speciation of heavy metals in sewage sludge after mesophilic and thermophilic anaerobic digestion

Chemical Papers 66 (6) 598–606 (2012)DOI: 10.2478/s11696-011-0128-9

ORIGINAL PAPER

Speciation of heavy metals in sewage sludge after mesophilicand thermophilic anaerobic digestion‡

Lidia Dabrowska*

Department of Chemistry, Water, and Wastewater Technology, Faculty of Environmental Protection and Engineering,

Czestochowa University of Technology, Dabrowskiego 69, 42200 Czestochowa, Poland

Received 13 June 2011; Revised 21 October 2011; Accepted 9 November 2011

Two types of sewage sludge anaerobic digestion were carried out: mesophilic and thermophilic.Metal speciation analysis was performed revealing some changes in the chemical form of the metalsduring the stabilization process of sludge. After both methane fermentation processes, a comparablelevel of organic matter distribution was obtained (≈ 40 %). The amount of produced methane duringthermophilic and mesophilic digestion was 560 mL of CH4 and 580 mL of CH4 from 1 g of removedorganic matter, respectively. Low concentration of heavy metal ions in the liquid phase of sludgewas observed. Metal ions precipitated and remained bound throughout the stabilization process. Noaccumulation of heavy metals in the mobile fractions of sludge (exchangeable and carbonate) wasobserved for either digestion process. The highest increase of zinc, copper, nickel, cadmium, andchromium concentration was observed in the organic–sulfide fraction, whereas the highest increaseof lead was found in the residual fraction.c© 2011 Institute of Chemistry, Slovak Academy of Sciences

Keywords: heavy metals, sewage sludge, mesophilic digestion, thermophilic digestion

Introduction

Methane anaerobic digestion is used to stabilizesewage sludge in large and medium wastewater treat-ment plants. It is mainly carried out under mesophilicconditions. After the digestion, inorganic suspendedmatter and hardly decomposed, similar to humus, or-ganic compounds remain in the sewage sludge. Modi-fication of the sewage sludge digestion process focuseson several areas, e.g.: searching for more efficient bio-gas production methods, improvement of the degreeof organic substances decomposition, but also short-ening of the time of the process duration and hygien-ization of sludge without the use of chemicals. Theseaims can be achieved by, inter alia, thermophilic di-gestion or the application of a two-stage process ofthermophilic–mesophilic digestion (Song et al., 2004;Carballa et al., 2009; Rubio-Loza & Noyola, 2010).

During anaerobic digestion, the decomposition oforganic substances results in an increase of heavymetals’ concentrations in the dry matter of stabilizedsewage sludge. Zinc was found to be the main heavymetal present in the stabilized and dehydrated sewagesludge. Relatively high concentration of copper, andin some cases of chromium and lead, were also found(Alonso et al., 2006; Dai et al., 2007; Pathak et al.,2009a; Mosquera-Losada et al., 2010; Werle & Wilk,2010). In general, the concentration of heavy metalsin sludge increases as follows: Cd, Ni, Pb, Cr, Cu, Zn.Heavy metals are usually transferred from sewage

sludge to the food chain via crops and plants grownon the land fertilized with sewage sludge. Despite thefact that traces of some heavy metals are necessary forproper metabolism of plants and animals, their higherconcentrations can be toxic for plants, animals, andendangering human health (da Silva et al., 2005; Dai

*Corresponding author, e-mail: [email protected]‡Presented at 38th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 23–27May 2011.

L. Dabrowska/Chemical Papers 66 (6) 598–606 (2012) 599

et al., 2006; Giller et al., 2009; Nagajyoti et al., 2010).The total amount of heavy metals in sewage sludge

shows possible risk, however, it is not fully indicativeof the consequences for the ecosystem. The availabilityof heavy metals for organisms is a more informativeparameter. It is dependent on the mobility of heavymetals which is determined based on the chemicalforms of their occurrence. In order to determine theforms of heavy metals in the sewage sludge, the speci-ation analysis is performed. The analysis is based onthe sequential extraction of metals with increasinglyaggressive solvents. Reagents for each step are chosento extract metal groups with known properties. Thefive step extraction proposed by Tessier et al. (1979) iswidely appreciated. The Tessier procedure was mod-ified many times and the modifications were mainlyrelated to the applied reagents and extraction condi-tions. As a result of the studies carried out, a shorter,three-stage, extraction was accepted by the Standards,Measurements, and Testing Programme of the Euro-pean Commission, formerly the Community Bureau ofReference under the name of BCR procedure (Gleyzeset al., 2002; Mossop & Davidson, 2003). The BCRmethod is faster, simpler, and validated for routinelaboratory unlike the Tessier scheme (Frentiu et al.,2009). It is currently the most spread and commonlyused extraction method for metals from sewage sludge(Fuentes et al., 2008; Chen et al., 2008; Hanay et al.,2008; Jamali et al., 2009; Yuan et al., 2011). It dividesmetals into the following groups: exchangeable, asso-ciated with carbonates, associated with hydrated ironoxides and manganese oxides, associated with organicmatter, and metals that can be found in the residualfraction. Metals that can be found in the two first frac-tions (exchangeable and carbonate) are believed to bemobile. The release of these metals can occur withthe change in pH or in the ionic composition of liq-uid. However, also metals bound to hydrated forms ofiron and manganese oxides and to organic matter areavailable. The fraction of iron and manganese oxidesis sensitive to redox changes, whereas metals bound toorganic matter are released during substrate mineral-ization. Heavy metals considered to be almost entirelyinaccessible are bound to residual fraction (residue).These metals are present in a crystallographic struc-ture of minerals in dry mater of sewage sludge.So far, there has been no research done on the

determination of chemical forms of heavy metals insewage sludge during thermophilic methane digestion.Research on the subject is therefore considered to bejustified. Changes in the form of heavy metals occur-rence in sludge after both mesophilic and thermophilicdigestion processes were determined. The main focuswas put on the possible increase of the content ofheavy metals in the stable fractions.The speciation analysis of heavy metals in bio-

chemically stabilized sewage sludge provides informa-tion significant for the determination of the rate at

which heavy metals pass into the soil solution andalso, as a consequence, of the rate of their uptake byplants. This information is especially important whenconsidering agricultural usage of sludge.

Experimental

Materials

Sewage sludge samples were taken from the munici-pal mechanical–biological wastewater treatment plantin Czestochowa (Silesia Voivodeship, Poland). The in-flow contains industrial wastewater (ϕr ≈ 1 : 5) Thisplant has the capacity of 45000 m3 per day. The pu-rification process was performed using the activatedsludge method, which takes into account nitrifica-tion, denitrification, and chemical/biological removalof phosphorus. Mesophilic digestion of the sludge fromthe settlement tanks (both raw and excess) was carriedout in closed fermentation chambers; sludge retentiontime in the digester was equal to 20 days. Stabilizedsludge was mechanically dewatered on tape presses us-ing a cationic polyelectrolyte.For the study, two randomly chosen single sludge

samples were collected: from primary clarifier (rawsludge) and from closed fermentation chamber (di-gested sludge). Before the sludge was introduced tobioreactors, it was first sieved through a 3 mm sieveto ensure uniformity of the studied material.

Anaerobic digestion

Two types of sewage sludge digestion were carriedout: mesophilic and thermophilic. For the growth ofthermophilic anaerobic microorganisms, glass bioreac-tors (a model GLS 80, WITKO Sp. z o.o., Poland; vol-ume of 1.0 L) were filled with 0.5 L of sludge collectedfrom the anaerobic digester. Bioreactors were placedin a thermostat at (55 ± 1) ◦C. Starting from the fifthday of the incubation, 1 mL of the growth medium (ad-vised for fermentation microorganisms) was fed every48 h into the culture (International Organization forStandardization, 1995). In addition, 200 g L−1 of glu-cose were added into the medium. Incubation lastedfor 30 days.First, inoculum was mixed with the sludge from the

digester (ϕr = 1 : 10). Next, the inoculum was mixedwith raw sludge (ϕr = 2 : 1). Twenty bioreactors (usedpreviously for the growth of inoculum) were then filledwith 0.5 L of the obtained sludge mixture and tenbioreactors placed in an incubator at (55 ± 1)◦C; tenbioreactors were incubated at (37 ± 1)◦C. Incubationlasted for 14 days.

Analysis procedure

Manometric measurements of the amount of pro-duced biogas were carried out at 24-hour intervals,

600 L. Dabrowska/Chemical Papers 66 (6) 598–606 (2012)

Table 1. Sequential extraction procedure, BCR, per 1 g of dry matter of sludge

Temperature TimeStep Reagent Fraction Nominal target phase(s)

◦C h

1 40 mL 0.11 M CH3COOH 22 16 Exchangeable,acid soluble

Soil solution, exchangeable cations,carbonates

2 40 mL 0.5 M NH2OH · HCl (pH 2) 22 16 Reducible Iron and manganese oxyhydroxides

10 mL 8.8 M H2O2 (pH 2–3) 22 1

3– 85 1

Oxidizable Organic matter and sulfides10 mL 8.8 M H2O2 (pH 2–3) 85 150 mL 1 M CH3COONH4 22 16

4 2 mL HNO3 (65 %) 6 mL HCl (37 %) 120 2 Residual Non-silicate minerals

whereas, the biogas composition (content of CH4 andCO2) was analyzed at 72-hour intervals. Analyses ofthe biogas composition were performed by gas chro-matography (gas chromatograph with a thermal con-ductivity detector, a model Agilent 6890N, AgilentTechnologies, USA). Separation was performed on aHayeSep MS 13X column. During the analyses, thetemperature was increased from 50◦C to 100◦C at20◦C per min, final temperature of 100◦C was achievedin 5 min. Split/splitless injection and standard mix-ture of the following composition: CH4 – 71.2 %, CO2– 28.2 %, CO – 0.36 %, O2 – 0.89 %, H2 – 0.89 %, wereused. The inert gas flow of N2 was 19.2 mL min−1.One reactor was emptied on the 1st, 3rd, 5th, 7th,

10th, and 14th day of both digestion processes. Prop-erties of sludge samples, like hydration, total solids,volatile, and non-volatile solids concentration (gravi-metric method) were determined. Alkalinity (titrationmethod, defined as mg of CaCO3 per L), pH (poten-tiometrically), volatile fatty acids (VFA; distillationmethod, defined as mg of CH3COOH per L), totalorganic carbon (TOC; infrared spectrometry using acarbon analyzer multi N-C, Analytic Jena, Germany),and heavy metals (atomic absorption spectrometry,using a spectrometer novAA 400, Analytic Jena, Ger-many) were determined for the liquid phase separatedin a centrifuge. The determination was performed withthree repetitions using commonly accepted methodol-ogy (Clesceri et al., 1998).Before and after the digestion, overall content of

heavy metals in the sludge was determined (Zn, Cu,Ni, Cd, Pb, Cr), and also their concentration in theparticular chemical fractions of the sludge was ana-lyzed. Preparation of sludge required centrifugation ofthe samples in a laboratory centrifuge (rotary speed6000 min−1, duration 10 min), evaporation in a heatedbath, and drying in a laboratory dryer at 105◦C. Driedsludge was ground using a porcelain mortar and sievedon a 0.4 mm stainless steel sieve. Three different sam-ples of the same sludge were prepared for the analyses.

In order to determine the overall content of heavymetals, (500 ± 1) mg of dry sludge were placed into15 mL glass test tubes. Then, 2 mL of HNO3 (65 %)and 6 mL of HCl (37 %) (aqua regia) were added (Eu-ropean Committee for Standardization, 2000). Thesludge underwent mineralization at 120◦C for twohours.Sequential extraction was carried out according to

the BCR procedure in order to quantify the occur-rence of different forms of heavy metals in the sludge.Each step was carried out in a 100 mL glass test tubeinto which (1000 ± 1) mg of sludge were placed. Ageneral scheme of the procedure is shown in Table 1.Preparation of the necessary reagents and the extrac-tion procedure were carried out according to Rauretet al. (2000).Plastic containers, with all the obtained extracts,

were stored at 4◦C for later analyses. The concentra-tion of heavy metals (Zn, Cu, Ni, Cd, Pb, Cr) wasdetermined by means of the atomic absorption spec-trometry method with four repetitions.The overall concentration of metals in the sludge

(Total) determined after mineralization with aqua re-gia was compared with the sum of metal concentra-tions in the extracted fractions (F1 + F2 + F3 + F4).Recovery in the sequential extraction procedure wascalculated as follows:

Recovery (%) =F1 + F2 + F3 + F4

Total× 100 (1)

Results and discussion

Physicochemical properties of sewage sludge

Raw sludge used as a substrate for microfloraduring the digestion process contained 36.38 g L−1

of dry mater. The dry matter comprises 77 % oforganic matter. Total organic carbon in the liquidphase of sludge was 585 mg L−1. On the other


Table 2. Chosen physicochemical indexes of sludge before and on the 7th and 14th day of mesophilic and thermophilic digestion

Mesophilic digestion Thermophilic digestionIndexes Unit Before digestion

7 d 14 d 7 d 14 d

pH – 7.71 7.76 7.75 7.90 7.95Hydration % 97.38 97.95 98.10 97.83 98.12Total solids g L−1 26.17 20.50 19.01 21.68 18.79

Volatile solidsg L−1 17.89 12.26 10.62 13.56 10.78% 68.4 59.8 55.9 62.5 57.4

Non volatile solidsg L−1 8.28 8.24 8.39 8.12 8.01% 31.6 40.2 44.1 37.5 42.6

TOC mg L−1 484 412 304 1190 1020Alkalinity mg L−1 2280 3065 3450 3350 3600VFA mg L−1 910 414 386 980 524

hand, the digested sludge used as inoculum con-tained 61 % of organic mater on dry basis. Whereasthe content of VFA and TOC in the liquid wereequal to 630 mg L−1 and 428 mg L−1, respec-tively.The chosen physicochemical properties of the

sludge (the mixture of raw sludge and inoculum) be-fore and during the mesophilic and thermophilic di-gestion are shown in Table 2. Standard deviationsof the replications were below 3 % of the mean val-ues.When the digestion was carried out at 37◦C, the

content of TOC at first increased to 825 mg L−1, andafterwards decreased to 412 mg L−1 on the seventhday of fermentation, indicating that easily availableorganic substrate was rapidly consumed. The TOCcontent on the 14th day of digestion was 304 mg L−1.During the tests, the decrease of dry organic mat-ter from 17.89 g L−1 to 10.62 g L−1 was observed,which corresponds to the reduction of organic matterin sludge equal to 41 %.During thermophilic digestion, the content of

TOC in the liquid phase of sludge increased from484 mg L−1 (before the process) to 1210mg L−1 on thethird day of fermentation, and decreased afterwards to1190 mg L−1 on the seventh and to 1020 mg L−1 onthe 14th day of digestion. During thermophilic diges-tion, TOC in the liquid phase was approximately threetimes higher than that during mesophilic digestion. Itindicates more effective hydrolyses of the organic sub-strate during thermophilic digestion. Dry organic mat-ter of the sludge decreased from 17.89 g L−1 to 10.78g L−1, which equals to a 40 % reduction of organicmatter in the sludge during digestion.Alkalinity of the liquids of 3065–3600 mg L−1 and

VFA of 386–980 mg L−1 did not exceed the extremevalues (1000–5000 mg L−1 and > 2000 mg L−1, re-spectively) for proper methane fermentation process(Malina & Pohland, 1992). Higher alkalinity of sludgeliquid and higher content of VFA were observed duringthermophilic digestion.

Fig. 1. Daily biogas production during; – mesophilic and– thermophilic digestion.

Biogas

Daily biogas yields for mesophilic and thermophilicmethane digestion of sewage sludge are shown inFig. 1. When comparing the yields obtained during thefirst five days of the stabilization process, more bio-gas was produced during mesophilic than thermophilicdigestion; 900–990 mL L−1 and 866–510 mL L−1 ofsludge, respectively. After five days, the trend changedand more biogas was produced under thermophilicconditions. Overall yields after 14 days of digestionwere: 7060 mL L−1 of sludge under thermophilic and6570 mL L−1 of sludge under mesophilic conditions.The content of methane in biogas during mesophi-

lic fermentation, except for the first day, was at thelevel of 61–65 %, whereas, for thermophilic fermen-tation it was 55–59 % (Table 3). The amount of pro-duced methane during thermophilic and mesophilic di-gestion was 560 mL of CH4 and 580 mL of CH4 from1 gram of removed organic matter, respectively.Higher biogas production was observed for ther-

mophilic than for mesophilic digestion; however, atthermophilic conditions, lower content of methane inbiogas was reported by Nges and Liu (2010).


Table 3. Content of CH4 and CO2 in biogas

Content in biogasa/%Digestion Component

1 d 3 d 5 d 7 d 10 d 14 d

MesophilicCH4 42.4 ± 0.5 63.6 ± 0.3 65.0 ± 0.3 65.2 ± 0.3 64.3 ± 0.4 61.2 ± 0.3CO2 37.6 ± 0.4 34.7 ± 0.4 33.7 ± 0.2 33.1 ± 0.3 33.8 ± 0.3 35.8 ± 0.2

ThermophilicCH4 40.8 ± 0.7 55.2 ± 0.4 58.8 ± 0.2 57.2 ± 0.3 56.8 ± 0.4 56.7 ± 0.3CO2 35.4 ± 0.8 37.7 ± 0.5 37.5 ± 0.3 38.2 ± 0.4 39.8 ± 0.3 38.5 ± 0.3

a) Results are the mean values of three measurements ± standard deviation.

Table 4. Concentrations of heavy metals (mg L−1) in the sludge liquid on the chosen days of mesophilic and thermophilic digestion

Mesophilic digestion Thermophilic digestionHeavy metal Before digestion

1 d 3 d 5 d 7 d 10 d 14 d 1 d 3 d 5 d 7 d 10 d 14 d

Zn 0.113 0.120 0.185 0.220 0.208 0.256 0.295 0.263 0.457 0.480 0.433 0.637 0.566Cu 0.042 0.047 0.053 0.070 0.075 0.078 0.074 0.054 0.093 0.084 0.072 0.104 0.083Ni 0.093 0.067 0.066 0.074 0.069 0.072 0.073 0.081 0.081 0.085 0.078 0.082 0.104Pb 0.037 0.036 0.040 0.042 0.044 0.042 0.038 0.031 0.033 0.036 0.028 0.043 0.032Cd 0.005 0.009 0.011 0.006 0.006 0.008 0.007 0.009 0.013 0.013 0.013 0.015 0.017Cr 0.076 0.064 0.042 0.034 0.022 0.018 0.022 0.063 0.057 0.066 0.072 0.062 0.084

Fig. 2. Percentage

a b c

distribution of heavy metals in fractions of sludge before (a) and after mesophilic (b) and thermophilic digestion(c); – exchangeable/carbonates, – Fe/Mn oxides, – organic matter/sulfides, – residual.

Heavy metals in sludge liquid

Total concentration of heavy metal ions in thesludge liquid on the chosen days of mesophilic andthermophilic digestion are shown in Table 4. Analyti-cal precision of replicates (mean values of three mea-surements ± standard deviation) varied from 4 % to10 % relative to standard deviation.During thermophilic digestion, an increase of zinc

and copper concentration in the sludge liquid was ob-served. In case of zinc, the concentration increasedfrom 0.11 mg L−1 to 0.64 mg L−1, and for copper from0.04 mg L−1 to 0.1 mg L−1. Concentrations of othermetals: nickel, lead, cadmium, and chromium, wereat the level of 0.08–0.1 mg L−1; 0.03–0.04 mg L−1;0.01–0.02 mg L−1, and 0.06–0.08 mg L−1, respec-tively. It can be summarized that digestion underthermophilic conditions did not significantly influencethe release of heavy metal ions into the stabilizedsludge.

During mesophilic digestion, the concentrationof zinc and copper increased to 0.3 mg L−1 and0.08 mg L−1, respectively, but decreased for chromium(final concentration of 0.02 mg L−1). For the othermetals, the concentrations were as follows: nickel ofapprox. 0.07 mg L−1, lead of approx. 0.04 mg L−1,and cadmium of 0.006–0.01 mg L−1. Then, on the 14thday of mesophilic digestion, two times lower zinc andcadmium concentrations and four times lower concen-tration of chromium were determined in the sludgecompared to thermophilic digestion.

Speciation of heavy metals in sludge

Averaged concentrations of heavy metals in thesludge, before and after mesophilic and thermophilicdigestion processes, together with standard deviationsare shown in Table 5. Percentage distribution of themetals over the stabilized sludge fractions is shown inFig. 2.


Table 5. Chemical fractionation of heavy metals in sludge before and after mesophilic and thermophilic methane digestion

Content in sewage sludgeb

Metal Fractiona Before digestion After mesophilic digestion After thermophilic digestion

mg kg−1 % mg kg−1 % mg kg−1 %

I 397.0 ± 4 23.6 367.0 ± 5 16.2 323.0 ± 4 14.1II 653.0 ± 7 38.8 770.0 ± 4 33.9 852.0 ± 16 37.2III 587.0 ± 11 34.9 988.0 ± 13 43.5 1019.0 ± 21 44.5

Zinc IV 46.0 ± 3 2.7 144.0 ± 4 6.4 95.0 ± 1 4.2Sum 1683.0 100.0 2269.0 100.0 2289.0 100.0Total 1783.0 – 2490.0 – 2540 –

% recovery 94.4 – 91.1 – 90.1 –

I 2.1 ± 0.1 1.4 1.2 ± 0.2 0.5 0.9 ± 0.2 0.4II 1.8 ± 0.2 1.1 1.4 ± 0.1 0.6 1.5 ± 0.2 0.7III 145.3 ± 0.5 91.7 210.6 ± 3.1 91.3 204.0 ± 12 93.4

Copper IV 9.2 ± 0.2 5.8 17.4 ± 0.8 7.6 12.0 ± 0.4 5.5Sum 158.4 100.0 230.6 ± 0.1 100.0 218.4 100.0Total 170.2 – 248.2 – 240.8 –

% recovery 93.1 – 92.9 – 90.7 –

I 80.5 ± 0.5 48.1 76.8 ± 2.2 35.1 79.6 ± 0.4 36.0II 28.7 ± 1.2 17.1 34.2 ± 0.7 15.7 35.7 ± 0.4 16.1III 53.9 ± 2.7 32.2 93.2 ± 2.4 42.6 96.2 ± 6.8 43.5

Nickel IV 4.4 ± 0.4 2.6 14.4 ± 0.1 6.6 9.8 ± 0.8 4.4Sum 167.5 100.0 218.6 100.0 221.3 100.0Total 156.2 – 202.3 – 219.0 –

% recovery 107.2 – 108.1 – 101.1 –

I 3.4 ± 0.2 5.9 4.6 ± 0.1 6.3 4.6 ± 0.1 6.2II 1.9 ± 0.2 3.3 4.2 ± 0.2 5.8 3.1 ± 0.1 4.2III 36.3 ± 3.3 63.5 33.6 ± 0.9 46.0 38.5 ± 1.5 51.9

Lead IV 15.6 ± 2.7 27.3 30.6 ± 1.7 41.9 28.0 ± 4.0 37.7Sum 57.2 100.0 73.0 100.0 74.2 100.0Total 59.1 – 73.8 – 79.0 –

% recovery 96.8 – 98.9 – 93.4 –

I < 0.1 – < 0.1 – < 0.1 –II 0.96 ± 0.22 36.5 1.14 ± 0.16 29.1 0.92 ± 0.16 25.0III 1.67 ± 0.10 63.5 2.78 ± 0.28 70.9 2.65 ± 0.55 72.0

Cadmium IV < 0.1 – < 0.1 – 0.11 3.0Sum 2.63 100.0 3.92 100.0 3.68 100Total 2.80 – 4.18 – 4.02 –

% recovery 93.9 – 93.8 – 91.5 –

I 1.1 ± 0.3 0.7 0.3 ± 0.1 0.1 0.4 ± 0.1 0.2II 0.4 ± 0.1 0.3 < 0.1 – < 0.1 –III 149.0 ± 3.1 97.5 202.6 ± 4.2 97.4 202.4 ± 8.0 93.6

Chromium IV 2.3 ± 0.6 1.5 5.2 ± 1.3 2.5 13.5 ± 2.8 6.2Sum 152.8 100.0 208.1 100.0 216.3 100Total 181.0 – 245.0 – 248.2 –

% recovery 84.4 – 84.9 – 87.1 –

a) Fraction: I – exchangeable and carbonates-bound, II – Fe/Mn oxides-bound, III – organic matter/sulfides-bound, IV – residual;b) results are the mean values of three measurements ± standard deviation (mg of metal per kg of dry matter).

The sum of zinc, copper, nickel, lead, and cadmiumcontent in four analyzed fractions was 90–108 % oftotal concentration in the sludge determined withoutfractionation, confirming the correctness of the usedmethodology and reliability of the obtained results(Walter et al., 2006; Fuentes et al., 2008; Chen et al.,2008; Yuan et al., 2011). Only in case of chromium,the balancing result was 84–87 %.

Sequential extraction revealed that the highestconcentration of zinc in the sludge before anaerobic di-gestion was found in the organic–sulfide fraction andin the fraction of hydrated oxides of iron and man-ganese. For both fermentation processes (thermophilicand mesophilic), the enrichment of zinc was observedmainly for the organic–sulfide fraction, much lower en-richment was observed for the fraction of hydrated


iron and manganese oxides. In case of the sludge stabi-lized with mesophilic digestion, an increase of zinc con-centration in the residual fraction was also found. Thetotal content of zinc in the iron and manganese oxides,organic–sulfide, and residual fractions was 86 % forthermophilic digestion, whereas after the mesophilicdigestion it was 84 % of the total content of the metalin the sludge. This is supported by other studies per-formed on sludge stabilized under anaerobic condi-tions (Alonso Álvarez et al., 2002; Fuentes et al., 2004,2008; Pathak et al., 2009b).Copper was bound to organic matter and sulfides

(92 %) before digestion and this fraction was enrichedafter the fermentation process. Copper shows highaffinity for organic compounds (Alonso et al., 2006;Lasheen & Ammar, 2009).In case of cadmium concentration, an increase was

observed in the organic–sulfide fraction in the stabi-lized sludge. The distribution of cadmium in this frac-tion was 71–72 %, while 25–29 % of the total cadmiumcontent was found together with that of iron and man-ganese oxides.The highest concentration of chromium was found

in the organic–sulfide fraction (97 %). After the diges-tion, the concentration of the metal increased in thisvery fraction. This is also confirmed by other studiesFuentes et al. (2004), Stylianou et al. (2007), Hanayet al. (2008); however, the concentration of chromiumin the organic fraction of sludge in these papers didnot exceed 85 %.The highest concentration of nickel in the sludge

before the stabilization was observed in the exchange-able–carbonate and organic–sulfide fractions. Nickelenrichment was mainly observed in the organic–sulfidefraction after both thermophilic and mesophilic di-gestion processes. A small increase of the metalconcentration was observed in the fraction of hy-drated iron and manganese oxides but also in theresidual fraction. The distribution of nickel overthe exchangeable–carbonate, hydrated iron and man-ganese oxides, organic–sulfide, and residual fractionsafter mesophilic digestion was 35 %, 16 %, 42 %, and7 %, respectively. The distribution after thermophilicdigestion was 36 %, 16 %, 44 %, and 4 %, respectively.High concentration of nickel in the exchangeable–carbonate fraction was also reported by Walter etal. (2006), Hanay et al. (2008), and Jamali et al.(2009).Lead was before the stabilization of the sludge

present mainly in the organic–sulfide fraction (64 %of the total content); much lower amount was presentin the residual fraction (27 %). After mesophilic diges-tion, 46 % of the total lead content was found in theorganic–sulfide fraction and 42 % in the residual frac-tion of sludge. Also after thermophilic digestion, anincrease to 38 % of the metal in the residual fractionwas observed. It is a relatively low percentage whencompared to (82–89 %) achieved for different sludges

after mesophilic digestion (Alonso Álvarez et al., 2002;Fuentes et al., 2004, 2008).The distribution of heavy metals over the sewage

sludge fractions for both thermophilic and mesophilicdigestion was as follows (from highest to lowest con-tent): Zn: organic + sulfide, reducible, exchangeable+ carbonate, residual; Cu: organic + sulfide, resid-ual, reducible, exchangeable + carbonate; Ni: organic+ sulfide, exchangeable + carbonate, reducible, resid-ual; Pb: organic + sulfide, residual, exchangeable +carbonate, reducible; Cd: organic + sulfide, reducible,residual, exchangeable + carbonate; Cr: organic + sul-fide, residual, exchangeable + carbonate, reducible.The residual fraction is considered to be chemically

stable and biologically inactive. Metals found in thisfraction are not harmful to the aquatic ecosystem. Themost mobile, meaning easily dissolved in the soil so-lution and assimilated by plants, are considered to beexchangeable metals and those that are bound to car-bonates. The highest concentration of nickel was foundin this fraction for both thermophilic and mesophilicdigestion.

Conclusions

Two types of sewage sludge anaerobic digestionwere carried out: mesophilic and thermophilic. Thedigestion progress was evaluated based on the mea-surements of biogas quantity and composition butalso on the decrease of organic matter content in thesludge after fermentation. Metal speciation analysiswas performed revealing some changes in the chemi-cal forms of the metals during the stabilization processof sludge.Based on the analyses, the following conclusions

have been drawn: (i) during thermophilic digestion,higher biogas yields were obtained compared to thoseof mesophilic digestion, 1 L and 0.9 L from 1 gof removed dry organic matter of sludge, respec-tively; however, the content of methane in biogas fromthermophilic digestion was on average approximately57 %, which is slightly less than that achieved inmesophilic digestion (about 64 %), after both methanefermentation processes, a comparable level of organicmatter distribution was obtained (about 40 %); (ii)application of thermophilic fermentation did not sig-nificantly influence the release of heavy metal ions tothe stabilized sludge liquid; concentration of zinc inthe liquid was below 0.6 mg L−1 during thermophilicdigestion and below 0.3 mg L−1 during mesophilicdigestion; concentration of other metals was below0.1 mg L−1 for both fermentation processes; (iii) noaccumulation of heavy metals in the mobile fractionsof sludge (exchangeable and carbonate) was observedin either fermentation process; after the digestion wascompleted, the highest increase of zinc, copper, nickel,cadmium, and chromium concentrations was observedin the organic–sulfide fraction, for lead, the highest


concentration was found in the residual fraction; thesechanges have been observed for both conditions: ther-mophilic and mesophilic; in case of zinc, a signifi-cant increase was observed also in the hydrated ironand manganese fraction; (iv) stabilization process ofthe sludge using thermophilic fermentation provideda similar distribution of heavy metals over the sludgechemical fractions as that obtained for mesophilic fer-mentation.

Acknowledgements. Funding for this work was provided byBS 402-301/07/R and Project no. N N523 410635.

References

Alonso, E., Villar, P., Santos, A., & Aparicio, I. (2006). Frac-tionation of heavy metals in sludge from anaerobic wastew-ater stabilization ponds in Southern Spain. Waste Manage-ment, 26, 1270–1276. DOI: 10.1016/j.wasman.2005.09.020.

Alonso Álvarez, E., Callejón Mochón, M., Jiménez Sánchez, J.C., & Ternero Rodríguez, M. (2002). Heavy metal extractableforms in sludge from wastewater treatment plants. Chemo-sphere, 47, 765–775. DOI: 10.1016/s0045-6535(02)00021-8.

Carballa, M., Omil, F., & Lema, J. M. (2009). Influence of dif-ferent pretreatments on anaerobically digested sludge char-acteristics: Suitability for final disposal. Water, Air, & SoilPollution, 199, 311–321. DOI: 10.1007/s11270-008-9880-z.

Chen, M., Li, X., Yang, Q., Zeng, G., Zhang, Y., Liao, D.,Liu, J., Hu, J., & Guo, L. (2008). Total concentrationsand speciation of heavy metals in municipal sludge fromChangsha, Zhuzhou and Xiangtan in middle-south region ofChina. Journal of Hazardous Materials, 160, 324–329. DOI:10.1016/j.jhazmat.2008.03.036.

Clesceri, L. S., Greenberg, A. E., & Eaton, A. D. (1998). Stan-dard methods for the examination of water and wastewater.Washington, DC, USA: American Public Health Association.

Dai, J., Chen, L., Zhao, J., & Ma, N. (2006). Characteristicof sewage sludge and distribution of heavy metal in plantswith amendment of sewage sludge. Journal of EnvironmentalSciences, 18, 1094–1100. DOI: 10.1016/s1001-0742(06)60045-4.

Dai, J., Xu, M., Chen, J., Yang, X., & Ke, Z. (2007). PCDD/F,PAH and heavy metals in the sewage sludge from six wastew-ater treatments in Beijing, China. Chemosphere, 66, 353–361.DOI: 10.1016/j.chemosphere.2006.04.072.

International Organisation for Standardization (1995). Waterquality – Evaluation of the “ultimate” anaerobic biodegrad-ability of organic compounds in digested sludge – Methodby measurement of the biogas production. ISO 11734:1995.Geneva, Switzerland.

European Committee for Standardization (2000). Characteriza-tion of sludges – Determination of trace elements and phos-phorus – Aqua regia exctracion methods. EN 13346:2000.Brussels, Belgium.

Frentiu, T., Ponta, M., Levei, E., & Cordos, E. A. (2009). Studyof partitioning and dynamics of metals in contaminated soilusing modified four-step BCR sequential extraction proce-dure. Chemical Papers, 63, 239–248. DOI: 10.2478/s11696-008-0102-3.

Fuentes, A., Lloréns, M., Sáez, J., Aguilar, M. I., Orutno, J. F.,& Meseguer, V. F. (2008). Comparative study of six differentsludges by sequential speciation of heavy metals. BioresourceTechnology, 99, 517–525. DOI: 10.1016/j.biortech.2007.01.025.

Fuentes, A., Lloréns, M., Sáez, J., Soler, A., Aguilar, M.I., Orutno, J. F., & Meseguer, V. F. (2004). Simple

and sequential extractions of heavy metals from differ-ent sewage sludges. Chemosphere, 54, 1039–1047. DOI:10.1016/j.chemosphere.2003.10.029.

Giller, K. E., Witter, E., & McGrath, S. P. (2009). Heavy metalsand soil microbes. Soil Biology and Biochemistry, 41, 2031–2037. DOI: 10.1016/j.soilbio.2009.04.026.

Gleyzes, C., Tellier, S., & Astruc, M. (2002). Fractionation stud-ies of trace elements in contaminated soils and sediments:a review of sequential extraction procedures. TrAC Trendsin Analytical Chemistry, 21, 451–467. DOI: 10.1016/s0165-9936(02)00603-9.

Hanay, O., Hasar, H., Kocer, N. N., & Aslan, S. (2008). Eval-uation for agricultural usage with speciation of heavy met-als in a municipal sewage sludge. Bulletin of EnvironmentalContamination Toxicology, 81, 42–46. DOI: 10.1007/s00128-008-9451-4.

Jamali, M. K., Kazi, T. G., Arain, M. B., Afridi, H. I., Jal-bani, N., Kandhro, G. A., Shah, A. Q., & Baig, J. A.(2009). Speciation of heavy metals in untreated sewagesludge by using microwave assisted sequential extration pro-cedure. Journal of Hazardous Materials, 63, 1157–1164. DOI:10.1016/j.jhazmat.2008.07.071.

Lasheen, M. R., & Ammar, N. S. (2009). Assessment ofmetals speciation in sewage sludge and stabilized sludgefrom different wastewater treatment plants, Greater Cairo,Egypt. Journal of Hazardous Materials, 164, 740–749. DOI:10.1016/j.jhazmat.2008.08.068.

Malina, J. F., Jr., & Pohland, F. G. (1992). Design of anaero-bic processes for the treatment of industrial and municipalwastes (Vol. 7). Lancaster, PA, USA: Technomic PublishingCo.

Mosquera-Losada, M. R., Munoz-Ferreiro, N., & Riqueiro-Rodríquez, A. (2010). Agronomic characterisation of differenttypes of sewage sludge: Policy implications. Waste Manage-ment, 30, 492–503. DOI: 10.1016/j.wasman.2009.09.021.

Mossop, K. F., & Davidson, C. M. (2003). Comparison of orig-inal and modified BCR sequential extraction procedures forthe fractionation of copper, iron, lead, manganese and zinc insoils and sediments. Analytica Chimica Acta, 478, 111–118.DOI: 10.1016/s0003-2670(02)01485-x.

Nagajyoti, P. C., Lee, K. D., & Sreekanth, T. V. M. (2010).Heavy metals, occurrence and toxicity for plants: a re-view. Environmental Chemistry Letters, 8, 199–216. DOI:10.1007/s10311-010-0297-8.

Nges, I. A., & Liu, J. (2010). Effects of solid retention time onanaerobic digestion of dewatered-sewage sludge in mesophilicand thermophilic conditions. Renewable Energy, 35, 2200–2206. DOI: 10.1016/j.renene.2010.02.022.

Pathak, A., Dastidar, M. G., & Sreekrishnan, T. R. (2009a).Bioleaching of heavy metals from sewage sludge: A review.Journal of Environmental Management, 90, 2343–2353. DOI:10.1016/j.jenvman.2008.11.005.

Pathak, A., Dastidar, M. G., & Sreekrishnan, T. R. (2009b).Bioleaching of heavy metals from sewage sludge by indige-nous iron-oxidizing microorganisms using ammonium ferroussulfate and ferrous sulfate as energy sources: A comparativestudy. Journal of Hazardous Materials, 171, 273–278. DOI:10.1016/j.jhazmat.2009.05.139.

Rauret, G., López-Sánchez, J. F., Sahuquillo, A., Barahona, E.,Lachica, M., Ure, A. M., Davidson, C. M., Gomez, A., Luck,D., Bacon, J., Yli-Halla, M., Muntau, H., & Quevauviller, P.(2000). Application of a modified BCR sequential extracion(three-step) procedure for the determination of extractabletrace metal contents in a sewage sludge amended soil ref-erence material (CRM 483), complemented by a three-yearstability study of acetic acid and EDTA extractable metalcontent. Journal of Environmental Monitoring, 2, 228–233.DOI: 10.1039/b001496f.


Rubio-Loza, L. A., & Noyola, A. (2010). Two-phase (acidoge-nic–methanogenic) anaerobic thermophilic/mesophilic diges-tion system for producing Class A biosolids from munic-ipal sludge. Bioresource Technology, 101, 576–585. DOI:10.1016/j.biortech.2009.08.066.

da Silva, A. L. O., Barrocas, P. R. G., Jacob, S. C., & Moreira,J. C. (2005). Dietary intake and health effects of selectedtoxic elements. Brazilian Journal of Plant Physiology, 17,79–93. DOI: 10.1590/s1677-04202005000100007.

Song, Y. C., Kwon, S. J., & Woo, J. H. (2004). Mesophilic andthermophilic temperature co-phase anaerobic digestion com-pared with single-stage mesophilic- and thermophilic diges-tion of sewage sludge. Water Research, 38, 1653–1662. DOI:10.1016/j.watres.2003.12.019.

Stylianou, M. A., Kollia, D., Haralambous, K. J., Ingleza-kis, V. J., Moustakas, K. G., & Loizidou, M. D. (2007).Effect of acid treatment on the removal of heavy met-als from sewage sludge. Desalination, 215, 73–81. DOI:10.1016/j.desal.2006.11.015.

Tessier, A., Campbell, P. G. C., & Bisson, M. (1979). Se-quential extraction procedure for the speciation of partic-ulate trace metals. Analytical Chemistry, 51, 844–851. DOI:10.1021/ac50043a017.

Walter, I., Martínez, F., & Cala, V. (2006). Heavy metal spe-ciation and phytotoxic effects of three representative sewagesludges for agricultural uses. Environmental Pollution, 139,507–514. DOI: 10.1016/j.envpol.2005.05.020.

Werle, S., & Wilk, R. K. (2010). A review of methodsfor the thermal utilization of sewage sludge: The Pol-ish perspective. Renewable Energy, 35, 1914–1919. DOI:10.1016/j.renene.2010.01.019.

Yuan, X., Huang, H., Zeng, G., Li, H., Wang, J., Zhou, C.,Zhu, H., Pei, X., Liu, Z., & Liu, Z. (2011). Total concen-trations and chemical speciation of heavy metals in liquefac-tion residues of sewage sludge. Bioresource Technology, 102,4104–4110. DOI: 10.1016/j.biortech.2010.12.055.

speciation of heavy metals in sewage sludge after mesophilic and thermophilic anaerobic digestion

Documents