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INVENT - Integrated Waste Management modules for different courses of graduate studies
Chapter Five
Waste Processing, Treatment and Recycling
Joe Green
Dr Chris Wooldridge
Cardiff University
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Learning Outcomes:
By completing this section you should:
•Be aware of the options for waste separation
•Be aware of the stages in thermal, biological, chemical and physical waste treatment
•Have knowledge of the use of material in waste recycling
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Manual Separation
• Manual separation sorts different qualities of wastepaper, colour specific glass, clear and coloured polyethylene foil or removes contaminating materials out of household or industrial waste streams.
• The core disadvantage of manual separation is the cost.
• Manual separation can distinguish between positive(removal of recyclables) and negative (removal of unusable products) sorting
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Manual Separation
Asia:
- Recycled material is used if it costs less than the raw product.
-Waste is handled formally and informally.
-Formal separation involves the separation of the waste in the treatment facility after collection.
-Informal sorting involves waste pickers sorting it prior to the authorized collection vehicles arrived. They can sell the materials which they collect.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Mechanical Separation:
Definition: the use of sorting or separation aggregates to recover single materials for reuse from municipal solid waste.
Size reduction considerations:
• The physical characteristics of the material to be reduced (e.g. particle size, structure, hardness, brittleness and fissionability),
• The future use of the material.
• The required properties of the final material (e.g. particle size, particle size distribution and average particle size)
Manual Separation
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INVENT - Integrated Waste Management modules for different courses of graduate studies
http://www.smh.com.au/ffximage/2007/03/10/recycling_wideweb__470x348,0.jpg
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste Classification:
Screening- Screening of material involves segregation into specific particle size categories.
- It is performed by separating particles based on the size of the openings on a screened surface.
- If the particles are smaller than the openings, they fall through the moving screen and become a part of the fine faction.
- The remaining material is considered the oversize fraction.
Air Classifier- The classifier separates according to the falling velocity of the particles.
- The falling velocity depends on particle form and the specific gravity.
- Apparent density, moisture content, waste composition, agglomeration tendency of the waste and any previous size reduction have an influence on the efficiency of the classifier.
Manual Separation
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste Sorting:
-Techniques include:
Density, flotation, optical, magnetic and electrical sorting.
- Waste separation technology includes the use of:
Magnetic separators, hydrocyclones, density sorters and flotation separators. http://www.recycling.bg/english/images/machines/magnetic%20Drums.JPG
Mechanical Separation
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste Compaction:
-The primary goal of compaction is to reduce surface area and increase bulk.
-Two typical methods for compaction are used:
-Build-up agglomeration – this technique uses binding agents or added fluids after drying the material.
-Compaction agglomeration - this is produced by outside acting forces.
Mechanical Separation
http://www.exeter.gov.uk/media/image/a/f/Presort_Waste_Compactor_image.jpg
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste Recycling
- Recycling involves reprocessing a waste item into a usable itemeither in the same form, as the original product, or into a different product.
- It is widely assumed to be environmentally beneficial, althoughcollecting, sorting and processing materials does give rise to environmental impacts and energy use.
- To improve resource recovery of source separated or commingled wastes in the facilities, appropriate processing equipment should be chose to separate the components into usable fractions.
- Recycling includes collecting, sorting, processing or converting of materials
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste RecyclingRecovery and use of secondary materials:
Recycling Household Waste
- A range of systems are used:
-Drop-off system
-Pick-up systems
-Household waste is categorized into groups including: paper/paperboard, ferrous/non ferrous metal, vegetative matter and composite materials
Recycling of Household-like Commercial Waste
- Contaminants reduce the sorting ability and resource quality.
- Source separation of commercial waste such as large or more recyclables containers can be carried out in a manner similar to household waste collection.
- Other processing technologies such as screens, air classifiers, ballistic and magnetic separator can be applied to remove contaminants
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste RecyclingProduction of RDF:MSW is not optimal fuel, because:
-The low heating value of household waste is caused by high ash and moisture content.
-The energy content of the source material varies not only throughout the year but also in the long-term based on changes in lifestyle.
- Heavy metals and halogenated compounds are found in household waste.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Waste RecyclingMarketing of Recyclable Products:
- For effective recycling a market survey for potential buyers, specifications, and prices of recyclables should be conducted as well as developing a concept.
- In market surveys, it is necessary to identify quantities of potential recyclables in a given collection area so that the existing collection, disposal methods and costs can be determined.
- The marketing of recyclables can be support and promoted by manufacturing industries, the retail sector, consumers, and the public sector.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste Treatment
Incineration for MSW and Commercial Waste:
Incineration is designed to:
- Make hazard waste residues inert while minimizing emissions
- Destroy organic contaminants and concentrating inorganic contaminants
- Minimize the quantity of waste requiring disposal
- Recover the waste’s heat value
- Transform the residues into usable secondary products to replace raw materials and resources.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentA Waste Incinerator:
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentStorage, pre-processing and charging:
Due to delivery of waste in bulk, and continuous feed into the incinerator storage is necessary.
There are three types of bunkers:
- Deep bunker
- Plate conveyor bunker
- Shallow bunker
- Cranes operate as pre-processors and can sort incompatible materials.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentCombusting Process of Grate Firing:
In grate firing the waste undergoes the following:
-Drying
-Pyrolysis
-Final combustion
-Gasification
-Afterburning
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentFluidised bed incinerators:
- In order to form a stabled fluidized-bed at a high temperature, the silica sand is filled in the incinerator furnace.
- The product to be incinerated is supplied on the sand fluidized-bed
- When the heated fluidizing air is blown from the bottom of the furnace, the fluidized-bed is actively formed in the sand layer.
- The ash produced by incineration is crushed into small pieces and discharged out of the system accompanies with the exhaust gas and collected.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentFluidised bed incinerators:
- The incinerator consists of a lined, cylindrical combustion chamber, with air distribution nozzles on the bottom.
- The nozzles supply the air for combustion.
- This results in a relatively large surface area for heat transfer and substance exchange.
Further considerations include:
- Incinerator slag removal
- Boiler and waste Heat Recovery
- Control and monitoring
- Emission control
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentPyrolysis and Gasification:
Pyrolysis:
- Pyrolysis is the thermal decomposition of organic materials without gasification aids such as oxygen, air, CO2, steam, etc.
- In the temperature ranges between 150 and 900 °C, volatile compounds are expelled, and complex carbohydrates are converted into simpler ones
Gasification:
- Gasification is a process whereby carbon-containing materials are converted at high temperatures into gaseous fuels.
- Reactive gases are added to oxidize the residual carbon from the glowing embers of the pyrolysis coke at temperatures in excess of 800 °C
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentPyrolysis and Gasification:
Advantages
-Uncomplicated and cost effective processes.
-A potential for energy and resource recovery.
-The recoverable product (energy) is easily stored.
-Flexibility with respect to various wastes and changing waste compositions.
-Almost complete prevention of environmental pollution.
Environmental Considerations
- Hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), ammonia (NH3), hydrogen sulphide (H2S), and hydrogen chloride (HCl) are produced
- As the gas cools, they condense into a tarry substance and become concentrated.
-The wastewater contains organics including: oils, tars and phenols. As the contaminants can only be partially decomposed in biological sewage treatment plants, a chemical/physical pre-treatment is recommend.
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste TreatmentHydrogenation and Hydrolysis:
-Hydrogenation is the reaction of certain substances with hydrogen under pressure and rising temperature.
-Hydrogenation is operated at temperature of 440 - 480 °C and at a pressure of about 300 bar and in presence of hydrogen.
-The input materials are carbon-containing waste such as coal, distillation residues from crude oil processing, waste oil, waste oil containing PCBs, spent activated carbon (containing solvent), spent paint thinner, and scrap plastics
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Thermal Waste Treatment Hazardous waste incinerators:
- Hazardous wastes are delivered and stored in containers and barrels of varying shapes and sizes.
- Hazardous waste that can not be incinerated due to their physical, chemical, or toxicological properties must be pre-treated prior to incineration.
Furnaces include:
-Rotary kiln furnace
-Multi-hearth furnace
-Fluidized bed furnace, and
-Combustion chamber
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INVENT - Integrated Waste Management modules for different courses of graduate studies
http://ec.europa.eu/dgs/jrc/site_images/normal_4890.jpg
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Physical and Chemical Waste TreatmentFiltration:
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Physical and Chemical Waste TreatmentSeparation by density differences:
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Other techniques include:
-Immobilisation through encapsulation
-Shredding and grinding
-Compacting
Physical and Chemical Waste Treatment
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Chemical Waste Treatment
-Chemical treatment involves altering a waste’s chemical composition, structure, and properties through chemical reactions.
-Chemical treatment can consist of mixing the waste with other materials (reagents), heating the waste to high temperatures, or a combination of both.
-Options for chemical precipitation include:
- Hydroxide precipitation
- Sulphide precipitation
- Carbonate precipitation
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Chemical Waste TreatmentSolidification and stabilisation
-Solidification reduces water content of waste by adding binders and additives.
-The aim is to reduce mobility and toxicity of contaminants, it is regarded as a treatment step for the preparation of waste for disposal into landfills
Processes include:
-In situ treatment
-In-drum processing
-In-plant processing
-Mobile-plant processing
-Ex situ treatment
Chemical oxidation and reduction
-Oxidation-reaction (redox) reactions can treat a range of organic compounds
- Chemical oxidation processes include:
-Oxidation with chlorine (Cl-) and hypochlorite salt (OCl-)
-Oxidation with chlorine dioxide
-Chemical reduction processes include:
-Reduction of sulphur dioxide (SO2) and sulfites (SO3-)
-Reduction with metals (iron, zinc)
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INVENT - Integrated Waste Management modules for different courses of graduate studies
Self Assessment Questions:
• Name the 5 types of screen used in waste screening
• Name the 4 binding mechanisms for MSW
• How it is possible to increase resource efficiency in recycling
• Explain how a fluidised bed incinerator works
• Define pyrolysis and chemical precipitation