The Choosing Of A Landfill Site Essay, Research Paper

The Choosing of a Landfill Site

There is currently much debate on the desirability of landfilling particular

wastes, the practicability of alternatives such as waste minimisation or pre-

treatment, the extent of waste pre-treatment required, and of the most

appropriate landfilling strategies for the final residues. This debate is likely

to stimulate significant developments in landfilling methods during the next

decade. Current and proposed landfill techniques are described in this

information sheet.

Types of landfill

Landfill techniques are dependent upon both the type of waste and the landfill

management strategy. A commonly used classification of landfills, according to

waste type only, is described below, together with a classification according to

landfill strategy.

The EU Draft Landfill Directive recognises three main types of landfill:

Hazardous waste landfill

Municipal waste landfill

Inert waste landfill

Similar categories are used in many other parts of the world. In practice, these

categories are not clear-cut. The Draft Directive recognises variants, such as

mono-disposal – where only a single waste type (which may or may not be

hazardous) is deposited – and joint-disposal – where municipal and hazardous

wastes may be co-deposited in order to gain benefit from municipal waste

decomposition processes. The landfilling of hazardous wastes is a contentious

issue and one on which there is not international consensus.

Further complications arise from the difficulty of classifying wastes accurately,

particularly the distinction between ‘hazardous’/'non-hazardous’ and of ensuring

that ‘inert’ wastes are genuinely inert. In practice, many wastes described as

‘inert’ undergo degradation reactions similar to those of municipal solid waste

(MSW), albeit at lower rates, with consequent environmental risks from gas and

leachate.

Alternatively, landfills can be categorised according to their management

strategy. Four distinct strategies have evolved for the management of landfills

(Hjelmar et al, 1995), their selection being dependent upon attitudes, economic

factors, and geographical location, as well as the nature of the wastes. They

are Total containment; Containment and collection of leachate; Controlled

contaminant release and Unrestricted contaminant release.

A) Total containment

All movement of water into or out of the landfill is prevented. The wastes and

hence their pollution potential will remain largely unchanged for a very long

period. Total containment implies acceptance of an indefinite responsibility for

the pollution risk, on behalf of future generations. This strategy is the most

commonly used for nuclear wastes and hazardous wastes. It is also used in some

countries for MSW and other non-hazardous but polluting wastes.

B) Containment and collection of leachate

Inflow of water is controlled but not prevented entirely, and leakage is

minimised or prevented, by a low permeability basal liner and by removal of

leachate. This is the most common strategy currently for MSW landfills in

developed countries. The duration of a pollution risk is dependent on the rate

of water flow through the wastes. Because it requires active leachate management

there is currently much interest in accelerated leaching to shorten this

timescale from what could be centuries to just a few decades.

C) Controlled contaminant release

The top cover and basal liner are designed and constructed to allow generation

and leakage of leachate at a calculated, controlled rate. An environmental

assessment is always necessary to that the impact of the emitted leachate is

acceptable. No active leachate control measures are used. Such sites are only

suitable in certain locations and for certain wastes. A typical example would be

a landfill in a coastal location, receiving an inorganic waste such as bottom

ash from MSW incineration.

D) Unrestricted contaminant release

No control is exerted over either the inflow or the outflow of water. This

strategy occurs by default for MSW, in the form of dumps, in many rural

locations, particularly in less developed countries. It is also in common use

for inert wastes in developed countries.

Options C and D might be considered unacceptable in some European countries.

Landfill techniques

Landfill techniques may be considered under seven headings:

location and engineering

phasing and cellular infilling

waste emplacement methods

waste pre-treatment

environmental monitoring

gas control

leachate management

1) Location and engineering

Site specific factors determine the acceptability of a particular landfill

strategy for particular wastes in any given location. In theory an engineered

total containment landfill could be located anywhere for any wastes, given a

high enough standard of engineering. In practice, the perceived risk of

containment failure is such that many countries restrict landfills for hazardous

wastes, and perhaps for MSW, to less sensitive locations such as non-aquifers

and may also stipulate a minimum unsaturated depth beneath the landfill. In

other cases, acceptability is dependent on the results of a risk assessment that

examines the impact on groundwater quality of possible worst-case rates of

leakage.

For the controlled contaminant release strategy, the characteristics of the

external environment in the location of the landfill, particularly its

hydrogeology and geo-chemistry, are integral components of the system. As such

they need to be understood in more detail than for any other strategy.

An environmental impact assessment (EIA) is essential and it must include

estimation of the maximum acceptable rates of leachate leakage. This estimation

will determine the degree of engineered containment necessary for the base liner

and top cover and any associated restrictions on leachate head within the

landfill.

The principal components of landfill engineering are usually the containment

liner, liner protection layer, leachate drainage layer and top cover. The most

common techniques to provide containment are mineral liners (eg clay), polymeric

flexible membrane liners (FMLs), such as high density polyethylene (HDPE), or

composite liners consisting of a mineral liner and FML in intimate contact.

Other materials are also in use, such as bentonite enhanced soil (BES) and

asphalt concrete.

Approximately 20 years experience has now accumulated in the installation of

engineered liners at landfills but there remains uncertainty over how long their

integrity can be guaranteed, and some disagreement as to the suitability of

particular liner materials for the containment of hazardous wastes and MSW, and

the gas and leachate derived from them.

At landfills with engineered containment it is necessary to make provision for

collection and removal of leachate. Often it is necessary to restrict the head

of leachate to minimise the rate of basal leakage. Head limits are typically set

at 300-1000mm leachate depth. This usually requires the installation of a

drainage blanket. This is a layer of high voidage free-draining material such as

washed stone, over the whole of the base of the landfill, to allow leachate to

flow freely to abstraction points. Drainage blankets are necessary because the

permeability of waste such as MSW is usually too low, after compaction, to

conduct leachate to abstraction points while maintaining the leachate head below

the stipulated maximum. The hydraulic conductivity of MSW can fall to less than

10-7m/s in the lower layers of even a moderately deep landfill. Under greater

compaction, values as low as 10-9m/s have been measured, which is of a similar

magnitude to that of mineral liner materials.

For the controlled release strategy the most critical engineered component is

the top cover, whose function is to control the rate of leakage by restricting

the rate of leachate formation. In any given location, percolation through the

top cover is a complex function of several factors, namely:

slope

the hydraulic conductivity of the barrier layer

the hydraulic conductivity of the soils or materials placed above the

barrier layer

the spacing of drainage pipes within the soil layer

Mineral barrier layers are typical for this application. They may also be used

for total containment sites, where FMLs or even composite liners have also been

used for the top cover. A review of mineral top cover performance (UK Department

of the Environment, 1991) found that percolation ranged from zero up to ~200mm/a.

To obtain very low percolation rates, protection of the barrier layer from

desiccation was necessary, drainage pipes should be at a spacing of not greater

than 20m, and the ratio of the hydraulic conductivity in the barrier layer to

that in the soil or drainage layer above it should be no greater than 10-4.

Under northern European conditions, protection of the barrier layer from

desiccation would typically require on the order of ~900mm of soil material.

Under hotter, drier conditions, a greater depth might be needed.

2) Phasing and cellular infilling

Landfills are often filled in phases. This is usually done for purely logistic

reasons. Because of the size of some landfills it is economical to prepare and

fill portions of the site sequentially. In addition, active phases are sometimes

further sub-divided into smaller cells which may typically vary from 0.5ha to

5ha in area. Often these cells may be engineered to be hydraulically isolated

from each other.

There are two main reasons for cellular infilling:

To allow the segregation of different waste types within a single landfill.

For example, one cell might receive MSW bottom ash, another inert wastes

and another non-hazardous industrial wastes. In hazardous waste landfills

different classes of hazardous waste may be allocated to dedicated cells.

To minimise the active area and thus minimise leachate formation, by

allowing clean rain water to be

discharged from unfilled areas while individual cells are filled.

Where cellular infilling is carried out, the landfill is effectively sub-divided

into separate leachate collection areas and each may need an abstraction sump

and pumping system. This can increase the physical complexity of leachate

removal arrangements and if the cells receive different waste types, each cell

may produce leachate with different characteristics. This may in turn influence

the design of leachate treatment and disposal facilities.

3) & 4) Waste emplacement methods and pre-treatment

Wastes are usually compacted at the time of deposit. This is done to gain

maximum economic benefit from the void space and to minimise later problems

caused by excessive settlement. The degree of compaction achieved depends on the

equipment used, the nature of the wastes and the placement techniques.

Equipment may vary from small, tracked bulldozers, up to specialised steel-

wheeled compactors. The latter are claimed to be able to achieve in situ waste

densities in excess of 1 tonne/m3 with MSW. Experience suggests that, to achieve

this, it is necessary to place wastes in thin layers, not more than 1m thick,

and to make many passes with the compactor. At many landfills, waste is placed

in much thicker lifts of 2.5m or more and receives relatively few passes by the

compactor. Densities of ~0.7 – 0.8t/m3 are more typical in such situations.

Some wastes are easier to compact to high densities than others. At some

landfills in Germany receiving final residues from MSW recycling facilities, it

has proved difficult to achieve densities greater than ~0.6t/m3 because the

residual materials tend to spring back after compaction. This low density has

led to problematic leachate production patterns because the waste allows very

rapid channelling during high rainfall, so that leachate flow rates exhibit more

extreme variability than at conventional landfills.

Common practice at MSW landfills in some EU countries is to place the first

layer of waste across the base of the site with little or no compaction and

allow it to compost, uncovered, for a period of six months or more. Subsequent

lifts are then placed and compacted in the usual way. This practice was

developed from research studies in Germany and has been found to generate an

actively methanogenic layer very rapidly. Leachate quality is found to be

methanogenic (1) from the start, and as a result, leachate management and

treatment is more straightforward.

Some operators of MSW landfills add moisture, or wet organic wastes such as

sewage sludge, at the time of waste emplacement, to encourage rapid degradation,

and in particular to encourage the early establishment of methanogenesis. There

is ample experimental and field evidence to show that this can be effective.

The covering of wastes with inert material at the end of each working day has

been an integral feature of sanitary landfilling techniques as developed in the

USA during the 1960s and 1970s. It is common practice at MSW landfills in many

countries around the world but is by no means universal practice within the EU.

Its continued use is increasingly being questioned, particularly where enhanced

leaching is to be undertaken to accelerate stabilisation, because many materials

used as daily cover can form barriers to the even flow of leachate and gas. The

primary role of daily cover is to prevent nuisance from smell, vectors (eg rats,

seagulls), and wind blown litter and this remains an important objective. No

universally applicable alternative has yet been found but the following measures

have been successful in some cases:

Pre-shredding of wastes, combined with good compaction, is said to render

them unattractive to vectors and to reduce wind pick-up. Spraying of lime has

also been used with the same benefits.

Commercial systems that spray urea-formaldehyde foam, or similar, onto the

wastes. The foam collapses when subsequent lifts are applied. This technique has

been slow to be accepted, mainly because of cost and convenience factors, but it

is now used at several sites in the EU.

Commercial systems that apply a spray-on pulp made from shredded paper,

usually separated from the

incoming wastes. Removable membranes such as tarpaulins.

5) Monitoring

Monitoring is an essential part of landfill management and has two important

functions:

It is necessary in order to confirm the degradation and stabilisation of

the wastes within the landfill

It is necessary to detect any unacceptable impact of the landfill on the

external environment so that action can be taken.

Monitoring can be divided into a number of distinct aspects, as follows:

Gas – Landfill gas quality within the site; soil gas quality outside the

site; air quality in and around the site

Leachate – Leachate level within the site; leachate flow rate leaving the

site; leachate quality within the site;

leachate quality leaving the site

Water – Groundwater quality outside the site; surface water quality outside

the site

Settlement – Settlement of wastes after infilling

The relative importance of each of these areas of monitoring depends on the type

of waste and the landfill management strategy. A controlled release landfill for

inorganic wastes is likely to need much effort focused on groundwater quality. A

containment and leachate control landfill for MSW will require more monitoring

of conditions inside the landfill than many other types of site.

6) Gas control

At most landfills receiving degradable wastes such as MSW and many non-hazardous

industrial wastes, it is necessary to extract landfill gas in order to prevent

it from migrating away from the landfill. Landfill gas (LFG), a mixture of

methane and carbon dioxide, has the potential to cause harm to human health, via

explosion or asphyxiation, and to cause environmental damage such as crop

failure. Examples of all three have occurred both within and outside landfills.

The techniques for extracting and controlling LFG are now reasonably well

established and in common use. Vertical gas extraction wells are usually

installed after infilling has ceased in a particular area. Gas is extracted,

usually under applied suction, and routed either to a flare or to a gas

utilisation scheme. It is now quite common to generate electrical power from LFG

and to recover heat. In some cases LFG has been used directly as a fuel source

in brick kilns, cement manufacture and for heating greenhouses.

In conjunction with extraction wells it is often necessary to install passive

control systems, in the form of barriers and venting trenches around the

perimeter of land-fills. An appropriate barrier will often be provided by the

continuation of basal leachate containment engineering or in some cases by in

situ clay strata. Reliance on the latter has, however, occasionally been

misplaced. Where ‘clays’ have included mudstone and siltstone layers, migration

of LFG has sometimes occurred and has proved particularly difficult to remedy.

An area of continuing development is in the control of LFG at older sites, where

methane concentrations may become too low to be flared, but are still high

enough to require control. One technique being studied is methane oxidation, in

which bacteria in aerobic surface soils oxidise methane to carbon dioxide as it

diffuses into the atmosphere. These techniques, and design criteria for the soil

layers, are not fully developed, but research results have indicated great

potential.

7) Leachate management

There are two aspects to active leachate management:

the treatment and disposal of surplus leachate abstracted from the base of

the landfill

the flushing of soluble pollutants from waste until they reach a non-

polluting state.

Treatment techniques depend on the nature of the leachate and the discharge

criteria. Leachates may broadly be divided into five main types, described by

Hjelmar et al (1995).

Leachate types

1) Hazardous waste leachate

Leachate with highly variable concentrations of a wide range of components.

Extremely high concentration of substances such as salts, halogenated organics,

and trace elements can occur.

2) Municipal solid waste leachate

Leachate with high initial concentrations of organic matter (COD >20,000 mg/l

and a BOD/COD ratio >0.5) falling to low concentrations (COD in the range of

2,000 mg/l and a BOD/COD ratio 1000 mg/l) of which more than 90% is Ammonia-N.

This type of leachate is relatively consistent for landfills receiving MSW,

mixed non-hazardous industrial and commercial waste and for many uncontrolled

dumps.

3) Non-hazardous, low-organic waste leachate

Leachate with a relatively low content of organic matter (COD does not exceed

4,000 mg/l and it has a typical BOD/COD ratio of