Conversion of Solid Waste to Methane

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Methane Production from Municipal Solid Waste
Introduction
Archaeological investigations of landfills have revealed that biodegradable wastes can be found — virtually intact — 25 years after burial. We know that landfills contain bacteria with the metabolic capability to degrade many of the materials that are common components of municipal refuse. The persistence for decades of degradable materials in the presence of such organisms appears somewhat paradoxical. In this experiment students will explore the factors that influence biodegradation of waste materials in landfills. Although recycling has significantly reduced the amount of landfill space dedicated to paper and other lignocellulosics, paper products are still a significant fraction of the solid waste stream. In this laboratory students will measure the rate and extent of anaerobic degradation of newsprint, Kraft paper, coated paper, and food scraps.

Theory
Over 150 million tons of municipal solid waste (MSW) are generated every year in the United States, and more than 70% of the MSW is deposited in landfills (Gurijala and Suflita 1993). Paper constitutes the major weight fraction of MSW, and this laboratory will focus on the biodegradation of that component. Anaerobic biodegradation of paper produces methane and carbon dioxide. Methane is a fuel and is the major component of natural gas. Methane produced in sanitary landfills represents a usable but underutilized source of energy. Energy recovery projects are frequently rejected because the onset of methane production is Table 5-1. Typical physical composition unpredictable and methane yields of residential MSW in 1990 vary from 1-30% of potential yields excluding recycled materials and based on refuse biodegradability data food wastes discharged with (Barlaz, Ham et al. 1992). The low wastewater (Tchobanoglous, methane yields are the result of Theisen et al. 1993) several factors that conspire to inhibit Component Range Typical anaerobic biodegradation including (% by weight) (% by weight) Organic low moisture levels, resistance to food wastes 6-18 9.0 biodegradation, conditions that favor paper 25-40 34.0 bacterial degradation pathways that cardboard 3-10 6.0 plastics 4-10 7.0 do not result in methane as an end textiles 0-4 2.0 product, and poor contact between rubber 0-2 0.5 bacteria and the organic matter. leather 0-2 0.5 Characteristics of municipal solid waste The physical composition of residential municipal solid waste (MSW) in the United States is given in Table 5-1. The fractional yard wastes wood Inorganic glass tin cans aluminum other metal dirt, ash, etc. 5-20 1-4 Organic total 4-12 2-8 0-1 1-4 0-6 Inorganic total 18.5 2.0 79.5 8.0 6.0 0.5 3.0 3.0 20.5

Methane Production from Municipal Solid Waste

48 contribution of the listed categories has evolved over time, with decrease in food wastes because of increased use of kitchen food waste Table 5-2. Percentage grinders, an increase in plastics weight of paper through the growth of their use for (Tchobanoglous, packaging, and an increase in yard 1993) wastes as burning has ceased to be Range allowed by most communities Type of paper newspaper 10-20 (Tchobanoglous, Theisen et al. 1993). books and 5-10 Excluding plastic, rubber, and leather, magazines the organic components listed in commercial 4-8 printing Table 5-1 are, given sufficient time, office paper 8-12 biodegradable. Although recycling efforts divert a significant fraction of paper away from landfills, paper continues to be a major component of landfilled waste. The types of paper found in MSW are listed in Table 5-2. The elemental composition of newsprint and office paper are listed in Table 5-3. The major elements in paper are carbon, hydrogen, and oxygen that together constitute 93.5% of the total solids. The approximate molecular ratios for newspaper and office paper and C6H9.5O4.5 are C6H9O4 respectively. Biodegradation of cellulose, hemicellulose, and lignin Cellulose and...
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