56 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 unpredictable and methane yields vary from 1-30% of potential yields based on refuse biodegradability data (Barlaz, Ham et al. 1992). The low methane yields are the result of several factors that conspire to inhibit anaerobic biodegradation including low moisture levels, resistance to biodegradation, conditions that favor bacterial degradation pathways that do not result in methane as an end product, and poor contact between bacteria and the organic matter. Characteristics of municipal solid waste The physical composition of residential municipal solid waste (MSW) in the United States is given in Table 6-1. The fractional Table 6-1. Typical physical composition of residential MSW in 1990 excluding recycled materials and food wastes discharged with wastewater (Tchobanoglous, Theisen et al. 1993) Component Range Typical Organic (% by weight) (% by weight) food wastes 6-18 9.0 paper 25-40 34.0 cardboard 3-10 6.0 plastics 4-10 7.0 textiles 0-4 2.0 rubber 0-2 0.5 leather 0-2 0.5 yard wastes 5-20 18.5 wood 1-4 2.0 Organic total 79.5 Inorganic glass 4-12 8.0 tin cans 2-8 6.0 aluminum 0-1 0.5 other metal 1-4 3.0 dirt, ash, etc. 0-6 3.0 Inorganic total 20.5 CEE 453: Laboratory Research in Environmental Engineering Spring 200357 contribution of the listed categories has evolved over time, with a trend toward a decrease in food wastes because of increased use of kitchen food waste grinders, an increase in plastics through the growth of their use for packaging, and an increase in yard wastes as burning has ceased to be allowed by most communities (Tchobanoglous, Theisen et al. 1993). Excluding plastic, rubber, and leather, the organic components listed in Table 6-1 are, given sufficient time, 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 6-2. The elemental composition of newsprint and office paper are listed in Table 6-3. Table 6-2. Percentage distribution by weight of paper types in MSW (Tchobanoglous, Theisen et al. 1993) Type of paper Range Typical newspaper 10-20 17.7 books and magazines 5-10 8.7 commercial printing 4-8 6.4 office paper 8-12 10.1 other paperboard 8-12 10.1 paper packaging 6-10 7.8 other nonpackaging paper 8-12 10.6 tissue paper and towels 4-8 5.9 corrugated materials 20-25 22.7 Total 100.0 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 are C6H9O4 and C6H9.5O4.5 respectively. Biodegradation of cellulose, hemicellulose, and lignin Cellulose and hemicellulose are the principal biodegradable constituents of refuse accounting for 91% of the total methane potential. Cellulose forms the structural fiber of many plants. Mammals, including humans, lack the enzymes to degrade cellulose. However, bacteria that can break cellulose down into its subunits are widely distributed in natural systems, and ruminants, such as Table 6-3. Elemental composition of two paper types on a dry weight basis (Tchobanoglous, Theisen et al. 1993). Constituent Newsprint Office Paper C 49.1% 43.4% H 6.1% 5.8% O 43.0% 44.3% NH4-N 4 ppm 61 ppm NO3-N 4 ppm 218 ppm P 44 ppm 295 ppm PO4-P 20 ppm 164 ppm K 0.35% 0.29% SO4-S 159 ppm 324 ppm Ca 0.01% 0.10% Mg 0.02% 0.04% Na 0.74% 1.05% B 14 ppm 28 ppm Zn 22 ppm 177 ppm Mn 49 ppm 15 ppm Fe 57 ppm 396 ppm Cu 12 ppm 14 ppm Methane Production from Municipal Solid Waste58 cows, have these microorganisms in their digestive tract. Cellulose is a polysaccharide that is composed of glucose subunits (see Figure 6-1). Another component of the walls of plants is hemicellulose, which sounds similar to cellulose but is unrelated other that that it is another type of polysaccharide. Hemicelluloses made up of five carbon sugars (primarily xylose) are the most abundant in nature. Lignin is an important structural component in plant materials and constitutes roughly 30% of wood. Significant components of lignin include coniferyl alcohol and syringyl alcohol subunits (Figure 6-2). Figure 6-1. Cellulose (two glucose subunits are shown). The exact chemical structure of lignin is not known but its reactivity, breakdown products, and the results of spectroscopic studies reveal it to be a polymeric material containing aromatic rings with methoxy groups (-OCH3) (Tchobanoglous, Theisen et al. 1993). One of the many proposed structures for lignin is shown in Figure 6-3. -C-C-C-HO-CH O3CH O3HO-CH O3 Figure 6-2. Coniferyl (left) and syringyl (right) subunits of lignin. -C-C-C-Degradation of lignin requires the presence of moisture and oxygen and is carried out by filamentous fungi (Prescot, Harley et al. 1993). The biodegradability of lignocellulosic materials can be increased