Page 127 - Materials Chemistry, Second Edition

CAT3525_C04.qxd 1/27/2005 11:12 AM Page 98
98 Waste Management Practices: Municipal, Hazardous, and Industrial
calculate the arithmetic mean, the geometric mean and the weighted mean.

100 75 50 25 5
arithmetic mean D 51 mm
5

geometric mean D (100 75 50 25 5) 1/5 34.2 mm

(2 100) (6 75) (12 50) (4 25) (4 5)
weighted mean D
2 6 12 4 4
48.9 mm

(225 100) (310 75) (500 50) (2000 25) (5750 5)
number mean D
225 310 500 2000 5750
17.0 mm

Note: The term diameter is defined to reflect a spherical particle shape; therefore, the above
equations must serve only as an approximation.
Other calculations of particle size distribution incorporate particle surface area and volume as
well.

4.6.4 FIELD CAPACITY
Field capacity may be defined as the total amount of moisture retained by mixed solids against the
force of gravity. Water in excess of field capacity will be released by the force of gravity as leachate.
The field capacity of a waste stream is of critical importance for two reasons; first, aerobic micro-
bial activity is optimized at or slightly below the field capacity. Therefore, biological processing
such as composting is optimized near this point. Secondly, field capacity is important in predicting
leachate formation in landfills, compost piles, or storage piles.
Field capacity varies with the degree of pressure applied to the waste and the state of decom-
position of the waste. The field capacity of uncompacted commingled wastes from residential and
commercial sources may range from 50 to 60% (Tchobanoglous et al., 1993; Kiely, 1997).
One equation for field capacity of MSW is

FC 0.6 – 0.55(W/[4500 W]) (4.18)

where FC is the field capacity (percent of dry weight of waste) and W the overburden weight cal-
culated at midheight of the waste in lift (kg).

4.6.5 HYDRAULIC CONDUCTIVITY OF COMPACTED WASTE
The hydraulic conductivity, designated K, of compacted wastes is a physical property that strongly
influences the movement of liquids (especially leachate) and gases in a landfill. Dense materials
such as sludges tend to resist rainfall infiltration and promote runoff from a landfill cell. In contrast,
paper and yard waste, by virtue of having large particles and therefore large void space, exhibit lit-
tle resistance to rainfall infiltration.
Loose samples of MSW have a hydraulic conductivity value of 15 10 5 m/s, while dense
baled waste may have a K of 7 10 6 m/s. The hydraulic conductivity for shredded waste ranges
from 10 4 to 10 6 m/s (Kiely, 1997). Since MSW is very heterogeneous, these values serve only as
an approximation.