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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


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


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


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.


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


http://www.smh.com.au/ffximage/2007/03/10/recycling_wideweb__470x348,0.jpg


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


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


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


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


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


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.


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.


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.


Thermal Waste TreatmentA Waste Incinerator:


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.


Thermal Waste TreatmentCombusting Process of Grate Firing:

In grate firing the waste undergoes the following:

-Drying

-Pyrolysis

-Final combustion

-Gasification

-Afterburning


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.


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


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


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.


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


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


http://ec.europa.eu/dgs/jrc/site_images/normal_4890.jpg


Physical and Chemical Waste TreatmentFiltration:


Physical and Chemical Waste TreatmentSeparation by density differences:


Other techniques include:

-Immobilisation through encapsulation

-Shredding and grinding

-Compacting

Physical and Chemical Waste Treatment


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


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)


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

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