Energy From Solid and Liquid Wastes - VII

8/14/2019 Energy From Solid and Liquid Wastes - VII

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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 effectsdepend 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 CO 2 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 CO 2

produced from combustion of methane is neutral with respect to global warming. Part of themethane that is not collected will be oxidized biologically in the upper aerobic layers of the


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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 CO 2 and H 2S. 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 anaerobicconditions. 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 H 2S 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


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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 withwooden 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


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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


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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.


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Average background soil concentrations of methane and carbon dioxide at the location were 0

and 18 g/m 3, 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 differenttemporal duration. The temporal duration can best be discussed based on the state of the


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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 dependupon 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


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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.


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


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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 membraneThe 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.


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The membrane should be constructed such that accidental losses of percolate can bemapped 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 1-

2.5 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.


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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 membranesshould 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


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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 in-

situ 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 Darcys law.

Diffusive transport is governed by Ficks 2 nd 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


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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 J D (g/m 2) of a compound with diffusion coefficient

in free water, D w (m 2/d), retardation factor, R and percolate concentration C (g/m 3) through a

membrane with porosity (m 3/m 3) and thickness L (m) can be estimated as follows (Olesenet al. 1996).

Dw C

JD= 0.45 ---- ---- ---------(16.1)

R L

If the drop in hydraulic head across the clay membrane is h (m) and the hydraulicconductivity of the clay material is K (m/d), the advective flux J A (g/m 2) is found as follows.

C K h

JD= ------ ---- ---------(16.2)

R 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.


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The drainage system shall ensure that the hydraulic head over the membrane is as lowas 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. Thehydraulic 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


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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

Y a (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/(m 2 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) (m 3/(m d)) is found as

dy I

Q (x) = y (x) v(x) = K y(x) --- = ( --- - x) q ---------(16.5)

dx 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)


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2 K

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

f

Ya = ------------------------- ---------(16.7)

1 + K a 2

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).


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

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


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16.1.3. Gas venting

Biodegradable wastes deposited at landfills will cause anaerobic conditions to developin 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

CO 2 and CH 2 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 manylandfills 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.


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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 CO 2 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 hasseveral 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


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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.


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Fig 16.9. Elements of top covers at landfills in case of permeable and non-

permeable 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 aregulation 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 basedon climatic data.


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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. Thewater 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 thesurface 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

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Energy From Solid and Liquid Wastes - VII