Composition of Organic and Inorganic Contaminants in Compost Produced from Different Organic Wastes
Mohamed Anis El Hammadi
The aim of this study was to evaluate xenobiotics contents (Detergent, heavy metal and chloride) of final compost made from different wastes (Textile sludge, municipal sludge, municipal waste and garden waste). After composting, there was an increase in total heavy metal contents in compost because of loss of mass due to biodegradation of organic matter. In addition, we recorded a slight increase of chloride content. On the other hand, this study demonstrates that aerobic biodegradation has important environmental consequence for detergent contents in the pile that biodegrade in the presence of air during the composting process. However, proper consideration should be given to the compost metal content and further field studies are required to examine the long-term effect of the final product in soils, particularly in relation to biological indicators of soil quality.
April 26, 2010; Accepted: July 09, 2010;
Published: August 30, 2010
The presence of organic and inorganic contaminants in the organic wastes pose
a grave danger to the environment. To overcome the risks incurred by the direct
use of some of these wastes in agriculture, treatment is required to minimise
and eliminate the undesirable effects and to optimise the efficiency of the
materials once applied to the soil. Among the organic wastes recycled in agriculture,
residual sludge generated by wastewater treatment is a source of organic matter
rich in both phosphorus and nitrogen. It can contribute to the rehabilitation
of degraded soils by its fertilising and other soil-improving qualities (Martinez
et al., 2003). Recycling of sludge for agricultural purposes seems
to be an appealing solution that enables valuable components to be recycled
(Dolgen et al., 2007) and sewage sludge has been
used as an amendment to agricultural soils (Kidd et al.,
2007) and the application of sludge also increase soil organic matter content
that contributing to the structural stability of the soil and to its resistance
to erosion (Ortiz and Alcaniz, 2006). Composting is
considered to be the best pretreatment for overcoming these problems (Amir
and Hafidi, 2001). Composted organic wastes used as substrates could be
a feasible option, especially sewage sludge due to its high production (Jouraiphy
et al., 2005). Composting is defined as a process of aerobic thermophilic
microbial degradation or an exothermic biological oxidation of various wastes
by many populations of the indigenous microorganisms which lead to a stabilized,
mature, deodorized, hygienic product, free of pathogens and plant seeds, rich
in humic substances, easy to store and marketable as organic amendment or fertilizer
(Ouatmane et al., 2000). A number of toxicants
discharged through the sewers tend to accumulate in sewage sludge and limits
the general use of this waste in agriculture. Besides the necessity to have
information about substance groups, which had been characterized as highly toxic,
there was a demand for having information about their contents in composts made
from materials of water treatment. sources. From these, a vast group of heavy
metals is usually the most considered one. Sewage sludge compost is a waste
derived material with increased concentrations of heavy metals. Consequently,
its use in agriculture and horticulture is problematic with strict regulations
determining both the amount and the method of application (Manios
et al., 2003). Chloride content in sludge compost are also of concern,
since in high concentrations this ion can be toxic to plant tissues. Additionally,
chloride concentration was one of the main factors influencing vegetable growth
(Garcia and Bernal, 2001). On the other hand and because
of their broad application in households and industry, detergents can be classified
as a relevant compound in sewage sludge. The detergent concentration is influenced
by the content in the sewage, the treatment process (aerobic, anaerobic), the
water hardness, the age of the sludge after storage etc and composting leads
generally to a decrease of detergent concentration in the composted sludge (Laguardia
et al., 2001).
Accordingly, the objective of this study was to examine the levels of total heavy metals (Cr, Cd, Cu, Fe and Mn), detergent and chloride contents in the final compost produced from different wastes.
MATERIALS AND METHODS
Justification of the proposed selection of the wastes for composting:
The use in agriculture of good quality compost is an efficient means of re-establishing
the balance between the withdrawal and restitution of organic matter in the
biosphere. This is particularly important in Tunisian soils, which are generally
poor in organic matter. In addition, the return of organic residential waste
such as compost to the cycle of nature is one of the most important ecological
objectives of recycling and reuse of substances. In this context, the increasingly
an growing sewage sludge production in Tunisia justify the choice of a biological
treatment for the production of compost as a way of utilizing the waste.
Site and climatic conditions: The field study was conducted from May 2006 to January 2007 in Tunis International Center for Environmental Technologies (CITET). The climatic characteristics of the study area are as following: annual precipitation did not vary obviously year by year within the study time, the average mean air temperature was 30°C, the lowest air temperature was 0°C in January and the highest air temperature was 45°C in August.
Composting design and sampling: The textile sewage sludge came from a textile-wastewater treatment plant in Ras Jebel (in the north of Tunisia) and the municipal sludge was collected from the wastewater treatment plant of Charguia in Tunis town (Capital of Tunisia). Municipal greenwastes were collected selectively from the central market of Tunis (in Tunis town) and garden greenwastes were taken from the CITET park area. The Typical characteristics of the sludges used in the composting process are shown in Table 1.
A mixture of sludges and greenwastes was composted on a composting platform in periodically turned outdoor piles and the pile comprised a layer of greenwaste followed by a layer of sludge. Composting, on a purpose-built platform, was followed for 120 days and the mixture was turned every 15 days for aeration. The following graph shows the composition of the pile according to the specific volume of sludges and green matter (Fig. 1).
|| Physico-chemical features of the sludges used for composting
|aResults expressed in g kg-1 of dry
matter, bTKN: Total Kjeldahl nitrogen, cResults expressed
as colony forming units 100 mL fresh material, dResults expressed
in mg kg-1 of dry matter, eResults expressed in μg
kg-1DW (dry basis)
|| Composition of the pile according to the specific volume
of the wastes
Compost parameters: Nitrogen was determined by the Kjeldahl method (NF ISO 11261), the organic matter by Gravimetry (Rodier 8th edition). Total organic carbon is measured according to Colorimetry method (ISO 14235). The C/N ratio was calculated from contents of Total Organic Carbon (TOC) and total nitrogen (Kjeldahl) in air-dried samples. The pH was determined with a glass electrode. Fe was analyzed by emission spectrometry -ICP (NF EN ISO 11885). The elements Cd, Cr, Cu and Mn were analyzed by emission spectrometry-ICP (NF EN ISO 11885). Mercury was determined by atomic absorption analysis (NF EN 1483). The detergent contents was determined by the colorimetric method. Chloride is measured according to the colorimetric-test method (ISO 7393).
Germination test: Germination tests were performed with (Helianthus
annuus L.). The germination index was determined by placing a layer of compost
or sludge sample in a Petri dish covered with a filter paper and water was subsequently
added until the filter paper was completely submerged. Seeds of sunflower (Helianthus
annuus L.) were then rinsed many times with distilled water and placed on
the filter paper. The percentage of germination was measured after incubating
the covered petri dishes (three replicates for each sample of the compost) in
the dark at 25°C for 96 h (Table 2).
|| Physico-chemical characteristics of the produced compost
|aResults expressed in g kg-1 DW dry
basis; bResults expressed in %; cResults expressed
as colony forming units 100 mL fresh material; OM: Organic matter, TKN:
Total kjelahl nitrogen, TOC: Total organic carbon
The germination index (GI) was computed by the formula:
Statistical analysis: In order to calculate the sample means and standard deviations between the different parameters, all data were statistically analysed using a Wessa System Software through a Fujitsu computer and each sample was considered as an individual observation. Values are mean of three independent replicates ±SE (n = 3).
RESULTS AND DISCUSSION
Physical and chemical properties of the produced compost are shown in Table
2, compost was near neutral pH (7.63), organic matter content was considerably
high (37.2%) and several heavy metals such as Cr, Cd, Cu, Fe and Mn were detected
in the compost. The heavy metal concentrations in the final compost deserve
consideration since they may affect the final product quality and the change
of use and uptake by soil flora will relate to total heavy metal content. The
results are presented in Table 3; the results indicate a general
increasing trend of compost metal contents in the final compost. Total heavy
metal content of Cr, Fe and Cu do not degrade in the composting process and
rose above those observed in the first mixture. This is due both to the concentrating
effect caused by the weight loss associated with mineralisation of the OM (Sanchez-Monedero
et al., 2004) and to phenomena of rapid decomposition of the organic
matter, with consequent release of soluble heavy metals (Chaney
and Ryan, 1993), although losses through drainage can not be excluded. In
addition, Mn is the heavy metal most represented in the final compost and no
difference has been detected in the total contents of Cd between the first mixture
and the final compost. The content of total Fe ions was 4.63 g kg-1
DW in matured compost, which was much lower than its respective initial concentration
of 62.33 and 7.49 g kg-1 DW in textile sludge and municipal sludge.
This was simply due to the addition of large amount of municipal and garden
greewaste into the compost. This enlarged the volume and diluted the metal content
in compost. Moreover, the reduction in volume of pile after composting also
resulted in an increased concentration of heavy metals. In addition, the wide
range of biochemical compounds dissolved in the wastewater and the industrial
processes used yielding the sewage sludge may lead to considerable differences
in the heavy metal content of the subsequent composted sewage sludge (Casado-Vela
et al., 2006).
|| Amount of heavy metals in first mixture and the produced
compost (Results expressed in dry basis)
|| Amounts of detergent and chloride in the produced compost
(Results expressed in dry basis)
The metal concentrations in this study were below the maximum permissible
levels for organic products made with materials of water treatment recommended
by the French norm (NF U 44-095). Hence the application of this compost derived
from different wastes may be considered as safe and suitable for soil improvement
in the agricultural plant production. The initial levels of chloride and detergents
in the first mixtures and the produced compost are shown in Fig.
2. In the first mixtures, chloride content was very low (0.2 mg kg-1
DW). Comparing this value with the chloride amount in the final compost (0.36
mg kg-1 DW), we recorded a slight increase of this organic chemical.
This is due to the fact that chloride is present in the water used in the maintaining
of the moisture of the composts during the composting process (Iiyama
et al., 1996). Also, the first mixture contains an average of approximately
56.6% organic matter on a dry weight basis. Following components addition, the
first mixture undergoes decomposition to carbon dioxide, water, low molecular
weight soluble organic acids, residual organic matter and inorganic constituents.
Although most of the organic fraction of the sludge is converted to carbon dioxide
and water, some becomes part of the stable soil humus layer (Hernandez
et al., 1990) and serves to increase the soil's net negative charge.
Chloride released from sludge following decomposition may be put into more soluble
anions C1¯. It was observed by Paredes et al.
(2005) that chloride level in compost can increased the soil salinity after
land application and that the levels of C1¯ were always higher in the soils
with compost, particularly with the high dose of C1¯. The results showed
also the degradation of an important masses of the detergent present in the
first mixture. The enumeration of microbial populations is typically performed
to gain information on the biodegradation potential of the of the detergent
and chloride contents. Microbiological analysis of the first mixture showed
a count of E. coli of 1.1°cfu g-1 fresh compost. It is
seen that the E. coli counts decreased to ˜104 cfu g-1 fresh
compost in the produced compost.
|| Evolution of sunflower germination parameters in the sludges
and produced composts ( All Result expressed in %)
|All values are reported as mean ± standard deviation
between three replicates; a Germination index
Thus, the composting process allowed to a decrease in the microbial counts.
This drop can be attributed to the exhaustion of nutrients from the medium and/or
to the temperature peak during the thermogenic phase (Jouraiphy
et al., 2005). The results in this study demonstrate that aerobic
biodegradation has important environmental consequence for detergent contents
in the pile that biodegrade in the presence of air during the composting process.
It could be seen that the level of detergent in the first mixture was 2.74 mg
kg-1 DW. After composting, the total removal rates of detergent was
beyond 94.89%. The composting process is generally marked by a degradation of
some detergent components during composting of sewage sludge with agricultural
waste products, i.e., straw, saw dust, tree clippings and the pile is aerated
by mixing and these steps may facilitate further aerobic degradation of detergent
(Laguardia et al., 2001). A number of studies
have shown that aerobic treatment is a wellsuited solution for reducing the
level of detergents (Prats et al., 1999; Solbe,
1999) and that composting sludge leads to a decrease of detergent concentration
(Elenaa et al., 2000). Under aerobic conditions,
some detergent compounds will be degradated by ω-oxidation followed by
a β-oxidation with subsequent degradation of the alkyl chain. Degradation
will be influenced by oxygen availability (Leschber, 2006).
Generally, in untreated soils, the detergent content is normally less than 0,2
mg kg-1 DW. Although sewage sludge is the main source for detergents
in soils, some detergent concentrations in soils may originate from compost
application (Carlsen et al., 2002). Moreover,
the increase of the germination index in the produced product (Table
4) suggested that the composts did not pose any toxicity on the plant growth,
that the maturity was sufficient and that composting is an important process
that has to eliminate toxic compounds present in sewage sludges (Tiquia
et al., 1996). Therefore, the produced composts can benefit plant
growth and is suitable for agricultural use.
From these data, it can be concluded that composting is a suitable alternative for the recycling of different wastes (sludge/municipal sludge/garden greenwaste/municipal greenwaste) and can safely be used to get a good quality of organic fertilizer. The experiment showed a reduction of the levels of detergent and an increase of total heavy metal and chloride contents in the mature compost. This compost met the French norm on composts made with materials of water treatment for heavy metals (NFU 44-095). In addition, the amounts of detergent and chloride in the final product are very low. Furthermore, a greater effort should be made to select wastes for composting to reduce metal content in the final compost, thus minimising the risks of soil pollution.
My thanks go to all of the people involved in obtaining the results included in this study. This work was supported by the CITET (Tunis International Center for Environmental Technologies).
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