Simultaneous removal of perchlorate and nitrate from drinking water using the ion exchange membrane bioreactor concept

ARTICLE IN PRESS WAT E R R E S E A R C H

40 (2006) 231 – 240

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Simultaneous removal of perchlorate and nitrate from drinking water using the ion exchange membrane bioreactor concept Cristina T. Matos, Svetlozar Velizarov, Joa˜o G. Crespo, Maria A.M. Reis CQFB/REQUIMTE, Chemistry Department, FCT, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal

art i cle info

A B S T R A C T

Article history:

This work evaluates the feasibility of the ion exchange membrane bioreactor (IEMB)

Received 18 January 2005

concept for the simultaneous removal of perchlorate and nitrate from drinking water,

Received in revised form

when nitrate is present in the ppm range and perchlorate in the ppb range. The IEMB

20 September 2005

concept combines Donnan dialysis and simultaneous biological degradation of both

Accepted 20 October 2005

pollutants. Membrane transport studies showed that Donnan dialysis is suitable for

Available online 15 December 2005

obtaining water with concentrations of perchlorate and nitrate below the recommended

Keywords:

levels. However, the pollutants were accumulated in a receiving stream, thus requiring

Drinking water

additional treatment before disposal. On the other hand, the IEMB process operated with

Ion exchange

hydraulic retention times ranging from 1.4 to 8.3 h in the water compartment, proved to

Membrane bioreactor

remove effectively perchlorate and nitrate while preserving the water composition with

Donnan dialysis

respect to other ions, thus avoiding secondary contamination of the treated water. For a

Perchlorate Nitrate


polluted water stream containing 100 ppb of ClO
4 and 60 ppm of NO3 , the concentrations of

both ions in the treated stream were kept below the recommended levels of 4 ppb for ClO
4 and 25 ppm for NO
3 . The IEMB system was operated under ethanol limitation, but even under these conditions, an increase of the perchlorate and nitrate concentrations in the treated water was not observed for up to 6 days. & 2005 Elsevier Ltd. All rights reserved.

1.

Introduction

Perchlorate is a drinking water pollutant that represents a risk for public health because it can disturb the production of metabolic hormones by the thyroid gland, affecting development, and can even induce thyroid gland tumors (Urbansky, 2002; Logan, 2001). Therefore, the US Environmental Protection Agency (EPA) recommends the use of a provisional cleanup level for this water pollutant in the range of 4–18 ppb (US Environmental Protection Agency, USEPA, 2003). Nitrate is a well-known water pollutant harmful to human health (Bauchard et al., 1992). The European Community limits the concentration of nitrate in public drinking Corresponding author. Tel./fax: +351 212 948 385.

E-mail address: [email protected] (M.A.M. Reis). 0043-1354/$ - see front matter & 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.10.022

water supplies to a maximum of 50 ppm, but recommends a level below 25 ppm (European Community EC, 1980). Nitrate is often found in drinking water resources also contaminated with perchlorate (Logan and LaPoint, 2002); however, while perchlorate is usually present at low concentrations (o100 ppb), nitrate is often found in much higher concentrations (in the ppm range). It is a challenge to develop a remediation technology able to economically remove perchlorate from contaminated water resources to this very low recommended level. The existing water treatment technologies for the removal of oxyanions like perchlorate and nitrate can be divided into physical/chemical and bioremediation technologies. Within the first group, ion

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40 (2006) 231– 240

Nomenclature

Subscripts

d A C D dw F J L Pi,m Q

1 2 i

boundary layer thickness (mm) membrane area (m2) ion concentration (mmol/l; ppb; ppm)

water compartment biocompartment target polluting ion

diffusion coefficient (cm2/s) dry weight

Superscripts

water flow rate (l/h) flux (g/m2 h) membrane thickness (mm)

in out

entering the water compartment leaving the water compartment

membrane permeability of ion i (cm2/s) membrane ion exchange capacity (mol/l)

exchange (Gu et al., 2003) as well as membrane processes such as electrodialysis (Roquebert et al., 2000), nanofiltration or reverse osmosis (Amy et al., 2003) are commonly used. The pollutants, however, are accumulated in a brine solution or a concentrate stream, which have to be treated before disposal. On the other hand, perchlorate and nitrate can be used as electron acceptors by some bacteria under anoxic conditions. Perchlorate is sequentially reduced to chloride according to


the following reaction: ClO
4 ! ClO3 ! ClO2 ! Cl . The final
product of this reduction is the innocuous Cl ion (Logan et al., 2001; Xu et al., 2003). Nitrate can also be biologically reduced to nitrogen gas in four sequential steps:
NO
3 ! NO2 ! NO ! N2 O ! N2 . It has been found that many perchlorate respiring bacteria are also capable of reducing nitrate, but it has not been yet clarified whether the reduction of perchlorate and nitrate is catalyzed by a single reductase; there are indications suggesting that these two ions are reduced by individual enzymes in some microorganisms (Xu et al., 2003, 2004). Biological reduction allows for the selective removal of perchlorate at a relatively low cost (Urbansky and Schock, 1999), but additional treatment is necessary to eliminate secondary contamination of the treated water by microbial cells, excess of nutrients and/or metabolic by-products. The ion exchange membrane bioreactor (IEMB) concept was introduced in the late 1990s (Crespo et al., 1999) and successfully tested for drinking water denitrification (Fonseca et al., 2000; Velizarov et al., 2001, 2002). A European patent was granted in 2003 (Crespo and Reis, 2003). The IEMB is an integrated process that combines the transport of charged pollutants from water streams (water compartment) through an appropriate ion exchange membrane with their simultaneous biodegradation by a suitable microbial culture in a separate compartment (biocompartment). Figure 1 illustrates
the transport of ClO
4 and NO3 from the water compartment to the biocompartment through a membrane with positively charged fixed groups (anion exchange membrane), followed by reduction of perchlorate to chloride, and nitrate to nitrogen by a suitable anoxic microbial culture. Additionally, chloride is added to the biological medium (biomedium) as a so-called driving counter-ion, which is transported through the membrane in a direction opposite to
that of the target counter-ions (NO
3 and ClO4 ). This type of transport is commonly referred to as Donnan dialysis (Lake

Anion Exchange Membrane

Treated Water

Biofilm Na +

Cl

Na +

-

Cl

-

CO2 + Cl -

ClO4-

ClO4

CO2 + N2

NO3-

NO3-

Biocompartment

Carbon Source + Nutrients + Additional Counterion (Cl - )

Water Compartment

Polluted Water

Figure 1 – Schematic diagram of ion transport and bioreduction in the ion exchange membrane bioreactor (IEMB).

and Melsheimer, 1978), which in the IEMB is integrated with the simultaneous biological degradation of the pollutants. The main advantage of the IEMB process is the selective ionic pollutant removal that results in a high quality treated water. Due to the physical separation of the microbial culture and the water stream, the treated water production rate does not depend on the hydraulic retention time (HRT) in the biocompartment. This feature is especially important for the removal of micropollutants, because the biological reduction rates in the ppb concentration range are generally low and require high HRTs. This work aims at evaluating the feasibility of the IEMB concept to the removal of a micropollutant such as perchlorate (in the ppb range) from contaminated water, containing nitrate at much higher concentrations (in the ppm range). This scenario occurs frequently in drinking water treatment systems, due to the large difference in the recommended concentration limits for each anion. Initially, the transport of nitrate and perchlorate with the system operating without biological reduction (Donnan dialysis) was studied. Then, the

ARTICLE IN PRESS WAT E R R E S E A R C H

biological reduction of perchlorate and nitrate, simultaneously present in the medium, was studied in a batch reactor. Finally, the performance of the integrated IEMB process was evaluated in a series of experiments operated under different HRTs in the water compartment, with and without carbon source limitation in the biocompartment.

2.

Materials and methods

2.1.

Experimental setup

The experimental setup used in both Donnan dialysis and IEMB studies was facilitated by a flat parallel-plate module with an anion exchange membrane (Neosepta ACS, manufactured by Tokuyama Soda, Japan) separating two rectangular channels with a defined geometry (channel length/ hydraulic diameter ratio of 52). One of the channels of this module was connected through a re-circulation loop to a reactor (stirred by means of a magnetic bar) with a total volume of 550 ml (biocompartment), which was continuously fed with a solution rich in chloride at a flow rate of 0.0048 l/h, corresponding to an hydraulic retention time (HRT) of 5 days. The other module channel was connected to a recirculation loop, which was continuously fed with polluted water at different flow rates to achieve various HRTs in the water compartment (see Table 1). The re-circulation was maintained by two gear pumps (ZP 140 Ismatec) operating at a volumetric flow rate of 97.2 l/h, providing Reynolds numbers of 3000 in both channels of the module. All experiments were run for at least 16 days and samples were taken periodically from the polluted water feed, treated water outlet, biofeed and biocompartment for conductivity measurements as well as ion, ethanol and cell concentration analyses. The experiments were performed at a temperature of 2371 1C in a air-conditioned laboratory. This installation was operated in two different ways depending on the aim of the experiment:

2.1.1.

IEMB studies

The stirred vessel was filled with 400 ml of fresh biomedium, which was inoculated with 100 ml of enriched microbial culture (see Section 2.3). After inoculation, the reactor was continuously fed with a biomedium containing ethanol and nutrients, and operated as a CSTR with an HRT of 5 days. Polluted water was continuously fed to the water compartment at different HRTs as shown in Table 1. All other operational conditions were kept constant during each experiment. The pH and ORP values were monitored on-line by appropriate electrodes. Samples taken from the biocompartment were centrifuged and filtrated (0.2 mm) before analyses. At the end of experiment 9 (after more than 2 months of IEMB operation) the membrane was removed from the module and the thickness of the biofilm was calculated by subtracting the thickness of the membrane from the membrane thickness with biofilm formation. Both measurements were performed with a micrometer at a precision of 0.001 mm.

40 (20 06) 23 1 – 240

2.1.2.

233

Donnan dialysis studies

In these studies, the same hydrodynamic conditions described for the IEMB (see Section 2.1.1) were used. However, the stirred vessel was continuously fed by a 0.1 mol/l solution of NaCl. Synthetic polluted water (Water 1, Table 1) was continuously fed to the water compartment.

2.2. Bioreduction of the target pollutants in a batch reactor A reactor of 500 ml was filled with 400 ml of fresh biomedium, containing 100 ppb of perchlorate and 60 ppm of nitrate, added in the form of sodium salts, as well as 400 ppm of ethanol. The reactor was inoculated with 100 ml of an enriched microbial culture (see Section 2.3). The initial medium pH was adjusted to 7 and anoxic conditions were maintained using a continuous flux of argon in the headspace of the reactor. The temperature was controlled at 2371 1C. Samples were taken periodically during 5 h for nitrate and perchlorate analyses and cell dry weight measurements.

2.3.

Microbial culture

The culture used to inoculate the batch bioreactor and the IEMB was obtained by enrichment of a primary inoculum taken from a municipal wastewater treatment plant. The enrichment was made in sealed oxygen-free 100 ml flasks with 50 ml of sterile biomedium in an incubator at 2571 1C. When the cultures became visually turbid, they were transferred to new flasks with fresh biomedium. This procedure was repeated 3 times. After enrichment, the culture was stored at 4 1C. For longer storage periods (more than 2 months), the culture was kept in 50% (v/v) glycerol in cryoscopy tubes at
80 1C. The culture was revived by transferring it to a rich biomedium (Nitrate Broth with
0.01 mol/l of NO
3 and ClO4 ).

2.4.

Biomedium

The biomedium used was a sterile solution with the following composition: 1 g/l of K2HPO4, 0.592 g/l of KH2PO4, 0.5 g/l of NaH2PO4, 0.233 g/l of NH4Cl, 0.1 g/l of MgSO4  7H2O and 5.84 g/l of NaCl. Ethanol was added, after sterilization of the medium, at different concentrations, depending on the aim of the experiment.

2.5.

Polluted water

Three different polluted water solutions were used: deionized
water supplemented with 100 ppb of ClO
4 and 60 ppm of NO3 , (Water 1), deionized water supplemented with 100 ppb of ClO
4 (Water 2) and water from the Lisbon public distribution network with a composition described in Table 5, to which
100 ppb of ClO
4 (1 mmol/l) and 60 ppm of NO3 (0.968 mmol/l) were added as sodium salts (referred to as tap water).

2.6.

Analytical methods



The concentrations of ClO
were determined 4 , NO3 and Cl using a Dionex ion exchange chromatography system con-

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40 (2006) 231– 240

Table 1 – Summary of the experimental conditions Exp.

No. exp.

Polluted water

F/A (l/m2 h)

HRT (h)

Donnan dialysis

1 2 3

Water 1 Water 1 Water 1

69.6 17.4 8.7

0.25 2 4

IEMB

4a 5 6 7 8a 9a

Water 1 Water 1 Water 1 Water 2 Water 1 Tap water

18.5 15.4 7.7 7.7 3.1 3.1

1.4 2 4 4 8.3 8.3

a

Experiments 4, 8 and 9 were performed consecutively without opening the module for cleaning or for changing the membrane.

sisting of an ED 50 electrochemical detector, Ionpac AG16 Guard and Analytical AS16 (4 mm) columns and an Anion Suppressor-ULTRA (4 mm) at 30 1C. For the analysis of ClO
4 concentrations in the water compartment, the mobile phase (flow rate 1.5 ml/min) was a 50 mM solution of NaOH; the ClO
4 detection limit was 1 ppb, when injecting 1 ml of sample. The analysis of ClO
4 in the batch bioreactor study and biocompartment (the samples taken from the biocompartment were first passed through Ag Dionex filters to remove Cl
) was made using a mobile phase of 25 mM of NaOH (flow rate 1 ml/min). Under these conditions, the detection limit was 10 ppb because it was necessary to inject a smaller sample volume (250 ml) due to the presence of other anions in the biomedium, which interfere with the perchlorate analysis by overlapping the perchlorate peak. For the determination of Cl
and NO
3 , the mobile phase (flow rate 1 ml/min) was a 5 mM solution of NaOH. The
detection limits were: 0.1 ppm for NO
3 and 1 ppm for Cl . The ethanol concentration in the biocompartment was measured by HPLC using a differential refractometer detector RI-71 and an Aminex HPX-87H column (BioRad, USA). The mobile phase was 0.01 N of H2SO4 (flow rate 0.5 ml/min) and the detection limit was 1 ppm. Samples taken from the water compartment were pre-concentrated by means of solid-phase microextraction (using the Supelco Carboxen/polydimethylsiloxane fiber) and the ethanol concentration was measured in a GCMS-QP2010 Shimadzu equipment mounted with a Supelco Wax-10 column (30 m length, 0.32 mm diameter, 0.25 mm film thickness). The oven temperature program started at 25 1C for 5 min, then a 80 1C/min ramp was used up to 100 1C. A calibration curve was done with standards between 50 and 1000 ppb of ethanol. The quantification limit of this method was 50 ppb of ethanol. The data presented in Table 5 was obtained in a certified laboratory (NP EN ISO/IEC 17025).

3.

Results and discussion

3.1.

Anion transport

exchange membrane, under Donnan dialysis operational conditions with Na+ as the main common co-ion, can be calculated by Eq. (1). When developing this model, it was considered that the ion exchange occurring in the membrane is instantaneous and, therefore, does not offer resistance to the target ion transport: Ci;1 Ci;2
CNaþ ;1 CNaþ ;2 , Ji ¼ L d1 d2 þ þ Pi;m  Q Di;w  CNaþ ;1 Di;w  CNaþ ;2

where the subscripts 1 and 2 refer to the water ðCNaþ ;1 ¼ 0:969 mmol=lÞ and the receiving compartment ðCNaþ ;2 ¼ 100 mmol=lÞ, respectively; d1 and d2 are the thickness of the corresponding boundary layers next to the membrane surfaces at the water and biomedium sides, respectively ðd1 ¼ d2 ¼ 38 mmÞ. This thickness is equal to the ratio dh/Sh where dh is the hydraulic diameter (dh ¼ 0:5 cm for both channels), Sh is the Sherwood number, calculated using the empirical correlation for membrane modules with parallel flat channels (Sh ¼ 0:04 Sc0.75 Re0.33), L is the membrane thickness (130 mm); Pi,m is the membrane permeability; Q is the ion exchange capacity of the membrane (1.5 mol/l), and Di,w is the diffusion coefficient of ion i in water (Dw;ClO
4 ¼ 1:79  10
5 cm2 =s and Dw;NO
3 ¼ 1:9  10
5 cm2 =s) (Lide, 1997). The following mass balance for the water compartment at steady state also gives the overall transport flux of a target ion (Ji): Ji ¼

F in ðC
Cout i;1 Þ, A i;1

Transport modelling aspects

As previously described (Velizarov et al., 2003), at steady state, the transport flux of a trace counter-ion i across an ion

(2)

where F is the flow rate of the treated water leaving the water compartment, A is the membrane area, Cin i;1 is the pollutant concentration in the water compartment feed and Cout i;1 is the outlet concentration of the same pollutant. Considering that Ci,1 in Eq. (1) is the same as Cout i;1 in Eq. (2), it is possible using Eqs. (1) and (2), to eliminate these two parameters and obtain a relationship that allows the prediction of the transport flux of a target anion in the IEMB.

3.1.2. 3.1.1.

(1)


Transport of ClO
4 and NO3 in Donnan dialysis

The objective of these studies was to evaluate the system potential for simultaneous transport of perchlorate and nitrate from model polluted water streams under Donnan

ARTICLE IN PRESS

achieved in about 5 days, which corresponds to the HRT for this compartment. As a consequence of this relatively long HRT, an accumulation of nitrate and perchlorate in the

Receiving Compartment

700 -

Concentration of NO3- (ppm)

Concentration of ClO4- (ppb)

ClO 4 NO 3

600 500 400 300 200 100 0 0

2

4

6

8

10

12

14

16

Time (days)

(a)

Water Compartment

100 Concentration of NO3- (ppm)

dialysis operating conditions. Several experiments were performed using Water 1 (see Section 2.5) at different F/A ratios (water flow rate per unit of membrane area), to study the effect of this parameter on the treated water flux and quality. The F/A ratio is an important parameter in the process design, because it determines the amount of water that can be treated by the system; this parameter can be adjusted independently from the hydrodynamic conditions imposed, keeping the water re-circulation rate constant. As a receiving solution, 0.1 mol/l of NaCl was used for all experiments. The Donnan dialysis data obtained were verified by charge balance calculations in the water compartment (Table 2). Considering that there is no co-ion (sodium) flux through the membrane and that electro-neutrality has to be conserved during the ion transport, the molar amount of sodium in the treated water should be equal to the total sum of perchlorate, nitrate and chloride concentrations. These charge balances were well satisfied for all Donnan dialysis experiments, as can be seen comparing the data in the second and last columns of Table 2. Furthermore, during all experiments there was a preservation of the conductivity values in both compartments, which is an indication of the efficient Donnan exclusion of co-ions (sodium ions) by the membrane used in these studies.
The steady-state ClO
4 and NO3 concentration values and the corresponding transport fluxes are presented in Table 3. As expected, if the F/A ratio increases the concentration of perchlorate and nitrate in the treated water stream also increase due to the reduced HRT in the water compartment. The fluxes of nitrate were about three orders of magnitude higher than those for perchlorate, which is a consequence of the higher nitrate loading rates (also three orders of magnitude higher). The dynamics of the system in the receiving and water compartments for experiment 1 is shown in Figure 2, at a F/A ratio of 8.7 l/m2 h that corresponds to an HRT of 4 h. The receiving compartment steady state was

235

40 (20 06) 23 1 – 240

Concentration of ClO4- (ppb)

WAT E R R E S E A R C H

-

80

ClO4

60

NO 3

-

40 20 10 8 6 4 2 0 0

2

(b)

4

6

8

10

12

14

16

Time (days)

Figure 2 – Time course of perchlorate and nitrate concentrations in the receiving (a) and water (b) compartments for the Donnan dialysis (experiment 1) at HRT ¼ 4 h (F/A of 8.7 l/m2 h).

Table 2 – Steady-state charge balances in Domain dialysis experiments at different F/A ratios in the treated water stream Exp.

F/A (l/m2 h)

Na+ in (mmol/l)

NO
3 out (mmol/l)

ClO
4 out (mmol/l)

Cl
out (mmol/l)

Total (mmol/l)

1 2 3

69.6 17.4 8.7

0.97 0.97 0.97

0.3970.030 0.1570.004 0.0470.002

0.4070.030 0.1470.019 0.0770.007

0.5470.03 0.8470.03 0.9070.04

0.9370.04 0.9970.04 0.9470.04

Table 3 – Steady-state nitrate and perchlorate concentration and fluxes in the Donnan dialysis experiments for different F/A ratios HRT (h)

4 2 0.25

F/A (l/m2 h)

8.7 17.4 69.6

Concentration of target anions in the receiving compartment

Concentration of target anions in the water compartment

Flux (g/m2 h)

ClO
4 (ppb)

NO
3 (ppm)

ClO
4 (ppb)

NO
3 (ppm)

ClO
4

NO
3

458727 731764 25547354

260713 655766 17887323

6.670.7 14.671.9 40.072.7

2.370.1 8.970.3 24.371.8

0.81  10
3 1.46  10
3 4.18  10
3

0.50 0.89 2.49

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120

1.0 Cell ClO4NO3-

100

0.9 0.8

80

0.7 60 0.6 40 0.5 20

0.4

0

Cell concentration (g dw/l)

receiving compartment was observed with steady-state concentrations of perchlorate and nitrate of 458727 and 260713 ppm, respectively. Despite this accumulation, the Donnan dialysis system was able to reduce the target ion concentrations in the treated water to low values. This can be explained by the sufficiently high amount of chloride continuously added to the receiving compartment, thus, favoring the transport of perchlorate and nitrate from the water against their own concentration gradients (see Eq. (1)). The steady-state in the water compartment for experiment 1 (Figure 2) was achieved rapidly, since the HRT for this compartment was set to 4 h. The final concentrations of perchlorate and nitrate were 6.670.7 and 2.370.1 ppm, respectively. It is worth noting that, in experiments 1 and 2 (Table 3), it was possible to remove both ions below the maximum recommended levels, which proved that Donnan dialysis was suitable for obtaining water with sufficiently low concentrations of these pollutants. However, their accumulation in the receiving stream would require additional treatment for their removal before disposal. Perchlorate separation from drinking water by other membrane processes has been investigated recently and a nanofiltration study demonstrated that 25 l/m2 h of polluted water with 100 ppb of perchlorate were treated with separation efficiencies between 75% and to 90% (Amy et al., 2003), while with reverse osmosis, separation efficiencies of 96% were found for the same water. However, pressure-driven membrane processes (especially reverse osmosis) require high trans-membrane pressures, thus increasing the process energy demands. Furthermore, these separation methods have generally lower selectivity compared to Donnan dialysis, which uses ion exchange membranes.

40 (2006) 231– 240

Concentration of NO3- (ppm)

WA T E R R E S E A R C H

Concentration of ClO4- (ppb)

236

0.0 0

1

2

3

4

5

Time (h)

Figure 3 – Time course of simultaneous nitrate and perchlorate bioreduction under batch operational conditions.

microbial communities in these studies or differences in the operational conditions.


3.3. Removal of ClO
4 and NO3 in the ion exchange membrane bioreactor

The previous results demonstrate that perchlorate and nitrate were efficiently transported through the selected membrane and that the enriched microbial culture is able to biodegrade them simultaneously. The next step was to evaluate the IEMB potential for integrated transport and removal of perchlorate and nitrate. Six experiments were performed with two different water sources and different F/A ratios. The hydrodynamic conditions for these studies were the same as those used for the Donnan dialysis experiments.


3.2. Biological reduction of ClO
4 and NO3 in a batch reactor

In order to evaluate the ability of the selected culture to simultaneously reduce perchlorate and nitrate, a batch reactor was operated with 100 ppb of perchlorate and 60 ppm of nitrate. The ethanol concentration (4375 ppm) in the bioreactor was never limiting. Figure 3 shows that the culture was able to remove perchlorate and nitrate in about 4 h. The final residual
concentrations were 0.4 ppm of NO
3 and 12 ppb of ClO4 , respectively. The maximum specific growth rate of the microbial culture was 0.13 h
1, while the maximum specific 2 reduction rates were 7170.1 mg NO
3 =ðg dw cell hÞ (r 40.99)
2 and 86713 mg ClO4 =ðg dw cell hÞ (r 40.97), respectively. The difference in the reduction rates was about three orders of magnitude, which is in agreement with the initial difference in concentrations of both ions. This study demonstrates that the selected culture was able to simultaneously remove perchlorate and nitrate in the range of concentrations needed for the IEMB studies. Simultaneous reduction of nitrate and perchlorate was also reported for an autotrophic, gas phase, packed-bed bioreactor (Logan and LaPoint, 2002), while in other studies (Nerenberg et al., 2002) nitrate inhibited the perchlorate reduction. These contrasting results could perhaps be attributed to dissimilar

3.3.1. Performance of IEMB with synthetically concocted polluted water The results presented in Table 4 show that, for all experiments, it was possible to remove both ions to concentrations within the recommended ranges of 4–18 ppb for perchlorate, and much below 25 ppm of nitrate, respectively. Furthermore, the presence of NO
2 in the treated water was never detected.
For the same initial concentrations of NO
3 and ClO4 , their concentrations in the treated water depended on the F/A
ratio. Indeed, the concentrations of ClO
4 and NO3 in the treated water increased with the F/A ratio. The volume of water that can be treated, with the same membrane area, depended on the quality desired in terms of ClO
4 , because the nitrate was always removed to a level much lower than the recommended one of 25 ppm. Comparing the experiment in which perchlorate was the only pollutant (experiment 7) with that in which ClO
4 and were both present (experiment 6), where the F/A and NO
3 ClO
4 concentration in the polluted water were both kept constant, it can be observed that the final concentration of perchlorate was lower in the experiment with the both ions. This could be explained by the higher transport resistance in the boundary layer of the water compartment (see Eq. (1)) for the more diluted water used in experiment 7.

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237

40 (20 06) 23 1 – 240


Table 4 – Treated water concentrations and fluxes of ClO
4 and NO3 for the IEMB studies and also theoretical predictions of the same fluxes using Eqs. (1) and (2)

Water

Exp.

F/A (l/m2 h)/m2h

Concentration of target anion in the treated water ClO
4 (ppb)

Experimental flux (g/m2 h)

NO
3 (ppm)

ClO
4

Predicted flux (g/m2 h)

NO
3

ClO
4

NO
3

Deionized

4 5 6 7 8

18.5 15.4 7.7 7.7 3.1

1672 1273 6.471.5 1172.5 2.570.6

5.770.2 2.870.5 1.670.9 N.A. 0.570.1

1.60  10
3 1.36  10
3 0.72  10
3 0.69  10
3 0.30  10
3

1.0 0.9 0.4 — 0.2

1.50  10
3 1.29  10
3 0.70  10
3 0.72  10
3 0.30  10
3

1.0 0.8 0.4 — 0.2

Tap

9

3.1

3.671.1

0.370.2

0.30  10
3

0.2

0.30  10
3

0.2

N.A.:- not added.

In this experiment, the IEMB was operated under conditions identical to those of experiment 8, but using tap water (instead of deionized water) supplied with 100 ppb of ClO
4 and 60 ppm NO
3 . The objective was to evaluate if the presence of other ions in drinking water would affect the removal of the target ions. The system performance, in terms of perchlorate and nitrate concentrations in the two compartments, is shown in Figure 4. This experiment started with a small amount of perchlorate (46 ppb of ClO
4 ) in the biocompartment, which was completely removed in 5 days, corresponding to the HRT set for this compartment. The culture could remove perchlorate to below its detection limit and nitrate to 2.370.3 ppm. The target ion concentrations in the treated water were reduced to 3.671.1 ppb of perchlorate and 0.370.2 ppm of nitrate (Table 4) in a short period of time (less than 4 h) and were maintained stable for at least 16 days. Furthermore, nitrite, which is more toxic than nitrate, was detected neither in the treated water nor in the biocompartment. The concentra-

300

40

250 200

30 150 20 100 10

Ethanol (ppm)

Concentration of NO3- (ppm)

Concentration of ClO-4 (ppb)

Ethanol ClO4NO3-

50 0

0 0

2

4

6

(a)

8

10

12

14

16

Time (days) Water Compartment 100

Concentration of NO3- (ppm)

3.3.2. Performance of IEMB with tap water supplemented with pollutant ions

Biocompartment 50

Concentration of ClO-4 (ppb)

At the end of experiment 9 the membrane was removed and a thin biofilm (average thickness of 0.01770.006 mm) was observed to be attached to the membrane surface in contact with the biocompartment. This biofilm is formed due to the ion exchange that creates a region with favorable conditions for culture growth near the membrane surface. In previous studies (Velizarov et al., 2003), it has been reported that the IEMB efficiency decreased slowly over time because of excessive biofilm growth. In the present study, such behavior was not observed because the concentrations of nitrate were lower (60 ppm of NO
3 ) than those in the previous studies ) and therefore, the biofilm formed in the (4150 ppm of NO
3 present study was visually much thinner. The system was operated with the same membrane for more than 2 months (consecutive experiments 4, 8 and 9, Table 1) with no decrease in the membrane performance; hence, its cleaning was not necessary.

ClO4NO3-

80 60 40 20 6 4 2 0 0

(b)

2

4

6

8

10

12

14

16

Time (days)

Figure 4 – Time course of perchlorate, nitrate and ethanol concentrations in the biocompartment (a) and water (b) compartment for the IEMB (experiment 9) with tap water, supplied with 100 ppb of ClO
4 and60 ppm of NO
3 at HRT ¼ 8.3 h (F/A ratio of 3.1 l/ m2 h). The ethanol concentration in the water compartment was below the quantification limit of 50 ppb.

tion of perchlorate in the treated water was higher for the tap water as compared to the synthetic one (Table 4). This behavior can be explained by the lower transport driving

ARTICLE IN PRESS WA T E R R E S E A R C H

Table 5 – Ethanol, microbial and ion species analyses of the tap water supplied with 100 ppb of ClO
4 and 60 ppm of NO
3 (polluted water) and the water leaving the IEMB (treated water), after an operation period of 16 days Components

Polluted water (ppm)

Treated water (ppm)

Anions

NO
3 ClO
4 Cl
HCO
3 NO
2 SO24 PO34

58 0.1 16 45.8 o0.005 30 0.34

0.18 0.004 84 8.1 o0.005 20 0.8

Cations

Na+ K+ Ca2+ Mg2+ NH+4

35 1.4 18.8 3.9 o0.05

37 1.8 19.1 3.9 o0.05

Microbial analyses Coliform bacteria Escherichia coli Faecal streptococci Clostridium perfringens

0/100 ml 0/100 ml 0/100 ml 0/100 ml

0/100 ml 0/100 ml 0/100 ml 0/100 ml

Ethanol

—

o0.05

40 (2006) 231– 240

3.3.4.

The IEMB response to ethanol limitation

The ethanol concentration in the IEMB biocompartment must be controlled in order to prevent its limitation for perchlorate and nitrate reduction. However, high concentrations of ethanol could result in ethanol transport to the water compartment (Velizarov et al., 2001). In order to study the response of the IEMB to ethanol limitation, the supply of ethanol to the bioreactor was reduced and the system was operated under carbon limitation. Figure 5 shows that when the amount of ethanol added to the system was insufficient, perchlorate started to accumulate in the biocompartment. A slight nitrate accumulation in this compartment (below 0.5 ppm) was also observed during the first days of limitation. It increased significantly as the ethanol limitation persisted. This accumulation had, however, no influence on the perchlorate and nitrate concentrations in the water compartment for at least 6 days, because the transport of these ions from the water compartment to the biocompartment was always favored by the higher concentration of Cl
in the biocompartment. Therefore, regarding the quality of the treated water only and not that of the bioreactor effluent, the IEMB operation does not require strict control of the electron donor. Therefore, an unexpected disturbance in the ethanol supply would not affect the water quality for a period of time, which is likely sufficient for the detection and correction of that operational problem. This is

force calculated by Eq. (1) as compared to that for experiment 8. Concentration of NO3- (ppm)

Concentration of ClO-4 (ppb)

320 140

ClO4NO3Ethanol

120 100

280 240 200

80

160

60

120

40

80

20

40

0

0 0

2

4

(a)

6 8 10 Time (days)

12

14

16

Water Compartment 100

Concentration of NO3- (ppm)

In order to use Eqs. (1) and (2) for predicting a target ion flux, the membrane permeability (Pi,m) to this ion must first be determined under the test conditions. The membrane permeability may be also viewed as a lumped parameter since it accounts also for any other possible effects not explicitly recognized by the model. The results of the Donnan dialysis studies were used to calculate the membrane permeabilities for nitrate and perchlorate through Eq. (1). The calculated Pi,m values were 3.35  10
8 cm2/s for nitrate and 3.58  10
8 cm2/s for perchlorate. With these pre-determined values, Eqs. (1) and (2) can be used to predict the nitrate and perchlorate fluxes in the IEMB process (Table 4). The predicted target ion fluxes were similar to the experimental ones, which indicates that the model was applicable when using the Pi,m values obtained by independent Donnan dialysis experiments. It should be noted that the model was sensitive to relatively small variations in the F/A ratio, such as the change from experiment 4 to 5, where the predicted fluxes corresponded with the change observed experimentally. The fact that this simplified model predicted the transport of both perchlorate and nitrate very well, could be explained by the relatively low ppb and ppm concentration ranges studied. Under such conditions, it is possible to calculate the fluxes of perchlorate and nitrate independently without significant deviation from the experimental results.

Concentration of ClO-4 (ppb)

3.3.3. Determination of membrane permeability and flux prediction

Biocompartment 160

Ethanol (ppm)

238

90

ClO4NO3-

80 70 60 50 40 30 20 10 0 0

(b)

2

4

6 8 10 Time (days)

12

14

16

Figure 5 – Time course of perchlorate, nitrate and ethanol concentrations in the biocompartment (a) and water (b) compartment for the IEMB operated at HRT ¼ 1.4 h (F/A ratio of 18.5 l/m2 h). The ethanol concentration in the water compartment was below the quantification limit of 50 ppb.

ARTICLE IN PRESS WAT E R R E S E A R C H

an advantage of the IEMB system in comparison to other proposed processes (such as fluidized bed bioreactors), in which a decrease of the electron donor supply to limiting concentrations would lead to production of water with higher pollutant concentrations. Periods of incomplete perchlorate removal at lower ethanol dosing levels were indeed observed in several studies, as discussed recently (Hatzinger et al., 2002; Xu et al., 2003).

3.4.

40 (20 06) 23 1 – 240

239

contamination by organic electron donors is often mentioned. This problem was overcome by some authors, using hydrogen as the electron donor (Nerenberg et al., 2002; Logan and LaPoint, 2002). However, secondary contamination by cell metabolic byproducts could be anticipated. Overall, the IEMB process can remove the two target polluting anions simultaneously and selectively, while preserving the water composition with respect to other ions and avoiding secondary water contamination.

Treated water quality

An important concern related to the use of biological processes in drinking water treatment is the possible secondary contamination of the treated water by microbial cells, excess of nutrients and/or metabolic by-products. Therefore, microbiological analyses were performed on the polluted (tap water) and treated water used in experiment 9 after 16 days of operation. Additionally, quantification of ethanol in the treated water was carried out. No bacterial contamination and no ethanol (quantification limit of 50 ppb) were found in the water after being treated in the IEMB (Table 5). Even with a concentration as high as 250 ppm of ethanol in the biocompartment (Figure 4), ethanol was never detected in the water compartment (o50 ppb) This was due to the very low diffusion coefficient of ethanol through the Neosepta ACS membrane (1.8  10
8 cm2/s (Fonseca et al., 2000), which is about three orders of magnitude lower than the ethanol diffusion coefficient in water 1.28  10
5 cm2/s at 20 1C). This was also due to the presence of a biofilm on the membrane surface contacting the biocompartment, which acted as a reactive barrier that decreased the concentration of ethanol at the membrane surface. Thus, ethanol proved to be a good choice as a carbon source and electron donor for pollutant reduction in the IEMB. Furthermore, the possible presence of methanol and/or other trace toxic compounds in the ethanol feed would not be a limitation of the IEMB process, due to its addition to the biocompartment only. On the other hand, the presence of impurities in ethanol, if any, may prohibit its use as a substrate in other types of bioreactors (e.g., fluidized or packed bed bioreactors) for drinking water treatment applications. Table 5 also shows the treated water quality for experiment 9. There was an effective Donnan exclusion of co-ions (cations) by the membrane, which was proved by the preservation of their concentrations in the water. Since a mono-anion perm-selective membrane (Neosepta ACS) was used, nitrate, perchlorate and some bicarbonate were transported from the polluted water through the membrane exchanging them for chloride. If necessary, the bicarbonate flux from the polluted water to the biocompartment can be prevented by changing the concentration of chloride added to the biocompartment, as previously demonstrated (Velizarov et al., 2002). Several other studies aiming at the simultaneous removal of perchlorate and nitrate from drinking water have been performed at both bench and full-scale (Herman and Frankenberger, 1999; Xu et al., 2003). Almost all of them were done using bioreactors with cell retention such as packed bed and fluidized bed, but the quality of water produced has not been reported in detail, although the problem of secondary

4.

Conclusions

The results presented clearly show that the IEMB process allows for a simultaneous removal of perchlorate and nitrate from contaminated drinking water to below their recommended limits. It was found that it is the quality needed for the treated water in terms of perchlorate that determines the treated water production rate in the IEMB. Under the experimental conditions of this study (polluted water with 100 ppb of perchlorate and 60 ppm of nitrate) it was possible to produce 3.1 l/m2 h of drinking water with removal efficiencies of 96.5%
for ClO
4 and 99.6% for NO3 , without secondary contamination of the treated water by microbial cells and residual organic compounds. Contrary to the traditional biological removal processes, the water treatment rate in the IEMB process does not depend on the pollutant biodegradation rate but rather on the transport rate of the target polluting ion through the membrane. Therefore the IEMB process proved to be rather robust even when operating under extreme conditions such as prolonged periods of carbon source limitation. In comparison to other membrane-assisted processes like reverse osmosis and/or nanofiltration, the IEMB process shows higher selectivity due to the use of a mono-anion permselective membrane and lower energy requirements (only what is necessary for pumping the solutions). It is also less complex compared to electrodialysis, which uses electrodes and an external power supply, and is particularly more effective when dealing with streams that have low concentration of ionic pollutants. Compared to ion exchange treatment only, the IEMB process allows for a continuous operation that eliminates brine and resin regeneration problems. Further work is currently being performed on the process design, considering a possible large-scale application of the IEMB process. Parameters such as membrane module configuration and the effect of different spacers on the hydrodynamic conditions in the water compartment are being investigated. Studies on the removal of other charged anionic drinking water pollutants, such as bromate and arsenate, as well as cationic pollutants, e.g., ionic mercury, have been initiated.

Acknowledgments The authors express their gratitude to Dr M. Rosa´rio Bronze and Professor L.F. Vilas Boas at Instituto de Tecnologia

ARTICLE IN PRESS 240

WA T E R R E S E A R C H

Quı´mica e Biolo´gica for the vital support on the GCMS ethanol analyses. The financial support by Fundac-a˜o para a Cieˆncia e a Tecnologia (FCT), Portugal through Projects POCTI/BIO/43625/ 2000 and POCTI/EQU/39482/2001 is gratefully acknowledged. Cristina Matos acknowledges FCT for the PhD scholarship SFRH/BD/9087/2002. R E F E R E N C E S

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