...Eqtn
1
The important physical properties of MSW include density (sometimes referred to as specific weight), moisture content, particle size and distribution, field capacity, and porosity. Although talking about MSW, it is important to note that the same fundamentals apply to all types of solid wastes.
Density
This is the weight per unit volume and is expressed as kg/m3. Density varies because of the large variety of waste constituents, the degree of compaction, the state of decomposition, and in landfills because of the amount of daily cover and the total depth of waste. Inert wastes such as construction and demolition materials may have higher densities, and density can change as in landfills where the formation of landfill gas and decomposition may bring about significant mass loss.
Density is important because it is needed to assess the total mass and volume of waste which must be managed. For example, the average density of loose refuse in the USA is 115kg/m3, but loose refuse is frequently compacted on collection so the density changes to 235 to 300kg/m3.
The density of MSW is often referred to as loose, as found in containers, uncompacted, compacted etc. so it is important to specify what sort of waste is being referred to. Density varies not only because of the type of treatment it gets (collection vs compaction etc) but also because of geographic location, season, and length of time in storage. For example, waste collected from outlying areas is likely to have some degree of compaction just through the travel process. In autumn there might be more loose leaves vs grass clippings in summer. Material stored for a long time will tend to consolidate thus occupying less volume. Some typical density values are presented in the table.
Table 1: Typical Properties of Uncompacted Wastes (USA Data)
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DENSITY(kg/m3) |
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| Food wastes |
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| Paper |
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| Plastics |
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| Garden trimmings |
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| Glass |
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| Ferrous metals |
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Moisture Content
The most commonly used method of expressing moisture content is as a percentage of the wet weight of material. Moisture content is important in regards to density (as above), compaction, the role moisture plays in decomposition processes, the flushing of inorganic components, and the use of MSW in incinerators. Pre-treatment of waste to ensure uniform moisture content can be carried out prior to landfill disposal. The wet weight moisture content can be determined using the following equation:
...Eqtn
1
Where M = moisture content (%)
w = initial weight of sample (kg)
d = weight of sample after drying at 105°C (kg)
Some typical moisture contents are shown in Table 2.
Table 2: Typical Moisture Contents of Wastes
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Range (%) |
Typical (%) |
| RESIDENTIAL | ||
Food wastes (mixed) |
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Paper |
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Plastics |
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Yard Wastes |
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Glass |
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| COMMERCIAL | ||
Food wastes |
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Rubbish (mixed) |
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| CONSTRUCTION & DEMOLITION | ||
Mixed demolition combustibles |
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Mixed construction combustibles |
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| INDUSTRIAL | ||
Chemical sludge (wet) |
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Sawdust |
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Wood (mixed) |
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| AGRICULTURAL | ||
Mixed Agricultural waste |
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Manure (wet) |
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Learning Question
Using these values, Equation 1 and wastes of a given composition it is possible to determine the moisture content of a solid waste sample.
Particle Size and Distribution
The size and distribution of the components of wastes are important
for the recovery of materials, especially when mechanical means are used,
such as trommel screens and magnetic separators. For example, ferrous items
which are of a large size may be to heavy to be separated by a magnetic
belt or drum system. The size of waste components can be determined using
one the following equations:
Sc = l ...Eqtn 2
Sc =
...Eqtn
3
Sc =
...Eqtn
4
where
The field capacity of MSW is the total amount of moisture which can be retained in a waste sample subject to gravitational pull. It is a critical measure because water in excess of field capacity will form leachate, and leachate can be a major problem in landfills as we will discuss next week. Field capacity varies with the degree of applied pressure and the state of decomposition of the wastes, but typical values for uncompacted comingled wastes from residential and commercial sources are in the range of 50 - 60%.
Permeability of Compacted Wastes
The hydraulic conductivity of compacted wastes is an important physical property because it governs the movement of liquids and gases in a landfill. Permeability depends on the other properties of the solid material include pore size distribution, surface area and porosity.
Proximate Analysis
Proximate analysis includes four tests - loss of moisture when heated
to 105°C for 1 hour; volatile combustible matter (loss on ignition);
fixed carbon; and ash (weight of residue after combustion). Some typical
values are shown in Table 3.
Table 3: Typical Proximate Analysis Values (% by weight)
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| Mixed food |
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| Mixed paper |
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| Mixed plastics |
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| Yard wastes |
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| Glass |
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| Residential MSW |
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Fusing Point of Ash
This is the temperature at which the ash resulting from the burning of waste will form a solid (clinker) by fusion. Typical fusing temperatures are from 1100 - 1200°C.
Elemental Analysis
This is also known as ultimate analysis and involves the determination of carbon, hydrogen, oxygen, nitrogen, sulphur, and ash. Because of concern about halogens these are also often determined as well. The results of this analysis is used to characterise the composition of the organic matter in wastes. This is important for C/N ratios for biological decomposition. Typical values are shown in Table 4.
Table 4: Typical data in Elemental Analysis (% by weight)
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| Mixed food |
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| Mixed paper |
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| Mixed plastic |
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| Yard waste |
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| Refuse Derived Fuel |
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Energy Content
The energy content of the components of waste can be determined using a boiler system, laboratory bomb calorimeter, or by calculation using elemental composition. The SI unit of measurement is kJ/kg. The energy content will be looked at later in this course when talking about incineration.
Essential Nutrients
If the organic content of MSW is to be used for biological conversion either for compost, methane or ethanol production, then the essential nutrient content is required. Of most importance are the major nutrients in their various forms - nitrogen (as nitrates, ammonium N) phosphorus and potassium.
Table 5: Transformation Processes in Solid Waste Management
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| Physical | ||
separation |
manual and/or mechanical | individual components found in comingled MSW |
volume reduction |
Force or pressure | original waste reduced in volume |
size reduction |
Shredding, grinding, or milling | altered in form and reduced in size |
| Chemical | ||
combustion |
thermal oxidation | CO2, SO2, oxidation products, ash |
pyrolysis |
destructive distillation | a variety of gases, tar and/or oil |
gasification |
starved air combustion | gases and inerts |
| Biological | ||
aerobic compost |
aerobic biological conversion | compost |
anaerobic digestion |
anaerobic biological conversion | methane, CO2, trace gases, humus |
anaerobic composting (in landfills) |
anaerobic biological conversion | methane, CO2, digested waste |
Physical Transformation
These include component separation, mechanical volume reduction, and mechanical size reduction. Component separation is used to describe the separation processes (manual and/or mechanical) in comingled waste. It can include such things as magnetic separation. The usual materials recovered include separation of recyclables, the removal of hazardous wastes, and the recovery of energy products. Volume reduction refers to the processes whereby waste volumes are reduced, usually by force or pressure. Collection vehicles frequently have compaction mechanisms - or compaction can take place at a transfer station. The baling of plastics, paper, and aluminium is another means of volume reduction, as is the compaction that takes place in landfills. Pressure can be used, eg with paper and cardboard, to form fireplace logs. Size reduction is used to reduce the size of wastes. It usually involves some form of shredding, grinding or milling.
Chemical Transformation
This usually involves a change of phase, eg solid to liquid, solid to gas etc. The main processes are combustion, pyrolysis, and gasification. Combustion is the chemical reaction with oxygen of organic materials accompanied by the emission of light and heat. The process can be represented as:
organic matter + excess air = N2 + CO2 + H2O + O2 + ash + heat
Pyrolysis involves combustion in an oxygen free atmosphere, while gasification involves partial combustion to form a gas. These processes will be examined in more detail later in the course.
Biological Transformation
The biological transformation of the organic fraction bother reduces the volume and weight of material but also produces compost. When carried out anaerobically methane is produced - a typical component of landfill gas. This will be examined in more detail later.
Importance of Waste Transformation
Typically waste transformations are used: