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Effect of linear alkylbenzene sulphonates (LAS) on theanaerobic digestion of sewage sludge

M.T. Garcia�, E. Campos, J. Sanchez-Leal, I. Ribosa

Department of Surfactant Technology, IIQAB-CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain

a r t i c l e i n f o

Article history:

Received 20 October 2005

Received in revised form

11 May 2006

Accepted 18 May 2006

Available online 17 July 2006

Keywords:

LAS

Sewage sludge

Anaerobic digestion

Toxicity

Bioavailability

nt matter & 2006 Elsevie.2006.05.033

hor. Tel.: +34 93 400 61 00;tgbet@iiqab.csic.es (M.T

A B S T R A C T

Batch anaerobic biodegradation tests with different alkylbenzene sulphonates (LAS) at

increasing concentrations were performed in order to investigate the effect of LAS

homologues on the anaerobic digestion process of sewage sludge. Addition of LAS

homologues to the anaerobic digesters increased the biogas production at surfactant

concentrations p5–10 g/kg dry sludge and gave rise to a partial or total inhibition of the

methanogenic activity at higher surfactant loads. Therefore, at the usual LAS concentration

range in sewage sludge, no adverse effects on the anaerobic digesters functioning of a

wastewater treatment plant (WWTP) can be expected. The increase of biogas production at

low surfactant concentrations was attributed to an increase of the bioavailability and

subsequent biodegradation of organic pollutants associated with the sludge, promoted by

the surfactant adsorption at the solid/liquid interface. When the available surfactant

fraction in the aqueous phase instead of the nominal surfactant concentration was used to

evaluate the toxicity of LAS homologues, a highly significant relationship between toxicity

and alkyl chain length was obtained. Taking into account the homologue distribution of

commercial LAS in the liquid phase of the anaerobic digesters of a WWTP, an EC50 value of

14 mg/L can be considered for LAS toxicity on the anaerobic microorganisms.

& 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Currently, special attention is paid to the environmental

acceptability of those detergent surfactants which are readily

biodegradable under aerobic conditions but not under anae-

robic conditions. The most used anionic surfactant, a

commercial mixture of linear alkylbenzene sulphonates

(LAS) (Cavalli et al., 1999), belongs to this group. LAS found

in wastewater is removed in wastewater treatment facilities

by sorption and aerobic biodegradation (Waters and Feijtel,

1995; Matthijs et al., 1999). Due to the hydrophobic character

of LAS, sorption leads to significant loads of this chemical in

sewage sludge (Sanchez-Leal et al., 1994; de Wolf and Feitjel,

1998; Jensen, 1999) which is often subjected to an anaerobic

digestion process as stabilization technique before final

r Ltd. All rights reserved.

fax: +34 93 204 59 04.. Garcia).

disposal. Anaerobic recalcitrance of LAS has been reported

by McEvoy and Giger (1986) as well as Federle and Schwab

(1992) and recently by Garcia et al. (2005). This conclusion is

supported by the relatively high concentrations of LAS found

in anaerobically treated sludge whereas aerobically treated

sludge contain low LAS amounts (Berna et al., 1989; Prats et

al., 1997). On the other hand, recent studies reported that LAS

could be anaerobically biodegradable in up-flow anaerobic

sludge blanket (UASB) reactors (Mogensen et al., 2003;

Angelidaki et al., 2004). These authors suggest that anaerobic

biodegradation of LAS is possible when the concentration of

LAS is low enough and that many unsuccessful attempts to

demonstrate anaerobic LAS biodegradation could be due to

lack of bioavailable LAS. In any case, the presence of high

amounts of undegraded LAS adsorbed on the sludge can

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WAT E R R E S E A R C H 40 (2006) 2958– 2964 2959

affect the functioning of the anaerobic digesters of waste-

water treatment plants (WWTPs) and should be evaluated.

The purpose of the present work has been to investigate the

effect of the LAS alkyl chain length and the surfactant

concentration on the anaerobic sludge digestion process.

2. Materials and methods

2.1. Linear sodium alkylbenzene sulphonates

Linear sodium alkylbenzene sulphonates were synthesized by

Petresa (Spain). The alkyl chain distribution, determined by

gas chromatography after desulphonation, for each LAS

homologue was: C10LAS (C10 99.9%), C11LAS (C11 95%, C12

4.3%), C12LAS (C12 99.9%), C13LAS (C12 4.2%, C13 88.7%, C14

7.1%), C14LAS (C14 99.9%).

2.2. Inoculum

Sludge samples from the anaerobic digester of a municipal

WWTP (Manresa, Barcelona) were used as inoculum. Total

and volatile solids of the anaerobic sludge samples were

determined according to Standard Methods (APHA, 1998) and

the values obtained were, respectively, 30–50 g/L and 45–60%.

After collection, sludge was washed with a mineral salt

solution, as described in the ECETOC test (ECETOC, 1988), to

reduce the amount of inorganic carbon to a value p10 mg/L.

A final resuspension step allowed adjusting the dried solids

concentration between 3 and 4.5 g/L. The LAS average content

in sludge samples, determined by the analytical method

described in Section 2.4, was 4310 mg LAS/kg dry sludge

(SD ¼ 275).

2.3. Anaerobic batch test system

A batch test system based on the ECETOC test (ECETOC, 1988),

method proposed by Birch et al. (1989), was applied. This

method evaluates the extent of ultimate anaerobic biodegra-

dation of a chemical based on the production of biogas

(methane and carbon dioxide) as compared to a blank

without the addition of the test substance (endogenous

biogas production). LAS homologues were tested at concen-

trations ranging 10–200 mg C/L. Batch digesters were inocu-

lated with anaerobic sludge samples from the Manresa

WWTP. Three replicates of each experiment (control and

LAS spiked digesters) were performed. All samples were

incubated in 250 mL pressure-resistant glass bottles at

3671 1C and the gas/liquid volume ratio was 3:7. The bottles

were fitted with gas tight septa and aluminium crimp seals.

After sealing the vessels and incubating them for about 1 h at

36 1C, excess gas was released to the atmosphere. The

incubation proceeded in the dark. The evolved pressure was

measured with a digital manometer connected to a syringe

needle and the increase in headspace pressure was used to

follow the mineralization process. At the end of the test, after

allowing the sludge to settle, the vessels were opened and

suitable volumes were withdrawn by a syringe from the clear

supernatant of each vessel and kept in small beakers care-

fully filled to the brim and covered with a cap to prevent CO2

exchange with the air. The dissolved part of carbon

dioxide was determined as the concentration of inorganic

carbon (IC) in the liquid phase using a carbon analyzer

(Shimadzu TOC-5050).

2.4. LAS analysis

Specific analysis of LAS both in the supernatant liquor and in

the settled sludge was carried out to determine the extent of

primary biodegradation. The LAS extraction procedure was

based on the method described by Matthijs and de Henau

(1987). At the end of the biodegradation assays, aqueous and

solid phases were separated by centrifugation at 4000 rpm for

15 min. Liquid samples: dissolved LAS was concentrated by

solid-phase extraction using octadecyl (C18) reversed-phase

silica columns. The eluted solutions were then analysed for

the LAS content. Solid samples: sludge samples from anaerobic

digesters were dried at 105 1C and Sohxlet extracted with

methanol for 8 h. These extracts were passed through a

strong anion exchange column, eluted with methanol/HCl

solution, neutralized and then passed over a C18 reversed-

phase silica column. The eluted solutions were then analysed

for the LAS content. LAS was determined by high-perfor-

mance liquid chromatography (HPLC) using a Waters chro-

matograph with UV detector and a C18 reversed-phase

column (m-Bondapack C18, 300�4.6 mm and 10mm particle

diameter, Waters Associates). The mobile phase consisted of

20% of solvent A (water) and 80% of solvent B (0.15 M NaClO4

in acetonitrile/water 80/20) and the flow rate was maintained

at 1 mL/min. The column effluent was monitored at 223 nm.

LAS quantification was conducted on the basis of external

standards. Recoveries ranged from 85% to 99% for the LAS

homologues. The standard deviation of the results, as a

measure of the reproducibility, was from 2% to 8%.

2.5. Surface tension measurements

Surface tension measurements were made at 36 1C by the

Wilhelmy plate technique using a Kruss K-12 tensiometer

with a sand-blasted platinum plate (ISO-304, 1985).

3. Results

The effect of the alkyl chain length of LAS on the anaerobic

digestion process was investigated performing batch degra-

dation tests with different alkyl chain length homologues

(C10LAS, C11LAS, C12LAS, C13LAS and C14LAS). Surfactant

concentration in the batch experiments (10–200 mg C/L)

corresponded to 5–100 g surfactant/kg dry sludge. Therefore,

typical values usually found in anaerobic sludge of WWTPs,

3–16 g LAS/kg dry sludge in Western Europe (Waters and

Feijtel, 1995; Prats et al., 1997; Fraunhofer Report, 2003), were

included in the tested concentration range. In addition,

considerably higher concentrations of LAS were also tested

to assess its potential toxicity.

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3.1. Biogas production in the digesters spiked with LAShomologues

The evolution of the biogas production in the digesters

spiked with different amounts of surfactant is represented

in Fig. 1 for C12LAS. In general, biogas production increased

with time until a plateau was reached. For the rest of LAS

homologues, a similar pattern of biogas production was

observed (curves not shown). The inhibition of the biogas

production caused by each LAS homologue was calculated

comparing the biogas production for the anaerobic digesters

spiked with different surfactant amounts and the endogen-

ous biogas production, when the biogas production curves

reached the plateau (1500 h). Results are plotted in Fig. 2.

For all LAS homologues, low surfactant concentrations

slightly increase the biogas production whereas high surfac-

tant loads clearly decrease the biogas production in the

anaerobic digesters. At the same nominal concentration,

the most severe effects on biogas production were observed

for the most hydrophilic LAS homologues. Thus, a complete

inhibition of the methanogenic microorganisms activity

was obtained for C10–C12LAS homologues at the highest

-80

-60

-40

-20

0

20

40

60

80

100

C10LAS C11LAS C

surfactant

%

10 mgC/L 25 mgC/L 50 m

Fig. 2 – Effect of the surfactant concentration on the anaerobic di

rate of the spiked digesters to the control digesters, for the LAS

00 10 20 30

25

50

75

100

125

150

175

200

biog

as p

rodu

ctio

n (m

L)

tim

Fig. 1 – Evolution of biogas (CH4+CO2) production in anaero

(m) 25 mg C/L, (.) 50 mg C/L, (&) 100 mg C/L, (� ) 200 mg C/L, and

standard temperature and pressure. The standard deviation of

surfactant concentrations tested (100 and 200 mg C/L). For

the most hydrophobic homologues (C13–C14LAS), no complete

inhibition of the biogas production was obtained even at the

highest concentration tested (200 mg C/L). Sensitivity of

methanogenic microorganisms to LAS has been already

described (Alexander, 1999). However, anaerobic digesters

spiked with low surfactant concentrations exhibited

stimulation of the microbial activity since a higher biogas

production than control digesters was observed. A significant

increase of biogas production in relation to the control

digesters was obtained for the longest LAS homologues

at a surfactant concentration of 10 mg C/L and even of

25 mg C/L. These results are in good agreement with those

previously reported by Garcia et al. (2006) on the enhance-

ment of the biogas production produced by C14LAS and allow

to generalize on this fact to all LAS homologues when the

surfactant concentration is p10 mg C/L (p5 g surfactant/kg

dry sludge).

The toxicity of LAS homologues on the anaerobic

microorganisms was assessed from the reduction of

the maximum biogas production for each surfactant concen-

tration. The surfactant concentration estimated to reduce

12LAS C13LAS C14LAS

concentration

gC/L 100 mgC/L 200 mgC/L

gestion, expressed as the percentage of the biogas inhibition

homologues.

40 50 60 70 80e (days)

bic batch digesters spiked with C12LAS at (K) 10 mg C/L,

in control digesters (’). Biogas production was calculated at

experimental data ranged from 2% to 11%.

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8 9 10 11 12 13 14 15 160.0

0.2

0.4

0.6

0.8

1.0

methanogenic microorganisms(r=0.9942, p=0.0005)

log

(1/E

C50

(mM

))

CnLAS homologue

Fig. 3 – Toxicity of LAS homologues on the methanogenic microorganisms of the anaerobic digesters considering nominal

surfactant concentrations.

WAT E R R E S E A R C H 40 (2006) 2958– 2964 2961

50% of the biogas production (EC50) ranged from 30

to 180 mg C/L for the LAS homologues. In Fig. 3, the toxicity

values, expressed as log(1/EC50), are plotted versus the

alkyl chain length of the LAS homologue. As it can be

observed, the higher the alky chain length, the lower the

toxicity value.

3.2. Primary biodegradation of LAS homologues

Primary biodegradation of LAS homologues in the anaerobic

tests was determined from the specific analysis of the

surfactant in liquid and solid phases. All the percentages

obtained over the test period ranged from 0% to 12%, in good

agreement with the results of the LAS mass balance over full-

scale anaerobic digesters (0–35%) reported in monitoring

studies (Berna et al., 1989).

3.3. Critical micelle concentration CMC of LAS homologues

CMC values were determined in the supernatant of the

anaerobic digesters. Measurements of surface tension

were carried out at 36 1C in the liquid phase obtained by

centrifugation of the anaerobic digesters content. From

the surface tension versus concentration curves, CMCs

were determined (Table 1). As described for different

surfactant families (Rosen, 2004; Garcia et al., 2004), the

CMC value decreases with increasing the alkyl chain

length of the LAS homologue. The CMC values of LAS

homologues in the supernatant liquid of the anaerobic

digesters were smaller than those reported in deionized

water (Garcia et al., 2002). This was attributed to the fact

that the presence of electrolytes in the ionic surfactant

solution diminishes the repulsion between identically

charged surfactant molecules favouring the surfactant mi-

cellization at a lower concentration.

4. Discussion

4.1. Enhancement of the biogas production by LAShomologues

For the digesters spiked with low surfactant amounts, not

only was toxicity on methanogenic bacteria not detected but

the biogas production was also increased (Fig. 2). However,

the low extent of LAS removal obtained by its specific analysis

provided evidence for a negligible microbial transformation of

the LAS molecule under anaerobic conditions. The slight

surfactant depletion obtained could be attributed to some

oxygen diffusion in the handling of sludge samples in the

initial steps of the experiments, resulting in a small extent of

LAS degradation by the o-oxidation mechanism. Therefore,

the increase of biogas production cannot be attributed to

the anaerobic biodegradation of the surfactant molecule. The

addition of LAS to the anaerobic system might increase

the bioavailability of other organic compounds sorbed on the

anaerobic sludge enhancing their biodegradation and, there-

fore, increasing the biogas production. Two main mechan-

isms have been proposed by Mackkar and Rockne (2003) to

explain the effect of a surfactant on the availability of organic

compounds: (i) increased apparent solubility of the pollutant

caused by the presence of micelles and (ii) facilitated

transport of the organic compound from the solid phase,

which can be caused by lowering of the surface tension of the

soil particle pore water, interaction of the surfactant with

solid interfaces or interaction of the organic compounds with

single surfactant molecules. Comparison of the CMC of each

LAS homologue and its concentration in the liquid phase of

anaerobic digesters showed that the LAS homologues were

present in the aqueous solution at concentrations 10–30 times

lower than their corresponding CMC value. Therefore, LAS

molecules were mainly present in the aqueous phase as

monomers and not as micellar aggregates and provided

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Table 1 – CMC values in the supernatant liquid of the anaerobic digesters at 36 1C, partition coefficients for the sorption onsewage sludge and EC50 values on methanogenic microorganisms, daphnia and fish for LAS homologues

LAS homologue CMC (M) alog Ki (L/kg) EC50 (mg/L)

Methanogenicmicroorganisms (95% CI)

bDaphnia(Daphniamagna)

bFish(Pimephalespromelas)

C10LAS 1.5�10�3 2.7 23 (20–26) 16.7 39.6

C11LAS — 3.1 14 (12–16) 9.2 19.8

C12LAS 2.0�10�4 3.6 8 (6–10) 4.8 3.2

C13LAS — 4.0 5 (4–6) 2.4 1.0

C14LAS 2.0�10�5 4.5 3 (2–4) 1.5 0.5

a Data reported by Garcia et al. (2005).b Data reported in the LAS-HERA report (2004).

0.08 10 12 14 16

0.5

1.0

1.5

2.0

2.5

3.0

methanogenic microorganismslog (1/EC50)=-1.58 x 0.27.n(r=0.9980, p=0.0001)

Daphnia magnalog (1/EC50)=-1.48 x 0.28.n(r=0.9975, p=0.0002)

log

(1/E

C50

(mM

))

CnLAS homologue

Fig. 4 – Toxicity of LAS homologues on the methanogenic microorganisms considering effective (bioavailable) surfactant

concentrations (’) and on daphnia (K).

WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 9 5 8 – 2 9 6 42962

evidence that the micellar solubilization was not the

mechanism responsible for enhancing the bioavailability

and subsequent biodegradation of organic compounds ad-

sorbed on the anaerobic sludge. The increase of bioavail-

ability of some organic molecules associated with anaerobic

sludge could be due to the surfactant adsorption on the

liquid/solid interface resulting in a decrease of the interfacial

tension and promoting their transport from the sludge.

4.2. Toxicity of LAS homologues

Surprisingly, toxicity seems to decrease with increasing

hydrophobicity of the surfactant molecule (Fig. 3). However,

the specific sorption on sludge of each homologue has to be

considered to properly evaluate its ‘‘effective’’ concentration

in the aqueous medium, i.e., its bioavailability. When LAS is

added to the anaerobic digesters, its sorption on sludge

produces an increase of the surfactant concentration in the

solid phase and a surfactant removal from the aqueous phase

(Garcia et al., 2005). The surfactant concentration in the

liquid phase decreases significantly as the LAS chain length

increases, i.e., the intensity of sorption on sludge increases

with the hydrophobicity of the LAS molecule as indicated by

the partition coefficient values, Ki (Table 1).

Due to the different sorption intensity of LAS homologues,

their available fraction in the aqueous phase will present

significant differences on basis of the alkyl chain length.

Considering the surfactant concentration in the aqueous

phase instead of the total added surfactant, the EC50 values

were recalculated and log (1/EC50) values plotted versus the

corresponding number of carbon atoms of the LAS alkyl

chain (Fig. 4). It can be observed that toxicity increases with

increasing hydrophobicity of the surfactant molecule and

there is a highly significant relationship (p ¼ 0:0002) between

EC50 and the alkyl chain length. These toxicity results were

compared with the toxicity values of LAS homologues on

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WAT E R R E S E A R C H 40 (2006) 2958– 2964 2963

daphnia and fish, the commonly used organisms for aquatic

toxicity assessment (Table 1). These toxicity data indicate that

the three species have a similar sensitivity against LAS

homologues. The toxicity values on Daphnia magna are plotted

in Fig. 4 together with the toxicity values on anaerobic

bacteria. As can be observed, a similar increment of toxicity

by increasing the alkyl chain length of LAS homologue is

found against both organisms.

4.3. Effect of commercial LAS on the anaerobic digestion ofsewage sludge

From the results here obtained and taking into account the

LAS average background concentration in the anaerobic

sludge samples from the WWTP (�4 g/kg dry sludge), LAS

concentrations up to 10–12 g surfactant/kg dry sludge can be

present without significant negative effects on the methano-

genic activity.

In the aqueous phase of the anaerobic digester of a WWTP,

different LAS homologues are present and, as mentioned

above, each of them has a different degree of toxicity. In a

previous work (Garcia et al., 2006), the analysis of LAS

homologue distribution in the supernatant of anaerobic

digesters showed that the average chain length of the

fingerprint of LAS is about C11.0, as a consequence of the

highest adsorption on suspended matter of the LAS homo-

logues with the highest alkyl chain length. Bearing in mind

the LAS homologue distribution in the liquid phase of the

anaerobic digesters, an EC50 value of 14 mg/L can be con-

sidered for LAS toxicity on the methanogenic microorganisms.

It is worth to remark that no adverse effects on the

anaerobic microorganisms and even stimulation of the

methanogenic activity were observed in the anaerobic batch

digesters for surfactant concentrations corresponding to the

usual LAS concentrations in anaerobic sludge of WWTPs. In

the toxicity context, this phenomenon could be attributed

to acclimation, understood as a higher tolerance to this

surfactant as a result of the prolonged exposure of the

anaerobic biomass to this chemical. However, in the biode-

gradation context, acclimation, understood as the period of

time before a clear disappearance of a chemical is observed, is

not produced because no evidence for LAS removal was

obtained.

5. Conclusions

The addition of LAS homologues to the anaerobic digesters

resulted in an increase of the biogas production at surfactant

concentrations p5–10 g/kg dry sludge and in a partial or total

inhibition of the methanogenic activity at higher surfactant

loads. Therefore, no adverse effects on the anaerobic

digesters functioning of a WWTP can be expected at the

common LAS concentration range in sewage sludge.

When the available surfactant fraction in the aqueous

phase instead of the nominal surfactant concentration was

used to evaluate the toxicity of LAS homologues, a highly

significant relationship between the toxicity and the alkyl

chain length was obtained. Taking into account the homo-

logue distribution of commercial LAS in the liquid phase of

the anaerobic digesters of a WWTP, an EC50 value of 14 mg/L

could be considered for LAS toxicity on the anaerobic

microorganisms.

Acknowledgements

This research was supported by the Spanish Ministerio de

Ciencia y Tecnologıa (MCYT) Fondo Europeo de Desarrollo

Regional (FEDER), project PPQ2001-2322.

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