Comparison of Specific Nutrients of Tunisian Textile Sludge and Produced Composts
Mohamed Anis El Hammadi ,
The composting of textile sludge is an alternative to the traditional disposal of residues and also involves a commitment to reducing the production of waste products. A comparative study was made for major nutrient elements or macronutrients (include carbon, nitrogen, phosphorous, potassium, calcium) and the micronutrient elements mostly used (sodium, magnesium) present in the textile sludge taken from the a textile-wastewater treatment plant in the north of Tunisia. The two produced composts were made from the above sludge, mixed in two proportion (1:1 and 1:3 sludge and green plant waste (v/v)). The two produced composts showed higher C, N, Ca, Mg, K, P and lower Na contents than the above sludge. Sludge composting reduce the Na content of the sludge, provide a stabilized organic matter and improve the physicochemical properties of the textile sludge. However, other studies are necessary for a complete evaluation of the two produced composts before land application.
Actually, a great problem in Tunisia is the final destination of some types of residues. Sedimentation both before and after wastewater biotreatment produces sewage sludge. Additionally, the upgrading and expansion of wastewater treatment plants have greatly increased the volume of sludge generated along with developing industry, agriculture and in improving human life. Textile sludge is an inevitable by-product of textile wastewater treatment processes (Yongjie and Yangsheng, 2005). Recently, large volumes of this waste is generated by the textile industry and released into the environment. Therefore, there is a growing need to find alternative solutions for textile sludge management. Textile sludge has a variable composition (Balan and Monteiro, 2001) and contains heavy metals and inorganic salts. Additionally, dyes, heavy metals and pathogenic microorganisms may be present and composting is often required for utilization and transformation of these wastes into a soil amendment. (Balasubramanian et al., 2006). Composting is defined as the aerobic biological decomposition and stabilization of organic substrates, under conditions that allow development of thermophilic temperatures as a result of biologically produced heat, to obtain a final product that is stable, free of pathogens and plant seeds and can be beneficially applied to land (Bertran et al., 2004). Kaushik and Garg (2003) evaluated the composting of textile sludge and concluded that the waste could be converted into a fertiliser product for agricultural use. Other advantages of composting include killing pathogens and improving handling characteristics of sludge by reducing volume and weight. Composting can decrease or eliminate the toxicity of textile sludge (Araujo and Monteiro, 2007). The quality of compost is related to its agronomic and commercial value as an organic solid conditioner and the application of composts to agricultural soils has many advantages, which include providing a whole array of nutrients to the soil (Tejada et al., 2001). The aim of the physico-chemical analyses in the present study is to conduct an analytical survey of nutrient elements and to compare their contents in selected textile sewage sludge and the produced compost.
MATERIALS AND METHODS
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
Composting Design and Sampling
The physico-chemical sewage sludge came from a textile-wastewater treatment
plant in Ras Jebel (in the north of Tunisia). A mixture of sludge and greenwaste
was composted on a composting platform in periodically turned outdoor piles.
Prior studies suggested that the most appropriate piles proportion was 1:1 v/v
(pile 1) and 1:3 v/v (pile 2) sludge/greenwaste ratios for the best integration
of materials. Piles were sampled for analysis after the beginning of the composting
process and monitored over 90 days. The compost piles were built following the
same protocol and comprised a layer of green waste followed by a layer of sludge
and according to the design of the experiment. 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.
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. Na2O, K2O,
CaO, MgO, P2O5 were analyzed by emission spectrometry-ICP
(NF EN ISO 11885). The elements Cd, Cu and Cr were analyzed by emission spectrometry-ICP
(NF EN ISO 11885).
RESULTS AND DISCUSSION
The pH values tended to diminish to a neutral pH in the two mixtures (Table
1). The initial pH value was lower for sludge than produced composts. After
pile 1 formation (Fig. 1), a rapid increase in temperature
was recorded from 25 to 67°C during the first 6 days and then fluctuated
until day 36 (thermophilic phase) at which point the temperature stabilized
gets lower values. These distinct phases were also observed during the process
after pile 2 formation, but a rapid increase in temperature was recorded and
temperature reached the same as that of the ambient temperature after 32 days
of composting. Sludge and produced composts showed variability in total carbon
(C), nitrogen (N) and Organic Matter (OM) contents and the two matured composts
recorded higher C, OM and N values than the textile sludge. OM rose throughout
the composting process indicating the humification of the OM (Paredesa et
al., 2005). In addition, the total C content in the sludge was 18.6 g kg-1
DW, increased considerably in the produced composts C1 and C2 to 179 and 166
g kg-1 DW, respectively.
|| Changes in ambient air temperatures and pile temperatures
|| Typical characteristics of the sludge and the produced composts
|aResults expressed in g kg-1 dry basis.
bResults expressed in %. cColony Forming units g-1
fresh material. OM: organic matter, TKN: total Kjeldahl nitrogen, TOC: total
|| Specific nutrient contents of the sludge and the produced
composts (Results expressed in g kg-1 dry basis)
||Amount of heavy metals in the produced composts (Results expressed
in dry basis)
|aResults expressed in mg kg-1 of dry
During composting, new water soluble carbon compounds of microbial origin may
be formed, since composting is a synthesis process (Charest et al., 2003).
The comparative concentrations of specific nutrients in the sludge and the two
produced composts are of interest (Table 2). The two Composts
showed potentially toxic heavy metal contents lower than the limits established
by the French norm (NF U 44-095) (Table 3) and contain generally
high amounts of nutrients than the originated sludge and their composition is
variable according to their respective sludge ratio. Thus, they have a generally
slightly alkaline pH value (pH 7.5 to 7.6). The nutrient contents in the two
produced composts with various percentage of sludge indicate that the fertiliser
value of the mixtures was generally increasing with increasing percentages of
sludge. In addition, the alkaline components of composts correlates with their
Ca and Mg contents. The concentration of Ca and Mg in the sludge are, respectively
5.46 and 0.82 g kg-1 DW. In the mature composts C1 and C2, there
are higher amounts of Ca (76.9 and 86.7 g kg-1 DW, respectively)
and Mg (6.46 and 5.91g kg-1 DW). The textile sludge is carried out
using physico-chemical system and had a high calcium and magnesium contents,
which came mainly from coagulating chemicals (magnesium salts and lime) (Balasubramanian
et al., 2006). It can be assumed that a substantial part of the Ca and
Mg is generated during the composting process, by precipitation of CaCO3
and MgCO3, respectively. Generally, where sludges are applied to
land, the amount of phosphorus applied is more than sufficient to supply the
needs of the crop and high P levels could significantly increase the risk of
surface water contamination. However, In this study, the P concentration is
very low in the textile sludge (0.12 g kg-1 DW) and increased in
the two final composts. The results show that P is a limitant factor for the
above sludge to be used in land application (Walter et al., 2006). In
addition, the sludge used in this study came from a physico-chemical wastewater
plant and the use of chemical flocculants during treatment processes can decrease
P mineralization from sludges and composts (Warman and Termeer, 2005). The originated
sludge contains relatively small amount of K because most of the K present in
the influent readily passes through the wastewater treatment facility and is
discharged with the effluent. Generally, When sewage sludge is applied to agricultural
land, the sludge usually does not provide enough K to meet the requirements
for forage and corn growth even though the availability of K in septic waste
and sewage sludge is rated very high (Warman and Termeer, 2005). In this experiment,
the compost treatment provided a relatively high amounts of K. The two produced
composts C1 and C2 had lower levels of Na than the native sludge (3.58 and 2.06
g kg-1 DW, respectively) compared with the produced composts. Comparing
these results with study previously carried out by Eghball et al. (1997)
from composting windrows not protected from rainfall, it was observed that the
Na content appear to be in lower concentration. In fact, the large salt concentration
(especially Na) might limit the use of compost (Contreras-Ramos et al.,
2003). In the present study, the Na contents of these composts are considered
to be very satisfactory for their further agricultural use.
It can be concluded that both chemical and microbiological parameters provide valuable information on the nutrient contents of sludge and produced composts. Present experiment showed that co-composting textile sewage sludge with greenwaste could provide a potential means for improving the low sludge nutrient contents. We found considerable difference in the fertilizer value of the waste material and the two produced composts. In view of their beneficial effects, this kind of compost could be successfully used to restore degraded areas and for soil remediation; and potential use for the textile sludge composting technology should be further explored, but these results must be investigated to evaluate the impact of the produced composts in a natural ecosystem, so that these resources can be effectively utilized for crop production without adverse effects on the environment.
The author is indebted to all of the people involved in obtaining the results included in this study. Special thanks are extended to CITET (Tunis International Center for Environmental Technologies) for the financial support of this study. We are also very thankful to Dr. Yangsheng Liu (University Beijing, China) for providing valuable technical information.
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