Abstract: During composting, the humification of organic matter will have a significant effect on the total heavy metal contents in the produced compost(s).Therefore the present study was conducted to Investigate the effect of co-composting textile sewage sludge with greenwaste on heavy metal amounts in the produced composts. This experiment examined co-composting of textile sludge with greenwaste at 1:1 v/v (pile 1) and 1:3 v/v (pile 2)) ratios. After the 90 days composting period, the produced compost 1 led to the greater concentration of heavy metals. An important observation was that metal contents in the two composts was similar for Cd and Fe and different for Mn, Cu and Cr. Although both mature composts had total heavy metal lower than the French limit values of composts to be used as soil fertilizer, it is necessary to take precautions when spreading teese produced products in natural areas.
INTRODUCTION
Large amounts of sewage sludge have been accumulated and expected to be generated in Tunisia due to the increasing volume of textile wastewater treated. The variety of chemicals, processes and technologic variations applied to the wastewater purification processes in textile industry cause many environmental problems affecting the quality of produced sewage sludge. The textile sludge contains organic matter, chemical nutrients and also a relative amount of heavy metals and aromatic dyes (Balan and Monteiro, 2001). The safe utilization of the sewage sludge has to be ensured by its preliminary treatment (Sanchez-Monedero et al., 2004). One of the possibilities to convert textile sewage sludge is co-composting it with different bulking agents as greenwastes. Very little data is available about composting of textile sewage sludge, but with this practice, a potential source of pollution could be reduced in size. 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). Unfortunately, the presence of high levels of heavy metals often hinders agricultural land application of the composted sludge. The presence of organic and inorganic contaminants in compost, pose a grave danger to the environment of which heavy metal is the main obstacle factor leading to restricted agricultural use of compost. Uptake of heavy metals (Cu, Cd, Cr, Hg …..) by plants and subsequent accumulation along the food chain is a potential threat to animal and human health. Consequently, focus on the improvement of the composting process to minimize the mobility of heavy metals using various additives is receiving more attention (Zorpas et al., 2000; Chiang et al., 2001).
In the study reported here, two composts made from textile sludge and green matter representing the respective sludge ratios delivered to the initial composting mixtures (1:1 v/v (pile 1) and 1:3 v/v (pile 2)) were analyzed for heavy metals to assess variations in their concentrations as a function of the delivery of the initial sludge to the composting site.
MATERIALS AND METHODS
Selection of Parameters to Analyze
To determine the range of heavy metal to analyze, we used two criteria:
1-It must be listed us high priority contaminant by the health and the environmental
authorities. 2-Included in other countries sludge or compost regulation guidelines.
Site and Climatic Conditions
The field study was conducted from May 2006 to January 2007 in Tunis International
Center for Environmental Technologies. 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 and Sampling
The textile sewage sludge came from a textile-wastewater treatment plant
in Ras Jebel (in the north of Tunisia). Green wastes were collected selectively
from CITET garden. A mixture of sludge and greenwastes was composted on a composting
platform and monitored over 90 days. The compost piles were built following
the same protocol and comprised a layer of greenwaste followed by a layer of
sludge and according to the design of the experiment. The composition of the
piles according to the specific volume of sludge and green matter is shown in
(Fig. 1).
The well progress of composting and microbial activities was followed by measuring
with a portable thermometer the pile temperature and external temperature during
the composting process. The mixture was turned over periodically to ensure aerobic
conditions. Numerous samples from various points of the compost heaps were collected.
The two selected times of sampling were T = 0 (initial mixture) and after 90
days. The samples were kept deep frozen until analysis. The typical characteristics
of the sludge used in the composting process are shown in Table
1.
| Fig. 1: | Composition of the piles according to the specific volume of sludge and green matter |
| Table 1: | Physico-chemical features of the sewage sludge used for composting |
| aResults expressed in g kg-1 of dry matter, b TKN: Total Kjeldahl Nitrogen, cResults expressed as colony forming units 100 mL fresh material, d Results expressed in mg kg-1 of dry matter, e Results expressed in μg kg-1 DW (dry basis) | |
Compost Parameters
Nitrogen was determined by the Kjeldahl method (NF ISO 11261), the Organic
Matter (OM) by Gravimetry (Rodier 8th Edition). Total organic carbon (COT) was
measured according to Colorimetry method (ISO 14235). 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).
RESULTS AND DISCUSSION
Physical and chemical properties of the produced composts are compiled in Table 2 and the addition of increasing amounts of greenwaste increased the Organic Matter (OM) content in the produced composts (Table 3). During composting, the OM decreased, respectively by 42.57 and 52.29% in the compost 1 and 2. In addition, the results indicate an increasing trend of compost metal contents with the increasing rate of the OM decomposition. Total heavy metal contents rose above those observed in previous weeks perhaps due to the concentrating effect caused by the weight loss associated with mineralisation of the OM (Sanchez-Mondero et al., 2001). For comparative purposes, the Table 4 shows the maximum levels of the heavy metals allowed in french norms for land-applicable waste (NF U 44-095). The produced compost 1 met the French norm on composts made with materials of water treatment for pathogenic microorganisms and heavy metals (NF U 44-095) as well as compost 2. Low total Cd and Hg concentrations were found in the textile sludge, so the risks to environmental impacts are low (McGrath, 1995). Also, no difference has been detected in the total contents of these two heavy metals between sludge and the two produced composts. Generally, composts originating from industrial centres do contain comparatively high Cd concentrations (Fricke and Vogtmann, 1993). Fe is the heavy metal most represented in the sludge. Generally, acids from leather manufacturing and the coagulant used in the water treatment sludge may solubilize partially insoluble forms of Fe (Teixeira et al., 2005). The sludge Fe content decrease considerably by 91.89 and 91.36% in the final composts 1 and 2, respectively. Comparing with the results of the two first mixtures, the total Fe contents showed an increase in the final produced composts with 7.58 and 7.5, respectively pH. In fact, this last effect is enhanced when PH is above 6.6 (Teixeira et al., 2005).
Total contents of Mn, Cu and Cr increased as a function of sludge doses in
both produced composts effect that was not observed for the total contents of
Fe. It can be caused by the reduction in volume of pile and therefore an increase
in the concentration of some heavy metal contents after composting. The composting
process is generally marked by an increase in some metal concentrations due
to the evident reduction of compost mass by decomposition (loss of matter) (Lazzari
et al., 2000).
| Table 2: | Evolution of physico-chemical parameters during composting of the produced composts |
| aResults expressed in g kg-1 of dry matter, bTKN: Total Kjeldahl Nitrogen, cResults expressed in% dry weight | |
| Table 3: | Amounts of heavy metals in the first mixtures (T = 0) (Results expressed in dry basis) |
| aResults expressed in g kg-1 of dry matter, bResults expressed in mg kg-1 of dry matter | |
| Table 4: | Amount of heavy metals in the produced composts (Results expressed in dry basis) |
| aResults expressed in g kg-1 of dry matter, bResults expressed in mg kg-1 of dry matter | |
Total Mn content increased more than the other metals in the two final products and proportionally to sludge doses. Regarding Cu, The initial amount in the sludge was not important. In fact, some forms of copper in high concentrations from leather industry can slow down the degradation of organic substances to minerals and humic acids, since copper salts have a fungicidal effect (Püntener, 2003). Comparing to values observed in the final compost 2, Sludge ratio in the first mixture increased the total Cu content in the produced compost C1. Total Cr contents recorded in the two compots for treatments with sludge were below the limits established by French norms (NF U 44-095). In fact, high levels of chrome are a major obstacle in agricultural use and an excessive level of chrome application may cause accumulation in the soil as well in farm crops and possibly endanger human health (Gui-ju et al., 2005). In regard to Cr contents, the compost number 1 that received ½ sludge ratio presented higher content of this heavy metal than compost 2 indicating that Cr increased with increasing sludge doses. This effect was not observed regarding Mn, heavy metal most dominant in comparison to the other total contents in the two final composts (Table 4). The produced composts had low levels of Cr compared with this found by (Contreras-Ramos et al., 2003) in compost made with tannery wastewater from the leather industry. Thus, in three months, it was possible to obtain from two mixtures of textile and greenwastes two adequate composts for application to the soil that met the French norm on composts made with materials of water treatment for heavy metals (NF U 44-095).
CONCLUSIONS
According to the experimental results, it is shown that the majority total heavy metal contents of the two produced sludge composts were similar. However, the composting process affected Mn amounts and led to considerable variation between them. Heavy metal analysis showed that metal contents in the two composts was similar for Cd and Fe and different for Mn, Cu and Cr. The composting procedure increased the amounts of heavy metals compared to the begining of the process. Therfore, both composts could be used without problems as soil amendment agents at proper rates since their heavy metal contents are lower than the maximum levels permitted by French legislation. From an agricultural point of view, the results obtained in this study may contribute to the prevention of environmental damage, although they cannot be extrapolated directly for making predictions about in situ metal behaviour in the soil. Therefore, to obtain the optimum application rates and to minimize negative environmental impacts, field experiments should be performed.
ACKNOWLEDGMENTS
We appreciate the effort of all people involved in obtaining the results included in this study. Special thanks to the CITET (Tunis International Center for Environmental Technologies) for the financing of this study.