fate of organotins in sewage sludge during anaerobic digestion

ent 371 (2006) 373–382www.elsevier.com/locate/scitotenv

Science of the Total Environm

Fate of organotins in sewage sludge during anaerobic digestion

Nikolaos Voulvoulis a,⁎, John N. Lester b

a Centre for Environmental Policy, Imperial College London, London SW7 2AZ, UKb School of Water Sciences, University of Cranfield, Cranfield, Bedfordshire, MK43 0AL, UK

Received 25 April 2006; received in revised form 2 August 2006; accepted 12 August 2006Available online 17 October 2006

Abstract

Adsorption onto sewage sludge is an important process for the elimination of tributyltin (TBT) from wastewater. However asthe disposal of sewage sludge to agricultural land is a significant route for recycling biosolids, there exists an issue as to whetherthe potential long-term build-up of organotins in agricultural soil is acceptable, from a human health and environmental point ofview. For the sustainable use of biosolids in agriculture it is essential to control and reduce the quantities of persistent pollutantssuch as organotins in sewage sludge. In this study, a sampling program was designed to establish the levels of TBT (and otherorganotins) in sewage sludge and their reduction during anaerobic treatment and processing prior to disposal. Experiments werealso undertaken to assess the fate of TBT in laboratory scale anaerobic digesters where the influence of digester operatingparameters could be evaluated. Organotin concentrations were determined using capillary gas chromatography with flamephotometric detection. The results demonstrated that the majority of TBT remained concentrated in the solid phase (sewagesludge). Concentrations of TBT in sewage sludge were approximately 18 mg kg−1 (dry weight) and both laboratory experimentsand fieldwork demonstrated that degradation of TBT during anaerobic digestion of sludge was minimal.© 2006 Elsevier B.V. All rights reserved.

Keywords: Organotin; Tributyltin; Removal; Sewage; Wastewater; Treatment

1. Introduction

During wastewater treatment those substances with arelatively high ability to adsorb to sludge particles andwhich are not fully mineralised during the retention timein the treatment plant will end up in the sludge. As thedisposal of sewage sludge to agricultural land is animportant route for recycling biosolids, it is essential toensure that there is no long-term build-up of these per-sistent contaminants in agricultural soil to protect bothhuman health and the environment (Clark et al., 1988).

⁎ Corresponding author. Tel.: +44 20 7594 7459; fax: +44 20 75810245.

E-mail address: [email protected] (N. Voulvoulis).

0048-9697/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2006.08.024

Since the 1960s organotin (OTs) compounds such aspolyvinyl chloride (PVC) stabilisers, fungicides, bacter-icides, insecticides, industrial catalysts and wood pre-servatives have been extensively used for industrial andagricultural purposes (Hoch, 2001).These activitiesinclude the use of monobutyltin (MBT) and dibutyltin(DBT) as heat and light stabilisers in PVC processing, theuse of TBT in antifouling formulations, as a general-purpose wood preservative as well as the use of tri-phenyltins (TPhT) in agriculture. The current climate ofhostility towards the use of organotins and TBT in par-ticular is based on their environmental impact (Gadd,2000). The direct entry of TBT into the aquatic envi-ronment as a result of its use as an antifouling agent hasbeen linked to the development of imposex characteristics

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http://dx.doi.org/10.1016/j.scitotenv.2006.08.024

Fig. 1. Location of sampling points at selected STWs.

374 N. Voulvoulis, J.N. Lester / Science of the Total Environment 371 (2006) 373–382

in some species of marine molluscs (Voulvoulis et al.,1999). Despite their widespread use as biocides in anti-fouling paints, organotins also reach aquatic environ-ments viamunicipal wastewater through their use inwoodpreservation and as plasticisers (Bancon-Montigny et al.,2004; Díez et al., 2005). Legislation has brought about areduction in the presence of TBT in both freshwater andmarine environments, but with limited success in the latter(de Mora and Pelletier, 1997). A ban on the use of TBTglobally in antifouling paints has been introduced by the

Fig. 2. Schematic representation of laboratory scal

International Maritime Organisation (IMO) in order toprotect the marine environment (Champ, 2003).

Strengthening the general feeling of hostility towardsthe use of organotins is their persistence in environ-mental systems. Organotins have the ability to accumu-late in living organisms and concern has been expressedover their ability to accumulate through the food chain(Alzieu, 1996). These compounds also persist interrestrial and aquatic environments, partly as a resultof adsorption onto particulate matter. They readily bindto sediments and soils and therefore degrade at a veryslow rate (Bosselmann, 1996). As a consequence, re-strictions on the use of TBT were imposed during the1980's, first in France and later in the UK, other Euro-pean countries and throughout most of the developedworld (Abbott et al., 2000). An Environmental QualityStandard (EQS) of 2 ngl−1 for marine waters has alsobeen widely agreed based on concentrations at whichreproductive and other abnormalities occur in marinemolluscs (Environment Agency, 2000).

If not efficiently removed during treatment the efflu-ent from wastewater treatment plants may become apoint source for the release of organotins or lead tocontamination of sewage sludge (Voulvoulis et al.,2004). Whilst the removal of heavy metals in the form ofdivalent cations has been extensively studied (Lesteret al., 1979; Stoveland et al., 1979; Goldstone et al.,1990a,b) during wastewater treatment and the mechan-isms involved have been elucidated (Stephenson andLester, 1987a,b; Stephenson et al., 1987) the removal of

e anaerobic digester (after Kirk et al., 1982).

Table 2Summarised data on concentrations of TBT, DBT and MBT inthickened and digested sludges in mg kg−1 as Sn

Date and time Thickened sludge(sample point 3)

Digested sludge(sample point 4)

MBT DBT TBT MBT DBT TBT

Fri 16:15 7.587 1.614 1.412 3.414Fri 19:15 3.067 4.681 11.298 5.878 3.537Fri 22:15 5.085 1.614 0.726 6.441 2.939 1.769Sat 1:15 3.753 1.130 1.291 1.584 – –Sat 4:15 2.381 0.323 1.735 1.496 – –

375N. Voulvoulis, J.N. Lester / Science of the Total Environment 371 (2006) 373–382

organometallic compounds has received only limitedattention (Goldstone et al., 1990c; Plagellat et al., 2004;Stasinakis et al., 2005). Consequently there exists apaucity of information on the fate of organometallicssuch as organotin compounds when they are dischargedto the municipal sewer for subsequent treatment.

When TBT enters sewage sludge in significant quan-tities, it is important to consider the potential for itsdegradation during sludge treatment processes. Twotreatment processes that could potentially cause the

Table 1Summarised data on concentrations of TBT, DBTand MBT in primaryand waste activated sludges in mg kg−1 as Sn

Date andtime

Primary sludge(sample point 1)

Waste activated sludge(sample point 2)

MBT DBT TBT MBT DBT TBT

Fri 16:15 4.134 0.415 – 3.738 1.329Fri 19:15 4.190 0.547 – 9.678 1.288 13.666Fri 22:15 7.550 0.680 – 5.483 – 0.332Sat 1:15 6.720 0.415 – 5.981 2.243 1.163Sat 4:15 12.911 2.680 – 4.403 0.498 2.409Sat 7:15 2.265 0.302 0.415 1.412 1.329 1.661Sat 10:15 2.076 1.095 – 2.077 1.661 1.661Sat 13:15 4.379 – – 1.911 – –Sat 16:15 19.743 3.586 2.416 14.039 3.157 2.243Sat 19:15 12.458 2.114 0.302 32.814 2.243 0.914Sat 22:15 1.321 1.208 – 1.911 0.582 1.994Sun 1:15 3.398 – 0.566 1.911 0.415 33.645Sun 4:15 15.232 0.264 – 8.390 1.163 0.498Sun 7:15 2.982 0.717 4.908 13.583 2.201 1.745Sun 10:15 0.944 0.868 – 22.845 2.534 4.403Sun 13:15 1.019 0.264 – 2.825 0.582 0.665Sun 16:15 8.381 1.095 – 4.195 0.748 0.291Sun 19:15 18.951 1.623 0.680 5.151 1.080 1.745Sun 22:15 5.398 0.434 – 1.246 0.665 0.498Mon 1:15 2.982 1.397 1.548 35.722 5.400 0.415Mon 4:15 1.586 – – 7.061 – 3.240Mon 7:15 3.926 – 0.529 3.655 – 3.738Mon 10:15 6.172 0.340 0.849 15.202 0.997 3.821Mon 13:15 6.493 0.982 7.777 2.658 0.665 3.572Mon 16:15 5.776 0.415 0.264 8.141 0.582 0.582Mon 19:15 3.473 – – 237.923 2.243 –Mon 22:15 3.247 – – 19.356 0.914 3.323Tues 1:15 12.533 0.566 12.458 4.569 – 6.314Tues 4:15 38.883 1.057 43.564 9.221 – 2.741Tues 7:15 31.861 0.491 24.462 5.400 0.831 7.809Tues 10:15 52.397 0.868 42.167 9.969 – 2.160Tues 13:15 7.361 0.529 10.570 6.314 – 9.470Tues 16:15 1.755 0.340 0.302 5.566 0.748 11.880Tues 19:15 2.416 0.642 4.681 10.218 1.828 21.184Tues 22:15 9.891 – 3.586 3.738 1.163 5.732Wed 1:15 6.153 – – 3.240 0.914 1.329Wed 4:15 8.985 0.302 – 37.549 8.557 2.409Wed 7:15 0.415 – – 3.988 2.991 3.572Wed 10:15 2.492 – – 10.218 0.415 0.831Wed 13:15 0.415 0.302 – 2.741 1.246 2.991

– Indicate values below limits of detection.

Sat 7:15 – – – 1.408 – –Sat 10:15 – – – 2.182 – –Sat 13:15 – – 0.525 3.414 – –Sat 16:15 – – – 4.611 – –Sat 19:15 – – – – – –Sat 22:15 3.148 0.484 – 3.907 – 2.112Sun 1:15 3.713 0.686 0.363 5.754 – 0.915Sun 4:15 10.048 0.686 0.363 2.393 0.282 0.352Sun 7:15 14.407 3.551 0.565 16.542 – 5.666Sun 10:15 4.318 0.383 0.303 0.598 – –Sun 13:15 9.322 0.525 1.291 2.798 – –Sun 16:15 12.833 0.807 0.363 4.998 – –Sun 19:15 4.641 0.686 – 2.094 – 1.830Sun 22:15 13.156 0.404 – 9.467 0.985 1.161Mon 1:15 7.849 1.352 – 6.599 1.056 –Mon 4:15 5.165 – – 4.716 0.845 0.880Mon 7:15 6.981 – – 5.561 0.458 –Mon 10:15 5.690 0.323 – 7.813 0.317 0.317Mon 13:15 3.349 – – 3.695 0.915 1.373Mon 16:15 – – 5.004 1.584 4.118 5.561Mon 19:15 0.323 – – 1.337 4.540 4.786Mon 22:15 25.747 – – 1.672 4.206 10.382Tues 1:15 – – – 0.704 – –Tues 4:15 9.685 2.462 0.686 3.308 4.611 4.364Tues 7:15 8.475 1.433 0.424 2.200 3.115 3.449Tues 10:15 7.264 0.404 – 1.091 1.619 2.534Tues 13:15 9.403 – – 1.654 2.393 4.294Tues 16:15 5.367 0.646 0.525 4.294 1.513 2.428Tues 19:15 9.322 – – 2.393 3.344 4.329Tues 22:15 9.605 – 0.484 0.880 0.774 1.197Wed 1:15 10.028 – – – –Wed 4:15 5.206 0.282 – 1.197 0.598 0.493Wed 7:15 7.627 0.646 – 1.760 – –Wed 10:15 7.062 – – – – –Wed 13:15 7.910 – – 3.238 – –

– Indicate values below limits of detection.

breakdown of TBT are anaerobic digestion and thermaldrying. Breakdown in sediments has been reported byDowson et al. (1996) under anoxic and anaerobic condi-tions. However, only limited information has been pub-lished describing the fate of TBT during anaerobicdigestion. A study of full-scale anaerobic sludge diges-tion at Zurich Sewage Treatment Works Fent (1996)found minimal removal of TBT. In laboratory scale di-gestion experiment it was also established that aerobicthermophilic, anaerobic thermophilic, and anaerobic

Table 3Statistical comparisons of log-transformed organotin concentrationdata

Sludge HA T P D.F.

Primary sludge MBTNTBT 5.22 8.22×10−6 35TBT≠DBT 0.0499 0.48 32

Waste activated sludge MBTNTBT 1.67 2.04×10−9 74TBT≠DBT −0.714 0.477 70

Thickened sludge MBTNDBT 4.14 0.006 6TBT≠DBT 1.892 0.054 6

Digested sludge MBTNDBT 2.46 0.0169 10TBT≠DBT 1.833 0.076 9


mesophilic digestion all achieved approximately 8%removal. These poor removals were attributed to thepresence of competing organic substrates and the sorptionof much of the organotin to particulate matter, limiting itsbioavailability. The low removal efficiency in theselaboratory-based experiments was not considered to bedue to the inhibition of metabolism by TBT, as thelaboratory experiments used sludges containing butyltinsat concentrations of 0.3–0.8 mg kg−1, which were belowconcentrations found to be inhibitory (Gadd, 2000).

The aim of this study was to determine the fate oforganotin compounds in sewage sludge during anaero-bic treatment, in laboratory experiments, and at a full-scale sewage treatment works (STW) to establish theirlevels and fate in treated sludge. This will contribute tobetter management of the risks posed by organotins insewage sludge disposed to agricultural land.

2. Materials and methods

2.1. Sampling

The aim of the sampling strategy was to establishlevels of TBT in sewage sludge prior to anaerobictreatment in order to assess the effect of treatment on

Fig. 3. Observed concentrations (mg l−1 as

sludge contamination. The STW studied had a dischargeconsent of 4.8 μgl−1 of TBT in the final effluent.Primary treatment was by lamella separators andsequence batch reactors (SBRs) provided secondarybiological treatment. The locations of the four samplingpoints are shown in Fig. 1 (Voulvoulis et al., 2004).

Sampling occurred over a period of five days fromFriday July 27th to Wednesday August 1st 2001, withsamples being taken at all points every 3 h. A three hourperiod was selected to account for the three hourresidence time within the lamella separators and 6 hresidence time within the SBRs. The monitoringprogram allowed for coverage of day night andweekday/weekend variations.

2.2. Laboratory scale digestion of sludges

Four laboratory scale digesters were used to deter-mine the fate of TBT spiked into sewage sludge asdescribed by Kirk et al. (1982). Each digester compriseda borosilicate glass bowl and lid containing five accessports; one for input of sludge, one a sludge outlet, one tohold a thermometer, one to hold a stirrer and one toallow the release of gas evolved to a gas collector. Thedigesters were kept in a water bath at 37 °C, which wasthe operating temperature of the full-scale digesters atthe selected STW, and stirred automatically for 10 minevery hour at approximately 100 rpm (see Fig. 2).

The digesters were initially filled with 1.5 l ofdigested sludge from the selected STW. In addition,sixty litres of undigested sludge were collected, spikedwith TBT and frozen in 1 l containers for future use.These sub-samples of sludge were then used to feed thedigesters, allowing extended operation with sludge ofhomogeneous composition. On start-up, nitrogen wasblown into the digesters to expel air and ensure ana-erobic conditions. To feed the digester, frozen, raw

Sn) of TBT in digester A (unspiked).

Fig. 4. Observed and expected concentrations (mg l−1 as Sn) of TBT in digester B (spiked with 150 ml of sludge containing 0.1 mg l−1 TBT as Sn).


sludge was thawed overnight and warmed to 35 °C(close to the operating temperature of the digester). Thissludge was then blown into the digester through siliconerubber tubing of the sludge inlet, using nitrogen to avoidingress of air. An equivalent volume (150 ml) of di-gested sludge was also removed from the digester. Thisoperation was carried out on each digester once every48 h. During these experiments, digested sludge wasremoved before fresh sludge high in TBT was added toensure analysis of digested material did not containfreshly added TBT.

After 10 days operation in which gas production wasmonitored to ensure that digestion was occurring, spikedsludge was added. Three of the digesters were spikedwith TBT while one system (digester A) was left un-spiked. Digester B was spiked with 150 ml of sludgecontaining 0.1 μgl−1 TBT, digester C was spiked with150 ml of sludge at a TBT concentration of 0.5 μgl−1,and digester D was spiked with 150 ml of sludge at aTBT concentration of 2 μgl−1.

The suspended solids content of each sludge sampleremoved from the digester was determined. In addition20 ml portions of each sludge sample removed from thedigester were placed in centrifuge tubes, acidified andfrozen prior to organotin analysis.

Fig. 5. Observed and expected concentrations (mg l−1 as Sn) of TBT in diges

Gas production was a measure of biochemical activi-ty and was also used to identify potential toxic effectscaused by TBT. The gas produced during each 48 hperiod between spiking and sample removal was mea-sured by downward displacement of acidified waterfrom the gas collectors.

2.3. Analysis

Organotin stock solutions were prepared in methanoland stored in the dark below 4 °C (Carlier Pinasseauet al., 1997). Working solutions were prepared fort-nightly in methanol. A working solution of 2% w/vsodium tetraethylborate (97%) (NaBEt4) was made updaily in ultrapure water. A 0.25% solution of tropolonein diethyl ether was used for sludge extractions. Sodiumacetate buffer was made up from 8.2% w/v sodiumacetate and 6.0% acetic acid. Sludge samples (20 ml)were placed into polyethylene centrifuge tubes, 2 ml ofconcentrated HCl were added, the tubes were allowed tostand for 30 min to allow CO2 and H2S to vent beforebeing sealed and then frozen.

The extraction procedure was based on that of Muller(1987) as modified by Fent and Müller (1991). Sludgesamples from the wastewater treatment plant were

ter C (spiked with 150 ml of sludge containing 0.5 mg l−1 TBT as Sn).

Fig. 6. Observed and expected concentrations (mg l−1 as Sn) of TBT in digester D (spiked with 150 ml of sludge containing 2 mg l−1 TBT as Sn).


analysed for organotins, however, due to the greateraffinity of the phenyl tins for solids, the method was notsuitable for the determination of the phenyl tincompounds.

Aliquots of 10 ml were pipetted into a graduatedcentrifuge tube. Samples were extracted three times with5 ml portions of diethyl ether containing 0.25% tropo-lone. After the addition of each portion, the sludge washomogenised mechanically, centrifuged at 5000 g for5 min and the organic phase removed. The three organicphases were combined from each sample in a 250 mlglass bottle and the solvent was evaporated with a steamof nitrogen.

Owing to their low volatility and high polarity,derivatisation of the organotins is required prior to gaschromatography. The organotins were ethylated withNaBEt4 at pH 4.5 in accordance with the method devel-oped by Carlier Pinasseau et al. (1997). Water (100 ml)was poured into the bottle containing the extractedorganotins. An internal standard tripropyltin was addedwith 3 ml of acetate buffer; NaBEt4 (100 μl of workingsolution) and tri-methylpentane (2 ml) were then addedsuccessively. The 250 ml glass bottle was closed andmechanically shaken for 30 min. After shaking, theorganic phase was transferred to a crimp-capped vial for

Fig. 7. Concentration of MBT in laborator

analysis by a Perkin-Elmer Autosystem XL Gas Chro-matograph (Perkin-Elmer, Beaconsfield, UK). It wasfitted with a programmable split/splitless injector, a BP5capillary column from SGE (SGE, Milton Keynes, UK)(length 30 m, film thickness 0.25 μm, internal diameter0.22 mm) and a flame photometric detector (FPD) oper-ating with a 610 nm optical filter. The detector was oper-ated with an air/hydrogen flame and helium carrier gas.

All samples were labelled appropriately to ensurecorrect identification. Recoveries were dependent onsample type and demonstrated variation between orga-notin species. ‘Recoveries for primary sludges were 70±10% and for activated sludge 90±10%’. Standard devi-ations for all samples were within 10%. Field and ana-lytical duplicates were analysed on 10% of the samplestaken. The quantification limit for the organotins in allsludge samples was 0.25 mg kg−1 as Sn.

3. Results

In primary sludge, MBT was present at the highestconcentration, followed by TBTand then DBT (Table 1).Most of the TBT detected was found in a single peakbetween 22:15 Monday and 16:15 on Tuesday. Much ofthe MBT found was also detected at the same time,

y anaerobic digesters (μg l−1 as Sn).

Fig. 8. Concentration of DBT in laboratory anaerobic digesters (μg l−1 as Sn).


although significant quantities of MBTwere detected onother occasions. The concentration of DBT was gener-ally lower and its fluctuations did not seem to besynchronised with those of MBT and TBT.

Concentrations of organotins in waste activated sludgewere similar to those reported for primary sludge. Inaddition, there was a clear link with the concentrations ofthe organotins in returned liquors, which enter the SBRswith the settled sewage from the lamellas (Voulvouliset al., 2004). MBT concentrations again demonstratedthe greatest variation. In contrast to the primary sludge,changes in MBT concentrations were not correlated withchanges in TBT concentration. DBTwas again present atthe lowest concentrations. In the returned liquors, as inprimary and waste activated sludge, MBTwas present atthe highest concentrations followed by TBT and thenDBT.

In thickened and digested sludges (Table 2), in mostof the cases, MBT was the most abundant organotincompound and TBTwas present at lower concentrationsthan DBT.

In all sludges, MBT was present in the highestconcentrations. T-tests on log-transformed data estab-lished that its concentration (in μgl−1) to be significantlyhigher than that of the next most concentrated organotinat a significance level of b0.05 (Table 3). In primarysludge, waste activated sludge and returned liquors themean concentration of TBT was higher than the mean

Fig. 9. Gas produced in dig

concentration of DBT but the difference was small andnot statistically significant. In primary sludge, the medi-an concentration of DBT exceeded that of TBT but theopposite was true when the means were compared. Inthickened sludge and digested sludge the concentrationof DBT was higher than that of TBT.

3.1. Laboratory digestion of sludge

Concentrations of TBT predicted in the sludge, assum-ing it to behave as a conservative substance (i.e. non-biodegradable) in the four laboratory-scale anaerobicdigesters were compared to the actual concentrations ofTBT detected in sludge removed from the digesters at 48 hintervals. Expected concentrations were calculated fromthe quantity of TBT added during spiking, the quantity ofTBT contained in sludge removed from the digester(assuming zero breakdown) and the mean TBTcontent ofunspiked digested sludge. The difference between ex-pected and observed concentrations of TBT in sludgerepresents the reduction in the concentration of TBTbecause of anaerobic digestion.

In digester A (fed with unspiked sludge), the concen-tration of TBT remained low throughout the experiment.Thus, it is hard to distinguish any trends in TBT con-centration from experimental error (Fig. 3). In digester Bwhich was fed with sludge spiked with 0.1 mg l−1 TBTas Sn, there was some variation in the extent of removal

esters A, B, C and D.


but no substantial evidence of breakdown was observed(Fig. 4). In digester C, spiked with 0.5 mg l−1 as Sn, theobserved and expected values were very similar (Fig. 5),a trend that becomes more evident in digester D whichwas spiked with 2 mg l−1 TBT as Sn (Fig. 6).

The concentrations of the two probable breakdownproducts of TBT-DBTand MBT-detected in sludge sam-ples from the anaerobic digesters, are shown in Figs. 7and 8. The concentrations of both MBT and DBT arevery low (low μg l−1 range). The concentration of MBTis not much different in the spiked and unspikeddigesters. The concentration of DBT in digesters C andD is slightly higher, suggesting some breakdown of TBT(around 9%), which was at higher concentrations inthose digesters. This is in agreement with the work ofFent (1996).

The volume of gas produced by the digesters wasmeasured as an indicator of microbial activity (Fig. 9).Gas production from the unspiked sludges (digester Aand all digesters on day 0), exhibited higher levels todigester B, C and D, where the quantity of gas produceddecreased thereafter and then remains stable at lowerlevels for the duration of the study.

The unspiked digester produced gas at an extremelyconsistent rate of between 15 and 17 cm per 96 h. Gasproduction in digester B fell by 50% after the first 2spiked additions, and in digesters D and C the rate of gasproduction fell by 70%, and remained stable. A possibleexplanation is that the high levels of TBT in the digest-ers inhibited anaerobic digestion of sludge by poisoningmicro-organisms responsible for this process. If TBTcontamination were to poison the anaerobic digestionprocess at a wastewater treatment plant, it would haveserious consequences for sludge treatment and disposal.

4. Discussion

In terms of contamination of sewage sludges, TBTposes three main risks:

• Damage to the agricultural land or the receivingenvironment through disposal of contaminatedsludge to land

• Damage to human health through subsequentincorporation into crop plants

• Reduction in the efficiency of the anaerobic digestionprocess through inhibition of the bacteria responsiblefor that process.

The risk associated with TBT contamination dependson three factors—the concentrations of TBT enteringthe wastewater treatment works, the extent to which it is

concentrated into the sludge, and the danger posed byTBT in the receiving environment.

In this study, the mean concentration of TBT enteringthe treatment works in raw sewage over the 5 daysduring which sampling occurred was 0.13 mg kg−1

(Voulvoulis et al., 2004). The concentration entering, orpossibly re-entering, the sludge treatment system of theworks in activated sludge was broadly similar (0.08 mgkg−1). The concentration of TBT leaving the works indigested sludge was 0.06 mg kg−1. This most probablyreflects re-partitioning of the TBT content of the sludgeduring treatment.

Generally, following digestion, sewage sludge isnormally dewatered and sometimes dried. Theory sug-gests that little TBT will be lost during dewatering,because of TBT's strong tendency to adsorb to particu-late matter. There is a paucity of data on the effects ofthermal drying on organotins in digested sludge. Hoch(2001) notes the boiling points ofMBT, DBTand TBT tobe 145 °C, 135 °C and 172 °C respectively and cites thefinding of Zuckerman et al. (1978) that TBT is thermallystable up to 200 °C. This suggests that there would berelatively little vaporisation or thermal breakdown oforganotins during drying at 105 °C, (the temperaturenormally used in sludge treatment). In some driers wherehot air enters at a temperature of 300 −800 °C and leavesat 100 °C (Institute of Water Pollution Control, 1981),there might be some breakdown of TBT.

If all the TBT present in digested sludge remains inthe sludge phase then the concentration of tin in sludgeapplied to land will be the same in terms of mg TBT perkg dry solids as is present in digested sludge. Therefore,the environment will receive sludge at an estimated rateof 0.024 mg kg−1 dry solids.

There is no statutory limit on the quantity of TBT thatmay be disposed to land. However, it is desirable toestimate the environmental risk associated with the useof sludge containing TBT. TBTon the surface of soils oron the surface of plants grown above ground is likely tobe broken down by sunlight. TBT is also likely to bebroken down by both microbial and abiotic processes inthe soil, since its breakdown in river sediments has beenobserved (de Mora, 1996). The high adsorbancy of TBTfor particulate matter should ensure that it remains in thesoil and does not migrate to, for example, groundwater.Guidance on the permissible levels of TBT in sewagesludge applied to land could come in the future. Forth-coming EU regulations may specify a limit of 100 mgkg−1 of diethylhexylphthalate, a plasticiser with possibleendocrine disrupting effects. However, a recent study forthe EU proposes a PNEC of 2.5 μg kg−1 soil for TBTwhich could be problematic for some sites depending on


farming practices (EUSES, 2002). Based on these, it isrecommended that concentrations of TBT in the finalproduct before application to land should be determinedin order to establish any possible risks associated with itsagricultural application on a site by site basis.

The function of TBT is to act as a microbial inhibitorpreventing the development of bacterial and algal films, itmight be expected to be toxic to the bacteria responsiblefor biological wastewater treatment and, therefore, toinhibit its own breakdown. TBT resistant strains have,however, been isolated in the laboratory, although thesebacteria metabolised a range of other substrates in pre-ference to TBT (Abbott et al., 2000). These results suggestthat there is potential for metabolism of TBT during thefinal stages of treatment of liquid effluent, when thewastewater contains very little organic material, but notduring anaerobic digestion of sludge, as the sludge will berich in organic matter throughout the process. Gadd(2000) also observed inhibition of bacterialmetabolism byTBT, at concentrations of 0.33–16 μM (equivalent to0.039–1.9 mg l−1 as Sn or 0.78–38 mg kg−1 dry solidsassuming 50 g l−1 (DS). These observations are confirmedby this study, where it was demonstrated that concentra-tions of TBT in sewage sludge might reach critical levelsat which inhibition of bacterial metabolism can occur.

5. Conclusions

Removal efficiency of TBT from wastewater is relat-ed to the removal efficiency of suspended solids andtherefore TBT is expected to concentrate in sludge.Concentrations of TBT in sewage sludge were approx-imately 18 mg kg−1 (dry weight) and both laboratoryexperiments and fieldwork demonstrated that degrada-tion of TBT during anaerobic digestion of sludge islimited, in agreement with other studies. Care must betaken in establishing the risks associated with recyclingbiosolids to agricultural land. Levels of TBT in sewagesludge should be assessed in the light of detailedenvironmental and health risk assessments.

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