14 Physical, Chemical, and Biological Properties of Municipal Solid Waste 14- 1 Physical Properties of MSW Important physical characteristics of MSW include specific weight, moisture content, particle size and size distribution, field capacity, and compacted waste porosity. The discussion is limited to an analysis of residential, commercial, and some industrial solid wastes. Note, however, that the fundamentals of analysis presented in this and the following chapter are applicable to all types of solid wastes. Additional details on the various physical, chemical, and microbiological methods of testing for solid wastes may be found in the various publications of the American Society for Testing and Materials (ASTM). Specific Weight Specific weight is defined as the weight of a material per unit volume. (It should be noted that specific weight expressed as lb/yd3 is commonly referred to in the solid waste literature incorrectly as density. In U.S. customary units density is expressed correctly as slug/f t3.) Because the specific weight of MSW is often reported as loose, as found in containers, uncompacted, compacted, and the like, the basis used for the reported values should always be noted. Specific weight data are often needed to assess the total mass and volume of waste that must be managed. Unfortunately, there is little or no uniformity in the way solid waste specific weights have been reported in the literature. Frequently, no distinction has been made between uncompacted or compacted specific weights. Because the specific weights of solid wastes vary markedly with geographic location, season of the year, and length of time in storage, great care should be used in selecting typical values. Municipal solid wastes as delivered in compaction vehicles have been found to vary from 300 to 700 lb/yd3; a typical value is about 500lb/yd3. Moisture Content The moisture content of solid wastes usually is expressed in one of two ways. In the wet-weight method of measurement, the moisture in a sample is expressed as a percentage of the wet weight of the material; in the dry-weight method, it is expressed as a percentage of the dry weight of the material. The wet-weight method is used most commonly in the field of solid waste management. In equation form, the wet-weight moisture content is expressed as follows:  (14- 1) where M = moisture content, % w = initial weight of sample as delivered, lb (kg) d = weight of sample after drying at 105℃, lb (kg) For most MSW in the United States, the moisture content will vary from 15 to 40 percent, depending on the composition of the wastes, the season of the year, and the humidity and weather conditions, particularly rain. Particle Size and Size Distribution The size and size distribution of the component materials in solid wastes are an important consideration in the recovery of materials, especially with mechanical means such as trommel screens and magnetic separators. The size of a waste component may be denned by one or more of the following measures: (14- 2) (14- 3) (14- 4) (14- 5) (14- 6) Field Capacity The field capacity of solid waste is the total amount of moisture that can be retained in a waste sample subject to the downward pull of gravity. The field capacity of waste materials is of critical importance in determining the formation of leachate in landfills. Water in excess of the field capacity will be released as leachate. The field capacity varies with the degree of applied pressure and the state of decomposition of the waste. A field capacity of 30 percent by volume corresponds to 30 in/100 in. The field capacity of uncompacted commingled wastes from residential and commercial sources is in the range of 50 to 60 percent. 14- 2 Chemical properties of MSW Information on the chemical composition of the components that constitute MSW is important in evaluating alternative processing and recovery options. For example, the feasibility of combustion depends on the chemical composition of the solid wastes. Typically, wastes can be thought of as a combination of semimoist combustible and noncombustible materials. If solid wastes are to be used as fuel, the four most important properties to be known are: 1. Proximate analysis 2. Fusing point of ash 3. Ultimate analysis (major elements) 4. Energy content Where the organic fraction of MSW is to be composted or is to be used as feedstock for the production of other biological conversion products, not only will information on the major elements (ultimate analysis) that compose the waste be important, but also information will be required on the trace elements in the waste materials. Proximate Analysis Proximate analysis for the combustible components of MSW includes the following tests : 1. Moisture (loss of moisture when heated to 105°C for 1 h) 2. Volatile combustible matter (additional loss of weight on ignition at 950°C in a covered crucible) 3. Fixed carbon (combustible residue left after volatile matter is removed) 4. Ash (weight of residue after combustion in an open crucible) Fusing Point of Ash The fusing point of ash is denned as that temperature at which the ash resulting from the burning of waste will form a solid (clinker) by fusion and agglomeration. Typical fusing temperatures for the formation of clinker from solid waste range from 2000 to 2200°F (1100 to 1200°C). Ultimate Analysis of Solid Waste Components The ultimate analysis of a waste component typically involves the determination of the percent C (carbon), H (hydrogen), 0 (oxygen), N (nitrogen), S (sulfur), and ash. Because of the concern over the emission of chlorinated compounds during combustion, the determination of halogens is often included in an ultimate analysis. The results of the ultimate analysis are used to characterize the chemical composition of the organic matter in MSW. They are also used to define the proper mix of waste materials to achieve suitable C/N ratios for biological conversion processes. Data on the ultimate analysis of individual combustible materials are resented in Table 14- 2. Essential Nutrients and Other Elements Where the organic fraction of MSW is to be used as feedstock for the production of biological conversion products such as compost, methane, and ethanol, information on the essential nutrients and elements in the waste materials is of importance with respect to the microbial nutrient balance and in assessing what final uses can be made of the materials remaining after biological conversion. The essential nutrients and elements found in the principal materials that compose the organic fraction of MSW are reported in Table 14- 3. 14- 3 Biological Properties of MSW Excluding plastic, rubber, and leather components, the organic fraction of most MSW can be classified as follows: 1. Water-soluble constituents, such as sugars, starches, amino acids, and various organic acids, 2. Hemicellulose, a condensation product of five- and six-carbon sugars, 3. Cellulose, a condensation product of the six-carbon sugar glucose, 4. Fats, oils, and waxes, which are esters of alcohols and long-chain fatty acids, 5. Lignin, a polymeric material containing aromatic rings with methoxyl groups (-OCH3, the exact chemical nature of which is still not known (present in some paper products such as newsprint and fiberboard), 6. Lignocellulose, a combination of lignin and cellulose, 7. Proteins, which are composed of chains of amino acids. Perhaps the most important biological characteristic of the organic fraction of MSW is that almost all of the organic components can be converted biologically to gases and relatively inert organic and inorganic solids. The production of odors and the generation of flies are also related to the putrescible nature of the organic materials found in MSW (e.g., food wastes). Biodegradability of Organic Waste Components Volatile solids (VS) content, determined by ignition at 550°C, is often used as a measure of the biodegradability of the organic fraction of MSW. The use of VS in describing the biodegradability of the organic fraction of MSW is misleading, as some of the organic constituents of MSW are highly volatile but low in biodegradability (e.g., newsprint and certain plant trimmings). Alternatively, the lignin content of a waste can be used to estimate the biodegradable fraction, using the following relationship : BF = 0.814- 0.028LC (14- 7) Where BF = biodegradable fraction expressed on a volatile solids (VS) basis 0.83 = empirical constant 0.028 = empirical constant LC = lignin content of the VS expressed as a percent of dry weight Wastes with high lignin contents, such as newsprint, are significantly less biodegradable than the other organic wastes found in MSW. The rate at which the various components can be degraded varies markedly. For practical purposes, the principal organic waste components in MSW are often classified as rapidly and slowly decomposable. Production of Odors Odors can develop when solid wastes are stored for long periods of time on-site between collections, in transfer stations, and in landfills. The development of odors in on-site storage facilities is more significant in warm climates. Typically, the formation of odors results from the anaerobic decomposition of the readily decomposable organic components found in MSW. For example, under anaerobic (reducing) conditions, sulfate can be reduced to sulfide (S2-), which subsequently combines with hydrogen to form H2S. The formation of H2S can be illustrated by the following two series of reactions. 2CH3CHOHCOOH + SO42- → 2CH3COOH + S2- + 2H2O + 2CO2 (14- 8) 4H2 + SO42- → S2- + 4H2O (14- 9) S2- + 2H+ → H2S (14- 10) The sulfide ion can also combine with metal salts that may be present, such as iron, to form metal sulfides. S2- + Fe2+ → FeS (14- 11) The black color of solid wastes that have undergone anaerobic decomposition in a landfill is primarily due to the formation of metal sulfides. If it were not for the formation of a variety of sulfides, odor problems at landfills could be quite significant. The biochemical reduction of an organic compound containing a sulfur radical can lead to the formation of malodorous compounds such as methyl mercaptan and aminobutyric acid. The reduction of methionine, an amino acid, serves as an example. CH3SCH2CH2CH(NH2)COOH → CH3SH + CH3CH2CH2(NH2)COOH (14- 12) The methyl mercaptan can be hydrolyzed biochemically to methyl alcohol and hydrogen sulfide: CH3SH + H2O → CH4OH + H2S (14- 13) Breeding of Flies In the summertime and during all seasons in warm climates, fly breeding is an important consideration in the on-site storage of wastes. Flies can develop in less than two weeks after the eggs are laid. The life history of the common house fly from egg to adult can be described as follows: Eggs develop 8-12 hours First stage of larval period 20 hours Second stage of larval period 24 hours Third stage of larval period 3 days Pupate stage 4-5 days Total 9-11 days The extent to which flies develop from the larval (maggot) stage in on-site storage containers depends on the following facts: If maggots develop, they are difficult to remove when the containers are emptied. Those remaining may develop into flies. Maggots can also crawl from uncovered cans and develop into flies in the surrounding environment. 14- 4 Physical, Chemical, and Biological Transformations of Solid Waste The purpose of this section is to introduce the reader to the principal transformation processes that can be used for the management of MSW. These transformations can occur either by the intervention of people or by natural phenomena. Solid waste can be transformed by physical, chemical, and biological means (Table 14- 4). One must understand the transformation processes that are possible and the products that may result because they will affect directly the development of integrated solid waste management plans. Physical Transformations The principal physical transformations that may occur in the operation of solid waste management systems include (1) component separation, (2) mechanical volume reduction, and (3) mechanical size reduction. Physical transformations do not involve a change in phase (e.g., solid to gas), unlike chemical and biological transformation processes. Component Separation. Component separation is the term used to describe the process of separating, by manual and/or mechanical means, identifiable components from commingled MSW. Component separation is used to transform a heterogeneous waste into a number of more-or-less homogeneous components. Component separation is a necessary operation in the recovery of reusable and recyclable materials from MSW, in the removal of contaminants from separated materials to improve specifications of the separated material, in the removal of hazardous wastes from MSW, and where energy and conversion products are to be recovered from processed wastes. Mechanical Volume Reduction. Volume reduction (sometimes known as densification) is the term used to describe the process whereby the initial volume occupied by a waste is reduced, usually by the application of force or pressure. In most cities, the vehicles used for the collection of solid wastes are equipped with compaction mechanisms to increase the amount of waste collected per trip. Paper, cardboard, plastics, and aluminum and tin cans removed from MSW for recycling are baled to reduce storage and handling costs and shipping costs to processing centers. Recently, high-pressure compaction systems have been developed to produce materials suitable for various alternative uses such as production of fireplace logs from paper and cardboard. To decrease the costs associated with the transport of waste materials to landfill disposal sites, municipalities also may use transfer stations equipped with compaction facilities. To increase the useful life of landfills, wastes are usually compacted before being covered. Mechanical Size Reduction. Size reduction is the term applied to the transformation processes used to reduce the size of the waste materials. The objective of size reduction is to obtain a final product that is reasonably uniform and considerably reduced in size in comparison with its original form. Note that size reduction does not necessarily imply volume reduction. In some situations, the total volume of the material after size reduction may be greater than that of the original volume (e.g., the shredding of office paper). In practice, the terms shredding, grinding, and milling are used to describe mechanical size-reduction operations. Chemical Transformations Chemical transformations of solid waste typically involve a change of phase (e.g., solid to liquid, solid to gas, etc.). To reduce the volume and/or to recover conversion products, the principal chemical processes used to transform MSW include (1) combustion (chemical oxidation), (2) pyrolysis , and (3) gasification. All three of these processes are often classified as thermal processes. Combustion (Chemical Oxidation). Combustion is defined as the chemical reaction of oxygen with organic materials, to produce oxidized compounds accompanied by the emission of light and rapid generation of heat. In the presence of excess air and under ideal conditions, the combustion of the organic fraction of MSW can be represented by the following equation: Organic matter + excess air → N2 + CO2 + H2O + O2 + ash + heat (14- 14) Excess air is used to ensure complete combustion. The end products derived from the combustion of MSW, Eq. (14- 14), include hot combustion gases—composed primarily of nitrogen (N2), carbon dioxide (CO2), water (H2O, flue gas), and oxygen (O2)— and noncombustible residue. In practice, small amounts of ammonia (NH3), sulfur dioxide (SO2), nitrogen oxides (NOx), and other trace gases will also be present, depending on the nature of the waste materials. Pyrolysis. Because most organic substances are thermally unstable, they can be split, through a combination of thermal cracking and condensation reactions in an oxygen-free atmosphere, into gaseous, liquid, and solid fractions. Pyrolysis is the term used to describe the process. In contrast with the combustion process, which is highly exothermic, the pyrolytic process is highly endothermic. For this reason, destructive distillation is often used as an alternative term for pyrolysis. The characteristics of the three major component fractions resulting from the pyrolysis of the organic portion of MSW are (1) a gas stream containing primarily hydrogen (H2). methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and various other gases, depending on the organic characteristics of the waste material being pyrolyzed; (2) a tar and/or oil stream that is liquid at room temperature and contains chemicals such as acetic acid, acetone, and methanol; and (3) a char consisting of almost pure carbon plus any inert material that may have entered the process. For cellulose C6H10O5) the following expression has been suggested as being representative of the pyrolysis reaction: 3C6H10O5 → 8H2O + C6H8O + 2CO + 2CO2 + CH4 + H2 + 7C (14- 15) In Eq. (14- 15), the liquid tar and/or oil compounds normally obtained are represented by the expression C6H8O. Gasification. The gasification process involves partial combustion of a carbonaceous fuel so as to generate a combustible fuel gas rich in carbon monoxide, hydrogen, and some saturated hydrocarbons, principally methane. The combustible fuel gas can then be combusted in an internal combustion engine or boiler. When a gasifier is operated at atmospheric pressure with air as the oxidant, the end products of the gasification process are (1) a low-Btu gas typically containing carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), methane (CH4), and nitrogen (N2); (2) a char containing carbon and the inerts originally in the fuel, and (3) condensible liquids resembling pyrolytic oil. Other Chemical Transformation Processes. In addition to the various combustion, pyrolysis, and gasification processes under investigation and/or construction, a variety of other public and proprietary processes are being developed and evaluated for the transformation of solid waste. The hydrolytic conversion of cellulose to glucose, followed by the fermentation of glucose to ethyl alcohol, is an example of such a process . Biological Transformations The biological transformations of the organic fraction of MSW may be used to reduce the volume and weight of the material; to produce compost, a humus-like material that can be used as a soil conditioner; and to produce methane. The principal organisms involved in the biological transformations of organic wastes arc bacteria, fungi, yeasts, and actinomycetes. These transformations may be accomplished either aerobically or anaerobically, depending on the availability of oxygen. The principal differences between the aerobic and anaerobic conversion reactions are the nature of the end products and the fact oxygen must be provided to accomplish the aerobic conversion. Biological processes that have been used for the conversion of the organic fraction of MSW include aerobic composting, anaerobic digestion, and high-solids anaerobic digestion. Aerobic Composting. Left unattended, the organic fraction of MSW will undergo biological decomposition. The extent and the period of time over which the decomposition occurs will depend on the nature of the waste, the moisture content, the available nutrients, and other environmental factors. Under controlled a stable organic residue known as compost in a reasonably short period of time (four to six weeks). Composting the organic fraction of MSW under aerobic conditions can be represented by the following equation: Organic matter + O2 + nutrients → new cells + resistant organic matter + CO2 + H2O + NH3 + SO42- + heat (14- 16) In Eq. (14- 16), the principal end products are new cells, resistant organic matter; carbon dioxide, water, ammonia, and sulfate. Compost is the resistant organic matter that remains. The resistant organic matter usually contains a high percentage of lignin, which is difficult to convert biologically in a relatively short time. Lignin, found most commonly in newsprint, is the organic polymer that holds together the cellulose fibers in trees and certain plants. Anaerobic Digestion. The biodegradable portion of the organic fraction of MSW can be convened biologically under anaerobic conditions to a gas containing carbon dioxide and methane (CH4). This conversion can be represented by the following equation: Organic matter + H2O + nutrients → new cells + resistant organic matter + CO2 + CH4 + NH3 +H2S + heat (14- 17) Thus, the principal end products are carbon dioxide, methane, ammonia, hydrogen sulfide, and resistant organic matter. In most anaerobic conversion processes carbon dioxide and methane constitute over 99 percent of the total gas produced. The resistant organic matter (or digested sludge) must be dewatered before it can be disposed of by land spreading or landfilling. Dewatered sludge is often composted aerobically to stabilize it further before application. Other Biological Transformation Processes. In addition to the aerobic composting and anaerobic digestion processes, a variety of other public and proprietary processes are being developed and evaluated for the biological transformation of solid waste. Importance of Waste Transformations in Solid Waste Management Typically, physical, chemical, and biological transformations are used (1) to improve the efficiency of solid waste management operations and systems, (2) to recover reusable and recyclable materials, and (3) to recover conversion products and energy. The implications of waste transformation in the design of integrated solid waste management systems can be illustrated by the following example. If composting is to be an element of a solid waste management plan, the organic fraction of the MSW must be separated from the commingled MSW. If the organic fraction must be separated, should it be done at the source of generation or at a materials recovery facility? If separation of wastes is to occur at the source, what components should be separated to produce an optimum compost? Improving Efficiency of Solid Waste Management Systems. To improve the efficiency of solid waste management operations and to reduce storage volume requirements at medium- and high-rise apartment buildings, wastes are often baled. For example, waste paper, recovered for recycling, is baled to reduce storage volume requirements and shipping costs. In some cases, waste materials are baled to reduce haul costs to the disposal site. At disposal sites, solid wastes are compacted to use the available landfill capacity effectively. If solid wastes are to be transported hydraulically or pneumatically, some form of shredding is normally required. Mechanical size reduction (shredding) has also been used to improve the efficiency of disposal sites. Hand separation at the point of generation is now considered an efficient way to remove small quantities of hazardous waste from MSW, thereby making landfills safer. Chemical and biological processes can be used to reduce the volume and weight of waste requiring disposal and to produce useful products. Recovery of Materials for Reuse and Recycling. As a practical matter, components that are most amenable to recovery are those for which markets exist and which are present in the wastes in sufficient quantity to justify their separation. Materials most often recovered from MSW include paper, cardboard, plastic, garden trimmings, glass, ferrous metal, aluminum, and other nonferrous metal. Recovery of Conversion Products and Energy. The organic fraction of MSW can be converted to usable products and ultimately to energy in a number of ways, including (1) combustion to produce steam and electricity; (2) pyrolysis to produce a synthetic gas, liquid or solid fuel, and solids; (3) gasification to produce a synthetic fuel; (4) biological conversion to produce compost; and (5) biodigestion to generate methane and to produce a stabilized organic humus.