. Even for an area landfill, some excavation for removal and stockpiling of topsoil, for smoothing the base of the landfill, for placement of any liner or retardation layers, or for grading or sloping the base, will be necessary).

The base of the landfill must be sloped, typically at a minimum of 2% if a liner is used to retard release of leachate downward and to collect the leachate. The leachate must flow horizontally to a low point from which it can be pumped or drained from the landfill. Even if no liner is used, however, it is usually, wise to grade the base of the landfill at a similar slope to promote drainage of rainfall during operation so it does not pond and cause operational difficulties. This also promotes some lateral leachate flow to a low point from where it can be pumped or drained, if necessary, at a later date to protect groundwater quality. Obviously, if the soil is very permeable so little lateral flow will occur and operational problems are unlikely during wet weather, such sloping is less important.

The slope of the base should normally follow the general slope of the original ground surface because this is usually the slope of the groundwater as well. This maintains a separation distance between the waste and the groundwater. Such a slope also reduces excavation requirements and brings both surface water and any leachate to the same part of the site for monitoring or control. Exceptions would be if cover soil is needed so excavation must be designed to maximize the amount of soil obtained, or if the closest sewer line is near another part of the landfill. Even if a liner and leachate collection system is not planned, placing the low point where any leachate will collect at the best location for possible removal for treatment is good planning.

Finally, both location of the footprint and base elevations of the landfill should take into account natural or existing features of the area to make use of them or to minimize disruption or loss of desirable features. For example, trees, hills, ponds, etc., should be maintained if at all possible for visibility, noise and dust control, and in the case of ponds to improve the appearance of the site and to provide for sedimentation of suspended matter in surface water runoff prior to drainage off-site.

If a hill is to be constructed, there are a few suggestions which should be considered. Since the upper lifts will involve potential visibility, and waste control problems, it is wise to construct a perimeter berm of cover soil around each lift, then fill the interior, constructing the next perimeter berm before the next lift is filled to provide visibility control and to better protect the waste from wind. If possible, the exterior slope of the berms should be planted with vegetation immediately after construction to reduce erosion and to improve site appearance. This is illustrated in Figure 6. Note that road access to upper elevations can be difficult. The road should be located on the side of the hill where it is most protected and the least visible, and should be left after closure to provide access for long-term maintenance.

Final Use and Topography

If plans are to use a sanitary landfill for some specific purpose in addition to waste disposal, it may be advisable to adapt ("customize") the landfill design to fit both the waste disposal and the desired use. Examples are topographical contouring, land reclamation for agricultural use, reclamation of aquatic environments, strip mine reclamation, and gas (methane) recovery.

 

 

 

 

 

Topographical Contouring

An example of topographical contouring is the construction of a fill that becomes a hill in a land area unrelieved by variation in elevation. An advantage of such an approach is more efficient usage of land area, i.e., more waste can be disposed within a given area. The completed fill would form a vertical series of more or less circular lifts tapered to achieve the contour of a hill. The area method would be used in the construction of the lifts, as shown in Figure 6.

It should be emphasized that constructing such a hill is difficult because of the following:

· It is inherently easier to operate equipment on level ground than on a slope; and equipment "wear-and-tear" is less.

· Although the slope may be within the angle of repose, some slippage takes place during normal operation. The slippage intensities the difficulty, of achieving the degree of compaction required for the refuse and cover material.

· Blowing of litter is accentuated.

· Abrasion of soil cover by wind, and erosion by downflowing surface water during rainfall easily reaches problem levels.

· To the usual problems encountered in establishing a vegetative cover on a completed fill must be added those of planting and maintaining vegetation on a hillside.

· Scavenging activities will be hindered as access to landfill site is made difficult by an incline. Also, problems may arise in transporting containers, pushcarts and other vehicles which may be used to convey items retrieved from the site.

Land Reclamation for Agriculture

Sanitary landfilling designed to accomplish land reclamation for agriculture combines satisfactory waste disposal with very practical land reclamation. The approach is applicable to a wide variety of situations. Examples are abandoned quarries, problem canyons, strip mined areas, agricultural lands no longer workable because of excessive soil erosion, and other land areas severely degraded through exploitation of natural forces.

Despite the diversity suggested by the preceding list of examples, the method of sanitary landfilling recommended in all cases is essentially the area method adapted to fit the specific situation. For nonworkable agricultural land, a single lift may be sufficient, whereas, several lifts would be required for abandoned quarries, canyons, and exhausted strip mines. In all cases, the depth of the final coyer should be such that plant roots do not enter the buried waste mass before the .Wastes have been sufficiently stabilized. Required depth and type of soil will vary with the crop to be grown on the fill, but a common depth is 0.6 to 1.6 m.

Measures must be taken to prevent or minimize unfavorable impacts upon the environment. Precautions against ground water contamination by leachate are the same as those applicable to all sanitary landfills in general. Design concepts addressed to minimize or prevent adverse environmental impacts from leachate generation are describes elsewhere, but note that landfills in sand or gravel mines or in rock quarries have caused some of the worst groundwater contamination problems from improper landfill practice in the past. Such sites must have liners, or otherwise protect against groundwater contamination because of location in an arid climate or having a large depth to groundwater, etc. A good final cover with a well constructed hydraulic barrier layer is especially critical to minimize leachate generation.

If the landfill is to be used for agricultural purposes, the final cover should be sloped to drain properly, and the vegetation layer should be thick to support crops. A thickness of 2 meters or more should be adequate for this purpose. Steps must be taken to prevent or dissipate accumulations of biogas because of the safety hazards (fire and explosions) associated with such accumulations. In addition to the safety hazards, accumulated biogas is likely to inhibit root development. Gas control is presented in more detail elsewhere.

Reclamation of Aquatic Environments

Refuse is often dumped into rivers on the pretext of land reclamation (examples in China and India abound). Solid waste should not be disposed near potential sources of water supply. In some cases it may be acceptable to reclaim marshes and areas with pockets of water having high salinity. In these situations, the water should be removed or allowed to evaporate and the appropriate evaluations carried out (geological, hydrological, etc.). Consideration should be given to the ecological conditions of the site. Since this practice can result in severe contamination of surface water, it should be used only when necessary and with careful consideration of the design and operation to minimize and control impacts.

Surface Water Drainage

Good landfill design and operation requires surface water management. Placing cover and contouring the land to promote surface water runoff will greatly improve operations, especially in wet weather, and will automatically improve the appearance of the site and force planning, which will in turn improve other aspects of site management. Access roads that are muddy, washed out, flooded, or generally inaccessible certainly, impair operation of a landfill and may, force the use of a separate, unplanned dump area, or even worse, cause random dumping of solid waste. Mud cracking on nearby public roads will result, with the danger of accidents on slippery, surfaces etc. Water not drained from the working face will make access to the working face difficult or impossible, and will make it difficult to place and compact both the solid waste and cover soil. Ponding of water at the cover soil source will make excavation difficult. All of these problems can be minimized or avoided with good surface water control.

The overall requirement in surface water control is that all surfaces in the landfill should be sloped a minimum of 1 to 2% on natural, undisturbed soil, and 4 to 5% on surfaces over solid waste which are subject to settling over a period of time. Intermediate cover, which will be exposed for only a few months at most, may be sloped less, but even then should be sloped at least at 2 to 3% and smoothly graded to promote runoff.

The landfill designer will normally choose to use pre-landfill surface water drainage paths, and route landfill generates surface water to them. Obviously, they will be at topographic low points around the landfill property boundary. A check should be made to be sure these pre-exiting streams, channels, culverts, ditches, etc., have the capacity to continue to take surface water from the landfill property, and if the landfill design calls for changing drainage paths compared to the natural, pre-landfill situation, a careful check on the ability of these pathways to take additional surface water must be made.

Once off-site drainage locations and capacities are determined, berms and drainage ditches are designed, certainly around the base of the landfill but at other areas such as soil excavation or stockpiling areas, berms, etc., to allow no unplanned ponding of surface water on-site. Ditches draining large areas and subject to large flows may have to be lined or protected, and may need rocks or other devices to slow the water velocity and limit damage. Roads should be graded with a crown or high point in the center and ditches on both sides, with culverts under the road as necessary to drain water freely off-site.

One of the most difficult parts of landfill design is to plan surface water drainage every day, of the life of the landfill. At all times the landfill and working face, access roads, soil excavation areas, and soil stockpiles must be located to promote runoff. When low points are unavoidable, as when landfilling below the surface elevation in a trench, or excavating cover soil below the original surface, slope the excavation so even here surface water will run to a low point from which it can be pumped to the nearest (and planned) drainage path. In order to minimize the amount of water to be pumped, surfaces around the excavation are sloped away from it so only water falling directly on the trench or excavation needs to be pumped.

For the portion of the landfill above the surrounding land surface (hill), special care to minimize surface water contamination and erosion is necessary. In the past it was common to have each lift, and its intermediate cover, horizontal. The problem was that if this cover was less permeable than the solid waste, which is common, or as leachate and its constituents reduce the permeability of the cover soil, which is also common, water can accumulate in layers on intermediate cover. This leachate builds up and eventually can flow out the side of the hill, leading to "leachate seeps" or "leachate weeps". The result is surface water contamination, staining of the cover, limited vegetation growth and odors. Once this happens, it is expensive and difficult to repair, as a subsurface drainage system is required which may require additional repairs for many years. To avoid this problem, lifts should be designed to slope towards the center of the hill, keeping any leachate accumulation as far as possible from the sides of the landfill. Further, it may be useful in wet climates to excavate some intermediate cover, or use gravel, at designed low areas to promote downward flow and to limit ponding.

The other difficulty regarding surface water control in hill landfills is the problem of bringing large amounts of runoff from upper elevations to the drainage system at the base of landfill without causing erosion. Experience has suggested that cutoff berms and ditches be located every 30 meters or so along the steeper slopes, and that these structures be sloped at 5% or so to gradually collect and bring the runoff down to the base of the hill. For large landfills, a series of enclosed culverts or lined spiraling ditches with velocity lowering devices, such as rocks, will be necessary. No one ditch alone spiraling around the hill will be able to handle the required volumes of water. Even with these runoff control features, a slope steeper than 4/1 horizontal to vertical likely lead to erosion problems in wet climates and should be avoided.

The last surface water control device to be discussed is the sedimentation or equalization pond. Surface water runoff will unavoidably carry sediment, which may eventually clog off-site surface water drainage systems. A simple pond, with removal of sediment as necessary, will solve the problem. In addition, depending on rainfall intensity patterns and the ability of surface water pathways on and off site to handle water volumes associated with major storm events, it may be cheaper, or necessary, to promote on-site storage of surface water. Such a pond should be designed to handle a major storm, perhaps accepting the runoff from the entire landfill for release over time. Creative planning can place such ponds at locations where coyer soil is to be excavated anyway, and will also locate and shape the ponds to improve the appearance of the landfill. A good location, if land topography makes it possible, is near the entrance road, or along a major road, etc. The pond will need to be designed to allow pumping as well as access for sediment removal.

Phasing

It is not possible to construct the entire landfill over many years with all activities operating continuously. Trenches are prepared, areas of land are cleared and graded, cover soil excavations move from location to location, and portions of the landfill are completed periodically over the life of the landfill. To spread the cost over time, to minimize the area of the site exposed to excavation or filling, and to generally provide better control, the landfill is constructed in phases. A phase is typically a portion of the landfill taking one to three years to complete. Two years is common. If climate is seasonal, so one season is better for excavation, for example, this allows most of the excavation to be done at the best time of the year, etc.

Each phase is designed as a small landfill, coordinating all the activities such as excavation and base preparation; construction of berms, roads, and drainage systems; waste placement; soil excavation and stockpiling; and final cover of part or all of the phase within the two year active lifetime, for example. The phases are designed to work together in sequence so the entire landfill meets final contours and specifications at closure. Prompt final covering of each phase seals portions of the landfill as they are completed, promoting runoff and limiting leachate generation and providing excellent opportunities for visibility control and improved appearance during much of the operational life of the landfill. For this reason, earlier phases can be placed along major roads or on the most sensitive sides of the area. Once completed with final cover and vegetation, these earlier phases provide excellent protection throughout the remainder of site operation.

Fencing and Entrance Design

The function of fencing and the entrance area is to limit site access to people and vehicles with reason or permission to be there, to limit access to the designated entrances, and to facilitate movement of traffic into the site. It is possible to achieve good and safe landfill operation without limiting access, as in remote sites far from housing areas, but generally fencing or natural boundaries (such as railroad tracks or steep hills) will be necessary. If there is a charge for waste disposal, or if the site is open only part of the day, people will bring in waste after hours or without paying, and will undoubtedly not place the waste in the working face. This leads to piles of exposed waste around the landfill, and the accompanying difficulty of maintaining a good operation. Fencing also helps control blowing litter, providing it is cleaned frequently so litter does not blow over the fence. A 3 meter mesh fence around the site is good for both access and liter control.

The access area should be clearly marked with the landfill name, owner or operator, hours of operation, fee structure, and any, special rules regarding acceptable or prohibited waste, etc. It should have a lockable gate and should have adequate space for vehicles to wait in line on-site and not on public roads. The road should be paved to minimized mud tracking and problems with vehicle movement in wet weather. There will usually be a gate house, with a scale in some cases, to control access, collect fees and provide instruction. The entrance area gives a great opportunity for innovative design to improve the appearance of the landfill to the public and to promote a sense of pride for persons using or working at the facility. A clean, well-maintained entrance area is related to the care people will take to maintain the rest of the landfill. Berms, vegetation, fencing, curved roads, sedimentation ponds, and topography should be used to maximum advantage. Gardens and a park-like atmosphere are common at well-designed and operated entrances, and can be used innovatively for special floral displays, etc., if carefully designed so as to not interfere with landfill operation and vehicle movement.

The turn-in area from public roads to the landfill should be designed to minimize accidents and promote traffic movement. Special turn lanes may be needed at some larger landfills. The on-site entrance road should be at least 8 meters wide to accommodate two-way traffic, and should be sloped to promote runoff with drainage ditches on one or both sides. It should be cleaned frequently, especially on wet or very dry and dust), days.

Roads to the working face will range from semi-permanent over portions of the landfill to

be used throughout the life of the site, to temporary in areas providing only access to the working face. This presents a design problem, for the road system must allow easy vehicle movement under all weather conditions, yet be financially and technically feasible. Roads to be used for several years, especially if they are included in the final use of the site, should be permanent and normally be paved. In some cases a gravel road will be satisfactory, but note that gravel roads are more difficult to clean as mud and dirt is tracked onto them. Temporary roads to the working face can be gravel, or in dry areas hard packed soil. Semi-permanent roads, between these two extremes, may be used over a period of months to years, according to the landfill phasing sequence. Depending on weather, the amount of traffic, and eventual use, if any, they may range from paved to gravel, but oftentimes may be constructed with selected incoming wastes, such as broken road pavement, broken concrete, demolition debris, excavated soil, or certain industrial wastes such as combustion residues, etc. The designer should evaluate wastes entering the site, and wastes which could enter the site if necessary, and be sure proper procedures and adequate equipment are available to make prompt and controlled use of such wastes. Piles of road-building materials stored for later use can be unsightly and should be controlled accordingly.

All roads should be clearly marked to route traffic to and from the working face. They should be elevated and sloped or crowned to promote runoff to ditches on both sides, and culverts should be placed under them to move surface water to a sedimentation pond or directly off-site. They should be watered on dusty days, and cleaned, especially nearer the entrance area, to avoid mud tracking and to promote vehicle movement. They should be at least 3 to 4 meters wide for one-way or small amounts of traffic (with passing areas as appropriate), or 7 to 8 meters wide for two way traffic. Some landfills prone to mud tracking problems may have special wheel cleaning locations, so trucks don't crack mud from the working face. These devices can include a wash pond, a water spray, mud knock- off bumps, or a long paved road (which is frequently cleaned). With sticky clays, however, even these devices may prove inadequate. Local experience is the best guide for what will work -- the function here is to simply point out that the designer must consider the need for such devices.

Groundwater and Gas Migration Monitoring

Depending on local regulations, groundwater use, the proximity to buildings and built-up areas, and the types of soil and location of groundwater, it may be important for the designer to place monitoring probes around the landfill. Monitoring wells should be placed at least up and down gradient of the landfill, and in the direction of any nearby wells, and gas probes should be placed in the directions of nearby buildings. The design of wells and probes is covered elsewhere, the point here is to emphasize the importance of getting background soil gas and groundwater quality information before any landfill activity takes place. If problems develop in the future, it will be known whether the landfill is the likely source, which in turn will help determine who is responsible and how to best solve the problem.

PROVISIONS FOR SCAVENGING

Introduction

Since sanitary landfilling is the subject of this course, the present section focuses on material recycling (scavenging) performed at the landfill site and does not include scavenging at the point of waste generation, during collection, or during transport.

Presently, the sequence commonly followed with respect to scavenging at the disposal site is as follows:

1. Incoming refuse is dumped, as usual, at or near the working face, i.e., immediately behind or at the foot (toe) of the working face.

2. Scavengers sort through the dumped load.

3. Scavengers separate the retrieved materials into organized lots.

4. Machinery spreads and compacts the waste remaining after the scavenging activity.

Although this discussion of scavenging is restricted to that which takes place at the disposal site, it does not affect fundamental arguments for or against the practice as a whole. Typical materials recycled in this manner include: unbroken bottles, metals, plastics, cardboard, paper products, textiles, and glass.

Associated Issues

The case for scavenging must be strong enough to counterbalance the objections that can be raised against it at the site. These objections stern from the safety hazards to personnel of both the scavenging group and the landfill employees, and from the interference caused by scavenging activity that prevents the efficient conduct of work at the fill. Scavenging activities have severe negative impacts on the productivity of equipment as well as the efficiency, of operations in general. Hazards caused by the intermingling of manual scavenging activity and equipment-oriented sanitary-landfilling activity increase when heavy equipment is involved. Furthermore, scavenging results in delays and often interferes with compaction and application of soil cover. Therefore, the problem is essentially one of developing a safe interface between scavenger and landfill equipment that allows for efficient operation of the landfill.

Designation of a Separate Scavenging Area

The problem of developing an interface between scavenging and landfill operations can be minimized or even eliminated by treating the scavenging activity as a first step in a sequence of steps that make up the landfill activity. Such an approach makes feasible a physical separation of the two activities of perhaps one or more kilometers. Unfortunately, such a separation adds a step to the overall operation. Solid waste handling now has two parts: 1) discharge of incoming wastes at the scavenging area of the disposal site, and 2) transfer of the residue remaining after scavenging to the burial site.

If the scavenging area is kept relatively close to the burial site, transfer of residue from one site to the other may be done quickly by means of a bulldozer. Such an arrangement would demand that the scavenging area be movable to be close to the working face. Unfortunately, this is probably so close as to cause mutual interference between man and machinery. The other extreme would be to locate the scavenging area a kilometer or more away from the working face. In this case, the waste to be disposed could be transported by means of dump trucks.

A fixed scavenging site for the life span of the fill would be desired when transfer by bulldozer is no longer feasible. A fixed scavenging area would be neither feasible nor advisable for a small disposal site. Dedication of a fixed portion of the disposal site for scavenging takes on many, of the characteristics and advantages of a transfer station. For instance, scavenging done in a fixed area can be sheltered from the elements (wind, rain, etc.) and undesirable impacts upon the environment can be avoided or minimized. The operation itself can be kept orderly and controlled closely, and abuses can be discouraged. Furthermore, efficiency can be improved by including a certain amount of mechanization

conveyor belts and screens). Best of all, encounters between scavengers and landfill equipment are more easily avoided. These advantages combine to enhance efficiency. This alternative also allows for sanitary facilities and a better working environment for the scavengers.

The strongest objection to designating a fixed site is probably the added step of pick up and transfer of waste to the working face. This objection does not come into play until the distance between the scavenging and burial sites becomes great enough to make transfer by bulldozing no longer feasible. Of course, the capital expenditure associated with the erection of a building and introduction of added equipment would be another disadvantage. From the preceding discussion it can be noted that the size of the disposal site is the decisive factor regarding the advisability and necessity for dedicating a portion solely to scavenging. In general, a minimum life span of 10 years would justify the incorporation of a fixed scavenging area.

Management of Scavenging Activity

Important factors when managing scavenging activities are the relative priorities of the scavenging and waste burial activities. Burial should have precedence over scavenging since the main purpose of the fill is the effective disposal of wastes. Therefore, scavenging must be managed in a way that does not unduly interfere with the disposal activity of the landfill. Alternately, consideration must be given to the potential income from scavenging for the scavengers, who are generally at the bottom of the economic ladder, as well as the importance of secondary materials to local industry.

Traffic

Unless carefully managed, traffic to and from the disposal site can be disruptive to the interface between scavenging, and burial (disposal). Among the obvious causes of

disruption are the increase in number of vehicles using the same road and the different moving speeds that result from the different types of vehicles involved. Scavenger vehicles

may be as small as a pushcart or as large as the vehicles used to transport the larger loads of recycled materials. Conversely, waste collection and haul vehicles normally surpass scavenger vehicles in terms of size, weight and speed. Unfortunately. the best way to separate the traffic is to provide separate access roads, but this could be an expensive approach.

The degree of access to the disposal site by scavengers depends upon the magnitude of separation between scavenging traffic and disposal traffic. If separation is complete, the access could range from unlimited to somewhat limited. Alternately, if the two traffic patterns are not separated, unlimited access is immediately ruled out because of the excessive interference with disposal traffic. If access is to be restricted, the problem arises as to which individuals are to be excluded. In arriving at such decisions, it should be remembered that political and social expediency would inevitably enter into any decision that would limit access.

Supervision

The scavenger activity should be under the direction of a supervisor who has the responsibility to see to it that the activity proceeds efficiently and fairly, yet with a minimum of interference with the disposal operation. Accomplishing the latter implies working closely with the director of the disposal operation. The latter should have the final say in decisions that affect the disposal operation (landfilling). The supervisor of the scavenging activity may be assisted by subordinates, if efficiency of operation requires such a provision. Efficiency and safety demand that good housekeeping be rigorously enforced.

Guidelines

A relatively fixed set of guidelines should be established. Among the subjects that could be regulated are:

1. Assignment of space, refuse loads, etc., to individual scavengers or groups thereof.

2. Removal of scavenged material from the site - - i.e., the promptness, frequency, and manner in which everything from separation of scavenged material to loading and hauling by cart or motorized vehicle is performed.

3. Ideally, the municipality should be responsible for the sale of the recovered materials.

4. The laborers should be provided with uniforms and safety equipment, bathrooms, showers, eating facilities, and first aid equipment.

The above guidelines should be enforced by the supervisor in a fair and responsible manner. As the supervisor may come under pressure to take bribes, however small, from different groups or individuals, the person in this position should be a scrupulous individual who is rewarded according to the quality and performance of scavenging activity.

PROVISIONS FOR SPECIAL WASTES

Baling

Because of the technology involved and its high costs, the baling of municipal wastes is generally not a practical disposal option for a developing country. However, because it may be possible under specific circumstances, this section briefly describes landfilling baled wastes.

Waste characteristics, in particular, moisture content, determine the cohesiveness and density of the bales. The optimum moisture content is between 15 and 25 percent. With the present baling technology and suitable moisture content, densities of bases range from 950 kg/m3 to 1130 kg/m3. Bale dimensions range between 0.9 and 1.2 m in the minimum dimensions and from 1.2 to 1.8 m in length. To keep recoil (expansion after pressure is released) at a minimum, baling pressure should be greater than 1.4 x 107N/m2. Even under optimum baling conditions, the volume of the bases eventually expands 10 to 15 percent.

The bales should be tightly stacked in the fill, usually with a fork lift, and covered with cover material. Equipment efficiency dictates that each lift be no higher than three layers of bales. Stability is attained by arranging the layers in a manner similar to bricklaying, in which each layer is offset so that the ends of bases in one layer are not directly under those in the next layer. Maximum stability requires that bases be stacked cross-wise from layer to layer or lift to lift. Each lift would then consist of three layers of bases covered with a thin layer of soil to accommodate truck and equipment traffic. The contours of the floor of the site should reflect the contours desired for the completed site.

Proponents of balefilling (landfilling of baled wastes) claim that the following advantages can be attributed to the use of baling in MSW disposal when the site is designed and operated properly:

1. Baling ensures a higher effective density, thereby reducing the land requirement and extending the useful life of a landfill.

2. The use of on-site equipment and personnel is less intensive in a balefill.

3. Damage to the environment is diminished. For example leachate strength is reduced because some percolating water is diverted to the spaces between the bases, diluting the leachate.

4. Problems related to vectors, dust, blowing litter, traffic, and moisture are considerably reduced in number and severity. For example, vector (birds, rats, flies, etc.) activity is notably diminished at balefills due to the smaller working face and the case of achieving complete daily soil cover.

5. Baling of solid waste improves the future usefulness of the disposal site by enhancing foundation- bearing factors. Also, the waiting period for land to stabilize is lessened.

Co-disposal

As the term implies, "co-disposal" involves the mixing of one type of waste with another and the subsequent disposal of the mixture. Although co-disposal as describes in this section applies no most types of non-industrial sludges, the following is directed primarily to sludges associated with the storage, treatment, and disposal of human body wastes (primarily fecal wastes). Examples of such sludges and wastes are those produced by a conventional wastewater (sewage) treatment facility, septic tank pumpings, sludge from the storage pits of unsewered public toilets, and nightsoil in general.

Despite the many hazards to public health and nuisances attributed to the practice, untreated

nightsoil is frequently co-disposed with municipal solid wastes in developing countries. These hazards and nuisances are amplified by the presence of scavengers and the prevalence of the open dump method of disposal. Although not as pronounced, the same hazards attend the open dump co-disposal of primary (i.e., raw) sewage sludge from a sewage treatment facility. The hazards can be substantially reduced by using good sanitary landfill practice.

In an operation involving co-disposal by sanitary landfilling, one approach is to deposit the sludge (20 to 30 percent solids) on top of the refuse at the working face of the landfill. The sludge and refuse are then thoroughly mixed and the mixture is spread, compacted, and covered. Liquid in the sludge is absorbed by the refuse. The mixing of the wastes must be done with care so as to not exceed the liquid holding capacity of the solid waste, otherwise a wet, muddy landfill will result. Sludges having a low solids content (2 to 4 percent solids) may be spray-applied from a tank truck to a layer of refuse at the working face. The refuse serves as a bulking agent, but once again we must be taken to not exceed the holding capacity of the solid waste.

It is clear that it is not easy to co-dispose sludges without greatly affecting the success of the facility as a solid waste landfill. The handling, placement and mixing of the sludge in reasonable proportions is key, requiring special design and operational provisions. Note that scavengers should not be permitted to come in contact with the wastes.

A different approach involves the use of sludge/soil mixture as an interim or final cover over completed areas of the refuse landfill. The approach has some advantages:

1. Sludge is removed or reduced from the working face of the fill.

2. Because of the nitrogen and phosphorus contents of the sludge, the mixture promotes the growth of vegetation over the completed fill area, thereby reducing fertilizer requirements.

3. The development of sanitation and erosion problems may also be mitigated. A major disadvantage is the limitation of this approach to well-stabilized, digested, sludge. The limitation arises from the incomplete burial of the sludge and its resulting exposure to the atmosphere and people.

An operational difficulty that may be encountered is vehicle movement problems due to the presence of and the high moisture content of the sludge. A possible solution is to mix sludge with ash from power plants or similar sources.

Hazardous Wastes (Secure Landfill)

Introduction

Hazardous wastes (mercury, and arsenic based wastes, pesticides, heavy metal waste, acid

wastes, oil-based wastes, cyanides, etc.) are equally dangerous and toxic whether in developed or developing countries. The place of origin or occurrence has no bearing on the degree of hazard inherent in a particular hazardous waste. The possibility exists that a given hazardous waste may pose a greater threat in a developing country, since "legal" definitions, standards, and safeguards tend to be more relaxed than those specifications found in a developed country, and because of the accessibility of sites to more people if located in congested areas or if scavenging is practiced. The result is that: 1) measures required in the disposal of hazardous wastes in developing countries should not differ materially from those imposed in developed countries; and 2) the "secure landfill" approach describes in this section applies equally, in developed and developing settings. The only differences would be those arising from conditions peculiar to the individual sites.

Definition and Specifications

A "secure landfill" is a sophisticated engineered earthen excavation especially designed to contain and prevent hazardous wastes from escaping into the environment. Therefore, a genuinely, secure landfill must have the following features:

1. Waste disposed is completely enclosed by a layer or liner of impervious material.

2. The distance between the bottom of the liner and the groundwater is sufficient to prevent contamination of the groundwater.

3. Leachate and all other liquids are not allowed to accumulate inside the containment layers.

4. Groundwater is monitored such that leakage from the fill can be detected.

5. The fill is located such that it is isolated from surface and subsurface water supplies. is free from flooding, earthquake, or other disruptions; and the site is not needed for other uses after the facility is closed.

Design

As with all sanitary landfills, design is largely dependent upon the hydrogeological characteristics of the site. Thus, if the distance to the groundwater table is substantial and the soils are very impermeable, compaction of the soils at the site coupled with the placement of single liner either of natural or of synthetic material would be sufficient to contain hazardous wastes. In such a case, soil or bentonite could serve as a natural material and polyvinyl chloride, high density polyethylene, or chlorinated polyethylene could serve as a synthetic material. If conditions are not ideal, but do meet minimum standards, it would be necessary to excavate the soil presently at the landfill site and replace it with a sand/gravel layer followed by a compacted clay liner, a synthetic liner, a leachate drainage layer, and perhaps even a second clay and drainage layer combination to form a so-called double liner system. In all cases, provision should be made for preventing the various wastes from mixing together and thereby triggering a chemical reaction (e.g., highly caustic waste with a strong acid waste). This is done by separating different areas from one another by forming subcells using earthen dikes.

Arrangements must be made for collecting and withdrawing leachate as it accumulates in the basin. This is done through a network of pipes installed in the drainage layer. Groundwater quality should be monitored by means of monitoring wells placed along the perimeter of the fill. Monitoring of groundwater should begin prior to any disposal of waste and should continued thereafter until the chances of a pollution problem become non-existent.

The design, operation, and monitoring of a secure fill is a highly sophisticated process which requires the participation of skilled professionals. Details of the various requirements of a secure landfill are given elsewhere.

The closure of a secure landfill must be designed such that total and complete decontamination of the facility is assured, and the completed fill does not pose a threat to the public safety and the environment. This objective is attained by adhering to the

following procedure:

1. At termination, cover the upper surface of the completed fill with impermeable soils, e.g. clays. This layer should be at least 0.6 m thick.

2. Cover this layer with a synthetic liner, if available, and then with at least 0.3 m of sand to provide horizontal drainage of percolate and to protect the impermeable soil layer and underlying wastes.

3. Cover the sand layer with a minimum of 0.6 m of vegetation support soil, of which at least the top 10 cm is topsoil. Then seed the topsoil to produce vegetation and to complete the closure operation. Leachate and gas collection pipes should protrude through the final cover.

The functions of a final cover with respect to hazardous waste containment are as follows:

a. minimize infiltration of precipitation

b. prevent contamination of surface run-off

c. deter wind scatter of waste

d. present contact of waste with humans and animals

e. promote surface drainage

f. minimize erosion

g. prevent build-up of gas pressures in the fill

h. accommodate settling and subsidence

i. protect the impermeable or barrier layer from freezing, drying, or any, other surface effects, and

j. support vegetation growth.

Finally, it is extremely important that the completed fill not be excavated in any way, since most buried hazardous wastes continue to be dangerous for extended periods of time, and the consequences of untimely exposure could be disastrous. A properly closed hazardous waste landfill may be utilized for general purposes, such as parking areas and open spaces. However, it is advisable that a hazardous waste site be closely monitored for surface cover quality, gas emissions, leachate collection, groundwater, erosion and other events for at least 30 years. This is an arbitrary time period which can be extended or shortened depending on site characteristics, the wastes disposed, monitoring results, and other pertinent mechanical information available.

 

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