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Effects of Aeration and C:N Ratio on
Household Waste Composting in Egypt
S. Sadakaa & A. El – Taweela
a Agricultural Engineering Dept., Faculty of Agriculture, Alexandria
University, Egypt
Published online: 23 Jul 2013.
To cite this article: S. Sadaka & A. El – Taweel (2003) Effects of Aeration and C:N Ratio on
Household Waste Composting in Egypt, Compost Science & Utilization, 11:1, 36-40, DOI:
10.1080/1065657X.2003.10702107
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Compost Science & Utilization, (2003), Vol. 11, No. 1, 36-40
Effects of Aeration and C:N Ratio on
Household Waste Composting in Egypt
S. Sadaka and A. El – Taweel
Agricultural Engineering Dept.,
Faculty of Agriculture, Alexandria University, Egypt
The goal of this research was to investigate the effects of aeration and C:N ratio on the
composting of Egyptian residential waste. Controlling aeration and carbon to nitrogen
ratio may help in enhancing the performance of the composting process. Three airflow
3
rates (0.000, 0.003 and 0.006 m /h/kg) and carbon to nitrogen ratio (11, 26 and 39) were
studied using a laboratory scale composting unit. Increasing the compost time result-
ed in a peak of the temperature up to the thermophilic range between the second to
third weeks. The compost temperature reached its maximum value of 68°C at the aer-
3
ation level of 0.003 m /h/kg and C:N ratio of 26. At this temperature, the plant and
animal pathogens may be killed or at least suppressed to acceptable levels. Moisture
content of compost material was reduced during the composting process. The reduc-
tion of the moisture content of the compost material due to change in C:N ratio was
small except in the case of no aeration. Moisture content reached its minimum value
3
of 20% d.b after four weeks at aeration level of 0.006 m /h/kg and C:N ratio of 39. The
pH values of the household waste were near neutral showing insignificant increase
with composting time. At the lower level of initial C:N ratio (11), composting time did
not affect the C:N ratios. The maximum reduction of the C:N value (33.1%) was ob-
3
served with aeration rate of 0.003 m /h/kg and initial C:N of 39.
Introduction
Egyptian household wastes contain kitchen residues and other refuse. Besides
nitrogen (N), phosphorus (P) and potassium (K), household waste contains calcium
and magnesium, which are important for their fertilizing value. Recycling these ele-
ments in the soil through composting greatly reduces the difficulty of meeting the
yearly NPK deficient.
Composting involves actions of enzymes, microorganisms and oxygen. It is impor-
tant to maintain aerobic conditions during the composting process through continuous
aeration. The main objective of supplying air is to provide oxygen for aerobic biodegra-
dation as well as to remove excess water vapor, heat and gases.The recommended range
of aeration in literature is quite wide, with suggested rates as low as 0.04 to 0.08 L air/min-
kg of volatile solids (Lau et al. 1992), up to 0.87 to 1.87 L air/min-kg of volatile solids
(Hong et al. 1983). Mathur (1991) stated that the optimal rate is considered to be a supply
3
of 0.6 - 1.8 m air/day/ kg volatile solids during the thermophilic stage of composting.
Mathur (1991) and Barington et al. (1997) stated that different materials have to be
mixed or layered to achieve an optimal C:N ratio, and thus optimal composting. The ac-
tivity of microorganisms involved in composting is directed at synthesizing microbial
protoplasm which contains about 50% C, 5% N and 0.25% P on a dry weight basis
(Alexander 1977). The effects of C:N ratio on composting were investigated by many re-
searchers and the range of 20 to 30 is generally recommended as the optimum C:N ratio.
Moisture content of Egyptian household waste varies depending upon the waste
type, collection method, and geographical site. Moisture content represents a signifi-
cant characterization in the growth of microorganisms and in the composting process.
Sadaka and Engler (2000) reported an increase in volatile solids and chemical oxygen
36 Compost Science & Utilization
Winter 2003

Effects of Aeration and C:N Ratio on Household Waste Composting in Egypt
demand reductions as initial moisture content in the reactor increased. Posey et al.
(1999) evaluated biodegradation of waste with moisture contents ranging from 13 to
75% in a landfill system. They found that biodegradation of waste began after 7, 14 and
21 days for moisture contents of 80, 73 and 65%, respectively. Waste biodegradation
was minimal after 70 days at moisture contents of 50% or less.
The aim of this research was to investigate the effects of aeration and initial carbon
to nitrogen ratio on household waste composting as measured by temperature, mois-
ture content, pH and C:N.
Materials and Methods
Three wood boxes (40 x 40 x 40 cm) were constructed in the Agricultural Engi-
neering Department workshop, Faculty of Agriculture, Alexandria University. A per-
forated plate was placed at the bottom of the box. Each box was divided into nine sec-
tions to offer twenty-seven experimental units. The boxes were insulated to reduce the
heat loss. Bulk density, moisture content and C:N ratio of the initial waste material av-
3
eraged between 345 47 kg/m , 45 4% and 11, respectively. Air was supplied by small
pre-calibrated air pumps to supply positive pressure to the compost. Three-air flow
3
rates namely 0.000, 0.003 and 0.006 m /h/kg waste and three carbon to nitrogen ratios
namely 11, 26 and 39 were used as shown in Table (1). The required carbon to nitro-
gen ratios were achieved by adding the needed amount of sawdust. Each box con-
tained three C:N ratios and was subjected to a single air flow rate under the perforat-
ed plate. Three replicates were used for each treatment.
Temperature, moisture content, pH and carbon to nitrogen ratio were measured
during the experimental run. Temperatures were measured using a digital ther-
mometer (Comark, 9006, Beaverton, Oregon, USA) and thermocouple with accuracy
of 0.1°C. The thermocouple
TABLE 1.
was injected to the center of
The experimental design
the experimental unit. Ade-
Air Flow Rate
(m /h/kg waste)
Carbon To
Nitrogen Ratio
Initial Moisture
Content % (d.b.)*
quate time was taken to read
the temperature until no fur-
ther temperature changes. A
3
0.000
0.003
0.006
11
26
39
11
26
39
11
26
39
49.2 1.1
47.4 0.9
45.3 1.2
47.6 1.3
45.5 0.8
46.4 0.9
45.2 1.1
43.1 0.7
41.4 0.7
3
known volume (50 cm ) of
compost was taken and oven
dried. The dried materials
were weighed and the mois-
ture content (d.b.) was deter-
mined. The pH and carbon to
nitrogen ratio were found in
the Soil Laboratory (Alexan-
dria University, Alexandria).
*Measured, n = 3
Results and Discussion
Composting temperature profile is a very good indicator of the composting com-
petency. The results of the effects of aeration level and C:N ratio on the composting
temperature are shown in Figure 1. The results show that increasing the compost
time resulted in a peak of the temperature up to the thermophilic range between the
second to the third weeks. After three weeks these temperatures began a gradual de-
cline. The results also showed that, increasing the aeration level from 0.000 to 0.003
Compost Science & Utilization
Winter 2003 37

S. Sadaka and A. El – Taweel
3
m /h/kg increased the com-
posting temperature. A fur-
ther increase in airflow rate
reduced the maximum tem-
perature. The higher temper-
ature might lead to higher
substrate decomposition. The
compost temperature in-
creased due to the activity of
the thermophilic bacteria. In-
creasing the airflow rate re-
duced the temperature due to
the heat transfer by convec-
tion with air and may be due
to reduction of microorgan-
isms. Aeration conditions
were achieved throughout
the second box after the sec-
ond week as suggested by the
rapid temperature rise. Gen-
erally, temperature remained
in the thermophilic range
from week two to four for
most of the experiments.
Figure 1. The effects of composting time on composting temperature at
different initial carbon to nitrogen ratios and aeration levels.
The compost temperature
reached its maximum value of
68°C at the aeration level of
3
0.003 m /h/kg and C:N of 26.
Either increasing or decreas-
ing the C:N ratio resulted in
decrease in maximum com-
posting temperature. The tem-
perature did not reach the de-
sired temperature of 68° in the
case of C:N of 39 may be due
to the lack of biodegradable
carbon and nitrogen. Where-
as, a C:N of 26 improved mi-
crobial activity, despite nitro-
gen losses. Increasing the C:N
ratio from 11 to 26 gave a bet-
ter nutrient balance for the mi-
croorganisms to be active and
increase in composting tem-
perature. Further increase in
C:N ratio to 39 reduced the
available nitrogen for the mi-
croorganisms to multiply. The
results also show that the com-
posting temperature reach the
Figure 2. The effects of composting time on composting moisture content
at different initial carbon to nitrogen ratios and aeration levels.
38 Compost Science & Utilization
Winter 2003

Effects of Aeration and C:N Ratio on Household Waste Composting in Egypt
room temperature at the sev-
enth week.
As with C:N ratio, proper
moisture content is also neces-
sary to support microbial ac-
tivity. However, no moisture
was added to the compost
throughout the course of this
study. The initial moisture con-
tents were in the range of 40-
50% d.b. The effects of aeration
level and C:N ratio are pre-
sented graphically in Figure 2.
The moisture content reduc-
tion rate was sharp at the be-
ginning of the composting pe-
riod. This reduction of
moisture content could be at-
tributed to the activity of the
microorganisms. A second rea-
son is the ability of air, to dry
the moist material at the begin-
ning. The temperature rise at
the first two weeks contributed
Figure 3. The effects of composting time on composting pH at different
initial carbon to nitrogen ratios and aeration levels.
to material drying. After five
weeks, moisture content
reached an almost steady state
for all experiments.
The difference between
the moisture content of the
compost material due to
change in C:N ratio was small
except in the case of no aera-
tion. The results showed that
the moisture content reached
its minimum value of 20%
(d.b.) after four weeks at aera-
3
tion level of 0.006 m /h/kg
and C:N ratio of 39. It can be
observed that, the higher the
aeration level the lower the
moisture content. The results
are in agreement with Rynk et
al. (1992). They reported that
composting reduced mois-
ture content. Also, Sartaj et al.
(1997) observed that moisture
content decreased from 76 to
70.2% while composting
poultry manure.
Figure 4. The effects of composting time on the composting C:N ratio at
different initial carbon to nitrogen ratios and aeration levels.
Compost Science & Utilization
Winter 2003 39

S. Sadaka and A. El – Taweel
The pH trends as affected by aeration level and C:N are shown in Figure 3. The
initial pH of the waste material was (6.8 - 7.0). The pH values of the medium increased
slightly with time for all treatments. The effects of aeration and C:N were not pro-
nounced. The results showed that adjustments of pH may not be necessary. Mathur
(1991) reported that most household refuse is between 5.0 and 7.0, more towards the
lower number if the material is partly putrefied as decomposition initially releases or-
ganic acids. On the other hand, if the compost material is high in substances like pro-
teins, urea and uric acid, enzyme hydrolysis initially will release ammonia, thus rais-
ing the reaction to alkalinity.
The effects of the composting time on the carbon to nitrogen ratio are presented
graphically in Figure 4. Neither aeration nor the composting time affects the C:N ra-
tio of the material at the initial C:N ratio of 11. Composting time affects the C:N ratio
for the initial C:N of 39. Rodrigues et al. (1995) and Martin et al. (1993) observed that a
compost with C:N of 20 - 35 is preferred ratio for composting. When C:N ratio is low,
the available carbon may be fully utilized and excess nitrogen may be lost in the form
of ammonia. If C:N ratio is high, nitrogen becomes the limiting factor and the process
slows down. Therefore, the C:N ratio of 26 is the optimum condition for composting.
Conclusions
It can be concluded that Egyptian household waste is a valuable material if it is com-
posted and converted to useful fertilizer. Aeration affects compost temperature. A max-
3
imum temperature of 68°C was achieved at the air flow rate of 0.003 m /h/kg and C:N
ratio of 26. Moisture content of compost material can be reduced by the means of com-
posting technology to reduce transporting costs. From the experiments, it can be con-
cluded that the optimum condition of Egyptian household waste composting process is
3
aeration level of 0.003 m /h/kg and C:N ratio of 26 and moisture content of 45% (d.b).
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