Ch. 14 Solid Waste – title
© 2010 Cengage Learning, Engineering
Chapter 14 Solid Waste
Environmental engineers design systems to manage solid
waste properly and consider the impacts or designs on the end-
of-life management options
Solid Waste • Solid waste generated in U.S.: 1.4 trillion lb of
garbage per day
• Refuse/Municipal solid waste is made up of:
o Garbage : which is food waste
o Rubbish : almost everything else in your
garbage can
o Trash : is larger items, such as old
refrigerators, tree limbs, mattresses, and
other bulky items, that are not commonly
collected with the household refuse
• A very important subcategory of solid waste,
called hazardous waste.
Solid Waste
The municipal solid waste problem can be
separated into three steps:
1. collection and transportation of household,
commercial, and industrial solid waste
2. recovery of useful fractions from this material
3. disposal of the residues into the environment.
Collection of refuge
In many locations in the United States and
other countries, solid waste from households and
commercial establishments is collected by trucks.
Sometimes these are open-bed trucks
that carry trash or bagged refuse.
These vehicles are called packers, trucks that
use hydraulic rams to compact the refuse to
reduce its volume and make it possible for the truck to carry larger loads (Figure 14.1)
Collection of refuge
Commercial and industrial collections are
facilitated by the use of containers (dumpsters or
roll-offs) that are either emptied into the truck by
using a hydraulic mechanism or carried by the
truck to the disposal site (Figure 14.2)
Vehicles for collecting separated materials,
such as newspaper, aluminum cans, and glass bottles, are also used (Figure 14.3)
Figure 14.1 pg 481
Collection of refuge
The entire operation is a study in inefficiency
and hazardous work conditions.
The safety record of solid waste collection
personnel is by far the worst of any group of
workers (three times as bad as coal miners, for
example).
Various modifications to this collection method
have been implemented to cut collection costs
and reduce accidents.
Compactors and garbage grinders in the
kitchen and semiautomated and fully automated
collection system can be used.
Figure 14.1 pg 481
Collection of refuge
Other alternate systems have been developed
for collecting refuse, one especially interesting
one being a system of underground pneumatic
pipes.
The pneumatic collection system at Disney
World in Florida has collection stations scattered
throughout the park that receive the refuse, and
the pneumatic pipes deliver the waste to a
central processing plant.
The selection of a proper route for collection
vehicles, known as route optimization,
can result in significant savings to the hauler.
Collection of refuge
Deposit your garbage at designated chutes
either on the street or in your building, which
transport the refuse (at high speeds!) through a
system of pressurized tubes hidden below the
streets.
Finally, it arrives at a central plant for
processing.
It’s cleaner, more efficient, and often less expensive than traditional truck collection,
Collection of refuge
https://www.google.com/search?q=Pneumatic+pipe+for+waste+collection&biw=
Table 14.1 pg 483
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14-13 Handbook of Solid Waste Management – Second Edition by George Tchobanoglous and Frank Kreith
Layout of Collection Routes
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14-15
The effectiveness of the collection routes can be assessed by the amount of
route overlap. (a) Route layout with overlap shown by the dotted lines. (b)
Route layout without overlap.
Handbook of Solid Waste Management – Second Edition by George Tchobanoglous and Frank Kreith
Figure 14.8 pg 486
© 2010 Cengage Learning, Engineering 14-17
Tipping fee: Waste management fees for vehicles crossing solid waste management facility scales
Figure 14.9 pg 487
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14.2 Generation of Refuse
Figure 14.11 pg 489
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The public can exercise three alternate means for
getting rid of its unwanted material once it is generated—
reuse, recycling, and disposal
In reuse an individual either uses products again for the
same purpose or puts products to secondary, often
imaginative, use.
Recycling, or material recovery, on the other hand,
involves the collection of waste and subsequent
processing of that waste into new products—for example,
turning plastic food containers into park benches.
A central processing facility is known as a material recovery facility, or MRF (pronounced “murf”)
Reuse and Recycling of Materials From Refuse
In both methods, reuse and recycling, the
primary goal is purity.
For example, the daily refuse from a city of
100,000 would contain perhaps 200 tons per day
of paper.
Secondary paper has sold for about $20/ton (but
it fluctuates greatly), so the income to the
community could be about $4,000 per day, or
about $1.5 million per year!
So why isn’t every community recovering the
paper from its refuse and selling it? The answer is
elementary— because processing the refuse to
recover the dirty paper costs more than producing paper from trees
Processing of Refuse
The obvious solution is to never dirty the paper
(and other materials that might have market value)
in the first place.
This requires the public to separate their waste,
a practice known as source separation.
Another option is to get rid of source separation
and let the MRF handle all the separation.
This reduces the public’s role and reduces
collection costs but increases the complexity of
and the processing costs at the MRF.
The most difficult operation in recycling is the identification and separation of plastics.
Processing of Refuse
Table 14.2 pg 493
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Reuse and Recycling of Materials From Refuse
Figure 14.13 pg 494
One product that always has a market is energy.
Because refuse is about 80% combustible
material, it can be burned as is, or it can be
processed to produce a refuse-derived fuel (RDF).
A cross section of a typical waste-to-energy
(WTE) facility is shown in Figure 14.15.
The hot gases produced from the burning refuse are cooled with a bank of tubes filled with water.
Combustion of Refuge
As the gases are cooled, the water is heated,
producing low-pressure steam.
The steam can be used for heating and cooling
or for producing electricity in a turbine.
The cooled gases are then cleaned by pollution
control devices, such as electrostatic precipitators
and discharged through a stack.
Combustion of Refuge
14-27
Solid waste can be combusted as is and it can also
be processed in many ways before combustion
There might be confusion as to what exactly is
being burned.
The American Society for Testing and Materials
(ASTM) developed a scheme for classifying solid
waste destined for combustion:
RDF-1 unprocessed MSW
RDF-2 shredded MSW (but no separation of
materials)
RDF-3 organic fraction of shredded MSW (usually
produced in a MRF or from source-separated organics, such as newsprint)
Combustion of Refuge
RDF-4 organic waste produced by a MRF that
has been further shredded into a fine, almost
powder, form, sometimes called “fluff”
RDF-5 organic waste produced by a MRF that
has been densified by a pelletizer or a similar
device and that can often be fired with coal in
existing furnaces
RDF-6 organic fraction of the waste that has
been further processed into a liquid fuel, such as
oil
RDF-7 organic waste processed into a gaseous fuel.
Combustion of Refuge
Particular concern to many people is the
production of dioxin in waste combustion.
Dioxin is actually a family of organic compounds
called polychlorinated dibenzodioxins (PCDD).
Members of this family are characterized by a
triple-ring structure of two benzene rings
connected by a pair of oxygen atoms (Figure
14.17).
A related family of organic chemicals are the
polychlorinated dibenzofurans (PCDF), which
have a similar structure except that the two benzene rings are connected by only one oxygen.
Combustion of Refuge
Dioxin
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All the PCDD and PCDF compounds (referred to
here as dioxins) have been found to be extremely
toxic to animals.
Neither PCDD nor PCDF compounds have
found any commercial use and are not
manufactured.
They do occur, however, as contaminants in
other organic chemicals.
Various forms of dioxins have been found in
pesticides and in various chlorinated organic chemicals (such as chlorophenols).
Combustion of Refuge
The only two realistic options for disposal are in
the oceans (or other large bodies of water) and on
land.
The former is presently forbidden by federal law
in the United States and is similarly illegal in most
other developed nations
Although the volume of the refuse is reduced by
over 90% in WTE facilities.
The remaining 10% still has to be disposed of
somehow, along with the materials that cannot be incinerated, such as old refrigerators.
Sanitary Landfills
A landfill is, therefore, necessary even if the
refuse is combusted, and a WTE plant is, therefore,
not an ultimate disposal facility.
The placement of solid waste on land is called a
dump in the United States and a tip in Great Britain
(as in “tipping”).
The dump is by far the least expensive means of
solid waste disposal and thus was the original
method of choice for almost all inland communities.
The operation of a dump is simple and involves
nothing more than making sure that the trucks empty at the proper spot.
Sanitary Landfills
Rodents, odor, air pollution, and insects at the
dump, however, can result in serious public health
and aesthetic problems, and alternate methods for
disposal are necessary.
Larger communities can afford to use an
incinerator for volume reduction.
But smaller towns cannot afford such capital
investment, so this has led to the development of
the sanitary landfill.
The sanitary landfill differs markedly from open
dumps in that the latter are simply places to dump
wastes while sanitary landfills are engineered
operations, designed and operated according to accepted standards.
Sanitary Landfills
The basic principle of a landfill operation is to
prepare a site with liners to deter pollution of
groundwater, deposit the refuse in the pit,
compact it with specially built heavy machinery
with huge steel wheels, and cover the material at
the conclusion of each day’s operation (Figure
14.18).
Developing a proper landfill requires planning and engineering design skills.
Sanitary Landfills
Figure 14.18 pg 502
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Table 14.3 pg 503
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Imagine a town where 10,000 households each
fill up one 80-gallon container of refuse per week.
To what density would a 20-cubic-yard packer
truck have to compact the refuse to be able to
collect all the households during one trip?
[Mass IN] = [Mass OUT]
VLCL = VPCP where V and C are the volume and density of the
refuse and the subscripts L and P denote
loose and packed refuse. Assume that the density in the cans is 200 lb/yd3 (Table 14.3).
Example 14.2
[(10,000 households)(80 gal/household)(0.00495
yd3/gal)] × [200 lb/yd3] = (20 yd3)CP
CP = 39,600 lb/yd 3(!)
Clearly impossible. Obviously, more than one truck and/or more than one trip is required.
Example 14.2
An added complication in the calculation of
landfill volume is the need for the daily cover,
which may be removable (such as a plastic ‘tarp’)
and not use any of the volume or may not be
removable (such as dirt).
The more permanent cover material (e.g., dirt)
that is placed on the refuse, the less volume there
is available for the refuse itself, so the shorter
the life of the landfill.
Commonly, engineers estimate that the volume
occupied by cover dirt is one-fourth the total landfill volume.
Sanitary Landfills
Sanitary landfills are not inert.
The buried organic material decomposes,
first aerobically and then anaerobically.
The anaerobic degradation produces various
gases, such as methane and carbon dioxide, and
liquids (known as leachate) that have extremely
high pollution capacity when they enter the
groundwater.
Liners made of either impervious clay or
synthetic materials, such as plastic, are used to
try to prevent the movement of leachate into the groundwater.
Sanitary Landfills
Figure 14.19 pg 504
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Synthetic landfill liners are useful in capturing
most of the leachate, but they cannot be perfect.
No landfill is sufficiently tight that groundwater
contamination by leachate is totally avoided.
Wells have to be drilled around the landfill to
check for groundwater contamination from leaking
liners, and if such contamination is found,
remedial action is necessary.
The use of plastic liners substantially increased
the cost of landfills to the point where a modern
landfill costs nearly as much per ton of refuse as a WTE plant
Sanitary Landfills
Modern landfills also require the gases to be
collected and either burned or vented to the
atmosphere.
The gases are about 50% carbon dioxide and
50% methane, both of which are greenhouse
gases.
In the past, when gas control in landfills was
not practiced, the gases were known to cause
problems with odor, soil productivity, and
even explosions.
Now the larger landfills use the gases for
running turbines to produce electricity to sell to
the power company. Smaller landfills simply burn the gases at flares.
Sanitary Landfills
Reducing the generation of Refuge: Source Reduction Pages 505 – 509
The EPA developed a national strategy
for the management of solid waste called
“integrated solid waste management”
(ISWM).
The intent of this plan is to assist local
communities in their decision making by
encouraging strategies that are the most
environmentally acceptable but providing
flexibility to manage wastes efficiently.
Integrated Solid Waste Management
It is based on the solid waste management
hierarchy, with the most-to-least-desirable solid
waste management strategies being
• source reduction
• recycling
• combustion
• landfilling
Read Pages 509-511
Integrated Solid Waste Management
Figure 14.24 pg 514
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