less effectively, depending on the means by which aeration is provided to the bulk
substrate. Hereby, the former approach is used in agreement with Gajalakshmi and
Abbasi (2008).
Windrow composting is probably the most traditional and widespread approach.
In this system, the input material is reduced in average size by means of
bio-shredders and is moistened to approximately 50–60%. The resulting composting
substrate is heaped up to form rows of defined width (3–4 m) and height (2–2.5 m),
while the length varies greatly according to the available space in the composting
facility. The specified cross-sectional dimensions are dictated primarily by the nature
of the substrate; the taller windrows would in fact result in “packing down” the
matrix, and, therefore, in anaerobiosis, while the smaller ones would dissipate heat
and dry out too quickly. Aeration is generally provided by mechanized turning
equipment (Fitzpatrick et al. 2005; Gajalakshmi and Abbasi 2008; Manderson
2009) that allows oxygen to access the matrix and promotes the homogenization
of the bulk material. Turnings are generally more frequent (twice a week) during the
initial stages when the oxygen demand is high and the temperature still exceeds
50
C, while turning on a weekly basis is typical of the cooling phase. Complete
composting is accomplished within 3–4 weeks. Thereafter the compost is allowed to
cure for another 3–4 weeks without turning (Gajalakshmi and Abbasi 2008). One
variant of the windrow system, where turning is not envisaged, consists of embedding pipes in the basement underlying the windrow in order to increase the air
exchange within the matrix. This, however, resembles aerated static piles
approaches, which are discussed next.
(Aerated) static pile composting is akin to windrow systems, except that turning is
not contemplated and the heaps are far larger than those specified above. This allows
to accommodate a consistent amount of material and keep the operational costs low.
On the other hand, static piles are “passively aerated” and the extent of the gas
exchange highly depends on the matrix porosity, thus resulting in prolonged incubation periods. One means to accelerate the composting process is the aerated static
pile system that was developed by the Beltsville Agricultural Research Center (MD,
USA) (Manderson 2009). A network of perforated pipes is allocated longitudinally,
along the base of the pile; blowers can be connected at the pipes’ end for an effective
delivery of oxygen to the substrate. When the blowers are turned off, the aeration is
“passive”. However, the “forced” aeration is achieved by running blowers. Blowers
can be operated both in negative and positive mode. In the former case, air is forced
to enter the pile and the collected off-gases are further treated through ripe compostbased biofilters; in the latter case, however, air is delivered to the compost heap and a
layer of mature compost is usually overlaid to the heap to prevent emission of
odorous off-gases.
An additional variant of the static pile is the “contained” pile, otherwise called
“enclosed static” pile (Gajalakshmi and Abbasi 2008) or “composting chamber”
(Šašek et al. 2003). In such a system the compost substrate is contained between
walls over a perforated floor through which air is blown. Advantages offered by this
approach include: efficient aeration of the composting substrate, effective collection
of leachate, and capturing of odorous emission.
23 Composting Practices for the Remediation of Matrices Contaminated by. . .
475
substrate. Hereby, the former approach is used in agreement with Gajalakshmi and
Abbasi (2008).
Windrow composting is probably the most traditional and widespread approach.
In this system, the input material is reduced in average size by means of
bio-shredders and is moistened to approximately 50–60%. The resulting composting
substrate is heaped up to form rows of defined width (3–4 m) and height (2–2.5 m),
while the length varies greatly according to the available space in the composting
facility. The specified cross-sectional dimensions are dictated primarily by the nature
of the substrate; the taller windrows would in fact result in “packing down” the
matrix, and, therefore, in anaerobiosis, while the smaller ones would dissipate heat
and dry out too quickly. Aeration is generally provided by mechanized turning
equipment (Fitzpatrick et al. 2005; Gajalakshmi and Abbasi 2008; Manderson
2009) that allows oxygen to access the matrix and promotes the homogenization
of the bulk material. Turnings are generally more frequent (twice a week) during the
initial stages when the oxygen demand is high and the temperature still exceeds
50
C, while turning on a weekly basis is typical of the cooling phase. Complete
composting is accomplished within 3–4 weeks. Thereafter the compost is allowed to
cure for another 3–4 weeks without turning (Gajalakshmi and Abbasi 2008). One
variant of the windrow system, where turning is not envisaged, consists of embedding pipes in the basement underlying the windrow in order to increase the air
exchange within the matrix. This, however, resembles aerated static piles
approaches, which are discussed next.
(Aerated) static pile composting is akin to windrow systems, except that turning is
not contemplated and the heaps are far larger than those specified above. This allows
to accommodate a consistent amount of material and keep the operational costs low.
On the other hand, static piles are “passively aerated” and the extent of the gas
exchange highly depends on the matrix porosity, thus resulting in prolonged incubation periods. One means to accelerate the composting process is the aerated static
pile system that was developed by the Beltsville Agricultural Research Center (MD,
USA) (Manderson 2009). A network of perforated pipes is allocated longitudinally,
along the base of the pile; blowers can be connected at the pipes’ end for an effective
delivery of oxygen to the substrate. When the blowers are turned off, the aeration is
“passive”. However, the “forced” aeration is achieved by running blowers. Blowers
can be operated both in negative and positive mode. In the former case, air is forced
to enter the pile and the collected off-gases are further treated through ripe compostbased biofilters; in the latter case, however, air is delivered to the compost heap and a
layer of mature compost is usually overlaid to the heap to prevent emission of
odorous off-gases.
An additional variant of the static pile is the “contained” pile, otherwise called
“enclosed static” pile (Gajalakshmi and Abbasi 2008) or “composting chamber”
(Šašek et al. 2003). In such a system the compost substrate is contained between
walls over a perforated floor through which air is blown. Advantages offered by this
approach include: efficient aeration of the composting substrate, effective collection
of leachate, and capturing of odorous emission.
23 Composting Practices for the Remediation of Matrices Contaminated by. . .
475