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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 General Manager of Sludge and Reuse.

3 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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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 bio-

treatment. 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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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 m

3/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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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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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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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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0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12

Composting time (Weeks)

Tem

pera

ture

(oC

)

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 co-

composting 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

8/13

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

6.306.406.506.606.706.806.907.007.107.207.307.40

Sample

(1)

Sample

(2)

Sample

(3)

Sample

(4)

Sample

(5)

average

DS/MC DS/MC/G

Figure (2): pH changes in the final composts from composting and co-composting at site 9N.

Organic Matter Content

0

5

10

15

20

25

30

35

40

45

50

Sample (1) Sample (2) Sample (3) Sample (4) Sample (5) average

Pe

rce

nt

%

DS/MC DS/MC/G

Figure (3): Organic Matter content in the final composts from composting and co-composting at site 9N.

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N,P and K content

0.0

0.5

1.0

1.5

2.0

2.5

3.0

DS/MC DS/MC/G

Perc

en

t (%

)

N P K

Figure (4): N,P and K content in the final composts from composting and co-composting at site 9N.

Electric Conductivity E.C

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Sample

(1)

Sample

(2)

Sample

(3)

Sample

(4)

Sample

(5)

average

ds/m

DS/MC 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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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 0.0027 0.0038

Pb 0.218 0.224

Cr 0.013 0.0187

Average of heavy metals content

0

200

400

600

800

1000

1200

1400

Zn Cu Ni Cd Pb Cr

(mg/k

g)

DS/MC 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 1.7x10

5 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 1.8x10

2 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)

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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

Count/100/ ml

Sewage Grit

Count/100 ml

Fecal Coliform Bacteria 7x109

1.7x105

Salmonella 2.1x104

1.8x102

Ascaris ova +ve +ve

Table (3): Typical pathogens count after the composting and co-composting. .

Pathogens DS/MC

Count/100/ ml

DS/MC/G

Count/100 ml

Fecal Coliform Bacteria -ve -ve

Salmonella <2 <2

Ascaris ova -ve -ve

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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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