basics of landfills

7/28/2019 basics of landfills

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BASICS OF LANDFILLS:

How they are constructed andwhy they fail?

MANDEEP KAUR

M.Sc (SWM)

Roll no. 9


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What is a Landfill?

There are two ways to bury trash:

Dump - an open hole in the ground where trash is buried and thathas various animals (rats, mice, birds) swarming around.

Landfill - carefully designed structure built into or on top of the

ground in which trash is isolated from the surrounding environment(groundwater, air, rain). This isolation is accomplished with a bottomliner and daily covering of soil.

Sanitary landfill - landfill that uses a clay liner to isolatethe trash from the environment

Municipal solid waste (MSW) landfill - uses a synthetic(plastic) liner to isolate the trash from the environment


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PARTS OF A LANDFILL

The basic parts of a landfill, as shown in Figure , are:

Bottom liner system - separates trash and subsequent leachatefrom groundwater

Cells (old and new) - where the trash is stored within the landfill

Storm water drainage system - collects rain water that falls on thelandfill

Leachate collection system - collects water that has percolatedthrough the landfill itself and contains contaminating substances(leachate)

Methane collection system - collects methane gas that is formedduring the breakdown of trash

Covering or cap - seals off the top of the landfill


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This cross-section drawing shows the structure of a municipal solid waste

landfill. The arrows indicate the flow of leachate


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Bottom Liner System

The bottom liner prevents the trash fromcoming in contact with the outside soil,

particularly the groundwater.

There are three types of liners:

Clay

Plastic composite

If the bottom liner fails, wastes will migrate directly into the environment.


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WHAT IS WRONG WITH A CLAY LINER?

Natural clay is often fractured and cracked.

A mechanism called diffusion will move organic chemicalslike benzene through a three-foot thick clay landfill liner in

approximately five years.

Some chemicals can degrade clay.


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WHAT IS WRONG WITH COMPOSITE LINERS?

A Composite liner is a single liner made of two parts-1. a plastic liner and2. compacted soil (usually clay soil).

Reports show that all plastic liners (also called FlexibleMembrane Liners, or FMLs) will have some leaks.

It is important to realize that all materials used as liners are atleast slightly permeable to liquids or gases and a certain amountof permeation through liners should be expected.

Additional leakage results from defects such as cracks, holes,and faulty seams.

Studies show that a 10-acre landfill will have a leak ratesomewhere between 0.2 and 10 gallons per day.


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Cells (Old and New)

Perhaps, the most precious commodity and overridingproblem in a landfill is air space.

The amount of space is directly related to the capacityand usable life of the landfill. If the air space isincreased, then the usable life of the landfill can be

extended. To do this, trash is compacted into areas, called cells,

that contain only one day's trash. The amount of trashwithin the cell is compressed which is done by heavyequipment that go over the mound of trash several

times. Once the cell is made, it is covered with six inches of

soil and compacted further. Cells are arranged in rowsand layers of adjoining cells (lifts).

.


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A bulldozer prepares a new cell in a landfill


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Storm Water Drainage It is important to keep the landfill as dry as possible to

reduce the amount of leachate. This can be done in twoways:

Exclude liquids from the solid waste. Solid wastemust be tested for liquids before entering the landfill.

Keep rainwater out of the landfill. To excluderainwater, the landfill has a storm drainage system.Plastic drainage pipes and storm liners collect waterfrom areas of the landfill and channel it to drainageditches surrounding the landfill's base.

The ditches are either concrete or gravel-lined and carrywater to collection ponds to the side of the landfill. In thecollection ponds, suspended soil particles are allowed tosettle and the water is tested for leachate chemicals.Once settling has occurred and the water has passedtests, it is then pumped or allowed to flow off-site.


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This storm drainage pipe empties into a

drainage ditch.

Drainage ditches run along the base of a

landfill. The black pipe carries landfill gas to a

pumping station.

This collection pond is for catching storm water. The black liner helpschannel the water and protect the underlying cells.


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Leachate Collection System The water percolates through the cells and soil in the landfill. As the

water percolates through the trash, it picks up contaminants (organicand inorganic chemicals, metals, biological waste products ofdecomposition). This water with the dissolved contaminants is calledleachate and is typically acidic.

To collect leachate, perforated pipes run throughout the landfill.These pipes then drain into a leachate pipe, which carries leachateto a leachate collection pond. Leachate can be pumped to thecollection pond or flow to it by gravity.

The leachate in the pond is tested for acceptable levels of variouschemicals (biological and chemical oxygen demands, organicchemicals, pH, calcium, magnesium, iron, sulfate and chloride) and

allowed to settle. After testing, the leachate must be treated like anyother sewage/wastewater; the treatment may occur on-site or off-site.

Some landfills recirculate the leachate and later treat it. This methodreduces the volume of leachate from the landfill, but increases the

concentrations of contaminants in the leachate.


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WHAT ARE SOME OF THE PROBLEMS WITH

LEACHATE COLLECTION SYSTEMS?

Leachate collection systems can clog up in less than a decade.

The resulting liquid pressure becomes the main force drivingwaste out the bottom of the landfill when the bottom liner fails.

They fail in several known ways: 1) they clog up from silt or mud;

2) they can clog up because of growth of microorganisms in thepipes;

3) they can clog up because of a chemical reaction leading to the

precipitation of minerals in the pipes; or 4) the pipes become weakened by chemical attack (acids,

solvents, oxidizing agents, or corrosion) and may then be crushedby the tons of garbage piled on them.


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A leachate collection pond is designed to catch the contaminants that can

get into water that goes through the trash in a landfill.


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Methane Collection System

Bacteria in the landfill break down the trash in the absence ofoxygen (anaerobic) because the landfill is airtight. A byproductof this anaerobic breakdown is landfill gas.

This presents a hazard because the methane can explode and/orburn. So, the landfill gas must be removed. To do this, a series of

pipes are embedded within the landfill to collect the gas. In somelandfills, this gas is vented or burned.

More recently, it has been recognized that this landfill gasrepresents a usable energy source. The methane can beextracted from the gas and used as fuel.

The extraction system is a split system, meaning that methanegas can go to the boilers and/or the methane flares that burn thegas.


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A methane collection pipe helps

capture the hazardous gasA methane "flare" is used for

burning landfill gas


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Covering or Cap Each cell is covered daily with six inches ofcompacted soil. This

covering seals the compacted trash from the air and prevents

pests (birds, rats, mice, flying insects, etc.) from getting into thetrash. This soil takes up quite a bit of space.

When a section of the landfill is finished, it is coveredpermanently with a polyethylene cap(40mil). The cap is thencovered with a 2-foot layer of compacted soil.

The soil is then planted with vegetation to prevent erosion of the

soil by rainfall and wind. No trees, shrubs or plants with deeppenetrating roots are used so that the plant roots do not contactthe underlying trash and allow leachate out of the landfill.

Occasionally, leachate may seep through weak point in thecovering and come out on to the surface. It appears black andbubbly. Later, it will stain the ground red. Leachate seepages arepromptly repaired by excavating the area around the seepageand filling it with well-compacted soil to divert the flow of leachateback into the landfill.


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An experimental tarp provides daily cover of

the landfill cells.

Grass and other plants cover the municipal

solid waste landfill.

Seepage of leachate (black) can be seen

through a weak spot in the cover.


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WHAT ARE THE PROBLEMS WITH COVERS?

Covers are vulnerable to attack from at least seven sources:

1) Erosion by natural weathering (rain, hail, snow, freeze-thaw cycles, andwind);

2) Vegetation, such as shrubs and trees that continually compete withgrasses for available space, sending down roots that will relentlessly seekto penetrate the cover;

3) Burrowing or soil- dwelling mammals (woodchucks, mice, moles,

voles), reptiles (snakes, tortoises), insects (ants, beetles), and worms willpresent constant threats to the integrity of the cover;

4) Sunlight (if any of these other natural agents should succeed inuncovering a portion of the umbrella) will dry out clay (permitting cracksto develop), or destroy membrane liners through the action of ultravioletradiation;

5) Subsidence--an uneven cave-in of the cap caused by settling of wastes

or organic decay of wastes, or by loss of liquids from landfilled drums--can result in cracks in clay or tears in membrane liners, or result inponding on the surface, which can make a clay cap mushy or can subjectthe cap to freeze-thaw pressures;

(6) Rubber tires, which "float" upward in a landfill; and

(7) Human activities of many kinds.


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Groundwater Monitoring At many points surrounding the

landfill are groundwater monitoringstations.

These are pipes that are sunk intothe groundwater so water can besampled and tested for thepresence of leachate chemicals.

The temperature of the groundwateris measured. Because thetemperature rises when solid wastedecomposes, an increase ingroundwater temperature couldindicate that leachate is seepinginto the groundwater.

Also, if the pH of the groundwaterbecomes acidic, that could indicateseeping leachate A groundwater monitoring pipe stands in the

center. The two yellow markers on either side

make it more visible so that equipment

operators will not run into the monitoring

station.


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Main Landfill Environmental ProblemsEmissions from landfills, are arguably the biggest landfill environmental

problems and can be categorised as:

Emissions to atmosphere

Emissions to the water environment

1. Emissions to Atmosphere

These comprise:

noise, dust, odour, and possibly bio-aerosols, predominantly fromlandfill site operation;

landfill gas - from soon after opening and for possibly severalhundred years thereafter.

2. Emissions to Water

These comprise the potential emission of leachate and contaminatedsurface water run-off to:

watercourses (ditches, streams, rivers etc)

groundwater in permeable strata below the landfill.


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