ENVIRONMENTAL PHYSICS
ASSESSMENT OF TOXIC ELEMENTS Cu, Cr, Ni, Pb, Cd, Hg, Zn, As
AND HEXAVALENT CHROMIUM IN SEWAGE SLUDGE FROM
MUNICIPAL WASTEWATER TREATMENT PLANTS BY COMBINED
SPECTROSCOPIC TECHNIQUES
THOMAS SPANOS1, ANTOANETA ENE2,*, IRINA B. KARADJOVA3
1
Eastern Macedonia and Thrace Institute of Technology, Department of Petroleum
and Mechanical Engineering Sciences, St. Lucas, 654 04 Kavala, Greece
2, *
Dunarea de Jos University of Galati, Faculty of Sciences and Environment,
Department of Chemistry, Physics and Environment, 47 Domneasca St., 800008 Galati,
Romania; e-mail: [email protected]
3
Sofia University “St Kliment Ohridski”, Faculty of Chemistry, 1 James Bourchier Blvd.,
1164 Sofia, Bulgaria
Received June 23, 2014
The aim of this study was to determine the concentrations of toxic elements Cu, Cr, Ni,
Pb, Cd, Zn, Hg, As and Cr(VI) employing spectroscopic techniques ICP–AES, ETAAS
and CVAAS, and to assess the possibility of using the sludge for application on
agricultural land. Sewage sludge (n=21 samples) was collected in different seasons in
the period 2007–2011 from three municipal wastewater treatment plants (WWTPs) in
Northern Greece (Kavala, Drama and Palio). Due to the low concentration values
(mg/kg dry matter) found for all the metals (Cd: 0.8–7.3, mean 2.13±1.61; Cr: 13.2–355,
mean 103.8±100.8; Cr(VI): 0.28–4.30, mean 1.56±1.32; Cu: 51–198, mean 136.5±45.5; Hg:
<0.2; Ni: 8.8–64, mean 29.2±15.5; Pb: 12–102, mean 62.0±23.1, Zn: 810–1880, mean
1256.1±334.3 and Cr(VI): 0.28–4.3, mean 1.56±1.32), the sewage sludge produced in
the WWTPs of all three cities may be used as fertilizer for agricultural soil according to
the European Council Directive 86/278/EEC and Greek legislation 80568/4225/91.
Although the As content of sludge is not regulated in Greece, the concentrations found
in selected sludge samples (6.3–9.2 mg/kg dry matter) must be corroborated with the As
concentration in background soil in case that agricultural disposal of sludge is the
chosen route.
Key words: toxic elements; Cr(VI); sewage sludge; ICP–AES, ETAAS, CVAAS techniques;
Northern Greece.
1. INTRODUCTION
Sewage sludge (biosolids) is a by–product of wastewater treatment plants
(WWTPs) which receive sewage from municipal, industrial and/or rural sources.
Wastewater is a combination of the liquid- or water-carried wastes removed from
residential, commercial and industrial establishments along with groundwater,
Rom. Journ. Phys., Vol. 60, Nos. 1–2, P. 237–245, Bucharest, 2015
238
Thomas Spanos, Antoaneta Ene, Irina B. Karadjova
2
surface water and rainwater [1–3]. Due to the large palette of inputs in the sewers,
it contains certain undesirable components, including organic, inorganic and toxic
substances, as well as pathogenic or disease-causing micro-organisms. Sludge
resulting from residential wastewater treatment process may concentrate from
influent wastewater large amounts of potentially toxic elements, such as heavy metals
(cadmium, copper, chromium, nickel, lead, cadmium, zinc and mercury) and
metalloids, derived from the very nature of the sewage and from plumbing [2, 4–7].
Population growth, diversification and increasing number of consumed goods
are responsible for a massive increase in the amount of sewage sludge produced
daily and the safe disposal of sludge is a serious environmental issue worldwide
[8]. The use of sewage sludge as fertilizer in agricultural land is an attractive
solution for sustainable management of this waste product, as it involves recycling
of organic matter, nutrients and valuable macroelements and provides a costeffective disposal option of the product [9–11]. However, sewage sludge containing
hazardous substances such as heavy metals poses danger to the environment,
increasing the risk of toxic element accumulating in the soil and subsequently
contamination of surface and groundwater, crops and food chain, having serious
consequences for animals and human health [4, 6, 8]. In 1986 the European Union
adopted the European Council Directive 86/278/EEC [12] on the protection of the
environment, when sewage sludge is used in agriculture in order to prevent harmful
effects on plants, soils, animal and human beings, containing ranges of allowed
concentrations for the metals Cu, Zn, Cd, Ni, Pb and Hg. At the moment, the
European Commission is working on improvements of the Directive 86/278/EEC,
publishing in 2000 the EU Working Document on Sludge, 3rd Draft [13] which
comprises microbiological criteria for the control of hygiene or stabilization
processes and lower limit values for heavy metals in sewage sludge, including Cr.
Greek legislation 80568/4225/91 [14] adopted the same ranges of metal
contents in sludge used for land application as stipulated by Directive 86/278/EEC
[12], setting supplementary limits for chromium species Cr(III) and Cr(VI). The
heterogeneous nature of sewage sludge produced at different WWTPs and the
seasonal variations necessitate the knowledge of its metal composition when
deciding on the suitability of sludge for agricultural application [8] and accurate
analytical techniques should be employed for this task. The aim of this study was
to determine the concentrations of toxic elements Cu, Cr, Ni, Pb, Cd, Zn, Hg, As
and Cr(VI) in sewage sludge produced in different periods in three municipal
WWTPs in Kavala, Palio and Drama, Northern Greece, by using combined
spectroscopic techniques Inductively Coupled Plasma Atomic Emission
Spectroscopy (ICP–AES), Cold Vapor Atomic Absorption Spectroscopy (CVAAS)
and Electrothermal Atomic Absorption Spectroscopy (ETAAS), in order to assess
the possibility of using the sludge for application on agricultural land in accordance
with the European and Greek legislation.
3
Assessment of toxic elements in sewage sludge
239
2. EXPERIMENTAL
The sewage sludge was collected from three municipal WWTPs in the cities
of Kavala (KA) and Palio (PA) (touristic area 12 km outside of Kavala), on the
North Aegean Sea coast, and Drama (DR), located in the Kavala and Drama
Prefectures, respectively, in the region of East Macedonia and Thrace, Northern
Greece. Their populations are approximately 80,000; 8,000 and 50,000,
respectively. These WWTPs receive only domestic wastewater. The monitoring
was carried out from 2007 until the beginning of 2011 and consisted of seven
sampling campaigns in different seasons: 1) March/April 2007; 2) August 2007; 3)
December 2007; 4) April 2008; 5) April 2010; 6) August/September 2010; 7)
January/February 2011.
The primary and secondary residual sludge from the municipal wastewater
treatment were pretreated as follows: centrifugal thickness; anaerobic fermentation;
treatment with aeration; dewatering of the residual sludge with centrifugation in the
presence of cationic polyelectrolyte. The sludge resulting from dewatering goes to
the waste dump. Our samples were taken from this sludge and prepared as follows:
each mud sample was created in the station by mixing equivalent amounts of mud
for three consecutive production days, total approximate weight is 5–8 kg. In the
certified by ISO 9001/2009 Laboratory of Instrumental Analysis of TEI Kavala,
Greece, after homogeneous mixing, a portion of each sample (about 1–1.5 kg) was
dried in ambient conditions for one week and next placed in the oven at 90°C for
about 12 hours, until completely dried. Next, approximately half of the dry sample
was grinded and then sifted in order to prepare a sample of powder of about 10–15 g
for the final analysis. Only for the first analysis three sub-samples (labeled 1a, 1b,
1c) were created with different weights in order to check the recovery of the
measurement. We found that the recovery was equivalent. For this reason, only
one sample was prepared for the next analyses, weighing approximately 10 g.
A sample amount of 0.5 g was accurately weighed in a Teflon vessel for
microwave (MW) digestion. 6 mL aqua regia was added and the solution was left
for one night. On the next day, MW digestion was performed according to the
following program: 100 W 5 min, 0 W 2 min, 300 W 10 min, 0 W 2 min, 600 W
10 min, 0 W 10 min. After cooling, the samples were transferred and diluted in a
volumetric flask of 25 mL.
The concentrations of the metals Cu, Cr(total), Ni, Pb, Cd, Zn and As were
determined by ICP–AES under optimal instrumental parameters, given in Table 1.
The ICP–AES measurements were performed with a Jobin Yvon ICP spectrometer
(JY ULTIMA 2, plasma generator frequency 40.67 MHz), equipped with a
cyclonic spray chamber and a concentric nebulizer. Quantification was done
against standard calibration curve prepared with aqueous standards.
Hg content was measured by CVAAS under optimal instrumental parameters
which are given in Table 1, using a Varian AA 240 atomic absorption spectrometer
equipped with a continuous flow VGA–77 Vapor Generation Accessory.
240
Thomas Spanos, Antoaneta Ene, Irina B. Karadjova
4
Quantification was done against standard addition calibration curve prepared for
each sludge sample.
Cr(VI) content was measured after pretreatment by ETAAS. For the
determination of Cr(VI), 0.5 g of the mud sample was shaken with a 20 mL
mixture that consists of 0.5 M NaOH + 0.28 M Na2CO3 for 3 hours. After filtration
the solution was neutralized with 0.5 M CH3COOH till pH 3-4. Cr(VI) was
extracted as ion associate with Aliquat 336 (0.5% v/v solution in isomethylbutyl
ketone) and determined by ETAAS under optimal instrumental parameters (sample
injection aliquots 20 µL, pretreatment temperature 1200°C, atomization
temperature 2400°C). ETAAS measurements were carried out on a Perkin-Elmer
(Norwalk, CT, USA) Zeeman 3030 spectrometer with an HGA-600 graphite
furnace. Only peak areas were used for quantification.
For checking the precision of the analyses, ICP–AES results were always
compared with ETAAS and some with FAAS (Flame Atomic Absorption
Spectroscopy) carried out by associated laboratories. Results obtained by the
different laboratories generally were in good agreement.
Table 1
Instrumental parameters for ICP-AES and CVAAS measurements
Instrumental parameters for ICP-AES measurements
Spectrometer
JY ULTIMA 2
Argon flow:
plasma 11 L/min
sheath 0.2 L/min
carrier 0.35 L/min
Nebulizer
concentric
Sample uptake rate
1.2 mL/min
Integration time
2s
Replicates
3
Element:
wavelength (nm):
Cu
324.754
Cr(total)
267.716
Ni
231.604
Pb
220.353
Cd
226.502
Zn
213.856
As
188.979
Instrumental parameters for Hg analysis using CVAAS
Instrument mode
Absorbance
Calibration mode
Concentration
Measurement mode
Integration
Bandpass
0.5 nm
Wavelength
253.7 nm
Quartz tube temperature
room
5
Assessment of toxic elements in sewage sludge
241
Table 1 (continued)
Delay time
Measurement time
Replicates
NaBH4 flow rate
Sample flow rate*
Acid flow rate*
35 s
2.0 s
3
1 mL/min
7 mL/min
1 mL/min
*Both sample and acid channels used for sample introduction
3. RESULTS AND DISCUSSION
The concentrations of Cu, Cr(total), Cr(VI), Ni, Pb, Cd, Zn, Hg and As in the
investigated sludge samples from WWTPs in the three cities Kavala (KA), Drama
(DR) and Palio (PA) are presented in Table 2, together with the legislated values in
Sewage Sludge Directive 86/278/EEC [12], EU Working Document on Sludge 3rd
Draft [13] and Greek National Legislation 80568/4225/91 [14]. Table 2 also
presents for each element the limit of detection (LOD), calculated according to 3σcriteria (3 times standard deviation of blank sample), the limit of quantification
(LOQ), according to 10σ-criteria (10 times standard deviation of blank sample),
and relative standard deviation (RSD) obtained for all measured samples. RSD was
within run precision for three parallel measurements of the sample.
Because of the detection of toxic element arsenic in some drinking waters in
concentrations higher than the permitted level in certain regions in Northern
Greece [15], this element was analyzed in this work in the sewage sludge collected
in the first sampling campaigns (years 2007, 2008), although arsenic is not
regulated either by the Directive 86/278/EEC [12], or by Greek National
Legislation 80568/4225/91 [14].
Table 2
Concentrations of Cu, Cr, Cr(VI), Ni, Pb, Cd, Zn, Hg and As in the investigated sludge samples (in
mg kg-1 dry matter), and LOD, LOQ and RSD for all types of measurements obtained in this work
Sample Date of
code
sampling
Kavala (KA)
16/04/07
KA 1a
KA 1b
KA 1c
KA 2
23/08/07
KA 3
19/12/07
KA 4
18/04/08
KA 5
18/04/10
KA 6
10/09/10
KA 7
19/01/11
Weight
g
Cu
Cr *
Cr(VI)
Ni
Pb
Cd
Zn
1.4138
1.1486
0.7797
101
111
98
69±5
106±12
108±16
158±9
198±9
128±7
18.2
16.6
15.8
14±2
65±6
32±2
73±3
85±3
46±2
NA
NA
NA
0.5±0.07
0.9±0.2
0.7±0.1
2.1±0.2
1.3±0.1
0.31±0.03
11.4
12.2
10.8
17±2
34±4
8.8±2
28±1
16.1±1
23±1
55
58
54
37±2
68±4
52±3
62±3
67±3
96±4
1.24
1.19
1.21
1.4±1
0.9±0.3
0.8±0.1
2.4±0.2
2.5±0.2
1.5±0.3
895
994
908
1061±25
998±89
1056±91
810±42
1095±46
987±56
Hg
As
NA
NA
NA
NA
< 0.2 6.3±1.3
< 0.2
NA
< 0.2
NA
< 0.2
NA
< 0.2
NA
< 0.3
NA
< 0.2
NA
242
Thomas Spanos, Antoaneta Ene, Irina B. Karadjova
6
Table 2 (continued)
Drama (DR)
DR 1a
16/03/07 1.3060
DR 1b
1.0693
DR 1c
0.7550
DR 2
23/08/07
DR 3
19/12/07
DR 4
18/04/08
DR 5
18/04/10
DR 6
23/09/10
DR 7
03/02/11
Palio (PA)
PA 1a
16/04/07 1.4994
1.0306
PA 1b
0.7138
PA 1c
PA 2
23/08/07
PA 3
19/12/07
PA 4
18/04/08
PA 5
18/04/10
PA 6
10/09/10
PA 7
19/01/11
Limit of detection
Limit of quantification
RSD of the method (%)
Proposed limits
Directive 86/278/EEC [12]
108
114
106
53±3
134±11
122±131
148±6
165±8
191±12
38
NA
40
NA
41
NA
24±3
0.7±0.1
180±9 1.9±0.4
149±11 1.3±0.3
239±16 3.8±0.3
227±16 3.2±0.2
42±2 0.28±0.02
20.1
21.3
19.9
14±1
62±3
34±3
64±3
33±2
37±2
36
34
33
43±2
74±6
53±4
41±3
49±2
70±4
1.43
1.49
1.52
1.8±1
1.8±0.5
1.7±0.4
7.3±0.4
2.2±0.2
1.3±0.3
856
798
808
1120±28
1340±105
1131±102
1080±86
1058±36
1327±73
NA
NA
NA
NA
< 0.2 6.5±1.4
< 0.2
NA
< 0.2 7.9±1.8
< 0.2 6.2±1.2
< 0.2
NA
< 0.3
NA
< 0.2
NA
127
121
131
51±5
148±14
193±24
184±9
187±7
185±11
0.3
0.9
3–7
12.2
13.4
14.1
17±3
65±6
185±12
355±23
275±12
37±2
0.3
0.8
2–6
13.1
12.5
14.0
21±2
42±3
22±3
45±2
25±1
42±2
0.3
0.8
4–8
57
60
58
12±1
99±7
73±5
94±5
62±3
102±5
0.5
1.4
5–8
1.64
1.53
1.71
1.4±1
1.5±0.4
1.6±0.3
6.1±0.4
2.8±0.2
1.4±0.2
0.2
0.6
3–7
1393
1442
1432
1840±31
1880±132
1540±93
1684±105
1346±76
1850±67
0.2
0.6
4–8
NA
NA
NA
NA
< 0.2 9.2±1.8
< 0.2
NA
< 0.2
NA
< 0.2
NA
< 0.2
NA
< 0.3
NA
< 0.2
NA
0.005
0.2
0.015
0.6
5–10
5–8
300400
300
7501200
750
20-40
25004000
2500
16-25
10
–
300400
7501200
20-40
25004000
16-25
–
–
NA
NA
NA
0.4±0.1
0.8±0.2
1.6±0.2
4.3±0.3
3.6±0.4
0.4±0.1
0.02
0.05
6–9
EU 3rd Draft [13]
10001750
1000
–
1000
–
Greek Legislation
80568/4225/91 [14]
10001750
Cr(III):500
Cr(VI):10
10
10
–
Note: * Cr (total) = (Cr(III) + Cr(VI)]; NA: not analyzed; –: not specified.
Figure 1 presents the average contents of toxic metals in WWTP sludge
originating from the three cities in the period 2007–2011.
Fig. 1 – Mean values of metal concentrations in sludge samples studied at different sites
in the period 2007–2011. Error bars indicate standard deviation of the mean.
7
Assessment of toxic elements in sewage sludge
243
From Table 2 and Fig. 1 it can be seen that the sludge metal loading is sitespecific and varies by the period of sample collection for most elements. This fact
demonstrates that the accumulation of metals in urban wastewater sludge depends
upon local factors, such as people’s lifestyles in the area served by the treatment
plant [3,7], irregularity of water inputs from urban sources [5,10,16], wastewater
composition, as well as the operating/treatment processes and cleaning
performance of each WWTP [7,8,10,17].
Zinc was the most abundant while cadmium had the lowest occurrence.
Mercury was not detected in any sample. Overall, the order of metal concentrations
is Zn>Cu>Cr>Pb>Ni>As>Cd, found approximately by other investigators for
sludge produced in various WWTPs from China [11,18,19], Rio de Janeiro, Brasil
[20], Salamanca, Spain [21], and Thessaloniki, Greece [5].
On average, comparison of sites for the elemental and Cr(VI) contents leads
to the following sequence PA>DR>KA for most metals.
For all the investigated metals and chromium species (VI and III) the
concentrations do not exceed the values stipulated in European [12,13] and Greek
[14] legislation in any analyzed sludge sample, being much lower than the
proposed limits. Cr(III) concentrations were calculated by the difference between
total chromium and Cr(VI) concentrations, being in the range 13.5–350.7, with a
mean of 115.7±101.7 mg/kg dry matter. The current results suggest that the sludge
produced in the three WWTPs from Northern Greece might be used in agriculture,
under the assumption that the natural soilborne heavy metal concentrations do not
limit the farmland application.
4. CONCLUSIONS
The determined data give information concerning the toxic element content
of anaerobic fermented and dewatered sludge resulting from three municipal
WWTPs in Northern Greece, in order to evaluate sludge suitability for land
application. It is important to note that the results of this work are important
statistical elements for the quality of the produced sludge in Greece and future
comparisons. Despite the inherent annual/seasonal variability due to various
effluents discharged into sewers, the total concentrations of the metals Cd, Cr, Cu,
Hg, Ni, Pb, Zn and of hexavalent chromium determined in all the urban sludge
samples are very low according to the Directive EEC and/or Greek legislation and
thus the sewage sludge meets the requirements for agricultural use. However, a
strict monitoring of toxic elements (including As) in the soil in the application area
is required before the sludge disposal, due to the fact that the Northern Greece
region is characterized by elevated metal concentrations in soil and groundwater.
244
Thomas Spanos, Antoaneta Ene, Irina B. Karadjova
8
Acknowledgements. The authors wish to thank the chemical laboratories at TEI Kavala, Greece
and “St. Kliment Ohridski” University of Sofia, Bulgaria, where the measurements were made.
Special thanks are extended to Municipal Authorities of Kavala, Drama and Palio for providing the
sewage sludge samples. Assoc. Prof. dr. Antoaneta Ene acknowledges the financial support of
Lifelong Learning Programme Erasmus (2010-2013) at “Dunarea de Jos” University of Galati, Romania.
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