Page 125 - Materials Chemistry, Second Edition

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96 Waste Management Practices: Municipal, Hazardous, and Industrial
SOLUTION
The dry weight of each MSW component is calculated using the equation

Dry weight [(moist weight)(100 % moisture)]/100

Component Moisture Content (%) Wt% Moist Weight (kg) Dry Weight (kg)
Paper waste 7 25 25 (1.0 – 0.07)(25) 23.25
Yard waste 55 18 18 (1.0 – 0.55)(18) 8.10
Food waste 65 20 20 (1.0 – 0.65)(20) 7.00
Plastic 2 5 5 (1.0 – 0.02)(5) 4.9
Wood 20 8 8 (1.0 – 0.2)(8) 6.4
Glass 3 7 7 (1.0 – 0.03)(7) 6.79
Metals 3 9 9 (1.0 – 0.03)(9) 8.73
Textiles 12 8 8 (1.0 – 0.12)(8) 7.04
Total 100 72.21

Totaling the values in the final column, the average percent moisture content of the MSW is
equal to [(100 – 72)/100](100%) 28%.

4.6.3 PARTICLE SIZE DISTRIBUTION
The size distribution of solid waste components is important for improving the rate of chemical
reactions; in other words, smaller particle sizes provide greater surface area and thus more rapid
reaction with microorganisms in a compost pile, or more rapid combustion in an incinerator. Size
distribution is also an important consideration in the recovery of materials, for example, with the
use of processing equipment such as a trommel screen or a magnetic separator (see Chapter 7).
MSW tends to stratify vertically when mixed, with smaller and denser components migrating to
the bottom of a pile and lighter, bulkier objects migrating to the top. Such stratification has implica-
tions for efficient combustion on a traveling grate in a boiler or for materials separation in a MRF.
Size distribution is measured by passing samples of MSW over a series of screens, beginning
with a coarse screen and continuing down to a fine screen. As discussed earlier, MSW is extremely
heterogeneous; therefore, neither MSW nor any of its components are considered to possess a char-
acteristic particle size (Liu and Liptak, 2000).
The size (i.e., “diameter”) of a waste component may be calculated by any of the following
equations:

D l (4.13)
D ( l w h ) / 3 (4.14)

D (l w) / 2 (4.15)
D (lw) ½ (4.16)
D (lwh) 1/3 (4.17)

where D is the diameter, l the length, w the width and h the height.
Particle size distributions of various MSW components are given in Table 4.23.

EXAMPLE 4.8
A mixture of nonspherical waste particles are uniformly sized as follows: l 4 units; w 1.2 units,
and h 1.5 units.