Co-composting of primary sewage sludge with matured compost, together with grit and sand was carried out to solve the problem of grit and sand disposal, this technique improved the sludge composting efficiency and the final compost quality

1. USING MIXTURE OF GRIT AND MATURE COMPOST AS BULKING
AGENT: ITS EFFECT ON COMPOSTING EFFICIENCY AND
COMPOST QUALITY.
*Dr. Helalley A.H. Helalley 1, Chem. Hussein M. Elashqar 2, Dr.Samaa M.Z. Abdel Aziz 3
1
Chief of Industrial Drainage, Sludge and Reuse sector.
2
3
General Manager of Sludge and Reuse.
Manager of Industrial Wastewater Research Dept.,
Alexandria Sanitary Drainage Company, Alexandria, Egypt
Keywords
Sewage grit, sewage sludge, windrow composting, co-composting.
ABSTRACT
Primary wastewater treatment plants in Alexandria produce large amounts of Grit and
sand as one of the major solid wastes arising from the preliminary treatment stage, which
represents a problem in disposal of these large quantities which are around 811 m3/month, (27
m3/day). On the other hand, one of the main sludge composting management problems is to
provide the bulking agents with affordable costs.
Co-composting of primary sewage sludge with matured compost, together with grit and
sand was carried out to solve the problem of grit and sand disposal, this technique improved
the sludge composting efficiency and the final compost quality.
Dewatered primary sludge was mixed with matured compost and grit and sand as a
bulking agent at 9:2:1 (v/v), and the mixtures were composted for 60 days. Addition of grit
and sand raised slightly the inorganic portion of the sludge compost, and rose significantly the
temperature in the first composting stage, therefore significantly increased the decomposition
activity due to the facilitation of homogenicity of the mixture which resulted in high mixture
porosity which improved air exchange within the mixture. The numbers of microbial
pathogens were significantly decreased to the safe permitted limits as a result of the raise in
temperature, thus produced a final safe product.
This technique not only improved the composting operation in terms of temperature,
moisture content, mixture porosity, oxygen content (up to 5%). But also reduce the cost of
composting operation and solve the problem of grit and sand disposal.
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2. 1. INTRODUCTION:
Sewage grit and sludge are inevitable by-products of wastewater treatment processes.
Sewer grit is collected by traditional removal at wastewater treatment works. Its production is
highly variable (0.5 to 2.0 t/year per 1000 population) depending on the catchment area
(higher grit production in coastal or sandy areas), sewer type (more from combined sewers
through road run-off than separate sewers), and weather conditions (grit deposited in the
sewer during dry weather is washed through during storm events). Grit quality has wide
variations in moisture content (7.4% to 75%) and organic content (30 – 65% total organic
carbon). Sewage sludge is produced by sedimentation, both before and after wastewater biotreatment. The upgrading and expansion of wastewater treatment plants have greatly
increased the volume of sludge generated. (Yongjie and Yangsheng, 2005, Lewin, et.al., 2011)
Quantitative and qualitative composition of the sewage sludge is very complicated. It is
rich in organic matter, nitrogen, phosphorus, calcium, magnesium, sulphur and other
microelements necessary for plants and soil fauna to live. So it is characterized by the large
manurial and soil-forming value. Except the indispensable elements to live, sludge can
contain toxic compounds (heavy metals, pesticides) and pathogenic organisms (bacteria, eggs
of parasites) (Kosobucki, et.al., 2000). The handling of sewage sludge is one of the most
significant challenges in wastewater management. In many countries, sewage sludge is a
serious problem due to its high treatment costs and the risks to environment and human
health. Although, the volume of the produced sewage sludge represents only 1 % to 2% of the
treated wastewater volume, its management costs are usually ranging from 20% to 60% of the
total operating costs of the wastewater treatment plant (Marcos and Carlos, 2005).
Due to the currently low capacities of wastewater treatment prevailing in many developing
countries, a future increase in the number and capacities of wastewater treatment plants can
be expected. As a consequence, the amount of produced sewage sludge is also expected to
increase (Zaini and Mogens, 2006). These aspects show that, sewage sludge management is an
increasing matter of concern in many countries due to the fast increases in sludge production,
which increases the resulting environmental threats accordingly (Murphy, et.al., 2004). The
beneficial use of sewage sludge in most of the developing countries as a soil conditioner or
for land application can be considered as a good option. Especially, the land degradation
problems and insufficient food production as well as the financial problems are considered
(Ghazy, et.al., 2009).
Grit consists mainly of mineral matter such as sand, gravel, glass and plastics. Owing to
this high proportion of sand and gravel-sized inorganic material, sewer grit has been used in
several recycling applications (Lewin, et.al., 2011). Untreated grit has been used as top cover/
infill for pipe trenches within the cartilage of wastewater treatment works during construction
projects. Certain soils, particularly heavy clay soils, can benefit from the addition of grit
materials to aid in drainage, to help break-up those soils and to improve compaction
properties. It may be used in horticulture and agriculture. Also, grit is used as a base material
for soil manufacture. Grit has also been successfully trialled as an input material for the
cement industry as an alternative source of silica rich material (Lewin, et.al., 2011).
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3. Under aerobic conditions and in admixture with other waste, the organics which are
present in sewer grit are biologically decomposed. The production of heat further sanitises the
input material. In addition to sanitisation, the composted product is stabilized and becomes an
important source of nutrients and humic substances that can be applied to land (Lewin, et.al.,
2011). With higher grit disposal costs and a legislative requirement to “reduce, reuse and
recycle”, there is a strong incentive for sewer grit recycling opportunities. This prioritizes
waste prevention and recycling ahead of disposal (Lewin, et.al., 2011).
Sewage sludge composting is being increasingly considered by many municipalities
throughout the world because it has several advantages over other disposal strategies.
Additionally, the application of composts to agricultural soils has many advantages, which
include providing a whole array of nutrients to the soil, decreasing soil acidification,
preventing soil erosion, increasing beneficial soil organisms, reducing the need for fertilizers
and pesticides, improving soil physical and biological properties, and helping keep organic
wastes out of landfills. All these effects and probably also additional ones (e.g. suppression of
pathogenic microorganisms) are advantageous for plant health (Yongjie and Yangsheng, 2005).
Primary wastewater treatment plants in Alexandria produce large amounts of Grit and sand
as one of the major solid wastes arising from the preliminary treatment stage, which
represents a problem in disposal. The quantities of grit which received at site 9N are around
811 m3/month, (27 m3/day). On the other hand, one of the main sludge composting
management problems is to provide the bulking agents with affordable costs.
Co-composting of primary sewage sludge with matured compost, together with grit was
carried out to solve the problem of grit disposal; and testing this technique in improving the
sludge composting efficiency and the final compost quality.
2. MATERIALS AND METHODS
2.1 Sewage Sludge
The sewage sludge used in this study was raw dewatered municipal sewage sludge
received at the landfill composting facility (site 9N) from the Mechanical Dewatering Facility
(MDF) located at the West Treatment Plant in west of Alexandria (Elkabary). MDF receives
all sludge resulting from primary treatment of sewage water at East and West Treatment
Plants in Alexandria and small quantities from new secondary treatment plants. This raw
sludge was mechanically dewatered by belt filter press to total solids of about 30%.
2.2 Sewage Grit
The grit used in this study was received at site 9N from all treatment plants in Alexandria.
The average of moisture content range of grit was 18-25% and the average of organic matter
content range was 20-30%.
2.3 Composting Process
The composting process was done at the composting facility (site 9N) of Alexandria
Sanitary Drainage Company (ASDCO). It is located approximately 40 km west of
Alexandria. Windrow-composting method was used throughout this study. The site 9N
traditional composting process was done using a mixture of raw dewatered sewage sludge
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4. with mature compost (DS/MC) with a ratio of 3:1 (v/v) respectively. Five windrows all over
the year were established to examine the effect of using a mixture of mature compost with
grit on the composting process and the quality of compost produced. Co-composting of
primary sewage sludge with matured compost, together with grit (DS/MC/G) was carried out
to solve the problem of grit disposal; and to examine if this technique will effect on the sludge
composting efficiency and the final compost quality. Dewatered primary sludge was mixed
with matured compost and grit as a bulking agent at 9:2:1 (v/v) respectively. Both the
(DS/MC) and the (DS/MC/G) mixtures were composted for 60 days with turning
mechanically by using turning machine and left for another one month without turning for
curing. The dimensions of each windrow were 4.2 m in width, 250 m in length, and 1.5 m in
height in the triangular shape. The mixtures were mixed by turning once a day for three
consecutive days and after that they were turned every 7-10 days for two months
(fermentation period) and after that they were stored for another one month without turning as
a curing period. The composting process consisted of two periods, the fermentation period
(two months) and the maturation period (one month) without turning.
2.4 Sampling Procedure
Samples were taken weekly from each pile of compost. Each composite sample was taken
from ten places at random at a depth of 70- 80 cm and mixed together. Samples were
transferred aseptically to the laboratory in a cold box and analyzed chemically and
microbiologically.
2.5 Laboratory Analysis
2.5.1 Physical analysis
Temperature was monitored near the centre of the pile with a metal probe thermometer
(Poincelot and Day, 1973). It was checked every week at five points along the pile. The colour
was assessed visually, while the odour was sensed by smelling (Khalil, 1996). The colour and
odour were tested by three persons.
2.5.2 Chemical analysis
Samples were oven-dried (60-70 °C), ground in a porcelain mortar and then by a hammer
mill. The ground samples were stored in dry, airtight containers until use. pH and Electric
conductivity were determined by shaking 5.0 g compost in 50.0 ml distilled water (1:10, w/v)
for 30 min, then pH was measured by a pH meter and electric conductivity meter (Albonetti
and Massari, 1979). Moisture content and dry weight were determined after drying the sample
at 105 °C. Ash was determined after drying the sample at 105 °C and ashing at 550 °C, in a
muffle furnace for about 3 h (WHO, 1978, APHA, 1998). Percentages of organic matter were
estimated as follows:
Organic matter (%) = 100 - ash (%)(WHO, 1978, Okalebo, et.al., 1993)
Total nitrogen was determined by the Kjeldahl method (WHO, 1978). Phosphorus was
determined by spectrophotometer at 470 nm, while potassium and heavy metals was
determined by Atomic Absorption Spectroscopy (WHO, 1978).
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5. 2.5.3 Detection of pathogenic bacteria
Multiple tube fermentation tests were used for fecal coliforms according to the method of
APHA (2005). Salmonella was determined as described by APHA (2005) and (ISO 6579,
1993).
2.6 Statistical Analysis
Standard analysis of variance (ANOVA: two-factor with replication) was used. Analysis
of variance was computed using the Microsoft Excel software. Differences of means were
based on the least significant difference (LSD) at P= 0.05.
3. RESULTS AND DISCUSSION:
Physical and chemical changes were examined periodically in this study together with the
heavy metals content and the pathogenic microorganisms.
3.1 Physical changes:
3.1.1 Temperature:
Temperature changes that occurred in the different windrows are shown in Figure (1). The
temperature of raw dewatered sewage sludge before mixing was about 29°C and after mixing
the temperature began to rise to 44°C in average at the third day of composting. During the
composting process, the temperatures of the windrows increased rapidly after mixing where it
reached about 52.3 and 53.4°C in DS/MC and DS/MC/G respectively after 7 days. The
maximum temperatures during the fermentation period of the DS/MC reached 68.4 °C after 8
weeks, while for the DS/MC/G it reached 70.4 °C after 5 weeks. Afterwards the temperatures
decreased gradually during the curing period as it reached an average temperature of 51.6 and
51.4 °C, respectively.
Although in the case of DS/MC it is noticed that the temperature increased gradually and
reach the maximum at week 8 but in the case of DS/MC/G it was observed that the
temperature increased rapidly in the first 3 weeks and reach maximum at week 5. This mainly
is due to the improving of the efficiency of mixing and aeration which provide good
ventilation conditions inside the composting windrow as a result of grit and raw sludge
mixing. Also, the warming up in the first stage of DS/MC/G was faster as related to high
porosity of the mixture which led to better aeration conditions thus improving the activity of
the aerobic microorganisms which is responsible for the activity increase of composting
process and temperature elevation. The temperature in both cases began to decrease below
55°C at the end of composting process.
It was mentioned that observing windrow temperature is the best way to monitor the
composting process (Diaz, et.al., 1993). It was also demonstrated that the single best way to
monitor the composting process is by observing temperature at points along the windrow
(Kuhlman, 1985). It was also mentioned that compost is matured enough when its temperature
remains more or less constant and does not vary with the turning of the material (Iacoboni,
1984). Therefore, this parameter may be considered as a good indicator for the end of the
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6. biooxidative phase in which the compost achieves some degree of maturity (Jiménez, and
Garcia, 1989).
In the present study, the mesophilic condition prevailed during the first three days, and
then the thermophilic stage dominated all over the composting process. Maturation period,
characterized by high temperature, began as temperature cooled down slowly but it was still
above 55 °C. These results are comparable with a study which stated that maturation piles
when left unturned for 4-10 weeks maturation, the temperatures remained at 65-75 °C
(Purcell, and Stentiford, 2001).
According to Fogarty and Tuovinen (1991) aeration during composting has multiple
functions: 1) it supplies O2 to support aerobic metabolism (oxygenase functions and aerobic
respiration), 2) it controls the temperature, and 3) it removes excess moisture as well as CO2
and other gasses. Moreover, it was mentioned that the overall goal of the aeration is to
maintain compost temperature in the range of 50- 55 °C to obtain efficient thermophilic
decomposition of organic wastes and pathogen destruction (Jeris, and Regan, 1973, Mckinley,
and Vestal, 1984, Schulze, 1962).
1.2 Odour
Odour was recorded during the composting processes. It was found that the unpleasant
odour of composting materials decreased with time. Odour increased immediately after
turning, but within a short time (30-45 min) it became as before turning. The unpleasant
odour decreased after 40-50 days of composting but did not disappear completely. At the end
of the composting process, the odour of both windrows were low and the odour of composts
after storage for another one month were similar to the odour of earth especially at low levels
of moisture contents (less than 10%). These results are in agreement with other studies which
mentioned that composting is at the end when unpleasant odours disappear and changed to an
earthy odour (Diaz, et.al., 1993, Jiménez, and Garcia, 1989).
1.3 Colour
During the composting process, a gradual change in colour from black to brownish black
took place and this gave indication of the maturity progress. Morphologically, the composts
of all windrows were nearly homogenous with fine grain powder and had a black-brown
colour or greyish black at the end of process. It is noticed that the compost of DS/MC/G was
more friable than DS/MC compost. These results are in agreement with other studies which
mentioned that by the time the process is finished it has become a dark grey to brown in
colour or brownish-black (Diaz, et.al., 1993, Gotaas, 1956).
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7. 80
o
Temperature ( C)
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Composting time (Weeks)
DS/MC
DS/MC/G
Figure (1): Changes in temperature during the composting and co-composting of sewage sludge and
sewage grit at site 9n.
- Values are means of three replicates±standard deviations.
- DS/M C: Windrow composting of dewatered sludge and matured compost.
- DS/MC/G: Windrow co-composting of dewatered sludge and matured compost with grit.
2. Chemical changes:
The chemical parameters measured in the final composts after the composting and cocomposting processes were organic matter (OM), nitrogen (N), Phosphorus (p), Potassium
(p), pH, electric conductivity and heavy metals. Results of chemical changes in all parameters
measured are shown in Figures (2-6).
The final results of composting and co-composting obtained showed that all chemical
parameters results of DS/MC/G decreased than DS/MC except for pH, P and K which
increased slightly. The pH of DS/MC compost was 6.9 and DS/MC/G compost was 7.15.
Both results are neutral and indicate good maturity. The OM content of DS/MC/G compost
less than DS/MC compost and it was 31.35% and 39.5% respectively. These results indicate
that the addition of grit in this study increased the porosity of the windrows, thus improving
the aeration process. The good aerobic conditions are recommended to maintain a rapid and
complete breakdown of readily decomposable organic compounds. Also, the organic content
of sewage grit is low and so decreased the OM content when co-composted with sludge. Also,
the N content of DS/MC/G compost was less than DS/MC compost and it was 1.6% and 2.6%
respectively. The P and K content of both DS/MC/G compost and DS/MC compost appear to
be nearly the same. The P content was 0.6% and 0.5% for DS/MC/G compost and DS/MC
compost, respectively. Also, the results indicate that the K content was 0.16% and 0.1% for
DS/MC/G compost and DS/MC compost, respectively. The N, P and K contents in both
composts are good percentages and adequate for using the compost as soil conditioner and
fertilizer. Electric conductivity measured in both composts is illustrated in Figure (5). The
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8. electric conductivity of DS/MC/G compost was 2.63 and of DS/MC compost was 3.5. The
decrease in electric conductivity of DS/MC/G compost is related to the addition of grit to
sewage sludge. The sewage grit mainly content is sand which has low electric conductivity.
The results are in agreement with other studies which stated that from fermentation
studies it is known that aerobic conditions prompt rapid and complete degradation of organic
materials by microorganisms. If anaerobic conditions prevail, slow degradation takes place
accompanied by accumulation of intermediary products, which give an offensive odour.
Therefore, aerobic conditions are recommended to maintain a rapid and complete breakdown
of readily decomposable organic compounds (Jeris, and Regan, 1973, Jann, et.al., 1959). It is of
prime importance to stabilize the organic waste rapidly. It can be concluded that frequent
turning can achieve this (Jann, et.al., 1959). Inbar et al., (1990) mentioned that about 50% of
the organic matter is metabolized to CO2 and H2O during the composting of separated cattle
manure.
Also, other studies stated that maturation phase produced further reduction in moisture
content to 30- 40% and volatile solids to 40% (Purcell, and Stentiford, 2001). The curing and
any subsequent storage can be considered as an extension of the composting process and are
associated with elevated temperatures (Willson, et.al., 1980). Also, it was mentioned that, when
the composted sludge has reached at least less than 40% moisture and the volatile solids has
been reduced to below 40%, the material is dry and stable enough to be used as a fertilizer
(Iacoboni, et.al., 1984).
pH
7.40
7.30
7.20
7.10
7.00
6.90
6.80
6.70
6.60
6.50
6.40
6.30
Sample
(1)
Sample
(2)
Sample
(3)
DS/MC
Sample
(4)
Sample
(5)
average
DS/MC/G
Figure (2): pH changes in the final composts from composting and co-composting at site 9N.
Organic Matter Content
50
Percent %
45
40
35
30
25
20
15
10
5
0
Sample (1) Sample (2) Sample (3) Sample (4) Sample (5)
DS/MC
average
DS/MC/G
Figure (3): Organic Matter content in the final composts from composting and co-composting at site 9N.
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9. N,P and K content
3.0
Percent (%)
2.5
2.0
1.5
1.0
0.5
0.0
DS/MC
DS/MC/G
N
P
K
Figure (4): N,P and K content in the final composts from composting and co-composting at site 9N.
Ele ctric Conductivity E.C
4.5
4
3.5
d /m
s
3
2.5
2
1.5
1
0.5
0
Sa mple
(1)
Sa mple
(2)
Sa mple
(3)
DS/MC
Sa mple
(4)
Sa mple
(5)
average
DS/MC/G
Figure (5): Electric Conductivity changes in the final composts from composting and co-composting.
3. Heavy metals content
As illustrated in Figure (6), the heavy metals content in the final composts of both
composting and co-composting of swage sludge and sewage grit are lower than the
permissible levels of Egyptian Sludge Regulation (Directive 254/2003)(Egyptian Code, 2005)
concerning processing and safe use of sludge in agricultural purposes. Also, the addition of
grit to sewage sludge led to decrease the all heavy metals content detected in case of
DS/MC/G compost than DS/MC compost except for lead metal. The slightly increase of Pb in
case of DS/MC/G compost than DS/MC compost result from the addition of grit which
contain high content of Pb and Cu than other metals as shown in Table (1).
The results obtained showed that the average of Zn content in the final composts was
1143.9 and 807.26 mg/kg in case of DS/MC and DS/MC/G, respectively, the Cu content was
252.6 and 209.28 mg/kg in case of DS/MC and DS/MC/G, respectively, the Ni content was
32.4 and 26.37 mg/kg in case of DS/MC and DS/MC/G, respectively, the Cd content was 1.3
and 0.74 mg/kg in case of DS/MC and DS/MC/G, respectively, the Pb content was 245.1 and
261.3 mg/kg in case of DS/MC and DS/MC/G, respectively, and the Cr content was 67.0 and
27.54 mg/kg in case of DS/MC and DS/MC/G, respectively. As mentioned above, the results
indicated that the mixing of grit which contains low levels of heavy metals as shown in Table
(1) with sewage sludge is the reason for decrease the heavy metals content in DS/MC/G
compost than DS/MC compost.
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10. Table (1): Heavy metals content of raw sewage grit before composting
Metals (mg/kg) Sample 1 Sample 2
Zn
0.072
0.0395
Cu
0.1771
0.2006
Ni
0.0297
0.0249
Cd
Pb
0.0027
0.0038
0.218
0.224
Cr
0.013
0.0187
Average of heavy metals content
1400
1200
(mg/kg)
1000
800
600
400
200
0
Zn
Cu
Ni
DS/MC
Cd
Pb
Cr
DS/MC/G
Figures (6): The heavy metals content in the final composts from composting and co-composting.
4. Detection of pathogens
In respect to pathogen and parasite destruction, the relative elimination efficiencies of the
composting process were evaluated on the basis of existence of indicator microorganisms
(fecal coliforms), enteric pathogenic bacteria (Salmonella spp.) and Ascaris ova.
The counts of fecal coliforms during the composting and co-composting of sewage sludge
and swage grit are shown in Table (2) and (3). The counts of fecal coliforms in raw sewage
sludge and raw grit were 7x109 and 1.7x105 MPN/g, respectively, and with the advance of
composting and co-composting processes, the counts became zero in both DS/MC compost
and DS/MC/G compost. The results of fecal coliform counts had lower number than the
permissible levels of fecal coliforms in the Egyptian Sludge Regulation (Decree 254/2003)
(Egyptian Code, 2005), which specifies that the fecal coliforms MPN should be less than
1000 cells/g dry solids.
In respect to Salmonella spp., the results indicated that the count in raw sewage sludge
and raw grit were 2.1x104 and 1.8x102 MPN/g, respectively, and at the end of composting and
co-composting processes, the counts became <2 in both DS/MC compost and DS/MC/G
compost. It is noticed also that all results were within the permissible levels of Salmonellae
specified in the Egyptian Sludge Regulation (Decree 254/2003)( Egyptian Code, 2005)
10/13

11. concerning processing and safe use of sludge in agricultural purposes, which specify that the
Salmonella MPN count should be less than 3 cells/100 ml at a sludge concentration of 4% dry
solids.
Ascaris ova count was positive in both raw sewage sludge and raw sewage grit which
disappeared after composting and co-composting processes and became negative in both
DS/MC compost and DS/MC/G compost. The results of Ascaris ova count indicate that they
had lower number than the permissible levels of Ascaris ova in the Egyptian Sludge
Regulation (Decree 254/2003)( Egyptian Code, 2005), which specifies that the Ascaris ova MPN
should be 1 cells/100 ml at a sludge concentration of 5% dry solids.
To discuss the results of the present study it should be mentioned that total and fecal
coliforms, Salmonella, Ascaris counts were selected because of their value as indicator
organisms, and because they are enteric pathogens known to be present in relatively high
numbers (54,122). These results agree with those reported in other study, which stated that
coliforms are more resistant to inactivation than Salmonella spp. and thus are good indicator
organisms (124). There is extensive data indicating that the densities of fecal coliforms are
good indicators of the effectiveness of the composting process in destroying enteric
pathogens such as Salmonella spp. Studies at the Los Angeles county facility determined that
the fecal coliforms concentrations of less than 1000 MPN/g indicated a high probability of
destruction of bacteria and parasitic and viral pathogens (146). Generally, the results of all
final composts in this study were negative for the tested pathogens and parasites. This meant
that these composts are safe for handling as previously reported that when an intensive
control program for composting process is carried out with regard to viruses, bacteria and
parasites, the hygienic quality becomes satisfactory (13).
Table (2): Typical pathogens count in raw sewage sludge and sewage grit
Pathogens
Raw Sewage Sludge Sewage Grit
Count/100/ ml
Count/100 ml
7x109
2.1x104
+ve
Fecal Coliform Bacteria
Salmonella
Ascaris ova
1.7x105
1.8x102
+ve
Table (3): Typical pathogens count after the composting and co-composting. .
DS/MC
DS/MC/G
Count/100/ ml Count/100 ml
-ve
Fecal Coliform Bacteria
-ve
<2
Salmonella
<2
-ve
Ascaris ova
-ve
Pathogens
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12. 4. CONCLUSIONS:
1- Co-composting of sewage sludge with sewage grit is an effective and safe method for
treatment of grit and disposal method buy reuse.
2- Addition of grit to sewage sludge as bulking agent during composting process increases the
porosity and improves aeration inside the windrows that made composting process faster
in time.
3- The temperature during the co-composting of sewage sludge and sewage grit was high
enough and it was within the effective range for pathogen and parasite destruction.
4- The compost produced from co-composting of sewage sludge and sewage grit is free of
pathogens and safe enough for using as soil conditioner and fertilizer for landscape and
ornamental plants.
5- The lower heavy metals content in grit that mixed with sewage sludge in this study
decreased the heavy metals content in compost produced.
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