DIPARTIMENTO DI INGEGNERIA CIVILE E AMBIENTALE UNIVERSITÀ DI CATANIA

DIPARTIMENTO DI INGEGNERIA IDRAULICA, AMBIENTALE, INFRASTRUTTURE VIARIE,

RILEVAMENTO POLITECNICO DI MILANO

DIPARTIMENTO DI INGEGNERIA IDRAULICA,

GEOTECNICA ED AMBIENTALE UNIVERSITA’ DI NAPOLI

FEDERICO II

DIPARTIMENTO DI SANITÀ PUBBLICA

UNIVERSITÀ DI FIRENZE

DIPARTIMENTO AMBIENTE-SALUTE-SICUREZZA UNIVERSITÀ INSUBRIA

DIPARTIMENTO DI INGEGNERIA,

UNIVERSITÀ DI FERRARA

DIPARTIMENTO DI INGEGNERIA CIVILE E AMBIENTALE, UNIVERSITÀ DI PALERMO

DIPARTIMENTO DI INGEGNERIA CHIMICA UNIVERSITA’ DI NAPOLI

FEDERICO II

DIPARTIMENTO DI INGEGNERIA CIVILE E

AMBIENTALE, UNIVERSITÀ DI FIRENZE

Workshop

Salvaguardia dei corpi idrici dalla

contaminazione da composti

xenobiotici: nuovi strumenti per

l'analisi, il controllo ed il

trattamento nelle acque reflue civili

ed industriali

Sala Abete, ECOMONDO, Rimini 4 Novembre 2010

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Confronto tra bioerattori a membrana e impianti convenzionali per la rimozione

di composti xenobiotici

Claudio Lubello*, Riccardo Gori Dipartimento di Ingegneria Civile e Ambientale – Università di Firenze

Via S.Marta 3, 50139 Firenze *claudio@dicea.unifi.it

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Lubello, Gori 1

CONFRONTO TRA BIOREATTORI A

MEMBRANA E IMPIANTI CONVENZIONALI

PER LA RIMOZIONE DI COMPOSTI

XENOBIOTICI

Claudio Lubello, Riccardo Gori

Dipartimento di Ingegneria Civile e Ambientale – Università di Firenze

Sommario. Nel presente lavoro sono stati messi a confronto due

bioreattori a membrana (MBR), uno utilizzante membrane a fibra

cava e l’altro piane, con un impianto operante in parallelo di tipo

convenzionale (CASP). L’obiettivo è quello di confrontare

l’efficienza di rimozione e il grado di ripartizione fra matrice

solida e liquida di alcuni composti xeno biotici, nello specifico

LAS (alchilbenzene sulfonati a catena lineare) e NPnEC

(nonilfenoli etossilati, n=1,15). Lo studio si è indirizzato anche

nella valutazione dei prodotti di degradazione. Allo scopo di

quantificare il contributo dovuto all’adsorbimento sono stati

analizzati alcuni campioni di fango estratti dal mixed liquor. I

risultati sono molto interessanti: la rimozione dei LAS si è

mostrata pari a circa il 99%, con differenze non significative fra

gli impianti MBR ed il CASP. Gli stessi composti appaiono

altamente adsorbibili sul fango, il che fa supporre un ruolo

rilevante nella rimozione, da parte della sedimentazione

primaria. I nonilfenoli mostrano una tendenzialmente migliore

eliminazione negli MBR a causa di migliore biodegradazione dei

composti. Infine dal punto di vista dell’adsorbimento il fango del

CASP appare possedere una più spiccata attitudine alla

ritenzione dei composti esaminati rispetto ai due MBR. Tale

capacità appare inoltre ben correlata con l’età del fango con cui

l’impianto opera.

INTRODUCTION

Anionic and nonionic synthetic surfactants are widely used for many

different purposes and applications: household detergents (60%),

industrial and technical cleaning applications (30%), industrial and

institutional (7%) and personal care (3%) for a worldwide production of

12.5 M tonnes/y (Edser, 2006).

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The main environmental impacts of surfactants arise from LAS and

APEO constituting a large part of respectively anionic and non ionic

surfactant market.

In wastewater treatment plants (WWTPs) surfactants, according to their

physiochemical properties and to the operational conditions of the plant

(for example sludge retention time (SRT), hydraulic retention time

(HRT), temperature) are partially biodegraded and partially sorbed to

sludge (Ahel et al., 1994; Ying and Fate, 2006; Cirja et al., 2008).

Residual surfactants and their degradation products are vehiculated into

the environment through WWTPs effluent and land application of sludge

(Schwarzenbach et al., 2003).

For some compounds with hydrophobic character (such as surfactants),

sorption to the organic matter can play a major role in their removal

through the excess sludge of the biological process (Ying and Fate, 2006;

Jensen and Fate, 1999; Temmik et al, 2004). In addition, the presence of

surfactants in sludge directed to agricultural land application, has arisen

major concerns in these last years, as showed by the European and local

legislation production (EU, 2000).

Surfactants toxicity has received considerable attention and many works

focus on effects and risk assessment on terrestrial, aquatic environment

by means of in vivo and in vitro experiments (WHO, 1996; Scott and

Jones, 2000; Soares et al, 2008).

In particular alkylphenols attract a significant research interest due to

endocrine disrupting properties of their biodegradation by-products.

Nonylphenols can mimic the natural hormone 17β-estradiol due to their

structure similarity (Joblin and Sumpter, 1994; White et al, 1994). For

these reasons they were voluntary banned since 1995 in northern Europe

in households cleaning products at first and in industrial applications

afterward (Renner, 1997).

Parent LAS and APEO are quite efficiently removed under aerobic

conditions in CASPs, but several studies show that the biotransformation

is always incomplete. Mineralization rarely occurs and breakdown

products are in some cases more toxic and persistent than the parent

compounds. For example nonylphenol (NP) is approximately 10 times

more toxic than its ethoxylates precursors (Renner, 1997).

For all these reasons the understanding of the behaviour and distribution

(in terms of solid/liquid phase partitioning) of surfactants during the

wastewater treatment is of main concern as well as the identification of

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Lubello, Gori 3

new technologies in order to minimize the amount of surfactants

discharged in the total environment.

MBRs are recognized as an efficient system to control the quality of the

effluent discharged into the environment (Stephenson et al., 2000;

Lubello and Gori, 2005; Melin at. Al., 2006). The main advantage is the

high SRT that can be maintained, and the total retention of the biomass

which allow the development of slow growing micro-organisms that may

remove low-biodegradable contaminants. Furthermore, the effluent is

characterized by the complete absence of suspended solid, and,

consequently, of the fraction of contaminants adsorbed to the solid phase.

Sludge of MBRs can have quite different characteristics than that of

CASPs; in particular MBRs’ sludge are usually characterized by smaller

flocs size and the mean particle size and specific surface area can have a

dramatic and nonlinear effect of the partitioning coefficient of organic

compounds (Yi and Harper, 2007).

This work is aimed to compare removal efficiency and solid/liquid

partitioning behavior of some surfactants and their degradation products,

in a CASP and two pilot scale MBRs operated in parallel. Target

compounds included LAS, APnEO (n=1-15), their acidic degradation

products alkylphenoxy carboxylates (APECs) and neutral degradation

products alkylphenols (APs).

MATERIALS AND METHODS

Full scale WWWTP and MBR pilot plants

The study was carried out at the WWTP of Terrassa (Barcelona, Spain)

which treats on average 50,000 m3/d of sewage wastewater originated

from domestic (80%) and industrial (mainly textile and pharmaceutical)

activities (20%).

As a consequence, the influent is characterized by the presence of

xenobiotic substances and particularly surfactants. The main

characteristics of the Terrassa WWTP influent and effluent are showed in

Table 1.

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Table 1. Main characteristics of Terrassa WWTP influent and effluent

during the experimental period

The Terrassa WWTP is a CASP which consists of: preliminary treatment,

primary sedimentation, pre-denitrification, oxidation-nitrification and

secondary settling. The biological section has a volume of 25,000 m3 and

is operated at a sludge retention time (SRT) of 10 d. Mixed liquor

suspended solids (MLSS) is 2.7 g/L and ratio between volatile suspended

solids (VSS) and total suspended solids (TSS) is 87%.

The two MBR pilot plants consisted of an aerated bioreactor equipped

with a Koch (Massachusetts, USA) submerged unit of hollow fibres

ultrafiltration (UF) membranes (indicated as MBR-HF) and a Kubota

(Osaka, Japan) submerged unit of plate and frame microfiltration (MF)

membranes (indicated as MBR-PF).

Both MBRs were inoculated with activated sludge from Terrassa WWTP

and were fed with the effluent of primary treatment. See table 2 for the

main characteristics and operating conditions of both pilot plants.

Table 2. Characteristics of MBR pilot plants.

Unit MBR-HF MBR-PF

Influent Primary effluent Final effluent

Total suspended solids (mg/L) 262 131 16

COD (mg/L) 651 457 71

BOD5 (mg/L) 363 241 15

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Aerobic reactor volume m3 3.6 7.5

Hydraulic retention time h 7.2 9-18

MLSS g/L 6-10 7-10

SRT d 30-40 65-75

Pore size µm 0.05 0.4

Effective membrane area m2 30 40

Average flux L/m2 h 17 10-20

The SRT is the more remarkable difference CASP and MBR pilot plants

in terms of operating conditions.

Analytical methods

A daily composite sample was taken once a week during two months

(totally 8 samples for each stream) of CASP and MBR pilot plants

influent and effluent. The sampling was made taking into account the

HRT of the CASP and MBRs. All samples were analyzed for LAS, NP1-

15EO, NP1EC, NP2EC, NP, OP.

In the same period, 7 instant samples of sludge coming from the

biological reactors of CASP and MBRs were also collected and analyzed

separately for LAS, NP3-15EO, NP2EO, NP1EO, NP1EC, NP, OP both

in the liquid and the solid phase.

All solvents (water, acetonitrile and methanol) were high performance

liquid chromatography (HPLC) grade and were purchased from Merck

(Darmstadt, Germany).

The standards used in this study were of the highest purity available.

High purity (98%) 4-tert-OP and 4-NP were obtained from Aldrich

(Milwaukee, WI, USA). NP1EC, NP1EO and NP2EO was obtained from

Dr. Ehrenstorfer (Augsburg Germany). Additionally, a technical mixture

of NPEOs containing chain isomers and oligomers with an average of 10

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ethoxy units (Findet 9Q/22) was from Kao Corporation (Barcelona,

Spain). Commercial LAS with low dialkyltetralinsulfonate content

(<0.5%) were supplied by Petroquimica Espanola S.A. in a single

standard mixture with the proportional composition of the four

homologues of: C10: 3.9%, C11: 37.4%, C12: 35.6%, C13: 23.1%. 4-

NP1EO-d2 and 4-n-NP-d8 which were used as the internal standard were

obtained from Dr. S. Ehrenstorfer (Augsburg, Germany). Stock solutions

(1 mg/mL) of individual standards and standard mixtures were prepared

by dissolving accurate amounts of pure standards in methanol. Working

standard solutions were obtained by further dilution of stock solutions

with methanol.

Sludge samples were collected in pre-cleaned glass bottles. The

suspension was centrifuged and the solid phase was separated and frozen

at -20 °C and finally lyophilized. The dried samples were stored at -20°C

until extraction. A 0.25 g sludge sample was sonicated and concentrated

as described elsewhere (Petrovic et al., 2001).

All samples were analyzed by solid phase extraction followed by liquid

chromatography-mass spectrometry (SPE-LC-MS-MS). The complete

procedure is described by Gonzalez et al. (Gonzalez et al., 2004;

Gonzalez et al., 2008) and Petrovic et al., 2006.

RESULTS AND DISCUSSION

Removal efficiency of surfactants by CASP and MBR pilot plants

The CASP and MBR pilot plants were very effective in the elimination of

LAS. On the basis of data showed in table 3, the overall elimination

efficiency was about 99% regardless the type of treatment, even if

removal efficiency clearly increased for higher SRTs. It confirms many

other works (Terzic et al., 2005, Petrovic and Barcelo, 2004; Clara et al.,

2007) where a high removal of LAS under aerobic conditions was

observed.

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Lubello, Gori 7

Table 3. Occurrence of target compounds in influent and effluent

samples of CASP and MBRs pilot plants (n = 8).

Unit Influent CASP

effluent

MBR-HF

effluent

MBR-PF

effluent

mean St.dv Mean St.dv Mean St.dv Mean St.dv

LAS µg/L 1362.9 187.1 15.06 14.44 12.32 9.33 7.31 5.05

NP(1-

15)EO

µg/L 16.44 5.67 1.65 1.25 0.33 0.32 0.45 0.16

NP µg/L 4.58 1.12 1.06 0.15 0.47 0.70 0.80 0.46

NP1EC µg/L 0.60 0.20 1.47 0.20 0.82 0.32 0.75 0.27

NP2EC µg/L 0.67 0.59 1.68 0.48 3.32 0.68 1.79 1.09

OP µg/L - - 1.55 0.44 - - - -

Concerning NP1-15EO, the concentrations detected in the influent were

typical of wastewater containing a fraction originated from industrial

activities, with an average value of 16.44 µg/L.

The removal obtained for NP1-15EO was significantly different between

CASP and MBR pilot plants: about 86% in the case of CASP and more

than 97% in both MBRs. A better efficiency of MBR with respect to

CASP, in the removal of NPnEO is consistent with previous studies (22).

Taking into account the total suspended solids (TSS) concentration of

MBR and CASP, and the partitioning coefficients estimated for NP1-

15EO (see below), the different efficiency in the removal of NP1-15EO

of CASP and MBRs cannot be fully attributed to the difference in the

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solids retention of the two systems. Thus, it appears reasonable

hypothesize a beneficial effect of high SRT on the biodegradation of

NP1-15EO.

It is well known that the biodegradation of APnEO lead to the formation

of more persistent metabolic products such as NP and NP1-2EC (Ahel et

al., 1994). In our study NP, the ultimate degradation product of NPnEO,

was found in the influent at a mean concentration of 4.58 µg/L. This

significant presence may be due to the utilization of NP for pesticide,

plastic formulations, and/or to a preliminary degradation of NP1-15EO in

the sewages which can partially occur also under anaerobic conditions

(Ying and fate, 2006). NP removal was about 75% for the CASP and

90% and 87% for MBR-HF and MBR-PF respectively. The higher

standard deviation characterizing both MBRs was probably due to the

unevenness of the operational conditions that affected the stability of the

biological process.

Average concentration of NP1EC and NP2EC in the influent was 0.60

µg/L and 0.67 µg/L respectively. Both carboxylic metabolites increased

their concentration in the effluents of all plants as a consequence of the

biodegradation of parent compounds in aerobic conditions (see Table 3).

The composition of CASP and MBRs influent and effluent in terms of

nonylphenolic compounds, were calculated, on the basis of molar

concentration of individual compounds. In the influent, the parent

compounds NPnEO and the final metabolite NP represented on average

51% and 41% respectively, of the total nonylphenolic compounds, while

the NPECs fraction was only the remaining 8%. In the effluent the

percentage of NPECs was shifted to 58% for CASP and 84% and 65%,

respectively, for the MBR-HF and MBR-PF.

Moreover, in order to calculate the percentage of nonylphenolic

compounds (NP1-15EO) in the effluent, in the surplus sludge and the

amount biodegraded, a mass balance on a molar basis was carried out

using the biological section as control volume.

Results are summarized in Table 4 and showed that a much higher

biodegradation was observed in MBR pilot plants which showed also a

better performance in terms of removal from the liquid phase.

Octylphenol is often of minor relevance because a minor fraction of

APnEO are OPnEO (Ying et al., 2002) and usually low concentration are

found in the influents and effluents of WWTPs (Clara et al., 2007). In the

present case, OP was only detected in the CASP effluent thus supporting

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Lubello, Gori 9

the hypothesis that operating with an high SRT, as in MBR pilot plants, it

can be degraded below the analytical detection limit.

Table 4. Fate of total NP(1-15)EO calculated as percentage of molar

content across the biological section.

Unit CASP MBR-

HF

MBR-

PF

Effluent

% of molar

content of NP1-

15EO

24.3 10.6 13.3

Excess sludge 38.9 14.9 9.8

Biodegradation 36.8 74.5 76.9

Removal from

liquid phase

75.7 89.4 86.7

Surfactants occurrence in the sludge

In order to evaluate the solid/liquid distribution behaviour of compounds

listed in Table 4, their concentration onto primary sludge of Terrassa

WWTP and biological sludge of CASP and MBRs as well as their

dissolved concentration were determined.

As showed in Table 5, high concentrations of LAS were detected in the

primary sludge as reported elsewhere (Clara et al., 2007).

Table 5. Contaminants concentration in solid (mg/kgTSS) and liquid

phase (µg/L) of sludge samples (n = 7).

u.m. Primary CASP MBR-HF MBR-PF

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Mean st.dv Mean St.dv Mean St.dv Mean St.dv

LAS mg/kg 12711 7662 447.9 926.7 97.8 101.8 46.5 34.3

µg/L 936.3 160.9 26.8 31.5 37.8 73.0 38.3 47.4

NP3-

15EO

mg/kg 11.22 6.53 4.71 3.26 3.71 2.84 3.73 3.82

µg/L 14.91 8.23 0.99 0.84 0.53 0.55 0.60 0.65

NP2EO mg/kg 18.61 11.63 23.49 16.09 27.01 18.19 22.80 8.12

µg/L 1.71 0.10 0.16 0.23 0.07 0.10 0.12 0.15

NP1EO mg/kg 10.36 7.17 0.61 0.57 0.47 0.79 0.38 0.50

µg/L 0.72 1.91 0.08 0.57 0.10 0.79 0.20 0.50

NP mg/kg 8.58 2.79 17.49 12.35 7.85 5.94 8.58 5.77

µg/L 1.40 0.52 0.96 0.20 0.87 0.14 0.88 0.12

NP1EC mg/kg 0.40 0.15 9.47 9.76 2.83 2.06 2.66 1.99

µg/L 0.22 0.13 1.36 0.61 0.51 0.26 0.98 0.25

OP mg/kg - - - -

µg/L 0.13 0.22 0.23 0.10 0.19 0.08 0.18 0.09

It confirms that a significant proportion of LAS in raw sewage adsorbs to

particulate matter and is removed through the sludge withdrawn from

primary settling tank, thus confirming the importance of primary

treatment in the removal of surfactants and in the mass balance of

surfactants in WWTPs.

An average LAS level in biological sludge of CASP and MBRs was

447.9 mg/kg, 97.81 mg/kg (for MBR-HF) and 46.54 mg/kg (for MBR-

PF), showing a great difference in the concentrations of primary and

secondary sludge. A similar relation was found in Terzic et al. (2005),

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Lubello, Gori 11

where against a primary sludge concentration of 7000 mg/kg, it was

found a quite lower level of 30 mg/kg onto biological sludge. It could be

explained not only by the dissolved concentration of LAS but also by the

difference in the homologs composition between primary and secondary

effluent. In fact, LAS sorption on suspended sediment increases with

increasing length of alkyl chains. This is confirmed in the present study

by the analysis of the different homologs of LAS in solid and liquid for

the same sample. Figure 1 shows the different composition of LAS

between the solid and the liquid phase in a sample of CASP sludge which

is representative of all sludge samples. The concentration in the solid

phase of C12 and C13 homologs is much higher than that of C10 and C11

homologs, while a more balanced composition of homologs was found in

the liquid phase.

In addition concentration of LAS in CASP sludge was much higher than

in MBRs sludge thus supporting the hypothesis that a better

mineralization of LAS occurred in MBRs than in the CASP. In fact a

lower concentration of LAS was detected both in the effluent and in the

sludge of MBRs than those of CASP.

solid phase C10

3%C11

7%

C12

49%

C13

41%

C10

C11

C12

C13

liquid phase C10

9%

C11

42%

C12

38%

C13

11%

C10

C11

C12

C13

Figure 1. Example of compositional change of LAS in solid and liquid

phase for CASP biological sludge sample.

Concerning nonylphenolic parent compounds, for NP3-15EO, in the

primary sludge a mean content of 11.22 mg/kg was found while levels of

3.71 and 3.73 mg/kg were found for MBRs sludge and a similar level

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(4.71 mg/kg) for CASP sludge.

Due to the lack of works focusing on the presence of long chain NPnEO

in primary and secondary sludge, the concentration of NP1-15EO was

calculated as sum of the different homologs NP3-15EO, NP2EO and

NP1EO in order to compare our results with levels found in other studies.

Concentration of NP1-15EO in the primary sludge was on average 40.19

mg/kg, which is lower than those observed by Fountoulakis et al. (2005)

in some Greek WWTPs (90 mg/kg for Athens WWTP and 233.5 mg/kg

for Heraklion WWTP).

Concentrations of NP1-15EO detected onto secondary sludge of CASP

and MBRs were very similar and all belonging to the range of 12.8–45

mg/kg found by Fountoulakis et al. (2005) in secondary sludge of Greek

WWTPs.

The NP2EO resulted the most abundant degradation product both in the

primary sludge and in the biological sludges. As observed in previous

works it is possible to find its accumulation in the sludge due to the

different degradation level achieved in the biological reactor (Ahel et al.,

2000; Hou et al., 2006).

A concentration of 10.36 mg/kg was detected in the primary sludge for

NP1EO with a perfect concordance with Clara et al. (2007) (10.86 mg/kg

in sorbed phase of influent samples having an average TSS concentration

of 396 mg/L). On the other hand concentrations of NP1EO below 1

mg/kg were detected in all biological sludges showing a not significant

accumulation of this metabolite, conversely to other works where the

NP1EO was the predominant by-product of APnEO (Stasinakis et al.,

2008; Hou and Sun, 2007).

NP average value in the primary sludge was 8.58 mg/kg, very similar to

the value of 4.06 mg/kg observed by Clara et al. (2007), while higher

concentrations were reported in Fountoulakis et al. (2005). In secondary

sludge an average level of 17.49 for CASP was found while in both

MBRs sludge a lower concentration of 7.85 and 8.58 mg/kg was sorbed

onto solid phase showing also for this compound a similar behaviour of

MBRs sludges.

The nonylphenoxy acetic acid (NP1EC) was only detected in few of the

seven collected samples in primary sludge with an average value of 0.40

mg/kg. This intermediate is produced via oxidation of the EO chain

during aerobic processes and for this reason was detected in greater

amount in secondary sludge with a concentration of 9.47 mg/kg for

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Lubello, Gori 13

CASP sludge and lower values for MBR sludges (2.83 and 2.66 mg/kg).

Finally any OP trace was not detected neither in primary nor in secondary

sludges. This is due to the relatively low production volume of OPnEC in

comparison with NPnEC that account for about 80% of total APnEO use

(Ying et al., 2002).

Partitioning of surfactants

Partitioning behaviour of LAS, NP3-15EO and NP was investigated on

the basis of concentrations detected in the liquid and in the solid phase.

Considering the primary and biological sludges as heterogeneous sorbent

phases, it was assumed the presence of multiple types of sorption sites

with different free energy and abundance, acting in parallel. According to

Schwarzenbach et al. (2003), a Freundlich isotherm was considered for

representing such a situation. In Figure 2 and Figure 3, log of

concentration in the solid phase (Cs) and in the liquid phase (Caq) are

plotted and fitted using a linear regression.

On the basis of data available, CASP sludge shows an higher adsorption

capability for LAS. The same result appears, even if with minor evidence,

for NP and NP3-15EO compounds.

In order to determine if the differences in the solid/Liquid phase

partitioning between biological reactors are significant, the analysis of

covariance (ANCOVA) for comparing linear regression was performed

for LAS, NP and NP3-15EO. If the regression lines for each reactor were

found to be statistically different (α=0.05) then a Tukey multiple

comparison procedure was used to determine between which reactors the

difference existed. The ANCOVA and Tukey’s method were computed

using MINITAB software.

The ANCOVA analysis indicated that a different behaviour in the

solid/Liquid partitioning of LAS, NP and NP3-15EO was observed in

CASP and MBRs reactors. In fact the p-values found for each compound

tested was below 0.05. Results of the Tukey test are presented proving

that only between CASP and MBR-PF plants the regression equations

were statistically different (considering α=0.10) with a p-value of 0.01,

0.07 and 0.10 for LAS, NPEO and NP respectively. For comparison

between equations of MBR-HF and MBR-PF all p values were found

higher than 0.35 and it can be concluded that the behaviour in the solid-

liquid partitioning of investigated compounds is statistically not

significant. In order to simplify the results obtained and compare them

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with those found in other works, solid/Liquid partitioning coefficients Kd

were calculated as ratio between concentration in the solid phase

(expressed as mg/kg) and the concentration in the liquid phase (expressed

as mg/L). Values are summarized in Table 6.

Figure 2. Sorption behaviour of LAS, NP and NP3-15EO on biological

sludge from CASP and MBRs.

CASP MBR-HF

MBR-PF

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Lubello, Gori 15

The Kd of LAS in primary sludge samples resulted on average 13211

L/kg which is higher than values of 2504 L/kg and 600-5000 L/kg

respectively found by Clara et al. (2007) and Jensen (1999). The present

study confirms that adsorption in the primary treatment plays a

significant role in the removal of LAS in WWTPs.

Concerning Kd of LAS in the biological reactors values of 13316 L/kg

was found for CASP while for MBR-HF and MBR-PF were found to be

6681 and 1949 L/kg respectively.

In the CASP reactor increasing Kd values corresponding to shorter

alkylphenolic intermediates were found (4774 L/kg for NP3-15EO, 6904

L/kg for NP1EO, 17366 L/kg for NP). In fact shorter homologs for

APEO are characterised by higher hydrophobicity and in general the

more hydrophobic is a chemical the greater is the amount that adsorbs

onto solid phase. This trend was not confirmed for both MBRs where

NP1EO showed the lowest Kd among NPnEO degradation products.

Furthermore a very high value of Kd for NP2EO was found in all

reactors; a similar behaviour was found by Hou et al. (2007).

Calculated Kd value for NP in the primary sludge (6380 L/kg) is higher

in comparison with the one found by Clara et al. (2007), while the range

of values found in biological reactors (17366 L/kg for CASP, 8377 and

7347 L/kg for MBR-HF and MBR-PF respectively) is similar to the value

of 10500 L/kg obtained by Ahel et al. (1994) in a biological reactor of a

CASP.

For NP, NP1EO and NP1EC a greater adsorption ability (higher Kd) was

showed in the CASP rather than in MBR pilot plants. It could be

explained by the importance in sorption mechanism of the organic carbon

(OC) content characterizing the sludge (John et al., 2007). Hou et al.

(2006) reported a strong relationship between Kd and OC content for

small metabolites of NPnEO. In particular the kd value for NP, NP1EO

and NP2EO increased with the OC% content. NP and NP1EO Kd values

increased from 2570 to 33600 and from 1480 to 38800 L/kg respectively

with the OC% content increasing from 1.3% to 25.2%. For the present

study VSS data are not available (with the exception of VSS/TSS ratio

for CASP of 87%), but it is known that OC content of sludge depends on

the SRT (with the same inlet characteristics). Since MBR pilot plants

were operated at a significant higher SRT than CASP, a significantly

lower OC% is expected in MBRs sludge and a lower Kd can be expected.

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16 Lubello , Gori

In the case of NP3-15EO, Kd values estimated in this study are lower

than those found in previous works; for example John et al. (2007)

reported Kd values in the range 12000-33000 L/kg in CASP reactors.

With respect to NP, NP1EO and NP1EC, an opposite behaviour was

registered for NP3-15EO with higher Kd values in MBRs rather than in

the CASP and decreasing values for increasing SRT. It could be supposed

that higher SRTs in MBRs are responsible for a more efficient

degradation of NPnEO causing a stronger shift in the homologs

composition (from NP15EO to NP3EO). Consequently, an higher

fraction of short chains NPnEO (characterized by higher Kd values)

could be present in MBRs, according to the typical biodegradation

pathway shown by Ahel et al. (1994).

Table 6. Kd values of LAS, NP3-15EO, NP2EO, NP1EO, NP and NP1EC

in CASP and MBRs reactors

u.m. Primary CASP sludge MBR-HF

sludge

MBR-PF

sludge

Mean St.dv Mean St.dv Mean St.dv mean St.dv

LAS L/Kg 13211 6559 13316 10871 6681 7932 1949 1274

NP(3-

15)EO

L/Kg 805 355 4774 2422 6252 3388 7045 1777

NP2EO L/kg 9695 291510 118635 124504

NP1EO L/kg - - 6904 3464 3416 3046 3821* -

NP L/Kg 6387 2172 17366 1079 8377 5928 7347 5772

NP1EC L/Kg - - 7906 8828 5862 3748 3115 2587

(*) n=1.

It is important to highlight that on the basis of concentration of

“Confronto tra bireattori a membrana e impianti convenzionali …”

Lubello, Gori 17

surfactants in the effluent and onto the sludge and suspended solids in the

effluents and partitioning coefficients of surfactants, it is possible to

affirm that the higher efficiency of MBRs in removal of NP1-15EO, can

be attributed to a better biodegradation rather than to the absence of

solids in the MBRs effluent.

RINGRAZIAMENTI. Questo lavoro trae origine da un più ampio

progetto di Ricerca finanziato anche da: GIDA SpA di Prato e il

Ministero della Scienze e dell’innovazione CEMAGUA (CGL2007-

64551/HID) cui hanno partecipato direttamente Francesca Malpei, Mira

Petrovic, Susana Gonzalez e Damia Barcelo. Si ringrazia in particolare

l’ing. Laura Cammilli per la preziosa attività di Ricerca sviluppata, che è

base rilevante di questo lavoro.

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