treatment of solid waste

7/31/2019 Treatment of Solid Waste

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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramcos

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Environmental For additional information on this subject, contact

File Reference: ENV10202 K. Hibrawi on 873-0211

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Treatment of Solid Waste


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Engineering Encyclopedia Environmental


Saudi Aramco DeskTop Standards

CONTENTS PAGES

METHODS OF PHYSICAL TREATMENT OF SOLID WASTE......................... 1

Manual Separation.......................................................................................6

Mechanical Separation ................................................................................ 6

Density.............................................................................................7

Optical............................................................................................10

Magnetic ........................................................................................ 10

Screening................................................................................................... 11

Vibratory Screens .......................................................................... 11

Rotary Screen.................................................................................12

Disc Screen .................................................................................... 12

Shredding .................................................................................................. 12

Mills...............................................................................................13

Shears.............................................................................................13

Grinding .................................................................................................... 15

Tubs ............................................................................................... 15

Mills...............................................................................................16Baling ........................................................................................................ 16

Example Problem 1. Volume reduction by materials separation

and recovery of recyclables...................................... 17

Filtration .................................................................................................... 19

Filter beds ...................................................................................... 19

Drums ............................................................................................ 19

Presses............................................................................................20

Flocculation...............................................................................................21

Precipitation...............................................................................................21

Flotation .................................................................................................... 22


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Saudi Aramco DeskTop Standards

Evaporation ............................................................................................... 22

Ponds ............................................................................................. 22

Drying Beds ................................................................................... 23

METHODS OF CHEMICAL TREATMENT OF SOLID WASTE...................... 27

Oxidation................................................................................................... 28

Reduction .................................................................................................. 29

Neutralization ............................................................................................ 29

Calcination.................................................................................................31

Ion Exchange.............................................................................................31

Other Chemical Methods........................................................................... 32

LANDFARMING AND COMPOSTING AS SOLID WASTE

TREATMENT METHODS .................................................................................. 34

THERMAL DESTRUCTION METHODS OF SOLID WASTE TREATMENT. 41

Incineration................................................................................................41

Pyrolysis.................................................................................................... 58

Emerging Technologies............................................................................. 58

Microwaving..................................................................................58

Plasma Arc.....................................................................................59

SOLIDIFICATION AND STABILIZATION AS SOLID WASTE

TREATMENT METHODS .................................................................................. 60

Cement-Based ........................................................................................... 61

Thermoplastic-Based.................................................................................63

Silicate-based.............................................................................................65

Vitrification ............................................................................................... 66

GLOSSARY ......................................................................................................... 68

REFERENCES...................................................................................................... 69


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Saudi Aramco DeskTop Standards 1

METHODS OF PHYSICAL TREATMENT OF SOLID WASTE

The primary purpose of physical treatment methods is to reduce the total quantity of solid

waste that requires disposal. This reduction in weight and/or volume can be accomplished by

one, or a combination of the following methods:

Manual separation

Mechanical separation

Screening

Shredding

Grinding

Baling

Filtration

Flocculation

Precipitation

Flotation

Evaporation

Source separation in many cases will result in less overall processing of waste and in therecovery of a higher quality product. Removing materials such as glass from the source

stream will reduce incinerator maintenance problems. The high thermal temperatures of

incinerators can actually melt glass. The molten glass then causes airflow through the

incinerator to decrease; this reduced airflow adversely affects combustion.

Figures 1 through 4 illustrate possible flow diagrams for the separation and recovery of

selected materials from commingled and source-separated municipal solid waste. All these

systems use a variety of the physical methods listed above. All the designs would be

improved by the addition of weighing scales throughout the process. It is essential to know

what materials are entering the plant and also to know the recovery efficiencies for the sorting

processes.


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Bagged

commingled

recyclable

materials

Second stage

manual sorting

Conveyer

Oversized material

Conveyor

Magnetic

separation

Conveyor

Conveyor

Screening

(trommel or disc)

Conveyor

First stage

manual sorting

Inclined conveyor

Bag breaker

Second-stage

manual presorting

Front-end loader,

floor and inclined

conveyor system

First-stage

manual presorting

Receiving areaFront-end loader

used to spread

waste for

presorting

Collection vehicle

Commingled MSW

Cardboard

Source-separated materials

in see-through bags

Bulky items

White goods

Other contaminants

Conveyor

Shredding

Combustion Compost for

intermediate landfill coverLandfill

Truck

Ferrous

metals

Paper

Plastics

Glass

Aluminum cans

Tin cans

Collection

vehicle

Source-separated

materials

Cardboard

Other large items

Paper

Cardboard

Plastics

Glass

Aluminum cans

Tin cans

Flow diagram for the separation and recovery of selected materials from commingledmunicipal solid waste

Figure 1


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Shipping

Forklift

Glass crusher

Forklift

Glass crusher

Mixed glass conveyor

Vibrating screen

Manual sorting

Conveyor

Conveyor

Manual sorting

Conveyor

Receiving hopper

Collection vehicle

Commingled

plastic and glass

Residual

materials

to landfill

Clear glass

crushed for

storage

Plastics

separated

accordingto type

Flow diagram for the separation and recovery of source separated waste: commingled glass

and plasticFigure 3


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Shipping

Forklift

Storage of baled

or crushed cans

Baler or

can crusher

Pulley magnet

Aluminum cans

Overhead magnetic

Conveyor

Conveyor

Receiving hopper

Collection vehicle

Commingled

aluminum and tin cans

Forklift or

pneumatic

conveyor

Tin cans,

baled or

crushed

Tin cans,

baled or

crushed

Flow diagram for the separation and recovery of source-separated waste: aluminum and steel

(tin) cans

Figure 4


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Manua l Separ ation

Manual separation of solid waste can either be accomplished at the source (for example, at the

home) or at a central collection station. "Curbside collection" at the home has the primary

advantage of eliminating the resident's need to drive to a central collection point.

Some cities provide multiple containers for the homeowner to sort the waste by types, such as

glass, plastic, paper and wet garbage. This separation process minimizes the labor of the

sanitation employees, but waste generators may not like the extra sorting effort. A second

method of curbside recycling is for the city to provide only a single recyclable waste

container. The homeowner bags wet garbage, or other materials the city cannot recycle,

separately. The waste collector performs the sorting of the recyclables at the time of the

pickup. The frequency of waste collection will help to determine if sanitation workers will

agree to this method of collection. A once-per-week collection frequency makes hand

sorting of garbage an undesirable activity. Concern for the possibility of transmittingcommunicable disease is another negative for handsorting.

Manual separation can be undertaken at a central collection point called a Materials Recovery

Facility (MRF). Waste generators can bring the waste to the MRF and do most of the sorting

by placing the material in separately marked bins. Another method is to have the sanitation

workers do the sorting at the MRF. The Rolla, Missouri MRF uses a combination of these

methods. Wastes at the Rolla MRF are placed in separate bins by the waste generators. Bins

are available for paper, plastic milk jugs, other plastics, glass (by color), aluminum and other

metals. Sanitation workers then ensure the sort has been done correctly, remove colored milk

caps (that can't be locally recycled) and prepare the sorted wastes for further processing steps.

If the bulk of the waste separation is performed by the sanitation workers, the MRF will often

have a conveyor belt system that brings the unsorted waste past a worker, who then removes

the recyclable materials as the waste passes the worker's station. These conveyor systems

haven't changed in concept since the turn of the century. The worker's protective equipment

has increased, and the conveyor facilities are now mostly air conditioned for worker comfort

and odor/pest control.

Mechanical Separ ation

Materials can be sorted by their physical properties: density (to sort light and heavy fractions);

optical clarity ( to sort glass or plastic by color); and electrostatic charge/magneticpermeability (to separate ferrous and non-ferrous metals).


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Density

Density separation can divide wastes into light fractions (such as paper, plastics and organics)

and heavy fraction (such as metals, wood and other heavy inorganic materials). Air

classification is the most popular of the methods, but devices called stoners and heavy media

separators are also used. A technology that is called "flotation" will be discussed in another

section.

In an air classifier, as shown in Figure 5, refuse enters the upper part of a chamber with air

blowing into the chamber from below. If the upward air velocity is high enough to overcome

the force of gravity on the particle, the particle will rise; small, heavy particles will fall to the

bottom of the chamber. Components suspended in the air stream are called "air classified

light fraction." Materials that settle out of the air stream are called "air classified heavy

fraction." Paper and plastics will concentrate in the light fraction, causing the light fraction to

have a higher heating value than the heavy fraction.

Unfortunately, air classifiers are sensitive to drag forces on the irregularly shaped particles.

Spherical particles are collected at different efficiencies than are flat particles of the same

weight and density. The more uniform the feed waste is the more consistent will be the

collection efficiency. Air classifiers are also sensitive to moisture content, which is absorbed

differently by various waste materials. Wet paper tends to stick together and will attract dirt .

The larger, relatively light paper fraction will carry into the light fraction with its load of dirt.

Shattered glass from a hammer mill can become imbedded in the paper and carry into the

light fraction.

A variation of the air classifier is called the air knife (shown in Figure 6). The device workswell with a waste stream that has already undergone some separation. The air current carries

paper and plastic with the air, while heavier materials fall more directly. Another density

classifying variation, known as a stoner, is shown in Figure 7. Air blows through a porous

deck and separates the material by differences in terminal velocity. Stoners work well over a

relatively narrow size range.

So-called heavy media separators have found popularity in the automobile recovery industry.

A pre-shredded feedstock with a high aluminum percentage is placed in a liquid with a high

specific gravity. The aluminum floats on the liquid, and the higher density materials settle or

remain below the surface.


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Heavyfraction

Ajustablebaffle

Ajustablebaffle

ShreddedrefuseLight fraction

Schematic of the air classification concept

Figure 5


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Air KnifePlenum

Refuse

Air from fan

Heavy

material

Light

material

Schematic of an air knife

Figure 6

Discharge

of light

material

Uniform supply

of fluidizing air applied

below porous deck

Porous deck sloped(about 4 ) and

vibrated in a

straight line

reciprocating motion

o

Light material that

is suspended and stratified

floats downslope

Light and heavy

materials to be

separated

Heavy material that

sinks to the bottom of

stratified bed

is conveyed upslope

by the deck's vibration

Discharge

of heavy

material

Schematic of a stoner

Figure 7


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Optical

Optical separation was popular in the 1970s and 1980s, but has fallen out of favor and is

rarely mentioned in current texts. The optical sensors were intended to sort glass by color, but

the sensors proved unreliable because the glass was frequently dirty or because the labels on

the glass interfered with the device that measured the optical properties. An optical sorter

might prove useful on a waste stream that was already presorted as glass and had been pre-

cleaned before the sorter.

Magnetic

Magnetic systems are used to separate ferrous and non-ferrous materials. In the three- magnet

system shown in Figure 8, the pickup magnet lifts ferrous material from the shredder belt;

then the transfer magnet brings the material around the curve of the belt and agitates the

material. The "no magnet" space before the discharge magnet is critical: as the material fallsfrom the belt, non-ferrous material that is caught behind the magnetic material is released.

The third magnet lifts the ferrous material back to the belt where it is discharged to the next

step in the process.

from

shredder

Solid wastes

Continuous belt

Discharge magnet

Ferrous

material

Transfer

magnetPickup

magnet

Nonferrous

materials

Typical magnetic separation system

Figure 8


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Screening

Many components in the waste stream have a characteristic size. Therefore, it is possible to

sort waste by size to achieve a series of waste streams with a higher percentage of certain

components. However, sorting by screening can never be 100% effective because of the

overlap in component sizing. Placing a screening mechanism before a shredder will reduce

loads on the shredder by removing small particles first. Various screening devices have been

developed, including vibratory screens, rotary screens and disc screens.

Vibr ator y Screens

Vibratory screens are excellent at separating relatively dry materials such as glass and metals.

A typical vibrating screen is shown in Figure 9.

Typical vibrating screen used for size separating commingled waste

Figure 9


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Mills

Mills are of several types: flail mills, vertical shaft hammer mills, and horizontal shaft

hammer mills. The vertical shaft hammer mill (Figure 13) is the most successful at achieving

size reduction, but at the cost of high maintenance and energy consumption. The steel

hammers that are pinned to the central shaft beat the waste to smaller and smaller sizes.

Shears

The shear shredder is made up of two parallel counterrotating shafts with a series of discs

mounted perpendicularly that serve as cutters. Waste material is reduced in size by the

shearing or tearing action of the cutter discs.

Small fraction

Large fraction

Schematic of a disc screenFigure 11


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200 4 8 12 16

Typical component size (in.)

10

20

30

40

50

60

70

80

90

100News-

print

Card-

board

Paper

Garbage

Yard

and

garden

waste

Metal

Glaass

Plastic

Inerts

Miscellaneous

organics

Percen

tcompos

ition

Typical size distribution of raw refuse

Figure 12


18/73




Drive motor

Infeed

Discharge

Neck section

Ballastic

ejection

Schematic of a vertical shaft hammer mill

Figure 13

Grinding

Grinders are actually another form of the shredders that were discussed in the preceding

section. Some varieties are tub grinders and mills.

Tubs

A tub grinder resembles a large diameter wash tub on wheels. It is a variety of mobile,

hammermill shredder. The tub grinder is often used for yard wastes, pruning and construction

debris. The waste is fed into the top of the tub, and then falls into a horizontal hammermill.

The grinders are often diesel-powered and may even have their own attached crane for

loading material in the tub.


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Mills

A grinding mill operates much the same as a shredding mill but has a lower power input and

uses time and multiple passes to do the work rather than brute force.

Baling

Baling is used at recycling centers to reduce the volume of waste. This volume reduction

eases storage requirements and cuts transportation costs. Some new equipment can produce

usable commercial products (such as fireplace logs) directly from waste cardboard and paper.

Balers are an alternative to compactors and operate at 100-200 pounds per square inch

pressure to produce compact bales of solid waste or recovered materials. Bale weight is in

the range of 500 to 800 kg.

Cardboard and Paper

Since up to 50% of municipal solid waste (MSW) is paper, significant reductions in landfill

volume can be obtained through cardboard and paper recycling. Larger department stores and

grocery stores often have their own cardboard balers. The baler compresses the waste paper

and automatically bands the material together with wire to ease transport methods. A forklift

and pallet system is normally used for transport within the facility. A baler used for paper and

cardboard is shown in Figure 14.

Aluminum Cans

Aluminum is one of the easiest and cost effective materials to recycle. The cans aresometimes baled, using the same baling equipment that is used for cardboard. They can also

be crushed and blown into trailers for shipping. Single can crushers are available for home

and office use to reduce waste volume at the source.


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Baler used for paper, cardboard and cans

Figure 14

The following example problem illustrates the effectiveness of recycling in extending landfill

life.

Example Problem 1. Volume reduction by materials separation and r ecovery of

recyclables.

A community with a population of 40,000 has a per-capita solid-waste disposal rate of 7

pounds per day (as collected). The distribution of components and nominal density and

compaction factors are for a typical landfill disposal. Determine the annual landfill area

requirements without any materials recovery and with a recovery of 75% of the glass and

paper and 80% of the metals and cardboard. By what factor could such recovery extend the

useful life of the landfill?


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Solution:

Computation table. *Given or measured values.

As collected (no recovery) Collected with recovery

Component %

by wt.

*

lb.

1000 lb.

*

density,

lb./ft.3

*

volume,

ft.3 per

1000 lb.

normal

comp.

factor *

fill

volume,

ft.3

wt. for

disposal,

lb.

density,

lb./ ft.3

*

disposed

volume,

ft.3

normal

comp.

factor *

fill

volume,

ft.3

Food Waste 12 120 18 6.7 0.35 2.3 120 18 6.7 0.35 2.3

Paper 44 440 5 88.0 0.20 17.6 110 5 22.0 0.20 4.4

Cardboard 5 50 3 16.7 0.25 4.2 10 3 3.3 0.25 0.8

Plastics 4 40 4 10.0 0.15 1.5 40 4 10.0 0.15 1.5

Glass 10 100 12 8.3 0.60 5.0 25 12 2.1 0.50 1.3

Ferrous Metal 5 30 20 2.5 0.35 0.9 10 20 0.5 0.35 0.2

Tin Cans 5 50 6 8.3 0.18 1.5 10 6 1.7 0.18 0.3

Aluminum 4 40 4 10.0 0.18 1.8 8 4 2.0 0.18 0.4

Miscellaneous 11 110 10 11.0 0.25 2.8 110 10 11.0 0.25 2.8

100% 1000 lb. 161.5

ft.3

37.6

ft.3

443 lb. 59.3

ft.3

14.0

ft.3

Calculation of solid waste density:

D withoutrecovery =1000 lb.(27ft.3/ yd.3 )

37.6 ft.3=718 lb./ yd.

3

D withrecovery = 443 lb.(27ft.3/ yd.3 )

14.0ft.3

= 854.4lb./yd.3

Calculation of approximate annual landfill area without recovery:

Average weight of daily waste collected = (40,000 persons)(7 lb./day/p.) = 280,000 lb./d.

Dailyfill volumerequired=280,000lb. / d.

718lb./yd.3= 390 yd.3/ d.

Assume a landfill cell cross-section of 2 yd. depth by 8 yd. wide = 16 yd.2

Dailycell length=390yd.3

16yd.2= 24.375 yd. lengthor 73.1 ft. + .5 ft. facecover = 73.6 ft.

Daily area = (73.6 ft.)(24.0 ft. + 2.0 ft. partition wall between trenches) = 1913.6 ft.2/d.

Annual area = (1913.6 ft.2/d.)(365 d./yr.) = 698,464 ft.2/yr.

or698,464ft.2/ yr.

43,560ft.2/ac.= 16.03 acres/yr.

The change in area (fill life) with material recovery is proportional to the volume change:37.6 ft.3

14.0 ft.3= 2.69or withmaterialrecovery thelandfilllife canbeextendedby a factor

of about2.7


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Filtration

The physical treatment methods that have been discussed so far are meant primarily for

relatively dry materials. For high liquid content wastes, it is appropriate to use: filtration,

flocculation, precipitation, flotation and evaporation/drying beds. These methods are all

meant to concentrate or separate solids from the liquid wastes.

Filtration is the physical process of retardation, resulting from the clogging of pore spaces in

the filter media. This clogging occurs when solid particles are trapped in the pores. Filtration

also occurs due to precipitation and accumulation of dissolved matter. There are three

common filtration methods: filter beds, drums and presses.

Filter beds

Sand filter beds were first used in water treatment plants, but the technology has found its wayinto wastewater treatment as a final "polishing" of sewage effluent. Beds are now made of

sand, coal, dual media (sand plus coal), or mixed media (sand, coal and granite).

A filter can reduce suspended solids in wastewater from 25 mg/L to 10 mg/L. If the solids

concentration is much higher than 25 mg/L, it is better to remove the solids with treatment

such as rapid mix-flocculation-sedimentation than it is with sand. If the sand pores load up

too quickly, the drop in pressure across the filter becomes excessive. A sand filter cross

section is shown in Figure 15. As the contaminant is captured on the sand, the pressure drop

increases, and the filter must be backwashed. The waste flow to the filter is shut off, and

water is pumped upward through the filter to rinse the filter media and to wash the

contaminant to a collection point. Backwashing normally lasts only a few minutes, finally thewashwater is shut off, and the clean filter is returned to operation.

A sand filter can also be used to capture petroleum type wastes. The finer the sand and the

higher the product viscosity, the greater the product retention. Up to 20,000 parts per million

(mg/L) of No. 6 diesel oil can be retained on fine sand. The filters are up to 625 square feet

in area, and flow rates over the beds average 3 gallons per minute per square foot of bed area.

Drums

Drum, or vacuum, filters are often used to dewater sludges. A cylindrical drum is covered

with a filter fabric. The drum rotates partially submerged in a vat of conditioned sludge. Avacuum is applied to the inside of the drum through the porous filter media. This vacuum

draws the water into the drum, leaving the dewatered filter cake on the outside of the drum. A

blade scrapes the drying filter cake from the rotating drum, and the cycle starts over.


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Outlet Main

Inlet Main

Wash Water

Outlet

Graded

Filter

Sand

Perfirated LateralsGraded Gravel

Wash Troughs

Typical cross section of a rapid sand filter

Figure 15

Presses

A continuous belt filter press(CBFP) operates on the shear and compressive forces

introduced in the sludge cake when the cake is pressed between two moving belts, as shownin Figure 16.

Washwater

FiltrateWash Spray

Conditioned

Sludge

Sludge Mixer

Polyelectrolite

Solution

Chemical

Conditional

Stage

Sludge

Cake

Compression

Dewatering

Stage

Gravity

Drainage

Stage

Continuous belt filter press

Figure 16


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Flocculation

Heavy metals removal from wastewater can be greatly increased by the addition of various

water-soluble chemicals and polymers in the dual process of coagulation and flocculation.

The two processes are used when normal settling rates of suspended particles are too slow for

effective clarification.

Coagulation is the addition and rapid mixing of a coagulant to neutralize charges and to

collapse the colloidal particles . This chemical addition and mixing causes agglomeration and

settling of the particles. Rapid mixing destroys the stability of the colloidal system. For

agglomeration to occur, the particles must collide; mixing helps that occur. Rapid mix times

of 30 seconds to 5 minutes are typical for the 8 cubic meter (maximum size) mixing basins.

Flocculation is the agglomeration of colloidal particles that have received coagulant

treatment. Flocculation takes place under slower mixing conditions in a 30-minute detentiontime tank.

Common coagulants are alum and iron salts for surface waters, and lime-soda mixes to soften

"hard" groundwater. Organic polymers, called polyelectrolytes, are often more effective than

these natural materials, at least for surface water treatment. There is no substitute for

laboratory jar testing to find the best combination of treatments for a particular water or waste

stream. Coagulants can also be added to wastewater settling tanks to promote better removal

of suspended solids.

The floc settles out in a sedimentation basin. The basin is sized for a 2 to 4 hour detention

time; again, this detention time depends on the treated material.

The agglomerated, flocculated waste must be handled properly for disposal. If the process is

designed to remove heavy metals, the waste will likely be classified as hazardous. Water and

most wastewater sludges can be recycled. The high calcium content in hard water sludge

makes it useful for concrete and gypsum additives.

Precipitation

Heavy metals are often present in liquid waste streams. Metals will precipitate at different pH

levels; this precipitation results in an insoluble salt that can be removed as a sludge. Lead,

for example, is soluble at acid pH, so lead in pipes is more of a problem at low pH. When anacid waste stream containing heavy metals (such as lead) is neutralized, the metals can be

removed by clarification, sedimentation, or filtration.

Lime is commonly used as a precipitate; sodium sulfide and sodium bisulfide can also be

used, but they present some danger of hydrogen sulfide release during the precipitation

process.


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Flotation

Flotation works well for low-density solids and hydrocarbon solids that can be separated from

liquids by air injection. Finely divided bubbles of air are injected into the liquid and attach to

the particles to be "floated." The particles, with their air "life preservers," rise to the water

surface and can be mechanically skimmed. Several mechanisms account for the air bubble

effectiveness, including actual bubble attachment to the particles, bubble formation at the

solid-liquid interface, and air entrapment in larger particles or air integration into the floc

itself. Care must be taken to avoid corrosion with air injection. The biology of the well and

the use of biocides are factors in flotation effectiveness.

Evaporation

Evaporation has been used for centuries to separate materials. Our ancestors evaporated salt

water in large pans to recover the valuable salt for trade. The technology changed little overthe years. Evaporation is defined as a treatment method by which wastewater is dispersed in a

vaporous form.

Ponds

The SAES-A-104, Wastewater Reuse and Land Disposal Engineering Standard addresses two

types of evaporation ponds: percolation/evaporation ponds and wastewater evaporation

ponds.

Percolation is a disposal method by which wastewater is allowed to pass through the soil base

of the pond. A percolation pond for wastewater requires the approval of the Chief Engineer,with the concurrence from the General Manager, Exploration; Chief, Preventive Medicine

Services; and Chief, Environmental Affairs. Percolation should not be used if the wastewater

contains hazardous waste, because of the potential danger of groundwater contamination.

If a pond is to be used for evaporation of wastewater, the SAES-A-104 Standard requires the

wastewater to be of "at least primary effluent quality," and the pond to be lined to prevent

percolation. Primary effluent is defined as the effluent from a process that provides removal

of sewage solids so that the effluent contains not more than 500mg/L of suspended solids.

The pond liner may be naturally occurring or man-made, and the coefficient of permeability

for the liner system must be less than or equal to 1 x 10-8 cm/sec. Wind velocity has a strong

effect on pond evaporation rates.


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Drying Beds

Drying beds have been used for decades at wastewater treatment plants. The purpose of the

beds is to dewater digested sewage sludge economically. The sludge is removed from the

beds after drying. The sludge is then disposed of in a landfill or used as a soil conditioner in

some countries. Saudi Aramco regulations prohibit sludge use for fertilizer. Drying beds are

low cost and low maintenance, and provide a high solids content in the dried product. Nature

does the drying work, rather than vacuum or horsepower. Four types of beds are used:

conventional sand, paved, artificial media, and vacuum-assisted.

A conventional sand bed is shown in Figure 17. Sand beds are useful for cities up to 20,000

persons. Larger cities should consider alternate methods because of the large space

requirements for sand beds. Typical area requirements are shown in Table 1.

Table 1. Typical area requirements for open sludge drying beds.

Type of sludge

Area,

ft2/per son

a

Sludge-loading r ate,

lb dr y solids/ft2.yr

Primary digested

Primary and trickling-filter humus digested

Primary and waste activated digested

Primary and chemically precipitated digested

1.0-1.5

1.25-1.75

1.75-2.5

2.0-2.5

25-30

18-25

12-20

20-33aCorresponding area requirements for covered beds vary from about 70 to 75 percent of those for the openbeds.

Note: ft2

x 0.0929 = m2

lb/ft2

x 4.8828 = kg/m2

. yr

Two types of paved drying beds are in use: a drainage type and a decanting type. The former

type is similar in size or larger than a conventional sand bed. Drying is encouraged by

frequent agitation with mobile equipment, and the dried sludge is removed with front end

loaders. There is generally less maintenance because there is no sand to be replaced

periodically. The decanting bed is shown in Figure 18. The slope of the bed allows

supernatant to be drawn off at the bed corners. The drying sludge is also mixed, similarly to

the drainage type bed. The decanting design works well for arid, or semi-arid areas. The

design requires evaporation and precipitation rates, plus pilot studies, to determine the

effectiveness for a particular sludge.


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Artificial media beds have been developed to overcome some of the cost and space

disadvantages of the sand and paved beds. Stainless steel wire mesh and polyurethane mesh

are used with some success. The polyurethane system works well for dilute sludges, and the

system produces a filtrate with low suspended solids.

Vacuum drying beds use vacuum on the underside of a stationary, porous, media plate to

assist gravity sludge dewatering. Dewatering proceeds over a 2-day period, which is much

shorter than with conventional sand filters. Filter size advantages are balanced by the need

for further post processing to obtain additional moisture reduction.


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METHODS OF CHEMICAL TREATMENT OF SOLID WASTE

Chemical treatment of municipal solid waste usually involves a form of chemical conversion

of the waste to a product, following some pre-processing. Chemical treatment is helpful in

promoting resource recovery of hazardous substances. Chemical treatment can also produce

useful by-products and residual effluents that are environmentally acceptable, which makes

chemical treatment a better method of waste management than the traditional landfill or

incinerator. Several processes are in common use:

Acid hydrolysis

Alkaline hydrolysis

Oxidation

Reduction

Neutralization

Calcination

Ion exchange

Other methods, such as dehydrohalogenation

As was discussed briefly in Module ENV 102.01, hydrolysis processes may be used torecover compounds such as glucose and furfural. The chemical process known as hydrolysis

occurs when chemical substances react with waste molecules. For example, acid hydrolysis

can convert cellulose into glucose. A cellulose molecule (consisting of about 3000 glucose

units) is soluble in water and in many organic solvents and is poorly degradable by

microorganisms. However, when the cellulose molecule is acid hydrolyzed, glucose and

other sugars may be recovered. The process involves treating a suspension of small cellulose

particles with a weak acid, heating the particles to between 180 and 230 C under pressure (to

prevent boiling and to allow for achieving those high temperatures). The quantity of glucose

that is recovered will depend on the characteristics of the waste and the process efficiency.

With good process efficiency, for example, and with kraft paper as the cellulose source,

approximately 80% of the weight of the kraft paper may be recovered as sugar. Therecovered sugars may be converted by biological processes and other chemical processes into

alcohols and other industrial chemicals. The general acid hydrolysis reaction for cellulose to

glucose is:

(C6H

10O

5)n + H

2O acid nC

6H

12O

6

cellulose glu cose


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Oxidation

Chemical reduction-oxidation (redox) reactions can take place with both organic and

inorganic chemicals. A redox reaction traditionally involves the gain or loss of oxygen. An

oxidation reaction adds oxygen to a compound. An example of an oxidation reaction is

FeSO4 changing to Fe2 (SO4)3. However, it is possible to have a redox reaction without

oxygen. With organic chemicals, oxidation is defined as the loss of electrons by a chemical (

the chemical donating the electron is oxidized). For inorganic materials, oxidation is better

defined as a reaction that raises the oxidation state of an atom. In the previous example, iron

(Fe) went from a +2 valence to +3, so it was oxidized.

Redox reactions in solid waste are mostly the result of biological activity, but some chemical

redox reactions can occur between organic materials and the soil. Some researchers have

found the oxidation of aromatic chemicals at soil and clay surfaces, catalyzed by adsorbed

oxygen and trace metals. These reactions can particularly occur in aerated unsaturated soil.Chlorinated organics do not seem to undergo such oxidation. Thus, one would expect that it

is easier to degrade aromatics than chlorinated solvents under natural aerobic conditions in

clay soils.

A common method of treating aqueous cyanide wastes is alkaline chlorination. The cyanide

is initially oxidized to a less dangerous cyanate form and then to carbon dioxide and nitrogen

in the following pair of reactions:

a +2

+ a a + a +2

2NaCNO + 3Cl2 + 4NaOH 2CO2 +N2 + 6NaCl+ 2H2O

This reaction is pH sensitive: The first reaction must be above pH = 10 for the cyanate to be

formed. The second reaction works better at pH = 8. Since hydrogen cyanide gas can be

released at acid pHs, one must really watch this reaction! The process can utilize

hypochlorite bleach, peroxide or ozone, so the process is useful for cleaning up spills of

cyanide or for disposing of cyanide wastes in a treatment method.

Another practical oxidation process for waste destruction is the Zimmerman wet air oxidation

method. Most organic compounds can be air-oxidized, if there is sufficient temperature and

pressure present. This process is an aqueous phase oxidation of dissolved and suspended

organics at 175 to 325 degrees Centigrade with enough pressure to prevent excessiveevaporation. With air bubbled through the liquid, the process is fuel efficient and usually

self-sustaining. Even some pesticides can be destroyed by this method.


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Reduction

A reduction reaction is a reaction that involves a decrease in valence from a gain of electrons.

A reduction-type reaction can be useful for the disposal of hexavalent chrome, a highly toxic

waste that is dangerous to both man and the environment. The hexavalent chrome can be

reduced to the much less toxic trivalent chrome by reacting the hexavalent form with sulfur

dioxide and lime. Reduction can also be used with most other metals to create an easily

removed precipitate. Redox treatment methods can, in general, be used for both in-situ and

ex-situ waste disposal procedures.

Neutralization

Many wastes, especially liquids, sludges and slurries, may be highly acidic or alkaline. The

best first step in treatment is to bring the waste to almost neutral pH. This neutralization

process will ease handling and improve the effectiveness of follow-on treatments.Neutralization effectiveness is easily measured by pH change, and acid-base reactions are

among the most often used processes in wastewater treatment. Several methods of

neutralization are practical:

Mixing multiple waste streams to achieve a neutral pH

Adding lime slurries to acidic wastes

Adding caustic soda or soda ash to acidic wastes

Adding carbon dioxide to alkaline waste

Adding sulfuric acid to alkaline wastes

All of these methods are effective, but it should be noted that, under RCRA regulations, such

operations cause a facility to be regulated as a treatment-storage-disposal (TSD) operation.

Even where RCRA does not apply, indiscriminate neutralization can be dangerous. The

reactions can give off considerable heat (exothermic reactions). In the case of cyanide at high

pH, overly enthusiastic neutralization could cause cyanide gas release, if the pH were to

become acidic during neutralization.

Materials can be neutralized in a simple reaction vessel as shown in Figure 19. Occasionally,sludges are treated in situ by mixing soda ash or lime with the sludge in a pond or lagoon.

Neutralization on a small scale is also useful for disposing of laboratory chemicals.

Since stack flue gases often have a high carbon dioxide content, such gases can be used to

neutralize alkaline wastes.

Good practice calls for bringing the waste to a near neutral pH (6 to 8), rather than to simply a


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RCRA non-hazardous waste condition (2 < pH < 12.5) .


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MIXER

pHmeter

Neutralizedeffluent

Neutralizer

Wastestream

Chemical neutralization treatment system for waste management

Figure 19

Calcination

The recovery of the large amount of calcium that is needed for the softening of hard waters iseconomically attractive. To be used again, the dried, calcined, dewatered sludge must not

contain much magnesium. Calcination is the process of heating dried solids to drive off

carbon dioxide. Both calcium oxide (CaO) and Magnesium Oxide (MgO) are produced by

the calcining process. Incidentally, the carbon dioxide driven off in the calcining may be used

for carbonation of the softened water.

Ion Exchange

An ion exchange system selectively exchanges ions from a chosen material in the waste

stream for the exchange medium. Two types of exchange systems are available: cationexchangers and anion exchangers. Cation exchangers are employed to remove positively

charged materials (metals), and anion exchange units are used to remove negatively charged

materials, such as organics. The primary use for ion exchange has been to remove metals

from ground waters and surface waters. Metals are concentrated in the resin column and can

then be removed by backflushing with water. The resin bed is then recharged by passing

dilute acid through the bed. It is possible to achieve removal efficiencies of better than 99 %,

with an effluent quality of less than 100 ppm of metal.


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Most water hardness is due to the presence of calcium and magnesium hardness. The water

can be softened by passing the water through an ion exchange resin. Generally, the ion that is

exchanged with the hardness is sodium, as indicated in the following equation:

Ca(HC3 )2 + 2NaR CaR2 + 2NaHCO3

where R is the ion exchange material. With this reaction, calcium or magnesium has been

removed from the water and replaced by an equivalent amount of sodium. This process

removes almost 100 % of the water hardness until the exchange capacity of the resin is

reached. At this point, "breakthrough" has taken place. The resin is taken off line and

backwashed with water containing a high sodium content. The hardness on the resin will

enter the backwash water as follows:

CaR2+

2NaCl

2NaR+

CaCl2

The resin bed is now ready to remove additional hardness; however, the CaCl 2 must be

disposed of, or a recycle use for it must be developed.

Ion exchange can also occur naturally in soil, particularly clay soils. The capability of a soil

to retain and exchange cations is called the cation exchange capacity. Clays have a higher

cation exchange capacity than other inorganic particles because clays have a large surface

area, with numerous charge sites.

It is important to note that ion exchange of metallic ions with soil may be partially reversible:

exchange sites which are saturated can release cations as concentrations of contaminantsdecrease in groundwater. This means that, as groundwater contamination is remediated, more

and more contamination that was bound up in a clay soil can be released: consequently, the

clean-up effort will take far longer than expected. The exchange sites can also release cations

if the pH is lowered. Ion exchange in the natural environment is, therefore, more of a

retardation, than an attenuation process. In addition, the exchange capacity of the subsurface

material can be so saturated that the contaminant transfer is not affected by ion exchange, and

contaminant transfer goes unretarded.

Other Chemical Methods

The safe treatment of PCBs and dioxins is a major concern when handling solid waste. Adechlorinization process was developed as an alternative to incineration of chlorinated

compounds. The underlying principle of dechlorinization is the removal of the chlorine atoms

from the compounds. The dechlorinization process employs synthesized reagents to break

down the chlorinated molecules or to form other compounds that are less toxic to the

environment and man.


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The most often used dechlorinization processes use polyethylene glycols (PEG) that have

reacted with alkali metals to bring about the dechlorinization. The two most common

processes are called the Alkali Polyethylene Glycol Process (APEG) and the Potassium

Polyethylene Glycol Process (KPEG). Either process will work in the field or the traditional

plant. Removal efficiencies of 99.5 % have been demonstrated. This efficiency, although

certainly desirable, is well below the efficiency of the best incinerators.

Three factors must be considered for any chlorine destruction process:

Temperature of the material

Contact time between the contaminant and the reagent

Moisture content of the soil

Elevating the waste temperatures from 20 to 80 Centigrade increases the reaction efficiency

from 50 % to 90 %. Contact time varies from 4 to 8 hours for ex-situ work, up to an average

of 7 days for in-situ treatment. Water adversely affects the reaction; soil moisture is,

therefore, a factor in the extended contact time required for in-situ treatment. Both APEG and

KPEG appear to have equal effectiveness. At this point in time, the major concern with

dechlorinization is that the reaction byproducts have not yet been well defined. Since U.S.

EPA has stated that dioxin is one of the most toxic substances known, it is well to proceed

with caution when attempting to destroy the material.


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LANDFARMING AND COMPOSTING AS SOLID WASTE TREATMENT

METHODS

Landfarming and composting are so similar in concept and requirements that the two

processes will be discussed together.

A landfarm is a biological treatment method that employs soil and associated microorganisms

as the treatment medium. The natural physical, chemical, and biological actions all come

together to degrade and immobilize the waste constituents. Some types of waste are not

actually degraded, and are incorporated only into the soil. When that occurs, landfarming

becomes a shallow, unlined landfill and is, therefore, a method of disposal, not treatment.

Composting is the process of making a nonputrescible (non-rotting) soil amendment from

solid waste. Many components of municipal solid waste are organic in origin and are

therefore biodegradable. Paper, yard waste, food waste and some textiles, such as cotton andwool, can all be composted. Composting of waste will occur readily, if there are present

appropriate microorganisms, adequate aeration, temperature in the correct range, required

nutrients, pH and sufficient moisture.

Landfarming has been used for:

Oily sludges and waste oils, such as separator sludge, slop oil emulsion solids,

induced and dissolved air flotation float, and leaded and unleaded tank

bottoms. Half-lives of the materials are typically 200 to 500 days. Oil loading

rate is usually the governing factor, although heavy metals in wastes such as

tank bottoms are sometimes limiting.

Organic sludges and liquids. Any waste that can be degraded biologically can

be landfarmed if regulations permit it. Halogenated sludges must be handled

very carefully to avoid problems during the sludges' slow decay time.

Sludges from wastewater treatment plants. Heavy industrial activity in a

community can cause high metals content in sewage sludge. Sludges are

monitored periodically to ensure the waste stays within the plant's operating

permit.

Careful attention must be made regarding the composition of the waste when considering aland disposal option. The US EPA has ranked all hazardous wastes based on their intrinsic

hazard, volume, and phased land disposal. "Land ban" restrictions were enacted in three steps

with the final enactment in May 1990. In order for land disposal (and also impoundment or

deep well injection) to receive approval, listed hazardous wastes must be pretreated to

pollutant concentrations below specified concentrations. It must also be demonstrated, to a

reasonable degree of certainty, that there will be no migration of hazardous constituents from

the disposal site.


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Saudi Aramco has established guidelines (Refinery Instruction Manual 5.881) for landfarming

wastes as follows:

1. Landfarmable wastes:

API separator sludges (less than 20 ppm organic lead)

Air flotation sludges

Biological waste treatment sludges

Recovered oil emulsions

Tank bottoms (excluding gasoline tanks)

Crude tank bottoms

MEA sludge

Oil contaminated soil (less than 20 ppm lead)

Desalter bottoms

Perhaps even more important than the list of what can be landfarmed, is the list of what the

Instruction Manual says can not be accepted for landfarming:

2. Wastes not acceptable for landfarming:

TEL tank bottoms

Leaded gasoline tank bottoms

Sweetening process sludges

Sludge from sulfuric acid treating

Caustic sludges

Spent acid

Catalyst from conversion processes

Asphalt waste


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Bacteria also are categorized by their atmospheric oxygen requirements as aerobic (require

atmospheric oxygen) anaerobic (require a lack of atmospheric oxygen) or facultative

anaerobic (function with or without oxygen). This environmental need will determine the

type of bacteria that are available to function in the treatment process. The oxygen

requirement also determines the rate at which treatment will progress and the resulting end

products, which differ with aerobic or anaerobic biological activity. For example, methane

will be a product of anaerobic activity but not of aerobic action. Most of the bacteria types

listed for petroleum product degradation are classified as aerobic bacteria.

Other critical environmental factors are temperature, pH, and moisture content. Optimal

biological activity occurs within a fairly narrow range of temperature and pH for specific

groups of bacteria although some activity can occur at less than an optimum rate outside the

preferred range of temperature and pH.

Generally, a warm environment (20-50 C) encourages biological activity. The optimum for

many bacteria is 35 C, although higher temperatures (about 55 C) may be optimal for some

bacteria. Fortunately, the bacteria most effective for petroleum products prefer temperatures

between 35 C and 45 C. The higher temperatures are also desired for the destruction of

pathogenic (disease-causing) microorganisms.

A near neutral pH range of about 6.5 to 7.5 is best for most bacteria. Soil pH of less than 5.5

is considered unacceptable.

Several other soil and site characteristics will determine the suitability of an area for

landfarming:

Soil depth must be greater than 2 feet.

Restrictive subsoils must be greater than 8 feet from the surface.

Highly permeable soil, such as sand, gravel, rocks and stone are undesirable.

High salinity or alkalinity can interfere with biological processes.

High background levels of certain metals or organics can make it difficult to

determine how "clean" the site is.

A water table closer to the surface than 8 feet (in sandy soil) or 6 feet (in loamy

sand) is unacceptable due to probability of off-site pollution migration.


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Moisture is critical to bacterial and other microbiological activity since about 80% of the cell

weight is water and since the nutrients for bacteria must be in solution to pass into the cell.

Supplemental water may be required for landfarming or composting municipal solid waste.

On a weight basis, the favorable moisture content for landfarming varies from 10-30%,

depending on the soil texture. This is equivalent to 50-70% of the soil water holding capacity.

It is important not to totally saturate the soil, or the bacteria will "drown," and the process will

become anaerobic. The desirable moisture content for composting is usually in the 50%

range. In non-arid climates with mild temperatures, these moisture contents can be achieved

with two inches per month of rainfall or applied water. With Saudi Arabia's summer

temperatures and minimal rainfall, considerable monitoring and adjustment will be needed to

reach these moisture contents.

In landfarming, wastes are applied uniformly to the land, either by spreading the wastes or by

injecting the wastes just beneath the surface. A disk harrow or rototiller (for small scale

operations) is used to mix the waste and soil. Cultivation maximizes the soil-waste contact.Furthermore, cultivation provides aeration to enhance the biological process. The cultivation

takes place in only the first 10 to 30 cm of soil depth (called the "zone of incorporation"), but

the "soil treatment zone" appears to extend to as much as 1.5 meters. Thus, landfarming is

primarily a shallow depth, aerobic process. It is common to till the soil at three week intervals

for at least the first three months of degradation.

Landfarming works best primarily with organic wastes, but landfarming can also remove

other contaminants. Suspended solids are removed by filtration on the soil and by physical

settling. Heavy metals can be adsorbed onto soil particles. Heavy metals also can be

precipitated or can undergo ion exchange with the soil. Some soils , particularly clays, have

a high sorption capacity for metals. Still, metals are often somewhat problematic. Tostabilize a heavy metal waste usually requires an alkaline pH. Such a high pH may not be

optimum for bacterial action. Metals may not be actually toxic to the microorganisms in the

soil. However, a high metals concentration may affect later plant growth when the site is

closed. Plant uptake can also act as a removal mechanism. Finally, the waste can simply

volatilize, or can be blown from the site on airborne dust. Thus, a heavy metal, such as lead,

can actually be transported from the site.

Both sandy and clay type soils have worked well for landfarms. Organic loadings as high as

5 to 10% of the soil volume (first 10 to 30 cm) are practical. Wastes are sometimes covered

in cold climates. Covering the waste raises the temperature and encourages bacterial growth.

Loading rates of 250 tons per acre per year are typical for petroleum wastes and are limited to300 dry tons per year per acre by SAES - A-104.

For landfarming to be successful, the rate and frequency of waste application must not exceed

the assimilative capacity of the site. This assimilative capacity is based on:

Capacity limits. Conservative, immobile wastes accumulate in the soil and

eventually reach a level at which the site must be closed.


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Rate limits. Non-conservative waste constituents will degrade, but the

cumulative amount applied must be compared with the cumulative amount

degraded over time. Refinery Operating Instruction 5.883 allows the

reapplication of sludge when the oil and grease content in a section of the

landfarm is lowered to 2% or less for two successive samplings.

Application limits. If the application rate is too high, waste can leave the site in

water runoff or waste can volatilize excessively.

Mathematical models are available to evaluate these various limits. A simple first order

reaction equation is often applied to determine the time required to degrade a waste to a final

soil concentration as follows:

t = ln(Ct/C0)/-K

where:

t = time to degrade

C0 = initial soil concentration

Ct = final soil concentration

K = first order rate constant

Removal efficiencies for common wastes can be remarkably high with landfarming:

In a Canadian study, a waste with 15,000 micrograms/gram each of benzene

and toluene was reduced to the required limits in soil of 5 and 30micrograms/gram for the two compounds, respectively, in less than 11 days.

Benzo(a)pyrene in the same waste took 294 days to degrade. Finally, arsenic, a

heavy metal that does not degrade, caused the abandonment of the site after

four years when the toxic limit (50 micrograms/gram) for arsenic was reached.

In a California laboratory experiment using soil contaminated with weathered

crude, the contamination was reduced from 3700 ppm to 1000 ppm in just 80

days. This experiment involved daily water addition, oxygenation and nutrient

addition. A "control" sample, which was untreated, went from 3700 to 3500

ppm in the same period. In a follow-up field pilot study, nutrient and surfactant

addition were required to get readings below 1000 ppm - water and aerationalone would not do it. The 1000 ppm standard is a common site clean-up limit

used by regulatory agencies in the United States.


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As stated previously, composting is used to turn MSW into a usable, useful product. Most

composting operations begin with the removal of the approximately 10% non-degradable

fraction, prior to starting the composting process. Glass, metal and plastic that do not

compost are all potentially valuable to sort and recycle. Removal of hazardous waste is

particularly important, although very difficult to achieve in practice. The wastes to be

composted are then shredded. The shredded materials are then placed in "windrows" on the

ground surface where the composting takes place. The primary difference between

composting and landfarming is that composting takes place on the ground, while landfarming

takes place in the ground. Composting, like landfarming is an aerobic process which requires

moisture control (40-65%, with 50% being optimal), temperature (55 C, or 131 F) for three

consecutive days to kill pathogens) and thorough oxygenation. The very high temperatures

are achieved even in winter due to the biological activity of the waste. The oxygenation can

be obtained by either of two methods: agitated piles, or static piles.

In the agitated method, the piles are 6-10 feet high and 14-25 feet at the bottom. Equipmentsuch as a Willibald side slicer is used to shave off a portion of the pile, moving it laterally

and aerating it at the same time. A "straddling turner" can also be used to aerate the pile in

place.

In the static method air is pulled or passed through the pile with an air blowing system.

Pulling the air through the pile by a system of surface grates or piping has the advantage of

allowing treatment of the exit air for odors. Blowing air through the pile allows the addition

of moisture or heat to the pile to control temperature and soil humidity.

Besides time, temperature and turbulence, the ratio of carbon-to-nitrogen has been found to be

rather critical in composting. The desirable C:N ratio is 20-30:1, with 30:1 often consideredas an optimum. Low C:N ratios can result in ammonia off gassing - a highly irritating odor.

High C:N ratios will eventually have a lower growth rate as the nitrogen is used up in the

process.

The whole composting process can take from 2 weeks to 18 months, depending on the degree

of process control. A 2.5 acre facility can handle approximately 50 tons per day of compost

(assuming a 2 month holding time), while each additional acre gains an additional 50 tons per

day capacity.


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THERMAL DESTRUCTION MET HODS OF SOLID WASTE TREATMENT

Great progress has been made in thermal destruction methods compared to the open burning

of trash that was practiced until recent times. Open burning of waste is still a factor in some

less populated rural areas. However, the pressure of urban and population growth has

virtually eliminated open trash burning in major population centers. In New York City, for

example, until the 1970s almost every multi-family housing unit (apartment house) had a

single chamber incinerator that was only a little improvement over open burning. Single

chamber incinerators were banned in New York in the 1970s, and replaced by trash

compactors at each apartment house. Open burning gives little volume reduction, compared

with more effective incineration methods, and sufficient organic residue remained in the

partially burned waste to still attract vermin and cause odors.

There is still a place in smaller communities for the open burning of yard clippings (grass and

small branches), but even a relatively small community should consider composting thesewastes for a mulch or fertilizer.

Thermal processes have lost some popularity recently as the method of choice for hazardous

waste destruction. The threat of incomplete waste combustion, even if the threat is

unfounded, has caused the U.S. public to oppose new incinerator locations. Three types of

thermal destruction methods will be discussed in greater detail: incinerators, pyrolysis, and

the "emerging technologies" of microwaving and plasma arc. Except for vitrification, which

will be discussed in the next section, thermal technologies are ex situ processes: the waste

must be transported to the processing unit.

Incineration

Thermal treatment, to most people, means the incineration of waste. Incineration can be

defined as the burning of substances by controlled flame in an enclosed area. The desirable

features of incineration are that incineration:

Detoxifies hazardous waste by destroying organic compounds

Reduces the volume of the waste

Converts liquid wastes to solids by vaporizing any fluids present in the waste

Sterilizes medical wastes

Recovers waste heat from high heat content wastes


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Incineration can be designed to treat waste in any physical state-solids, liquids, sludges,

slurries and gases. Incineration has been especially successful at destroying organic matter in

waste. Removal efficiencies as high as 99.9999 % have been reached. Air emission

regulators refer to such efficiencies as "six-9s" treatment level.

Incinerator effectiveness depends on three factors, which are often cited as the three "T"s of

combustion:

Temperature of the combustion chamber

Time -- the residence time of the waste in the chamber

Turbulence -- the amount of mixing of the material with air while in the

chamber.

In addition, oxygen (from the air), if present in excess, will drive the combustion reactions to

completion more quickly. If a stoichiometric quantity of oxygen is all that is available, the

reaction will still eventually go to completion. However, excess air will greatly increase the

rate at which combustion is achieved. Different materials also have different excess air

requirements. Air that is sufficient to burn natural gas cleanly, for example, is insufficient for

a heavy fuel oil. Black smoke would result from the incomplete fuel oil burn. Even propane

and natural gas (methane) have different air fuel requirements; a burner that is sized for

propane will produce excess carbon monoxide if it is used with natural gas. Normal

combustion temperatures vary between 900 1500 Centigrade, and occasionally much higher

temperatures are experienced. One of the major difficulties with burning unsorted municipal

waste is the tremendous range in heat content of the waste. Raw, wet garbage requires agreat deal of supplemental fuel to burn; plastics, in contrast, have so much heat content that

they can heat damage the incinerator walls.

Most modern incinerators employ two combustion chambers. The first chamber's function is

to convert the compound to a gas and initiate the combustion process. The second chamber

completes the combustion of the gases. Auxiliary fuel to maintain combustion is normally

added in the primary (first) chamber and sometimes in the secondary chamber. A two-

chamber incinerator is shown in Figure 20. The grate where the waste material is placed in

this incinerator is fixed in position. Such an incinerator can support a grate loading of about

30 lb/hr-ft2. If the as-received heating value of the waste is 4000 to 5000 Btu/lb, then the heat

release rate from the grate will be 120,000 to 150,000 Btu/hr-ft2. Although the fixed gratemight prove satisfactory for a small apartment house incinerator, most larger incinerators

employ a so-called "spreader stoker" to spread the waste on the grate and to ease ash removal.

Several varieties of spreader stokers are shown in Figure 21.


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Curtainwall port

Mixing

chamber

Cleanout doorsunderfire air ports

Fixed

grates

Primary combustionchamber

Secondarycombustion

chamber

Curtain

wall

Secondaryair port

Flame portCharging doorwith overfire

air ports

Cutaway of an in-line multi-chamber incinerator

Figure 20


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(a) Stationary

(c) Oscillating (d) Traveling

(b) Dumping

Types of grates available for combustion of solid fuels

Figure 21


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The spreader stokers in Figure 21 throw the fuel mechanically across the grate from the fuel

hopper in the right-hand side of the drawings. The grates in Figure 21 (a) are still fixed in

place. This fixed grate system requires the ash to fall through the grate without any grate

movement. Airholes in the grate for the underfire air can be blocked by the refuse, which

affects combustion efficiency. In Figure 21 (b), the grates can be rotated mechanically to

dump ash into the bottom ash pit. The air holes can be smaller in size; the smaller air holes

lessen the chance of unburned waste dropping into the ash pit. Two types of oscillating or

travelling grates are shown in Figure 21 (c) and (d). This style of grate lessens the blockage

of air holes and generally improves combustion. Heat release rates for a spreader stoker can

approach a million Btu/hr-ft2, which is six to eight times that of a stationary grate incinerator.

All of these systems, except for the stationary grate, could be considered for use in refuse-

derived fuel (RDF) combustion because of the uniformity of the fuel. Unprocessed fuel is so

variable that a different type of grate has proved to work better. Figure 22 illustrates a stair-

step grate arrangement, with every other step being held stationary. The moveable steps areslowly moved back and forth, pushing the refuse towards the ash hopper. As the refuse falls

from one step to the next, its surface area is exposed for a longer and longer time. It is

essential to provide this mixing or turbulence capability when working with unprocessed

refuse.

In Figure 23, a mass-fired municipal combustor is repeated here from Module ENV 102.01.

This incinerator is used for the large-scale conversion of municipal solid waste , with energy

recovery to electrical energy as used in Los Angeles, California. The detailed steps of the

process are given after the figure.

By using a conversion from megawatts (million watts ) to Btu/hr, one can estimate the amountof energy produced by municipal refuse. If the overall combustion-to-energy conversion

efficiency is 33% (typical), then one must put in 3 megawatts to get 1 megawatt output:

3 megawatts x 3.413 x 106 Btu/hr/megawatt = 1.02 x 107 Btu/hr

if refuse = 6000 Btu/lb, then

1.02 x 107 Btu/hr 6000 Btu/lb = 1700 lb/hr per megawatt out

Thus, almost a ton of refuse per hour is required to generate a megawatt of electricity.

Although this sounds like a great deal of refuse, a city of 25,000 persons producing 5lb/person/day of refuse will equate to 2.6 tons per hour of waste. The generation of 3

megawatts of electricity from this amount of waste should be compared very carefully to the

option of landfilling the waste.


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Mechanically fired incinerator with reciprocating grates

Figure 22


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Section through a typical continuous-feed mass-fired municipal combustor that is used for

energy production from municipal solid waste. (courtesy of County Sanitation Districts of

Los Angeles County, California)

Figure 23


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The operations of the process in Figure 23 are:

1. Solid wastes are received from the collection trucks.

2. Wastes are temporarily held in a storage pit (about 2 days waste volume

capacity).

3. An overhead crane transfers the waste from storage to the charging chute.

4. The charging chute conveys wastes to the furnace chamber.

5. The waste is burned in the furnace chamber.

6. The grates retain wastes while they are burned.

7. The combustion chamber burns gases and small organic particles that rise due

to a forced air draft. Temperatures are generally in excess of 1600 F.

8. The boiler recovers heat from the hot gases using water-filled tubes for the

production of steam.

9. The steam is used to drive a turbine-generator to produce electricity.

10. Ammonia injection is used to control the NOx (nitrogen oxides).

11. A dry scrubber is used to control the SO2 (sulfur dioxide and acid gas controlby neutralization with lime).

12. A fabric filter (bag house) system is used to collect particulates.

13. A fan is used for inducing a draft to help overcome head loss through the air

pollution-control equipment and to supply air to the combustion process.

14. Cleaned gases are discharged through a high stack for atmospheric dispersion.

15. Ash and unburned residue from the grates fall to a hopper where they are

quenched with water.

16. Furnace ash and fly ash from the dry scrubber and bag house are conveyed to

ash treatment and disposal facilities. Ash should be tested against hazardous

waste disposal criteria.


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In the preceding Figure 23, a baghouse was used for particulate control. Baghouses will

likely work better than electrostatic precipitators (ESP) for applications in mass burn

incinerators because of the variability of the fuel input. ESP's are very sensitive to a particle

property called resistivity. The ESP efficiency will vary as the fuel feed resistivity varies,

and the fuel feed naturally will vary greatly with municipal waste.

Solid wastes principally contain these elements: carbon, hydrogen, oxygen, nitrogen, and

sulfur. Lesser quantities of other elements (various metals, for example) may be found in the

ash residue. When the waste is burned with the addition of stoichiometric quantities of

oxygen (or air) that is needed for complete combustion, the typical gaseous products are

carbon dioxide (CO2), water (H2O, flue gas), nitrogen (N2) and small amounts of sulfur

dioxide (SO2). The reactions for the oxidation or combustion of C, H, and S (and their

atomic weights) as contained in municipal solid waste are as follows:

for carbon: C+

O2

CO212 32

for hydrogen: 2H2 + O2 2H2O

4 32

for sulfur: S + O2 SO232.1 32

The stoichiometric amount of air that is required for each of the above combustion reactions

may be calculated as follows:

If it is assumed that dry air contains 23.15% oxygen by weight, the amount of air

required for the oxidation of 1 pound of carbon, hydrogen and sulfur, respectively, will

be:

(32/12) (1/0.2315) = 11.52 lb. air/lb. carbon

(32/4) (1/0.2315) = 34.56 air/lb. hydrogen

(32/32.1) (1/0.2315) = 4.31 lb. air/lb. sulfur


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An example of the calculation for determining the amount of air required for the combustion

of an organic solid waste follows. Note that in step 2 of the Problem, the %'s are given in %

by volume; in step 3 - 5, the %'s are by weight.

Example Problem 2. Determ ination of the stoichiometr ic amount of air r equired for the

combustion of an organic solid waste.

Determine the amount (lbs and ft3) of air that is required for the complete combustion of one

ton of an organic solid waste. Assume that the composition of the organic waste to be

combusted is given by C5H12. Assume the specific weight of air is 0.075 lb/ft3

Solution:

1. Write a balanced stoichiometric equation for the oxidation of the organic compoundbased on oxygen:

C5H

12 + 8O2 5CO2 + 6H2O

72 256

2. Write a balanced equation for the oxidation of the organic compound with air. In

combustion calculations, dry air is assumed to be comprised of 21% oxygen and 79%

nitrogen. Thus, the corresponding reaction to that given in Step 1 for air is

C5H12 + 8O2 + 30.1N2 5CO2 +6H2O + 30.1N2

3. Determine the amount of air required for combustion, assuming air contains 23.15%oxygen by weight

O2 required =256

72 (2000lb / ton) = 7111lb / ton

Air required =7111lb/ ton

0.2315= 30,717 lb/ ton

4. The amount of air required for combustion can also be computed using the factors, given

previously.

Air required for carbon, C =60

72 (2000lb / ton)11.52 = 19,200lb / ton

Air required for hydrogen, H =12

72 (2000lb / ton) 34.56 = 11,520 lb/ ton

Totalair required = 19,200 +11,520 = 30,720lb / ton

5. Determine the volume of air required for combustion.

Volume of air = (30,717 lb/ton)/(0.075 lb/ft3) = 409,560 ft3/ton


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Solid waste combustors are designed to operate as either mass-fired units or RDF units. A

mass fired combustor uses commingled, unseparated solid waste with little or no processing.

An RDF unit uses refuse-derived fuel, a processed waste which has had much of the

noncombustible material removed. Approximately 23% of the U.S. units are of this type.

Mass-fired units have some design considerations that may not be important when using

RDF:

The nature of the waste can be quite variable and may contain unsuitable

materials. The crane operator can reject large unsuitable objects or attempt to

blend high- and low-moisture content materials.

Potentially hazardous materials may be contained in the waste. The combustor

design should be such that the risk of damage to equipment and the risk of

injury to personnel will be minimized.

The use of RDF can provide a combustible product that is more consistent and may be

processed (although more expensive) to a pellet or cube form (sometimes called densified

RDF or d-RDF) that can be burned by itself or mixed with other fuels. RDF combustion units

are typically smaller than mass-burn combustors, but some space will be required for the

processing of the fuel. Because of RDF's greater uniformity of materials and heat value, an

RDF combustor burns more efficiently than a mass-burn combustor. Air pollution control is

also

treatment of solid waste

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