Vol.3, No.2, 124-135 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.32018
Comparison of methods for evaluating stability and
maturity of co-composting of municipal solid wastes
and sewage sludge in semi-arid pedo-climatic condition
Olfa Fourti*, Naceur Jedidi, Abdennaceur Hassen
Centre des Recherches et des Technologies des Eaux, Laboratoire Traitement et Recyclage, Cité Mahrajène, Tunisie; *Corresponding
Author:
Received 28 October 2010; revised 29 November 2010; accepted 3 December 2010.
ABSTRACT
The step of maturation (around 60 days) ap-
peared less active as compared to the first step
of pre-fermentation (around 90 days) since the
values of temperature, recorded during the
second step of maturation, are found generally
less important than those recorded during the
first step. A similar tendency of C/N ratio values
decrease is generally observed in the two piles
of wastes during the two steps of composting
and these C/N ratio values determined in the
pile of wastes free of sewage sludge (W1) are
generally slightly higher than those observed in
the pile added with dry sewage sludge (W2). The
amount of total heavy metals (order of content:
Zn > Pb > Ni > Cu > Cr > Cd) appeared very het-
erogeneous and showed a large variation in the
two piles of wastes. The use of sewage sludge
in the pile W2 showed generally no apparent
impact on the whole amount of total heavy met-
als recorded in the finished product and the
values recorded are usually lower than the
metal concentration limits imposed by several
countries. Microbial inventory and total DNA
extracted from composting materials followed
during all the two steps of composting showed
a net variation over time, and revealed specifi-
cally a good parallel progress according to the
ambient temperature recorded inside the waste
materials. It appeared also from this study that
the microbial diversity is much nuanced in the
case of windrow W2 added with sewage sludge
as compared to that observed in the case of
windrow W1 free of sewage sludge.
Keywords: Temperature; Compost; Heavy Metals;
Microbial Diversity; Microbial DNA Concentration
1. INTRODUCTION
Composting is defined as a process of aerobic ther-
mophilus microbial degradation or an exothermic bio-
logical oxidation of various organic wastes (sewage
sludge, animal manures, yard waste, crop residues, mu-
nicipal solid waste, fish scraps and mortality, and food
waste and food process wastes) by many populations of
the indigenous microorganisms which lead to a stabi-
lized, mature, deodorized, hygienic product, rich in hu-
mic substances, easy to store and marketable as organic
amendment or manure [1].
Composting aimed to reduce the volume of wastes
going to the landfills and to facilitate recycling of nutri-
ents in the organic wastes as a useful method of produc-
ing a stabilized product that can be used as a source of
nutrients and soil conditioner in the field. The composted
product has the advantage of improving soil structure,
increasing soil organic matter, suppressing soil-borne
plant pathogens and enhancing plant growth. Immature
composts may contain more growth inhibiting sub-
stances than mature composts. Some of these growth
inhibiting compounds include salts, free ammonia, phe-
nolic substances, heavy metals, and organic acids.
For optimum aerobic composting, moisture is neces-
sary to support the metabolic processes of microorgan-
isms. Composting materials should be maintained within
a range of 40 to 65% moisture. A higher percentage will
increase anaerobic decomposition, while lower moisture
content will slow down the composting process as mi-
croorganisms die or become dormant. Microbial diver-
sity and activity during the composting process, mediat-
ing stabilization of organic wastes, is an important factor
and key to consider in composting. A large variety of
mesophilic, thermotolerant and thermophilic aerobic
microorganisms (including bacteria, actinobacteria, yeast,
moulds and various other fungi) have been extensively
reported in composting and other self-heating organic
materials at temperatures between 20 and 60°C [2,3].

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125
Many factors determine the microbial communities dur-
ing composting. Under aerobic conditions, temperature
is a major factor determining the type of microorganisms,
species diversity, and the rate of metabolic activities [3].
On the other side, waste stream from biosolids or mu-
nicipal solid waste can contain elevated concentration of
various metals as potential toxic elements (PTEs), and
the impact of metals on compost quality and use is a
major concern throughout the world [4]. In small
amounts, many of these elements may be essential for
plant growth; however, in higher concentrations they are
likely to have a detrimental effect upon plant growth.
A molecular detection procedure, using ribosomal in-
tergenic spacer analysis, was tested for microbial com-
munity diversity assessment. Analysis of compost mi-
crobial communities is one of the challenging areas of
research due to the enormous complexity of biodiversity
caused by the heterogeneity of the physical and chemical
structure of compost environments. Recently, a number
of molecular biological methodologies have been suc-
cessfully developed and introduced to reveal the biodi-
versity of microbial communities in a wide range of en-
vironments including soils [5,6] and solid wastes [7-9].
This diversity concerned pathogens and saprophytic mi-
croorganisms useful for a good procedure and organiza-
tion of the composting process.
The aim of this study was to assess, in a semi-indus-
trial pilot plant and under semi-arid pedo-climatic condi-
tions, a new process of co-composting of municipal solid
wastes and sewage sludge. So, some physico-chemical,
biochemical and microbiological parameters are fol-
lowed during the two steps of pre-fermentation (uncon-
trolled fermentation) and of maturation (controlled fer-
mentation), respectively. Also, total heavy metals con-
tent as potential toxic elements (PTEs) and microbial
diversity are evaluated.
2. MATERIALS AND METHODS
Preparation and monitoring of windrows. The study
was performed in a semi-industrial pilot plant and using
two types of windrow W1 and W2. The process of com-
posting proceeds in two successive separately steps:
pre-fermentation (uncontrolled fermentation) and matu-
ration (controlled fermentation), and with two different
raw composting materials: municipal solid waste from
the Erriadh city of Beja were pre-selected at source
(household pre-selection with average physical-chemical
characteristics, humidity = 60%, organic matter = 30%
dry weight, and C/N = 32) and dry sewage sludge (stabi-
lized from anaerobic digestion of the urban wastewater
treatment plant of Beja was integrated in the process at
the dry state (with average physical-chemical character-
istics, humidity = 30%, organic matter = 65% dry weight,
and C/N = 12.5) and primarily for the cover of windrow
W2. During the first step, the two windrows W1 and W2
of pyramidal form (of approximately 7 × 3 × 1.5 m3,
length × width × height, respectively) are constituted
exclusively with raw waste materials (100% of munici-
pal solid wastes) for W1 and (60% of municipal solid
wastes and a superficial layer of dry sewage sludge of
about 40% of dried sewage sludge) for W2, respectively,
and deposed in the platform of composting, under natu-
ral conditions, for 3 months of pre-fermentation. Sec-
ondly, at the end of this period of pre-fermentation, the
waste material of each windrow is submitted to a deep
manual sorting and fermentable organic material is
chipped, shred, sieved to 40-mm stitch sieve to reduce
heterogeneity and subjected for approximately 2 months
to a second controlled fermentation in pile of about 1.5
m of height. The pile of wastes during the first step of
process is left under natural condition without any main-
tenance, and on the contrary the maintaining of waste
pile during the second step of composting consists to
monitor temperature inside the pile in order to maintain
a temperature around 25-55°C, by turning as needed (1-3
times per month), and to preserve pile moisture at the
consistency of a well-wrung sponge by watering. The
finished product is sieved with a 10-mm stitch sieve.
Temperature and humidity were controlled daily. When
the mean temperature recorded in the different depths
(depths 20, 40 and 60 cm) of the pile averaged 55°C
(using a thermo-couple iron-constantan type J), the wind-
row was turned and watered. These operations of turning
and watering with tap water were performed almost
twice monthly in considering ambient temperature.
Temperature inside the pile is regularly determined (at
least 1 time per week) at many different places and at
two different depths of 20 and 100 cm, using a thermo-
couple thermometer; iron-constantan type J and humid-
ity (approximately 50 g of the waste sample was dried at
105°C in an oven for 24 h) is regularly controlled.
Analytical methods. Samples of wastes are in general
taken every 2 weeks and sampled as described by [10].
These samples are subjected to different determinations
after screening through 12 mm (0.5 inch) mesh.
Electrical conductivity (EC) and pH are measured in
10:50 (dry compost: deionized water) extracts using a
standard pH-meter (LPH 230 T, Tacussel electronic,
France) and a standard conductivity-meter (Orion re-
search, model 150, USA).
Total nitrogen (N) and total organic carbon (C) are
determined by the Kjeldahl method [11] and by the wet
dichromate oxidation method [12], respectively.
Organic matter is determined by ashing at 550°C for 2
h in a muffle furnace.
Total heavy metal contents are measured by an atomic

O. Fourti et al. / Natural Science 3 (2011) 124-135
126
absorption spectrophotometer (Perkin-Elmer, Model 560)
after digestion of the samples in concentrated HNO3-
HClO4 (2:1) according to [13].
Biological parameters. Total indigenous viableculti-
vable mesophilic and thermophilic bacteria from differ-
ent suspension-dilution of waste samples are enumerated
by the pour plate technique on Tryptic soy agar (Pasteur
Production, Paris). Enumerations are performed after 3
days of incubation at 28 and 55°C for mesophilic and
thermophilic bacteria, respectively. The count of spore-
forming bacteria includes several steps from selection by
applying heat to destroy vegetative bacteria (80°C for 10
min) to the indication by spreading volumes of the sam-
ple into Tryptic soy agar followed by incubation at 37°C
for 3 days. Spore-counting bacteria were expressed as
colony-forming units per gram (CFU s/g) of dry weight
of compost.
The deshydrogenase activity expressed as triphenyl-
formazan unit is measured according to Tabatabai [14].
Solid waste microbial biomasses C (BC) and N (BN)
are evaluated using the Fumigation-Extraction method
and total microbial DNA extraction method [15].
DNA was extracted and purified from equivalent dry
weights of each waste materials sample (500 mg fresh
materials), using the Bio 101 Fast DNA Kit for Soil
(Biogene, France), according to the manufacturer in-
structions. Purified DNA was quantified by spectropho-
tometry (Bio-RAD Smart SpecTM Plus, France) [16]. The
spectrophotometric A260/A280 and A260/A230 ratios
were determined to evaluate levels of protein and humic
acid impurities, respectively, in the extracted DNA [17].
These last methods recommended at first for soil micro-
bial biomass analysis is used in this study for microbial
biomass determination in the compost [18].
The microbial diversity in compost is evaluated by
DNA extraction and polymerase chain reaction (PCR) of
rRNA intergenic spacer. Total DNA extraction and puri-
fication are performed on waste extracts according to [5].
Each DNA sample was used as template to amplify the
intergenic spacer between the genes encoding for the
large (23S) and small (16S) subunits of ribosomic RNA.
Universal eubacterial primers are used. Amplification
products were resolved in a 5% non-denaturing acryla-
mide gel, stained with SYBR green I and photographed
under UV illumination.
Pearson’s correlation coefficients between selected
parameters were calculated.
3. RESULTS AND DISCUSSION
3.1. Temperature Profiles
Temperature profiles established during the two steps
of composting (pre-fermentation and maturation) in the
two windrows W1 and W2 revealed the three classical
temperature steps of composting the mesophilic, ther-
mophilic and cooling phases, respectively (Figure 1).
The step of maturation (around 60 days) appeared less
active as compared to the first step of pre-fermentation
(around 90 days) since the values of temperature, re-
corded during the second step of maturation, are found
generally less important than those recorded during the
first step. Differences in temperature averaged 15°C.
3.2. Evolution of the Physico-Chemical
Parameters of Composting
3.2.1. Acidity
Ideally, the pH should be in the range of 6-8 to allow
the highest rates of decomposition. If the pH is outside
this range, microbial activity will be compromised and
decomposition will be slowed or even stopped. The pH
values recorded in the two piles W1 and W2 showed in
general a weak variation from the beginning until the
end of pre-fermentation, and present a slight acidic com-
post increase during the step of maturation (Table 1).
3.2.2. Conductivity
The change in value of electrical conductivity (EC),
used as chemical indicators and as an indirect measure of
the soluble salts in a growing medium, for the two wind-
rows may be due to the characteristics and the amounts
of the materials used for constructing the pile of wastes.
The electrical conductivity showed no major variation for
the two windrows during the two steps of composting,
and limit values averaged 60 ms/m (Table 1). The addi-
tion of dry sewage sludge to windrow W2 seems have no
apparent impact on the conductivity value of the compost.
3.2.3. Organic Matter
The degradation rate of organic matter during com-
posting is importantly used to evaluate the compost ma-
turity. The organic matter decreased notably over time
during the first step of pre-fermentation in the two
windrows W1 and W2, 20.9 to13.5 and 20.44 to 13.60%,
respectively (Table 1), pointing to active degradation of
the organic materials during this step. On the other side,
organic matter degradation appeared no so active during
the second step of maturation in the two windrows,
12.24 to 10.8 and 12.4 to 11.45%, respectively.
3.2.4. C/N Ratio
At the beginning of the first step of pre-fermentation,
the C/N ratio values averaged 30 and these values de-
creased remarkably at the end of this step to 23.22 and
20.2 for windrows W1 and W2, respectively. These re-
sults, both valid for the two piles of wastes, indicate a

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127
Figure 1. Progress of average temperature values in the two windrows during the two steps of composting.
Table 1. Limit values of the main physical-chemical parameters determined during the two steps of composting.
Step 1 of pre-fermentation Step 2 of maturation
Parameters
Initial waste Final waste Precompost Final compost
pH (water) 7.07 ± 0.30 6.66 ± 0.08 7.02 ± 0.2 8.58 ± 0.01
EC (ms/m) 62.08 ± 26.80 70.40 ± 24.70 75.24 ± 16.54 68.57 ± 20.55
MO (%) 20.90 ± 1.60 13.50 ± 1.02 12.24 ± 0.82 10.80 ± 0.80
W1
C/N 32 ± 3.2 23.22 ± 2.1 22.2 ± 1.6 15.2 ± 1.4
pH (water) 6.56 ± 0.12 7.10 ± 0.12 7.20 ± 0.08 8.34 ± 0.01
EC (ms/m) 58.01 ± 14.44 51.14 ± 31.26 73.39 ± 15.85 68.84 ± 20.30
MO (%) 20.44 ± 1.85 13.60 ± 1.66 12.40 ± 0.14 11.45 ± 0.78
W2
C/N 28.72 ± 2.9 20.2 ± 2.2 21.4 ± 1.2 13.6 ± 1.2
EC: Electrical conductivity. OM: Organic matter content. C/N ratio. n = 3. ± : Deviation standard.

O. Fourti et al. / Natural Science 3 (2011) 124-135
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high rate of biological activities. The similar tendency of
result is observed during the second step of maturation
since there is a net decrease of C/N ratio between the
beginning and the end of this step of maturation, 22.2 to
15.2 and 21.4 to 13.6 in the two piles of wastes, respec-
tively.
The C/N ratio values determined in the pile of wastes
free of sewage sludge (W1) are generally slightly higher
than those observed in the pile added with dry sewage
sludge (W2). This result is certainly due to the special
composition of sewage sludge used (around 30% of the
total composting wastes) in the windrow W2. This sub-
strate is therefore loaded in degradable carbon and
compost must lead to a very steady and slowly biode-
gradable compound (low C/N).
3.2.5. Heavy Metals
Table 2 illustrates the total concentration of metals
(Zn, Pb, Cu, Ni, Cr, Cd) during the two steps of com-
posting. The amount of total metal appeared very het-
erogeneous and showed a large variation. This tendency
is valid for the two piles of wastes, and could be ex-
plained by the high bio-physical-chemical complexity of
this environment; since there will be a loss by leaching
and a concentration of metals by adsorption. The order
of total metal content in the composted material and fi-
nal compost was Zn > Pb > Ni > Cu > Cr > Cd. The use
of sewage sludge in the waste of pile W2 showed gener-
ally no apparent impact on the whole amount of total
heavy metals recorded in the finished product (Table 2).
3.2.6. Microbial Biomass
BC/BN ratio values showed usually a net increase dur-
ing the thermophilic phase, 30-70 and 20-50 days, for
the steps of pre-fermentation and maturation, respec-
tively (Figure 2). The BC/BN ratio values are on average
around 4-6 and 3-4 during the first and second steps of
composting, respectively.
3.2.7. Deshydrogenase Activity
The deshydrogenase activity generally showed a net
increase between 20-70 and 15-35 days through the pre-
fermentation and maturation steps, respectively. A good
positive correlation is observed between the values of
deshydrogenase and of temperature recorded (r = 0.96, P
= 0.01). These values of deshydrogenase activity fluctu-
ated between 3.2-5.84 and 1.7-5.66 triphenylformazan
(TPF)/g of waste dry weight during the step of prefer-
mentation in the two windrows W1 and W2, respectively
(Figure 2). By the same, these values varied between
3.6-4.9 and 1.96-3.22 TPF/g of waste dry weight during
the step of maturation in the two windrows W1 and W2,
respectively. So variation of the deshydrogenase activity
appeared very narrow (around 1.2 TPF/g of waste dry
weight on average for W1 and W2) during the second
step of composting and indicated a certain degree of
homogeneity of the waste materials used in this second
step of maturation; and in the opposite, a high heteroge-
neity of the waste used during the first step of pre-fer-
mentation since variation of deshydrogenase values re-
corded were relatively large (around 2.64 and 4 TPF/g of
waste dry weight for W1 and W2, respectively).
On the other hand, this activity appeared slightly
higher in the windrow free of sludge than in the one with
sewage sludge.
3.3. Biological Parameters of Composting
3.3.1. Total Viable-Cultivable Mesophilic,
Thermophilic and Spore-Forming
Bacteria
The changes in main microbial population, total viable-
cultivable mesophilic, thermophilic and spore-forming
bacteria, in the two piles of composting materials during
the two steps of process are presented in Figure 3. The
number of mesophilic microorganisms during the first
step of pre-fermentation varied, and during the first
weeks of this step the number of these microorganisms
remained stable around 8.5 Ulog10 CFUs/g of compost
dry weight in the two windrows W1 and W2. The lowest
level of mesophilic bacteria in compost materials was
recorded between 65 and 75 days of pre-fermentation
(around 5.5 Ulog10 CFUs/g of compost dry weight). From
that moment, the number of mesophilic bacteria began to
increase and remained at the level of 7 Ulog10 CFUs/g of
compost dry weight for the two windrows. In the case of
step of maturation, the number of mesophilic bacteria
did not varied remarkably and it fluctuated on average
around 6 Ulog10 CFU s/g of compost dry weight.
On the other side, the number of thermophilic bacteria
started to increase, from 4 Ulog10 CFUs/g of compost
dry weight, since the first weeks of pre-fermentation step,
and it remained constant around 6 Ulog10 CFUs/g of
compost dry weight from the days 35 to 85. At the end
of this step of pre-fermentation, the number of thermo-
philic bacteria reached 4 Ulog10 CFUs/g of compost dry
weight for the two windrows. For the second step of
maturation, the number of thermophilic bacteria showed
low variation, particularly in the case of windrow W2,
and a little increase is observed between 25-48 days of
maturation (around 6 Ulog10 CFUs/g of compost dry
weight). For the number of spore-forming bacteria, the
variation appeared very weak (around 4.5 Ulog10
CFUs/g of compost dry weight) during the two steps of
composting for the two windrows W1 and W2, and a
little increase is always remarked specially during the

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129
Table 2. Heavy metal contents obtained during the two steps of composting and the metal concentration limits for composts in sev-
eral countries.
Step 1 of pre-fermentation Step 2 of maturation Canadian nor-
malization
French nor-
malization
German nor-
malization
Spanish nor-
malization
US nor-
malization
Initial waste
content
Final waste
content
Precompost
content
Final compost
content Limit values
Class AClass B
(mg kg-1 dry wt)
Cu 24,54 ± 5,18 42,83 ± 7,37 28,06 ± 9,7343,25 ± 2,66100 757 100 100 450 100
Cd 4,97 ± 2,28 1,75 ± 0,50 2,78 ± 1,8 3,6 ± 1,5 3 20 8 1,5 10 2
Cr 53,26 ± 14,7 24,15 ± 5,76 17,41 ± 3,0526,68 ± 1,44210 1060100 100 400 100
Pb 90,47 ± 27,80 66,19 ± 25,54 80,01 ± 56,94102,98 ± 15,40150 500 800 150 300 150
Zn 195,60 ± 20,30 237,82 ± 19,68 160,30 ± 30,65188,40 ± 11,40500 1850500 400 1100 400
W1
Ni 64,34 ± 20,30 75,80 ± 4,70 37,47 ± 11,5060,10 ± 10,1062 180 200 50 120 50
Cu 42,32 ± 9,60 76,31 ± 24,52 71 ± 12,54 97,39 ± 17,5100 757 100 100 450 100
Cd 4,80 ± 2,78 3,98 ± 2,08 3,04 ± 2,08 1,07 ± 0,14 3 20 8 1,5 10 2
Cr 30,15 ± 7,98 31,92 ± 5,82 26,73 ± 11,6923,2 ± 2,07 210 1060100 100 400 100
Pb 112,53 ± 28,40 82,20 ± 17,42 64,28 ± 34,3092,16 ± 27,75150 500 800 150 300 150
Zn 141,90 ± 23,69 191,29 ± 32,12 203,08 ± 17,70229,15 ± 36,50500 1850500 100 1100 400
W2
Ni 44,91 ± 10,11 62,85 ± 16,56 60,80 ± 20,5574,0 ± 5,10 62 180 200 50 120 50
Values for Class A of composts ‘‘which have no restrictions in use’’ and Class B of compost ‘‘which can be used on forest lands and road sides and for other
landscaping purposes’’, according to Canadian normalization [19]; n = 3, ± : Deviation standard.
days 34-35 and 25-48 of pre-fermentation and matura-
tion steps, respectively (around 5 Ulog10
CFUs/g of
compost dry weight).
3.3.2. Microbial DNA Concentration
Microbial total DNA extracted from composting ma-
terials and followed during all the two steps of compost-
ing process showed a net variation over time, and re-
vealed a good parallel increase with the temperature pro-
gress inside the waste materials (Figure 4). This increase
varied between 13.2 to 26.1 µg of total DNA per g dry
weight. The lowest values of DNA are observed at the
start and the end of each steps of process (around 13.2
µg of total DNA per g dry weight) and the highest values
are always recorded during each thermophilic phase
(around 26 µg of total DNA per g dry weight). The de-
termination of A260/A230 and A260/A280 ratios for
compost DNA showed a significantly lower values (0.96
and 1.2) than those for DNA solutions of pure cultures
(1.57 and 1.89).
3.3.3. Microbial Diversity
Changes in microbial diversity are assessed during the
two steps of process by DNA extraction and polymerase
chain reaction (PCR) of rRNA intergenic spacer. The
RISA analysis of microbial communities change and
diversity in compost showed an equivalent diversity at
the start of the first step of composting process in the
two windrows W1 and W2 (Figure 5, Lanes 1 and 2).
During the thermophilic phases of the first step of proc-
ess, there is a net variation in microbial diversity, this
diversity appeared very rich and obvious in case of
windrow W2 added with sewage sludge (Lanes 3 and 4).
At the end of this first step of composting (Lanes 5 and
6), the RISA profile appeared equivalent. The start of the
second step of composting showed a varied profile
(Lanes 7 and 8) and diversity is richer also in case of
windrow W2. The end of step of maturation exhibit a
high microbial diversity materialized in the RISA profile
by a various big number of bands (Lanes 9 and 10).
Generally, this microbial diversity is much nuanced in
the case of windrow W2 added with dry sewage sludge
as compared to that observed in the case of windrow W1
free of sewage sludge. Also, it is important to mention
that there were a temporal dynamics of the microbial
communities with modifications of the RISA profile

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130
Figure 2. Progress of the deshydrogénase activity and BC/BN ratio values in the two windrows during the two steps of composting.
over time of composting for the same type of composts.
unity structures as follow: Lane 1, Composting raw
materials of W2; Lane 2, Composting raw materials of
W1. Lane 3, Composting materials of W2 during ther-
mophilic step; Lane 4, Composting materials of W1
during thermophilic step; Lane 5, Composting materials
of W2 at the end of step of pre-fermentation; Lane 6,
Composting materials of W1 at the end of step of
pre-fermentation; Lane 7, Composting materials of W2
at the start of step of maturation; Lane 8, Composting
materials of W1 at the start of step of maturation; Lane 9,
Composting materials of W2 at the end of step of matu-
ration; Lane 10, Composting materials of W1 at the end
of step of maturation.
3.4. Relationship between Some Selected
Parameters of Composting
A correlation matrix (Table 3) showed some signifi-
cant relationships between the temperature, biomasses C
and N, deshydrogenase activity and DNA concentration.
There was a good positive linear relationship between
ambient temperature recorded inside the waste heap and
deshydrogenase activity and BC/BN ratio (r = 0.93, r =
0.96, r = 0.87, r = 0.71, P = 0.01, respectively). So, in-
crease of temperature during each thermophilic phase is
equivalent to a good deshydrogenase activity and a lot of
active microbial biomass. Also, BC/BN ratio is well
correlated to the extracted DNA concentration only in
the case of windrow W2 (r = 0.63, P = 0.01). The values
of DNA concentration recorded in the case of windrow
W1 is well correlated to those observed in case of wind-
row W2 (r = 0.61, P = 0.02). This result is valid for the
two steps of composting, pre-fermentation and matura-
tion. Moreover, stability of the temperature values inside
the mass of waste means the stability of the microbial
activity during the process (low deshydrogenase activity
and BC/BN ratio), and this fact could be exploited as an
indicator of maturity of the compost.
4. DISCUSSION AND CONCLUSIONS
The present study investigates, in a semi-industrial
pilot plant for compost production and under semi-arid

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131
Figure 3. Progress of the number of mesophilic, thermophilic and spore-forming bacteria in the two windrows during the two steps
of composting.
Tunisian pedo-climatic condition, a new and a sim-
pleprocess of co-composting of municipal solid wastes
and sewage sludge, and some physical-chemical and
biological parameters are followed during two succes-
sive steps of pre-fermentation and of maturation using
two types of pyramidal windrows W1 (100% of munici-
pal solid wastes) and W2 (60% of municipal solid
wastes and 40% of sewage sludge in weight). Results
revealed that temperature profiles established during the
two steps of composting in the two windrows W1 and
W2 showed the three classical temperature steps of
composting the mesophilic, thermophilic and cooling
phases, respectively.
The optimal duration of pre-fermentation was evalu-
ated to three months on average. Consequently, a maxi-
mum organic material recovery is ensured during a
minimum time interval.
The second step of process of controlled fermentation

O. Fourti et al. / Natural Science 3 (2011) 124-135
132
Figure 4. Progress of the total compost DNA concentrations and agarose gel electrophoresis of total DNA extracted from com-
posting materials (W1) during the two steps of composting.
(step of maturation) is characterized by a degradation of
the organic materials by microorganisms. The purpose of
management of this second step of composting is to
ensure favorable conditions in order to force the micro-
biological activity. The procedure of microbes during
this second step of composting appeared on average less
active than the one recorded during the first step of pre-
fermentation. All parameters considered in the present
study are in favour of this conclusion. We could explain
this result by several factors mainly by the decrease or
exhaustion and depletion of readly degradable compounds
in the mass of waste materials.
This study confirm amongst others that temperature is
the main parameter to consider in composting. This

O. Fourti et al. / Natural Science 3 (2011) 124-135
133
1 2 3 4 5 6 7 8 9 10
Figure 5. Example of the ARISA analysis of microbial
communities change and diversity in compost of wind-
row 1.
important factor condition the funtionning of all the
others parameters, specifically the microbial activity. As a
result, the level of temperature condition and depends on
the type of microorganisms operating during composting.
So mesophilic microbes dominate at the beginning and
at the end of each step of composting process; on the
other hand, thermophilic microbes control the important
step of composting, the thermophilic phase.
The values of temperature recorded during the two
steps of composting revealed a good relashionship of
this parameter with others studied here such as the
deshydrogenase activity, the BC/BN ratio and the total
extracted DNA. In fact, statiscal analysis showed a good
positive correlation of all these last parameters with tem-
perature values recorded inisde the pile of waste. Also,
the preliminaly molecular study using RISA method
confirmed the high complexity of the microbial com-
munauties acting during the two steps of composting in
the two windrows W1 and W2 and underlined by several
authors [20-23]. In addition, this preliminaly molecular
study demonstrated a temporal dynamics of the micro-
bial communities according to the modifications of the
RISA profile over time of composting for the same type
of composts. It appeared also from the result of this
study that the microbial diversity is much nuanced in the
case of windrow W2 added with sewage sludge as com-
pared to that observed in the case of windrow W1 free of
sewage sludge.
Furthermore, contaminants in feedstock can impact
the quality, marketability, and use of finished composts.
Overuse and persistence of some heavy metals, herbi-
cides and insecticides could result in heavy metals and
pesticide contamination of compost. The viability of
composting depends very much on the quality and con-
sistency of compost produced. Heavy metals content is
of primary concern, and generally the values recorded
during the two steps of composting are usually lower
than the metal concentration limits imposed by several
countries. So, it appeared from this study that the level
of heavy metals contamination of the finished product of
compost is satisfactory and suitable for soil application
and enrichment. These low values of heavy metal con-
tent are related to many factors, mainly the low level of
metal contaminants in feedstock used as raw composting
materials in this plant as compared to those usually men-
tioned in several developed countries. This low level of
heavy metal contaminants of the finished product is con-
firmed by the study of [18] in the same plant.
Compost is often contaminated by heavy metals due
to inadequate separation of biodegradable fractions from
non-degradable or inert materials. Composting can con-
centrate or dilute heavy metals present in waste material
by adsorption and leaching, respectively. Lowering the
amounts of heavy metal present depends on metal loss
through leaching. The increase of metal level is due to
weight loss in the course of composting following or-
ganic matter decomposition, release of carbon dioxide
and water and mineralization processes [24,25]. As a
consequence, potential utilisation of compost in agricul-
ture might be restricted, and therefore, it is essential to
monitor the magnitude of heavy metal contamination in
soils, which urgently requires detailed chemical analysis
of the soil.
Finally, this study reviews a new and simple com-
posting process tested and used in a pilot plant for com-
post production under semi-arid Tunisian pedo-climatic
condition, and some important chemical, physical and
biological monitoring parameters are investigated in
order to understand and to master the process of com-
posting of municipal solid waste.
5. ACKNOWLEDGMENTS
This study was supported by the Japan International Cooperation
Agency (JICA) within the framework of a research program. We thank
the staff of a semi-industrial composting plant, Mr K. Ben Khedija and
Ms Tounsi N. at ANPE, for their assistance, cooperation and par-
ticipation in this study. We are most grateful to Pr H.W. Ackermann of

O. Fourti et al. / Natural Science 3 (2011) 124-135
134
Table 3. Pearson’s correlation coefficients between the main parameters studied during the two steps of composting.
T°C. W1 T°C. W2 Deh. W1 Deh. W2 BC/BN W1BC/BN W2 DNA W1 DNA W2
Temp. W1 r 1.000 0.924** 0.930** 0.963** 0.873** 0.718** 0.100 0.509*
S 0.000 0.000 0.000 0.000 0.001 0.724 0.053
Co 96.053 112.217 11.136 8.969 11.247 9.394 0.996 4.416
Temp. W2 r 1.000 0.999** 0.938** 0.862** 0.593** 0.008 0.494
S . 0.000 0.000 0.000 0.007 0.977 0.061
Co 153.619 15.116 11.042 14.035 9.818 0,098 5.267
Dehy. W1 r 1.000 0.943** 0.875** 0.593** 0.003 0.480
S 0.000 0.000 0.007 0.992 0.070
Co 1.492 1.095 1.404 0.967 0,003 0.503
Dehy. W2 r 1.000 0.830** 0.639** 0.047 0.503
S 0.000 0.003 0.867 0.056
Co 0.903 1.036 0.811 0,046 0.427
BC/BN W1 r 1.000 0.638** 0.080 0.424
S 0.003 0.776 0.115
Co 1.727 1.120 0,098 0.450
BC/B W2 r 1.000 0.472 0.711**
S 0.076 0.003
Co 1.782 0.640 0.837
DNA W1 r 1.000 0.610*
S 0.016
Co 0.907 0.480
DNA W2 r 1.000
S
Co 0.683
** Correlation is significant at level 0.01 (bilateral). * Correlation is significant level at 0.05 (bilateral). r: Pearson correlation, Co : Covariance, S: Sig. (bilat-
eral), Microbial C biomass (BC), Microbial N biomass (BN), DNA: DNA concentration, Deh: Deshydrogenase activity, W1 and W2 : Windrows 1 and 2.
the Department of Microbiology, Faculty of Medicine, Laval University
in Quebec, Canada, for helpful discussion and critical review of the
manuscript.
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