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Fate of aromatic hydrocarbons in Italian municipalwastewater systems: An overview of wastewater treatmentusing conventional activated-sludge processes (CASP)and membrane bioreactors (MBRs)

Francesco Fatone*, Silvia Di Fabio, David Bolzonella, Franco Cecchi

Department of Biotechnology, University of Verona, Strada Le Grazie 15, I-37134 Verona, Italy

a r t i c l e i n f o

Article history:

Received 21 February 2010

Received in revised form

7 August 2010

Accepted 9 August 2010

Available online 14 August 2010

Keywords:

Municipal wastewater systems

Polycyclic aromatic hydrocarbons

Volatile organic compounds

Membrane bioreactor

Solideliquid partitioning

a b s t r a c t

We studied the occurrence, removal, and fate of 16 polycyclic aromatic hydrocarbons

(PAHs) and 23 volatile organic compounds (VOCs) in Italian municipal wastewater treat-

ment systems in terms of their common contents and forms, and their apparent and actual

removal in both conventional activated-sludge processes (CASP) and membrane bioreac-

tors (MBRs). We studied five representative full-scale CASP treatment plants (design

capacities of 12 000 to 700 000 population-equivalent), three of which included MBR

systems (one full-scale and two pilot-scale) operating in parallel with the conventional

systems. We studied the solideliquid partitioning and fates of these substances using both

conventional samples and a novel membrane-equipped automatic sampler. Among the

VOCs, toluene, ethylbenzene, xylenes, styrene, 1,2,4-trimethylbenzene, and 4-chlor-

otoluene were ubiquitous, whereas naphthalene, acenaphthene, fluorene, and phenan-

threne were the most common PAHs. Both PAHs and aromatic VOCs had removal

efficiencies of 40e60% in the headworks, even in plants without primary sedimentation.

Mainly due to volatilization, aromatic VOCs had comparable removal efficiencies in CASP

and MBRs, even for different sludge ages. MBRs did not enhance the retention of PAHs

sorbed to suspended particulates compared with CASPs. On the other hand, the specific

daily accumulation of PAHs in the MBR’s activated sludge decreased logarithmically with

increasing sludge age, indicating enhanced biodegradation of PAHs. The PAH and aromatic

VOC contents in the final effluent are not a major driver for widespreadmunicipal adoption

of MBRs, but MBRs may enhance the biodegradation of PAHs and their removal from the

environment.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The increasing worldwide contamination of freshwater

systems by hundreds of anthropogenic chemicals is a key

environmental problem (Schwarzenbach et al., 2006). To face

this challenge, two priority research aimshave beenproposed:

(a) selection of the most serious target compounds based on

their actual occurrence and toxicological concerns and (b)

definition of the most appropriate water and wastewater

treatment technologies to remove these compounds (Fatta-

Kassinos et al., 2010). Among the target nonconventional

pollutants, oil-derived aromatic substances are hazardous

* Corresponding author. Tel./fax: þ39 045 802 7965.E-mail address: francesco.fatone@univr.it (F. Fatone).

Avai lab le a t www.sc iencedi rec t .com

journa l homepage : www.e lsev ie r . com/ loca te /wat res

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0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.watres.2010.08.011

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both for the environment and for human health (An, 2004;

Luch, 2005; Farhadian et al., 2008; CDC, 2009), and they are

ubiquitous, since most originate from nonpoint sources such

as internal combustion engines (Fernandez-Martinez et al.,

2000). As a result, oil-derived aromatic compounds constitute

a major concern for municipal wastewater treatment plants

(WTPs), where their removal and final fate have been studied

by several investigators who focused on individual WTPs

(Blanchard et al., 2001; Busetti et al., 2006; Vogelsang et al.,

2006; Manoli and Samara, 1999, 2008).

Volatile organic compounds (VOCs) are a related class of

organic compounds with a vapor pressure greater than

0.1�mm Hg at 20 �C and 1�atm. These compounds are exten-

sively used by many industries, and like PAHs, they can

adversely affect both human health and the environment.

VOCs are key ingredients in many consumer products such as

fuels, paints, aerosols, cosmetics, disinfectants, refrigerants,

and pesticides. Thus, they are often abundant in municipal

wastewater (Barcelo, 2004). In fact, the emissions of VOCs

from WTPs have been studied since the 1980s, and aromatic

VOCs typically account for more than 75% to the total VOC

load (Namkung and Rittmann, 1987). The fate of aromatic

VOCs at WTPs is a major concern because of their volatiliza-

tion and the resulting safety risk for the plant’s operators.

However, their bioaccumulation and biodegradation are also

important factors that define the best approach to waste

sludge treatment and disposal. On the other hand, the accu-

mulation of polycyclic aromatic hydrocarbons (PAHs) in

sewage sludge is an issue of major concern, together with the

potential ecological impacts related to the potential use of

these wastes as (for example) soil amendments (Villar et al.,

2006; Cai et al., 2007).

To date, there is insufficient knowledge to outline clear

scenarios for waste treatment in the heterogeneous system of

municipal WTPs. In practice, WTPs often receive both urban

(mostly combinedwastewater and rainfall runoff) wastewater

and a number of additional (often variable) waste flows. The

current Italian law (decree 152/06) states that liquid waste and

industrial wastewater may both be collected byWTPs, so long

as total hydrocarbon and aromatic organic solvent contents of

lower than 10 and 0.4�mg�L�1, respectively.

In addition, there is insufficient knowledge on whether

membrane bioreactors (MBRs) are an appropriate technology

for use in WTPs at scales that are representative of municipal

WTPs (Cirja et al., 2008), even though this technology is being

widely chosen both for treatment of industrial wastewater at

the source and for centralized and decentralized treatment of

municipal sewage (Fane and Fane, 2005; Judd, 2006; Lesjean

and Huisjes, 2008). With particular reference to fuel-derived

aromatic hydrocarbons, three recent large industrial refer-

ences are operating petrochemical and refinery sites in Italy.

Therefore, this appears to be a relatively new sector for MBR

technology (Lesjean et al., 2009), and the technology is

expected to effectively enhance the removal of aromatic

contaminants produced by the oil industries.

However, a number of recent papers have reported that the

effectiveness of MBR technology in the removal of xenobiotics

and persistent compounds is not sufficiently pronounced to

serve as the sole justification for employing MBRs in munic-

ipal wastewater treatment (DeWever et al., 2007; Weiss and

Reemtsma, 2008). Moreover, even volatile compounds could

be influenced in different ways by MBRs in which strong

coarse-bubble aeration is used to scour the submerged

membranes.

The present study was part of a national research project

aimed at identifying the most commonly occurring noncon-

ventional organic pollutants in Italy and the benefits provided

by the application of MBR technology, as the application of

this technology is rapidly growing in WTPs. In particular, we

present and discuss the occurrence, removal, and phase

distribution of PAHs and aromatic VOCs in five full-scale

WTPs, of which three include MBRs (one full-scale and two

pilot-scale), operating as conventional activated-sludge

process (CASP) or MBR plants, that are representative of

typical Italian WTP scenarios.

First, we evaluated the magnitude of the problem by

focusing on the total contents and solideliquid partitioning of

the aromatic compounds in sewage influents received by

WTPs. Next, we discuss the performance of MBRs and

compare this with the performance of CASP-based WTPs to

provide insights into the potential advantages of MBR tech-

nology in urban wastewater treatment systems, in terms of

the ability of MBRs to enhance the removal of PAHs and

aromatic VOCs.

2. Materials and methods

2.1. The analyzed WTPs and the sampling equipment

To account for the heterogeneity of the municipal WTPs in

Italy, we selected five representative WTPs in central and

northern Italy (Table 1, Fig. 1) and monitored levels of many

nonconventional organic contaminants in their influents and

effluents. The criteria used to select the representative WTPs

were based on: (1) the design (maximal) treatment capacity

(from 12 000 to 700 000 population-equivalent); (2) the types of

wastewater collected in the public sewer system; (3) the need

for co-treatment of municipal liquid wastes (mainly sewage

fromseptic tanks andmunicipal landfill leachate); (4) the types

Table 1eCurrent influent flowrate andwastewater originfor the five municipal WTPs analyzed.

WTP Averageinfluentflowrate(m3/d)

Rate ofmunicipalwastewater

(%)

Main industrial activities inthe catchment area

A 25 000 w30 Chemical-petrochemical,

pharmaceutical, agroindustry,

metal plating, shipyard

B 15 000 w100 e

Ca 4900 þ 15,000 w100 e

D 118 000 w60 Chemical-petrochemical, oil

refining, metal plating,

shipyard, thermoelectric

power plant

E 21 000 w90 Oil refining, metal plating

a 4900 and 15,000 to the MBR and CASP, respectively.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 494

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of biological processes (i.e., predenitrificationenitrification,

extended oxidation, multizone biological nutrient removal,

intermittent aeration of continuously fed bioreactors, MBRs).

In addition, as we discuss in Section 2.2, we studied MBR

technology inWTPs B, C, and D, where pilot-, demonstration-,

or full-scale MBRs operated in parallel with full-scale CASPs.

We analyzed the aromatic compounds by means of gas chro-

matographyemass spectrometry (GCeMS, Agilent technology

5975 inert Mass selective detector, Agilent technology 6890 N

network GCs, Agilent technology 7683B series Injector, e O.I.

analytical ECLIPSE 4660) according to the U.S. EPA methods (EPA

8270C/96 and EPA 8260B/96).

We selected target aromatic compounds from the widely

occurring BTEXS (benzene, toluene, ethylbenzene, xylenes,

styrene) VOC group, the 16 PAHs recommended by the

US-EPA, plus an additional 17 aromatic VOCs.

Fig. 1 e Block flow diagrams of the five analyzed publicly owned WTPs.

wa t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 4 95

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Toaccount forvariabilityof the influents,weobtainedsamples

distributed throughout the year (from 2005 to 2008), except in

August, when industrial production and municipal inputs were

drastically reduced. We obtained at least six daily composite

samples of raw wastewater using conventional refrigerated

samplers. We paid particular attention to the solideliquid parti-

tioning, since it is well established that the particulate fraction

may include the largest portion of the potential pollution load

(Ashley et al., 2004). Therefore, it is crucial to determine the levels

of pollutants associated with suspended solids in any evaluation

of the transport and fate of target nonconventional contaminants

in urban WTPs (Buzier et al., 2006).

To provide sufficient reliability in the composite sampling

operation, we complemented the classical samples by collecting

daily composite and concentrated samples using a membrane-

equipped automatic sampler (Fig. 2) that we designed and

engineered for this purpose. This sampler was equipped with

a ZeeWeed 10 (GE-Water and Process Technology) submerged

membrane module, so as to use the same polyvinylidene fluo-

ride hollow-fiber membrane for this special sampling that was

used for the actual wastewater treatment. To collect the

composite samples, we used a timer-controlled feeding pump

with the timing based on the specific local hourly variation in

influent sewageflows.Thepermeatepumpwascontrolledbased

on the wastewater level in the tank. The membrane-equipped

sampler let us analyze both the composite and concentrated

samples (2e12 g SS L�1, instead of the 0.1e0.6 g SS L�1 commonly

analyzed in conventional composite samples from raw urban

wastewater) of influent particulates based on a day-long ultra-

filtration of some 400 to 700 L of raw wastewater. As a conse-

quence, the calculation of the daily composite soluble fraction

was based on sufficiently reliable data to account for the high

variability in the characteristics of urban wastewater, including

both short- and long-term fluctuations.

Prior to analyzing the collected samples,wedried the influent

solids and sewage sludge to a constantweight in an oven at 40 �Cand ground these materials in an agate mortar, followed by

sieving to obtain particles smaller than 1 mm in diameter.

Higher drying temperatures are not recommended due to

possible volatilization of low-molecular-weight PAHs (those

with two or three rings). The fraction smaller than 1 mm was

stored at 4 �C until analysis. As PAHs are easily photodegraded

(Dabestani and Ivanov, 1999), exposure to direct sunlight and

other strong light was avoided during all steps of sample prep-

aration, including extraction and storage of the extracts.

2.2. The MBRs considered in this study

In addition to a full-scale MBR system at plant C, we observed

two pilot-scale plants that were operated in parallel with the

CASP-based WTPs at plants B and D.

The pilot-scale MBRs were stainless-steel tanks with

reaction volumes of 11 m3 (MBR-B; Fig. 3a) and 1.4 m3 (MBR-D;

Fig. 3b) and. Both were equipped with industrial modules

composed of submerged hollow-fiber membranes (manufac-

tured by GE Process and Water Technologies; nominal pore-

size of 0.04 mm) and had membrane areas of 21.6 and 69.9 m2,

respectively, which allowed them to treat real urban waste-

water volumes of up to 24 and 75, respectively. Both the pilot

MBRs were equipped with on-line meters that measured dis-

solved oxygen (DO), oxidation-reduction potential (ORP), pH,

and mixed-liquor suspended solids (MLSS), and could operate

inmultizone treatment schemeswith intermittent aeration in

automatically controlled or sequencing batch reactors. Fatone

et al. (2005, 2008) provide a full description of the pilot MBRs,

and discuss the occurrence and removal of conventional

pollutants and metals by the MBRs.

The full-scale MBR (Plant C) was implemented by upgra-

ding an existing municipal WTP, whose original construction

dated back to the 1970s. Some of the urban wastewater

(up to 6000) is treated in the membrane plant so that it can be

reused for irrigation, and the remainder of the inflow (up to

15,000m3 d�1) is diverted to the conventional part of the plant.

The ultrafiltration membrane has a membrane area of

12,130 m2; Fatone et al. (2007) provide further details.

3. Results and discussion

3.1. Occurrence and liquidesolid partitioning ofaromatic VOCs in influent

Except for benzene, the rest of the BTEXS group (toluene,

ethylbenzene, xylene, styrene) was the most commonly

Influent pump (timer-controlled)

Concentrated sample of suspended particulate

Backwashing line

Pressure gauge

Overflow weir ZW10 ultrafiltration membrane

Permeate line

Permeate pump

Fig. 2 e Membrane-equipped automatic sampler.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 496

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occurring compounds, together with 1,2,4-trimethylbenzene

and 4-chlorotoluene (Table 2). In particular, toluene was the

most relevant compound because of its high level and because

it is widely used as an industrial feedstock and a solvent to

replace the more toxic and carcinogenic benzene.

High BTEXS concentrations were expected since this is

a well-known characteristic of diffuse sources such as vehicle

emissions (e.g., exhaust, fuel evaporation). On the other hand,

the high concentration of 1,2,4-trimethylbenzene was not

expected, even though other researchers have reported that

this compound is sometimes unexpectedly present in air at

significant levels (Fernandez-Martinez et al., 2000). Along with

toluene, 1,2,4-trimethylbenzene occurs naturally in crude oil

and is not removed by oil refineries. Refineries pump this and

other “unrecovered” substances to other facilities that recover

the material and provide it for various uses, such as being

added directly to gasoline to improve combustion. As 4-

chlorotoluene is a high-volume chemical that is widely used,

even as a drain pipe solvent, it was found at higher levels in

pure municipal wastewater than in mixed municipal and

industrial systems.

Because aromatic VOCs are highly mobile, and are not

strongly absorbed by various media such as suspended

particulates, they are present primarily in the liquid phase of

contaminated water (Zytner, 1994). In fact, even in urban

wastewater, the fraction of VOCs associated with the sus-

pended particulate matter was always under our detection

limit (0.5 mg/kg TS).

3.2. Occurrence and solideliquid partitioning of PAHs ininfluent

PAHs are lipophilic (i.e., hydrophobic) chemicals, and the

larger compounds are poorly water-soluble and have lower

volatility than smaller compounds. Because of these proper-

ties, many studies have reported that PAHs are adsorbed onto

organic matter ranging from street particles to the waste

activated sludge produced by the WTPs (Dobbs et al., 1989;

Fig. 3 e a) The pilot-scale MBR operating in parallel with the publicly owned WTP B; (b) The pilot MBR operating in parallel

with the publicly owned WTP B.

wa t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 4 97

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Stringfellow and Alvarez-Cohen, 1999). Table 3 shows the

contents of the 16 PAHs that we monitored in the composite

influent samples at the five urban WTPs.

Table 3 shows that themaximumabundances of individual

PAH compounds in raw urban wastewater were all well lower

than 1.0 mg/L1 and the total values were all less than 2 mg/L1,

even for large WTPs where most of the surrounding catch-

ment area is urbanized. Most of the average concentrations

were well below 0.5 mg/L1 and were <2 mg/L1 even at the

highest level (WTP D). Napthalene was ubiquitous in the

analyzed samples, and fluorene and phenanthrene were

commonly found above the limit of detection (LOD) in the raw

urban wastewaters. Napthalene is a commercially important

aromatic hydrocarbon produced from coal tar and petroleum.

It is widely used to manufacture phthalic and anthranilic

acids, synthetic resins, lubricants, celluloid, lamp black,

smokeless gunpowder, and hydronaphthalenes. Napthalene

is also used in dusting powders, bathroom products,

deodorant discs, wood preservatives, fungicides, and moth-

balls, and is used as an insecticide. It is possibly carcinogenic

to humans (Group 2B) and toxic effects on animals and

humans have been demonstrated (IARC, 2002). The available

data are inadequate to permit an evaluation of the carcino-

genicity of phenanthrene in experimental animals, whereas

fluorene does not appear to be a human carcinogen (Group D).

It is important to note that the toxicity of the PAHs in Table 3

depends on their structure, with different isomers varying

fromnontoxic to extremely toxic. PAHs that arewell-known for

their carcinogenic, mutagenic, and teratogenic properties in-

clude benzo[a]anthracene, chrysene, benzo[b]fluoranthene, B[j]

FA, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene,

coronene, dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene, and

ovalene(Luch, 2005). These compounds were not the mostly

commonly occurring. In fact, theywere foundwith a frequency

of occurrence lower than 50%, and their maximum concen-

trations were all well under 1 mg/L1.

In terms of their solideliquid partitioning, PAHs are typi-

cally associatedwith particulatematter that is already present

in the sewer system (Lau and Stenstrom, 2005;Mansuy-Huault

et al., 2009). In agreement with the results in the literature

(except for napthalene), the samples collected by the

membrane-equipped sampler showed two things: First, the

PAHs were mainly associated (65% or more) with suspended

particulate matter in the influent that could be separated by

membrane micro- or ultrafiltration (Fig. 4). Second, the higher

the KP value of the PAH,which is expressed as a function of the

compound octanolewater partitioning coefficient KOW

(logKP ¼ 0.58 logKOW þ 1.14; reported by Dobbs et al., 1989) for

sorption to solids in municipal sewage, the higher the solid-

bound fraction (Fig. 5).

Taking into account the data fromWTPD,where PAH levels

were both stable and significant, Fig. 4 shows the typical par-

titioning of the analyzed compounds between the wastewater

solid and liquid phases and the associated partitioning coeffi-

cient for the compound (logKP).

3.3. Removal of PAHs and aromatic VOCs in CASP-based WTPs and WTPs with MBRs

The target aromatics undergo different removal mechanisms

in urban treatment systems: VOCs are subject to volatilization

(air stripping) and biodegradation or biotransformation,

whereas PAHs are subject to (bio)sorption and biodegradation

or biotransformation.

Table 2 e Influent concentrations of 23 aromatic VOCs to five Italian municipal WTPs (average over six samples per plant).

Parameter(mg/L)

WTPA

WTPB

WTPC

WTPD

WTPE

Gasperi et al.(2008)

Influent 100%municipal

Nikolaou al.(2002)

Influent 100%municipal

Benzene 0.21 <0.005 0.063 0.239 0.063a <1.0 <0.1e5.90

Toluene 3.544 4.885 3.008 7.169 3.008 <1.0e3.2 <0.05e13.70

m-Xylene þ p-Xylene 0.568 0.15 0.193 0.775 0.15 <0.05e36.40

o-xylene 0.035 0.192 0.313a 0.351 0.035 <0.05e0.60

Styrene 0.148 0.139 0.226 1.116 0.139

Ethylbenzene 0.238 0.093 0.132 0.216 0.093 <0.05e19.80

Chlorobenzene 0.020a 0.120a 0.031a 0.349a 0.020a

Isopropylbenzene 0.530a 0.063a 0.063a <0.005 0.063a

Bromobenzene <0.005 <0.005 <0.005 <0.005 <0.005

2-Chlorotoluene 0.058 0.208a 0.118 <0.005 0.058a <0.1e0.8

n-Propylbenzene 5.567 <0.005 0.110a 0.149 0.110a <0.05e0.6

4-Chlorotoluene 0.05 0.301 0.306 0.22 0.05 <0.25e1.60

1,3,5-Trimethylbenzene 0.053 <0.005 <0.005 0.360a 0.053a <0.25e64.80

1,2,4-Trimethylbenzene 0.168 1.018 0.764 0.81 0.168

Tert-butylbenzene 0.08 <0.005 <0.005 <0.005 0.080a

1,3-Dichlorobenzene 0.04 <0.005 0.090a <0.005 0.040a <0.05e0.20

Sec-butylbenzene <0.005 <0.005 <0.005 <0.005 <0.005

1,4-Dichlorobenzene 0.03 0.618 1.054 <0.005 0.030a <0.1e0.30

p-Isopropyltoluene 0.02 <0.005 0.098 0.56 0.02

1,2-Dichlorobenzene <0.005 <0.005 <0.005 <0.005 <0.005 <0.1e18.10

n-Butilbenzene 0.020a <0.005 0.156 <0.005 0.020a

1,2,4-Trichlorobenzene <0.005 <0.005 <0.005 <0.005 <0.005

a only one sample over the detection limit.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 498

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Table

3e

Influentco

nce

ntrationsof16PAHsreco

mm

endedbyth

eUS-E

PA

influentto

fiveItalianm

unicipalW

TPs.(avera

geoversixsa

mplesperplant).

Parameter(mg/L)

WTP

AW

TP

BW

TP

CW

TP

DW

TP

EAverage

�standard

deviation(mg/L)

Frequency

of

occ

urren

ce(%

)(LOQ,mg/l)

Kolpin

etal.,

2002

Influent100%

municipal

Gasp

erietal.

(200

8)Influent

100%

municipal

Naphth

alene,Np

0.25

0.096

0.113

0.634

0.103

0.24�

0.23

95(0.005)

0.02e0.04

<0.05

Ace

naphth

ylene,Acy

0.030a

0.017a

<0.005

0.022

0.011

0.02�

0.01

32(0.005)

Ace

naphth

ene,Ace

0.18

0.084a

0.027a

0.285

0.115

0.14�

0.10

56(0.005)

0.02e0.04

Fluorene,F

0.177

0.058

0.008a

0.148

0.043

0.09�

0.07

71(0.005)

0.01e0.14

Phenanth

rene,Ph

0.084

0.052

0.047

0.188

0.039

0.08�

0.06

85(0.005)

0.02e0.04

0.02e0.42

Anth

race

ne,An

0.014

<0.005

<0.005

0.038

0.013

0.01�

0.01

22(0.005)

0.07

0.02e0.06

Fluoranth

ene,Fl

0.028

0.018

<0.005

0.126

0.009

0.04�

0.05

44(0.005)

0.02e0.04

Pyrene,Py

0.025

0.018

<0.005

0.107

0.01

0.03�

0.04

44(0.005)

0.02e0.04

0.02e0.53

Benzo

[a]anth

race

ne,B[a]A

n0.023

0.02

0.008a

0.025

0.01

0.02�

0.01

42(0.005)

0.02e0.06

Chryse

ne,Chry

0.059

<0.005

<0.005

0.025

0.02

0.02�

0.02

43(0.005)

0.02e0.08

Benzo

[b]fluora

nth

ene,B[b]Fl

0.016a

<0.005

<0.005

0.014

0.02

0.01�

0.01

22(0.005)

0.02e0.08

Benzo

[k]fluora

nth

ene,B[k]Fl

0.032a

<0.005

<0.005

<0.005

0.040a

0.02�

0.02

7(0.005)

0.02e0.04

Benzo

[a]pyrene,B[a]Py

0.016a

<0.005

<0.005

0.014

0.020a

0.01�

0.01

22(0.005)

0.02e0.06

Indeno[1,2,3-cd]pyrene,I[1,2,3-cd]Py

<0.005

<0.005

<0.005

<0.005

0.020a

0.01�

0.01

4(0.005)

0.02e0.04

Dibenz[a,h]anth

race

ne,dB[a,h]A

n<0.005

<0.005

<0.005

<0.005

0.020a

0.01�

0.01

4(0.005)

<0.02

Benzo

[ghi]perylene,B[ghi]Pe

0.016a

<0.005

<0.005

0.020a

0.020a

0.01�

0.01

12(0.005)

P16PAHs

0.71

0.22

0.14

1.54

0.32

0.76�

0.57

0.02e0.07

aonly

onesa

mple

overth

edetectionlimit.

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3.3.1. Removal and fate of aromatic VOCsGenerally, the removal of aromatic VOCs in the conventional

WTPswas almost complete, and the secondary effluents often

showed VOC concentrations below the limits of quantification

(Table 4). Only toluene was still present in the effluents at

significant levels, with concentrations of 0.75e2.7 mg/L and

0.5e1.9 mg/L, after CASP andMBR treatment, respectively. This

is probably related to the high concentrations in the raw

influent and to the low bioavailability of this compound,

which has low volatility and low solubility in water (Nahar

et al., 2000). It is important to note that despite the heteroge-

neity of the WTP framework, the aromatic VOC contents in

the effluents were not clearly dependent on CASP or MBR

technology. In particular, only the aeration method (micro-

bubble diffusers) was used in all five WTPs. The plants also

differed in their biological processes for the wastewater

treatment line (see Fig. 1) and headworks (aerated grit cham-

bers in WTPs A, D, and E, versus vortex-type grit chambers in

WTPs B and C). In addition, the submerged MBRs operated

with considerable coarse-bubble aeration for membrane

scouring. Therefore, the final solideliquid separation might

reasonably involve different additional removal mechanisms

for aromatic VOCs: biosorption and biodegradation for the

CASPs (with gravitational clarifiers) and air stripping and

biodegradation for the MBRs (with membrane filtration).

As expected, sorption was not a relevant removal mecha-

nism. In fact, the concentration of aromatic VOCs in the

sewage sludge was lower than the detection limit (0.5 mg/kg

TS). Volatilization was likely to be the major removal mecha-

nism for aromatic VOCs, and has previously been observed as

Fig. 4 e Example of solid/liquid partition of the PAHs in a raw urban wastewater and dependence on the logKP.

y = 100,78e -1,2973x

R 2

= 0,8787

y = 227,44e -1,2108x

R 2

= 0,9465

y = 141,25e -0,4398x

R 2

= 0,8092

0

10

20

30

40

50

60

70

80

90

100

1 1,2 1,4 1,6 1,8 2 2,2 2,4 2,6 2,8 3

PAH classified by logKp @ 25°C

)

%

(

d

i

u

q

i

l

e

h

t

n

i

n

o

i

t

c

a

r

f

H

A

P

Secondary effluent Primary effluent Raw wastewater Espo. (Raw wastewater) Espo. (Primary effluent) Espo. (Secondary effluent)

Fig. 5 e PAHs liquidesolid partitioning over the treatment stages in a representative full-scale CASP.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 4100

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a consequence of the turbulence in the headworks, together

with air stripping in the case of aerated grit chambers (Metcalf

and Eddy, 2003). Our multiple sampling along the treatment

line, the relative VOC removal effectiveness varied among the

different sections of the system: 40e60% for the headworks

and primary treatments, and 10e50% of the remaining VOCs

(i.e., after removal by the headworks) for the secondary bio-

logical treatments. Because off-gassing control strategies are

not commonly implemented in urban WTPs, the removal of

aromatic VOCs by volatilization does not take advantage of

a green technology such as compound transfer from the liquid

to the gas phase (Farhadian et al., 2008).

Table 4 also shows that when CASP systems were com-

plementedby theadditionofMBRs, theMBRtechnologydidnot

provide any significant advantages in terms of VOC removal.

As the concentration of aromatic VOCs in the sludge was

always below the detection limit of 0.5 mg/kg TS, mass

balance calculations were not possible, and we could not

quantify the roles of the different removal mechanisms.

However, the xylenes can be used to evaluate the removal

mechanisms within the bioprocesses because of their

chemical and physical properties. Of these compounds,

o-xylene is the least volatile, but p-xylene is the most difficult

for microorganisms to detoxify. Given the removal efficien-

cies of the biological processes, o-xylene was the most

persistent VOC. Therefore, volatilization and air stripping

were likely the main removal mechanisms in both the CASPs

and the MBRs, and no additional significant effect was linked

to the major turbulence generated in the strongly aerated

filtration chamber.

3.3.2. Removal and fate of PAHsDue to their lipophilic and hydrophobic natures, PAHs tend to

adsorb on particulate organic matter and their removal

from the final effluent should be enhanced by advanced

solideliquid separation mechanisms such as MBRs. However,

the WTP effluents analyzed in this study showed PAH

contents in the permeates produced by the parallel MBR

systems (Table 5) suggesting that these compounds are

adsorbed to suspended particles that are already well sepa-

rated by the conventional gravitational clarifiers. During the

primary and secondary treatment stages, the PAHs were

removed at rates of 40e60% and 10e90% of the remaining

PAHs (i.e., after removal by the headworks), respectively.

These results agree with those of other studies of full-scale

WTPs (Manoli and Samara, 2008), in which PAH removal

ranged from 28 to 67% in the primary treatments, including

primary sedimentation. In addition, these results confirmed

our expectations, since PAHs adhere significantly to street

particles, which are usually removed in the conventional

headworks of a WTP. On the other hand, napthalene was

almost completely volatilized in the headworks due to the

amount of water turbulence.

Among the five WTPs we analyzed, only WTP D had suffi-

ciently high PAH contents that we could evaluate the final fate

of the PAHs. With a design (maximal) treatment capacity of

400 000 population-equivalent, WTP D0s conventional flow

scheme included headworks (gross and fine sieving, an

aerated grit chamber), secondary biological treatment

(conventional predenitrification and nitrification, activated

sludge reactors, and secondary gravitational clarifiers), and

final disinfection with peracetic acid.

The solideliquid partitioning of the PAHs in the different

treatment stages of a full-scale WTP was significantly linked

to the hydrophobicity of each polyaromatic compound, which

was given by the KP values (Fig. 5).

Since urban wastewater usually takes hours to reach the

WTPs, the interactions of PAHswith suspendedparticulates are

likely to be in equilibrium by the time the wastewater reaches

the headworks. Therefore, sorption equilibrium is likely to

Table 4 e Effluent concentrations and removal efficiencies of the mostly occurring aromatic VOCs in CASPs and MBRs(average over six samples per plant).

Parameter (mg/L) WTPA

WTPB

WTPC

WTPD

WTPE

MBRparallelto B

MBRparallelto C

MBRparallelto D

Benzene (mg/L) 0.06 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) 71 e >92 >98 >92 e >92 >98

Toluene (mg/L) <0.005 2.7 0.84 2.7 0.75 0.52 0.7 1.9

Removal (%) e 44 72 62 75 89 77 73

Ethylbenzene (mg/L) 0.03 <0.005 0.13 0.12 <0.005 0.12 <0.005 <0.005

Removal (%) 87 >95 2 44 >95 na >96 >98

m-Xylene þ p-Xylene (mg/L) 0.04 0.08 <0.005 0.45 <0.005 0.35 <0.005 0.25

Removal (%) 93 47 >97 42 >97 na >97 68

o-xylene (mg/L) 0.02 0.19 0.17 0.3 <0.005 0.2 0.1 0.35

Removal (%) 43 1 46 15 >86 na 68 na

Styrene (mg/L) 0.005 0.28 0.005 0.16 0.005 0.2 0.1 0.005

Removal (%) 97 na 98 100 96 na 56 100

1.2.4-Trimethylbenzene

(mg/L)

0.02 0.71 <0.005 0.41 <0.005 0.34 0.5 0.32

Removal (%) 88 30 >99 49 >97 67 na na

4-Chlorotoluene (mg/L) <0.005 <0.005 <0.005 0.1 <0.005 0.27 <0.005 <0.005

Removal (%) >90 >98 >98 55 >90 10 >98 >98

S BTEXS 0.16 3.26 1.16 3.74 0.78 1.40 0.92 2.52

Removal S BTEXS �82 �37 �68 �60 �90 �45 �81 �73

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exist within all the analyzed treatment stages, characterized

by decreasing bulk PAH concentrations. The correlation

between the hydrophobicity of the PAH compounds and their

solideliquid partitioning was clear, and values in the liquid

fraction were stably higher than 50% only in the secondary

effluent, where the bulk concentration of theP

PAHs was less

than0.2mg/L1. Inaddition, thecontentsof themost toxicPAHsat

this treatment stagewere below theLOD (data pointswith logKP

greater than 1.8 in Fig. 5).

Therefore, the PAH content in the liquid fraction depends

on the bulk concentrations and on the particular sorbent. As

expected based on the organic content of the sorbents, Fig. 5

confirms that the sorption affinity of the coupled PAH and

activated sludge is higher than that of the coupled PAH and

suspended particulate matter. It is also important to note that

since residual PAHs are dissolved in the secondary effluent,

there would be little advantage to incorporating the

membrane systems used in MBRs. In fact, the full-scale and

pilot MBR systems operating in parallel produced analogous

removal levels (80e95%). The two systems operated with

similar hydraulic retention times, but the solid retention time

was 12 days for the CASP system, and ranged from 200 tomore

than 500 days for the MBR systems. However, the impact of

high sludge age must be considered when evaluating the

biodegradation of PAHs (i.e., the actual removal from

a contaminated flow). Owing to the high variability of PAH

concentrations in the influents and the amount of available

data, the concentration of PAHs in the waste activated sludge

represents the system’s “historical memory”, and provides

Table 5 e Effluent concentrations and removal efficiencies of the 16 PAHs recommended by the US-EPA in CASPs andMBRs(average over six samples per plant).

Parameter(mg/L)

WTPA

WTPB

WTPC

WTPD

WTPE

MBRparallelto B

MBRparallelto C

MBRparallelto D

Np (mg/L) 0.056 0.073 0.043 0.074 0.037 0.042 0.054 0.035

Removal (%) 78 24 62 88 64 56 52 94

Acy (mg/L) <0.005 0.015 <0.005 0.011 <0.005 <0.005 <0.005 <0.005

Removal (%) >83 12 e 50 >55 >71 e >77

Ace (mg/L) <0.005 0.025 <0.005 0.023 0.02 0.014 <0.005 0.03

Removal (%) >97 70 >81 92 83 83 >81 89

F (mg/L) 0.019 <0.005 <0.005 0.011 0.016 <0.005 <0.005 0.009

Removal (%) 89 >91 >38 93 63 >91 >38 94

Ph (mg/L) 0.058 0.014 <0.005 0.012 0.01 0.012 0.008 0.022

Removal (%) 31 73 >89 94 74 77 83 88

An (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 0.007 <0.005 <0.005

Removal (%) >64 e e >87 >62 na e >87

Fl (mg/L) 0.017 <0.005 <0.005 0.011 <0.005 0.048 <0.005 0.017

Removal (%) 39 >72 e 91 >44 na e 87

Py (mg/L) 0.017 0.012 <0.005 0.015 <0.005 0.048 <0.005 0.013

Removal (%) 32 33 e 86 >50 na e 88

B[a]An (mg/L) 0.013 <0.005 0.007 0.011 <0.005 0.007 <0.005 <0.005

Removal (%) 43 >75 13 56 >50 65 >38 >80

Chry (mg/L) 0.015 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) 75 e e >80 >75 e e >80

B[b]Fl (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) >69 e e >64 >75 e e >64

B[k]Fl (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) >84 e e e >88 e e e

B[a]Py (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) 69 e e >64 >75 e e >64

I[1.2.3-cd]Py (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) e e e e >75 e e e

dB[a.h]An (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) e e e e >75 e e e

B[ghi]Pe (mg/L) <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005

Removal (%) >68.75 e e >75 >75 e e >75

P16 PAHs (mg/L) <0.195 <0.139 <0.05 <0.168 <0.083 <0.178 <0.062 <0.126

RemovalP

16 PAHs (%) �73 �37 �64 �89 �74 �19 �56 �92

Fig. 6 e PAHs daily specific accumulation (DSA) and solids

retention time (SRT) in MBRs (data from the pilot-scale

MBR-D).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 9 3e1 0 4102

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the most reliable way to evaluate the typical biodegradation

potentials of the two systems.

Given that the variation in influent PAH levels and

hydraulic retention times were equal in the CASP and MBR

systems, Fig. 6 shows a logarithmic relationship between the

daily specific accumulation of PAHs in the activated sludge

and the sludge age (i.e., the solid retention time). This

suggests that a high solid retention time improved the

elimination of PAHs, but that this was partly compensated

for by the lower biological activity. This is demonstrated by

two observations (Table 6): (1) the volatile to total solids ratio

decreased with increasing retention time in the MBR (from 80

to 52%) and (2) the maximum specific oxygen utilization rate

(sOUR; Spanjers et al., 1996) decreased from 17 to 8 mg O2/(g

VSS-h).

4. Conclusions

In this study, we reviewed five typical Italian scenarios for the

occurrence, treatment, and fate of aromatic organic

compounds (23 VOCs and 16 PAHs) in municipal conventional

activated-sludge plants and membrane bioreactors. Our

results suggest the following main conclusions:

- Among the aromatic VOCs, toluene, ethylbenzene, xylenes,

styrene, 1,2,4-trimethylbenzene, and 4-chlorotoluene were

most common. However, toluene showed the highest

concentrations, ranging from 3 to 7 mg/L; all the others had

concentrations less than 1 mg/L. The removal of aromatic

VOCs was comparable in the conventional and membrane

systems, despite the strong aeration used for membrane

scouring in the submerged MBRs. The biodegradation was

not significantly influenced by the high sludge age, prob-

ably due to the high water-solubility of these compounds;

this differs from the PAHs, which were more efficiently

biodegraded.

- Except for naphthalene (with 95% occurrence in the raw

urban wastewater), only acenaphthene, fluorene, and

phenanthrene occurred at detectable levels in more than

50% of the samples; however, the concentration of any

single compound except napthalene was generally less

than 0.3 mg/L, and more than 60% of these substances were

associated with the suspended particulate matter. The

apparent removal levels of PAHs in conventional CASP-

based plants and MBRs were comparable, but the actual

removals, which were related to biodegradation of the

PAHs, were enhanced by long sludge age (i.e., long solid

retention times), and showed a logarithmic relationship

with sludge age.

- Both PAHs and aromatic VOCs had removal efficiencies of

40e60% in the headworks, even in plants without primary

sedimentation. This demonstrated that PAHs adhere to

removed street particles and that VOCs easily volatilize as

a result of water turbulence in the headworks.

Acknowledgments

This research was supported by the Italian Ministry of

University and Research (projects PRIN 2003 and PRIN 2005).

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