Unit Operations for Processing Municipal Solid Waste
Course Learning Outcomes for Unit III Upon completion of this unit, students should be able to:
Reading Assignment Chapter 4: Mechanical Processes
Unit Lesson As the world begins to approach the peak rate at which natural resources can be delivered into commerce, the role of segregating, recycling, and reusing materials from waste refuse becomes more important to the global economy. In a world that has adopted the key principles of sustainable waste management, waste is now seen as a valuable resource and is no longer viewed as being without value. A high-level of resource recovery is accomplished by applying different processing strategies that extract valuable materials from waste refuse (Department for Environmental Food & Rural Affairs, 2013). No single technology can meet all recovery objectives. Therefore, multi-unit operations that operate in series to each other will be needed if materials are to be extracted that still have or can be converted into products having commercial value. Each of the different unit operations will exert its own positive impacts on the environment. The more material that can be extracted for recycling and reuse, the less residue remains that will need to be either thermally oxidized for heat recovery or placed into a landfill. While practicing engineers in the field generally talk about separation technologies, there is also the need to include a discussion about conversion processes. Conversion processes can be classified into the categories of physical, thermal, or biological. The physical technologies that are primarily used when refuse is to be prepared for energy recovery are comprised of the following: screening, picking, shedding, grinding, wet separation, drying, pressing, baling, and pelletizing. The end result is that the processed refuse is compressed and densified into a refuse-derived fuel (rdf) for combustion in an incineration unit. Thermal technologies convert solid materials into gaseous products that are commonly referred to as synthetic gas or syngas. When oxygen is present, the process is referred to as gasification. In the absence of oxygen, the process is known as pyrolysis. Pyrolysis units are designed to operate at temperatures greater than 925 degrees Fahrenheit in the absence of oxygen. In addition to generating a pyrolysis char and oil, pyrolysis units produce syngas comprised of methane, carbon monoxide and dioxides, hydrogen, and complex organics. The syngas that is generated can be combusted in boilers, gas turbines, or internal combustion engines to make electricity that is placed onto the grid and sold to the local utilities. Syngas can also be transformed and made into other more complex organic chemical substances. Gasification units operate in the presence of reduced oxygen levels at temperatures greater than 1,400
degrees Fahrenheit. These units convert the organic materials in the municipal solid waste (MSW) to produce a syngas along with carbon monoxide, hydrogen, and slag. Slag is derived from inorganic materials that are converted into a solid, glassy residue at high temperatures. Plasma gasification units are also capable of generating syngas in units that operate at temperatures greater than 7,000 degrees Fahrenheit by making an electrically conducting gas called a plasma. The plasma is
UNIT III STUDY GUIDE
Unit Operations for Processing Municipal Solid Waste
MEE 5901, Advanced Solid Waste Management 2
UNIT x STUDY GUIDE
generated by passing electricity through graphite electrodes using steam and a limited supply of oxygen. The plasma converts organic materials into tars, oils, char and syngas. Other conversion technologies include aerobic and anaerobic biological systems that utilize microbial metabolic processes to degrade the organic content of municipal solid waste. Composting is an example of a process that converts municipal solid waste to a soil conditioner, and other biological processes are able to produce ethanol and biodiesel for use in commerce. Municipalities are beginning to look more favorably at conversion technologies as compared to putting all refuse into landfills that throughout the country are being filled to capacity. Many cities are without an alternative location for building and operating a new facility. Even for municipalities that have a site that can be developed, the design, construction, and regulatory and permitting processes are costly and can take many years to complete before the first load of refuse is placed into the new landfill. In addition to having a small footprint, the primary benefit of implementing a conversion technology is that the outcome involves the generation of electricity when waste refuse is seen as a renewable resource. Conversion technologies produce energy from organic materials that cannot be recycled or composted (e.g., complex papers, plastics). As there are many conversion technologies to choose from, evaluation criteria must be used to help make a good selection. Decision criteria must consider both technical and environmental performances. Examples of criteria include the flexibility of the system to the composition and variability of the fuel, the level of commercial risk that the investors are willing to take, the level of water usage, and the conversion efficiency. Regulatory compliance obligations must be met, and how the byproducts and residual wastes will be managed must be assessed. Worker safety and nuisance odor issues must also be considered. In the U.S., many state authorities have complex and diverse regulatory requirements. Massachusetts, Vermont, and Rhode Island have passed legislation banning the disposal of food wastes and wood debris from landfills and requiring their placement into compost piles. California now allows MSW to be used as a fuel in cement kilns. These are a few examples of regulatory changes as waste management practices transition from landfills to conversion technologies. The primary issue is that there are only a few full scale units in the U.S. Most new technologies are demonstration or pilot facilities that have not yet been scaled up into a full size commercial unit. The European Union (EU) lacks landfill space; they have taken the initiative and have moved faster and further with implementing new conversion technologies. The U.S. and the rest of the world will need to catch up and build on the EU experience. As countries implement strict regulations that require communities to reduce greenhouse gases, local government are beginning to switch their vehicles and fleets from diesel and gas to syngas. As syngas is a salable byproduct of conversion technologies; communities will be able to produce their own syngas from the treatment of municipal refuse. They will also be able to run their vehicles on the syngas produced in these treatment units, making waste disposal a revenue generator for local governments. While it has traditionally been the role of municipalities to manage the collection and disposal of MSW, global companies are now offering to privatize these operations and provide integrated conversion technologies to local communities. More and more municipalities will bid out waste management contracts and sign long-term agreements with private companies. The revenue derived from these relationships will help these cities to fund and meet their budget obligations. Having in place a long term infrastructure plan for the future will improve the bargaining position of a city to entice companies and skilled workers to relocate into the community, which will have the benefit of growing the tax base and employment opportunities for the region.
Reference Department for Environment Food & Rural Affairs. (2013, February). Advanced thermal treatment of
municipal solid waste. Retrieved from https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/221035/pb13888- thermal-treatment-waste.pdf
MEE 5901, Advanced Solid Waste Management 3
UNIT x STUDY GUIDE
Suggested Reading This pdf explores different options to deal with municipal solid waste so that less of it has to go to landfills. The document focuses on different technologies that are referred to as Mechanical Biological Treatment (MBT). It will expand on the topics covered in the textbook and unit lesson. Department for Environmental Food & Rural Affairs. (2013 , February). Mechanical biological treatment
of municipal solid waste. Retrieved from https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/221039/pb13890- treatment-solid-waste.pdf
Learning Activities (Non-Graded) Practice the skills learned in this unit by answering the following questions:
relationship to a sanitary landfill and explain the reason for your selection. Non-graded Learning Activities are provided to aid students in their course of study. You do not have to submit them. If you have questions, contact your instructor for further guidance and information.
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