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Asian Jr. of Microbiol. Biotech. Env. Sc. Vol. 15, No. (1) : 2013 : 25-42
© Global Science Publications
ISSN-0972-3005
EVALUATION OF THE COMPOSTING PROCESS THROUGH
THE CHANGES IN PHYSICAL, CHEMICAL, MICROBIAL AND
ENZYMATIC PARAMETERS
A.I. KHALIL1,2*, M.S. HASSOUNA 1, M.M. SHAHEEN 1 AND M.A. ABOU BAKR 3
1 Department of Environmental Studies, Institute of Graduate Studies and Research,
University of Alexandria, Alexandria, Egypt (Permanent address)
2 Department of Agricultural Engineering, College of Food and Agriculture Sciences, King Saud
University, Saudi Arabia (Present address)
3 Research Institute of Soils, Water and Environment, Agricultural Research Center, Alexandria, Egypt.
(Received 6 January, 2013; Accepted 20 January, 2013)
Key words : Organic fraction of municipal solid wastes, Composting, Evaluation parameters
Abstract - Evaluation of the composting process at three compost plants (Abis-I, Abis-II and El-
Montaza) through the changes in physical, chemical, microbial and enzymatic parameters was
investigated. The results showed that the temperature increased and reached the maximum and then
slowly decreased. The temperature remained almost steady at high temperature throughout the
composting period except during the winter season. The unpleasant odour decreased with time but not
completely disappeared. A gradual darkening of colour was evident. The colour of the final composts
was light brown, except that found at El-Montaza plant in the winter season it was nearly dark brown.
Moisture content decreased with time. Marked changes in pH values were found. C/N, C/P and C/K
ratios decreased with time. This decrease was higher in the winter season than in the other seasons
and the highest decrease was found at El-Montaza plant. The mesophilic bacteria decreased with time,
whereas the thermophilic ones increased and reached the maximum after 10 days and then
decreased. The mesophilic fungi appeared at the beginning and then decreased and disappeared after
a period differed from 10 to 20 days, whereas the thermophilic ones disappeared at the beginning and
appeared after 10 days and then disappeared after a period differed from 20 to 30 days. The maximum
activity of α-amylase was found at the beginning and then decreased. The activity of CMCase
increased and reached the maximum after a period differed from 10 to 30 days and then decreased.
The activity of xylanase increased and reached the maximum after 30 days and then decreased. The
activity of enzymes was higher in the winter season than in the other seasons and the highest activity
was found at El-Montaza plant.
*Corresponding author’s email : khalilaii@yahoo.com
INTRODUCTION
The large amounts of municipal solid wastes
(MSW) produced in modern society, as well as its
disposal, represent a serious environmental, social
and economic problem (Castaldi et al. 2008).
Accumulation of a large amount of waste may
create several problems to inhabiting populations.
It requires application of some effective strategies
for proper disposal of MSW (Gautam et al. 2010).
Production of MSW, including organic waste is
increasing while soils are progressively losing
organic matter due to intensive cultivation and
climatic conditions. This makes the recycling of
organic waste as soil amendments a useful
alternative to incineration, landfill or rubbish
dumps (Massiani and Domeizel, 1996). Among the
techniques of disposal characterized by a low
environmental impact, composting of the organic
fraction of MSW is an environmentally and
economically interesting solution (Veeken and
Hamelers, 2002).
Composting, a widely-accepted technology for
organic waste recycling in agriculture, ensures the
organic matter stabilization and sanitization of
these wastes (Bernal et al., 1998). This waste
26 KHALIL ET AL.
treatment technology is based on the biological
transformation of the organic wastes under aerobic
conditions, with the participation of a wide range of
microbial groups (Ishii et al. 2000). Composting can
be regarded as the most usual method for recycling
the organic fraction of MSW, since it provides an
agricultural amendment capable of mitigating the
serious deficit of organic matter suffered by many
agricultural soils, caused by the low use of organic
materials in the fertilization programmes of crops
(Canet and Pomares, 1995).
During composting, organic matter is
transformed into a humus-rich product by the
action of microorganisms and their enzymes. Most
of the modifications that organic matter undergoes
during composting are mediated by enzymes
(Vargas-Garcia et al. 2010). Microbes and their
secreted enzymes play a key role in biological and
biochemical transformations of compost matrixes
in the process of composting (Guo et al. 2012).
Microbial enzymes are capable of degrading large
organic molecules with complex structures into
simple water-soluble compounds composed of
small molecules (Castaldi et al. 2008). Therefore,
characterizing microbial communities along the
composting process may provide valuable
information regarding the evolution of the process,
the rate of biodegradation and, finally, the maturity
of the product (Ryckeboer et al. 2003).
Characterizing and quantifying the enzymatic
activities during composting can reflect the
dynamics of the composting process in terms of the
decomposition of organic matter (Goyal et al. 2005)
and may provide information about the stability
(Mondini et al. 2004) and maturity of compost
(Tiquia, 2002). Since organic substrates are
characterized by a high complexity, their complete
biotransformation during composting demands the
joint action of many different enzymes (Vargas-
Garcia et al. 2010).
The composting process requires adequate
conditions of pH, temperature, moisture,
oxygenation and nutrients, to allow the adequate
development of the microbial population (De
Bertoldi, 1992). Therefore, changes in these
conditions during the process will affect the
proliferation of certain microflora, having different
enzymatic activities, which control the organic
matter degradation (Garcia-Gómez et al. 2003).
Composting as a successful strategy for the
sustainable recycling of organic wastes relies
mainly on the quality of the end products (Mondini
et al. 2004). Generally, most studies in composting
have focused on physico-chemical parameters to
evaluate both process evolution and compost
quality (Said-Pullicino et al. 2007; Albrecht et al.
2008). Microbiological and biochemical parameters
have also recently arisen as good indicators for the
characterization of the composting process (Raut et
al. 2008; Vargas-Garcia et al. 2010; Liu et al. 2011).
In Alexandria city, Egypt, there are three plants
for composting of the organic fraction of municipal
solid wastes (MSW). These plants are focusing on
the increase of the turnover rate of waste streams
and not on quality of the end product.
Consequently, the aim of this study was to evaluate
the composting process at the three compost
plants through the changes in physical, chemical,
microbial and enzymatic parameters.
MATERIALS AND METHODS
Composting methods
Municipal solid wastes (MSW) were collected over
24 h periods from the city of Alexandria, Egypt, and
transferred to the compost plants (Abis et al. 1997).
After sorting, the organic fraction was transferred
to a rotating drum (classifier) which tears off the
organic material into small pieces to produce the
compostable material. Then, a conveyor with a
tipper mechanism drops the compostable material
down to the fermentation area along the conveyor
to form a longitudinal windrow and left to be
processed under the compost plant normal
operating conditions (pile size, moisture content,
turning and temperature were not adjusted as
usual). After the fermentation period (1 month), the
fermented compost was transferred to the
maturation area for 1 month without turning or
adding water. Changes in physical, chemical,
microbial and enzymatic parameters were
monitored during the composting process.
Sampling procedure
Samples from each windrow were taken from six
places at random at a depth of 90 cm and mixed
together. Composite samples were transferred
aseptically in closed bags under cooling to the
laboratory for analyses. Sampling was twice a
week for determination of moisture content and pH
and every 10 days for the chemical, microbial and
enzymatic analyses.
27
Evaluation of the Composting Process Through the Changes in Physical, Chemical
Analytical methods
Physical analysis
The temperature was monitored near the center of
the pile with a metal probe thermometer (Poincelot
and Day, 1973). It was checked twice a week at five
points along the pile. The colour was assessed
visually, while the odour was sensed by smelling
(Khalil, 1996).
Chemical analysis
Samples were oven-dried (60-70oC), ground in a
porcelain mortar and then by a hammer mill. The
ground samples were stored in dry, airtight
containers until use. The pH was determined by
shaking 5.0 g compost in 50.0 mL distilled water
(1:10, w/v) for 30 min, then the pH was measured
(Albonetti and Massari, 1979). The ash content was
determined after drying the sample at 105oC and
ashing at 550oC in a muffle furnace for about 3 h.
The organic matter (OM) and organic carbon (OC)
were estimated as follows: OM (%) = 100 - ash (%),
OC (%) = OM (%)/1.8 (WHO, 1978). The total
nitrogen (N) was determined by the Kjeldahl
method, while the C/N ratio was calculated using
values of the organic carbon and the Kjeldahl total
nitrogen (WHO, 1978). Phosphorus (P) was
determined by spectrophotometer at 470 nm, while
potassium (K) was determined by atomic
absorption spectroscopy (WHO, 1978). All
determinations were carried out in triplicate.
Microbiological analysis
Quantitative estimation of different culturable
aerobic microorganisms was conducted during the
composting process by inoculating the appropriate
media with 0.1 mL volumes of different tenfold
serial dilutions. Bacteria and fungi, both mesophilic
and thermophilic, were isolated from the compost
samples as described by Nakasaki et al. (1992).
Nutrient agar (NA) and potato dextrose agar (PDA)
media were used for bacteria and fungi,
respectively. Incubation temperatures were 30oC for
isolation of mesophiles and 50oC for thermophiles.
The incubation time was 3 days for mesophilic
bacteria, 2 days for thermophilic bacteria and 7
days for fungi, either mesophilic and thermophilic.
All microbial counts were calculated on the dry
weight basis. The average number of microorga-
nisms isolated on three plates was expressed as
colony-forming units (CFU) per dry weight of
compost.
Enzymatic analysis
The activities of relevant enzymes were determined
using aqueous compost extracts. Thus, 10 g of each
fresh sample was transferred to a flask containing
50 mL acetate buffer (0.1 M, pH 5.0). The flask was
shaken at 150 rpm for 1 h. The flask content was
clarified by filtration through cheese cloth, and
then 10 mL of the filtrate were centrifuged at 4oC for
15 min at 5000 rpm. Subsequently, the supernatant
was used for measurement of enzymatic activity.
The assay of a-amylase, carboxymethylcellulase
(CMCase) and xylanase activity was carried out by
measuring the reducing sugars as described by
Shambe and Ejembi (1987). All determinations were
carried out in triplicate.
Statistical analysis
One-way analysis of variance (ANOVA) was used
to compare mean values from different samples.
Where significant differences were obtained,
individual means were tested using the Least
Significance Difference test (P < 0.05).
RESULTS AND DISCUSSION
Physical changes
Temperature
Composing is a biological and biochemical process
involving microbes and their secreted enzymes
(Zeng et al. 2007, 2010). The change in composting
temperature indicates the changes of microbes in
the compost matrix and the temperature increase
of the compost matrix is the result caused by the
accumulation of microbial metabolic warmth of
the compost matrix, thus indicating the metabolic
intensity and organic-matter transformation rate
of microbes of the matrix (Wang et al. 2011). The
temperature is one of the main parameters to
evaluate the composting process, since its value
determines the rate at which many of biological
reactions take place as well as the sanitation
capacity of the process (Bustamante et al. 2008). It
was mentioned that observing windrow
temperature is the best way to monitor the
composting process (Kuhlman, 1985). Therefore,
this parameter may be considered as a good
indicator for the end of the biooxidative phase in
which the compost achieves some degree of
maturity (Jiménez and Garcia, 1989).
28 KHALIL ET AL.
In the present study, changes in temperature
that occurred in the different windrows during the
composting period (60 days) at the three compost
plants during the four seasons are shown in Fig. 1.
The temperature increased and reached the
maximum values and then gradually decreased.
Among the three plants, there was a difference in
the time at which the temperature reached to the
maximum. The increase in temperature may be
attributed to the abundant and active indigenous
microorganisms in the raw composting materials.
On the other hand, the decrease in temperature
after that may be attributed to the decrease of
microbial and enzymatic activities due to most of
the easily degradable organic matter had been
metabolized and may be attributed to that the
moisture content and aeration are not suitable for
microbial and enzymatic activities. It was noticed
also that the temperature remained almost steady
at high level throughout the composting period
except during the winter season for the three
plants, and for Abis-I and El-Montaza plants to
some extent in the spring season as the temperature
decreased. The temperature of the windrow at El-
Montaza plant was lower than at Abis-I and Abis-
II plants. The temperature range during the winter
season was lower than that during the other
seasons for the three plants. The high temperature
of the windrows during the composting period
could be attributed to the lack of adjusting the
moisture content and turning frequency. Lower
temperature in the three plants was detected
during the winter season; this could be attributed
to the ambient temperature and the rain fall. The
temperature detected at El-Montaza plant was
lower than that at the other plants; this could be
also due to the nature of the composted materials.
Generally, the high temperature at the end of
composting (50-67oC) has indicated that the
compost did not reach the full maturity. It was
mentioned that compost is matured enough when
its temperature remains more or less constant and
does not vary with the turning of the material
(Harada et al., 1981; Alexander, 1990). It is generally
agreed that the temperature of the composting
process should not exceed 60oC to avoid rapid
thermal inactivation of the desired microbial
community necessary for the efficient degradation
of organic wastes (Fogarty and Tuovinen, 1991).
Nakasaki et al. (1985c) showed that the optimum
temperature for microbial activity was below 60°C.
This finding is in keeping with results of several
other studies. Temperatures higher than 65°C are
undesirable and can be lowered by aeration
(Kuhlman, 1990). Another report mentioned that
microbial processes and chemical reactions of
thermophiles are faster at higher temperatures but
beyond 70°C, the organisms and many exoenzymes
are inactivated and thermokilled, causing the
process to stall (Finstein and Miller, 1985). A
temperature of 55°C for 3 days is necessary to
destroy the pathogens in the compost while critical
temperature for killing most weed seeds is 63°C
(Rynk et al. 1992). Although high composting temp-
eratures assure adequate pathogenic reduction
(generally 55oC for 3 days), it is apparent that
temperatures higher than this also incapacitate the
microorganisms responsible for carrying out the
composting process. Thus, precise temperature
control is necessary to provide pathogenic
reduction while maintaining a healthy community
of composting microbes (Mckinley et al. 1985).
Generally, during the composting process the
decrease in temperature at the end of the
composting period was very slow. This may be
attributed to the decrease in moisture content of
compost where it decreased with time and
generally it was below the optimum range for the
microbial and enzymatic activities and therefore
the decrease in decomposition of organic matter. In
the present study, the mesophilic stage prevailed
only during the first 2-3 days, and then the
thermophilic stage dominated all over the
composting process. Maturation period,
characterized by high temperature, and this period
is considered as a cooling stage. Generally the
composting process has four overlapped stages.
These are mesophilic, thermophilic, cooling down
and maturation (Gray et al. 1971a; Poincelot, 1974;
Fogarty and Tuovinen, 1991). The first two stages
were evident in the present study in all seasons at
the three plants, while the cooling down stage was
evident only in the winter season at the three
plants and in the spring season at Abis-I and El-
Montaza plants. It could be concluded that the
composts did not reach the full maturity, except
that found at El-Montaza plant in the winter season.
Odour
Odour was observed during the composting
process. It was found that the unpleasant odour of
composting materials decreased with time.
Malodour increased immediately after turning, but
within a short time (≈30 min) it become as before
29
Evaluation of the Composting Process Through the Changes in Physical, Chemical
turning. Generally, the unpleasant odour decreased
after 30 days of composting but did not disappear
completely. The obtained observations are in
agreement with those reported by Haug (1980)
who mentioned that the odour emission rate
dropped significantly during the first stage and
then fluctuated somewhat during the remaining of
the composting period. The odour emission rate
increased immediately after turning, but within
about one hour, it returned to the rate before
turning. The final composts in the present study
were not completely odourless and this is not
agreed with that reported by other investigators. It
was stated that the final material of composting
should be odourless or have a slightly earthy
odour or the musty odour of moulds and fungi
(Gotass, 1956; Alexander, 1990). The disappearing
of the smell coincided with the onset of dark colour
of the waste and the levelling off the temperature
(Mbuligwe et al. 2002).
Colour
During the composting process, a gradual
darkening of the material took place.
Morphologically, the final composts were
heterogeneous with a light- brown colour and this
gave indication of the incomplete maturity, except
the final compost at El-Montaza plant in the winter
season it was nearly dark brown. This could be
attributed to the normal operating conditions
which were not adjusted as usual. The obtained
results are not agree with those reported in
previous studies which mentioned that the
matured compost should be greyish-black or
brownish-black in colour, depending on whether
tannins, melanin or other materials containing
brown pigments were originally present (Gotaas,
1956; Diaz et al. 1993). Also, Jiménez and Garcia
(1989) pointed that the final product, after a
sufficiently long period of maturation should have
a dark brown or almost black colour.
Chemical changes
Moisture content
During composting, the moisture content is
important for transporting the dissolved nutrients
required for the physiological and metabolic
activities of microorganisms (Liang et al. 2003). The
optimum moisture content depends on the specific
physicochemical properties and biological features
of the materials being composted (Guo et al. 2012).
Moisture content has significant effects on enzyme
activities and microbial respiration of the
composting process (Margesin et al. 2006). Biological
activity can be greatly reduced at a moisture
content of less than 30% (Gray et al. 1971b). On the
other hand, the higher moisture content could lead
to anaerobic conditions which results in slower
decomposition. As mentioned by Poincelot (1974),
above 60%, the compost tends to become anaerobic,
causing it to emit foul odors. It was mentioned that
the optimal moisture levels are usually between 40
and 60% (Hachicha et al. 2009). Other investigators
mentioned that 50-60% moisture content is
recommended to be the optimum for composting
(Poincelot, 1974; Fogarty and Tuovinen, 1991). It
was found that the moisture content at the start of
composting ranged between 42.3-50.8%, thereafter
it decreased with time. By the end of composting, it
ranged between 11-30.2%. It was noticed that the
moisture content was slightly higher in the winter
season than in the other seasons and also was
higher at El-Montaza plant than the other plants in
all seasons (Fig. 2a). The decrease in moisture
content during the composting period could be
attributed to the continuous elevated temperature
that occurred without any humidity adjustment
(moisture content was not adjusted during the
composting process) and also to the infrequent
turning. The observed higher moisture content in
the winter season is expected due to the rainfall
and lower temperature. The higher moisture
content at El-Montaza plant may be attributed to
the nature of the composted materials. Generally, it
could be concluded that the moisture content
decreased to a level that is not favourable for the
composting process and therefore, the final
composts were immature, except the final compost
at El-Montaza plant in the winter season it was
nearly mature.
pH
The average changes in pH value during the
composting process are shown in Fig. 2b. The pH
fluctuated in all the treatments in a range from 1 to
2 pHs. Through all the windrows the minimum
value reached was 6.37 (after 3 days at Abis-I plant
in the spring season), while the maximum value
was 8.7 (after 28 days at Abis-II plant in the winter
season). In most cases it was noticed that there was
a decrease of about 1 pH within the first week, and
then it increased. It was also observed that at the
end of composting, the pH was in the alkaline side
30 KHALIL ET AL.
(7.35- 8.43). There were no marked differences
between the three plants or between seasons. It is
noticed that the pH started near the neutrality,
decreased and then increased with time to the
alkalinity and returned again to near the neutrality
at the end of composting, especially in the winter
season at Abis-II and El-Montaza plants and
summer season at Abis-I and Abis-II plants. A
fluctuation in pH value was found during the
themophilic stage. The slight alkalinization
occurred as soon as the mass temperature
increased. Generally, the pattern of pH during the
composting process was typical for many
composting processes in some cases as described by
several authors (Chang and Hudson, 1967;
Poincelot, 1974; Inbar et al. 1993). It was reported
that the decrease in pH during the first period of
composting is expected because of the acids formed
during the metabolism of readily available
carbohydrates. After this initial stage, the pH is
expected to rise, with evolution of free ammonia
and to stabilize or drop slightly again to near
neutral as a result of humus formation with its pH-
buffering capacity at the termination of composting
activity (Poincelot, 1974; Fogarty and Tuovinen,
1991). It was also mentioned that the optimal pH
values for composting range from pH 5.5 to 8.0.
Bacteria favour a near-neutral pH, whereas fungi
favour an acidic range. The effects of extreme pH
on the composting process are directly related to
the effect of pH on microbial activity or, more
specifically, on microbial enzymes (Fogarty and
Tuovinen, 1991).
C/N, C/P and C/K ratios
The results obtained showed that organic matter
(OM) and organic carbon (OC) decreased gradually
by time, whereas ash, nitrogen (N), phosphorus (P)
and potassium (K) increased gradually and
consequently the C/N, C/P and C/K ratios
decreased. It is noticed that the composition of
wastes at the beginning of composting at the three
compost plants was different at the same season.
This variation could be attributed to the variation
in time and place of waste collection. It was
mentioned that the composition of biowaste (the
organic fraction of municipal solid waste) varies
widely, depending on the time and place of
collection (Van Roosmalen and Van de Langerijt,
1989). These variations in biowaste composition
lead to large fluctuations in the quality of the
compost as concerns, among other things, heavy
metal content, organic matter content, electrical
conductivity and stability of the compost (Veeken
et al. 2004). Generally, the changes in C/N, C/P and
C/K ratios are shown in Fig. 3. The results indicated
that the decrease in C/N, C/P and C/K ratios in the
winter season was higher than in the other seasons
at the three plants and the highest decrease was
found at El-Montaza plant and this could be
attributed to the suitability of moisture content
and temperature for composting process.
The initial carbon to nitrogen (C/N) ratio is one
of the most important factors influencing compost
quality (Michel et al. 1996). The optimal C/N ratios
for the microbiological decomposition of organic
material in composting processes have been
reported to be in the range of 26 to 35 (Poincelot,
1972). In general, initial C/N ratios of 25-30 are
considered ideal for composting (Kumar et al. 2010).
The C/N ratio is often used as an index of compost
maturity, despite many pitfalls associated with
this approach, but it seems to be a reliable
parameter for following the development of the
composting process (Inbar et al. 1990). The results
showed that the C/N ratio decreased with time in
all seasons at the three plants (Fig. 3a). As the
decomposition progressed due to losses of carbon
mainly as carbon dioxide, the carbon content of the
compostable material decreased with time and N
content per unit material increased, which resulted
in the decrease of C/N ratio (Goyal et al. 2005). It has
been stated that the C/N ratio of mature compost
should ideally be about 10 but this is hardly ever
achievable due to the presence of recalcitrant
organic compounds, or materials which resist
decomposition due to their physical or chemical
properties (Mathur, 1991). It was reported that a C/
N ratio below 20 is indicative of an acceptable
maturity (Poincelot, 1974), a ratio of 15 or even less
being preferable (Jiménez and Garcia, 1989). There
is no single parameter which can be used as a
suitable indicator of maturity of a wide range of
composts prepared from different materials. One
ratio, which is frequently used as an index of
maturity, is C/N ratio. When a waste is composted,
generally there is decrease in C/N ratio with time
due to losses of C as CO2 which stabilizes in the
range of 15-20 (Poincelot, 1974; Golueke, 1981).
However, Hirari et al. (1983) stated that the C/N
ratio cannot be used as an absolute indicator of
compost maturity, since the values for well-
composted materials present great maturity
variability, due to characteristics of the waste used.
31
Evaluation of the Composting Process Through the Changes in Physical, Chemical
Generally, the decrease in C/N ratio can be taken as
a reliable index of compost maturity when
combined with other parameters as mentioned by
Goyal et al. (2005).
The present results showed that P and K levels
of composts were very high when compared to the
starting values and consequently the C/P and C/K
ratios decreased with time (Fig. 3b and c). Similar
results were obtained by several investigators
(Inbar et al. 1993; Khalil, 1996; Shaheen, 2007). Inbar
et al. (1993) related similar results to concentration
of these elements since no leaching took place
during composting while there was a
corresponding loss in organic matter.
From the fermentation studies, it is known that
aerobic conditions prompt rapid and complete
degradation (stabilization) of organic materials by
microorganisms. Therefore, aerobic conditions are
recommended to maintain a rapid and complete
breakdown of readily decomposable organic
compounds (Jann et al. 1959; Jeris and Regan,
1973b). The aeration rate is considered to be the
most important factor influencing successful
composting (Diaz et al. 2002). Insufficient aeration
can lead to anaerobic conditions due to the lack of
oxygen, while excessive aeration can increase costs
and slow down the composting process via heat,
water and ammonia losses. The optimal aeration
rate depends on the composition of the raw
materials and ventilation methods (Guo et al. 2012).
It was concluded that frequent turning can achieve
this better than under less aeration (Jann et al.
1959). Moreover, it was mentioned that the overall
goal of the aeration is to maintain compost
temperature in the range of 50-55oC to obtain
efficient thermophilic decomposition of organic
wastes and pathogen destruction (Jeris and Regan,
1973a; Raut et al. 2008). Unfortunately, it was
noticed that the turning frequency during the
present study was not suitable for good aeration for
microbial populations and therefore the
degradation of organic matter was very slow and
the maturity of the final composts was not
complete.
Microbial changes
In the process of composting, the succession of
various microbial groups plays a crucial role, and
the appearance of nutritionally specialized
microbial groups reflects the maturity of composts
(Goyal et al. 2005). Monitoring of the microbial
succession may provide important information for
the effective management of the composting
process and the appearance of certain groups of
microorganisms is believed to reflect the degree of
stabilization of the organic matter during the
process (Ryckeboer et al. 2003). Therefore, changes
in the numbers of mesophilic and thermophilic
bacteria and fungi during the composting process
were determined. The changes in the numbers of
these microbes are illustrated in Fig. 4. The figures
show the logarithm of the number present.
The population of mesophilic bacteria was found
to be highest at the beginning of composting and
then declined with time (Fig. 4a). This trend was
found in all seasons at the three compost plants.
The time at which the slope of decline was sharper
differed at the different seasons and the different
plants. This was between day 10 and day 30,
afterwards the rate of decline was less pronounced.
Close results were found by several investigators
(Chang and Hudson, 1967; Khalil et al. 1999; Hassen
et al. 2001). Chang and Hudson (1967) noticed a
reduction in numbers of mesophilic bacteria
during the high temperature phase (55-70oC). They
also stated that the mesophilic bacteria behave
more like the thermophilic fungi than the
mesophilic ones and this may in part be due to the
fact that they have high maximum growth
temperatures. It was noticed that the count of
mesophilic bacteria in the winter season was
higher than in the other seasons at the three plants.
It was noticed also that the count of these bacteria
at El-Montaza plant was higher than at Abis-I and
Abis-II plants in all seasons. The decrease in the
total count of mesophilic bacteria could be
attributed to two main reasons: the maintained
high temperature degrees in all windrows and also
to the concomitant decrease in moisture content.
The higher count of mesophilic bacteria during the
winter season may be attributed to the lower
temperature range and to the higher moisture
content than the other seasons. This was more
evident at El-Montaza plant (Figs. 1 and 2a). On the
other hand, the thermophilic bacteria increased
gradually and reached the maximum after 10 days
of composting and then gradually decreased with
time (Fig. 4b). This trend was found in all seasons at
the three plants. It was noticed that the count of
thermophilic bacteria in the winter season was
higher than in the other seasons at the three plants.
It was noticed also that the count of these bacteria
at El-Montaza plant was higher than at Abis-I and
32 KHALIL ET AL.
Abis-II plants in all seasons. The observed higher
thermophilic bacterial counts during the winter
season may be attributed to the higher moisture
content than the other seasons and to the lower
temperature range. The higher thermophilic
bacterial count at El-Montaza plant may be
attributed to the same reasons. The population of
thermophilic bacteria reached its maximum after
10 days of composting matching the temperature
profile and then a decline was observed. Close
results were found by some investigators (Chang
and Hudson, 1967; Khalil et al. 2001; Goyal et al.
2005). Chang and Hudson (1967) stated that the
thermophilic bacteria increased during the first 2
days of composting and continued to increase
during the maximum temperature phase and then
gradually decreased by the end of composting. This
gives further support to the general thesis that
some thermophilic bacteria can tolerate higher
temperatures than thermophilic fungi.
As shown in Fig. 4c, the population of mesophilic
fungi was found at the beginning of composting
and then decreased sharply and completely
disappeared after a period differed from 10 to 20
days and this could be attributed to the rapid
increase in temperature that rapidly reached over
50ºC. These fungi appeared at the beginning as the
temperature was low but they disappeared when
the temperature increased. On the other hand, the
thermophilic fungi disappeared at the beginning of
composting and appeared after 10 days and then
disappeared after 20 days in the autumn season at
Abis-I plant, spring season at Abis-I, Abis-II and El-
Montaza plants, summer season at Abis-II and El-
Montaza plants and winter season at Abis-I and
Abis-II plants and after 30 days in the autumn and
winter seasons at El-Montaza plant (Fig. 4d). It was
noticed that there was no thermophilic fungi
during the autumn season at Abis-II plant and
during the summer season at Abis-I plant. These
fungi appeared after few days from the start of
composting concomitant to temperature elevation.
The disappearance of these fungi after 20-30 days
may be attributed to the high temperature (above
60oC). The absence of thermophilic fungi during the
autumn season at Abis-II plant and during the
summer season at Abis-I plant may be due to the
high temperature as it was 71.1 and 65.7oC after 10
days at Abis-II plant in the autumn season and
Abis-I plant in the summer season, respectively.
From the results obtained, it can be seen that the
most common microorganisms in the composting
process are bacteria (mesophilic and thermophilic).
It was mentioned that bacteria are usually
considered to be the main decomposers in the
composting process (Hassen et al. 2001). The
findings in the present study supported this idea,
as shown in Fig. 4a and b. The mesophilic and
thermophilic fungi had a short time span in the
composting process. Bacteria flourished because of
their ability to grow rapidly on soluble proteins
and other readily available substances and because
they are the most tolerant to high temperature as
mentioned by Miller (1992). Although a large
number of mesophilic bacteria could be isolated
from the composted materials even at thermophilic
range of 60oC, their respiratory activity at 60oC
was found to be negligible (Nakasaki et al. 1985a, b).
Moreover, it was stated that most fungi are
eliminated at temperatures above 50oC; only a few
have been isolated from compost that can grow at
all up to 62oC. Their survival was due to their
thermotolerance property. During composting,
temperatures above 55oC discourage fungal
growth. Fungi are excluded from waste composting
during the earlier high temperature stage (Miller,
1992). The disappearance or decrease of fungi could
be attributed to the effect of pH (> 7.0) as found in
this study, because the fungi favour an acidic pH
range as mentioned by Fogarty and Tuovinen
(1991). Generally, mesophilic microorganisms are
responsible for the initial decomposition of organic
materials and the generation of heat responsible for
the increase in compost temperature (Nakasaki et
al.1985c; Fogarty and Tuovinen, 1991). The
microbial biomass of some groups of microorga-
nisms, especially thermophilic bacteria, decreases
in the last phases of composting as the product
reaches maturity, so that a total count of
microorganisms (principally bacteria) throughout
the process can be indicative to the state of
compost maturity (Jiménez and Garcia, 1989).
Generally, it is noticed from the results obtained
that the compost maturity period (1month) was
not enough, because the number of thermophilic
bacteria was still high although their number
decreased during this period.
Enzymatic changes
Composting is mainly a degradation process
wherein complex organic molecules are broken
down to simpler components (Zameer et al. 2010).
Microbes reproduced in the composting pile
metabolize insoluble particles of organic matter by
33
Evaluation of the Composting Process Through the Changes in Physical, Chemical
secreting different hydrolytic enzymes. These
various hydrolytic enzymes are thought to control
the degradation rates of different substrates, and
they are the main mediators of various degradation
processes (Tiquia, 2002). Measurement of enzyme
activity is helpful in understanding microbial
metabolism during composting (Mondini et al.
2004). Thus, the monitoring of various enzyme
activities throughout the process provides useful
information on the dynamics of important
nutritional elements such as C and N and is
beneficial for understanding the transformations
occurring during composting (Vargas-Garcia et al.
2010). Therefore, different enzymatic activities were
measured to find out which enzymes participate in
the bioconversion of the given organic waste
material. Due to the complex composition of the
composting mixture, the enzymatic activities to be
measured were chosen according to the presence of
their possible inducers such as starch, cellulose
and hemicellulose. So, the changes in the activities
of three important enzymes; a-amylase, CMCase
and xylanase which are responsible for hydrolysis
of starch, cellulose and hemicellulose, respectively,
were studied to understand the degradation of
organic wastes during the composting process.
Amylases are commonly concerned with
breakdown of starch into glucose (Zameer et al.
2010). The maximum activity of a-amylase was
found at the beginning of composting which
ranged from 1.50 to 3.60 μmol/mL/min and then the
activity sharply decreased as it ranged from 0.29 to
0.42 μmol/mL/min by the end of composting (Fig.
5a). The activity was higher in the winter season
than in the other seasons at the three plants and
this could be attributed to the suitability of
moisture content and temperature for microbial
and enzymatic activities. The maximum activity at
the beginning of composting could be attributed to
the starchy materials which are degraded faster
than other materials (cellulose and hemicellulose).
This finding agrees with the fact that has been
stated by Chang and Hudson (1967) and Poincelot
(1974) that some substrates in natural materials
such as sugars, starch, protein and lipids are more
easily degraded than substrates such as cellulose,
lignin and other long chain polysaccharides. The
results are in conformity with the findings of other
researchers. It was stated that the maximum
activity of a-amylase occurred after 7 days (Khalil
et al. 1999), 9 days (Raut et al. 2008) and declined
after that. It was mentioned that early degradation
of starch could have been attributed as a result of
increasing microbial biomass during this initial
phase (Raut et al. 2008). Zameer et al. (2010) stated
also that the activity of amylase was highest
during the initial stage and lowest during the final
stage of composting. The lower activity during the
final stage indicates the mere completion of
decomposition process.
Cellulase activity is dependent on the types of
cellulolytic microorganisms that develop on the
organic waste (Goyal et al. 2005). The cellulase
activity is closely related to cellulose degradation
and carbon metabolism during the composting
process (Castaldi et al. 2008; Guo et al. 2012). As
shown in Fig. 5b, the activity of CMCase increased
gradually and reached the maximum values after
10 days at Abis-I and El-Montaza plants during the
spring season and after 20 days during the same
season at Abis-II plant, whereas during the other
seasons in all plants, the maximum activity was
found after 30 days and then decreased by the end
of composting. Close results were found by other
researchers. Khalil et al. (1999, 2001) mentioned that
the maximum activity of CMCase was found after
3 weeks, whereas Goyal et al. (2005) stated that the
maximum activity of CMCase was found after 30
days and declined later on. Generally, initial low
CMCase activity could be attributed to the
availability of simple compounds to the living
microorganisms and the subsequent higher
activity at the lower C/N ratio may be attributed
to the greater nitrogen availability as mentioned
by Ashbolt and Line (1982). It was noticed that the
activity of CMCase was higher in the winter
season than in the other seasons at the three plants
and this could be attributed also to the suitability
of moisture content and temperature for microbial
and enzymatic activities. The highest activity of
CMCase was found at El-Montaza plant in all
seasons and this may be attributed also to the
same reasons.
Xylanase is responsible for the hydrolysis of
hemicelluloses (Goyal et al. 2005), one of the major
components during the composting process (Liu et
al. 2011). The activity of xylanase increased
gradually and reached the maximum values after
30 days and then decreased by the end of
composting (Fig. 5c). The obtained results were in
contrary with those found by Goyal et al. (2005)
who mentioned that the maximum activity of
xylanase was found after 60 days and declined
after that. The higher activity of xylanase was
34 KHALIL ET AL.
Fig. 1 Changes in temperature (oC) during the
composting of municipal solid wastes at three compost
plants during the autumn (a), winter (b), spring (c) and
summer (d) seasons. Values are means of five
replicates ± standard deviations.
Fig. 2a Changes in moisture content (%) during the
composting of municipal solid wastes at three compost
plants during the autumn (a), winter (b), spring (c) and
summer (d) seasons. Values are means of three
replicates ± standard deviations.
found during the winter season at the three plants
and this may be attributed also to the suitability of
moisture content and temperature for microbial
and enzymatic activities. The highest activity of
xylanase was found at El-Montaza plant in all
seasons and this may be attributed also to the
same reasons. Initial low xylanase activities could
be attributed to the availability of simple
compounds to the living microorganisms.
It was found that the activity of such enzymes
towards the end of the composting period
decreased. The enzymes enumerated can be taken
into consideration in estimation of the compost
maturity. Generally, the hydrolytic activities
monitored in this study were able to reveal the
dynamics of the organic matter degradation during
the composting process.
CONCLUSION
From this study, it can be concluded that the
normal operating conditions at the three compost
plants were not suitable for the composting process
and therefore the final composts did not reach the
full maturity. Changes in microbial and enzymatic
activities could be used as suitable indicators to
35
Evaluation of the Composting Process Through the Changes in Physical, Chemical
Fig. 2b Changes in pH during the composting of
municipal solid wastes at three compost plants during
the autumn (a), winter (b), spring (c) and summer (d)
seasons. Values are means of three replicates ±
standard deviations.
Fig. 3a Changes in C/N ratio during the composting of
municipal solid wastes at three compost plants during
the autumn (a), winter (b), spring (c) and summer (d)
seasons. Values are means of three replicates ±
standard deviations.
characterize the composting process and compost
maturity when combined with some physical and
chemical parameters such as temperature and C/N
ratio. Adjustment of composting conditions such as
pile size, aeration, moisture content and
temperature is very important. This would allow
for microbial populations and their enzymatic
activities to increase and therefore the increase of
organic matter decomposition and then the
composting time can be reduced. Therefore,
composting could be an appropriate technology to
produce a useful product (compost) if the optimum
conditions are performed.
ACKNOWLEDGEMENTS
The authors acknowledge the technical support
provided by the working staff at the three compost
plants, Alexandria, Egypt.
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