Energy From Solid And Liquid Wastes - Vii

Lecture No: 15 15.1. Environmental effects Landfilling can have several negative impacts upon the surrounding environment both during construction (e.g. waste deposition) and after the landfill has been closed. The effects depend upon the conditions at the landfill, i.e., the waste composition and quantity, the quality of environmental protection activities, operation strategy, geographical location, hydrological conditions at the location and time. Figure 15.1 shows some of the most important environmental effects on soil, water and air caused by landfilling together with the typical distances over which the effects are significant.

Figure 15.1 Potential environmental effects on soil, water and air as a function of distance from a landfill 15.1.1 Atmospheric environment Global warming. Organic wastes deposited in landfills will typically decompose biologically under anaerobic conditions producing methane gas. Part of the methane will escape to the atmosphere and add to global warming because it is a much more powerful greenhouse gas compared to carbon dioxide. The CO2 produced from the organic wastes will not add to global warming, as the organic matter is in essence CO 2 neutral because it is synthesized via photosynthesis. Methane accounts for approximately 18% of the total quantity of greenhouse gases on a global scale (Christensen, 1998). Methane from landfills accounts for approximately 1-2% of global greenhouse gas emissions (Thorneloe 1996). Methane produced at landfills can be collected via gas extraction systems and used for energy production. This will reduce global warming potential, as the CO2 produced from combustion of methane is neutral with respect to global warming. Part of the methane that is not collected will be oxidized biologically in the upper aerobic layers of the

landfill cover. Preventing organic wastes from being deposited at landfills can also reduce methane emissions. Ozone depletion. Chlorine and fluorine containing gases released to the atmosphere are potentially harmful to the ozone layer. These gases are degraded photo chemically in the upper atmosphere producing free chlorine and fluorine that reacts with ozone and thereby deplete the ozone concentrations that protects the earths surface from the ultra violet rays of the sun. In connection with landfills the gases are primarily released from disposed refrigerators, freezers and other types pf cooling equipment, solvents and insulation materials. Many of the gases can potentially be degraded under the anaerobic conditions existing in landfills (Kromann et al. 1998) but since the gases are very volatile significant quantities significant quantities will escape to the atmosphere. Controlled collection and combustion of the landfill gas can reduce emissions of the ozone depleting gases. Toxic gases.

Landfill gas contains significant concentrations of compounds that are

potentially toxic to humans. These gases include mainly CO2 and H2S. Toxic gases are also present in trace amounts in the landfill gas. Here benzene and vinyl chloride, dioxins and furans are important due to their carcinogenic and toxic properties. Dioxins and furans are normally produced via uncontrolled combustion of the landfill gas. Benzene normally originates from gasoline and solvents disposed of at the landfill. Vinyl chloride is a degradation product from trichloroethylene, a solvent that can be degraded under anaerobic conditions. Vinyl chloride itself is not very degradable under anaerobic conditions and therefore has the potential to reach the atmosphere. Controlled collection and combustion of landfill gas will reduce the emissions of toxic gases to the environment. Odor. Problems with odorous and foul smelling compounds are typically significant only near the landfill. Important odorous compounds are H2S and organic sulfur compounds (mercaptans etc.). Odor problems are most significant during deposition of the wastes at the landfill. Odor can be a significant nuisance in areas near a landfill. Odor problems can be reduced by minimizing the amount of easily degradable material in the landfill, by keeping a small open waste front at the landfill, by operating as far away from inhabited areas during the summer as possible and by placing landfills under consideration of prevailing wind directions. Noise. Noise is one of the most significant nuisances near the landfill and is created by the traffic of waste trucks to and from the landfill. Also compactors and other large equipment in use at the landfill add to the noise problem. In special cases can birds especially seagulls create their own noise problem. Constructing noise barriers around the landfill area such as

earthen walls and dense plantations can reduce noise. Noise reduction can also be achieved by using equipment that creates less noise and restricting operation hours especially during seasons when resident uses outdoors facilities. 15.1.2.Soil environment Vectors. Landfills that receive organic (food) wastes usually attract animals and insects that seek food and tend to multiply in the area. It is especially flies, gulls, rats and cockroaches that are attracted to the wastes. Most of these animals can spread diseases and is therefore a hygienic problem. Large flocks of birds can also cause problems for air traffic. The presence of animals can be reduced by carefully covering the wastes after each day, using a thick top layer, using rat poison and by using bird nets over the landfill site. Fly waste and dust. Dust and fly waste (waste transported by the wind) can often be a nuisance near landfill sites. Dust is especially a problem at sites where ash and soil is deposited. Dust and fly waste can be reduced by using only a small open waste front, by watering dry wastes, by covering the wastes carefully and by regular cleaning of the landfill area. Fire and explosion hazard. Landfill gas can potentially cause fire and explosions, as the gas is highly combustible. The gas is explosive if between 5 and 15% methane is mixed with atmospheric air. This range is not very dependent upon the presence of other components in the gas (Gendebien et al. 1992). Landfill gas is normally not a problem with respect to explosion hazard if the gas is emitted directly to the atmosphere. It is however not uncommon that the gas can ignite and burn steadily at the location of emission. If the gas seeps into closed spaces such as basements in houses or sewers there is a potential explosion hazard. A spark from electrical installations can ignite the gas. Explosions in residential areas near landfills are not all that uncommon and people have been reported killed in such explosions. In March 1991 an explosion occurred in an older house near an old closed landfill at Skellingsted, Seland, Denmark killing two people. The house was constructed with wooden floors directly over the soil surface offering no gas flow barrier and was located 20 m away from the landfill edge. Figure 15.2 shows the weather pattern during the period. It is seen that the explosion occurred simultaneously with the passage of a low-pressure weather system and that significant rain fell the day before the explosion. The rain likely sealed the upper layers of the soil, restricting gas movement whereas the low pressure increased the gas pressure gradient across the soil formation. The gas was therefore forced to escape under the house (the only dry spot) and increased gas movement into the house. It is believed that a cigarette ignited the gas once concentrations became high enough. The fire and explosion

hazard can be reduced by collection of the landfill gas, by minimizing the amount of biodegradable waste deposited and by installation of gas alarms in buildings near the landfill.

Figure 15.2. Atmospheric pressure and precipitation variation in March 1991 when an explosion occurred in a house near an old landfill

Vegetation damage. Another aspect of landfill gas migrating into the soil formations surrounding the landfill is the displacement of the oxygen containing air from the soil pores. The gas can often cause displacement of oxygen from the upper soil layers including the root zone. These mechanisms cause damage to vegetation near the landfill because the root system cannot develop in an oxygen free atmosphere. This causes the plants to develop roots very near the soil surface, which results in vegetation damage and destruction during dry periods. Plants may also develop dwarfed growth patterns in such areas. Vegetation damage is often seen at or near landfills without gas collection as the landfill gas can migrate through the soil up to 100 m away from the landfill. Migration is most significant at older landfills without membrane systems. Gas migration also depends upon the surrounding soil type; sandy soils facilitate faster gas movement. Lenses or layers of low permeability materials in the soil can also cause farther gas movement away from the landfill edge. Variations in precipitation and atmospheric pressure also affect gas movement. Figure 15.3 shows methane and carbon dioxide concentrations in soil near the Skellingsted landfill as a function of time during the year 1999.

Figure 15.3 Methane and carbon dioxide concentrations in soil 10m from landfill edge at an old landfill near the Skellingsted.

Average background soil concentrations of methane and carbon dioxide at the location were 0 and 18 g/m3, respectively. Significant gas movement as far as 30 m away from the landfill edge as well as areas with significant vegetation damage were observed at the Skellingsted site. Vegetation damage can be reduced by collecting the landfill gas or by reducing the amount of organic waste deposited at the landfill. Soil pollution. Movement and deposition of contaminated dust (for instance from contaminated soil or ash) can pollute the soil near the landfill. Pollution can also spread by surface water runoff from the landfill. Soil pollution is best prevented by careful encapsulation of the waste and by irrigation of dry dusty wastes. Also surface water must be managed in a controlled manner to prevent erosion of the landfill surface.

15.1.3. Water environment Surface water. If the drainage system for percolate and surface water collection at the landfill site is overloaded for instance in connection with heavy rain or snow melting there is a chance that the contaminated water can reach nearby streams and lakes and cause severe damage to their ecosystems. Acute effects are oxygen depletion and ammonia toxicity. Effects of long-term contamination are changes in the flora and fauna of the water body and development of permanently oxygen free zones. Contamination of surface waters can be reduced by locating landfills away from lakes and rivers, by construction of trench systems for collection of runoff and by proper design and operation of percolate collection systems. Ground water. The ground water contamination potential is perhaps the most significant environmental hazard in connection with landfills. This has prompted the use of membrane systems and percolate collection at modern landfills. The contamination plumes observed at landfill are normally relatively short, less than 1 km (Christensen 1998) and they have a significant self-cleaning capacity due to high microbial activity. Because the use of membranes and percolate collection systems has been introduced some 20-30 years ago, percolate plumes in the groundwater has generally only been observed at older unprotected landfills. The knowledge about ground water pollution potential at modern landfills is therefore limited. Ground water contamination can be minimized by the use of membranes, percolate collection, limitation in the types of waste that are deposited and by minimizing the infiltration to the waste via the top layer. 15.2. Temporal duration of environmental effects The environmental effects caused by deposition of wastes at landfills have very different temporal duration. The temporal duration can best be discussed based on the state of the

landfill. The lifecycle of a landfill can be divided into three major phases. 1) The deposition phase, 2) The active phase, and 3) the passive phase. These three phases will be discussed in more detail in the following. It is noted that the division of the landfill life into these three phases is not a universal way of characterizing landfills but it is convenient as it is related to the emissions from the landfill as well as the conditions of the wastes. 15.2.1 Deposition phase The deposition phase is the period when the landfill is receiving waste. During this phase the wastes are built into the landfill and the different sections of the landfill are completed. During this phase also percolate and gas collection systems are being constructed and operation will start up as landfill sections are completed. The landscape is restored and vegetation is planted on the landfill cover. The duration of the phase depends on the capacity of the landfill and can typically vary between 5 and 50 years. For economic reasons it is desirable to have at least 15-25 years of capacity. Landfill sites for facilities this large, however are difficult to locate in many regions, the problems with locating suitable landfill sites also prompts a long life and efficient use of existing sites. The deposition phase is often divided into a sub-set of construction phases such that the construction of the entire landfill is not completed at once but is spread out over the deposition phase.

15.2.2 Active phase The active phase is the period after deposition has been completed but when the emissions are still significant enough to require active efforts for environmental protection. This is percolate and landfill gas collection, percolate treatment and energy production from landfill gas. It is difficult to assess the length of this phase as it depends upon the types of waste deposited at the landfill as well as the construction and condition of the landfill itself. The length of the phase may be determined based on the actual emissions from the landfill and the capacity of the surroundings to absorb the effects of the emissions. The emissions will depend upon the characteristics of the landfill, the waste, the size of the landfill, deposition technology and the time. The capacity of the surroundings depends upon where the landfill is located, the distance to and the type of environments near the landfills as well as the political regulations for land use and environmental protection in the area. 15.2.3 Passive phase When the activities for environmental protection are no longer operated actively, the landfill enters the passive phase. During this phase a proper choice of landfill site, past restrictions placed on the types of wastes deposited at the landfill, deposition technology and passive

environmental protection will ensure that the emissions to the environment are kept at an acceptable level. The passive environmental protection can for instance consist of sloping surfaces with good vegetation that will reduce the infiltration to the waste and oxidation zones in the top layer where landfill gas can be oxidized biologically. This phase will continue for as long as the emissions from the landfill are larger than from the surroundings. Because the emissions from the landfill are larger than the surroundings during the passive phase and the acceptance of these emissions is based on assumptions and past knowledge and that the passive phase covers many years into the future it is very likely that the passive phase will include monitoring of the landfill emissions. 15.2.4 Environmental effects The emissions and the environmental effects caused by them have very different temporal duration. Effects such as noise, dust and animals are linked to the presence of exposed waste materials and these effects are therefore only present during the deposition phase. Effects such as global warming and fire hazard on the other hand can be present during all three phases. Figure 15.4 gives an overview of the likely temporal duration of the most significant environmental effects of land filling.

Figure 15.4. Potential temporal duration of environmental effects caused by land filling of solid wastes

Lecture No: 16 16.1. Landfill construction The most important elements of a modern landfill facility are bottom membrane, percolate collection system, gas collection system, percolate irrigation system and top cover (Fig 16.1). These are all integrated parts of the landfill. In addition monitoring of incoming waste quality as well as air and groundwater quality in the area can be part of the facility. The following sections briefly describe the design of the different elements of the landfill.

Fig 16.1. Structural elements of a modern land filling facility 16.1.1. Bottom membrane The purpose of the bottom membrane is to reduce the leaching of contaminants out of the landfill. It is not practically and economically possible to ensure that the membrane is 100% effective. An acceptable emission is determined based on a weighting of the costs of constructing the membrane to a certain safety level against the costs of remediation of an accidental loss of percolate to the soil below the landfill. The membrane should fulfill the following demands: •

The membrane should function according to its purpose during the entire duration of landfill operation as long as the percolate contains contaminants in unacceptable levels for the surroundings. This means that the membrane should be functional during both the deposition and the active phases.

•

The membrane should be constructed such that the function can be changed when the percolate concentrations of contaminants have decreased to acceptable levels.

•

The membrane should be constructed as simple and robust as possible to reduce the occurrence of constructed and operation errors.

•

The membrane should be constructed such that accidental losses of percolate can be mapped and possibly controlled.

Membrane types and materials: The bottom membrane is composed of different elements each with its own function. These elements are: •

Watertight layer (the membrane)

•

Protecting layer

•

Supporting layer

The watertight layer is normally either an in-situ clay layer or constructed layers of clay and/or plastic. Constructed clay membranes are often made of bentonite (montmorillonite). Bentonite is also used to improve the water retaining capability of in-situ clay layers. The water retaining capability of clay membranes is based on their low water permeability. If the membrane is very thin or there is a large pressure gradient across the membrane, significant quantities of percolate may seep through. In Denmark law requires that clay membranes are at least 0.5 m thick and has a hydraulic conductivity that is less than 10-10 m/s. Plastic membranes are often made of polyethylene but there are also membranes made from other materials such as rubber or PVC on the market. Plastic membranes are usually 12.5 mm thick. The advantage of using plastic (or similar material) is that the material consumption is small and that the membrane is (at least in theory) completely watertight. The elasticity of most plastic membranes also means that they can withstand deformations without breaking. The disadvantage is the small thickness and thus, the high possibility for puncture plus the fact that construction of the membrane involves welding sections of membrane in the field under current weather conditions with the possibility for errors. Composite membranes of both plastic and clay combine the robustness but limited water retaining capability of the clay with the complete water tightness but limited strength of the plastic yielding a membrane with greater margin of safety against leaching of percolate. The composition of a composite membrane is illustrated in Fig.16.2.

Fig 16.2. Schematic of a composite membrane consisting of a plastic and clay membrane sandwich combination The membrane system is in some cases fitted with a protective layer for protecting the plastic membrane against sharp objects. Also a supporting layer may be established under the membrane if the existing soil is insufficient. Supporting and protective layers can be made from sand or from geo textile. There should be no water conducting layer between the plastic and the clay membranes in a composite membrane system. In case of a hole in the plastic membrane a water-conducting layer will allow the percolate to spread quickly over a large area resulting in increased emission to the soil below. The first layer of waste that is deposited over the membrane should also be regarded as a protective layer and visual inspection of this waste with respect to sharp objects should be conducted. Construction of the membrane. Land filling facilities should be constructed by companies who have the required knowledge and expertise. The weather conditions under which construction will take place should also be considered. It is of outmost importance that the quality of the construction is inspected as it will be difficult or impossible to locate an error when construction is completed. An approved quality assurance program including inspection and control of materials and of the actual construction work should be available when the contract is signed with the construction company. Clay membranes, that be either in-situ membranes or manufactured membranes should be constructed such that drying of the clay and the risk of crack formation is avoided. Also the risk of softening of the clay due to rain must be avoided. This means that weather conditions under which construction takes place must be carefully considered. It is not possible to work in rainy weather and sections of the membrane that are finished must be covered during sunny periods. Clay membranes should not be allowed to freeze, as this will also cause crack formation. In-situ clay membranes must have a certain homogeneity and quality. Parts that are of inferior quality for instance as determined by measurements of hydraulic conductivity must be replaced. Recommended maximum hydraulic conductivity for

clay membranes is 10-10 m/s. The surface must be smooth and sloping consistently to facilitate construction of the drainage system. If a composite membrane is desired the surface of the insitu clay membrane must be free of sharp rocks that can perforate the plastic membrane. The permeability of the clay is of outmost importance for the membrane function and core samples of the membrane materials should be taken for laboratory measurement of the permeability. If the permeability of the clay material is too high bentonite can be mixed into the in-situ clay formation. Plastic membranes are normally constructed as welded sections of 5 – 10 m width. If the plastic membrane is part of a composite construction it is important to ensure close contact between the plastic membrane and the clay membrane below. It is therefore important to avoid wrinkles in the plastic. Also changes in material length due to temperature changes during the construction period must be considered. The weather should not be cold, wet or windy, as this will cause difficulties in handling and welding of the membrane sections. It is very important that the welding seams are completely watertight. This can be ensured by using double seems and pressure test the space between seems as illustrated in Fig.16.3. A protective layer is constructed on top of the membrane system. This layer often has also the function of a drainage layer. When constructing the protective and drainage layer it is important not to drive in the same tracks over the membrane as this can cause damage to the membrane.

Fig 16.3. Schematic of welding technique and testing of plastic membranes Transport through membranes. Transport of contaminants and other compounds through clay membranes is controlled by advection and diffusion. The advective transport is facilitated by water movement through the membrane and is governed by Darcy’s law. Diffusive transport is governed by Fick’s 2nd law. For plastic membranes the advective transport is unimportant (if the membrane is tight that is), as water cannot penetrate the membrane. Certain molecules, however, are able to cross the membrane by diffusion even if

water molecules do not cross. Clay membranes retain pollutants by adsorption and ion exchange processes. This means that these compounds move through the membrane slower than water. The transport velocity for a given compound compared to that of water is characterized by the retardation factor R. For instance if the transport of heavy metals is 100 times slower than that of water (a typical value) R equals 100. For clay membranes that are free of structural errors, the diffusive flux JD (g/m2) of a compound with diffusion coefficient in free water, Dw (m2/d), retardation factor, R and percolate concentration C (g/m3) through a membrane with porosity φ (m3/m3) and thickness L (m) can be estimated as follows (Olesen et al. 1996). Dw JD= 0.45φ ---R

C ---- ---------(16.1) L

If the drop in hydraulic head across the clay membrane is ∆h (m) and the hydraulic conductivity of the clay material is K (m/d), the advective flux JA (g/m2) is found as follows. C K ∆h JD= -----R

---- ---------(16.2) L

Equations (16.1) and (16.2) assume that the transport is at steady state, that the concentration is zero below the membrane and that the percolate concentration and hydraulic head are constant in time. In reality it may take considerable time before steady state is reached and concentrations below the membrane can usually not be expected to be zero. The above equations therefore represent a worst-case estimate of the transport through the membrane. If cracks or holes are present in the membrane emissions can be orders of magnitude higher than for intact membranes. 16.1.2. Drainage system The purpose of the drainage system is to ensure an effective collection of percolate during the deposition and the active phases and minimize the risk of uncontrolled leaching from the landfill. The demands to the drainage system can be formulated as follows. •

The drainage system must function properly without blockages during its period of active operation.

•

The drainage system shall ensure that the hydraulic head over the membrane is as low as possible during the operational period.

•

The drainage system must be constructed such that it is possible to monitor its function and collection of percolate samples should be possible.

The drainage system is comprised of different components as illustrated in Fig.16.4. In addition to ensure percolate collection the drainage system also functions as a protective layer for the membrane system below. The system consists of the drainage layer, drainage pipes, inspection and collection wells and pumping stations. The materials for the system are chosen based on the operation conditions at the actual landfill and on the fact that it can be very expensive to excavate and repair a faulty drainage system after waste has been deposited. The hydraulic conductivity of the drainage layer should be at least 10 -3 m/s (Christensen 1998). In Denmark the thickness of the layer is typically 30 cm. The drainage layer can be constructed of two separate layers, a bottom layer of coarse gravel (20 cm) and a top layer of sand (10 cm). The drainage pipes are normally placed within a section of stabilizing gravel directly on top of the membrane. The capacity and strength of the drainage pipes is determined by the infiltration rate and the geo-technical pressure on the pipes. The pipes should have no abrupt changes in direction and the inside diameter should be at least 100 mm to allow for TV inspection. Also the pipes should have relative large perforations (minimum 2.5 mm wide) to minimize the possibility for clogging due to chemical precipitation. The thickness of the drainage layer is determined by its hydraulic conductivity, the infiltration rate, the distance between the drainage pipes and the maximum desirable percolate head over the membrane.

Fig 16.4. Schematic of the drainage system at a landfill Design of drainage systems. The monthly precipitation that is exceeded once a year is taken as basis for design of drainage system (Christensen 1998). This means that the system may be overloaded 1/12 of the time. If re-circulation of percolate is done this must be included when determining the design infiltration rate. Also the hydraulic conductivity of the drainage layer

may decrease due to clogging and chemical precipitation. This must also be included in the design procedure. A simplified procedure for calculating the capacity of the drainage system can be laid out as follows (Fig.16.5). The drainage pipes are located with distance I (m) from each other, the slope of the membrane is a (m/m), the constant design infiltration rate is q (m/d), hydraulic conductivity of the drainage layer is K (m/d) and the maximum percolate depth is Ya (m). The percolate depth should under normal conditions not be larger than the thickness of the drainage layer. The Darcy flux of percolate in the drainage layer v (m 3/(m2 d)) is found as dy V (x) = K ---- ---------(16.3) Dx

The percolate depth y on a horizontal membrane is given as

q

I

y(x) = x √-- (--- -1 ) ---------(16.4) K

x

Where I is the maximum theoretical distance between drainage pipes. The percolate flow in the drainage layer per m of drainage pipe Q(x) (m3/(m d)) is found as dy Q (x) = y (x) v(x) = K y(x) --dx

I = ( --- - x) q ---------(16.5) 2

The total amount of percolate that enters the drainage pipe per m is, thus 2Q(0) (or – 2Q(I)). Therefore each drainage pipe should be able to handle a flow of IqL where L is the length of the drainage pipe assuming that I is constant. The maximum percolate depth f (m) at the mid-point between two drainage pipes assuming horizontal membrane is I

q

f = --- √ --- ---------(16.6)

2

K

On a sloping membrane the maximum percolate depth, ya (m) is

f Ya = ------------------------- ---------(16.7) √ 1 + K a2 q

Where a is the slope m/m. Given the values of ya, K, q and a, the design distance, I, between the drainage pipes can be calculated using Eq.(16.7). If the membrane is constructed with slopes towards the drainage pipes (Fig. 16.6) a distance between drainage pipes of I/2, i.e., a safety factor of 2 is used i.e. I is chosen equal to 0.5 times the maximum value. If the system is constructed with uniform slope (Fig. 16.6) a safety factor of 4 is used. Typical distances between drainage pipes are 10 – 20 m depending on the design infiltration rate (Christensen 1998). Field investigations (Brune et al. 1991) indicate that one of the major problems with drainage systems is clogging due to chemical precipitation of calcium carbonate and iron sulfide in the drainage layer. This can not be entirely avoided but the problem can be reduced by using coarse materials for drainage layer construction and by operating the landfill such that percolate with a high content of degradable organic matter is prevented from reaching the drainage layer (the organic matter is degraded before reaching the drainage system or the percolate is collected above the drainage layer).

Fig 16.5. Infiltration, percolate flow and percolate depth in drainage layer on a horizontal membrane

Fig 16.6. Percolate depth as a function of distance between drainage pipes on horizontal, double sloping and uniformly sloping membrane

16.1.3. Gas venting Biodegradable wastes deposited at landfills will cause anaerobic conditions to develop in the wastes resulting in formation of biogas. The gas will spread to the surroundings including the atmosphere and surrounding soil formations if it is not collected. Landfill gas is transported by both diffusion and advection but advection is normally the primary transport mechanism (Poulsen et al. 2001). The advective flux of gas is driven by pressure differences between the landfill and the atmosphere (or surrounding soil formations). This means that variations in atmospheric pressure and fluctuations in wind speed and direction at the landfill/soil surface play major roles in landfill gas migration. As mentioned earlier also the gas permeability of the landfill material is very important. Figure 16.7 shows emissions of CO2 and CH2 during the passage of a low-pressure weather system.

Fig 16.7. Landfill gas flux to atmosphere during passage of a low-pressure weather system at Skellingsted lanfill Management of the landfill gas can be done by collection and combustion of the gas possibly with utilization of the energy to produce heat and electricity. This is done at many landfills throughout the world. In cases where combustion of the gas is not possible for instance due to low gas production rates a passive gas venting system can be used. Passive venting is driven by the gas pressure difference between the interior of the landfill and the atmosphere. Venting of the gas is done using perforated gas collection pipes (made of PVC or PEH) installed in the waste. The pipes may be installed either vertical or horizontally as illustrated in Fig.16.8. The perforated sections of the pipes are installed with a layer of gravel between the pipe and the waste to facilitate gas movement into the pipe and prevent clogging of the perforation.

Fig 16.8. Design of horizontal and vertical landfill gas collection systems

Recently

alternative passive solutions to gas collection and combustion have been proposed. These methods are based on improving the methane degradation capability of the soil cover. By ensuring a homogeneous soil cover without cracks the landfill gas can be biologically oxidized to CO2 in the soil layer before escaping to the atmosphere. Adding organic matter such as compost to the soil can accelerate the degradation process. The organic matter also is able to adsorb organic compounds found in the landfill gas thereby reducing odor emissions. The technology is still under development and more research and experimenting is required before it can be considered an alternative to gas combustion. 16.1.4. Percolate management The percolate collected at the bottom of the landfill can be either directly sent to a wastewater treatment plant or it can be recycled to the top of the landfill. Recycling has several benefits, it provides an initial cleaning of the percolate before it is sent to the wastewater treatment plant and if the percolate is applied on the surface of the landfill cover for instance by a sprinkler system the percolate production can be reduced by evaporation. If the percolate is applied below the landfill surface via a piping system reduction of percolate quantity will be minimal. Application below ground has the advantage of minimizing odor emissions and it can be used even during periods with frost. A disadvantage is that it is difficult to repair the system. Newer sections of the landfill typically have lower degradation capacity, the percolate from these sections can be lead to older sections where anaerobic

conditions have developed and the degradation potential is high, this will improve percolate cleaning. 16.1.5. Top cover The construction and design of the top cover is done based on the availability of construction materials in the surrounding area as well as on the function demands to the cover. Important demands to top covers are (Christensen 1998). •

Control infiltration to the waste and the production of percolate

•

Enhance and control surface runoff and evaporation

•

Control gas emissions

•

Provide a physical barrier between the waste and the surroundings.

•

Prepare the landfill area for its future use Top covers can be constructed as permeable or non-permeable as illustrated in Fig.

16.9. The advantage of using a non-permeable cover is that the amount of percolate will be very limited and limited percolate management is therefore necessary. Non-permeable covers should not be used at landfills where the wastes are not degraded to a level where the environmental emissions are insignificant. This is because most of the processes degrading the wastes require water and therefore is dependent upon precipitation and infiltration. Also many landfill sites will likely be used for recreational purposes after closure and the top cover should therefore be constructed such that different plant species can be planted after completion of the landfill. Also there should be as few as possible installations (gas pipes etc.) above the soil surface after closure of the landfill to facilitate future use of the site for other purposes.

Fig 16.9. Elements of top covers at landfills in case of permeable and nonpermeable covers The top cover consists of the following elements. •

Plant cover

•

Growth layer

•

Root barrier

•

Membrane (for non-permeable covers)

•

Regulation layer The purpose of the plant cover is protection against erosion and dust emissions and it

enhances evapotranspiration from the growth layer, reducing percolate formation. Suitable plants should be selected based on landscape conditions, climate and the future use of the area. If the landfill is placed in areas where strong winds occur sufficient thickness of the growth layer must be ensured for proper root development. It is generally advisable to use plants that are robust and able to grow in extreme soils. Care should also be taken with respect to landfill gas emissions that can cause oxygen deficiency and lead to insufficient root development for the plant cover making it more vulnerable to drought and strong winds. The growth layer can be constructed as two separate layers, an upper layer of organic rich soil and a lower layer of sand and silt containing soil. The upper layer can be 20 cm thick and the lower layer is normally 80 cm thick if a root barrier is present. If a root barrier is nto present the lower growth layer should be 1.7 m thick (Christensen et al. 1998). The root barrier is a 15 – 20 cm thick layer of coarse well draining gravel. In case of a non-permeable top cover a membrane (possibly a composite membrane) is constructed under the root barrier (Fig. 16.10), which then also functions as a drainage layer for the infiltrating precipitation. If a membrane is used a gas drainage layer (gravel or sand) may be constructed under the membrane to facilitate gas transport and collection. If the surface of the wastes is uneven a regulation layer with a smooth surface can be constructed directly on top of the wastes. 16.2. Landfill hydrology The hydrology of a landfill is of the outmost importance for management of the landfill. The quantities of percolate that is produced at the landfill are controlled by the hydrologic cycle at the landfill site. Knowing the flows of water in the different parts of the cycle allows for the estimation of percolate production. The following sections present the water balance for a landfill and a simple model for estimation of percolate production based on climatic data.

16.2.1. Water balance

The following considerations with respect to landfill water balance are applicable to a closed section of a landfill or an entire landfill where a top cover has been constructed. The water balance is illustrated in Fig.16.10.

Fig 16.10. Components of the water balance for a closed landfill where top cover has been constructed The precipitation (N) is perhaps the most important factor for percolate production at least at modern landfill facilities with membrane systems. When precipitation falls on the surface of the landfill some of the water will evaporate from the surface before it can infiltrate into the soil. Some of the water will also run off the surface to the surroundings (R) in cases of high levels of precipitation or in connection with snow melting. Surface runoff can also cause water from the surroundings to flow toward the landfill increasing the infiltration to the top cover (R is a negative value). The remaining water will infiltrate into the top cover. Part of this water will be taken up by plants and be transported to the atmosphere via evaporation and by transpiration is termed evapotranspiration (E). The water that has not evaporated or run off will infiltrate to the waste (I) where part of it may be consumed by the chemical and biological processes occurring in the landfill. Some of the water may also get hung up in the pores of the waste and increase the water content of the waste. The water that has not been removed by all of the above processes will then infiltrate to the bottom of the landfill as percolate (P). The water balance for the landfill can then be written as P = N - R - E - C - W - G