The treatment of sulfate-rich wastewater using an anaerobic sequencing batch biofilm pilot-scale reactor

Desalination 249 (2009) 241–246

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

The treatment of sulfate-rich wastewater using an anaerobic sequencing batch biofilm pilot-scale reactor Arnaldo Sarti ⁎, Ariovaldo J. Silva, Marcelo Zaiat, Eugenio Foresti Department of Hydraulics and Sanitation, School of Engineering of São Carlos, University of São Paulo, Av. Trabalhador São-carlense 400, CEP: 13566-590, São Carlos, SP, Brazil

a r t i c l e

i n f o

Article history: Accepted 19 August 2008 Available online 2 October 2009 Keywords: Anaerobic reactor Ethanol Sulfate reduction Mineral coal Industrial wastewater

a b s t r a c t This paper presents the results from 92 cycles of an anaerobic sequencing batch biofilm reactor containing biomass immobilized on inert support (mineral coal) applied for the treatment of an industrial wastewater containing high sulfate concentration. The pilot-scale reactor, with a total volume of 1.2 m3, was operated at sulfate loading rates ranging from 0.15 to 1.90 kgSO42−/cycle (48 h — cycle) corresponding to sulfate concentrations of 0.25 to 3.0 gSO42− l− 1. Domestic sewage and ethanol were utilized as electron donors for sulfate reduction. Influent sulfate concentrations were increased in order to evaluate the minimum COD/ sulfate ratio at which high reactor performance could be maintained. The mean sulfate removal efficiency remained between the range of 88 to 92% at several sulfate concentrations. Temporal profiles along the 48 h cycles were carried out under stable operation at sulfate concentrations of 1.0, 2.0 and 3.0 gSO42− l− 1. Sulfate removal reached 99% for cycle times of 15, 25, and 30 h, and the effluents sulfate concentrations were lower than 8 mgSO42− l− 1. The results demonstrate the potential applicability of the anaerobic configuration for the biological treatment of sulfate-rich wastewaters. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Increasing anthropogenic activity has contributed to local imbalances in the natural sulfur cycle, leading to serious environmental problems. Industrial wastewater containing sulfate has contributed to this sulfur imbalance [1]. Moreover, the discharge of sulfate-rich industrial wastewater into surface waters contributes to the increase of the corrosion potential of receiving waters due to the biological reduction of sulfate to hydrogen sulfide under anaerobic conditions. Sulfate can be removed from wastewaters by chemical precipitation or desalination processes, such as reverse osmosis and ion exchange, but at significantly high costs. On the other hand, the success of high-rate anaerobic technology has encouraged researchers to extend its application to the treatment of complex wastewaters. Hence, increasing attention has been given to biological methods for the removal of sulfate from industrial wastewaters. Biological processes including sulfate reduction to sulfide [H2S(g) + H2S(aq) + HS−] and its subsequent conversion to elemental sulfur [S0], have been successfully developed as a cost-effective method for sulfate removal from waste streams [2]. Since the addition of an appropriate electron donor is required for wastewaters containing insufficient electron donors for complete sulfate reduction (COD/sulfate ratios lower than 0.67), the cost of the electron donor must be low enough to keep the system competitive with conventional physical–chemical techniques [3]. Ethanol has

⁎ Corresponding author. Tel.: + 55 16 3373 8357; fax: + 55 16 3373 9550. E-mail address: [email protected] (A. Sarti). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.08.017

been used as electron donor in full-scale sulfate-reducing plants [4]. The primary disadvantage of using ethanol as the electron donor is the generation of acetic acid resulting in an effluent containing high residual COD [5]. Consequently, the residual pollution caused by the electron donor should be minimized and sulfide should be partially re-oxidized to elemental sulfur in a separate second reactor [6]. A variety of reactors have been reported for wastewater biological treatment promoted by sulfate-reducing bacteria, such as anaerobic filters [7], batch reactors [8], baffled reactor [9], fixed bed reactors [10,11], “gas-lift” reactors [3,12], fluidized bed reactors [6,13], and expanded granular sludge bed reactors [14]. Among the new proposed configurations of anaerobic reactors, the anaerobic sequencing batch reactor (ASBR) has been considered as a potential alternative for wastewater treatment [15,16]. Compared to other reactor configurations, the main advantages of the ASBR reactors are the possibility of achieving high solid retention times and avoiding the use of primary and secondary settling units. The anaerobic sequencing batch biofilm reactor (ASBBR) is a variant of the ASBR that contains support for biomass immobilization, thus eliminating uncertainties about biomass granulation and suppressing the settling step [17]. This paper presents and discusses the performance of a pilot-scale ASBBR in the treatment of industrial wastewater with a high sulfate concentration (~ 200 gSO42− l− 1). Domestic sewage was used as the primary electron donor and for diluting the industrial wastewater in order to obtain different sulfate concentrations (0.25, 0.5, 1.0, 2.0 and 3.0 gSO42− l− 1). Mineral coal was tested as the support material, and ethanol was used also as a supplementary electron donor for the SRB due availability and costs reasons. Although the application of ASBBR

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reactors seems to be promising based on bench-scale experiments [18], large scale studies are still needed to evaluate the applicability of this anaerobic technology for industrial wastewaters. 2. Materials and methods 2.1. Industrial wastewater Industrial wastewater originates from the washing of the products of the sulfonation of vegetable oils (rice, soy, and corn). Sulfonation occurs in the presence of sulfuric acid (H2SO4) and liquid ammonia (25%) in a batch reactor operated under controlled temperatures. Free acids are eliminated from the reaction product by a washing operation that produces a highly aggressive wastewater. The composition of the sulfate-rich washing wastewater is presented in Table 1. 2.2. ASBBR reactor and operation The pilot-scale ASBRR reactor was constructed from fiberglass with a total volume of 1.2 m3. The reactor was filled with 500 kg of irregular pieces of mineral coal (diameter of 40 to 80 mm) occupying a volume of 1.0 m3, resulting in a liquid volume of 0.5 m3 (bed porosity = 0.5). The head-space volume (0.2 m3) was filled with 0.1 m3 of liquid to keep the recirculation pipe immersed in the liquid. Therefore, the treatment volume available by cycle or batch mode was 0.6 m3 (0.5 m3 + 0.1 m3). The outlet biogas pipe was immersed in a hydraulic seal containing an alkaline solution (NaOH) for H2S removal. Silva et al. [18] utilized an ASBBR (bench-scale) filled with charcoal for the treatment of a sulfate-rich wastewater. These authors utilized a special device to retain the fixed bed inside the reactor due to the low density of the charcoal particles. The substitution of charcoal with a denser material (mineral coal) simplifies the reactor design by eliminating the need of an internal device to retain the bed particles. Additionally, mineral coal is cheaper than charcoal. The influent wastewater was pumped from a storage tank (0.6 m3) to a circular perforated tube located at the reactor's bottom for achieving a better liquid distribution. Mixing was provided by liquid recirculation (up-flow) with a centrifugal pump (Jacuzzi-model 5JL15) connected to the inflow distribution system. The cycle time was 48 h, including the steps of feeding (1 h), reaction with continuous liquid recirculation (46 h) and discharge (1 h). A schematic of the experimental apparatus is presented in Fig. 1. The reactor was maintained at an ambient temperature of 31 ± 2 °C in the Laboratory of Biological Processes (Universidade de São Paulo, São Carlos-Brazil). Domestic sewage was used to dilute the sulfate-rich industrial wastewater (Table 1), thus providing some organic matter for sulfate reduction. Table 2 summarizes the operational parameters applied to the ASBBR in different experimental periods. At the beginning of the operation, the reactor was operated with a sulfate loading rate (SLR) of 0.15 kgSO42−/cycle (period I — 0.25 gSO4− 2 l− 1). The SLR was increased in a stepwise manner up to 1.9 kgSO42−/cycle (period V − 3.0 gSO42− l− 1) throughout the experimental phase. Organic matter from domestic sewage was the sole electron donor in period I, and ethanol was added from periods II to V as a supplementary source of electrons for sulfate reduction. The added volume was varied

Table 1 Characteristics of the industrial wastewater (20 samples). Variables

Minimum

Maximum

Mean

pH CODTotal (g l− 1) CODFiltered (g l− 1) NH4+ (g l− 1) SO42− (g l− 1)

2.31 9.24 8.98 1.32 183

3.25 15.43 10.90 1.87 284

– 13.7±4.1 10.6±1.3 1.52±0.5 201±35

Fig. 1. Schematic representation of ASBBR with biomass immobilized on mineral coal.

according to the sulfate removal efficiencies obtained for the different COD/sulfate ratios (Table 2) aimed at maximizing the simultaneous sulfate reduction and ethanol utilization. 2.3. Reactor monitoring Monitoring (92 cycles) was carried out through physical–chemical analyses of the influent and effluent samples, such as chemical oxygen demand (COD) of total and filtered samples, ammonium ion (colorimetric method), total suspended solids (TSS), volatile suspended solids (VSS), and pH according to the Standard Methods [19]. Determinations of volatile fatty acids (VFA), such as acetic acid (HAC) and bicarbonate alkalinity (BA), followed the methodology described by Dilallo and Albertson [20] and modified by Ripley et al. [21]. The methylene blue method (method 4500 D) [19] was used to determine the total dissolved sulfide (TDS). Sulfate concentrations were measured by a turbidimetric method using the HACH sulfaver reagent. Influent and effluent samples were collected at the beginning and end of the same cycle, respectively. Both samples were collected every second cycle. Temporal profiles were carried out under stable condition of operation for three different sulfate concentrations (1.0, 2.0 and 3.0 gSO42− l− 1). In addition to the analyses of sulfate, TDS and COD (total), acetic acid [22] and ethanol concentrations were determined by gas chromatography (HP 6890). Temperature, pH and oxidation– reduction potential were assessed using a YELLOW SPRING-600 probe during the cycle. Methane concentration in the biogas was evaluated using a Gow-Mac gas chromatograph with a thermal conductivity detector (TC) and a “Poropak” Q column (2 m × 1/4″ of 80–100 mesh). The biogas samples were collected in glass samplers (300 ml) attached to the biogas pipeline. The biomass concentration in the inoculum was evaluated by analyzing the total solids (TS) and total volatile solids (TVS). Direct

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Table 2 Summary of average operational parameters applied to the ASBBR reactor. Parameters

Period I

Period II

Period III

Period IV

Period V

Cycle intervals SLR (kgSO42−/cycle) Sulfate (g l− 1) COD/sulfatea OLR (kgCOD/cycle) CODTotal (g l− 1) CODFiltered (g l− 1) BA (mgCaCO3 l− 1) VFA (mgHac l− 1) pH TSS (mg l− 1) VSS (mg l− 1) NH4+ (mg l− 1)

1–7 0.15 ± 0.02 0.25 ± 0.03 2.13 ± 0.35 0.60 ± 0.05 0.98 ± 0.08 0.54 ± 0.08 103 ± 15 67 ± 11 6.9 280 ± 240 241 ± 195 76 ± 5

8–31 0.30 ± 0.03 0.50 ± 0.05 1.89 ± 0.65 0.90 ± 0.17 1.49 ± 0.29 0.93 ± 0.28 103 ± 45 67 ± 16 7.0 177 ± 83 141 ± 70 134 ± 24

32–52 0.65 ± 0.07 1.08 ± 0.12 1.77 ± 0.26 1.40 ± 0.16 2.35 ± 0.27 1.89 ± 0.26 93 ± 19 112 ± 23 6.7 142 ± 111 123 ± 92 236 ± 67

53–61 1.30 ± 0.14 2.16 ± 0.24 1.64 ± 0.40 2.50 ± 0.60 4.12 ± 1.0 3.57 ± 1.0 84 ± 13 96 ± 11 6.9 115 ± 39 90 ± 14 412 ± 68

62–92 1.90 ± 0.13 3.12 ± 0.23 1.50 ± 0.25 3.0 ± 0.47 5.08 ± 0.78 4.63 ± 0.78 92 ± 26 119 ± 33 6.7 99 ± 45 79 ± 34 515 ± 82

OLR = organic loading rate (cycle); SLR = sulfate loading rate (cycle). a CODFiltered.

biomass measurement during the experiments was not feasible due to sample size restrictions. Biomass concentration in the mineral coal sample was assessed for each sulfate concentration (0.25 to 3.0 gSO42− l− 1), and estimated by drying the coal loaded-biomass at 105 °C, and subsequently heating it in an oven at 540 °C for 30 min to volatilize the biomass. The difference in the mineral coal weight before and after this process is reported as the biomass dry weight. The application of this methodology to mineral coal was adapted from Nagpal et al. [6]. 2.4. Inoculation The ASBBR was inoculated with 0.2 m3 of anaerobic sludge (47.2 kgTS m− 3 and 33.1 kgTVS m− 3) taken from a full-scale UASB treating domestic sewage installed at the Wastewater Treatment Plant of the Campus of University of São Paulo (São Carlos, SP, Brazil). Domestic sewage was pumped by recirculation to complete the reactor volume to be treated (0.6 m3). The reactor was re-fed with domestic sewage at each cycle. This operation took 24 days, or 12 cycles of 48 h. The total solid concentration in the start-up period of the ASBBR reactor was maintained at 15.7 kgTS m− 3 and 11.1 kgTVS m− 3 (TVS/TS = 0.7), considering the liquid volume. 3. Results and discussion 3.1. ASBBR performance Each operational parameter in the ASBBR reactor was monitored during 92 cycles in the several operational periods characterized by

different influent sulfate concentrations (Table 3). The reactor achieved the maximum sulfate removal rate (SRR) of 1.6 kgSO42−/cycle for a sulfate loading rate (SLR) of 1.9 kgSO42−/cycle (period V). The average sulfate reduction efficiencies were 92%, 88%, 89%, 87%, and 85% in the periods I, II, III, IV, and V, respectively (Fig. 2). At the end of each period, sulfate concentrations remained between 0.002 and 0.014 mgSO42− l− 1, reaching a sulfate removal of 99% for all the conditions assayed. The high sulfate reduction yields obtained in the ASBBR reactor indicate that sulfate reducing bacteria (SRB) were able to attach to the mineral coal. Operational stability for each condition was achieved after a short period of time following the changes in the operation conditions, thus indicating the high capacity of the mineral coal to retain the biomass (Fig. 3). A large difference in the biomass concentrations (0.06 kgSTV/kg support to 0.18 kgSTV/kg support) occurred between the periods I and II due to the addition of ethanol from the second operation condition. Several studies have demonstrated the ability of SRB to develop a biofilm under different conditions and carriers [5,18,23,24]. For this reason, one of the main hypotheses of this research was that mineral coal could be used as inert support for SRB biomass attachment in sulfidogenic ASBBR reactors. The results obtained can be taken as a confirmation of the initial hypothesis. High TSS and VSS removal efficiencies were observed for all the experimental periods (Table 3). The higher efficiencies were observed during periods I and II (82% and 88%, respectively). An increase of effluent TSS and VSS concentrations occurred in the subsequent periods, possibly due to biomass detachment, resulting in average removal efficiencies ranging between 60% and 70%.

Table 3 Summary of average operational parameters obtained in the ASBBR reactor. Parameters

Period I

Period II

Period III

Period IV

Period V

Cycle intervals Temperature (°C)a SRR (kgSO2− 4 /cycle) Sulfate (g l− 1) TDS (mg l− 1)b H2S (mg l− 1) ORR (kgCOD/cycle) CODTotal (g l− 1) CODFiltered (g l− 1) BA (mg CaCO3 l− 1) VFA (mgHac l− 1) pH TSS (mg l− 1) VSS (mg l− 1) −1 NH+ ) 4 (mg l

1–7 24 ± 3 0.15 ± 0.01 0.021 ± 0.01 4.0 ± 0.9 1.4 ± 0.2 0.50 ± 0.03 0.14 ± 0.03 0.11 ± 0.03 301 ± 48 24 ± 20 7.1 35 ± 24 25 ± 20 71 ± 17

8–31 31 ± 3 0.25 ± 0.04 0.064 ± 0.02 25 ± 8 8.2 ± 0.9 0.70 ± 0.11 0.33 ± 0.08 0.24 ± 0.07 471 ± 50 49 ± 45 7.3 32 ± 17 23 ± 12 128 ± 16

32–52 30 ± 3 0.60 ± 0.07 0.12 ± 0.05 132 ± 32 55 ± 1.0 0.98 ± 0.12 0.72 ± 0.16 0.57 ± 0.15 790 ± 171 172 ± 63 7.1 46 ± 32 38 ± 33 230 ± 65

53–61 35 ± 1 1.15 ± 0.12 0.26 ± 0.07 221 ± 71 82 ± 5 1.61 ± 0.32 1.45 ± 0.35 1.07 ± 0.33 1533 ± 197 238 ± 204 7.3 37 ± 15 32 ± 14 401 ± 54

62–92 34 ± 1 1.60 ± 0.21 0.48 ± 0.18 287 ± 132 177 ± 11 1.25 ± 0.46 3.01 ± 0.57 2.77 ± 0.56 765 ± 772 1552 ± 641 6.7 40 ± 12 27 ± 12 508 ± 86

ORR = organic removal rate (cycle); SRR = sulfate removal rate (cycle). a Liquid. b Total dissolved sulfide.

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Fig. 2. Sulfate removal (●), COD removal (■), and COD/sulfate ratio (▲) in several periods of ASBBR operation.

A gradual decrease of COD removal efficiencies was observed along the experiments (Fig. 2, Table 3). The mean removal efficiencies decreased from 86% to 70% (I–IV), reaching 41% (V) for the organic loading rates (OLR) ranging from 0.6 to 3.0 kgCODTotal/cycle. The mean organic removal rates (ORR) increased through period IV (0.5 to 1.61 kgCODTotal/cycle) and decreased significantly in period V (1.25 kgCODTotal/cycle). This decrease in the organic matter conversion is probably related to methanogenesis inhibition, due to the increase of the mean concentration of non-ionized sulfide (H2S) and TDS in the liquid medium. Concerning sulfide toxicity, it has been reported that the outcome of sulfide inhibition depends not only on the pH, which is directly related to H2S concentration, but also on the TDS concentration and the biomass characteristics [25]. This indicates that both TDS and H2S may inhibit microorganisms (SRB and MM). The effluent TDS and H2S concentrations obtained in this study are shown in Table 3. TDS mean concentrations increased from 4.0 to 287 mg l− 1 while H2S concentration increased from 1.4 mg l− 1 (40%) to 177 mg l− 1 (62%) in several periods of the ASBBR operation (I, II, III, IV and V). The inhibition was specifically critical in period V. There was a decrease in the mean value of the effluent pH from 7.3 to 6.7 (Table 3), indicating that the undissociated H2S was the predominant S specie. TDS and H2S concentrations achieved the highest values of 460 and 397 mg l− 1 (86%), respectively. These concentrations are above the reported inhibitory values for MM, which is 224 mg l− 1 for TDS with 50% of inhibition [26] and 250 mg l− 1 for H2S [27]. The increase of VFA concentration and the simultaneous reduction of BA occurred in period V (Table 3), supporting the presence of methanogenesis inhibition. In this period, the mean values of VFA and BA in the effluent reached 1552 mgHac l− 1 and 765 mgCaCO3 l− 1,

respectively. This significant concentration of VFA (as acetic acid) was generated as a result of the partial oxidation of ethanol to acetate [6]. Therefore, because VFA was not consumed by the MM, the residual COD in the ASBBR effluent increased, resulting in the low COD removal efficiency (41%) obtained. On the other hand, the BA generation and the low values of VFA in the previous periods were considered indicators of the balance between acidogenesis and methanogenesis. VFA values remained between 24 and 238 mgHac l− 1 (I–IV), while BA values were between 301 and 1533 mg CaCO3 l− 1 (I–IV). According to Hansen et al. [28], the metabolism of a particular substrate by SBR may involve incomplete oxidation up to an intermediary compound (acetate, for example). In fact, the organic matter was converted into acids, as highlighted by the high concentrations of VFA in period V. In contrast to the general case of anaerobic wastewater treatment, where the organic matter is the key target, sulfate reducing reactors are designed to have maximum sulfate reduction and, if possible, complete suppression of methanogenesis. It is expected that the organic substrates and their reducing equivalents are channeled to SRB in bioreactors operating under excess of sulfate [29]. As the COD/sulfate ratio was decreased from 1.89 (II) to 1.50 (V), the average values of ethanol consumed were 1.62, 1.35, 1.27, and 1.06 kg ethanol/kgSO42− removed/cycle. Therefore, lower amounts of ethanol were consumed when higher SLR's were applied. In this case, the cost (USD$/kgSO42− removed/cycle) to treat sulfate in a reactor ASBBR reactor would be of 0.0 (I), 1.0 (II), 1.08 (III), 1.20, and 1.51 (V). Current studies debate the value of COD/sulfate ratio to be applied for sulfate biological treatment; however, the predominance of sulfate reduction over methanogenesis in this study was observed when the COD/sulfate ratio achieved 1.50 and sulfate concentration was 3.0 gSO42− l− 1 (period V). A preliminary economic viability study showed that the operational cost of biological process sulfate reduction using added ethanol is less than half the operational cost of chemical precipitation processes using calcium salts. Moreover, a biological process makes it possible to recover elemental sulfur [30]. The average concentration of ammonium ion was monitored to evaluate the occurrence of interactions between the nitrogen and sulfur cycles, as described by some authors [1,31]. According to these authors, interactions between these cycles were detected in microbial ecosystems related to conventional anaerobic treatments of effluents containing high concentrations of organic matter, nitrogen, and sulfate. Simultaneous removal of sulfate and nitrogen was not observed in this study, even after increasing the SLR and the ammonium ion concentration. The average ammonium ion concentrations in the influent and effluent were similar (Tables 2 and 3). The difference between the average influent and effluent values remained in the range of 2% to 10% with removal efficiencies ranging from 2% to 7%. 3.2. Temporal profiles (periods III, IV and V)

Fig. 3. Biomass concentration in the mineral coal over several periods of ASBBR operation.

Temporal profiles were carried out after the ASBBR reactor achieved operational stability when the sulfate reduction efficiency did not vary significantly (Fig. 4). Samples from the suction pipe of the recirculation pump were collected over 48 h. Biogas single samples (glass ampoules — 300 ml) were collected and analyzed in order to evaluate the presence of methane during the profiles (12, 24 and 48 h). The dilution of the sulfate-rich wastewater with sewage affected the initial sulfate concentration of the profiles, such that the values obtained for conditions A (1.0 gSO42− l− 1), B (2.0 gSO42− l− 1) and C (3.0 gSO42− l− 1) were 1.45, 2.30, and 2.92 gSO42− l− 1, respectively; however, the values of the COD/sulfate ratio (in terms of CODFiltered) obtained from the profiles were similar to the average values (Table 2) in each period (III, IV, and V). The mean temperature (liquid medium) was 29± 1.5 °C (A), 28 ± 2.6 °C (B), and 28 ± 3.4 °C (C). The oxidation–

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Fig. 4. Temporal profiles (48 h-cycle): A — 1.0 gSO42− l− 1 (■), B — 2.0 gSO42− l− 1 (▲), and C — 3.0 gSO42− l− 1(●).

reduction potential values varied between −364 and −378 mV (anaerobiosis). The cycle times to attain 99% sulfate removal were 15, 25, and 30 h (Fig. 4a) for concentrations of 1.45, 2.32, and 2.92 gSO42− l− 1 (A, B, and C), respectively. Therefore, cycle times lower than 48 h can be applied to the ASBBR reactor. The reduction of the cycle time allows an increase in the number cycles, and consequently, the reactor's treatment capacity. Ethanol consumption (Fig. 4b) in profiles A, B, and C presents similar behavior to sulfate reduction (Fig. 4a). Ethanol was not detected in the effluent at 15 h (A) and 25 h (B), being totally utilized as an electron donor. After 30 h (sulfate removal N99%), a residual ethanol concentration of 230 mg l− 1 was detected in temporal profile C (Fig. 4b). The total COD removals were 81%, 78%, and 20%, respectively. The values of effluent COD (Fig. 4c) were 577 mg l− 1 (A), 1080 mg l− 1 (B), and 4220 mg l− 1 (C). The low organic matter conversion and high total residual COD in profile C are directly related to the high concentration of acetic acid (3530 mgHac l− 1) in the effluent (Fig. 4d). Profiles A and B (Fig. 4d) show the behavior of the reactor

regarding the generation and consumption of acetic acid, resulting in concentrations of 15 and 172 mgHac l− 1, respectively, at the end of the cycles. The deterioration of the acetic acid consumption process (Fig. 4d), corresponding to low methane concentrations in profile C (Fig. 4e), clearly demonstrates the occurrence of methanogenesis inhibition (period V). The pH values remained between 7.0 to 7.3, 6.8 to 7.5, and 6.8 to 5.7 during profiles A, B, and C, respectively. The pH values increased at the end of profiles A and B, whereas a reduction was observed in profile C due to the presence of VFA and H2S as the predominant S specie. Fig. 4f presents the temporal profile of undissociated H2S concentration. The maximum concentrations achieved in profiles A, B, and C were 115 mg l− 1 (H2S/TDS = 0.51), 138 mg l− 1 (H2S/TDS = 0.45), and 194 mg l− 1 (H2S/TDS= 0.88), respectively. Therefore, the inhibition of the MM growth occurring in the period V was primarily related to the presence of undissociated H2S (profile C — 88%) and not to the TDS [1]. This indicates that non-ionized sulfide exerted a higher inhibitory effect on the methanogenic microorganisms than on the SBR for sulfate concentrations equal to or higher than 2.0 gSO42− l− 1. The decrease of

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dissolved H2S concentrations occurred at the end of the profiles (40, 43, and 150 mg l− 1) and can be partially or totally attributed to “stripping,” considering that the ASBBR reactor was operated with a high upward velocity (20 m h− 1) imposed by the liquid recirculation. 4. Conclusions Our study has demonstrated that the ASBBR reactor is a novel option for sulfate removal, particularly in Brazil, where calcium salts (Ca(OH)2 and CaCl2) are primarily employed in the physical–chemical sulfate removal process. The application of a biological treatment to industrial effluent containing high sulfate concentrations (0.25 to 3.0 gSO42− l− 1) provided significant results in terms of sulfate reduction (88% to 92%). Therefore, the potential application of full-scale ASBBR reactors filled with mineral coal for the treatment of sulfate-rich wastewater was demonstrated. Mineral coal can be considered an effective inert support for biomass attachment, especially for methanogenic archeae and sulfate reducing bacteria. Based on the responses of the ASBBR reactor, it can be concluded that this reactor configuration can be used for the combined removal of sulfate (86%) and organic matter (70%) at sulfate influent concentrations up to 2.0 gSO42− l− 1 if ethanol is used as electron donor. At influent sulfate concentrations higher than 2.0 gSO42− l− 1, however, the formation of high concentrations of reduced sulfur compounds (TDS) and residual COD was observed. The methanogenesis inhibition observed for high influent concentrations was attributed to the high concentrations of undissociated H2S formed during the sulfate reduction process. The application of this process on an industrial scale would require a post-treatment system to adequately produce effluents to emission standards. The residual COD composed by organic acids (as acetic acid) can be easily removed in biological reactors (aerobic and anaerobic). However, the removal of reduced sulfur compounds from liquid effluents must be carried out in order to produce elemental sulfur. This product can be returned to the productive chain as sulfuric acid or soil conditioner. Acknowledgements Acknowledgements are due to the Brazilian Research funding institutions: Fundação de Amparo a Pesquisa do Estado de São PauloFAPESP (research grants 03/07799-2) and Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq (Edital Universal: number 019/2004 and process number: 478355/2004-1).

References [1] P.N.L. Lens, A. Visser, A.J.H. Janssen, L.W. Hulshoff-Pol, G. Lettinga, Crit. Rev. Environ. Sci. Tech. 28 (1998) 41–88. [2] L.W. Hulshoff-Pol, P.N.L. Lens, J. Weijma, A.J.M. Stams, Water Sci. Technol. 44 (2001) 67–76. [3] R.T. Van Houten, H. Van Der Spoel, A. Van Aelst, L.W. Hulshoff-Pol, G. Lettinga, Biotechnol. Bioeng. 50 (1995) 136–144. [4] C. Buisman, J. Boonstra, J. Krol, H. Dijkman, Proceedings IAWQ-NVA Conference, IWA Publishing, Amsterdam, The Netherlands, 1996, pp. 91–94. [5] A. De Smul, J. Dries, H. Goethals, W. Verstraete, Appl. Microb. Biotech. 48 (1997) 297–303. [6] S. Nagpal, S. Chuichulcherm, L. Peeva, A. Livingston, Biotech. Bioeng. 70 (2000) 370–379. [7] E.S.K. Chian, F.B. Walle, Proceedings 38th Industrial Waste Conference, Purdue University, West Lafayette, IN, 1983, pp. 920–927. [8] L.J. Herrera, P. Hernandez, S. Ruiz, S. Gantenbein, Environ. Toxic. Water 6 (1991) 225–238. [9] A. Grobicki, D.C. Stuckei, Water Res. 23 (1992) 371–378. [10] G. Stucki, K.W. Hansemann, A. Hürzeler, Biotech. Bioeng. 41 (1993) 303–315. [11] L.A. Du Preez, J.P. Maree, Water Sci. Technol. 30 (1994) 275–285. [12] E.N. Kaufman, M.H. Little, P.T. Selvaraj, Appl. Biochem. Biotech. 63–65 (1996) 677–693. [13] V. Federovich, M. Greben, S. Kalyuzhnyi, P. Lens, L. Hulshoff-Pol, Biodegradation 11 (2000) 295–303. [14] J. Weijma, T.M. Chi, L.W. Hulshoff-Pol, A.J.M. Stams, G. Lettinga, Process Biochem. 38 (2003) 1259–1266. [15] A. Sarti, E. Pozzi, F.A. Chinalia, M. Zaiat, E. Foresti, Chemosphere 62 (2006) 1437–1443. [16] A. Sarti, M.L. Garcia, M. Zaiat, E. Foresti, Resour. Conserv. Recycl. 51 (2007) 237–247. [17] S.M. Ratusznei, J.A.D. Rodrigues, E.F.M. Camargo, M. Zaiat, W. Borzani, Bioresour. Technol. 75 (2000) 127–133. [18] A.J. Silva, J.S. Hirasawa, M.B. Varesche, E. Foresti, M. Zaiat, Anaerobe 12 (2006) 93–98. [19] APHA, AWWA, WPCF, Standard Methods for the Examination of Water and Wastewater19th edition, American Public Health Association, 1998. [20] R. Dilallo, O.E. Albertson, J. WPCF 33 (1961) 356–365. [21] L.E. Ripley, W.C. Boyle, J.C. Converse, J. WPCF 58 (1986) 406–411. [22] E.M. Moraes, M.A.T. Adorno, M. Zaiat, E. Foresti, Assessment of total volatile acids by gas chromatography in anaerobic bioreactor liquid waste, Proceedings of VI Latin American Workshop and Seminar on Anaerobic Digestion, Recife-Brazil, vol. 2, 2000, pp. 235–238. [23] J. Weijma, A.J.M. Stams, L.W. Hulshoff-Pol, G. Lettinga, Water Res. 36 (2002) 1825–1833. [24] F. Glombitza, Waste Manage. 9 (2001) 23–42. [25] V. O'Flaherty, E. Colleran, Sulfur Problems in Anaerobic Digestion, in: P.N.L. Lens, L.W. Hulshoff-Pol (Eds.), Environmental Technologies to Treat Sulfur Pollution: Principles and Engineering, IWA publishing, London, 2000, pp. 467–489. [26] P.P. Karhadkar, J.M. Audic, G.M. Faup, P. Khanna, Water Res. 21 (1987) 1061–1066. [27] F. Omil, P.N.L. Lens, L.W. Hulshoff-Pol, G. Lettinga, Environ. Technol. Lett. 10 (1996) 815–822. [28] T.A. Hansen, A. Van Leeuw, J. Microb. 66 (1994) 85–165. [29] M.V.G. Vallero 2003. Sulfate reducing processes at extreme salinity and temperature: extending its application window. Ph.D. thesis, Wageningen Agricultural University, Wageningen, the Netherlands. [30] A.J. Silva, M.B. Varesche, E. Foresti, M. Zaiat, Proc. Biochem. 37 (2002) 927–935. [31] F. Fdz-Polanco, M. Fdz-Polanco, N. Fernandez, M.A. Urueña, P.A. Garcia, S. Villaverde, Water Sci. Technol. 44 (2001) 15–22.