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Post-treatment of effluents from the sulfate reduction process by anaerobic sequencing batch biofilm reactors
Desalination 237 (2009) 243–253
Post-treatment of effluents from the sulfate reduction process by anaerobic sequencing batch biofilm reactors Arnaldo Sarti*, Roberto S. Côrtes, Julia S. Hirasawa, Eduardo C. Pires, Eugenio Foresti Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos (EESC), Universidade de São Paulo (USP), Av. Trabalhador São-Carlense 400, CEP 13566-590, São Carlos, São Paulo, Brasil Fax: +55 (16) 3373 9550; email: [email protected] Received 22 August 2007; Accepted 27 December 2007
Abstract The main objective of this research was to evaluate the potential use of a bench-scale anaerobic sequencing batch biofilm reactor (ASBBR) containing mineral coal as inert support for removal of sulfide and organic matter effluents from an ASBBR (1.2 m3) utilized for treatment of sulfate-rich wastewater. The cycle time was 48 h, including the steps of feeding (2 h), reaction with continuous liquid recirculation (44 h) and discharge (2 h). COD removal efficiency was up to 90% and the effluents total sulfide concentrations (H2S, HS!, S2!) remained in the range of 1.5 to 7.5 mg.l!1 during the 50 days of operation (25 cycles). The un-ionized sulfide and ionized sulfides were converted by biological process to elemental sulfur (S0) under oxygen limited conditions. The results obtained in the benchscale reactor were used to design an ASBBR in pilot scale for use in post-treatment to achieve the emission standards (sulfide and COD) for sulfate reduction. The pilot-scale reactor, with a total volume of 0.43 m3, the COD and total sulfide removal achieved 88% and 57%, respectively, for a cycle time of 48 h (70 days of operation or 35 cycles). Keywords: Wastewater treatment; Anaerobic sequencing batch biofilm reactor; Sulfide removal; Sulfate reduction
1. Introduction Sulfate can be removed from wastewaters by chemical precipitation or desalination processes such as reverse osmosis and ion exchange at significant costs. On the other hand, the success *Corresponding author.
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 process for the removal of sulfate from industrial wastewaters [1]. Biological processes including sulfate reduction to sulfide [H2S(g) + H2S(aq) + HS!(aq) + S2! (aq)]
0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.12.034
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and its subsequent conversion to elemental sulfur [S0] were successfully developed as a costeffective method for removal of sulfur from waste streams [2]. In this case, the production of sulfide is the major problem associated with the anaero-bic treatment of sulfate-rich wastewaters [3]. Therefore, to achieve successful treatment of wastewaters containing sulfate, it is necessary to eliminate sulfide from the anaerobic reactor. For wastewater containing inorganic sulfate with no or insufficient electron donor and organic matter for the complete sulfate reduction, addition of an appropriate electron donor is required. The use of ethanol as an electron donor in sulfate-reducing systems is applied in full-scale plants [2]. However, the main drawback of using ethanol as electron donor is the generation of acetic acid, resulting in an effluent with high residual COD [4]. The residual pollution caused by the electron donor should be minimized by a second reactor where the sulfide can be partially reoxidized with air to elemental sulfur [5]. Oxidation processes for sulfide removal are aeration (uncatalyzed and (bio)catalyzed), electrochemical oxidation, chlorination, ozonization, and, treatment with potassium permanganate (KMnO4) or hydrogen peroxide (H2O2) [6]. In all these oxidation processes, elemental sulfur, thiosulfate, and sulfate are the end products in varying ratios depending on the pH. In practice, suitable sulfide removal methods are the precipitation of sulfide, sulfur removal from H2Senriched stripping gas [7], or partial oxidation of dissolved sulfide. The selection of the most suitable method depends on technical and economical considerations [3]. Biological sulfide oxidation does provides a clean alternative for the removal of low level H2S from both liquid and gas streams, as well as it has the potential of sulfur recovery. Under oxygen limited conditions, that is, dissolved oxygen concentrations below 0.1 mg.l!1, S0 is the major end product of the sulfide oxidation [Eq. (1)], while sulfate is formed under sulfide-limiting
conditions [8]. S0 formation requires four times less oxygen compared with complete oxidation and, consequently, lower energy consumption for aeration [Eq. (2)]. The oxidation of suldide to elemental sulfur can be carried out by chemolithoautotrophic bacteria belonging to the genus Thiobacillus [9]. 2HS! + O2 ÷ 2S0 + 2OH! ΔG0 = !169.35 kJ.mol (HS!)!1
(1)
2HS! + 4O2 ÷ 2SO4!2 + 2H+ ΔG0 = !732.58 kJ.mol (HS!)!1
(2)
Since the formation of sulfate yields more energy this reaction is carried out preferentially by the microorganisms. The formation of sulfur will only proceed under oxygen-limiting conditions or at high sulfide loading rates, whereas sulfate is the main product in the presence of an excess of oxygen [9]. The products of sulfide oxidation, such as sulfate and thiosulfate, are highly water soluble and difficult to separate. Therefore, sulfide removal studies have been focused mainly on the partial oxidation of sulfide to sulfur which could be efficiently separated from the waste stream [10]. Among the separation processes such as filtration, flotation, extraction, and membrane processes, sedimentation of the sulfur particles represents the technically and economically most attractive removal method. The formation of sulfur particles with good settling properties is the main prerequisite [11]. The removal of reduced compounds of sulfur from liquid and gaseous effluents, if carried out to produce elemental sulfur, with its subsequent recovery, yields a better effluent for the environment and the treatment economic sustainable. The elemental sulfur presents an availability of returning to the productive chain as sulfuric acid and soil conditioner [3].
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A conventional anaerobic sequencing batch reactor (ASBR) is operated with intermittent cycles of four steps: the feeding or loading of liquid influent, anaerobic biological reactions, biomass sedimentation and effluent discharge. The sedimentation step is directly dependent on the formation of biomass with good settling characteristics, as in granular form, avoiding losses of the metabolic adapted sludge during discharging of the treated effluent [12]. This type of anaerobic reactor offers distinct advantages over continuous reactor configurations: there is no liquid short-circuiting, influent distribution system at the bottom is not required and no external or internal device for separating solids is involved [13]. The feasibility of sequencing batch reactors to treat low-strength wastewaters (COD <1000 mg.l!1) was assessed by other authors [14,15]. Those studies indicated that the use of ASBR to treat low-strength wastewater, even at low temperatures, allows the treatment of industrial and municipal wastewaters at lower costs than those of aerobic systems. Additionally, the use of inert supports to immobilize cells in sequencing batch reactors appears to be a promising method to improve solids retention, suppressing the settling step and thus reducing the total cycle time. Moreover, the immobilization of biomass in an inert support eliminates uncertainties about sludge granulation [16]. This paper presents the performance of two ASBBR reactors. The first, a bench-scale ASBBR during 25 cycles (50 days) was used for posttreatment of effluents from an anaerobic reactor utilized for sulfate reduction. Although the application of ASBBR reactors seems to be promising based on bench-scale experiments, research in scaled-up studies are needed to evaluate the applicability of this technology for the treatment of sulfate-rich wastewaters. Thus, a second ASBBR in pilot scale (0.43 m3) was designed for the treatment of sulfide and organic matter generated in sulfate reduction process. Mineral coal was tested
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as material support due its low cost and availability in Brazil. 2. Materials and methods 2.1. Feed wastewater The influent came from an ASBBR (1.2 m3) utilized for the treatment of sulfate ion from a chemical industry wastewater. This industry has the sulfonation of vegetable oils (rice, soy and corn) as its main waste source and the above mentioned reactor with mineral coal as inert support demonstrated to be a novel option for sulfate treatment. The ASBBR (1.2 m3) provided significant results in terms of sulfate reduction (higher than 95%) with ethanol used as electron donor for sulfate reduction. The added ethanol was increased gradually, following the sulfate concentration (1.0–3.0 gSO42!.l!1). The composition of the wastewater after biological treatment for sulfate reduction (1.0–3.0 gSO42!.l!1) is presented in Table 1. In this experiment, the effluents showed increase of the residual COD composed mainly by acetic acid (partial oxidation of ethanol) [5]. 2.2. Reactors (post-treatment) The removal of the residual COD is relatively simple and can also proceed in an anaerobic biological reactor. A two-phase treatment system with the first phase for sulfate reduction and the second phase for methanogenesis (post-treatment) seems promising to eliminate the residual COD. Thus, ASBBR reactors were designed for COD removal in different scales. The bench-scale ASBBR reactor (ASBBRBS) for initial studies was constructed with PVC pipe with a total volume of 6.3 l. The reactor was filled with 1.9 kg of irregular pieces of mineral coal (10–20 mm of equivalent diameter) occupying a volume of 4.5 l (bed), resulting in a liquid volume of 2.6 l (bed porosity=57%). The head-
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Table 1 Characteristics of the wastewater for post-treatment (50 samples) Variables
Min.
Max.
Mean
Temperature (ºC) pH BA (mgCaCO3!2.l!1) VFA (mgHac.l!1) CODTotal (mg.l!1) OLRa (kgCOD/cycle) CODFiltered (mg.l!1) SO4!2 (mg.l!1) TDS (mg.l!1) SLRb (kgTDS/cycle) TSS (mg.l!1) VSS (mg.l!1)
32 6.1 60 381 1331 0.35 595 7 75 0.02 28 25
36 7.5 2070 2340 4310 1.12 3820 17 496 0.13 69 64
341 — 76572 1552641 3005996 — 2679965 108 287±132 — 7212 5814
a
Organic loading rate (cycle). Sulfide loading rate (cycle).
b
space volume (1.7 l) was filled with 0.9 l of liquid to keep the recirculation pipe immersed in the liquid. Therefore, the treatment volume available for each cycle of the batch mode was 3.5 l (2.6 l in the bed + 0.9 l in the headspace). A diagram of the experimental apparatus and operation of ASBBRBS is shown in Fig. 1. Mixing was provided by liquid recirculation (up-flow) with a peristaltic pump (23 l.h!1). The influent wastewater was drained from ASBBR (1.2 m3) also by the recirculation pump to a pipe located at the reactor’s bottom (Fig. 1). The cycle time was 48 h, including the steps of feeding (2 h), reaction with continuous liquid recirculation (44 h) and discharge (2 h). The pilot-scale ASBBR reactor (ASBBRPS) was designed in fiberglass with total volume of 0.43 m3. The reactor was packed with 160 kg of irregular pieces of mineral coal (20–40 mm of equivalent diameter) occupying a volume of 0.32 m3 (bed), resulting in a liquid volume of 0.16 m3 (bed porosity=50%). The head-space volume (0.11 m3) was filled with 0.070 m3 of
Fig. 1. Schematic representation of ASBBR reactor (bench-scale) containing biomass immobilized in mineral coal.
liquid to keep the recirculation pipe immersed. Therefore, the treatment volume available by cycle was 0.23 m3 (0.16 m3 in the bed + 0.070 m3 in the head space). The experimental set-up is showed in Fig. 2. The same operational cycle of the ASBBRBS was used for experimental with the ASBBRPS.
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2.3. Biomass seeding The ASBBRBS was inoculated with 1.0 l of anaerobic sludge (54 gTS.l!1 and 49 gTVS.l!1) from a full-scale UASB treating poultry slaughterhouse wastewater. The same anaerobic sludge (80 l) was utilized for inoculation of ASBBRPS. The inoculation process occurred before the start-up of the reactors. Both reactors were fed with a mixture of sludge and effluent of the ASBBR (1.2 m3) for sulfate treatment. These anaerobic reactors were maintained with continuous recirculation of the mixture (peristaltic, ASBBRBS or centrifugal pump, ASBBRPS) for 15 days. 2.4. Analytical methods
Fig. 2. Schematic representation of ASBBR reactor (pilot-scale) containing biomass immobilized in mineral coal.
The agitation or mixing was also provided by liquid recirculation (up-flow), but with a centrifugal pump (4.5 m3.h!1). The influent wastewater in the feeding step was pumped from ASBBR (1.2 m3) to the perforated pipe (distribution system) located at the reactor’s bottom (Fig. 2). The ASBBR reactors were located at the Laboratory of Biological Process–USP/São Carlos and exposed to ambient temperature variations of 30±5EC.
The ASBBRBS and ASBBRBS monitoring was carried out through physicochemical analysis of the influent and effluent samples. Chemical oxygen demand (COD) of total and filtered samples (membrane of 1.2 µm), total solids (TS), total volatile solids (TVS), total suspended solids (TSS) and volatile suspended solids were measured according to Standard Methods [17]. Determinations of volatile fatty acids (VFA) as acetic acid (HAC) and bicarbonate alkalinity (BA) were measured according to methodology previously described [18] and modified [19]. The methylene blue method, method 4500 D [17], was used for the determination of total dissolved sulfide (TDS). Sulfate concentrations were measured by a turbidimetric method using the Hach sulfaver reagent. The oxidation-reduction potential (ORP), dissolved oxygen (DO), pH and temperature were assessed using a Horiba OM-12 and Yellow Spring-600 probe. Influent and effluent samples were collected on alternate cycles. 3. Results and discussion 3.1. ASBBRBS performance The reactor start-up was very fast after the
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Fig. 3. Total COD removal (!) and TDS removal (#) in experimental period of ASBBR operation (bench-scale).
Fig. 4. Influent VFA (•), effluent VFA (!) and effluent BA (#) in experimental period of ASBBR operation (bench-scale).
acclimatization phase (10th cycle) with seeding as described, achieving 86% of total COD removal efficiency (Fig. 3). The values of CODTotal and CODFiltered effluent obtained after 50 days (25 cycles) were 377 and 106 mg.l!1, respectively. The mean influent VFA concentration (as acetic acid) was 1054 mg.l!1 (935 to 1238 mg.l!1) and the effluent concentrations decreased to 36 mg HAc.l!1 (Fig. 4). The stability of the methanogenic process was confirmed by VFA removal and by increasing of bicarbonate alkalinity (BA) in the effluent (580 to 910 mg
Fig. 5. Influent TDS (#) and effluent TDS (!) in experimental period of ASBBR operation (bench-scale).
CaCO3.l!1), which were higher than the influent (30–58 mg CaCO3.l!1) (Fig. 4). The pH ranged from 5.9 to 6.2 for the influent and 6.8 to 8.0 for the effluent. By day 20 (10th cycle), the TDS removal efficiency reached approximately 98% (Fig. 3), with an effluent TDS of 4.8 mg.l!1 and 2.8 mg.l!1 for maximum and minimum values, respectively (Fig. 5). During this period, the influent TDS ranged from 60 to 102 mg.l!1 (Fig. 5). Low values of sulfate concentrations (2–8 mg SO4!2.l!1) were obtained during the experiments in effluent of the ASBBRBS. These values indicate that biological sulfate regeneration does not occur [Eq (2)]. The ORP and DO values remained at around !310 mV/!325 mV and 0.02/0.05 mg O2.l!1, respectively, indicating that anaerobic conditions prevailed inside the ASBBRBS during the experimental period. Fig. 6a shows the presence of a sulfur layer at the top of the reactor (yellowish/sulfur particles). In this case, part of the sulfide was biologically oxidized to sulfur due to the air/liquid interface. The biological oxidation of sulfide to elemental sulfur [Eq. (1)] may be accomplished by the genus Thiobacillus, frequently observed in low oxygen concentrations [11]. In the ASBBRBS a small amount of sulfide might have escaped as gas; however, at a reactor pH of >6.5 (6.8–8.0), the residual sulfide (TDS)
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(a)
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(b)
Fig. 6. Results of the EDX analysis (a) of the sulfur particles inside the ASBBR reactors in the interface air/liquid (b).
should be presented as HS!. Other authors have achieved maximum sulfur recovery in a suspended growth system maintained at pH = 8 and ORP (corrected) in the range of !147mV/ !137 mV [20]. The inhibitory effect of sulfide for methanogenic organisms is presumed to be caused by unionized H2S [3]. According to the pH values as above mentioned, HS! was predominant in the liquid phase. Although we do not know which microorganisms were enriched, the COD removal and oxidation of sulfide results indicate that operation condition in the ASBBRBS led to the development of an active microbial community capable of degrading organic matter and oxidizing sulfide. Probably methanogenic microorganisms developed in the mineral coal (biofilm) and sulfide oxidizing bacteria in the top of the reactor (liquid phase) [21]. This zone does not have enough turbulence to disrupt sulfur particles [11]. In this case, the sulfur layer (particles) should be removed before the end of the cycle by an external unity to allow the separation of the liquid phase. EDX analysis (energy dispersive X-Ray) was performed to identify the yellowish layer observed in the air/liquid interface. This analysis
was assessed using the equipment Link model QX-2000 coupled to a Zeiss digital scanning microscope DSM-960. According to the EDX analysis, sulfur (S) was the predominant element. The EDX results are presented in Fig. 6b. The preliminary study showed that the ASBBRBS for COD and sulfide removal by biological process may be applied for post-treatment of effluents from sulfate reduction, especially using ethanol as electron donor. Moreover, the biological process makes the recovery of elemental sulfur feasible. Thus, the design and application of ASBBRPS was proposed (Fig. 2) with an external clarifier for sulfur recovery. 3.2. ASBBRPS performance Table 2 summarizes the operational parameters obtained with the ASBBRPS from 35 cycles (70 days) in different influent sulfide and COD concentrations (minimum, maximum and mean values). The reactor achieved the maximum sulfide removal rate (SRR) of 0.01 to 0.03 kgTDS/cycle for sulfide loading rates (SLR) ranging from 0.02 to 0.13 kgTDS/cycle (Tables 1 and 2). After 20 cycles the mean sulfide removal was 57% (Fig. 7)
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Fig. 7. Total COD removal (!) and TDS removal (#) in experimental period of ASBBR operation (pilot-scale). Table 2 Summary of operational variables obtained from the ASBBRPS (35 cycles) Variables
Min.
Max.
Mean
Temperature (ºC) pH BA (mgCaCO3!2.l!1) VFA (mgHac.l!1) CODTotal (mg.l!1) ORRa (kgCOD/cycle) CODFiltered (mg.l!1) SO4!2 (mg.l!1) TDS (mg.l!1) SRRb (kgTDS/cycle) TSS (mg.l!1) VSS (mg.l!1)
29 7.2 1074 22 239 0.66 111 2 5 0.01 24 21
35 7.9 1550 232 496 0.91 375 8 130 0.03 66 40
323 — 1350118 6723 38666 — 27658 52 66±19 — 3411 286
a
Organic removal rate (cycle). Sulfide removal rate (cycle).
b
and the effluent sulfide concentrations remained between 25 and 65 mgTDS.l!1 (Fig. 8). The effluent sulfate concentration ranged from 4 to 7 mg SO4!2.l!1 during 35 cycles; however, these values do not characterize sulfate regeneration. The ORP and pH values remained at around !325 mV/ !340 mV and 7.2/7.9, respectively (anaerobic condition). According to the literature, bacterial sulfide oxidation under low sulfide loading or without oxygen control produced sulfate as the major
Fig. 8. Influent TDS (#) and effluent TDS (!) in experimental period of ASBBR operation (pilot-scale).
product [22]. In the ASBBRPS only atmospheric oxygen in the top was accessible. After five cycles of fed batch operation, the reactor liquid at the air interface became turbid whitish-yellow due to the formation of elemental sulfur. From the sixth cycle on the sulfur layer in top of ASBBRPS were discharged to the clarifier before the end of each cycle (Fig. 2). This discharge to the clarifier allows the break of the sulfur layer and conse-quently the separation of sulfur particles by the sedimentation process. The main problem encountered was the amount of sulfur particles generated in the ASBBRPS in the end of the cycles. This reactor presented a reduced TDS removal (57%) and the sulfur layer had small thickness and dimension. Therefore, the separation in the clarifier was hindered by the low settling characteristic of these sulfur particles. Only small amount sulfur was removed from the liquid at the clarifier. According to the EDX analysis, sulfur (S) was also the predominant element in the particles as showed in the ASBBRBS. The high COD removal efficiencies obtained in the ASBBRPS during the whole operation period pointed out that methanogenic microorganisms were able to attach to mineral coal by an inoculation process. Pseudo-steady states were achieved after a short period of time (four cycles) following the changes in the influent COD con-
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centrations (Table 2), thus indicating the high capacity of the mineral coal to retain the biomass. The mean COD removal efficiency was 88% (Fig. 7) for organic loading rates (OLR) ranging from 0.35 to 1.12 kgCODTotal/cycle. The mean values of CODTotal and CODFiltered concentrations in the effluent were 386 and 276 mg.l!1, respectively. The organic removal rates (ORR) were maintained between 0.66 and 0.91 kgCODTotal/ cycle. The TSS and VSS concentration observed in the effluent were likely related to the biomass immobilization in the mineral coal (Table 2). The mean concentrations of 34 (±11) mgTSS.l!1 and 28 (±6) mgVSS.l!1 were achieved in the 35 cycles of the experiment. These low values do not indicate the occurrence of biomass detachment. The reduction of VFA concentration and also the increase of BA (Fig. 9) that occurred in the experimental period correlate with the supposition of high adhesion of methanogenic microorganisms in mineral coal. The BA generation and the low values of VFA may be considered as evidence of the presence these organisms in the biofilm. VFA values remained between 22 and 232 mgHac.l!1 and BA between 1074 and 1550 mgCaCO3.l!1. Therefore, the VFA (as acetic acid) was consumed by the methanogenic microorganisms resulting in the low COD concentration in the effluent and high removal efficiency. 3.3. Experimental profile (sulfate and TDS) In order to understand the lower TDS removal that occurred in the ASBBRPS in relation to ASBBRBS, a temporal profile (in terms of sulfate and TDS) was carried out under stable conditions of operation (after the 35th cycle). Samples from the suction pipe of the recirculation pump were collected during 48 h (cycle time). The behavior of sulfate and TDS concentrations can be observed in Fig. 10. The temporal profile in the ASBBRPS demonstrated the generation of the sulfate ion
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Fig. 9. Influent VFA (•), effluent VFA (!) and effluent BA (#) in experimental period of ASBBR operation (pilot-scale).
Fig. 10. Temporal profile (48 h) in an ASBBR reactor (pilot-scale): sulfate (•) and TDS (#).
(87 mg.l!1) in the feed step (2 h). After 16 h an effective sulfate reduction process was observed. Probably acetic acid was utilized as the source of organic matter to sulfate-reducing bacteria [4]. However, the problem is that TDS residual (72 mg.l!1) was produced along with a reduction of TDS removal (50%). The high sulfate concentration is correlated with the presence of atmospheric oxygen when the ASBBRPS is empty in the start of the feed step (Fig. 10). Therefore, part of TDS in the influent is converted to sulfate [Eq (2)]. Moreover, different cycle times can be applied to the ASBBRPS in order to avoid formation of sulfate and TDS residual. For example, the decrease of the cycle time is a strategy feasible and it allows increase
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the number of cycles and consequently the treatment capacity. On the other hand, the sulfate reduction process was observed. Also acetic acid could be used as a carbon source by sulfate-reducing bacteria. Such microorganisms can grow using this organic compound as source of energy for their metabolism and can oxidize this compound completely to CO2 [23]. Although we do not know which microorganisms were enriched, the sulfate reduction and COD removal could be attributed to the coexistence of sulfate-reducing bacteria and methanogenic archaea in the biofilm. These results indicate that mineral coal provided the proper conditions for the immobilization of the microorganisms responsible for coexistence of sulfate and organic matter removal. It has been stated that use of a complex microbial biofilm for the treatment of sulfide-containing effluents is necessary [24]. The coexistence of aerobic, anaerobic and chemolithoautotrophic microorganisms could be very important for the long-term functioning and versatility of the reactor.
organic matter in order to adequate the produced effluents to the emission standards. The removal of the residual COD, composed of organic acids (as acetic acid), was relatively simple and TDS removal can be improved by optimization of operational strategies applied to the ASBBR configuration. Mineral coal can be considered an effective inert support for biomass attachment, especially for methanogenic microorganisms. Therefore, the main goal was attained and with a possible solution to remove sulfide and COD biologically using technology and available materials in Brazil was shown. Further research focused on the biological conversion of sulfide to elemental sulfur and on its recovery is needed. By combining anaerobic sulfate-reducing and aerobic sulfide-oxidizing processes, almost any inorganic sulfur compound can be converted into elemental sulfur. However, to make the newly-developed technologies successful in practice, an adequate sulfur removal step is essential.
4. Conclusions
Acknowledgements
The high COD (86%) and TDS (98%) removal obtained with the ASBBRBS filled with mineral coal demonstrated the potential use of this technology as post-treatment from sulfate reduction process, with ethanol as the electron donor. On the other hand, the ASBBRPS provided only significant results in terms of COD removal (88%). The operating conditions of this reactor favored the occurrence of a sulfate reduction process with generation of sulfide and low performance for TDS removal (57%). However, this problem can be solved by decreasing the cycle time and still increasing the treatment capacity of the reactor. Based on the results of the ASBBRPS, it can be concluded that this reactor configuration can be used for the combined removal of TDS and
Acknowledgements are due to the Brazilian research fund institutions, Fundação de Amparo a Pesquisa do Estado de São Paulo-FAPESP (grant no. 03/07799-2) and Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq (Edital Universal: number 019/2004 and grant no. 478355/2004-1).
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Post-treatment of effluents from the sulfate reduction process by anaerobic sequencing batch biofilm reactors Arnaldo Sarti*, Roberto S. Côrtes, Julia S. Hirasawa, Eduardo C. Pires, Eugenio Foresti Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos (EESC), Universidade de São Paulo (USP), Av. Trabalhador São-Carlense 400, CEP 13566-590, São Carlos, São Paulo, Brasil Fax: +55 (16) 3373 9550; email: [email protected] Received 22 August 2007; Accepted 27 December 2007
Abstract The main objective of this research was to evaluate the potential use of a bench-scale anaerobic sequencing batch biofilm reactor (ASBBR) containing mineral coal as inert support for removal of sulfide and organic matter effluents from an ASBBR (1.2 m3) utilized for treatment of sulfate-rich wastewater. The cycle time was 48 h, including the steps of feeding (2 h), reaction with continuous liquid recirculation (44 h) and discharge (2 h). COD removal efficiency was up to 90% and the effluents total sulfide concentrations (H2S, HS!, S2!) remained in the range of 1.5 to 7.5 mg.l!1 during the 50 days of operation (25 cycles). The un-ionized sulfide and ionized sulfides were converted by biological process to elemental sulfur (S0) under oxygen limited conditions. The results obtained in the benchscale reactor were used to design an ASBBR in pilot scale for use in post-treatment to achieve the emission standards (sulfide and COD) for sulfate reduction. The pilot-scale reactor, with a total volume of 0.43 m3, the COD and total sulfide removal achieved 88% and 57%, respectively, for a cycle time of 48 h (70 days of operation or 35 cycles). Keywords: Wastewater treatment; Anaerobic sequencing batch biofilm reactor; Sulfide removal; Sulfate reduction
1. Introduction Sulfate can be removed from wastewaters by chemical precipitation or desalination processes such as reverse osmosis and ion exchange at significant costs. On the other hand, the success *Corresponding author.
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 process for the removal of sulfate from industrial wastewaters [1]. Biological processes including sulfate reduction to sulfide [H2S(g) + H2S(aq) + HS!(aq) + S2! (aq)]
0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.12.034
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and its subsequent conversion to elemental sulfur [S0] were successfully developed as a costeffective method for removal of sulfur from waste streams [2]. In this case, the production of sulfide is the major problem associated with the anaero-bic treatment of sulfate-rich wastewaters [3]. Therefore, to achieve successful treatment of wastewaters containing sulfate, it is necessary to eliminate sulfide from the anaerobic reactor. For wastewater containing inorganic sulfate with no or insufficient electron donor and organic matter for the complete sulfate reduction, addition of an appropriate electron donor is required. The use of ethanol as an electron donor in sulfate-reducing systems is applied in full-scale plants [2]. However, the main drawback of using ethanol as electron donor is the generation of acetic acid, resulting in an effluent with high residual COD [4]. The residual pollution caused by the electron donor should be minimized by a second reactor where the sulfide can be partially reoxidized with air to elemental sulfur [5]. Oxidation processes for sulfide removal are aeration (uncatalyzed and (bio)catalyzed), electrochemical oxidation, chlorination, ozonization, and, treatment with potassium permanganate (KMnO4) or hydrogen peroxide (H2O2) [6]. In all these oxidation processes, elemental sulfur, thiosulfate, and sulfate are the end products in varying ratios depending on the pH. In practice, suitable sulfide removal methods are the precipitation of sulfide, sulfur removal from H2Senriched stripping gas [7], or partial oxidation of dissolved sulfide. The selection of the most suitable method depends on technical and economical considerations [3]. Biological sulfide oxidation does provides a clean alternative for the removal of low level H2S from both liquid and gas streams, as well as it has the potential of sulfur recovery. Under oxygen limited conditions, that is, dissolved oxygen concentrations below 0.1 mg.l!1, S0 is the major end product of the sulfide oxidation [Eq. (1)], while sulfate is formed under sulfide-limiting
conditions [8]. S0 formation requires four times less oxygen compared with complete oxidation and, consequently, lower energy consumption for aeration [Eq. (2)]. The oxidation of suldide to elemental sulfur can be carried out by chemolithoautotrophic bacteria belonging to the genus Thiobacillus [9]. 2HS! + O2 ÷ 2S0 + 2OH! ΔG0 = !169.35 kJ.mol (HS!)!1
(1)
2HS! + 4O2 ÷ 2SO4!2 + 2H+ ΔG0 = !732.58 kJ.mol (HS!)!1
(2)
Since the formation of sulfate yields more energy this reaction is carried out preferentially by the microorganisms. The formation of sulfur will only proceed under oxygen-limiting conditions or at high sulfide loading rates, whereas sulfate is the main product in the presence of an excess of oxygen [9]. The products of sulfide oxidation, such as sulfate and thiosulfate, are highly water soluble and difficult to separate. Therefore, sulfide removal studies have been focused mainly on the partial oxidation of sulfide to sulfur which could be efficiently separated from the waste stream [10]. Among the separation processes such as filtration, flotation, extraction, and membrane processes, sedimentation of the sulfur particles represents the technically and economically most attractive removal method. The formation of sulfur particles with good settling properties is the main prerequisite [11]. The removal of reduced compounds of sulfur from liquid and gaseous effluents, if carried out to produce elemental sulfur, with its subsequent recovery, yields a better effluent for the environment and the treatment economic sustainable. The elemental sulfur presents an availability of returning to the productive chain as sulfuric acid and soil conditioner [3].
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A conventional anaerobic sequencing batch reactor (ASBR) is operated with intermittent cycles of four steps: the feeding or loading of liquid influent, anaerobic biological reactions, biomass sedimentation and effluent discharge. The sedimentation step is directly dependent on the formation of biomass with good settling characteristics, as in granular form, avoiding losses of the metabolic adapted sludge during discharging of the treated effluent [12]. This type of anaerobic reactor offers distinct advantages over continuous reactor configurations: there is no liquid short-circuiting, influent distribution system at the bottom is not required and no external or internal device for separating solids is involved [13]. The feasibility of sequencing batch reactors to treat low-strength wastewaters (COD <1000 mg.l!1) was assessed by other authors [14,15]. Those studies indicated that the use of ASBR to treat low-strength wastewater, even at low temperatures, allows the treatment of industrial and municipal wastewaters at lower costs than those of aerobic systems. Additionally, the use of inert supports to immobilize cells in sequencing batch reactors appears to be a promising method to improve solids retention, suppressing the settling step and thus reducing the total cycle time. Moreover, the immobilization of biomass in an inert support eliminates uncertainties about sludge granulation [16]. This paper presents the performance of two ASBBR reactors. The first, a bench-scale ASBBR during 25 cycles (50 days) was used for posttreatment of effluents from an anaerobic reactor utilized for sulfate reduction. Although the application of ASBBR reactors seems to be promising based on bench-scale experiments, research in scaled-up studies are needed to evaluate the applicability of this technology for the treatment of sulfate-rich wastewaters. Thus, a second ASBBR in pilot scale (0.43 m3) was designed for the treatment of sulfide and organic matter generated in sulfate reduction process. Mineral coal was tested
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as material support due its low cost and availability in Brazil. 2. Materials and methods 2.1. Feed wastewater The influent came from an ASBBR (1.2 m3) utilized for the treatment of sulfate ion from a chemical industry wastewater. This industry has the sulfonation of vegetable oils (rice, soy and corn) as its main waste source and the above mentioned reactor with mineral coal as inert support demonstrated to be a novel option for sulfate treatment. The ASBBR (1.2 m3) provided significant results in terms of sulfate reduction (higher than 95%) with ethanol used as electron donor for sulfate reduction. The added ethanol was increased gradually, following the sulfate concentration (1.0–3.0 gSO42!.l!1). The composition of the wastewater after biological treatment for sulfate reduction (1.0–3.0 gSO42!.l!1) is presented in Table 1. In this experiment, the effluents showed increase of the residual COD composed mainly by acetic acid (partial oxidation of ethanol) [5]. 2.2. Reactors (post-treatment) The removal of the residual COD is relatively simple and can also proceed in an anaerobic biological reactor. A two-phase treatment system with the first phase for sulfate reduction and the second phase for methanogenesis (post-treatment) seems promising to eliminate the residual COD. Thus, ASBBR reactors were designed for COD removal in different scales. The bench-scale ASBBR reactor (ASBBRBS) for initial studies was constructed with PVC pipe with a total volume of 6.3 l. The reactor was filled with 1.9 kg of irregular pieces of mineral coal (10–20 mm of equivalent diameter) occupying a volume of 4.5 l (bed), resulting in a liquid volume of 2.6 l (bed porosity=57%). The head-
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Table 1 Characteristics of the wastewater for post-treatment (50 samples) Variables
Min.
Max.
Mean
Temperature (ºC) pH BA (mgCaCO3!2.l!1) VFA (mgHac.l!1) CODTotal (mg.l!1) OLRa (kgCOD/cycle) CODFiltered (mg.l!1) SO4!2 (mg.l!1) TDS (mg.l!1) SLRb (kgTDS/cycle) TSS (mg.l!1) VSS (mg.l!1)
32 6.1 60 381 1331 0.35 595 7 75 0.02 28 25
36 7.5 2070 2340 4310 1.12 3820 17 496 0.13 69 64
341 — 76572 1552641 3005996 — 2679965 108 287±132 — 7212 5814
a
Organic loading rate (cycle). Sulfide loading rate (cycle).
b
space volume (1.7 l) was filled with 0.9 l of liquid to keep the recirculation pipe immersed in the liquid. Therefore, the treatment volume available for each cycle of the batch mode was 3.5 l (2.6 l in the bed + 0.9 l in the headspace). A diagram of the experimental apparatus and operation of ASBBRBS is shown in Fig. 1. Mixing was provided by liquid recirculation (up-flow) with a peristaltic pump (23 l.h!1). The influent wastewater was drained from ASBBR (1.2 m3) also by the recirculation pump to a pipe located at the reactor’s bottom (Fig. 1). The cycle time was 48 h, including the steps of feeding (2 h), reaction with continuous liquid recirculation (44 h) and discharge (2 h). The pilot-scale ASBBR reactor (ASBBRPS) was designed in fiberglass with total volume of 0.43 m3. The reactor was packed with 160 kg of irregular pieces of mineral coal (20–40 mm of equivalent diameter) occupying a volume of 0.32 m3 (bed), resulting in a liquid volume of 0.16 m3 (bed porosity=50%). The head-space volume (0.11 m3) was filled with 0.070 m3 of
Fig. 1. Schematic representation of ASBBR reactor (bench-scale) containing biomass immobilized in mineral coal.
liquid to keep the recirculation pipe immersed. Therefore, the treatment volume available by cycle was 0.23 m3 (0.16 m3 in the bed + 0.070 m3 in the head space). The experimental set-up is showed in Fig. 2. The same operational cycle of the ASBBRBS was used for experimental with the ASBBRPS.
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2.3. Biomass seeding The ASBBRBS was inoculated with 1.0 l of anaerobic sludge (54 gTS.l!1 and 49 gTVS.l!1) from a full-scale UASB treating poultry slaughterhouse wastewater. The same anaerobic sludge (80 l) was utilized for inoculation of ASBBRPS. The inoculation process occurred before the start-up of the reactors. Both reactors were fed with a mixture of sludge and effluent of the ASBBR (1.2 m3) for sulfate treatment. These anaerobic reactors were maintained with continuous recirculation of the mixture (peristaltic, ASBBRBS or centrifugal pump, ASBBRPS) for 15 days. 2.4. Analytical methods
Fig. 2. Schematic representation of ASBBR reactor (pilot-scale) containing biomass immobilized in mineral coal.
The agitation or mixing was also provided by liquid recirculation (up-flow), but with a centrifugal pump (4.5 m3.h!1). The influent wastewater in the feeding step was pumped from ASBBR (1.2 m3) to the perforated pipe (distribution system) located at the reactor’s bottom (Fig. 2). The ASBBR reactors were located at the Laboratory of Biological Process–USP/São Carlos and exposed to ambient temperature variations of 30±5EC.
The ASBBRBS and ASBBRBS monitoring was carried out through physicochemical analysis of the influent and effluent samples. Chemical oxygen demand (COD) of total and filtered samples (membrane of 1.2 µm), total solids (TS), total volatile solids (TVS), total suspended solids (TSS) and volatile suspended solids were measured according to Standard Methods [17]. Determinations of volatile fatty acids (VFA) as acetic acid (HAC) and bicarbonate alkalinity (BA) were measured according to methodology previously described [18] and modified [19]. The methylene blue method, method 4500 D [17], was used for the determination of total dissolved sulfide (TDS). Sulfate concentrations were measured by a turbidimetric method using the Hach sulfaver reagent. The oxidation-reduction potential (ORP), dissolved oxygen (DO), pH and temperature were assessed using a Horiba OM-12 and Yellow Spring-600 probe. Influent and effluent samples were collected on alternate cycles. 3. Results and discussion 3.1. ASBBRBS performance The reactor start-up was very fast after the
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Fig. 3. Total COD removal (!) and TDS removal (#) in experimental period of ASBBR operation (bench-scale).
Fig. 4. Influent VFA (•), effluent VFA (!) and effluent BA (#) in experimental period of ASBBR operation (bench-scale).
acclimatization phase (10th cycle) with seeding as described, achieving 86% of total COD removal efficiency (Fig. 3). The values of CODTotal and CODFiltered effluent obtained after 50 days (25 cycles) were 377 and 106 mg.l!1, respectively. The mean influent VFA concentration (as acetic acid) was 1054 mg.l!1 (935 to 1238 mg.l!1) and the effluent concentrations decreased to 36 mg HAc.l!1 (Fig. 4). The stability of the methanogenic process was confirmed by VFA removal and by increasing of bicarbonate alkalinity (BA) in the effluent (580 to 910 mg
Fig. 5. Influent TDS (#) and effluent TDS (!) in experimental period of ASBBR operation (bench-scale).
CaCO3.l!1), which were higher than the influent (30–58 mg CaCO3.l!1) (Fig. 4). The pH ranged from 5.9 to 6.2 for the influent and 6.8 to 8.0 for the effluent. By day 20 (10th cycle), the TDS removal efficiency reached approximately 98% (Fig. 3), with an effluent TDS of 4.8 mg.l!1 and 2.8 mg.l!1 for maximum and minimum values, respectively (Fig. 5). During this period, the influent TDS ranged from 60 to 102 mg.l!1 (Fig. 5). Low values of sulfate concentrations (2–8 mg SO4!2.l!1) were obtained during the experiments in effluent of the ASBBRBS. These values indicate that biological sulfate regeneration does not occur [Eq (2)]. The ORP and DO values remained at around !310 mV/!325 mV and 0.02/0.05 mg O2.l!1, respectively, indicating that anaerobic conditions prevailed inside the ASBBRBS during the experimental period. Fig. 6a shows the presence of a sulfur layer at the top of the reactor (yellowish/sulfur particles). In this case, part of the sulfide was biologically oxidized to sulfur due to the air/liquid interface. The biological oxidation of sulfide to elemental sulfur [Eq. (1)] may be accomplished by the genus Thiobacillus, frequently observed in low oxygen concentrations [11]. In the ASBBRBS a small amount of sulfide might have escaped as gas; however, at a reactor pH of >6.5 (6.8–8.0), the residual sulfide (TDS)
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(a)
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(b)
Fig. 6. Results of the EDX analysis (a) of the sulfur particles inside the ASBBR reactors in the interface air/liquid (b).
should be presented as HS!. Other authors have achieved maximum sulfur recovery in a suspended growth system maintained at pH = 8 and ORP (corrected) in the range of !147mV/ !137 mV [20]. The inhibitory effect of sulfide for methanogenic organisms is presumed to be caused by unionized H2S [3]. According to the pH values as above mentioned, HS! was predominant in the liquid phase. Although we do not know which microorganisms were enriched, the COD removal and oxidation of sulfide results indicate that operation condition in the ASBBRBS led to the development of an active microbial community capable of degrading organic matter and oxidizing sulfide. Probably methanogenic microorganisms developed in the mineral coal (biofilm) and sulfide oxidizing bacteria in the top of the reactor (liquid phase) [21]. This zone does not have enough turbulence to disrupt sulfur particles [11]. In this case, the sulfur layer (particles) should be removed before the end of the cycle by an external unity to allow the separation of the liquid phase. EDX analysis (energy dispersive X-Ray) was performed to identify the yellowish layer observed in the air/liquid interface. This analysis
was assessed using the equipment Link model QX-2000 coupled to a Zeiss digital scanning microscope DSM-960. According to the EDX analysis, sulfur (S) was the predominant element. The EDX results are presented in Fig. 6b. The preliminary study showed that the ASBBRBS for COD and sulfide removal by biological process may be applied for post-treatment of effluents from sulfate reduction, especially using ethanol as electron donor. Moreover, the biological process makes the recovery of elemental sulfur feasible. Thus, the design and application of ASBBRPS was proposed (Fig. 2) with an external clarifier for sulfur recovery. 3.2. ASBBRPS performance Table 2 summarizes the operational parameters obtained with the ASBBRPS from 35 cycles (70 days) in different influent sulfide and COD concentrations (minimum, maximum and mean values). The reactor achieved the maximum sulfide removal rate (SRR) of 0.01 to 0.03 kgTDS/cycle for sulfide loading rates (SLR) ranging from 0.02 to 0.13 kgTDS/cycle (Tables 1 and 2). After 20 cycles the mean sulfide removal was 57% (Fig. 7)
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Fig. 7. Total COD removal (!) and TDS removal (#) in experimental period of ASBBR operation (pilot-scale). Table 2 Summary of operational variables obtained from the ASBBRPS (35 cycles) Variables
Min.
Max.
Mean
Temperature (ºC) pH BA (mgCaCO3!2.l!1) VFA (mgHac.l!1) CODTotal (mg.l!1) ORRa (kgCOD/cycle) CODFiltered (mg.l!1) SO4!2 (mg.l!1) TDS (mg.l!1) SRRb (kgTDS/cycle) TSS (mg.l!1) VSS (mg.l!1)
29 7.2 1074 22 239 0.66 111 2 5 0.01 24 21
35 7.9 1550 232 496 0.91 375 8 130 0.03 66 40
323 — 1350118 6723 38666 — 27658 52 66±19 — 3411 286
a
Organic removal rate (cycle). Sulfide removal rate (cycle).
b
and the effluent sulfide concentrations remained between 25 and 65 mgTDS.l!1 (Fig. 8). The effluent sulfate concentration ranged from 4 to 7 mg SO4!2.l!1 during 35 cycles; however, these values do not characterize sulfate regeneration. The ORP and pH values remained at around !325 mV/ !340 mV and 7.2/7.9, respectively (anaerobic condition). According to the literature, bacterial sulfide oxidation under low sulfide loading or without oxygen control produced sulfate as the major
Fig. 8. Influent TDS (#) and effluent TDS (!) in experimental period of ASBBR operation (pilot-scale).
product [22]. In the ASBBRPS only atmospheric oxygen in the top was accessible. After five cycles of fed batch operation, the reactor liquid at the air interface became turbid whitish-yellow due to the formation of elemental sulfur. From the sixth cycle on the sulfur layer in top of ASBBRPS were discharged to the clarifier before the end of each cycle (Fig. 2). This discharge to the clarifier allows the break of the sulfur layer and conse-quently the separation of sulfur particles by the sedimentation process. The main problem encountered was the amount of sulfur particles generated in the ASBBRPS in the end of the cycles. This reactor presented a reduced TDS removal (57%) and the sulfur layer had small thickness and dimension. Therefore, the separation in the clarifier was hindered by the low settling characteristic of these sulfur particles. Only small amount sulfur was removed from the liquid at the clarifier. According to the EDX analysis, sulfur (S) was also the predominant element in the particles as showed in the ASBBRBS. The high COD removal efficiencies obtained in the ASBBRPS during the whole operation period pointed out that methanogenic microorganisms were able to attach to mineral coal by an inoculation process. Pseudo-steady states were achieved after a short period of time (four cycles) following the changes in the influent COD con-
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centrations (Table 2), thus indicating the high capacity of the mineral coal to retain the biomass. The mean COD removal efficiency was 88% (Fig. 7) for organic loading rates (OLR) ranging from 0.35 to 1.12 kgCODTotal/cycle. The mean values of CODTotal and CODFiltered concentrations in the effluent were 386 and 276 mg.l!1, respectively. The organic removal rates (ORR) were maintained between 0.66 and 0.91 kgCODTotal/ cycle. The TSS and VSS concentration observed in the effluent were likely related to the biomass immobilization in the mineral coal (Table 2). The mean concentrations of 34 (±11) mgTSS.l!1 and 28 (±6) mgVSS.l!1 were achieved in the 35 cycles of the experiment. These low values do not indicate the occurrence of biomass detachment. The reduction of VFA concentration and also the increase of BA (Fig. 9) that occurred in the experimental period correlate with the supposition of high adhesion of methanogenic microorganisms in mineral coal. The BA generation and the low values of VFA may be considered as evidence of the presence these organisms in the biofilm. VFA values remained between 22 and 232 mgHac.l!1 and BA between 1074 and 1550 mgCaCO3.l!1. Therefore, the VFA (as acetic acid) was consumed by the methanogenic microorganisms resulting in the low COD concentration in the effluent and high removal efficiency. 3.3. Experimental profile (sulfate and TDS) In order to understand the lower TDS removal that occurred in the ASBBRPS in relation to ASBBRBS, a temporal profile (in terms of sulfate and TDS) was carried out under stable conditions of operation (after the 35th cycle). Samples from the suction pipe of the recirculation pump were collected during 48 h (cycle time). The behavior of sulfate and TDS concentrations can be observed in Fig. 10. The temporal profile in the ASBBRPS demonstrated the generation of the sulfate ion
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Fig. 9. Influent VFA (•), effluent VFA (!) and effluent BA (#) in experimental period of ASBBR operation (pilot-scale).
Fig. 10. Temporal profile (48 h) in an ASBBR reactor (pilot-scale): sulfate (•) and TDS (#).
(87 mg.l!1) in the feed step (2 h). After 16 h an effective sulfate reduction process was observed. Probably acetic acid was utilized as the source of organic matter to sulfate-reducing bacteria [4]. However, the problem is that TDS residual (72 mg.l!1) was produced along with a reduction of TDS removal (50%). The high sulfate concentration is correlated with the presence of atmospheric oxygen when the ASBBRPS is empty in the start of the feed step (Fig. 10). Therefore, part of TDS in the influent is converted to sulfate [Eq (2)]. Moreover, different cycle times can be applied to the ASBBRPS in order to avoid formation of sulfate and TDS residual. For example, the decrease of the cycle time is a strategy feasible and it allows increase
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the number of cycles and consequently the treatment capacity. On the other hand, the sulfate reduction process was observed. Also acetic acid could be used as a carbon source by sulfate-reducing bacteria. Such microorganisms can grow using this organic compound as source of energy for their metabolism and can oxidize this compound completely to CO2 [23]. Although we do not know which microorganisms were enriched, the sulfate reduction and COD removal could be attributed to the coexistence of sulfate-reducing bacteria and methanogenic archaea in the biofilm. These results indicate that mineral coal provided the proper conditions for the immobilization of the microorganisms responsible for coexistence of sulfate and organic matter removal. It has been stated that use of a complex microbial biofilm for the treatment of sulfide-containing effluents is necessary [24]. The coexistence of aerobic, anaerobic and chemolithoautotrophic microorganisms could be very important for the long-term functioning and versatility of the reactor.
organic matter in order to adequate the produced effluents to the emission standards. The removal of the residual COD, composed of organic acids (as acetic acid), was relatively simple and TDS removal can be improved by optimization of operational strategies applied to the ASBBR configuration. Mineral coal can be considered an effective inert support for biomass attachment, especially for methanogenic microorganisms. Therefore, the main goal was attained and with a possible solution to remove sulfide and COD biologically using technology and available materials in Brazil was shown. Further research focused on the biological conversion of sulfide to elemental sulfur and on its recovery is needed. By combining anaerobic sulfate-reducing and aerobic sulfide-oxidizing processes, almost any inorganic sulfur compound can be converted into elemental sulfur. However, to make the newly-developed technologies successful in practice, an adequate sulfur removal step is essential.
4. Conclusions
Acknowledgements
The high COD (86%) and TDS (98%) removal obtained with the ASBBRBS filled with mineral coal demonstrated the potential use of this technology as post-treatment from sulfate reduction process, with ethanol as the electron donor. On the other hand, the ASBBRPS provided only significant results in terms of COD removal (88%). The operating conditions of this reactor favored the occurrence of a sulfate reduction process with generation of sulfide and low performance for TDS removal (57%). However, this problem can be solved by decreasing the cycle time and still increasing the treatment capacity of the reactor. Based on the results of the ASBBRPS, it can be concluded that this reactor configuration can be used for the combined removal of TDS and
Acknowledgements are due to the Brazilian research fund institutions, Fundação de Amparo a Pesquisa do Estado de São Paulo-FAPESP (grant no. 03/07799-2) and Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq (Edital Universal: number 019/2004 and grant no. 478355/2004-1).
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