Operation of the SHARON Denitrification Process to Treat Sludge Reject Water Using Hydrolyzed Primary Sludge to Denitrify

By Dosta, J Gali, A; El-Hadj, T Benabdallah; Mata-Alvarez, J

ABSTRACT: An efficient biological treatment to treat reject water from anaerobic digestion of wastewater sludge is the SHARON denitrification process, which takes place in a chemostat reactor, where aerobic/anoxic periods are alternated under specific hydraulic retention time (HRT) and temperature conditions that favor ammonium oxidizers growth and ensure the total washout of nitrite oxidizers, achieving the biological nitrogen removal over nitrite. An optimized performance of this process to treat Spanish reject water was obtained using methanol and working at an HRT of 2 days, 33[degrees]C, and cycle length of 2 hours. Supernatant of hydrolyzed primary sludge was tested to denitrify. Because biochemical oxygen demand was not extremely high in the primary sludge, the fluid dynamics of the system were changed, with respect to the strategy with methanol, but maintaining the reject water influent flowrate. The use of hydrolyzed primary sludge improved the process efficiency, because the alkalinity present in the primary sludge buffered the process until an optimum pH range. Water Environ. Res., 80, 197 (2008).

KEYWORDS: chemostat, denitrification, nitrite, primary sludge, reject water, SHARON.

doi:10.2175/106143007X184636

Introduction

In municipal wastewater treatment plants (WWTPs) with anaerobic digestion of wastewater sludge, the centrifuged effluent of anaerobic digestion (reject water) is recirculated and typically contains 10 to 25% of the total nitrogen loaded to the plant. Separate treatment of this high-strength flow could considerably reduce the total nitrogen concentration in the final effluent (Fux et al., 2003; Hans and Helle, 1997; Mace and Mata-Alvarez, 2002; Van Kempen et al., 2001), to meet new legislations with stricter effluent standards for nutrient release into aquatic environments. Fux et al. (2003) and Van Loosdrecht and Salem (2006) demonstrated that the most economical way to treat this effluent is the biological nitrogen removal (BNR) process when compared with other chemical or physical-biological treatments.

The BNR process is divided into two steps-(1) oxidation of ammonium to nitrate (NO^sub 3^^sup -^) (nitrification), and (2) nitrate reduction to nitrogen gas (denitrification). Nitrification is defined as a two-step process, where ammonium is first oxidized to nitrite (NO^sub 2^^sup -^) (nitritation), and, subsequently, nitrite is oxidized to nitrate (nitratation). Then, denitrification is the reduction of NO^sub 3^^sup -^ to NO^sub 2^^sup -^ and further on to nitrogen gas by the catabolism of heterotrophic bacteria. This process is carried out under anoxic conditions and with a biodegradable carbon source as electron donor. The performance of BNR over nitrite suggests a saving of 25% of the aeration costs and 40% of the external carbon source needed during denitrification and a reduction in the amount of sludge produced (Hellinga et al., 1997, 1999; Jetten et al., 1997).

An efficient biological process to treat reject water is the patented SHARON (Single reactor High activity Ammonia Removal Over Nitrite) process combined with denitrification (Hellinga et al., 1999; Van Loosdrecht and Salem, 2006), which is a continuous process where BNR is performed via nitrite. The SHARON process is based on the fact that, at temperatures above 20[degrees]C, nitrite oxidizers have a distinctly lower growth rate than ammonium oxidizers (Brouwer et al., 1996; Hellinga et al., 1999; Van Dongen et al., 2001; Van Huile et al., 2004). Therefore, if a short residence time and a high working temperature (typically 350C) are carefully selected, the total washout of nitrite-oxidizing biomass is feasible. In the SHARON denitrification process, intermittent aeration is imposed, and both denitrification and concomitant pH control are possible (Mulder and van Kempen, 1997; Mulder et al., 2001; Verstraete and Philips, 1998). However, because the reject water of anaerobic digestion of wastewater sludge is characterized by an unfavorable biochemical oxygen demand (BOD)/ammonia-nitrogen (NH^sub 4^^sup +^- N) ratio, an organic carbon source must be added during the anoxic periods to denitrify. Many organic compounds, including carbohydrates, alcohols, organic acids, and amino acids, can serve as effective carbon sources for heterotrophic denitrifying organisms. Methanol, ethanol, acetate, and/or glucose are often used for denitrification, because these chemicals are available at a relatively low cost (Constantin and Pick, 1997; Orhon and Artan, 1994). From an environmental point-of-view, the use of alternative carbon sources coming from internal flows from the same WWTP is recommended. Several studies focusing on the use of organic carbon sources from residual wastes or wastewaters have been conducted; Aesoy and Odegaard (1994) reported that biologically hydrolyzed sludge (volatile fatty acids [VFAs] constituted 66% of the soluble chemical oxygen demand [COD]) could be used as carbon source to denitrify in a rotating biofilm contactor. Canziani et al. (1995) studied the feasibility of using hydrolyzed primary sludge under mesophilic conditions (solids retention time of 1 day) from a WWTP for biological nitrogen (and/or phosphorous) removal, concluding that the addition of this stream could lead to an increase of the denitrification efficiency by 4 to 10%, with respect to the influent total nitrogen. Barlindhaug and Odegaard (1996) investigated the quality of thermically treated hydrolyzed sludge as carbon source for denitrification. Subsequently, Aesoy et al. (1998) tested thermically treated hydrolyzed sludge (180[degrees]C, 30 minutes) as an organic carbon source to denitrify synthetic wastewater in a packed-bed reactor, obtaining a denitrification efficiency similar to the one registered when using ethanol. Aravinthan et al. (2001) tested several pretreatments (namely, alkaline, acid, autoclaved, alkaline autoclaved, and acid autoclaved solubilization methods) to hydrolyze excess sludge from a sequencing fluidized bed bioreactor treating synthetic wastewater, to increase its biodegradability and use it in denitrification processes. These authors encountered that autoclaved alkaline sludge hydrolysate gave the fastest denitrification rate. Elefsiniotis et al. (2004) reported good denitrification efficiencies when using the VFAs naturally generated in a laboratoryscale acid-phase anaerobic digester (treating a 1:1 mixture of startrich industrial and municipal wastewater) as an organic carbon source to denitrify. Finally, Gali, Dosta, and Mata- Alvarez (2006) reported good denitrification efficiencies when hydrolyzed primary sludge was used as the carbon source to denitrify in a sequencing batch reactor (SBR) treating sludge reject water for biological nitrogen removal over nitrite.

Some studies of the SHARON denitrification process at laboratoryscale conditions have been carried out in past years. Among many others, Hellinga et al. (1998) reported that an average ammonium conversion in the range 80 to 85% were obtained in a SHARON denitrification process, with the following operational conditions: 35[degrees]C temperature, 1.5-day HRT, cycles of 2 hours (80 minutes aerobic and 40 minutes anoxic), and 60% of the total denitrification (1 g methanol/g converted NH^sub 4^^sup +^-N). Fux et al. (2003) reported removal rates of nitritation and denitrification of 0.3 kg NH^sub 4^^sup +^-NM^sup 3^ . d and 0.55 kg NO^sub 2^^sup -^-N/m^sup 3^ . d, respectively, for the treatment of reject water when working at 28.9[degrees]C, total HRT of approximately 4 days, 2.7 mg oxygen/ L, 0.885 mg mixed liquor suspended solids/L, cycle length 2 hours (50% aerobic and 50% anoxic) and using methanol to denitrify. Gali, Dosta, Van Loosdrecht, and Mata-Alvarez (2006) reported a total nitrogen removal above 95% in the treatment of 0.4 kg nitrogen/ m^sup 3^ . d when working with a SHARON denitrification process at 33[degrees]C, HRT of 2 days (50% aerobic and 50% anoxic), using methanol to denitrify, and without external control of pH.

This process has also been scaled up to full-scale conditions in the Utrecht WWTP (Netherlands) (900 kg total Kjeldahl nitrogen [TKN]/ d) and the Rotterdam Dokhaven WWTP (Netherlands) (850 kg TKN/d), where nitrogen removal efficiencies of more than 90% were achieved when working with a total retention time of 1.5 days, aerobic/ anoxic periods of 2 hours (1.33 hours for partial nitrification and 0.67 hour for denitrification), 35[degrees]C temperature, and using methanol to denitrify. The success of operations at Uldrecht and Rotterdam led to the construction of other similar facilities, such as the Zwolle (Netherlands) (410 kg TKN/d), the Beverwijk (Netherlands) (1200 kg TKN/d), and the Groningen (Netherlands) WWTPs. Moreover, a full-scale demonstration facility of the SHARON process will be implemented in the Wards Island plant of New York City by 2007 (Grontmij, 2004).

The aim of this study is to test the possible use of an internal flow of the WWTP rich in readily biodegradable COD to denitrify in anoxie periods of the SHARON-denitrification at laboratoryscale conditions. Moreover, the effluent quality obtained with a laboratory-scale SHARON denitrification process using methanol and using this internal flow to perform the denitrification steps is compared. Materials and Methods

Experimental Devices. A jacketed chemostat reactor (4 L) was used to test the biological nitrogen removal process at laboratoryscale conditions. The operating temperature was maintained by a heating system (Termotronic, Selecta, Barcelona, Spain). Mixing was provided by a mechanical stirrer (Heidolph RZRl, Heidolph Instruments GMBH & Co., Schwabach, Germany). Aeration was supplied by an air blower connected to a pumice stone system. A peristaltic pump (Ismatec, Reglo, Germany) was used to perform the feeding of wastewater to the reactor. Water was withdrawn by means of an overflow system. Another peristaltic pump (Ismatec, Reglo) performed the feeding of the electron donor used to denitrify. The digester was equipped with a dissolved oxygen (IX)) probe (WTW Oxi 34Oi, Weilheim, Germany) and a pH electrode (Crison pH 18, Barcelona, Spain). Moreover, it was controlled and monitored by a programmable logic controller (Logo Siemens, Germany) and a computer with a data acquisition card (PCL- 812PG), a control box, and an interphase card (PCL-743/745). The computer worked with Bioexpert Software (version l.lx, Applikon, Dietikon, Switzerland).

The closed intermittent-flow respirometer described in Dosta et al. (2007) was used to evaluate the characteristics of the COD of the supernatant from hydrolyzed primary sludge.

Substrate and Inoculum. Centrifuged reject water and hydrolyzed primary sludge were obtained from a municipal WWTP of the Barcelona, Spain, metropolitan area. Before using the primary sludge, it was sieved and then conserved at 4[degrees]C during 3 days to favor solids precipitation. To minimize changes in the supernatant of hydrolyzed primary sludge, 2 L of supernatant was prepared every day and kept at 4[degrees]C. With this pretreatment, the supernatant obtained contained few solids.

The seed to inoculate the SHARON denitrification reactor was secondary sludge of a municipal WWTP (Barcelona metropolitan area) previously acclimated to the nitrification/denitrification process (Gali, Dosta, Van Loosdrecht, and Mata-Alvarez, 2006).

Analytical Methods. Analyses of COD, alkalinity, suspended solids, and VSS were performed according to Standard Methods (APHA et al., 1998). Nitrates and nitrites were analyzed by capillary electrophoresis (Hewlett Packard 3D, Waldronn, Germany). Ammonium was determined by an ammonia-specific electrode (model pH 2002, Crison, Barcelona, Spain). Samples were centrifuged at 10 000 r/min for 10 minutes and filtered through 0.45-[mu]m paper filters to remove suspended solids before being fed to the capillary electrophoresis. The VFAs were analyzed by gas chromatography (Hewlet Packard, 5890 series II).

Results and Discussion

Reject Water Composition. Table 1 shows the composition of the real reject water used in the experiments. During the experiments, the exact composition of the wastewater changed, and, consequently, two periods are distinguished. However, this wastewater was always characterized by a high ammonium concentration (600 to 900 mg NH^sub 4^^sup +^-NTL), high temperature (35[degrees]C), and a bicarbonate- to-ammonium ratio in molar basis near 1.0, which means that the alkalinity of the wastewater is not capable to buffer the complete nitrification process, but only approximately the onehalf of the process.

Moreover, reject water from the anaerobic digestion of wastewater sludge from a municipal WWTP is commonly characterized by a low BOD- to-nitrogen ratio (Arnold et al., 2000; Dosta et al., 2007; Hellinga et al., 1999; Vandaele et al., 2000), which implies the use of an external carbon source to denitrify. As discussed above, this external carbon source could be synthetic or (if possible) the VFA present in another wastewater stream of the same municipal WWTP.

SHARON Denitriflcation Using Methanol to Denitrify. The SHARON denitrification process using methanol as the external carbon source to denitrify was carried out at laboratory-scale conditions. The strategy proposed by GaIi, Dosta, Van Loosdrecht, and Mata-Alvarez (2006) after an optimization study of the SHARON denitrification process for sludge reject water was implemented in this work. The temperature was maintained at 33 +- 0.1[degrees]C. The retention time was set at 2 days, and the duration of the nitrification/ denitrification cycles was 2 hours (1 hour aerobic and 1 hour anoxic). The stoichiometric amount of methanol per gram of NO^sub 2^^sup -^-N formed was added during the first 30 minutes of every anoxic period, to prevent its accumulation for the subsequent aerobic stage, which would reduce the nitrifying activity because of oxygen competition with heterotrophic biomass. To start up the process, the reactor was seeded with microorganisms from the withdrawals of an SBR for biological nitrogen removal. The behavior of the SHARON denitrification process without external pH control was analyzed during 3 months. During the first 2 months (period 1, P- 1), the ammonium load and the alkalinity-to-nitrogen ratio of the wastewater tested was slightly higher than that analyzed in the subsequent 1-month period (period 2, P-2). The most relevant operational parameters under steady-state operation obtained in this study are presented in Table 2, where it is observed that the nitrogen removal efficiency was above 96% during P-1 and slightly lower (approximately 87%) during P-2. This reduction in nitrogen removal efficiency also led to an increase of the dissolved oxygen level and a descent of the pH range related to a reduction in the alkalinity of the biological system.

Figure 1 shows representative pH and dissolved oxygen profiles during 2 days of operation under steady-state conditions of period 2. As observed, the pH range oscillated within 6.6 to 7.7, as a result of the restricted alkalinity of the wastewater tested. The dissolved oxygen concentration inside the reactor was increased during the aerobic step because of the reduction of nitrifying activity associated with the pH descent. Moreover, in Figure 2, the effluent concentrations of NH^sub 4^^sup +^-N and NO^sub 2^^sup -^- N during the 3 months of operation are presented. As observed, the average effluent concentration (obtained from daily integrated samples) of ammonium was 8.4 (P-1) and 60.2 mg NH^sub 4^^sup +^-N/L (P-2), and the average effluent concentration of nitrite was 11.5 (P- 1) and 28.5 mg NO^sub 2^^sup -^-N/L (P-2). Nitrate formation was not detected. Table 3 shows the composition of the effluent of the SHARON denitrification reactor in period 2. Considering that this effluent is recirculated to the main stream of the WWTP, the obtained effluent quality is acceptable. The percentages of soluble COD and nitrogen removal from the reject water were 37.3 and 87.3%, respectively. In Table 2, the average ammonium uptake rate (AUR) and nitrite uptake rate (NUR) are shown. The AUR and NUR were calculated according to eqs 1 and 2, respectively, as follows:

Where

Q^sub 1^ = daily influent flowrate of the reject water (L/d);

S^sub NH^sub 4^REJECT WATER^ = NH^sub 4^^sup +^-N concentration in reject water (mg/L);

S^sub NO^sub 2^REJECT WATER^ = NO^sub 2^^sup -^-N concentration in reject water (mg/L);

S^sub NH^sub 4^EFFLUENT^ = NH^sub 4^^sup +^-N concentration in the effluent (mg/L);

S^sub NO^sub 2^EFFLUENT^ = NO^sub 2^^sup -^-N concentration in the effluent (mg/L);

S^sub NO^sub 2^PRODUCED^ = NO^sub 2^^sup -^-N produced (mg/L) resulting from the oxidation of NH^sub 4^^sup +^-N calculated, as reported in the activated sludge models (Henze et al., 2000) considering Y^sub AOB^ = 0.20 mg cell COD/mg NH^sub 4^^sup +^-N consumed (Gali, Dosta, Van Loosdrecht, and Mata-Alvarez, 2006) and i^sub XB^ = 0.07 mg NH^sub 4^^sup +^-NMg COD;

t^sub N^ = time daily for nitrification (h/d); and

t^sub DN^ = time daily for denitrification (h/d).

As shown in Figure 2, during period 2, the NO^sub 2^^sup -^-N concentration was always lower than the NH^sub 4^^sup +^-N effluent concentration, which is in agreement with the experimental results obtained by Fux et al. (2003). Considering that the alkalinity-to- nitrogen ratio of this wastewater was very close to 1.0 (see Table 1) and that denitrification is only capable of recovering one-half of the alkalinity consumed during nitrification, this could be because the required alkalinity-to-nitrogen ratio (2.0) to oxidize all the NH^sub 4^^sup +^-N concentration could not be reached.

Selection of an Internal Flow of the Wastewater Treatment Plant to Denitrify. Because the wastewater tested had a very reduced BOD/ NH^sub 4^^sup +^-N ratio, and the use of methanol represents an additional cost to reduce the content of nitrogen in the effluent stream (0.27 to 0.41 U.S. dollars/kg nitrogen removed, according to STOWA, 1996), the possible use of an internal flow of the WWTP to denitrify was evaluated.

In previous work, Gali, Dosta, and Mata-Alvarez (2006) discussed the suitability of several streams of the plant to denitrify. In Table 4, the characterization of the internal flows evaluated in this study are presented. Although the carbon present in the influent stream of the WWTP and the influent wastewater of the secondary biological reactor would be useful to denitrify, its use was rejected, because a great amount of these wastewaters would be necessary, and, consequently, the reactor’s volume would be extremely large. The hydrolyzed secondary sludge obtained from the bottom of the secondary clarifier could also be used to denitrify, but its use was discarded, because the kinetics of denitrification using endogenous carbon at 30[degrees]C were too slow when compared with the use of methanol or acetic acid (which is in agreement with Henze et al., 2002).

The hydrolyzed primary sludge (from the bottom of the primary thickener, approximately after 0.5-day HRT) contains a high quantity of VFA (characterization shown in Table 4), which can be used to denitrify. However, because of its high content in total solids, this primary sludge should be centrifuged before being inserted to the SHARON denitrification process. To test the biological process performance when using the supernatant of hydrolyzed primary sludge to denitrify at laboratory scale, the pretreatment described in the Substrate and Inoculum section was applied. The main characteristics of the supernatant of primary sludge used in this work are presented in Table 5. Figure 3 shows a respirogram of the supernatant of hydrolyzed primary sludge, where 175 mL of primary sludge was added to 3.0 L of a mixed liquor with biomass from the withdrawals of an SBR reactor (Dosta et al., 2007) and 12 mg Allyl Thiourea (ATU)/L to inhibit nitrifying activity. In this plot, it can be noticed that the biodegradable COD of the hydrolyzed primary sludge can be divided into two fractions-a readily biodegradable COD related to short-chain VFAs and slowly biodegradable COD. As observed in Table 5, approximately 80% of the soluble COD was mainly composed by VFA, and more than 90% of the total COD was biodegradable. Moreover, the use of the supernatant of primary sludge to denitrify implies the introduction of an additional NH^sub 4^^sup +^-N load to the biological system (190 mg NH^sub 4^^sup +^-NTL) and a considerable volume of this internal wastewater stream to denitrify.

SHARON Denitrification Using Supernatant of Hydrolyzed Primary Sludge to Denitrify. Once the steady-state operation of the SHARON denitrification process using methanol as the external carbon source was well-defined, the methanol was substituted with the supernatant of hydrolyzed primary sludge. Because the biodegradable COD of this internal flow was not extremely concentrated, the fluid dynamics of the system were changed, with respect to the strategy with methanol, but the influent flowrate of the reject water was maintained. Considering that the supernatant of hydrolyzed primary sludge had nearly 4 g COD/L related to VFA concentrations, after calculations, it was decided to add 1 L/d of this carbon source. This would lead to a total retention time of 1.4 days, but the reject water treated per day was the same as the SHARON denitrification with methanol.

The substitution of methanol by supernatant of primary sludge was drastic, because it was considered that the VFAs contained in the primary sludge were as easily biodegradable as methanol by the facultative heterotrophs developed under the previous experimental conditions. However, because the supernatant of primary sludge had a low pH value, during the first day of operation, the alkalinity was controlled by dosing sodium bicarbonate when the pH dropped below 6.7. Then, the control of pH was totally suppressed, and the reactor performance evolved to a stationary state. In Table 2, the most relevant operational parameters are presented during the 2 weeks of operation.

In Figure 4, a representative pH and dissolved oxygen profile of 2 days of operation of the SHARON denitrification process using hydrolyzed primary sludge to denitrify is presented. The plot shows that use of the supernatant of hydrolyzed primary sludge improved the kinetics, with respect to the use of methanol, because the alkalinity present in the primary sludge buffered the process until an optimum pH range (7.4 to 8.6). Moreover, the shape of the dissolved oxygen profile of this SHARON denitrification process is considerably different from the one obtained when using methanol. The dissolved oxygen level inside the reactor (from 2.4 to 4.3 mg oxygen/L) was inversely proportional to the oxygen uptake rate (OUR). It is observed that, when the pH decreased, the dissolved oxygen level was increased, which means that the nitrifying activity was reduced. Because the oxygen supply was the same in both SHARON denitrification processes, it was noticed that biomass activity when using hydrolyzed primary sludge was higher than when using methanol. Moreover, the dissolved oxygen rise during aerobic periods of the SHARON denitrification process using primary sludge to denitrify was more pronounced than when using methanol. This can be explained because, since the pH dependency of nitrifying activity follows a Gaussian expression, with an optimum pH near 8.0 (Dosta et al., 2005; Grunditz and Dalhammar, 2001) for lower pH values, a lower increase in nitrifying activity is detected.

Figure 5 shows the results obtained for NH^sub 4^^sup +^-N and NO^sub 2^^sup -^-N concentrations in the effluent of the SHARON denitrification process using hydrolyzed primary sludge to denitrify. The first day corresponds to the changing of methanol to primary sludge as a carbon source. Therefore, it is stated that the process was rapidly adapted to the new COD substrate. The effluent quality obtained in this process was better than that obtained when methanol was used, and the average nitrogen removal efficiency (calculated by eq 3) was 96.7%.

Where

Q^sub 1^ and Q^sub 2^ = daily influent flowrates of reject water and hydrolyzed primary sludge (L/h), respectively;

t^sub N^ and t^sub DN^ = daily nitrification and denitrification rates (h/d), respectively; and

N^sub REJECT WATER^, N^sub PRIMARY SLUDGE^, ana N^sub EFFLUENT^ = total nitrogen (NH^sub 4^^sup +^-N + NO^sub 2^^sup -^-N) of the reject water, primary sludge, and effluent (mg/L), respectively.

Values of the average AUR and NUR are also provided in Table 2. These parameters were calculated according to eqs 4 and 5, where S^sub NH^sub 4^PRIMARY SLUDGE^ and S^sub NO^sub 2^PRIMARY SLUDGE^ = the NH^sub 4^^sup +^-N and NO^sub 2^^sup -^-N concentrations in hydrolyzed primary sludge, respectively. As observed, average AUR and NUR values were improved when hydrolyzed primary sludge was used to denitrify.

On the other hand, considering the quantity of solids in the pretreated hydrolyzed primary sludge (see Table 5), the increase of VSS was higher when the supernatant of hydrolyzed primary sludge was used to denitrify, because more biomass was formed, as a result of the consumption of more biodegradable COD by heterotrophic biomass.

Another aspect important to consider is the low temperature of supernatant of the hydrolyzed primary sludge in real conditions, because the saving in costs related to methanol (approximately 0.27 to 0.41 U.S. dollars/kg nitrogen removed) could not compensate for the possible need of external thermal control. On the other hand, Hellinga et al. (1998) reported that, at their process conditions (reject water concentration of 1000 mg NH^sub 4^^sup +^-N/L, 60% denitrification), the exothermic reactions led to a net temperature increase of 9[degrees]C. This heat production could maintain the desired temperature (33[degrees]C) in the studied biological system.

Conclusions

The application of the SHARON denitrification process to treat supernatant from anaerobic digestion of wastewater sludge can reduce its ammonium load to more than 85%. The customary use of methanol to denitrify can be avoided by the use of supernatant of hydrolyzed primary sludge, because it contains a proper amount of readily biodegradable COD.

When the supernatant of hydrolyzed primary sludge is used to denitrify, the biological efficiency is improved considerably, because it buffers the system within an optimal pH range (7.5 to 8.5).

Credits

The authors thank the CICYT (Comision de Ciencia y Tecnologia; Madrid, Spain) project CTM 2005-02877/TECNO for financial support. Joan Dosta and Alexandre Gali are grateful for the grants received from the Spanish Government and the University of Barcelona (Spain), respectively.

Submitted for publication August 3, 2006; revised manuscript submitted April 17, 2007; accepted for publication May 24, 2007.

The deadline to submit Discussions of this paper is June 15, 2008.

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J. Dosta, A. Gali, T. Benabdallah El-Hadj, J. Mata-Alvarez*

Department of Chemical Engineering, University of Barcelona, Spain.

* Department of Chemical Engineering, University of Barcelona. Marti i Franques, 1. 6th floor. 08028 Barcelona, Spain; e-mail address: jmata@ub.edu.

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