Role of a chemical/environmental engineer to manage solid wastes

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,

 

 

Table 14.1 pg 483

© 2010 Cengage Learning, Engineering 14-11

 

 

© 2010 Cengage Learning, Engineering 14-12

 

 

14-13 Handbook of Solid Waste Management – Second Edition by George Tchobanoglous and Frank Kreith

 

 

Layout of Collection Routes

© 2010 Cengage Learning, Engineering 14-14

 

 

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

© 2010 Cengage Learning, Engineering 14-18

14.2 Generation of Refuse

 

 

Figure 14.11 pg 489

© 2010 Cengage Learning, Engineering 14-19

 

 

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

© 2010 Cengage Learning, Engineering 14-23

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

© 2010 Cengage Learning, Engineering 14-31

 

 

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

© 2010 Cengage Learning, Engineering 14-37

 

 

Table 14.3 pg 503

© 2010 Cengage Learning, Engineering 14-38

 

 

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

© 2010 Cengage Learning, Engineering 14-43

 

 

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

© 2010 Cengage Learning, Engineering 14-49

 
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