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 *[email protected]
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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.
“Titolo”
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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