Page 251 - Materials Chemistry, Second Edition

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222 Waste Management Practices: Municipal, Hazardous, and Industrial
maximum acceptable moisture content is a level at which no nuisance conditions (e.g., anaerobio-
sis) will develop and at which the process will proceed satisfactorily.
Moisture content of the pile can be measured in the field using a range of analytical equipment
ranging from gypsum blocks to tensiometers. Alternatively, a sample can be taken to the laboratory
and measured field-moist and oven-dry (i.e., after 48 h in an oven at 105°C). These data are used to
calculate the gravimetric moisture content.
Moisture is rapidly depleted from an active compost pile and must be replaced by regular addi-
tions of water or, in some cases, application of wastewater sludge (also a rich source of het-
erotrophic microorganisms). The optimum amount of water to be applied to a compost pile can be
calculated from a mass balance equation (Vesilind et al., 2002)
M (M X 100X )/(X X ) (8.4)
w
s s
w
s
p
where M is the moisture content of the compost pile at the start of composting (%), M the mois-
s
p
ture content of the solids, for example shredded MSW (%), X the mass of solids (wet metric tons)
s
and X the mass of water, wastewater, or other source of water (metric tons).
w
EXAMPLE 8.2
At a municipal waste handling facility, a mixture of approximately 25 metric tons of food waste, yard
waste, and paper waste is to be composted. The moisture content of this feedstock measures 9.5%. It
has been previously determined that an ideal moisture content for the compost pile should be about 55%.
Calculate the metric tons of water to be added to the solids to achieve the optimum moisture content.
SOLUTION
M (M X 100X )/(X X ) [(25 9.5) (100 X )]/(25 X )
s
w
w
w
p
s s
w
X 25.3 MT of water to be applied to the pile.
s
8.5.7 TEMPERATURE
There is a direct relationship between microbial activity and temperature of the pile. High temper-
atures result from biological activity, i.e., heat liberated from microbial respiration and the result-
ant breaking of chemical bonds of substrate compounds. This heat builds up within the pile;
dispersal of this heat is limited due to the insulating effects of the pile.
Thermophilic vs. mesophilic temperature ranges have their own advantages and disadvantages
with respect to composting. The temperatures that enhance microbial activity are in the range 28 to
55°C (48 to 131°F). The highest O consumption also occurs within this range.
2
High temperatures are commonly considered as necessary conditions for good composting.
Excessively high temperatures, however, inhibit growth of most microorganisms, thus slowing decom-
position of feedstock. When the temperature rises beyond approximately 65 to 70°C (150 to 160°F),
the tendency is for spore formers (e.g., Bacillus and Clostridium) to convert into spores. This transi-
tion is undesirable, because the spore-forming stage is a resting stage and therefore the rate of decom-
position is reduced. Moreover, microbes incapable of forming spores are strongly inhibited or killed at
those temperatures. Consequently, the maximum temperature should be kept at about 65°C (150°F).
The temperature distribution within a composting mass is affected by the surrounding climatic
conditions and by the method of aeration. In static piles (see below), the highest temperatures
develop at the center of the mass and the lowest temperatures occur at the edges of the pile. These
temperature gradients promote a small degree of convection (i.e., natural airflow). The degree of air
movement is a function of ambient conditions as well as porosity of the composting mass. The prob-
lem of temperature control is best solved, however, by either periodically turning the pile or using
forced ventilation throughout the process.