1、CHAPTER 4 PHYSICAL, CHEMICAL,AND BIOLOGICAL PROPERTIES OF MUNICIPAL SOLID WASTE,4-1 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.,Specific Weight,Speci
2、fic Weight is defined as the weight of a material per unit volume. 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 asse
3、ss 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.,Moisture Content,The m
4、oisture 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
5、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:,Particle Size and size Distribution,The size and size distribution of the component materials in solid wastes are an important consideration in t
6、he recovery of materials, especially with mechanical means such as trommel screens magnetic separators.,The size of a waste component may be defined by one or more of the following measures:,Field Capacity,Definition: The field capacity(场地持水量) of solid waste is the total amount of moisture that can
7、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
8、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.,Permeability of Compacted Waste,The
9、hydraulic conductivity of compacted wastes is an important physical property that, to a large extent, governs the movement of liquids and gases in a landfill. The coefficient of permeability is normally written as,固有渗透性,水的动力黏度,渗透系数,4-2 CHEMICAL PROPERTIES OF MSW,Information on the chemical compositi
10、on 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 noncom
11、bustible 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
12、the production of other biological conversion products, the information we must know: 1）the major elements (ultimate analysis) that compose the waste, 2）the trace elements in the waste materials.,Proximate Analysis,Proximate analysis for the combustible components of MSW includes the following tests
13、: 1. Moisture (loss of moisture when heated to 105 for 1 h) 2. Volatile combustible matter (additional loss of weight on ignition at 950 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),
14、Fusing Point of Ash,definitionThe fusing point of ash is defined as that temperature at which the ash resulting from the burning of waste will form a solid (clinker) by fusion and agglomeration.Notation: Typical fusing temperatures for the formation of clinker from solid waste range from 2000 to 220
15、0 (1100 to 1200).,Ultimate Analysis of Solid Waste Components,Ultimate Analysis of Solid Waste Components typically involves the determination of the percent C,H,O,N, S and ash (sometimes halogen).The purpose: To characterize the chemical composition of organic matters in MSW.,Energy Content of Soli
16、d Waste Components,The energy content of the organic components in MSW can be determined (1) by using a full scale boiler as a calorimeter, (2) by using a laboratory bomb calorimeter (3) by calculation, if the elemental composition is known. Because of the difficulty in instrumenting a full-scale bo
17、iler, most of the data on the energy content of the organic components of MSW are based on the results of bomb calorimeter tests.,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,
18、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 f
19、ound in the principal materials that compose the organic fraction of MSW are reported in Table 4-6.,4-3 BIOLOGICAL PROPERTIES OF MSW,the organic fraction of most MSW can be classified as follows（ Excluding plastic, rubber, and leather components ）:1. Water-soluble constituents, such as sugars, starc
20、hes, 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(蜡), which are esters of alcohols and long-chain fatty acids,5. Lignin, a polymeric materia
21、l 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
22、 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
23、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, is often used as a measure of the biodegradability of the organic fraction of MSW. The use of VS in describing the biodegradabil
24、ity 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 foll
25、owing relationship:,The biodegradability of several of the organic compounds found in MSW, based on lignin content, is reported in Table 4-7. As shown in Table 4-7, wastes with high lignin contents, such as newsprint, are significantly less biodegradable than the other organic wastes found in MSW. T
26、he 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
27、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.,Breeding of
28、 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:,The
29、 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 i
30、nto flies in the surrounding environment.,4-4 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 in
31、tervention of people or by natural phenomena.,Solid waste can be transformed by physical, chemical, and biological means. 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 ma
32、nagement 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,(3) mechanical size ment:Physical transformations do not involve a change in phase (e.g.,
33、 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 heter
34、ogeneous waste into a number of more-or-less homogeneous components.,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. I
35、n 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.,Mechanical Size Reduction.,Size reduction is the term applied to the transformation processes used to reduce the size of the waste materials
36、. The objective of size reduction is to obtain a final product that is reasonably uniform and consider- ably reduced in size in comparison with its original form. In practice, the terms shredding, grinding, and milling are used to describe mechanical size-reduction operations.,Chemical Transformatio
37、ns,Chemical transformations of solid waste typically involve a change of phase . To reduce the volume and/or to recover conversion products, the principal chemical processes used to transform MSW include(I) combustion (chemical oxidation),(2) pyrolysis, (3) gasification. All three of these processes
38、 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 ac- companied by the emission of light and rapid generation of heat. In the presence of excess air and under ide
39、al conditions, the combustion of the organic fraction of MSW can be represented by the following equation:,Pyrolysis.,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,
40、 destructive distillation is often used as an alternative term for pyrolysis.,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. Th
41、e 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), hyd
42、rogen (H2), methane (CH4),nitrogen (N2);(2) a char containing carbon and the inerts originally in the fuel, (3) condensible liquids resembling pyrolytic oil.,Other Chemical Transformation Processes.,Example:The hydrolytic conversion of cellulose to glucose, followed by the fermentation of glucose to
43、 ethyl alcohol.,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; to produce methane. The principal organisms involved
44、 in the biological transformations of organic wastes are bacteria, fungi(真菌), yeasts, and actinomycetes（放线菌）. These transformations may be accomplished either aerobically or anaerobically, depending on the availability of oxygen.,Aerobic Composting.,Left unattended, the organic fraction of MSW will
45、undergo biological decomposition. The extent and the period of time over which the decomposition occurs will depend onthe nature of the waste, the moisture content,the available nutrients, other environmental factors. Under controlled conditions, yard wastes and the organic fraction of MSW can be co
46、nverted to 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:,Anaerobic Digestion.,The biodegradable portion of the organic fraction of MSW c
47、an be converted biologically under anaerobic conditions to a gas containing carbon dioxide and methane (CH4). This conversion can be represented by the following equation:,Other Biological Transformation Processes.,In addition to the aerobic composting and anaerobic digestion processes, a variety of
48、 other public and proprietary processes（专利技术） are being developed and evaluated for the biological transformation of solid waste. The high-solids(高固体分) anaerobic digestion process discussed in Chapter 14 is one such example.,Importance of Waste Transformations in Solid Waste Management,Typically, ph
49、ysical, 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, (3) to recover conversion products and energy.,1）Improving Efficiency of Solid Waste Management）Systems. wastes
50、are often baled（打包）. wastes are compacted Mechanical size reduction Hand separation at the point of generation-hazardous waste,2）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.,
