Energy-from-waste technologies are designed to reduce or eliminate waste that otherwise contributes to global warming, while simultaneously generating energy and reducing the need to consume nonrenewable resources.

Energy from waste, or waste-to-energy, refers to any waste treatment that generates energy from a waste source. Most waste-to-energy technologies make electricity directly through combustion, or they produce fuels, such as methane, hydrogen, biodiesel, or ethanol. These technologies reduce the amount of waste on the planet and also reduce the need to produce energy using technologies that create more waste.

As human populations and industrialization have increased, the amount of generated municipal solid waste (MSW) has grown steadily. In the early twenty-first century, the United States produced around 1.8 kilograms of such waste per person per day. Because it has high energy content, refuse material can be burned to release energy. The amount of energy generated by burning waste is about half that produced by burning the same mass of coal.

Several cities around the world use incinerators to convert refuse to electricity. Incinerators are huge furnaces, capable of handling 15 metric tons of waste per hour, in which temperatures are high enough to allow waste to be burned completely. The value of the electricity generated by these incinerators offsets the costs of MSW handling and burning. However, incineration has serious environmental consequences, including the production of greenhouse gases (GHGs). Burning MSW produces nitrogen oxide as well as carbon dioxide (CO2), the primary GHG. There is a worldwide awareness that CO2 emissions contribute to global warming. Some of the CO2 produced in incinerators is biomass-derived and is considered to be part of the Earth's natural carbon cycle, however.

Incineration is particularly popular in countries where land is a scarce resource. Sweden and Denmark have been leaders in using the energy generated from incineration for more than a century. Although incinerators reduce the volume of the original waste by 95-96 percent, the ash produced after incineration must still be disposed of in landfills. Ash often contains high concentrations of hazardous heavy metals, such as lead or cadmium. Ash may also include precious metals, such as aluminum, gold, copper, and iron. These metals are recycled before ash deposition into the landfills. Alternatively, ash can be used for road work or building construction, provided that it does not contain hazardous substances.

Buried in landfills, wastes do not have access to oxygen. The resulting anaerobic waste decomposition produces biogas, which is 30 percent methane. Methane is a very powerful GHG, but it is also a very good fuel. In order to avoid releasing methane into the atmosphere, a number of cities install "gas wells" in landfills to capture the methane they generate and use it as fuel. There are several such landfill gas facilities in the United States that generate electricity.

Biogas can also be generated from wastewater and from animal waste. Domestic wastewater consists of substances such as ground garbage, laundry water, and excrement. All these components are biological molecules that microbes can eat. Thus, one of the most common treatments of wastewater and manure employs microbial anaerobic digestion. Such digestion is very similar to the process of landfill waste decomposition, and it yields biogas.

In this process, wastewater or manure is fed into digesters (bioreactors), where microorganisms metabolize it into biogas. Biogas can be used to fuel engines connected to electrical generators to produce electricity. The nutrient-rich sludge remaining after digestion can be used as fertilizer. In many countries, millions of small farmers maintain simple digesters at home to generate energy. The only side effect of this technology is that burning methane in combusting engines produces CO2. Because methane has a greater global warming potential than does CO2, however, the process potentially results in a net decrease in GHG emissions.

Where energy is generated as a by-product of waste disposal, agricultural waste may have considerable merit. A great number of cellulosic wastes result from the cultivation of crops such as corn. This waste can be turned into ethanol. Use of ethanol as fuel has been vigorously promoted. Ethanol is mainly produced by fermentation of sugars derived from food crops with the help of baker's yeast. However, making ethanol from leftover materials such as corn stover is highly desirable, because such materials consist largely of sugars but they have no direct food use. Other cellulosic waste material such as sawdust, wood chips, cane waste (bagasse), and wastepaper can be converted into ethanol as well. In contrast to food-to-ethanol conversion, converting farm waste to ethanol involves little or no contribution to the greenhouse effect.

Plant waste material may also be gasified to produce syngas, a mixture of carbon monoxide (CO) and hydrogen (H2). Syngas is considered an alternative fuel, because it generates electricity with coproduction of water and CO2 when burned. Syngas can also be converted by certain microbes into other alternative fuels, such as ethanol and H2.

Exhaust streams from power plants and other manufacturing units contain high levels (up to 20 percent) of CO2. Typical coal-fired power plants account for up to 13 percent of anthropogenic CO2 emissions. Researchers are exploring the application of photosynthetic microalgae to remove CO2 from the emissions of power stations and other industrial plants. Algae utilize CO2 and, at the same time, produce oil and H2 as part of their growth process. Therefore, they can be used to generate environmentally friendly biofuels such as biodiesel and H2.

The biological potential of photosynthetic microalgae for CO2 removal and biofuel generation is determined by their cultivation techniques. Current industrial production of microalgae is achieved in open "raceway" ponds of some thousand square meters in size. These systems suffer from severe limitations, such as lack of temperature control, low attainable cellular concentrations, and difficulty in preventing contamination. The need to overcome these limits led to design and development of photobioreactors.

Photobioreactors are closed systems that are made of an array of tubes or tanks, in which microalgae are cultivated and monitored. The main challenge in photobioreactor design is to create a simple, inexpensive, high-cell-density, energy-efficient reactor that is scalable to meet the needs of industrial production. Several U.S. companies (GreenFuel Technologies, GreenShift, Solix, and Valcent Products) have created pilot-scale photobioreactors for CO2 mitigation and biofuel production by microalgae.

Contemporary society generates significant amounts of waste that affect the climate on Earth as the result of GHG release, as well as polluting the environment generally. One of the solutions to this situation is to turn this waste into energy. Energy-from-waste technologies are valuable for energy generation and may represent efficient means for removing waste while minimizing environmental and climatological side effects. However, energy-from-waste technologies should be used under strict environmental regulations to avoid generating additional pollutants or otherwise exacerbating the problems they seek to solve. For instance, responsible application of energy-from-waste technology would require recycling the CO2 released from incinerators.

Another solution to waste management is recycling. Making products such as paper from recycled materials requires 65 percent less energy and generates 75 percent less CO2 and methane emissions than does similar production using virgin raw materials. Therefore, waste recycling, although it does not generate energy, saves considerable amounts of energy and reduces GHG emissions.

Bibliography:

1)         Haag, Amanda L. "Algae Bloom Again." Nature 447 (May 31, 2007): 520-521.

2)         Nebel, Bernard J., and Richard T. Wright. Environmental Science: Towards a Sustainable Future. Englewood Cliffs, N.J.: Prentice Hall, 2008.

3)         Williams, Paul T. Waste Treatment and Disposal. Chichester, West Sussex, England: John Wiley and Sons, 1998.

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