Waste disposal and

processing

Contents1. Landfills

Landfill site consideration

Landfill gas

Leachate

2. Unit operations for processing

3. Thermal conversion technologies

combustion

dioxin

pyrolysis

gasification

4. Biological and chemical conversion technologies

aerobic composting

anaerobic digestion

5. Recycling at a glance

Landfills

These are the physical facilities for the disposal of residual solid waste in the

surface soils of the earth

Sanitary landfill refers to an engineered facility for the disposal of MSW

designed and operated to minimize public health and environmental impacts

Landfills for the disposal of hazardous waste are called secured landfills

The term cell is used to describe the volume of material placed in a landfill

during one operating period, usually one day

A lift is a complete layer of cells over the active area of landfill

Daily cover usually consists of 6-12 inches of native soil or alternative

materials

Landfill site consideration

1. Haul Distance

2. Location restrictions

3. Available land area

4. Site access

5. Soil conditions and topography

6. Climatologic conditions

7. Surface water hydrology

8. Geologic and hydro geologic conditions

9. Local environment conditions

10. Ultimate use for completed landfills

Landfill Gas

"Landfill gas" is not the same thing as "natural gas" or "methane." They are

three separate terms which mean different things. They should not be used

interchangeably. The term "landfill methane" is deceiving as it's usually used

to imply that landfill gas is simply methane.

Natural gas is approximately 80-99% methane, with the remainder being

mostly other hydrocarbons (ethane, propane, butane, etc.) as well as some

nitrogen, oxygen, water, CO2, sulfur and various contaminants.

Leachate

The liquid that collects at the bottom of the landfill is known as leachate. In

deep landfills leachate s often collected at intermediate points. In general

leachate is a result of percolation of precipitation, uncontrolled runoff and

irrigation water into the landfill

Leachate can also include water initially constituted in the waste. It is a

source of ground water pollution

Leachate contains a variety of chemical constituents derived from

solubilisation of the materials present in the landfill

Clay layer and geomembrane serves as a barrier for the movement of

leachate

Unit operations used for processing

1. Size reduction

It is the unit operation used to reduce the size of the materials in MSW. It is done to use the process material directly for composting.

Types of equipments used – hammermill shredders, shear shredders, tub grinders

2. Size separation

In this process screens are used to separate waste on the basis of size

3. Density separation

typical applications include air, classifiers for the preparation of RDF; inertial separation for the processing of commingled waste; floatation for processing of construction debris

Contd..4. Electric and magnetic field separation

Typical applications include, separation of plastics or paper; the separation of

ferrous from non ferrous materials

5. Densification (compaction)

These are the unit operations that are used to increase the density of recovered

materials to reduce transportation costs and simplify storage. Typical

applications include baling for cardboard, paper, plastics, and aluminium cans;

and the use of cubing and pelletizing for the production of densified RDF.

6. Materials handling

Typical applications include conveyors for transport of MSW and recovered

materials, storage bins; rolling stock such as forklifts, front-end loaders,etc,

Thermal conversion technologies

It is the conversion of waste into gaseous, liquid and solid conversion products.

On the basis of air requirement it is of three types

Combustion (Incinerators)

It is a process of chemical oxidation with excess air

End products include nitrogen, carbon dioxide and flue gas

Energy can be recovered by heat exchange from the hot combustion gases

Solid waste combustion systems are designed to operate with two types of

solid waste fuel

1. commingled solid waste(mass fired)

2. processed solid waste refuse-derived fuel (RDF-fired)

Dioxins

"Dioxins" refers to a group of toxic chemical compounds that share certain

chemical structures and biological characteristics. Dioxins can be released into

the environment through forest fires, backyard burning of trash, certain

industrial activities, and residue from past commercial burning of

waste(incinerators). Dioxins break down very slowly and past releases of dioxins

from both man-made and natural sources still exist in the environment.

Almost every living creature has been exposed to dioxins. Studies have shown

that exposure to dioxins at high enough levels may cause a number of adverse

health effects, including cancer. The health effects associated with dioxins

depend on a variety of factors including: the level of exposure, when someone

was exposed, and for how long and how often someone is exposed.

Pyrolysis

Thermal processing of waste in complete absence of oxygen

pyrolysis use an external source of heat

Gasification on the other hand is self sustaining

It is an endothermic process

Three major components resulting from pyrolysis process are:-

1. a gas steam(hydrogen, methane, carbon monoxide, carbon dioxide)

2. a liquid fraction( tar, oil stream of complex hydrocarbons)

3. a char(pure carbon)

Gasification

Process of partial combustion

Efficient technique for volume reduction and energy recovery

Generation of a combustible fuel gas rich in CO, hydrogen and some saturated

hydrocarbons

This gas can be used in IC engines, gas turbine, or boiler under excess-air

conditions

It is an exothermic process

Biological and chemical conversion

technologies

Aerobic composting

This is most commonly used for the conversion of organic portion of the MSW

to a stable humus like material known as compost

volume reduction is up to 50%

Processing time is 20 to 30 days

Applications of aerobic composting include

1. yard waste

2. separated MSW

3. commingled MSW

4. co-composting with waste water sludge

Anaerobic digestion

It is used to produce methane from organic fraction of MSW

The first step involves hydrolysis of higher-molecular mass compounds

The second step involves conversion to low-molecular mass compounds

The third step involves intermediate compounds to simpler end products,

principally, methane and carbon dioxide.

The process is net energy producer

Time required is 20-40 days

Volume reduction is up to 50%

Recycling at a glance

References

http://www.syngastechnology.com/

http://www.pyrolysisplant.net

http://Yokogava.com

Wikipedia.org

George Tchobanoglous Integrated solid waste management

http://www.environmentalchoice.co.za

Thanks..