chemical engineering research and design 9 2 ( 2 0 1 4 ) 752–757
Contents lists available at ScienceDirect
Chemical Engineering Research and Design
journa l h om epage: www.elsev ier .com/ locate /cherd
Sewage sludge digestion at increasedmicropollutant content
Agata Rosinska ∗, Lidia DabrowskaFaculty of Environmental Engineering and Biotechnology, Department of Chemistry, Water and Sewage Technology,Czestochowa University of Technology, Dabrowskiego 69, 42-200 Czestochowa, Poland
a b s t r a c t
Research was conducted, which aim was to evaluate quantitative and qualitative PCB changes in sewage sludge
during mesophilic digestion, and the influence of those compound concentration above acceptable level (1.9 mg kg−1
of sludge dry matter), also nickel and cadmium ions, on mesophilic digestion process. Before and after the digestion,
concentration of seven PCB congeners, Ni and Cd in particular chemical fractions of the sludge was analyzed.
During mesophilic digestion it was stated that increased PCB and heavy metal ions content in sewage sludge,
did not disturb correct progress of the process. Last days of digestion were important for intensive degradation of
higher chlorinated PCB to congeners containing less chlorine atoms in particle. In liquid of sludge with increased PCB
content, up until the 14th day of mesophilic digestion process, gradual increase in amount and sorts of indicator PCB
congeners was observed. First LCB were released into sludge liquid. Total amount of PCB in sludge after digestion was
decreased significantly, which indicate that biological degradation of those compounds under anaerobic conditions,
and/or biosorption of those compounds, proceeded. It has been proved that the digestion of sludge excessively loaded
with nickel ions can lead to an adverse accumulation of this metal in the exchangeable and carbonate fraction of
Keywords: Digestion; Sewage sludge; PCB; High concentration; Heavy metals; Biogas
sewage sludge with higher PCB concentration, rate and veloc-
1. Introduction
One of the methods of sewage sludge treatment is diges-tion, which result in decomposition of complex large moleculeorganic compounds, leading to sludge stabilization. Hydroly-sis is considered to be the stage limiting velocity of sludgedigestion (Khalid et al., 2011). Acceleration of sludge organiccompound transformation into soluble forms, results in signif-icant improvement of efficiency of following digestion stagesprogress. Increase in process temperature can act as an accel-erator. Therefore apart from mesophilic sludge digestion,thermophilic-mesophilic systems are also used (Oles et al.,1997). Optimal temperature range for mesophilic digestion isbetween 30 and 38 ◦C, whereas for thermophilic digestion 49and 57 ◦C.
Inhibiting substances are a threat for correct digestionprogress, they can be either incorporated in the biomass,
or emerge as products of substrate decomposition. Those
∗ Corresponding author. Tel.: +48 343250364; fax: +48 343250496.E-mail addresses: [email protected] (A. Rosinska), dabrowskaReceived 19 August 2013; Received in revised form 29 November 2013
include: ammonia, sulfide, heavy metal ions, chlorophenols,nitrobenzenes, halogenated aliphatics, LCFAs (Chen et al.,2008), and PCBs (Chang et al., 1999). Under certain condi-tions, PCBs are capable of abiotic and biotic transitions. Thestudies have shown that, under suitable conditions, PCBs canbe biodegraded with participation of microorganisms underaerobic and anaerobic conditions. During above mentionedprocesses, changes of PCBs result in decrease of their contentin sludge. According to earlier reported data, under anaerobicconditions, the reductive dechlorination of higher chlorinatedPCB (HCB) occurs, forming lower chlorinated PCB (LCB). Duringanaerobic degradation, microorganisms cause selective elim-ination of substituted chlorine, especially in meta- and para-positions. As a result of the process, a decline in toxicity ofPCB is obtained (Borja et al., 2005; Fava et al., 2003; Masteret al., 2002; Wu et al., 1996). In literature it is reported that in
@is.pcz.czest.pl (L. Dabrowska).; Accepted 16 December 2013
ity of dechlorination were low. Data was regarding sludge with
neers. Published by Elsevier B.V. All rights reserved.
chemical engineering research and design 9 2 ( 2 0 1 4 ) 752–757 753
P
fdobmctfetl3
iIesamwamoiDwd
2
2
Fa3miapsctd
yiwf
mS3wwmocScqo
CB concentration of: 1.5 and 10 mg L−1 (Chang et al., 1999).Metal toxicity depends on concentration and chemical
orm, which can change under anaerobic conditions duringigestion, and also on pH, Eh, temperature, and presence ofther cations. Soluble metal compounds are easier absorbedy organisms, therefore they are more toxic than insolubleetal salts. Concentration of heavy metals in sludge, which
ause decrease in efficiency or complete inhibition of diges-ion process, is defined by many authors in very wide rangerom a few up to several hundred mg in L. Amongst oth-rs, Malina and Pohland (1992), Appels et al. (2008), statedhat inhibiting or toxic concentration of zinc, copper, nickel,ead, and cadmium in sludge liquid amounted respectively to:–100; 5–100; 30–200; 5–30; 70 mg L−1.
A subject which is not yet well known, is the influence ofncreased PCB content on sewage sludge digestion progress.n this studies, research was conducted, which aim was tovaluate quantitative and qualitative PCB changes in sewageludge, and the influence of those compound concentrationbove acceptable level, also nickel and cadmium ions, onesophilic digestion process. Seven chosen PCB congenersere analyzed, marked with codes 28, 52, 101, 118, 138, 153,nd 180, identification of which is recommended by EPA,oreover The Sludge Directive introduces the allowable value
f total concentration of seven above mentioned congenersn sewage sludge intended for agricultural purposes (Workingocument, 2000). Metal speciation analysis was performed,hich revealed some changes in chemical forms of the metalsuring sludge stabilization process.
. Materials and methods
.1. Experimental setup
or the experiment the following types of sludge was used: mixture of primary and excess sludge (with ratio of ca.:1), and digesting sludge. Sewage sludge was sampled from aunicipal, mechanical–biological wastewater treatment plant
n Czestochowa (Poland). The treated wastewater containspproximately 20 vol% of industrial wastewater. In the WWTP,urification process is performed with the use of activatedludge method, which covers nitrification, denitrification, andhemical/biological removal of phosphorus. Mesophilic diges-ion of sludge from settling tank is carried out in closedigesters (temperature 37 ± 2 ◦C).
The first stage of the performed experiment was the hydrol-sis of the mixture of primary and excess sludge (P + E) at 55 ◦Cn closed, 1000 mL bioreactors for 4 days. The bioreactors filled
ith 650 mL of the sludge mixture were placed in a thermostat,acilitating proper temperature.
Hydrolyzed sludge mixture (P + E) was inoculated withesophilic digesting sludge (D) in volumetric ratio of 1:2.
ludge mixture (P + E + D) was obtained, which was placed into0 bioreactors. Next, the sludge in 10 bioreactors was enrichedith standard mixture PCB-mix3, which consists of congenersith codes: 28, 52, 101, 118, 138, 153, and 180. Sludge samplesarked with symbol (P + E + D)p were obtained. The amount
f added standard was selected in a way providing total PCBoncentration to be higher than acceptable level stipulated inludge Directive, for sludge designated for agricultural appli-ations (Aparcio et al., 2009; Working Document, 2000). The
uantity of introduced PCB standard amounted to 1.9 mg kg−1
f sludge dry matter. For further 10 reactors, marked as
(P + E + D)m, 1 mL of solution containing nickel and cadmiumions was added, which was prepared out of salts: NiCl2·6H2Oand Cd(NO)3·4H2O. Concentration of introduced nickel andcadmium ions amounted respectively to 20 and 5 mg L−1 ofsludge. Sludge in remaining 10 bioreactors was used as a ref-erence setup (P + E + D)c. Sludge (P + E + D)p, (P + E + D)m, and(P + E + D)c, were placed into thermostat at temperature of37 ◦C ± 1 ◦C, and mesophilic digestion process was carried outfor 21 days.
Before and during the mesophilic digestion at a specifiedtime intervals the following parameters of the sludge weredetermined: hydration, total solids, volatile solids. Alkalin-ity, pH, volatile fatty acids (VFA), total organic carbon (TOC),and heavy metals (Ni, Cd) were determined for the sludgeliquid separated in a centrifuge (rotary speed – 6000 rpm, dura-tion – 10 min). Three samples were measured, and commonlyaccepted methodology was used (APHA, 1998).
Analysis of PCBs was also carried out for sewage sludgeand sludge liquid before, during and after the process ofmesophilic digestion. Before and after the digestion, concen-tration of Ni and Cd in the particular chemical fractions of thesludge was analyzed.
2.2. Biogas
During digestion, pressure and biogas composition were mon-itored. Pressure measurement was conducted every 24 h. Thecomposition of biogas (content of CH4 and CO2) was measuredevery 72 h with the use of GC (model Agilent GC 6890N madeby Agilent Technologies).
2.3. PCB analysis
Methodology of sample preparation was described elsewhere(Dabrowska and Rosinska, 2012). The extract was compactedin vacuum and then subject to qualitative and quantitativeanalysis by means of gas CGC chromatography. Separationwas conducted on a DB-5 column (30 m × 32 mm × 1 �m). Aquadrupole mass spectrometer MS 800, working in a selectivemode of ion monitoring was used for detection. Determinationwas performed for each sample and each four injection of theobtained extract. In order to evaluate the procedure for deter-mination of PCBs in sewage sludge, the recovery values werealso determined. Sewage sludge was enriched with Ehren-storfer’s PCB MIX3 at the concentration of 10 ng �L−1. Thenbiphenyls determination was carried out using the abovemen-tioned procedure. The obtained values of recovery ranged from65% to 93% and were within the range typical of the references(Berset and Holzer, 1996). The precision of the determinationwas expressed with the values of standard deviation.
2.4. Fractionation of Ni and Cd in sewage sludge
Sequential extraction was carried out according to the BCRprocedure in order to quantify the occurrence forms of Ni andCd in the sludge. The preparation of necessary reagents andthe extraction procedure were carried out according to (Rauretet al., 2000).
In first stage, in order to extract exchangeable metals andmetals bound to carbonates, 0.11 M CH3COOH was used; insecond stage (extraction of metals bound to hydrated ironand manganese oxides) 0.5 M NH2OH HCl (pH 2) was used.
In next stage, in order to extract metals bound to organicmatter and sulfides, 8.8 M H2O2 (pH 2–3) and 1 M CH3COONH4
754 chemical engineering research and design 9 2 ( 2 0 1 4 ) 752–757
Table 1 – Selected physicochemical indexes of sludge (P + E + D)c, (P + E + D)p and (P + E + D)m before and during mesophilicdigestion.
Indexes Beforedigestion
Sewage sludge(P + E + D)c
Sewage sludge(P + E + D)p
Sewage sludge(P + E + D)m
7th d 14th d 21st d 7th d 14th d 21st d 7th d 14th d 21st d
Fig. 1 – Changes of PCB congeners concentration in sludge
TOC (mg L ) 1460 580 384
were used; in last stage (residual fraction) – HNO3 (65%) andHCl (37%) were used. The samples were stored at 4 ◦C for fur-ther analyses of nickel and cadmium with the use of atomicabsorption spectrometry method (spectrometer novAA 400,Analytik Jena).
3. Results and discussion
3.1. Physicochemical properties of sewage sludge
Mesophilic digestion process preceded by 4-day thermophilichydrolysis was conducted under substrate loading rate typ-ical for conventional digestion chambers, which amountedto 1.08 kg m−3 d−1. Share of organic substances in sludgeamounted to ca. 64%.
During conducted digestion processes, alkalinity ofsludge liquid was increasing gradually within the rangeof 2630–3450 mgCaCO3 L−1, while VFA concentration wasdecreasing from 1180 down to 112 mgCH3COOH L−1 (Table 1),and they did not exceed the extreme values (respectively1000–5000 mgCaCO3 L−1, and above 2000 mgCH3COOH L−1),which are recommended for correct proceeding of digestionprocess (biogas production) (Malina and Pohland, 1992). TOCcontent in liquids initially increased up to ca. 1520 mg L−1 onthe 3rd day of digestion, and then decreased down to 690, 524,and 580 mg L−1 on the 7th day of process, accordingly in liquidsof sludge (P + E + D)p, (P + E + D)m, (P + E + D)c, which indicatesthe consumption of easily accessible organic substrate. TOCcontent on the 21st day of stabilization amounted to 405, 332,and 358 mg L−1 respectively. After the completion of diges-tion, the sludge remained alkaline properties (pH 7.70–7.77).Biodegradation of organic compounds in sludge proceededcorrectly, which was proved by decrease in dry residue andloss on ignition. Research results showed decrease in organicsubstance content in sludge with higher PCB content, and ref-erence sludge, at level of 41–42% (41.3 ± 0.4%, and 41.4 ± 0.2%,respectively), whereas in sludge with higher nickel and cad-mium amount 38.7 ± 0.2%.
3.2. Biogas
Digestion process of sludge: enriched with PCB (P + E + D)p,addition of nickel and cadmium ions (P + E + D)m, and refer-ence (P + E + D)c, proceeded the most intensive during initial 4days, when daily biogas production amounted to respectively0.68–0.83, 0.72–0.91, and 0.77–0.89 L/L of sludge. In total, dur-ing 21-day digestion, 5.46 ± 0.11; 5.29 ± 0.14, and 5.38 ± 0.19 L
of gas out of 1 L of stabilized sludge was obtained. Unit pro-duction of biogas during conducted digestion amounted to
liquid (P + E + D)p.
respectively 0.90; 0.93; 0.88 L out of 1 g of removed organic drymatter. In reference reactors, the amount of methane in bio-gas (excluding the 1st day) maintained in the range of 60–66%,whereas in reactors with sludge (P + E + D)p 58–65%, and withsludge (P + E + D)m 61–65%.
3.3. PCB and metal ions in sludge liquid
Qualitative and quantitative changes of PCB during mesophilicdigestion in sludge (P + E + D)p liquid are presented in Fig. 1.It was demonstrated that in sludge liquid of reference setup,the PCB concentration was below detection level. In liquid ofsludge with increased PCB content, up until the 14th day ofmesophilic digestion process, gradual increase in amount andsorts of indicator PCB congeners was observed. First LCB werereleased into sludge liquid. On the 7th day of the process, inliquid there were detected LCB with codes 28 and 52, and PCB101 which represents HCB, while no presence of other HCBwas determined. Total concentration of three PCB amountedto 11.5 ng L−1, including 63% of LCB. On the 9th day of the pro-cess, PCB 28, 52, and 101 concentration increased, moreoveradditionally PCB 118 and PCB 138 were released from sludgeinto the liquid. As a result, total PCB concentration in sludgeliquid increased by 78%.
On the 14th day, further growth in PCB congener concen-tration in sludge liquid, including HCB, was demonstrated.After the completion of mesophilic digestion (21st day), pres-ence of two LCB congeners (PCB 28 and 52), and one HCBwith code 101, was showed, concentration of which decreased(with respect to data obtained on the 14th day of the process).
Total concentration of indicator congeners fell down by 80%with respect to results obtained on the 14th day. Changes of
chemical engineering research and design 9 2 ( 2 0 1 4 ) 752–757 755
Table 2 – Concentration of heavy metal ions (mg L−1) in sludge liquid (P + E + D)c and (P + E + D)m before and duringmesophilic digestion.
Fig. 2 – Changes in LCB and HCB content in sewage sludgeduring mesophilic digestion at increased concentration ofPCB: a) sludge (P + E + D)c, b) sludge (P + E + D)p.
Cadmium 4.86 0.11 0.08
otal indicator PCB, with regard to data prior to the process,ere not statistically significant. Comparing the concentra-
ion of leached congeners before and after leaching, it wasound that analyzed PCB are leached only at a low level – thisroves their strong adsorption on solids, which is especiallyurable when the investigated matrix is rich in organic sub-tances (Dercová et al., 1999; Sakai et al., 2000; Triska et al.,004).
During mesophilic digestion of sludge enriched withetals, nickel and cadmium ion concentration in liquidaintained at low level (Table 2). Just after the 1st day of
igestion, it fell down from 19.8 mg L−1 down to 0.4 mg L−1
n case of nickel, and from 4.9 mg L−1 down to 0.1 mg L−1
n case of cadmium, and it was decreasing until the 10thay. During further progress of the process, concentrationaintained at almost the same level, in case of nickel
t amounted to 0.14–0.11 mg L−1, cadmium 0.04–0.03 mg L−1.ntroduced to the sludge additional amount of nickel andadmium ions, immediately after the start of stabilization,recipitated and remained bounded until the end of diges-ion process. This finding is consistent with information aboutossibility of heavy metal ion precipitation in form of sul-des.
Also in case of mesophilic digestion of sludge, which con-tituted reference samples, during the whole time of processroceedings, nickel and cadmium concentration varied in theange of 0.09–0.06 and 0.03–0.02 mg L−1, respectively.
.4. PCB in sewage sludge
hanges of PCB in sewage sludge during digestion waseviewed with respect to transition of HCB into LCB undernaerobic conditions. LCB and HCB content in sludgeP + E + D)c and (P + E + D)p during mesophilic digestion is pre-ented Fig. 2. In reference sludge, presence of all indicator PCBongeners was demonstrated, in which HCB were dominantFig. 2a). The highest concentration was found for HCB withode 153 (Table 3). LCB constituted minor share, ca. 8%. LCBontent in sludge prior to digestion process was over eightfoldower than HCB content.
Changes in LCB and HCB in control sludge and in sludgeith increased PCB content, during digestion process were
tatistically significant, similar trends in changes of LCB andCB concentration were detected for both sludge. On the 7thay of the process, in examined sludge LCB share was still
ower than HCB. On the 9th day of the process further decreasen LCB and HCB content in sludge was demonstrated. Thehare of those compounds decreased significantly in sludgeP + E + D)p, as LCB and HCB content was lowered by 67% and3% respectively, in comparison to data prior to the process.n reference sludge, LCB and HCB content fell down by 40%
nd 51%, respectively. On the 21st day of the process in sludgeith increased PCB content, 75% decrease in LCB content was
demonstrated, whereas in reference sludge LCB content wascomparable to its level prior to digestion process. HCB contentin both sludge (P + E + D)c, and sludge (P + E + D)p fell down by64 and 86% respectively, with regard to data prior to the pro-cess. In sludge with increased PCB content, loss of PCB, higherthan in reference setup, was obtained (Fig. 2b). Chang et al.(1999) demonstrated that in sewage sludge with higher PCBconcentration (1.5 and 10 mg L−1), rate and velocity of dechlo-rination were low. This data suggest that in sludge with higherPCB content, degradation rate of those compounds shouldbe lower. Literature data regard sludge with 3- to 23-timehigher PCB concentration in comparison to examined samples(Chang et al., 1999). This information indicate that the rate andvelocity of PCB dechlorination can decrease at suitably highconcentration of those compounds in sewage sludge.
Last days of digestion were a “breakthrough” for intensive
degradation of HCB to congeners containing less chlo-rine atoms in particle, which is confirmed by literature
756 chemical engineering research and design 9 2 ( 2 0 1 4 ) 752–757
Table 3 – Concentration of PCBs (�g kg−1 d.m.) in sludge (P + E + D)c and (P + E + D)p before and during mesophilic digestion(mean ± standard deviation, n = 5).
Before digestion During digestion
Congeners 7th d 14th d 21st d(P + E + D)c (P + E + D)p (P + E + D)c (P + E + D)p (P + E + D)c (P + E + D)p (P + E + D)c (P + E + D)p
information, that under anaerobic conditions PCB biodegra-dation proceed by means of dechlorination of PCB containinghigher amount of chlorine in particle (Field and Sierra-Alvarez,2008; Wiegel and Wu, 2000). This fact was observed partic-ularly for sludge with increased PCB content, where 4- and3-fold increase in content of trichlorobiphenyl with code 28,and tetrachlorobiphenyl 52, with respect to the 9th day of theprocess, proceeded. Reductive dechlorination of HCB to LCB isadvantageous process, due to the fact that PCB containing lesschlorine atoms in particle are less toxic (Wiegel and Wu, 2000;Wu et al., 1996).
Based on conducted research, it was stated that sub-stitution of chlorine atoms in ortho position slowed downbiological degradation. Examples can be given: 2 PCB con-geners 138 and 153, in which chlorine atoms are in orthoposition but also in meta and para. After the anaerobicdigestion process of sludge (P + E + D)c concentration of thosecongeners was ca. 2,5-times lower, whereas for PCB 180, inwhich particle 3 chlorine atoms are substituted in meta posi-tion, above 4-fold decrease in content was observed. Obtainedresults confirmed Wiegel and Wu (2000) studies. The authorsdemonstrated that chlorine atoms substituted in meta andpara positions better and faster undergo biodegradation, whilesubstituted in ortho position inhibit biological decompositionof PCB.
3.5. Nickel and cadmium in sewage sludge
The highest nickel content in sludge prior to diges-tion occurred in organic-sulfide fraction (43%), and in
Table 4 – Chemical fractionation of heavy metals in sludge befodeviation, n = 3).
Metals Fraction Before digestion
mg kg−1 d.m. %
(P + E + D)c
Ni F1 88.2 ± 1.6 37.2
F2 37.8 ± 2.1 15.9
F3 101.0 ± 0.8 42.6
F4 10.3 ± 0.4 4.3 ∑237.3 100
Cd F1 0.34 ± 0.11 6.5
F2 0.57 ± 0.08 11.0
F3 3.45 ± 0.20 66.5
F4 0.83 ± 0.15 16.0 ∑5.19 100
Fraction: F1 – exchangeable/carbonates; F2 – Fe/Mn oxides; F3 – organic m
31.2 847.9 32.3 328.2
exchangeable-carbonate fraction (37%). Mesophilic digestionprocess conducted in reference set-up, caused mainly increasein this metal content in organic-sulfide fraction of the sludge(Table 4). Minor enrichment occurred also for fractions:exchangeable, carbonate, and residue. In case of digestion ofsludge with nickel ion addition, enrichment occurred for allof the fractions, mainly exchangeable-carbonate, and organic-sulfide. As the most mobile, and therefore easily migrating intosoil solution and up-taken by plants, are considered exchange-able and carbonate-bound forms of metals. Digestion of sludgewith additional nickel amount, confirmed the possibility ofenrichment of exchangeable-carbonate sludge fraction withthis metal. Share of this mobile fraction in nickel boundingduring mesophilic digestion process, was stated already inpreviously conducted studies (Fuentes et al., 2008; Lasheenand Ammar, 2009; Walter et al., 2006). Presence of signifi-cant amount of nickel in mobile fraction indicates that, dueto changes in external conditions, such as equilibrium insorption-desorption system, or decrease in pH value, releaseof nickel into water-soil environment can occur.
In case of cadmium, increase in its content after referencesludge mesophilic digestion was stated mainly in organic-sulfide fraction (Table 4). Share of this fraction in cadmiumbounding amounted to 70%. While after digestion of sludgewith additional amount of cadmium, enrichment occurredfor organic-sulfide fraction, and iron and manganese oxidefraction. Cadmium distribution in those fractions amountedto respectively 72% and 22% of total amount of this metal.
Metals bounded to iron and manganese oxide fraction, and toorganic matter, are temporarily immobilized. Oxide fraction is
re and after mesophilic digestion (mean ± standard
After digestion
mg kg−1 d.m. % mg kg−1 d.m. %(P + E + D)c (P + E + D)m
chemical engineering research and design 9 2 ( 2 0 1 4 ) 752–757 757
stps
4
-
-
-
-
A
TP
R
A
A
A
B
B
ensitive to changes in redox potential, whereas metals boundo organic matter are released in substrate mineralizationrocess. No enrichment of mobile exchangeable-carbonateludge fraction with cadmium was observed.
. Conclusions
During mesophilic digestion of sludge: thermophilichydrolyzed, unenriched, and enriched with PCB or nickeland cadmium ions, comparable amount of biogas (ca. 0.9 Lfrom 1 g of removed sludge organic dry matter), and organicsubstance decomposition rate ca. 41–39%, was obtained.
During mesophilic digestion it was stated that increased upto 1.9 mg kg−1 of dry matter, PCB content in sewage sludge,did not disturb correct progress of the process. Total amountof PCB in sludge after digestion was decreased significantly,which indicate that biological degradation of those com-pounds under anaerobic conditions, and/or biosorption ofthose compounds, proceeded. The most intensive processof PCB decomposition took place in last days of the process.With increased PCB content in sewage sludge, mesophilicdigestion process can be used for degradation of those com-pounds.
Introduction of additional amount of nickel ions (ca.20 mg L−1 of sludge), and cadmium (ca. 5 mg L−1 of sludge),did not disturb the proceeding of sewage sludge mesophilicdigestion. Low concentration of metal ions in liquid wasfound during the digestion: nickel lower than 0.4 mg L−1,cadmium lower then 0.1 mg L−1. Metals were bound insludge.
During digestion of sludge excessively loaded with nickelions, adverse accumulation of this metal in exchangeableand carbonate fraction can occur, whereas loading withcadmium ions can lead to its accumulation in iron and man-ganese oxide fraction of stabilized sludge.
cknowledgement
his work was carried out within the research project No. BS-B-402/301/2011.
eferences
merican Public Health Association, 1998. Standard Methods forthe Examination of Water and Wastewater, 20th ed. AmericanPublic Health Association, Washington, DC.
ppels, L., Baeyens, J., Degreve, J., Dewil, R., 2008. Principles andpotential of the anaerobic digestion of waste-activated sludge.Prog. Energy Combust. 34, 755–781.
parcio, I., Santos, J.I., Alonso, E., 2009. Limitation of theconcentration of pollutants in sewage sludge for agriculturalpurposes: a case study in South Spain. Waste Manag. 29,1747–1753.
erset, J.D., Holzer, R., 1996. Determination of coplanar and orthosubstituted PCBs in some sewage sludges of Switzerlandusing HRGC/ECD and HRGC/MSD. Chemosphere 32,2317–2333.
Chen, Y., Cheng, J.J., Creamer, K., 2008. Inhibition of anaerobicdigestion process: a review. Bioresour. Technol. 99, 4044–4064.
Dabrowska, L., Rosinska, A., 2012. Change of PCBs and forms ofheavy metals in sewage sludge during thermophilic anaerobicdigestion. Chemosphere 88, 168–173.
Dercová, K., Vrana, B., Baláz, S., 1999. A kinetic distribution modelof evaporation, biosorption and biodegradation ofpolychlorinated biphenyls (PCBs) in the suspension ofPseudomonas stutzeri. Chemosphere 38 (6), 1391–1400.
Fava, F., Zanaroli, G., Young, L.Y., 2003. Microbial reductivedechlorination of pre-existing PCBs and spiked2,3,4,5,6-pentachlorobiphenyl in anaerobic slurries of acontaminated sediment of Venice Lagoon (Italy). FEMSMicrobiol. Ecol. 44, 309–318.
Field, J.A., Sierra-Alvarez, R., 2008. Microbial transformation anddegradation of polychlorinated biphenyls. Environ. Pollut. 155,1–12.
Fuentes, A., Llorens, M., Saez, J., Aguilar, M.I., Ortuno, J.F.,Meseguer, V.F., 2008. Comparative study of six differentsludges by sequential speciation of heavy metals. Bioresour.Technol. 99, 517–525.
Khalid, A., Arshad, M., Anjum, M., Mahmood, T., Dawson, L., 2011.The anaerobic digestion of solid organic waste. Waste Manag.31, 1737–1744.
Lasheen, M.R., Ammar, N.S., 2009. Assessment of metalsspeciation in sewage sludge and stabilized sludge fromdifferent wastewater treatment plants, Greater Cairo, Egypt. J.Hazard. Mater. 164, 740–749.
Malina Jr., J.F., Pohland, F.G., 1992. Design of Anaerobic Processesfor the Treatment of Industrial and Municipal Wastes, vol. 7.Technomic Publishing Co., Lancaster.
Oles, J., Dichtl, N., Niehoff, H., 1997. Full scale experience of twostage thermophilic/mesophilic sludge digestion. Water Sci.Technol. 36, 449–456.
Rauret, G., Lopez-Sanchez, J.F., Sahuquillo, A., Barahona, E.,Lachica, M., Ure, A.M., Davidson, C.M., Gomez, A., et al., 2000.Application of a modified BCR sequential extracion(three-step) procedure for the determination of extractabletrace metal contents in a sewage sludge amended soilreference material (CRM 483), complemented by a three-yearstability study of acetic acid and EDTA extractable metalcontent. J. Environ. Monit. 2, 228–233.
Sakai, S., Urano, S., Takatsuki, H., 2000. Leaching behavior of PCBsand PCDDs/DFs from some wastematerials. Waste Manag. 20,241–247.
Triska, J., Kuncová, G., Macková, M., Nováková, H., Paasivirta, J.,Lahtiperä, M., Vrchotová, N., 2004. Isolation and identificationof intermediates from biodegradation of low chlorinatedbiphenyls. Chemosphere 54, 725–733.
Walter, I., Martinez, F., Cala, V., 2006. Heavy metal speciation andphytotoxic effects of three representative sewage sludges foragricultural uses. Environ. Pollut. 139, 507–514.
Working Document on Sludge, 2000. European CommissionEnvironment ENV.E.3/LM, 3rd draft, Brussels.
Wu, Q., Bedard, D.L., Wiegiel, J., 1996. Effect of incubationtemperature on the route of microbial reductivedechlorination of 2,3,4,6-tetrachlorobiphenyl in
