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| Waste Management |
Waste management is the collection, transport, processing, recycling or disposal of waste materials, usually ones produced by human activity, in an effort to reduce their effect on human health or local aesthetics or amenity. A subfocus in recent decades has been to reduce waste materials' effect on the natural world and the environment and to recover resources from them.
Waste management can involve solid, liquid or gaseous substances with different methods and fields of expertise for each.
Waste management practices differ for developed and developing nations, for urban and rural areas, and for residential, industrial, and commercial producers. Waste management for non-hazardous residential and institutional waste in metropolitan areas is usually the responsibility of local government authorities, while management for non-hazardous commercial and industrial waste is usually the responsibility of the generator.
reduce, reuse and recycle, which classify waste management strategies according to their desirability in terms of waste minimization. The waste hierarchy remains the cornerstone of most waste minimisation strategies. The aim of the waste hierarchy is to extract the maximum practical benefits from products and to generate the minimum amount of waste.
Waste collection methods
Waste collection methods vary widely between different countries and regions, and it would be impossible to describe them all. Many areas, especially those in less developed countries, do not have a formal waste-collection system in place.
For example, in Australia most urban domestic households have a 240-litre (63.4 U.S. gallon) bin that is emptied weekly from the curb using side- or rear-loading compactor trucks.
In Europe and a few other places around the world, a few communities use a proprietary collection system known as Envac, which conveys refuse via underground conduits using a vacuum system. Roosevelt Island has had this system since 1975.
In Canadian urban centres curbside collection is the most common method of disposal, whereby the city collects waste and/or recyclables and/or organics on a scheduled basis. In rural areas people usually dispose of their waste by hauling it to a transfer station. Waste collected is then transported to a regional landfill.
Waste management methods
3. Resource recovery
5. Composting and anaerobic digestion
6. Mechanical biological treatment
7. Pyrolysis & gasification
Waste management methods for vary widely between areas for many reasons, including type of waste material, nearby land uses, and the area available.
For example, in Australia the most common method of disposal of solid household waste is in landfill sites, due to it being a large country with a relatively low-density population (therefore landfill sites are relatively common).
By contrast, in Japan it is more common for solid waste to be incinerated because that country is smaller and population density greater (therefore less room is available for landfill sites).
Disposing of waste in a landfill is one of the most traditional method of waste disposal, and it remains a common practice in most countries. Historically, landfills were often established in disused quarries, mining voids or borrow pits. A properly-designed and well-managed landfill can be a hygienic and relatively inexpensive method of disposing of waste materials in a way that minimises their impact on the local environment.
Older, poorly-designed or poorly-managed landfills can create a number of adverse environmental impacts such as wind-blown litter, attraction of vermin, and generation of leachate where result of rain percolating through the waste and reacting with the products of decomposition, chemicals and other materials in the waste to produce the leachate which can pollute groundwater and surface water. Another byproduct of landfills is landfill gas (mostly composed of methane and carbon dioxide), which is produced as organic waste breaks down anaerobically. This gas can create odor problems, kill surface vegetation, and is a greenhouse gas.
Design characteristics of a modern landfill include methods to contain leachate, such as clay or plastic lining material. Disposed waste is normally compacted to increase its density and stablise the new landform, and covered to prevent attracting vermin (such as mice or rats) and reduce the amount of wind-blown litter. Many landfills also have a landfill gas extraction system installed after closure to extract the landfill gas generated by the decomposing waste materials. Gas is pumped out of the landfill using perforated pipes and flared off or burnt in a gas engine to generate electricity. Flaring off the gas is generally a better environmental outcome than allowing it to escape to the atmosphere, as this consumes the methane (which is a far more potent greenhouse gas than carbon dioxide).
Many local authorities, especially in urban areas, have found it difficult to establish new landfills due to opposition from owners of adjacent land. Few people want a landfill in their local neighborhood. As a result, solid waste disposal in these areas has become more expensive as material must be transported further away for disposal (or managed by other methods).
This fact, as well as growing concern about the impacts of excessive materials consumption, has given rise to efforts to minimise the amount of waste sent to landfill in many areas. These efforts include taxing or levying waste sent to landfill, recycling the materials, converting material to energy, designing products that use less material, and legislation mandating that manufacturers become responsible for disposal costs of products or packaging. A related subject is that of industrial ecology, where the material flows between industries is studied. The by-products of one industry may be a useful commodity to another, leading to a reduced materials waste stream.
Some futurists have speculated that landfills may one day be mined: as some resources become more scarce, they will become valuable enough that it would be economical to 'mine' them from landfills where these materials were previously discarded as valueless. A related idea is the establishment of a 'monofill' landfill containing only one waste type (e.g. waste vehicle tires), as a method of long-term storage.
Incineration is a waste disposal method that involves the combustion of waste at high temperatures. Incineration and other high temperature waste treatment systems are described as "thermal treatment". In effect, incineration of waste materials converts the waste into heat, gaseous emissions, and residual solid ash. Other types of thermal treatment include pyrolysis and gasification.
A waste-to-energy plant (WtE) is a modern term for an incinerator that burns wastes in high-efficiency furnace/boilers to produce steam and/or electricity and incorporates modern air pollution control systems and continuous emissions monitors. This type of incinerator is sometimes called an energy-from-waste (EfW) facility.
Incineration is popular in countries such as Japan where land is a scarce resource, as they do not consume as much area as a landfill. Sweden has been a leader in using the energy generated from incineration over the past 20 years. Denmark also extensively uses waste-to-energy incineration in localised combined heat and power facilities supporting district heating schemes.
Incineration is carried out both on a small scale by individuals, and on a large scale by industry. It is recognised as a practical method of disposing of certain hazardous waste materials (such as biological medical waste), though it remains a controversial method of waste disposal in many places due to issues such as emission of gaseous pollutants.
Breaking down complex chemical chains such as dioxin through the application of heat usually cannot be done by simply burning the material at the temperatures seen in an open-air fire. It is often necessary to supplement the combustion process with gas or oil burners and air blowers to raise the temperature high enough to result in molecular breakdown. Alternately, the exhaust gases from a natural air fire may pass through tubes heated to sufficiently high temperatures to trigger thermal breakdown.
Thermal breakdown of pollutant molecules can indirectly create other pollution problems. Dioxin breakdown begins at 1000°C, but at the same time poisonous nitrogen oxides and ozone begin to form when atmospheric nitrogen and oxygen break down at 1600°C. This undesired oxide formation may require further catalytic treatment of the exhaust gases.
3. Resourse Recovery:
A relatively recent idea in waste management has been to treat the waste material as a resource to be exploited, instead of simply a challenge to be managed and disposed of. There are a number of different methods by which resources may be extracted from waste: the materials may be extracted and recycled, or the calorific content of the waste may be converted to electricity.
The process of extracting resources or value from waste is variously referred to as secondary resource recovery, recycling, and other terms. The practice of treating waste materials as a resource is becoming more common, especially in metropolitan areas where space for new landfills is becoming scarcer. There is also a growing acknowledgement that simply disposing of waste materials is unsustainable in the long term, as there is a finite supply of most raw materials.
There are a number of methods of recovering resources from waste materials, with new technologies and methods being developed continuously.
In some developing nations some resource recovery takes place by way of manual labourers who sift through un-segregated waste to salvage material that can be sold in the recycling market. These unrecognised workers called waste pickers or rag pickers, are part of the informal sector, but play a significant role in reducing the load on municipalities' solid waste management departments. There is an increasing trend in recognising their contribution to the environment and there are efforts to try and integrate them into the formal waste management systems, which is proven to be both cost effective and also appears to help in urban poverty alleviation. However, the very high human cost of these activities including disease, injury and reduced life expectancy through contact with toxic or infectious materials would not be tolerated in a developed country.
Recycling means to recover for other use a material that would otherwise be considered waste. The popular meaning of ‘recycling’ in most developed countries has come to refer to the widespread collection and reuse of various everyday waste materials, such as newspapers and drink bottles. They are collected and sorted into common types so that the raw materials from these items can be used again to create new products. In many areas, material for recycling is collected separately from general waste using dedicated bins and collection vehicles. Other waste management processes can recover materials from mixed waste streams.
In developed countries, the most common consumer items recycled include aluminium beverage cans, steel, food and aerosol cans, HDPE and PET bottles, glass bottles and jars, paperboard cartons, newspapers, magazines, and cardboard. Other types of plastic (PVC, LDPE, PP, and PS: see resin identification code) are also recyclable, although these are not as commonly collected. These items are usually composed of a single type of material, making them relatively easy to recycle into new products. The recycling of complex products (such as computers and electronic equipment) is more difficult and costly, due to the separation and reprocessing required.
The economics of recycling waste products is complex, depending variously on the product, location and market forces. Recycled or used materials compete in the market with new (virgin) materials. The cost of collecting and sorting the materials often means that they are equally or more expensive than virgin materials. This is most often the case in developed countries where industries producing the raw materials are well-established. Practices such as trash picking can reduce this value further, as choice items are removed (such as aluminium cans). In some countries, recycling programs are subsidised by deposits paid on beverage containers (see container deposit legislation).
Most economic systems do not account for the benefits to the environment of recycling these materials, compared with extracting virgin materials. It usually requires significantly less energy, water and other resources to recycle materials than to produce new materials . For example, recycling 1000 kg of aluminum cans saves approximately 5000 kg of bauxite ore being mined (source: ALCOA Australia) and prevents the generation of 15.17 tonnes CO2eq greenhouse gases ; recycling steel saves about 95% of the energy used to refine virgin ore (source: U.S. Bureau of Mines). Some attempts have been made to account for these benefits using market interventions, such as mandatory recycling programs, container deposit legislation and extended producer responsibility laws.
When consumer-separated recycling is a government requirement, waste is often not well separated due of either ignorance or contempt of the rules. This results in glass containers that may have metal lids still attached and rotted food inside, aluminum cans full of chewing tobacco spit and cigarette butts, corrugated paper boxes soiled with oils, solvents, or rotting food, and inclusion of incompatible plastic types in a plastics recycling bin. This can all lead to process contamination, work stoppage, a system cleanout, and landfill disposal of the contaminated batch of otherwise recyclable materials. Re-sorting of consumer-separated wastes is often needed to prevent recycling process contamination.
A common method of machine sorting of complex waste streams is to shred the entire stream into a fine particulate of similar size. A magnetic conveyor belt removes ferrous metals from this particulate, and cyclonic separation towers separate objects from the waste stream by mass. Spectral imaging such as with X-rays can further separate glass and various metals from the stream by scanning for x-ray absorption and firing precise puffs of air at the falling pieces to push them sideways into various sorting bins.
The remainder of the unsorted shredded material is known as fluff and contains mostly plastics, paper and other organic materials. When vehicles are shredded and processed in this manner for recycling, often a large mass of fluff results from the plastics used in the seat cushions, dashboard, roof liner, carpeting, and so forth. There are not many well-established processes for further separation and recycling of fluff, other than incineration or pyrolysis.
5. Composting and anaerobic digestion:
Waste materials that are organic in nature, such as plant material, food scraps, and paper products, are increasingly being recycled using biogical composting and/or digestion processes to decompose the organic matter and kill pathogens. The resulting organic material is then recycled as mulch or compost for agricultural or landscaping purposes.
There are a large variety of composting and digestion methods and technologies, varying in complexity from simple windrow composting of shredded plant material, to automated enclosed-vessel digestion of mixed domestic waste. These methods of biological decomposition are differentiated as being aerobic in composting methods or anaerobic in digestion methods, although hybrids of the two methods also exist.
An example of waste management through composting is Green Bin Program in Toronto, Canada, where household organic waste (such as kitchen scraps and plant cuttings) are collected in a dedicated container and then composted.
6. Mechanical biological treatment:
Mechanical biological treatment (MBT) is a technology category for combinations of mechanical sorting and biological treatment of the organic fraction of municipal waste. MBT is also sometimes called BMT (Biological Mechanical Treatment), however this simply refers to the order of processing.
The "mechanical" element is usually a bulk handling mechanical sorting stage. This removes recyclable elements from the waste (such as metals, plastics and glass), and/or processes it to produce refuse derived fuel (RDF) that is burnt in power plants, boilers or kilns. The "biological" element refers to a biological digestion process, which breaks down the biodegradable component of the waste to produce biogas and/or organic matter. The biogas can be used to generate energy, and organic matter recycled as compost.
Edmonton, Canada, where 200,000 tonnes of residential solid waste and 22,500 tonnes of biosolids are composted each year to produce 80,000 tonnes of compost. The co-composter itself is 38,690 square metres in size, equivalent to 8 football fields.
Pyrolysis & gasification
Pyrolysis and gasification are two related forms of thermal treatment where waste materials are heated to high temperatures with limited oxygen availability. The process typically occurs in a sealed vessel under high pressure. Converting material to energy in a sealed environment is potentially more efficient than direct incineration, with more energy able to be recovered and used.
Pyrolysis of solid waste converts the material into solid, liquid and gas products. The liquid oil and gas can be burnt to produce energy or refined into other products. The solid residue (char) can be further refined into products such as activated carbon.
Gasification is used to convert organic materials directly into a synthetic gas (syngas) composed of carbon monoxide and hydrogen. The gas is then burnt to produce electricity and steam. Gasification is used in biomass power stations to produce renewable energy and heat.
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