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Methane Production from Municipal Solid Waste Essay

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 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 other paperboard paper packaging other nonpackaging paper tissue paper and towels corrugated materials 49 cows, have these microorganisms in their digestive tract. Cellulose is a polysaccharide that is composed of glucose subunits (see Figure 5-1). 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 (two glucose up of five carbon sugars (primarily Figure 5-1. Cellulose xylose) are the most abundant in subunits are shown). 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 5-2). The exact chemical structure of lignin is not known but its CH O CH O 3 3 reactivity, breakdown products, and -C-C-CHO-C-C-Cthe results of spectroscopic studies HOreveal it to be a polymeric material CH O 3 containing aromatic rings with methoxy groups (-OCH3) Figure 5-2. Coniferyl (left) and syringyl (Tchobanoglous, Theisen et al. (right) subunits of lignin. 1993). One of the many proposed structures for lignin is shown in Figure 5-3.

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 by an array of physical/chemical processes including pretreatment to increase surface area (size reduction), heat treatment, and treatment with acids or bases. Such treatments are useful when wood and plant materials are to be anaerobically degraded to Figure 5-3. A postulated formulation for produce methane. Research on this spruce lignin (by (Brauns 1962), as cited by topic has been performed by (Pearl 1967)). This structure is suggested by Cornell Prof. James Gossett spectroscopic studies and the chemical (Gossett and McCarty 1976; reactions of lignin.

Methane Production from Municipal Solid Waste

50 Pavlostathis and Gossett 1985a; Pavlostathis and Gossett 1985b). Three major groups of bacteria are involved in the conversion of cellulosic material to methane (Zehnder 1978): (1) the hydrolytic and fermentative bacteria that break down biological polymers such as cellulose and hemicellulose to sugars that are then fermented to carboxylic acids, alcohols, carbon dioxide and hydrogen gas, (2) the obligate hydrogen reducing acetogenic bacteria that convert carboxylic acids and alcohols to acetate and hydrogen, and (3) the methanogenic bacteria that convert primarily acetate and hydrogen plus carbon dioxide to methane.

Sulfate reducing bacteria (SRB) may also play a role in the anaerobic mineralization of cellulosic material. In the presence of sulfate, the degradation process may be directed towards sulfate reduction by SRB with the production of hydrogen sulfide and carbon dioxide (Barlaz, Ham et al. 1992). Cellular requirements for growth The availability of oxygen is a prime determinant in the type of microbial metabolism that will occur. Microbial respiration of organic carbon is a combustion process, in which the carbon is oxidized (i.e., is the electron donor) in tandem with the reduction of an electron acceptor.

The energy available to microorganisms is greatest when oxygen is used as the electron acceptor and therefore aerobic metabolic processes will dominate when oxygen is available. Some microorganisms require oxygen to obtain their energy and are termed “obligate aerobes.” In the absence of oxygen, other electron acceptors such as nitrate (NO3-), sulfate (SO4-2) and carbon dioxide (CO2) can by used. Organisms that can only exist in an environment that contains no oxygen are termed “obligate anaerobes.” Organisms that have the ability to grow in both the presence and the absence of oxygen are said to be “facultative.” The availability of nutrients can limit the ability of cells to grow and consequently the extent of biodegradation. Nitrogen and/or phosphorous constitute important nutrients required for cell synthesis. Inorganic bacterial nutritional requirements also include sulfur, potassium, magnesium, calcium, iron, sodium and chloride.

In addition, inorganic nutrients needed in small amounts (minor or trace nutrients) include zinc, manganese, molybdenum, selenium, cobalt, copper, nickel, vanadium and tungsten. Organic nutrients (termed “growth factors”) are also sometimes needed (depending on the microorganism) and include certain amino acids, and vitamins (Metcalf & Eddy 1991). Environmental conditions such as pH, temperature, moisture content, and salt concentration can have a great influence on the ability of bacteria to grow and survive. Most bacteria grow in the pH range from 4.0 to 9.5 (although some organisms can tolerate more extreme pH values), and typically grow best in the relatively narrow range from 6.5 to 7.5 (Metcalf & Eddy, 1991).

Microorganisms have a temperature range over which they function best, and are loosely characterized as phychrophilic (ability to grow at 0°C), mesophilic (optimal growth at 25-40°C) or thermophilic (optimal growth above 45-50°C) (Brock 1970). Many common methanogens are mesophilic. Elevated temperatures also favor faster reaction rates. While some microorganisms are very tolerant of low moisture conditions, active microbial growth and degradation of organic matter necessitates that water not be a scarce resource.

This uptake mechanism requires that the solute concentration inside the cell be higher than that of the outside media. Organisms that grow in dilute solutions can not tolerate high salt concentrations because their normal osmotic gradient is reversed and they can not take in water. Some cell strains, termed “halophiles” are adapted for growth at very high salt concentrations. The above factors suggest that bacterial degradation of MSW to produce methane will occur optimally at circumneutral pH, low ionic strength, in the absence of oxygen, nitrate and sulfate, in the presence of moisture and nutrients, and under mesophilic conditions. Estimates of paper biodegradability Volatile solids (VS) content (determined by weight loss on ignition at 550°C) has been used to estimate the biodegradability of MSW components, but this measure overestimates the biodegradability of paper. Paper products have a very high volatile solids content.

Newsprint, office paper, and cardboard have VS of 94%, 96.4%, and 94% respectively (Tchobanoglous, Theisen et al. 1993). Paper products also can have a high content of lignocellulosic components that are only slowly degradable. Lignin constitutes approximately 21.9%, 0.4% and 12.9% respectively of the VS in newsprint, office paper, and cardboard. Lignin content and biodegradability are strongly correlated and thus lignin content can be used to estimate biodegradability and potential methane production. Chandler et al. (1980) found a relationship between lignin content and biodegradable volatile solids using a wide variety of waste materials. The empirical relationship suggests that not only is lignin not easily biodegraded, but that lignin also reduces the biodegradability of the nonlignin components. This reduction in biodegradability may be caused by lignin polymeric material physically preventing enzymatic access to the nonlignin components.

The relationship is VSbiodegradable = −2.8VSlignin + 0.83 where VSbiodegradable is the biodegradable fraction of the volatile solids and VSlignin is the fraction of volatile solids that are lignin. From equation 5.7 the maximum destruction of VS is limited to about 83%, a limitation due to the Biodegradability of selected production of bacterial Table 5-4. components of MSW (Tchobanoglous, by-products. The high Theisen et al. 1993) concentration of lignin in VS/TS Lignin/VS VSbiodegradable* newsprint makes it much less % % % biodegradable than more highly Type of waste mixed food 7-15 0.4 82 processed office paper (Table 5-4). newsprint 94 21.9 22 office paper 96.4 0.4 82 Energy recovery from MSW cardboard 94.0 12.9 47 Energy could be recovered from MSW by direct combustion in an * Obtained by using equation 5.7 incinerator or by anaerobic biodegradation and production of methane. Proximate analysis is used to measure moisture content, volatile matter, fixed carbon (combustible but not volatile), and ash.


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