Characteristics of Municipal Solid Waste (MSW)
The following are the major characteristics of municipal solid waste (MSW):
1. Physical Characteristics
1a) Specific Weight
This is weight of waste material per unit volume (kg/cm3 or kg/m3)
Moisture content of MSW is expressed in two ways: wet weight method and dry weight method. In the wet weight method, the moisture in a sample is expressed as percentage of wet weight while in the dry weight method, the waste is expressed as the dry weight of the material. The wet weigh is the most commonly use in field for waste management. It is expressed thus:
M =(w-d)/w* 100
Where M= moisture content (%), w = initial waste sample as delivered (kg) and d = weight of sample after drying at 1050C
Example: The dry weight of waste sample after drying at 1050C is 22372 g. If the moisture content of the waste was 65%, what would be the initial weight of waste (in kg) sample as delivered?
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m = (w-d)/w*100
Where m = moisture content; w = initial weight of waste; d = weight of waste sample after drying
65 = (w-22.372)/w * 100
65w = 100w-2237.2
65w-100w = -2237.2
-35w = -2237.2
w = 63.92kg
1c) Particle size and size distribution
These are important properties for recovery of materials. The size of waste component may be defined by the following measures:
Sc = L, Sc = (L+ W)/2 , Sc = (L+W+H)/3
Where Sc = size of component (mm), L= length (mm), W= width (mm), H= height (mm).
1d) Field capacity
This is the amount of moisture that can be retained in a waste sample subject to downward pull of gravity. FC is important in determining the formation of leachate in landfill.
1e) Permeability of compacted waste
The hydraulic conductivity of compacted waste is an important physical property that governs the movement of liquid and gases in a landfill.
2. Chemical Characteristics
Information on the chemical composition of MSW is important for evaluating alternative processing and recovery options. If solid wastes are to be used as fuels, four most important properties to be determined are:
- Proximate analysis
- Fusing point of ash
- Ultimate analysis
2a) Proximate Analysis
- Moisture (loss of moisture when heated to 1050C)
- Volatile combustible matter (additional loss of weight on ignition at 9500C in a covered crucible).
- Fused carbon (combustible residue left after volatile matter is removed)
- Ash (weight of sample after combustion in an open crucible).
Table. Typical example of proximate analysis and energy data for materials found in residential, commercial and industrial solid wastes
|Types of waste||Proximate Analysis (% by weight)||Energy Content (KJ)|
|Moisture||Volatile Matter||Fixed carbon||Non Combust.||As collected||Dry||Dry ash free|
Source: Isirimah (2002).
2b) Fusing point of Ash
Is defined as that temperature at which the ash resulting from the burning of waste will form a solid (Clinker). Typical temperatures for the formation of clinker range from 110 to 12000C.
2c) Ultimate analysis of solid waste
This is the analysis of waste components. It involves the determination of % C, N, S, O, H and ash. Halogens are often included because of their importance. The result of ultimate analysis is used to characterize chemical composition of organic matter in MSW.
They are also used to define proper mix of waste material to achieve suitable C/N ratio for biological conversion processes. An example is given in table below.
Table: Typical date on ultimate analysis of the combustible materials found in residential, commercial and industrial solid wastes
Type of waste
|Percentage by weight (dry basis)|
3. Biological Characteristics
3a) Biodegradability of organic waste components
This is a biological characteristic of organic fraction of MSW that is measured by Volatile solid (VS) content (determined by ignition at 5500C).
This method of biodegradable is misleading because some organic fractions of MSW are highly volatile but low in biodegradability e.g. certain plant trimmings.
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Alternatively, lignin content of a waste can be used to estimate the biodegradable fraction using the following relationship:
BF = 0.83-0.028 LC
Where BF = biodegradability fraction expressed on VS basis
LC = lignin content of the VS expressed as percentage dry weight.
3b) Production of odors
Formation of odor results from the decomposition of organic compound found in MSW. When under anaerobic condition, sulphate is reduced to sulfide. The latter combined with hydrogen to form hydrogen sulphide.
The black color of solid waste is due to production of FeS.
The biochemical reduction of an organic compound containing sulphur radical can lead to the formation of malodorous compound such as methyl mercaptan and amino butynic acid.
3c) Breeding of flies
Fly breeding is important during storage of waste. Flies develop in less than 2 weeks after the eggs are laid. Life of common housefly from egg to adult can be described as follows:
Eggs development 8-12 hours
First stage of larval 20 hours
Second stage of larval 24 hours
Third stage of larval 3 days
Pupa stage 4-5 days
Total 9-11 days.
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