and O2 concentrations were detected after two weeks of composting
most likely because of the important dilution although the airflow
was decreased during the cooling phase. A wide range of
aeration rates leads to successful composting and after preliminary
tests, the used aeration rates were halfway between the optimal
aeration rate of 120 l h1 kg1 DM, proposed by Yamada and Kawase
(2006) and the optimal range of 8.5–16.6 l h1 kg1 DM, proposed
by de Guardia et al. (2008) during the composting of
sludge with bulking agents.
3.1.3. Moisture content
The moisture content tended to decrease due to the combination
of high temperature levels and aeration during the thermophilic
phase and was controlled by applying water at each
sampling date and by humidifying the inlet air. The initial moisture
content (66% of wet weight) was reduced in all experiments to
reach an average moisture content of 41 ± 7% (CV = 11%) of the
wet weight, remaining above the minimum moisture content of
40% suggested by Haug (1993) for optimal composting conditions
(Table 1). However, the composting process may have been temporarily
limited by low moisture content in a few cases due to compost
drying by the airflow although optimal moisture contents
were restored in all cases by water addition on sampling dates.
3.2. Evolution of compost characteristics
3.2.1. Organic matter loss during composting
Dry matter losses mainly occurred during the first 13 days but
varied among composting replicates at this stage of composting,
with a mean loss of dry matter of 31 ± 9% (Table 1) and a coefficient
of variation of 29%. The average dry matter loss reached 40 ± 7%
over the entire composting process and variability among composting
replicates decreased to 17%. The extent of dry matter loss
was similar to observed during sewage sludge composting on a
full-scale composting site over 110 days (Sánchez-Monedero
et al., 1999). Similar patterns were observed for TOM, with a loss
of 46 ± 6% (CV = 12%) of the initial TOM during the overall composting
process. Total organic carbon loss of 42 ± 6% (CV = 14 %)
of initial TOC at the end of the composting was similar to the
C1
0
10
20
30
40
50
60
70
80 C2 C3
O2 (%) and CO2 (%)
0
2
20
25
30
35
40
C4
0 10 20 30 40
Temperature (°C)
0
10
20
30
40
50
60
70
80 C5
Composting time (days)
0 10 20 30 40
C6
0 10 20 30 40 0
2
20
25
30
35
40
O2
CO2
T3
T1
Fig. 2. Evolution of the temperature profile (C) in the centre of the compost (T1) and in the thermostatic bath (T3), percentages of O2 and CO2 in the outlet gas flow during the
first 41 days of composting in the six composting reactors C1–C6.
and O2 concentrations were detected after two weeks of composting
most likely because of the important dilution although the airflow
was decreased during the cooling phase. A wide range of
aeration rates leads to successful composting and after preliminary
tests, the used aeration rates were halfway between the optimal
aeration rate of 120 l h1 kg1 DM, proposed by Yamada and Kawase
(2006) and the optimal range of 8.5–16.6 l h1 kg1 DM, proposed
by de Guardia et al. (2008) during the composting of
sludge with bulking agents.
3.1.3. Moisture content
The moisture content tended to decrease due to the combination
of high temperature levels and aeration during the thermophilic
phase and was controlled by applying water at each
sampling date and by humidifying the inlet air. The initial moisture
content (66% of wet weight) was reduced in all experiments to
reach an average moisture content of 41 ± 7% (CV = 11%) of the
wet weight, remaining above the minimum moisture content of
40% suggested by Haug (1993) for optimal composting conditions
(Table 1). However, the composting process may have been temporarily
limited by low moisture content in a few cases due to compost
drying by the airflow although optimal moisture contents
were restored in all cases by water addition on sampling dates.
3.2. Evolution of compost characteristics
3.2.1. Organic matter loss during composting
Dry matter losses mainly occurred during the first 13 days but
varied among composting replicates at this stage of composting,
with a mean loss of dry matter of 31 ± 9% (Table 1) and a coefficient
of variation of 29%. The average dry matter loss reached 40 ± 7%
over the entire composting process and variability among composting
replicates decreased to 17%. The extent of dry matter loss
was similar to observed during sewage sludge composting on a
full-scale composting site over 110 days (Sánchez-Monedero
et al., 1999). Similar patterns were observed for TOM, with a loss
of 46 ± 6% (CV = 12%) of the initial TOM during the overall composting
process. Total organic carbon loss of 42 ± 6% (CV = 14 %)
of initial TOC at the end of the composting was similar to the
C1
0
10
20
30
40
50
60
70
80 C2 C3
O2 (%) and CO2 (%)
0
2
20
25
30
35
40
C4
0 10 20 30 40
Temperature (°C)
0
10
20
30
40
50
60
70
80 C5
Composting time (days)
0 10 20 30 40
C6
0 10 20 30 40 0
2
20
25
30
35
40
O2
CO2
T3
T1
Fig. 2. Evolution of the temperature profile (C) in the centre of the compost (T1) and in the thermostatic bath (T3), percentages of O2 and CO2 in the outlet gas flow during the
first 41 days of composting in the six composting reactors C1–C6.
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