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The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants
Accepted Manuscript Title: The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants Author: T. Alvarino O. Komesli S. Suarez J.M. Lema F. Omil PII: DOI: Reference:
S0304-3894(16)30041-3 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.01.040 HAZMAT 17392
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
22-10-2015 13-1-2016 14-1-2016
Please cite this article as: T.Alvarino, O.Komesli, S.Suarez, J.M.Lema, F.Omil, The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.01.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants
T. Alvarinoa*, O. Komeslib,c, S. Suareza, J.M. Lemaa and F. Omila
a
Department of Chemical Engineering, Institute of Technology, University of Santiago de Compostela,
15782
Santiago
de
Compostela,
Spain
([email protected],
[email protected],
[email protected], [email protected]) b
Ataturk University, Department of Environmental Engineering, 25250 Erzurum, Turkey
c
Middle East Technical University, Department of Environmental Engineering, 06531 Ankara, Turkey
*
Corresponding author. Tel.: +34-881816739; Fax: +34-881816702
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Highlights: -
Complete OMPs mass balance in a combined system biological treatment plus PAC.
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Improvement of the denitrification after PAC addition.
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Enhancement of OMPs biotransformation after PAC addition.
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Relation between hydrophobicity (log D) and sorption onto the PAC.
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Progressive saturation of the activated carbon in the solid phase with the time.
ABSTRACT SeMPAC is an innovative process based on a membrane sequential batch reactor to which powdered activated carbon (PAC) is directly added. It was developed with the aim of obtaining a high quality effluent in terms of conventional pollutants and organic micropollutants (OMPs). High COD removal and nitrification efficiencies (>95%) were obtained already during the operation without PAC, although denitrification was enhanced by PAC addition. OMPs were followed in the solid and liquid matrixes so that biotransformation, sorption onto the sludge and adsorption onto the PAC could be assessed. Recalcitrant compounds, such as carbamazepine and diazepam, were readily removed only after PAC addition (>99%). Progressive saturation of PAC was observed, with increasing concentrations of OMPs in the solid phase. Removal efficiencies for recalcitrant compounds were used as indicators for new additions of PAC. An improvement in the moderately biodegradable OMPs removal was observed after PAC addition (e.g. fluoxetine, trimethoprim) which was attributed to the biofilm that grew onto the sorbent, as well as to adsorption onto PAC. Keywords: Organic micropollutants; activated carbon; MBR; sorption; biotransformation
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1. INTRODUCTION The removal of organic micropollutants (OMPs) in wastewater treatment plants (WWTPs) has been studied in past years [1,2] because of their potential environmental hazards: bioaccumulation tendency, toxicity and estrogenicity [3,4]. Consequently, the European Union has included seven pharmaceuticals (diclofenac, 17 alpha‐ethinylestradiol, 17 beta‐estradiol, estrone, erythromycin, azithromycin and clarithromycin) in its watch list for the Water Framework Directive in order to monitor and gather information regarding the potential risks they may cause to the aqueous environment. Recent technological developments have shown that a combination of biological processes bearing different redox potentials is a promising alternative for enhancing the amount of OMPs biodegraded in a wastewater treatment plant (WWTP). For example, ibuprofen, an anti‐inflammatory, is readily biodegraded under aerobic conditions only, whilst negative redox potentials favour the removal of trimethoprim, an antibiotic [5]. Presently, many innovative bioreactor configurations include membranes with the aim of maximizing biomass retention and thus, enhancing the microbial diversity responsible for the removal of organic pollutants both at colloidal and at soluble levels, to give a final effluent with higher quality [6]. Accordingly, Sipma et al. reported better removal performance in membrane biological reactors (MBRs) compared to conventional activated sludge (CAS) units for slowly biodegradable micropollutants which was attributed to the relatively longer sludge ages promoted in these systems [7]. Hence, it would be expected that the combination of anoxic‐aerobic processes and membrane filtration should favour the achievement of higher removal efficiencies for biodegradable OMPs. A large amount of OMPs present in sewage can be biodegraded in WWTPs [8] , although there are many compounds which have been reported as being recalcitrant, such as carbamazepine or diazepam. In order to ensure the highest removal degree for OMPs, physical‐chemical procedures based on the use of adsorbents or oxidants have been usually considered, both in WWTPs and in
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drinking water plants (DWPs). A relevant number of studies concerning OMPs are focused on the use of activated carbon (AC) in WWTPs, usually as post‐treatment [9,10]. Synergistic effects have been reported when biological transformation with adsorption processes are combined, such as supplying AC in the biological reactor [11], which may lead to additional benefits such as an enhancement of the microbial diversity [12]. In the specific case of MBRs, the growth of a biofilm onto the surface of the activated carbon particles, produced reduction in membrane fouling and enhancement in the removal efficiencies of conventional parameters [13]. This approach has proven to be suitable in the case of dyes or aromatic hydrocarbons with positive results [14,15]. Other studies have shown an effective removal of OMPs, even for the more recalcitrant compounds, in a process that combines adsorption by activated carbon and biological transformation [16,17,18, 19, 20]. Since these studies have focused on the removal efficiencies in the water line (without considering the OMPs present in sludge), further research providing information on the complete mass balances is needed in order to elucidate the role of the different removal mechanisms. Furthermore, the combination of different environmental redox potentials and the influence of AC on OMPs concentration in the solid phase (sludge and the AC) are also key issues. These are the pillars of the SeMPAC technology developed and patented by the authors (patent ES 2 362 298 B2, European Patent application EP12777603.7) [21]. This technology comprises a sequential batch reactor (SBR) which operates with a direct addition of Powdered Activated Carbon (PAC) coupled to an external submerged microfiltration membrane [16]. The aim of the present study was to determine the effect of PAC addition on the removal of OMPs during an anoxic/aerobic biological wastewater treatment process followed by an external microfiltration step. A complete mass balance was carried out in order to determine the OMPs concentration in the liquid and in the solid phase (a matrix of PAC and sludge). The research was focused on the effect of the PAC and the redox potential on the OMPs removal mechanisms (biotransformation and adsorption onto the solids), as well as on the relation between the physical‐ chemical characteristics of the OMPs and their trends to be adsorbed onto the PAC.
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2. MATERIAL AND METHODS 2.1. Experimental set‐up A sequential batch reactor (SBR) connected to an external submerged microfiltration flat sheet membrane provided by Kubota, with a pore size of 0.4 m and a total surface of 0.2 m2, was started up and operated (patent ES 2 362 298 B2) [21]. The hydrophilic membrane permeated during 7.5 min which was followed by a relaxation time of 1.25 min. The SBR and the membrane chamber were inoculated with 6 gVSS/L of biomass which was collected from the conventional activated sludge unit in the WWTP of Calo‐ Milladoiro (NW of Spain). The reactor, with a volume of 30 L, was operated in cycles of 6 h distributed in five stages: filling (14 min), anoxic reaction (70 min), aerobic reaction (210 min), settling (42 min) and effluent withdrawal (14 min). The exchange volume was fixed at 25% and the supernatant was discharged into the 18 L membrane chamber. The hydraulic retention time (HRT) was maintained at 1 d in the SBR, while a recycle ratio of 0.5 was maintained between the membrane chamber and the SBR in order to homogenise the PAC and sludge concentration in both chambers. The composition of the synthetic wastewater fed to the reactor was as follows (g/L): 2.29 CH3COONa∙3H2O, 0.33 NH4Cl, 0.04 KH2PO4 and 0.45 NaHCO3, with pH around 7.5. The PAC purchased to PANREAC (code 211237) was characterized by a specific surface area of 555 m2/g. Three antiphlogistics (ibuprofen (IBP), naproxen (NPX) and diclofenac (DCF)), four antibiotics (trimethoprim (TMP), sulfamethoxazol (SMX) erythromycin (ERY) and roxithromycin (ROX)), three estrogens (estrone (E1), estradiol (E2) and ethynilestradiol (EE2)), three musk fragrances (galaxolide (HHCB), celestolide (ADBI) and tonalide (AHTN)), one antidepressant (fluoxetine (FLX)), one tranquillizer (diazepam (DZP)) and one antiepileptic (carbamazepine (CBZ)) were spiked at concentrations between 1‐40 ppb. OMP concentrations were selected according to their presence in the hospital wastewaters [22], as well as the limits of quantification of analytical methods (LOQ, Table S.1). The substances were obtained from Sigma Aldrich (Steinheim, Germany).
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Three operation periods were considered: the start‐up during which no OMPs were spiked to the influent (P1, 32 days); the second period in which OMPs were spiked to the influent in order to assess their removal without activated carbon (P2, 76 days), and the third period (P3, 118 days) which started at day 108 with one single addition of powdered activated carbon (PAC) in the SBR, at a concentration of 1 g/L. No further addition of PAC was carried out with the aim of studying its progressive saturation with time. 2.2. Analytic methods The sampling routine comprised influent and effluent samples twice a week for solids, COD, ammonium, nitrite and nitrate determinations, while temperature, dissolved oxygen (DO) and pH were monitored daily with a Hach HQ40d multi‐parameter digital sensor. Soluble samples were obtained using 0.45 μm nitrocellulose membrane filters (HA, Millipore). Standard Methods (APHA) were used to determine temperature, pH, total suspended solids (TSS), volatile suspended solids (VSS), alkalinity, ammonium, nitrate, nitrite and chemical oxygen demand (COD) [23]. Several sampling campaigns were carried out during eight months in order to determine the dissolved and sorbed OMPs concentrations in the influent, effluent of the SBR and permeate. Punctual samples were taken with a time delay of one HRT. Liquid phase samples were prefiltered (AP3004705, Millipore), pre‐concentrated by a solid‐phase extraction (SPE) and analysed by gas chromatography– mass spectrometry (GC/MS) and liquid chromatography‐tandem mass spectrometry (LC/MS/MS). The analytical procedure was described by Alvarino et al. [5]. The solid phase samples were taken from the mixed liquor of the SBR and membrane chamber, centrifuged to collect the solids, frozen and lyophilized. The dried sludge was extracted in an ultrasonic bath three times with methanol and acetone and the solvent fractions were combined and evaporated in order to be resuspended in water, pre‐concentrated by SPE and analysed by GC/MS and LC/MS/MS.
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2.3. Micropollutants mass balances OMPs concentration in the liquid and the solid phase was determined to perform a mass balance under pseudo‐steady state conditions taking into account the main removal processes (sorption, volatilization and biotransformation) (Eq. 1). The OMPs concentrations in the liquid phase were determined in the influent and the permeate streams, while the concentrations in the solid phase were measured in the mixed liquor of the SBR and the membrane chamber.
(Eq. 1) where Finf, Feff, Fstripped, Fsor and Fbiod represent the mass flows (in g/d) in the influent, effluent, off‐gas released from the aerobic stage, sorbed onto solids (including PAC) and biotransformed, respectively. Fstripped (Eq. 2) was calculated in function of the dimensionless Henry’s law constant (H, Table S.1), the feed flow rate Q (L/d), the air supply per unit of wastewater treated q (2 Lair/Linf) and the liquid phase concentration in the effluent Cw (g/L). ∙
∙ ∙
(Eq. 2) Since no purges were carried out during this experimental study (226 days) the term corresponding to sorption was calculated taking into account the accumulation of OMPs in the solid phase (Eq. 3). Therefore, periods P2 and the P3 have been considered separately. In P2, sorption was only dependent on OMP sorbed onto the sludge; while in P3 accumulation inside the activated carbon was also taken into account. For this calculation the Kd constant sorption cannot be considered coefficient since solid‐ liquid equilibrium was not achieved.
∙
∙
∙
2 2
1 1
(Eq. 3)
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where Cs1 and Cs2 (in g/L) are the concentration on the solid phase corresponding to times t1 and t2 (d), HRT (d) the hydraulic retention time and VSS the volatile suspended solids inside the reactor (gVSS/L). The calculated OMPs mass flows in the influent, effluent, volatilized during aeration and sorbed onto the solids, were used to determine the OMPs biotransformation according to Eq. 1. The OMPs biological kinetic constants (kbiol, L/gVSS∙d) were obtained considering that the process follows a pseudo‐first order kinetic (Eq. 4).
(Eq. 4) where V is the volume of the reactor (L). Sorption batch test were performed in 1 L flasks at constant temperature (25ºC) in duplicate, using a phosphate buffer, with an initial OMPs concentrations of 2.5 ppb and a concentration of PAC in the range of 1‐10 ppm (0, 2.5, 5, 7.5 and 10 ppm). Continuous stirring was provided at 150 rpm. Samples were taken up to the point a constant OMPs concentration was measured in the liquid phase. The fit of the experimental data to the Freundlich isotherm model was done through linear regression analysis (Eq. 5). 1/
(Eq. 5)
where kf is the adsorption capacity constant and 1/n is related to the intensity of the adsorption, Cs (g OMP/g) and Cw (g OMP/L) are the concentration on the solid and liquid phase, respectively.
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3. RESULTS AND DISCUSSION 3.1. Reactor performance The SeMPAC system was operated at 17‐20ºC and the pH was maintained in the range 7‐8.5 without additional control. During the whole operation (226 days) the turbidity of the final effluent ranged 0.2‐ 0.6 NTU, below the limit established by the World Health Organization to ensure effectiveness of disinfection (1 NTU). COD removal efficiencies were maintained above 95% (Table 1), with biodegradation taking place mainly in the SBR (>90%). High phosphate removal efficiencies were measured (97%) during the whole operation. Nitrification efficiency was stable in the whole system (>98%), although it decreased from 95 to 75% in the SBR due to the lower dissolved oxygen (DO) concentrations (from 2 to 0.8 mg/L) observed during the last 76 days. The decrease in the DO was attributed to the increase in biomass concentration in the SBR (from 4.8 to 8 gVSS/L) as a consequence of the absence of sludge purges during the experiment. The concentration of biomass in the membrane chamber was stable during the whole operation (7±1 gTSS/L). Concerning denitrification, this was in fact the only conventional parameter which was influenced by PAC addition, with an increase from 76 to 87% (Table 1). When PAC was not added (periods P1 and P2) denitrification took place exclusively during the anoxic phase of the SBR cycle, reaching the highest possible value considering a volumetric exchange of 33% in each cycle. Therefore, the improvement in denitrification observed in P3 was attributed to act as support for the growth of a biofilm the capacity of the PAC to adsorb the nitrate ion [24] and to, which implies the existence of anoxic zones even during the aerobic cycles [25]. After PAC addition a decrease in the particle size of biomass agglomerates from 108.5 to 83.2 m was observed [26].
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3.2. Micropollutants fate Operation without PAC Before PAC addition, total removal efficiencies of OMPs in the system were determined by applying the mass balances to the liquid and solid phase concentrations, considering the influent and effluent of the SeMPAC system. IBP, FLX, SMX, HHCB and ADBI were the compounds readily removed (above 80% of total removal efficiency), whilst DCF, CBZ, DZP and TMP exhibited a recalcitrant behaviour with removal efficiencies below 20% (Fig. 1). Musk fragrances and fluoxetine, which are lipophilic compounds, were eliminated by sorption (with removal efficiencies associated to this mechanism of 26%, 5%, 15% and 8% for AHTN, HHCB, ADBI and FLX, respectively) and biotransformation (Fig. 1), while the other OMPs were only eliminated by biotransformation. Prior to PAC addition a study on the OMPs fate during a cycle in the SBR was carried out in order to get a deeper insight on the degradation kinetics at the different redox conditions (Fig. 2). Only FLX and SMX were degraded under aerobic and anoxic conditions [27,28], while for DZP, CBZ and DCF the flat concentration profile confirmed their recalcitrant behaviour. Musk fragrances were mainly removed in the first stage of the cycle (anoxic reaction), according to the results obtained by Xue et al. in an anaerobic‐anoxic‐aerobic system [29] with removal of musk fragrances only in the anaerobic and anoxic stages, whilst the concentration remained constant in the aerobic stage. The decrease on SMX concentration during the first five minutes of the anoxic phase is related to the dilution factor since this compound was only removed under aerobic conditions and requires the presence of an external carbon source [30]. Previous studies reported the influence of nitrification and heterotrophic activity in the co‐metabolic removal of OMPs [5,6]. For instance, IBP, ERY and ROX were mainly removed under aerobic conditions since their biotransformation was highly influenced by nitrification and also by organic carbon degradation in the case of IBP [31,32]. The OMPs were mainly removed in the SBR, being the lower removal efficiencies achieved in the membrane chamber attributed to the residual OMP and organic matter loading rates entering this unit
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(COD and ammonium fed to the membrane unit were lower than 40 mg COD/L and 2.5 mg N‐NH4+/L, respectively). This fact can be related to the fate that the removal of OMPs following a pseudo first order kinetic is modelled as a function of OMPs concentration [33], thus OMPs removal activities were expected to decrease simultaneously with OMPs concentration. Operation with PAC After one single PAC addition at day of operation 108, most of OMPs removal efficiencies were enhanced (Fig. 3, S.1), with sorption onto activated carbon especially relevant for the more recalcitrant compounds (CBZ, DZP, DCF and TMP). Very high OMPs removal efficiencies (>98%) could be maintained for around 50 days of operation after PAC addition (Fig. 3, S.1), although in the case of the removal of SMX and IBP no sorption onto the PAC was observed. The hydrophobicity of OMPs was a key factor in determining their affinity to sorb onto activated carbon [34,35,36]. This means that for neutral compounds removal by sorption could be correlated with the octanol‐water coefficient (log Kow), but in the case of ionizable compounds the pH and ionization constant (pKa) had to be considered also. In fact, in the case of compounds with the same log Kow, the adsorption was dependent on the charge of the compound [37]. Therefore, LogD calculation, which comprises pKa, pH and log Kow, was found as a better option for estimating the solid‐liquid phase distribution [38]. In this work, removal efficiencies above 99% were obtained for compounds with log D >1, such as CBZ, DZP or DCF, whilst IBP and NPX were not affected by the presence of activated carbon (Fig. S.1) in agreement with their log D of 0.94 and 0.73, respectively (at pH = 7) [39]. In order to corroborate the sorption capacity of PAC, several batch tests were carried out to determine the Freundlich isotherms (Table S.2). According to the low values of the measured kf coefficient for the antinflamatories IBP (967 (µg/g)(L/µg)1/n) and NPX (735 (µg/g)(L/µg)1/n), their removal efficiencies were not influenced by sorption onto PAC during P3, in contrast to DZP and CBZ whose high sorption affinity (kf 2850 and 1793 (µg/g)(L/µg)1/n, respectively) led to removal efficiencies during P3 above 99%. The parameter 1/n of the Freundlich isotherms, which measures the intensity of the adsorption, reconfirms
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the low trend of both antiinflamatories to be sorbed onto the PAC (n 4.6 and 2.64 for IBP and NPX, respectively), while the lower n value determined in the case of CBZ and DZP are an evidence of their higher sorption intensity (1.89 and 1.68, respectively). 3.3. Sorption The main removal mechanisms for OMPs in this process were sorption and biotransformation (Fig. 1), as volatilisation was only relevant for the musk fragrance celestolide (the compound with the highest Henry’s coefficient). As will be evidenced from the following results, the presence of PAC in P3 enhanced the sorption capacity of the solid matrix (Fig. 3&4). Operation without PAC During the first stage, the lipophilic compounds, namely musk fragrances and FLX were removed by both, sorption and biotransformation, while the other non recalcitrant OMPs were only removed by biotransformation (Fig. 1, 4 and S.2). In fact, the relative amount of musk fragrances and FLX sorbed onto sludge was above 2500 L/gTSS, whereas in the case of the other OMPs it was below 250 L/gTSS. Flat sheet microfiltration membranes (MF, pore size of 0.4 m) were used, thus ruling out OMPs removal based on size exclusion [40]. Depending on the properties of both the solute and the membrane, compounds with low molecular weight could be adsorbed on the membrane [41], although this was not considered as relevant in the present study due to the hydrophilic character of the membranes used. Accumulation of sorbed musk fragrance was observed in the sludge before PAC addition (Fig. 4&S.2), which could be attributed to two different effects exerted by the cake layer, namely sorption and size exclusion [42]. Operation with PAC After PAC addition, a quantitative extraction of OMPs from the solid phase (matrix composed of sludge and PAC) was carried out. The results showed a pronounced increase in the OMPs concentration in the solid phase with time in both chambers in P3, which was related to the sorption capacity of the
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activated carbon (Fig. 4, S.2). This observation confirmed that after PAC addition the equilibrium between the solid‐liquid phases was not reached in P3. As musk fragrances and fluoxetine were absorbed onto the sludge and the activated carbon in P3 (Fig. 4, S.2), their continuous increase in the solid phase concentration was due to the combination of the effect of retention of the suspended solids and those present in the cake layer, as well as the progressive saturation of activated carbon. The anti‐inflammatories IBP and NPX are compounds that at pH 7 include negatively charged chemical structures and are characterized by log D < 1. These properties make them scarcely adsorbable onto activated carbon [36, 37] and thus their removal was mainly driven by biotransformation during the whole operation. In the case of DCF, the presence of halogens, aromatic rings and heterocycles enhances its removal by sorption onto the PAC [43, 44]. Additionally, its high log D = 1.77 [39] makes the compound significantly adsorb onto PAC (Fig. 3). A progressive saturation of the PAC was observed and lead to a decrease in the removal efficiencies achieved for most OMPs with the time in P3 (Fig. 3, S.1), especially in the case of CBZ, DZP and DCF (from complete removal to below 50% at the end of the operation). The quickness of PAC saturation was related to the charge of the compound [45], being higher for the negatively charged compounds, as evidenced by PAC saturation first in the case of DCF (anion) and later for CBZ and DZP (neutral) (Fig. 3, S.1). In the case of FLX and estrogens complete sorption onto PAC was observed, according to their log D > 1 (FLX 1.83, E1 3.62, E2 4.15 and EE2 4.11) [39], being the removal efficiencies maintained above 98% even after 80 days of PAC addition, which could be related to their positive charge at the pH value in the plant [45]. In the case of the four antibiotics, removal efficiencies above 90% were obtained during the whole period of operation with PAC, although two different behaviours were observed: SMX (a negatively charged compound) was only removed by biotransformation even after PAC addition, maintain constant removal efficiencies in P2 and P3 (Fig. S.1), while TMP (a positively charged compound) was removed by sorption onto the activated carbon as well as by biotransformation.
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The trend of progressive sorption and saturation of the PAC was observed in the OMPs concentrations of the solid phase (Fig 4 and S.2) during P3. After 120 days of operation with PAC, the solid phase concentration of the negatively charged compounds (DCF, IBP or NPX) was lower than in the case of neutral (CBZ, DZP or HHCB) and positively charged compounds (TMP). Additionally, stabilization in the solid phase concentration of the negatively charged OMPs was observed, whereas the concentration of the neutral and positive compounds kept increasing during the whole operation (Fig. 4, S.2). 3.4. Biotransformation Biotransformation was the main removal mechanism in P2 (before PAC addition) for the complete set of selected OMPs (Fig. 1). In the case of antibiotics, removal efficiencies higher than 85% were achieved, except in the case of TMP, being the contribution of sorption onto the sludge lower than 2.5% in all cases. Although musk fragrances are lipophilic compounds, they were mainly removed by biotransformation during P2, representing 94, 82, 68% of the total removal for ADBI, HHCB and AHTN, respectively (Fig. 1). The recalcitrant compounds in biological processes, such as CBZ and DZP, were not removed during P2. After PAC addition, apart from sorption onto activated carbon, an enhancement in biotransformation was observed for some OMPs. The presence of anoxic and even anaerobic zones inside the biofilm could explain the enhanced biotransformation of FLX, which is readily biodegradable under anoxic conditions [28], and also of SMX that needs anaerobic conditions to be degraded [5]. Although this effect was observed for all the OMPs studied, it was more intense for those with moderate biotransformation kinetics (Fig. 5), such as antibiotics (Table 2). For instance, the biotransformed mass flows corresponding to TMP and SMX, which are readily biodegradable compounds under anaerobic conditions [5], increased from 50 and 70 to 270 and 240 µg/d, respectively, after PAC addition (Fig. 5). In both periods of operation, kbiol values were determined by application of mass balances in the solid and liquid phases (Eq. 1‐4, table 2). Before PAC addition, maximum kbiol were obtained in the case of
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IBP, FLX and SMX, in accordance to their high removal efficiencies (> 85%). Although IBP is a readily biotransformable compound under aerobic conditions, a slightly diminution in the biotransformation rate was observed. This fact might be due to the decrease in nitrification in the last 76 days of SeMPAC operation (P3) because the removal of this compound is dependent on the nitrification [5, 31]. In spite of quantifying AHTN removal efficiencies above 80% during P2, its kbiol was low (0.23 L/gVSS∙d) because also sorption was a relevant removal mechanism. Increase in kbiol for P3 served to quantify the enhancement in biotransformation. During P2 all kbiol values were lower than 4 L/gVSS∙d, while for ROX, HHCB, FLX, SMX, E1 and EE2 these biological kinetic coefficients were higher after PAC addition (table 2), reaching typical values for readily biodegradable compounds. CONCLUSIONS The SeMPAC process is a promising alternative to achieve high OMPs removal efficiencies by means of two main mechanisms: i) enhancing their biotransformation with respect to conventional processes due to the development of a more enriched sludge, and ii) retaining the more recalcitrant compounds through sorption onto the PAC. Both the solid and liquid matrixes were analyzed in order to carry out a complete OMP and nutrient mass balance in order to determine the fate and behaviour of each substance. The main results achieved can be summarised as follows: -
A high quality effluent was obtained in terms of conventional parameters such as suspended solids (turbidity around 0.2‐0.6 NTU), organic matter (COD removal of 95 %) and total nitrogen (removal efficiencies around 75 %).
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-
The presence of activated carbon enhanced denitrification due to the direct adsorption of nitrate, as well as to denitrification in the anoxic zones within the biofilm that grew on the activated carbon particles.
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The sorption trend onto PAC was explained by the hydrophobicity and dissociation constant of the selected OMPs.
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Musk fragrances and FLX, besides being sorbed onto the sludge and the activated carbon as lipophilic compounds, they were also removed by biotransformation; CBZ, DCF and DZP were only sorbed onto activated carbon.
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The antibiotics ERY, ROX and TMP were removed by both biotransformation and sorption onto activated carbon, but SMX, IBP and NPX were only influenced by biotransformation.
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A progressive saturation of the activated carbon was observed. In the case of the negatively charged compounds, such as DCF, concentration stabilization in the solid phase was observed after 100 days, indicating that the solid‐liquid equilibrium was reached. However in the case of neutral compounds, such as DZP, solid phase concentration increased with time during the whole operation being the complete saturation not achieved.
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An increase in the removal by both biotransformation and sorption for the moderate biodegradable compounds was observed after PAC addition, which could be related to the development of a biofilm on the activated carbon.
ACKNOWLEDGMENTS This research was supported by the Spanish Ministry of Economy and Competitiveness through HOLSIA (CTM2013‐46750‐R) project and by the Spanish Ministry of Education and Science through RedNovedar (CTQ2014‐51693‐REDC) project. The authors belong to the Galician Competitive Research Group GRC2013‐032, programme co‐funded by FEDER. The support of VIAQUA to this technology, through its licensing, is also acknowledged.
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[18] Z. Li, B. Dvorak, X. Li, Removing 17β‐estradiol from drinking water in a biologically active carbon (BAC) reactor modified from a granular activated carbon (GAC) reactor, Water Res. 46 (2012) 2828‐ 2836. [19] P. Lipp, H‐J. Groß, A. Tiehm, Improved elimination of organic micropollutants by a process combination of membrane bioreactor (MBR) and powdered activated carbon (PAC), Desalination Water Treat. 42 (2012) 65‐72. [20] M. Boehler, B. Zwickenpflug, J. Hollender, T. Ternes, A. Joss, H. Siegrist, Removal of micropollutants in municipal wastewater treatment plants by powder‐activated carbon, Water Sci Technol. 66 (2012) 2115‐2121. [21] D. Serrano, F. Omil, S. Suarez, J.M. Lema, Process for the removal of pharmaceutical products present in wastewater, Spain patent ES2362298B2, Feb. 6th 2012. [22] M. Al Aukidy, P. Verlicchi, N. Voulvoulis, A framework for the assessment of the environmental risk posed by pharmaceuticals originating from hospital effluents, Sci. Total Environ. 493 (2014) 54‐64. [23] APHA, Methods for the Examination of Water and Wastewater, twentieth ed. American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC, USA (1999). [24] H. Demiral, G. Gündüzoglu, Removal of nitrate from aqueous solutions by activated carbon prepared from sugar beet bagasse, Bioresour. Technol. 101 (2010) 1675‐1680. [25] C.A. Ng, D. Sun, J. Zhang, B. Wu, A.G. Fane, Mechanisms of Fouling Control in Membrane Bioreactors by the Addition of Powdered Activated Carbon, Sep. Sci. Technol. 45 (2010) 873‐889. [26] M.M.T. Khan, S. Takizawa, Z. Lewandowski, W.L. Jones, A.K. Camper, H. Katayama, F. Kurisu, S. Ohgaki, Membrane fouling due to dynamic particle size changes in the aerated hybrid PAC‐MF system, J. Membr. Sci. 371 (2011) 99‐107.
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[27] S. Suarez, F. Omil, J.M. Lema, Removal of pharmaceutical and personal care products (PPCPs) under nitrifying and denitrifying conditions, Water Res. 44 (2010) 3214‐3224. [28] F.I. Hai, X. Li, W.E. Price, L.D. Nghiem, Removal of carbamazepine and sulfamethoxazole by MBR under anoxic and aerobic conditions, Bioresour. Technol. 102 (2011) 10386‐10390. [29] W. Xue, C. Wu, K. Xiao, X. Huang, H. Zhou, H. Tsuno, H. Tanaka, Elimination and fate of selected micro‐organic pollutants in a full‐scale anaerobic/anoxic/aerobic process combined with membrane bioreactor for municipal wastewater reclamation, Water Res. 44 (2010) 5999–6010. [30] E. Müller, W. Schüssler, H. Horn, H. Lemmer, Aerobic biodegradation of the sulfonamide antibiotic sulfamethoxazole by activated sludge applied as co‐substrate and sole carbon and nitrogen source, Chemosphere 92 (2013) 969‐978. [31] E. Fernandez‐Fontaina, F. Omil, J.M. Lema, M. Carballa, Influence of nitrifying conditions on the biodegradation and sorption of emerging micropollutants, Water Res. 46 (2012) 5434‐5444. [32] M. Pomiès, J.M. Choubert, C. Wisniewski, C. Miège, H. Budzinski, M. Coquery, Lab‐scale experimental strategy for determining micropollutant partition coefficient and biodegradation constants in activated sludge, Environ. Sci. Pollut. Res. 22 (2015) 4383‐4395. [33] R.P. Schwarzenbach, P.M. Gschwend, D.M., Imboden, Environmental Organic Chemistry, John Wiley & Sons Inc., Hoboken, New Jersey, 2003. [34] C. Kazner, Advanced Wastewater Treatment by Nanofiltration and Activated Carbon for High Quality Water Reuse, Dissertation, RWTH‐Aachen University, 2011. [35] X. Li, F.L. Hai, L.D. Nghiem, Simultaneous activated carbon adsorption within a membrane bioreactor for an enhanced micropollutant removal, Bioresour. Technol. 102 (2011) 5319‐5324. [36] D.J. de Ridder, L. Villacorte, A.R.D. Verliefde, J.Q.J.C. Verberk, S.G.J. Heijman, G.L. Amy, J.C. van Dijk, Modeling equilibrium adsorption of organic micropollutants
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onto activated carbon, Water Res. 44 (2010) 3077‐3086. [37] J. Margot, C. Kienle, A. Magnet, M. Weil, L. Rossi, L.F. de Alencastro, C. Abegglen, D. Thonney, N. Chèvre, M. Schärer, D.A. Barry, Treatment of micropollutants in municipal wastewater: ozone or powdered activated carbon? Sci. Total Environ. 461‐462 (2013) 480‐498. [38] S.K. Bhal, K. Kassam, I.G. Peirson, G.M. Pearl, The rule of five revisited: applying log D in place of log P in drug‐likeness filters, Mol. Pharmaceutics 4 (2007) 556‐560. [39]American Chemical Society, 2009. Scifinder Scholar. [40] A.I. Schafer, L.D. Nghiem, T.D. Waite, Removal of the natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis, Environ. Sci. Technol., 37 (2003) 182–188. [41] D. Jermann, W. Pronk, M. Boller, A.I. Schäfer, The role of NOM fouling for the retention of estradiol and ibuprofen during ultrafiltration. J. Membr. Sci. 329 (2009) 75–84. [42] J. Wu, C. He, D. Bi, J. Yu, Y. Zhang, A bio‐cake model for the soluble COD removal by the back‐ transport, adsorption and biodegradation processes in the submerged membrane bioreactor, Desalination (2013) 322 1‐12. [43] R. Mailler, J. Gasperi, Y. Coquet, A. Buleté, E. Vulliet, S. Deshayes, S. Zedek, C. Mirande‐Bret, V. Eudes, A. Bressy, E. Caupos, R. Moilleron, G. Chebbo, V. Rocher, Removal of a wide range of emerging pollutants from wastewater treatment plant discharges by micro‐grain activated carbon in fluidized bed as tertiary treatment at large pilot scale, Sci. Total Environ. 542 (2016) 983‐996. [44] R. Mailler, J. Gasperi, Y. Coquet, S. Deshayes, S. Zedek, C. Cren‐Oliv, N. Cartiser, V. Eudes , A. Bressy, E. Caupos, R. Moilleron, G. Chebbo, V. Rocher, Study of a large scale powdered activated carbon pilot: Removals of a wide range of emerging and priority micropollutants from wastewater treatment plant effluents, Water Res. 72 (2015) 315‐330.
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[45] L.N. Nguyen, F.I. Hai, J. Kang, L.D. Nghiem, W.E. Price, W. Guo, H.H. Ngo, K. Tung, Comparison between sequential and simultaneous application of activated carbon with membrane bioreactor for trace organic contaminant removal, Bioresour. Technol. 130 (2013) 412‐417.
22
DCF NPX IBP DZP CBZ AHTN HHCB ADBI FLX TMP SMX ROX ERY EE2 E2 E1 0
20
40
biotransformation
60
% Removal sorption
80
100
volatilization
Fig. 1. OMPs average total removal efficiencies before PAC addition in the SeMPAC system taking into account the three removal mechanisms involved (n=3, average values and standard deviations)
21
Influent Aerob 60 min.
Anoxic 5 min. Aerob 180 min
Anoxic 60 min. Settling
OMPs concentration (µg/L)
18 15 12 9 6 3 0 CEL
GLX
TON
CBZ
DZP
IBP
NPX
DCF
ERY
FLX
Fig. 2. Evolution of the concentration of OMPs during a cycle in the SBR.
23
ROX
SMX
TMP
15
25
10
IDCF/L
DZP/L
20 15 10
5
5 0
0 0
50
100
150
200
effluent SBR
50
100
a)
time (d) influent
0
250
150
200
permeate
influent
20
effluent SBR
250
b)
time (d) permeate
12 10 FLX/L
HHCB/L
15 10 5
8 6 4 2
0
0 0
50 influent
100
150
time (d) effluent SBR
200 permeate
250
0
c)
50 influent
100
150
time (d) effluent SBR
200 permeate
250
d) Fig. 3.
FIG 3 DZP (a),DCF (b), HHCB (c) and FLX (d) concentration in the influent, effluent of SBR and permeate. PAC addition
24
25
80
20 gDCF/gTSS
gDZP/gTSS
60
15
40
10
20
5
0
0 0
81
116
122
199
213
78
81
116 122 136 190 213 225
Sludge_SBR
12
25
10
20
8
Time (d) b) Sludge_Membrane
gFLX/gTSS
30
15
6
10
4
5
2
0
0 35
53
225
Time (d) a) Sludge_Membrane
Sludge_SBR
HHCB/gTSS
136
42
53
116 122 136 190 213 225
Sludge_SBR
53
Time (d) c) Sludge_Membrane
78
81
136
190
213
225
Time (d) d) Sludge_SBR Sludge_Membrane
Fig. 4. DZP (a),DCF (b), HHCB (c) and FLX (d) concentration in the sludge of the SBR and the membrane chamber. PAC addition.
25
DCF NPX IBP FLX DZP AHTN HHCB ADBI TMP SMX ROX ERY 0
200
400
600
gbiotransformed/d Before PAC addition After PAC addition
800
Fig. 5. Biotransformation rate of OMPs before and after PAC addition in the SeMPAC system (n=3‐5, average values and standard deviations)
26
Table 1. Influent and effluent of macropollutants in the SeMPAC system COD (mg/L)
N-NH4+ (mg/L)
TN (mg/L)
P-PO43- (mg/L)
Influent
850±100
80±15
80±15
5.5±1.0
Effluent (P2)
25±15
<1±1
20±15
0±0.3
Effluent (P3)
30±15
<1±1
10±15
0±0.3
Table 2. OMPs biotransformation kinetics constants before and after PAC addition in the SBR reactor (P2 and P3, respectively) kbiol (L/gVSSd)
ERY
ROX
Before PAC addition 0.4±0.2 0.4±0.2 After PAC addition
1.9±1.0 6.0±1.0
HHCB 0.3±0.2
DZP
FLX
0.04±0.04 1.5±0.4
6.2±0.4
0.1±0.06
IBP
NPX
4.4±0.9
SMX
TMP
1.3±0.4 0.07±0.04 4.1±0.1 2.0±0.3
kbiol (L/gVSSd)
ADBI
Before PAC addition 0.5±0.3 After PAC addition
AHTN
0.2± 0.1 3.9± 0.7 0.2± 0.2
0.9±1.0 1.0±0.1
2.9±0.4
27
0.2±0.03
E1
E2
EE2
0.3± 0.2 0.3± 0.2 0.3± 0.2 17.8
2.6±1.0 4.2±0.4
S0304-3894(16)30041-3 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.01.040 HAZMAT 17392
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
22-10-2015 13-1-2016 14-1-2016
Please cite this article as: T.Alvarino, O.Komesli, S.Suarez, J.M.Lema, F.Omil, The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.01.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The potential of the innovative SeMPAC process for enhancing the removal of recalcitrant organic micropollutants
T. Alvarinoa*, O. Komeslib,c, S. Suareza, J.M. Lemaa and F. Omila
a
Department of Chemical Engineering, Institute of Technology, University of Santiago de Compostela,
15782
Santiago
de
Compostela,
Spain
([email protected],
[email protected],
[email protected], [email protected]) b
Ataturk University, Department of Environmental Engineering, 25250 Erzurum, Turkey
c
Middle East Technical University, Department of Environmental Engineering, 06531 Ankara, Turkey
*
Corresponding author. Tel.: +34-881816739; Fax: +34-881816702
1
Highlights: -
Complete OMPs mass balance in a combined system biological treatment plus PAC.
-
Improvement of the denitrification after PAC addition.
-
Enhancement of OMPs biotransformation after PAC addition.
-
Relation between hydrophobicity (log D) and sorption onto the PAC.
-
Progressive saturation of the activated carbon in the solid phase with the time.
ABSTRACT SeMPAC is an innovative process based on a membrane sequential batch reactor to which powdered activated carbon (PAC) is directly added. It was developed with the aim of obtaining a high quality effluent in terms of conventional pollutants and organic micropollutants (OMPs). High COD removal and nitrification efficiencies (>95%) were obtained already during the operation without PAC, although denitrification was enhanced by PAC addition. OMPs were followed in the solid and liquid matrixes so that biotransformation, sorption onto the sludge and adsorption onto the PAC could be assessed. Recalcitrant compounds, such as carbamazepine and diazepam, were readily removed only after PAC addition (>99%). Progressive saturation of PAC was observed, with increasing concentrations of OMPs in the solid phase. Removal efficiencies for recalcitrant compounds were used as indicators for new additions of PAC. An improvement in the moderately biodegradable OMPs removal was observed after PAC addition (e.g. fluoxetine, trimethoprim) which was attributed to the biofilm that grew onto the sorbent, as well as to adsorption onto PAC. Keywords: Organic micropollutants; activated carbon; MBR; sorption; biotransformation
2
1. INTRODUCTION The removal of organic micropollutants (OMPs) in wastewater treatment plants (WWTPs) has been studied in past years [1,2] because of their potential environmental hazards: bioaccumulation tendency, toxicity and estrogenicity [3,4]. Consequently, the European Union has included seven pharmaceuticals (diclofenac, 17 alpha‐ethinylestradiol, 17 beta‐estradiol, estrone, erythromycin, azithromycin and clarithromycin) in its watch list for the Water Framework Directive in order to monitor and gather information regarding the potential risks they may cause to the aqueous environment. Recent technological developments have shown that a combination of biological processes bearing different redox potentials is a promising alternative for enhancing the amount of OMPs biodegraded in a wastewater treatment plant (WWTP). For example, ibuprofen, an anti‐inflammatory, is readily biodegraded under aerobic conditions only, whilst negative redox potentials favour the removal of trimethoprim, an antibiotic [5]. Presently, many innovative bioreactor configurations include membranes with the aim of maximizing biomass retention and thus, enhancing the microbial diversity responsible for the removal of organic pollutants both at colloidal and at soluble levels, to give a final effluent with higher quality [6]. Accordingly, Sipma et al. reported better removal performance in membrane biological reactors (MBRs) compared to conventional activated sludge (CAS) units for slowly biodegradable micropollutants which was attributed to the relatively longer sludge ages promoted in these systems [7]. Hence, it would be expected that the combination of anoxic‐aerobic processes and membrane filtration should favour the achievement of higher removal efficiencies for biodegradable OMPs. A large amount of OMPs present in sewage can be biodegraded in WWTPs [8] , although there are many compounds which have been reported as being recalcitrant, such as carbamazepine or diazepam. In order to ensure the highest removal degree for OMPs, physical‐chemical procedures based on the use of adsorbents or oxidants have been usually considered, both in WWTPs and in
3
drinking water plants (DWPs). A relevant number of studies concerning OMPs are focused on the use of activated carbon (AC) in WWTPs, usually as post‐treatment [9,10]. Synergistic effects have been reported when biological transformation with adsorption processes are combined, such as supplying AC in the biological reactor [11], which may lead to additional benefits such as an enhancement of the microbial diversity [12]. In the specific case of MBRs, the growth of a biofilm onto the surface of the activated carbon particles, produced reduction in membrane fouling and enhancement in the removal efficiencies of conventional parameters [13]. This approach has proven to be suitable in the case of dyes or aromatic hydrocarbons with positive results [14,15]. Other studies have shown an effective removal of OMPs, even for the more recalcitrant compounds, in a process that combines adsorption by activated carbon and biological transformation [16,17,18, 19, 20]. Since these studies have focused on the removal efficiencies in the water line (without considering the OMPs present in sludge), further research providing information on the complete mass balances is needed in order to elucidate the role of the different removal mechanisms. Furthermore, the combination of different environmental redox potentials and the influence of AC on OMPs concentration in the solid phase (sludge and the AC) are also key issues. These are the pillars of the SeMPAC technology developed and patented by the authors (patent ES 2 362 298 B2, European Patent application EP12777603.7) [21]. This technology comprises a sequential batch reactor (SBR) which operates with a direct addition of Powdered Activated Carbon (PAC) coupled to an external submerged microfiltration membrane [16]. The aim of the present study was to determine the effect of PAC addition on the removal of OMPs during an anoxic/aerobic biological wastewater treatment process followed by an external microfiltration step. A complete mass balance was carried out in order to determine the OMPs concentration in the liquid and in the solid phase (a matrix of PAC and sludge). The research was focused on the effect of the PAC and the redox potential on the OMPs removal mechanisms (biotransformation and adsorption onto the solids), as well as on the relation between the physical‐ chemical characteristics of the OMPs and their trends to be adsorbed onto the PAC.
4
2. MATERIAL AND METHODS 2.1. Experimental set‐up A sequential batch reactor (SBR) connected to an external submerged microfiltration flat sheet membrane provided by Kubota, with a pore size of 0.4 m and a total surface of 0.2 m2, was started up and operated (patent ES 2 362 298 B2) [21]. The hydrophilic membrane permeated during 7.5 min which was followed by a relaxation time of 1.25 min. The SBR and the membrane chamber were inoculated with 6 gVSS/L of biomass which was collected from the conventional activated sludge unit in the WWTP of Calo‐ Milladoiro (NW of Spain). The reactor, with a volume of 30 L, was operated in cycles of 6 h distributed in five stages: filling (14 min), anoxic reaction (70 min), aerobic reaction (210 min), settling (42 min) and effluent withdrawal (14 min). The exchange volume was fixed at 25% and the supernatant was discharged into the 18 L membrane chamber. The hydraulic retention time (HRT) was maintained at 1 d in the SBR, while a recycle ratio of 0.5 was maintained between the membrane chamber and the SBR in order to homogenise the PAC and sludge concentration in both chambers. The composition of the synthetic wastewater fed to the reactor was as follows (g/L): 2.29 CH3COONa∙3H2O, 0.33 NH4Cl, 0.04 KH2PO4 and 0.45 NaHCO3, with pH around 7.5. The PAC purchased to PANREAC (code 211237) was characterized by a specific surface area of 555 m2/g. Three antiphlogistics (ibuprofen (IBP), naproxen (NPX) and diclofenac (DCF)), four antibiotics (trimethoprim (TMP), sulfamethoxazol (SMX) erythromycin (ERY) and roxithromycin (ROX)), three estrogens (estrone (E1), estradiol (E2) and ethynilestradiol (EE2)), three musk fragrances (galaxolide (HHCB), celestolide (ADBI) and tonalide (AHTN)), one antidepressant (fluoxetine (FLX)), one tranquillizer (diazepam (DZP)) and one antiepileptic (carbamazepine (CBZ)) were spiked at concentrations between 1‐40 ppb. OMP concentrations were selected according to their presence in the hospital wastewaters [22], as well as the limits of quantification of analytical methods (LOQ, Table S.1). The substances were obtained from Sigma Aldrich (Steinheim, Germany).
5
Three operation periods were considered: the start‐up during which no OMPs were spiked to the influent (P1, 32 days); the second period in which OMPs were spiked to the influent in order to assess their removal without activated carbon (P2, 76 days), and the third period (P3, 118 days) which started at day 108 with one single addition of powdered activated carbon (PAC) in the SBR, at a concentration of 1 g/L. No further addition of PAC was carried out with the aim of studying its progressive saturation with time. 2.2. Analytic methods The sampling routine comprised influent and effluent samples twice a week for solids, COD, ammonium, nitrite and nitrate determinations, while temperature, dissolved oxygen (DO) and pH were monitored daily with a Hach HQ40d multi‐parameter digital sensor. Soluble samples were obtained using 0.45 μm nitrocellulose membrane filters (HA, Millipore). Standard Methods (APHA) were used to determine temperature, pH, total suspended solids (TSS), volatile suspended solids (VSS), alkalinity, ammonium, nitrate, nitrite and chemical oxygen demand (COD) [23]. Several sampling campaigns were carried out during eight months in order to determine the dissolved and sorbed OMPs concentrations in the influent, effluent of the SBR and permeate. Punctual samples were taken with a time delay of one HRT. Liquid phase samples were prefiltered (AP3004705, Millipore), pre‐concentrated by a solid‐phase extraction (SPE) and analysed by gas chromatography– mass spectrometry (GC/MS) and liquid chromatography‐tandem mass spectrometry (LC/MS/MS). The analytical procedure was described by Alvarino et al. [5]. The solid phase samples were taken from the mixed liquor of the SBR and membrane chamber, centrifuged to collect the solids, frozen and lyophilized. The dried sludge was extracted in an ultrasonic bath three times with methanol and acetone and the solvent fractions were combined and evaporated in order to be resuspended in water, pre‐concentrated by SPE and analysed by GC/MS and LC/MS/MS.
6
2.3. Micropollutants mass balances OMPs concentration in the liquid and the solid phase was determined to perform a mass balance under pseudo‐steady state conditions taking into account the main removal processes (sorption, volatilization and biotransformation) (Eq. 1). The OMPs concentrations in the liquid phase were determined in the influent and the permeate streams, while the concentrations in the solid phase were measured in the mixed liquor of the SBR and the membrane chamber.
(Eq. 1) where Finf, Feff, Fstripped, Fsor and Fbiod represent the mass flows (in g/d) in the influent, effluent, off‐gas released from the aerobic stage, sorbed onto solids (including PAC) and biotransformed, respectively. Fstripped (Eq. 2) was calculated in function of the dimensionless Henry’s law constant (H, Table S.1), the feed flow rate Q (L/d), the air supply per unit of wastewater treated q (2 Lair/Linf) and the liquid phase concentration in the effluent Cw (g/L). ∙
∙ ∙
(Eq. 2) Since no purges were carried out during this experimental study (226 days) the term corresponding to sorption was calculated taking into account the accumulation of OMPs in the solid phase (Eq. 3). Therefore, periods P2 and the P3 have been considered separately. In P2, sorption was only dependent on OMP sorbed onto the sludge; while in P3 accumulation inside the activated carbon was also taken into account. For this calculation the Kd constant sorption cannot be considered coefficient since solid‐ liquid equilibrium was not achieved.
∙
∙
∙
2 2
1 1
(Eq. 3)
7
where Cs1 and Cs2 (in g/L) are the concentration on the solid phase corresponding to times t1 and t2 (d), HRT (d) the hydraulic retention time and VSS the volatile suspended solids inside the reactor (gVSS/L). The calculated OMPs mass flows in the influent, effluent, volatilized during aeration and sorbed onto the solids, were used to determine the OMPs biotransformation according to Eq. 1. The OMPs biological kinetic constants (kbiol, L/gVSS∙d) were obtained considering that the process follows a pseudo‐first order kinetic (Eq. 4).
(Eq. 4) where V is the volume of the reactor (L). Sorption batch test were performed in 1 L flasks at constant temperature (25ºC) in duplicate, using a phosphate buffer, with an initial OMPs concentrations of 2.5 ppb and a concentration of PAC in the range of 1‐10 ppm (0, 2.5, 5, 7.5 and 10 ppm). Continuous stirring was provided at 150 rpm. Samples were taken up to the point a constant OMPs concentration was measured in the liquid phase. The fit of the experimental data to the Freundlich isotherm model was done through linear regression analysis (Eq. 5). 1/
(Eq. 5)
where kf is the adsorption capacity constant and 1/n is related to the intensity of the adsorption, Cs (g OMP/g) and Cw (g OMP/L) are the concentration on the solid and liquid phase, respectively.
8
3. RESULTS AND DISCUSSION 3.1. Reactor performance The SeMPAC system was operated at 17‐20ºC and the pH was maintained in the range 7‐8.5 without additional control. During the whole operation (226 days) the turbidity of the final effluent ranged 0.2‐ 0.6 NTU, below the limit established by the World Health Organization to ensure effectiveness of disinfection (1 NTU). COD removal efficiencies were maintained above 95% (Table 1), with biodegradation taking place mainly in the SBR (>90%). High phosphate removal efficiencies were measured (97%) during the whole operation. Nitrification efficiency was stable in the whole system (>98%), although it decreased from 95 to 75% in the SBR due to the lower dissolved oxygen (DO) concentrations (from 2 to 0.8 mg/L) observed during the last 76 days. The decrease in the DO was attributed to the increase in biomass concentration in the SBR (from 4.8 to 8 gVSS/L) as a consequence of the absence of sludge purges during the experiment. The concentration of biomass in the membrane chamber was stable during the whole operation (7±1 gTSS/L). Concerning denitrification, this was in fact the only conventional parameter which was influenced by PAC addition, with an increase from 76 to 87% (Table 1). When PAC was not added (periods P1 and P2) denitrification took place exclusively during the anoxic phase of the SBR cycle, reaching the highest possible value considering a volumetric exchange of 33% in each cycle. Therefore, the improvement in denitrification observed in P3 was attributed to act as support for the growth of a biofilm the capacity of the PAC to adsorb the nitrate ion [24] and to, which implies the existence of anoxic zones even during the aerobic cycles [25]. After PAC addition a decrease in the particle size of biomass agglomerates from 108.5 to 83.2 m was observed [26].
9
3.2. Micropollutants fate Operation without PAC Before PAC addition, total removal efficiencies of OMPs in the system were determined by applying the mass balances to the liquid and solid phase concentrations, considering the influent and effluent of the SeMPAC system. IBP, FLX, SMX, HHCB and ADBI were the compounds readily removed (above 80% of total removal efficiency), whilst DCF, CBZ, DZP and TMP exhibited a recalcitrant behaviour with removal efficiencies below 20% (Fig. 1). Musk fragrances and fluoxetine, which are lipophilic compounds, were eliminated by sorption (with removal efficiencies associated to this mechanism of 26%, 5%, 15% and 8% for AHTN, HHCB, ADBI and FLX, respectively) and biotransformation (Fig. 1), while the other OMPs were only eliminated by biotransformation. Prior to PAC addition a study on the OMPs fate during a cycle in the SBR was carried out in order to get a deeper insight on the degradation kinetics at the different redox conditions (Fig. 2). Only FLX and SMX were degraded under aerobic and anoxic conditions [27,28], while for DZP, CBZ and DCF the flat concentration profile confirmed their recalcitrant behaviour. Musk fragrances were mainly removed in the first stage of the cycle (anoxic reaction), according to the results obtained by Xue et al. in an anaerobic‐anoxic‐aerobic system [29] with removal of musk fragrances only in the anaerobic and anoxic stages, whilst the concentration remained constant in the aerobic stage. The decrease on SMX concentration during the first five minutes of the anoxic phase is related to the dilution factor since this compound was only removed under aerobic conditions and requires the presence of an external carbon source [30]. Previous studies reported the influence of nitrification and heterotrophic activity in the co‐metabolic removal of OMPs [5,6]. For instance, IBP, ERY and ROX were mainly removed under aerobic conditions since their biotransformation was highly influenced by nitrification and also by organic carbon degradation in the case of IBP [31,32]. The OMPs were mainly removed in the SBR, being the lower removal efficiencies achieved in the membrane chamber attributed to the residual OMP and organic matter loading rates entering this unit
10
(COD and ammonium fed to the membrane unit were lower than 40 mg COD/L and 2.5 mg N‐NH4+/L, respectively). This fact can be related to the fate that the removal of OMPs following a pseudo first order kinetic is modelled as a function of OMPs concentration [33], thus OMPs removal activities were expected to decrease simultaneously with OMPs concentration. Operation with PAC After one single PAC addition at day of operation 108, most of OMPs removal efficiencies were enhanced (Fig. 3, S.1), with sorption onto activated carbon especially relevant for the more recalcitrant compounds (CBZ, DZP, DCF and TMP). Very high OMPs removal efficiencies (>98%) could be maintained for around 50 days of operation after PAC addition (Fig. 3, S.1), although in the case of the removal of SMX and IBP no sorption onto the PAC was observed. The hydrophobicity of OMPs was a key factor in determining their affinity to sorb onto activated carbon [34,35,36]. This means that for neutral compounds removal by sorption could be correlated with the octanol‐water coefficient (log Kow), but in the case of ionizable compounds the pH and ionization constant (pKa) had to be considered also. In fact, in the case of compounds with the same log Kow, the adsorption was dependent on the charge of the compound [37]. Therefore, LogD calculation, which comprises pKa, pH and log Kow, was found as a better option for estimating the solid‐liquid phase distribution [38]. In this work, removal efficiencies above 99% were obtained for compounds with log D >1, such as CBZ, DZP or DCF, whilst IBP and NPX were not affected by the presence of activated carbon (Fig. S.1) in agreement with their log D of 0.94 and 0.73, respectively (at pH = 7) [39]. In order to corroborate the sorption capacity of PAC, several batch tests were carried out to determine the Freundlich isotherms (Table S.2). According to the low values of the measured kf coefficient for the antinflamatories IBP (967 (µg/g)(L/µg)1/n) and NPX (735 (µg/g)(L/µg)1/n), their removal efficiencies were not influenced by sorption onto PAC during P3, in contrast to DZP and CBZ whose high sorption affinity (kf 2850 and 1793 (µg/g)(L/µg)1/n, respectively) led to removal efficiencies during P3 above 99%. The parameter 1/n of the Freundlich isotherms, which measures the intensity of the adsorption, reconfirms
11
the low trend of both antiinflamatories to be sorbed onto the PAC (n 4.6 and 2.64 for IBP and NPX, respectively), while the lower n value determined in the case of CBZ and DZP are an evidence of their higher sorption intensity (1.89 and 1.68, respectively). 3.3. Sorption The main removal mechanisms for OMPs in this process were sorption and biotransformation (Fig. 1), as volatilisation was only relevant for the musk fragrance celestolide (the compound with the highest Henry’s coefficient). As will be evidenced from the following results, the presence of PAC in P3 enhanced the sorption capacity of the solid matrix (Fig. 3&4). Operation without PAC During the first stage, the lipophilic compounds, namely musk fragrances and FLX were removed by both, sorption and biotransformation, while the other non recalcitrant OMPs were only removed by biotransformation (Fig. 1, 4 and S.2). In fact, the relative amount of musk fragrances and FLX sorbed onto sludge was above 2500 L/gTSS, whereas in the case of the other OMPs it was below 250 L/gTSS. Flat sheet microfiltration membranes (MF, pore size of 0.4 m) were used, thus ruling out OMPs removal based on size exclusion [40]. Depending on the properties of both the solute and the membrane, compounds with low molecular weight could be adsorbed on the membrane [41], although this was not considered as relevant in the present study due to the hydrophilic character of the membranes used. Accumulation of sorbed musk fragrance was observed in the sludge before PAC addition (Fig. 4&S.2), which could be attributed to two different effects exerted by the cake layer, namely sorption and size exclusion [42]. Operation with PAC After PAC addition, a quantitative extraction of OMPs from the solid phase (matrix composed of sludge and PAC) was carried out. The results showed a pronounced increase in the OMPs concentration in the solid phase with time in both chambers in P3, which was related to the sorption capacity of the
12
activated carbon (Fig. 4, S.2). This observation confirmed that after PAC addition the equilibrium between the solid‐liquid phases was not reached in P3. As musk fragrances and fluoxetine were absorbed onto the sludge and the activated carbon in P3 (Fig. 4, S.2), their continuous increase in the solid phase concentration was due to the combination of the effect of retention of the suspended solids and those present in the cake layer, as well as the progressive saturation of activated carbon. The anti‐inflammatories IBP and NPX are compounds that at pH 7 include negatively charged chemical structures and are characterized by log D < 1. These properties make them scarcely adsorbable onto activated carbon [36, 37] and thus their removal was mainly driven by biotransformation during the whole operation. In the case of DCF, the presence of halogens, aromatic rings and heterocycles enhances its removal by sorption onto the PAC [43, 44]. Additionally, its high log D = 1.77 [39] makes the compound significantly adsorb onto PAC (Fig. 3). A progressive saturation of the PAC was observed and lead to a decrease in the removal efficiencies achieved for most OMPs with the time in P3 (Fig. 3, S.1), especially in the case of CBZ, DZP and DCF (from complete removal to below 50% at the end of the operation). The quickness of PAC saturation was related to the charge of the compound [45], being higher for the negatively charged compounds, as evidenced by PAC saturation first in the case of DCF (anion) and later for CBZ and DZP (neutral) (Fig. 3, S.1). In the case of FLX and estrogens complete sorption onto PAC was observed, according to their log D > 1 (FLX 1.83, E1 3.62, E2 4.15 and EE2 4.11) [39], being the removal efficiencies maintained above 98% even after 80 days of PAC addition, which could be related to their positive charge at the pH value in the plant [45]. In the case of the four antibiotics, removal efficiencies above 90% were obtained during the whole period of operation with PAC, although two different behaviours were observed: SMX (a negatively charged compound) was only removed by biotransformation even after PAC addition, maintain constant removal efficiencies in P2 and P3 (Fig. S.1), while TMP (a positively charged compound) was removed by sorption onto the activated carbon as well as by biotransformation.
13
The trend of progressive sorption and saturation of the PAC was observed in the OMPs concentrations of the solid phase (Fig 4 and S.2) during P3. After 120 days of operation with PAC, the solid phase concentration of the negatively charged compounds (DCF, IBP or NPX) was lower than in the case of neutral (CBZ, DZP or HHCB) and positively charged compounds (TMP). Additionally, stabilization in the solid phase concentration of the negatively charged OMPs was observed, whereas the concentration of the neutral and positive compounds kept increasing during the whole operation (Fig. 4, S.2). 3.4. Biotransformation Biotransformation was the main removal mechanism in P2 (before PAC addition) for the complete set of selected OMPs (Fig. 1). In the case of antibiotics, removal efficiencies higher than 85% were achieved, except in the case of TMP, being the contribution of sorption onto the sludge lower than 2.5% in all cases. Although musk fragrances are lipophilic compounds, they were mainly removed by biotransformation during P2, representing 94, 82, 68% of the total removal for ADBI, HHCB and AHTN, respectively (Fig. 1). The recalcitrant compounds in biological processes, such as CBZ and DZP, were not removed during P2. After PAC addition, apart from sorption onto activated carbon, an enhancement in biotransformation was observed for some OMPs. The presence of anoxic and even anaerobic zones inside the biofilm could explain the enhanced biotransformation of FLX, which is readily biodegradable under anoxic conditions [28], and also of SMX that needs anaerobic conditions to be degraded [5]. Although this effect was observed for all the OMPs studied, it was more intense for those with moderate biotransformation kinetics (Fig. 5), such as antibiotics (Table 2). For instance, the biotransformed mass flows corresponding to TMP and SMX, which are readily biodegradable compounds under anaerobic conditions [5], increased from 50 and 70 to 270 and 240 µg/d, respectively, after PAC addition (Fig. 5). In both periods of operation, kbiol values were determined by application of mass balances in the solid and liquid phases (Eq. 1‐4, table 2). Before PAC addition, maximum kbiol were obtained in the case of
14
IBP, FLX and SMX, in accordance to their high removal efficiencies (> 85%). Although IBP is a readily biotransformable compound under aerobic conditions, a slightly diminution in the biotransformation rate was observed. This fact might be due to the decrease in nitrification in the last 76 days of SeMPAC operation (P3) because the removal of this compound is dependent on the nitrification [5, 31]. In spite of quantifying AHTN removal efficiencies above 80% during P2, its kbiol was low (0.23 L/gVSS∙d) because also sorption was a relevant removal mechanism. Increase in kbiol for P3 served to quantify the enhancement in biotransformation. During P2 all kbiol values were lower than 4 L/gVSS∙d, while for ROX, HHCB, FLX, SMX, E1 and EE2 these biological kinetic coefficients were higher after PAC addition (table 2), reaching typical values for readily biodegradable compounds. CONCLUSIONS The SeMPAC process is a promising alternative to achieve high OMPs removal efficiencies by means of two main mechanisms: i) enhancing their biotransformation with respect to conventional processes due to the development of a more enriched sludge, and ii) retaining the more recalcitrant compounds through sorption onto the PAC. Both the solid and liquid matrixes were analyzed in order to carry out a complete OMP and nutrient mass balance in order to determine the fate and behaviour of each substance. The main results achieved can be summarised as follows: -
A high quality effluent was obtained in terms of conventional parameters such as suspended solids (turbidity around 0.2‐0.6 NTU), organic matter (COD removal of 95 %) and total nitrogen (removal efficiencies around 75 %).
15
-
The presence of activated carbon enhanced denitrification due to the direct adsorption of nitrate, as well as to denitrification in the anoxic zones within the biofilm that grew on the activated carbon particles.
-
The sorption trend onto PAC was explained by the hydrophobicity and dissociation constant of the selected OMPs.
-
Musk fragrances and FLX, besides being sorbed onto the sludge and the activated carbon as lipophilic compounds, they were also removed by biotransformation; CBZ, DCF and DZP were only sorbed onto activated carbon.
-
The antibiotics ERY, ROX and TMP were removed by both biotransformation and sorption onto activated carbon, but SMX, IBP and NPX were only influenced by biotransformation.
-
A progressive saturation of the activated carbon was observed. In the case of the negatively charged compounds, such as DCF, concentration stabilization in the solid phase was observed after 100 days, indicating that the solid‐liquid equilibrium was reached. However in the case of neutral compounds, such as DZP, solid phase concentration increased with time during the whole operation being the complete saturation not achieved.
-
An increase in the removal by both biotransformation and sorption for the moderate biodegradable compounds was observed after PAC addition, which could be related to the development of a biofilm on the activated carbon.
ACKNOWLEDGMENTS This research was supported by the Spanish Ministry of Economy and Competitiveness through HOLSIA (CTM2013‐46750‐R) project and by the Spanish Ministry of Education and Science through RedNovedar (CTQ2014‐51693‐REDC) project. The authors belong to the Galician Competitive Research Group GRC2013‐032, programme co‐funded by FEDER. The support of VIAQUA to this technology, through its licensing, is also acknowledged.
16
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DCF NPX IBP DZP CBZ AHTN HHCB ADBI FLX TMP SMX ROX ERY EE2 E2 E1 0
20
40
biotransformation
60
% Removal sorption
80
100
volatilization
Fig. 1. OMPs average total removal efficiencies before PAC addition in the SeMPAC system taking into account the three removal mechanisms involved (n=3, average values and standard deviations)
21
Influent Aerob 60 min.
Anoxic 5 min. Aerob 180 min
Anoxic 60 min. Settling
OMPs concentration (µg/L)
18 15 12 9 6 3 0 CEL
GLX
TON
CBZ
DZP
IBP
NPX
DCF
ERY
FLX
Fig. 2. Evolution of the concentration of OMPs during a cycle in the SBR.
23
ROX
SMX
TMP
15
25
10
IDCF/L
DZP/L
20 15 10
5
5 0
0 0
50
100
150
200
effluent SBR
50
100
a)
time (d) influent
0
250
150
200
permeate
influent
20
effluent SBR
250
b)
time (d) permeate
12 10 FLX/L
HHCB/L
15 10 5
8 6 4 2
0
0 0
50 influent
100
150
time (d) effluent SBR
200 permeate
250
0
c)
50 influent
100
150
time (d) effluent SBR
200 permeate
250
d) Fig. 3.
FIG 3 DZP (a),DCF (b), HHCB (c) and FLX (d) concentration in the influent, effluent of SBR and permeate. PAC addition
24
25
80
20 gDCF/gTSS
gDZP/gTSS
60
15
40
10
20
5
0
0 0
81
116
122
199
213
78
81
116 122 136 190 213 225
Sludge_SBR
12
25
10
20
8
Time (d) b) Sludge_Membrane
gFLX/gTSS
30
15
6
10
4
5
2
0
0 35
53
225
Time (d) a) Sludge_Membrane
Sludge_SBR
HHCB/gTSS
136
42
53
116 122 136 190 213 225
Sludge_SBR
53
Time (d) c) Sludge_Membrane
78
81
136
190
213
225
Time (d) d) Sludge_SBR Sludge_Membrane
Fig. 4. DZP (a),DCF (b), HHCB (c) and FLX (d) concentration in the sludge of the SBR and the membrane chamber. PAC addition.
25
DCF NPX IBP FLX DZP AHTN HHCB ADBI TMP SMX ROX ERY 0
200
400
600
gbiotransformed/d Before PAC addition After PAC addition
800
Fig. 5. Biotransformation rate of OMPs before and after PAC addition in the SeMPAC system (n=3‐5, average values and standard deviations)
26
Table 1. Influent and effluent of macropollutants in the SeMPAC system COD (mg/L)
N-NH4+ (mg/L)
TN (mg/L)
P-PO43- (mg/L)
Influent
850±100
80±15
80±15
5.5±1.0
Effluent (P2)
25±15
<1±1
20±15
0±0.3
Effluent (P3)
30±15
<1±1
10±15
0±0.3
Table 2. OMPs biotransformation kinetics constants before and after PAC addition in the SBR reactor (P2 and P3, respectively) kbiol (L/gVSSd)
ERY
ROX
Before PAC addition 0.4±0.2 0.4±0.2 After PAC addition
1.9±1.0 6.0±1.0
HHCB 0.3±0.2
DZP
FLX
0.04±0.04 1.5±0.4
6.2±0.4
0.1±0.06
IBP
NPX
4.4±0.9
SMX
TMP
1.3±0.4 0.07±0.04 4.1±0.1 2.0±0.3
kbiol (L/gVSSd)
ADBI
Before PAC addition 0.5±0.3 After PAC addition
AHTN
0.2± 0.1 3.9± 0.7 0.2± 0.2
0.9±1.0 1.0±0.1
2.9±0.4
27
0.2±0.03
E1
E2
EE2
0.3± 0.2 0.3± 0.2 0.3± 0.2 17.8
2.6±1.0 4.2±0.4