Microalgal-Biotechnology As a Platform for an Integral Biogas

Article pubs.acs.org/est

Microalgal-Biotechnology As a Platform for an Integral Biogas Upgrading and Nutrient Removal from Anaerobic Effluents Melanie Bahr,† Ignacio Díaz,‡ Antonio Dominguez,‡ Armando González Sánchez,§ and Raul Muñoz*,† †

Department of Chemical Engineering and Environmental Technology, University of Valladolid, C/Dr. Mergelina s/n, 47011 Valladolid, Spain ‡ BIOGAS FUEL CELL S.A., Parque Tecnológico de Gijón, C\ Luis Moya 82, Edificio Pisa 1° izq, 33203 Gijón, Spain § Instituto de Ingeniería, Universidad Nacional Autónoma de México, Circuito Escolar, Ciudad Universitaria, 04510 Mexico City, Mexico S Supporting Information *

ABSTRACT: The potential of a pilot high rate algal pond (HRAP) interconnected via liquid recirculation with an external absorption column for the simultaneous removal of H2S and CO2 from biogas using an alkaliphilic microalgal-bacterial consortium was evaluated. A bubble column was preferred as external absorption unit to a packed bed column based on its ease of operation, despite showing a comparable CO2 mass transfer capacity. When the combined HRAP-bubble column system was operated under continuous mode with mineral salt medium at a biogas residence time of 30 min in the absorption column, the system removed 100% of the H2S (up to 5000 ppmv) and 90% of the CO2 supplied, with O2 concentrations in the upgraded biogas below 0.2%. The use of diluted centrates as a free nutrient source resulted in a gradual decrease in CO2 removal to steady values of 40%, while H2S removal remained at 100%. The anaerobic digestion of the algal-bacterial biomass produced during biogas upgrading resulted in a CH4 yield of 0.21−0.27 L/gVS, which could satisfy up to 60% of the overall energy demand for biogas upgrading. This proof of concept study confirmed that algal-bacterial photobioreactors can support an integral upgrading without biogas contamination, with a net negative CO2 footprint, energy production, and a reduction of the eutrophication potential of the residual anaerobic effluents.

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and exhibit prohibitive operating costs.4 On the other hand, and to the best of our knowledge, there is no single biological technology capable of simultaneously removing H2S and CO2 since aerobic or denitrifying biofiltration only removes H2S, while conventional microalgae photobioreactors are only efficient for CO2 capture.2,5,6 Algal-bacterial symbiosis in photobioreactors represents an opportunity to simultaneously remove both biogas pollutants at a low energy cost and environmental impact. In these systems, microalgae would use solar energy to fix the CO2 from biogas via photosynthesis, with the concomitant production of O2. This in situ generated O2 is subsequently used by sulfur oxidizing bacteria to oxidize H2S to sulfate.7 The operation of these algal-bacterial processes at high pH values (by using alkaliphilic sulfur oxidizing bacteria and high pH-tolerant microalgae) would significantly enhance the mass transport of the acidic gases H2S and CO2 from the biogas to the algalbacterial cultivation broth, thus allowing for an integral biogas upgrading.8,9 Another advantage of this novel biotechnology is

INTRODUCTION Biogas from the anaerobic digestion of solid wastes constitutes a valuable bioenergy source with the potential to partially alleviate the world’s dependence on fossil fuels. According to the EurObservER 2012 report,1 the production of electricity from biogas in the EU-27 increased by 18% from 2010 to 2011, while biogas heat sales to factories or heating networks increased by 16%. The primary biogas production in the European Union in 2011 accounted for 10.1 Mtoe, and there is still a huge potential for anaerobic digestion in the treatment of municipal and agricultural solid wastes in most EU countries.1 In this context, a decrease in the biogas CO2 content, which accounts for 25−50% of the biogas on volume basis, will result in lower transportation costs and an increase in the biogas energy content. Likewise, a reduction in the H2S content (0−2 %vol) is also crucial for biogas management since H2S is highly corrosive, toxic and malodorous.2 A wide range of technologies based on physical-chemical and biological mechanisms have been applied for the removal of CO2 or H2S from biogas, but there is a lack of technologies capable of simultaneously coping with both biogas contaminants.3,4 Physical/chemical technologies such as membrane separation or chemical scrubbing can cope simultaneously with both biogas pollutants, but are often less environmentally friendly © 2013 American Chemical Society

Received: Revised: Accepted: Published: 573

August 13, 2013 November 23, 2013 December 3, 2013 December 3, 2013 dx.doi.org/10.1021/es403596m | Environ. Sci. Technol. 2014, 48, 573−581

Environmental Science & Technology

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velocities cited above, flowing cocurrently with the rising biogas. All experiments were performed at 20 ± 1 °C. CO2 and O2 concentrations were monitored by GC-TCD at the inlet and outlet of the absorption columns. Simultaneous CO2 and H2S Removal in an Integrated HRAP-Bubble Column System. A 1.2 m2 180 L HRAP (2.02 cm length × 62.5 cm width × 15 cm depth) interconnected to a 0.8 L bubble column was used to evaluate the upgrading of biogas (Figure 1). The HRAP was continuously agitated using a

the possibility of using residual nutrients from wastewater to support the growth of the above-mentioned algal-bacterial consortia, and to produce significant amounts of biomass for the subsequent generation of biogas (=bioenergy), which would significantly improve the energy balance of the biogas plant.10 However, despite the above-mentioned advantages, the potential of algal-bacterial symbiosis as a core technology for the simultaneous removal of CO2 and H2S from biogas and nutrients from wastewater has been poorly explored. This work assessed the potential of an alkaliphilic microalgalbacterial consortium for the simultaneous removal of H2S and CO2 from biogas coupled with nutrient removal from centrates using an innovative process: a pilot high rate algal pond (HRAP) connected to an external CO2−H2S absorption column. The CO2 mass transfer capacity of a packed bed and a bubble column, and the biochemical CH4 potential of the algal-bacterial biomass produced, were also evaluated. Finally, a detailed mass and energy balance for a large-scale biogas upgrading unit was conducted based on the experimental results here obtained.

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MATERIALS AND METHODS Microorganisms and Culture Conditions. Spirulina platensis, a cyanobacterium with an optimum growth pH of 9−10, was purchased from the SAG Culture Collection (Germany), while the alkaliphilic H2S-oxidizing bacterial consortium was kindly provided by Dr. Sergio Revah (Universidad Autónoma Metropolitana, Mexico). Activated sludge was obtained from Valladolid domestic wastewater treatment plant (WWTP) (Spain). A synthetic mineral salt medium (MSM) was used in the characterization of CO2 mass transfer in the absorption columns and during the initial stages of the HRAP operation. The MSM was composed of (g/l): 6.80 NaHCO3; 2.01 Na2CO3; 0.25 K2HPO4; 1.25 NaNO3; 0.50 K2SO4; 0.50 NaCl; 0.10 MgSO4·7H2O; 0.02 CaCl22H2O; 0.005 FeSO4·7H2O; 0.04 EDTA and 5 mL of a micronutrient solution prepared according to the Spirulina mineral salt medium recommended by the SAG Culture Collection.11 The pH of the MSM was 10. Centrates from the centrifugation of the effluent from the sludge digesters of Valladolid WWTP were also used as a free nutrient source instead of MSM. The average concentrations of NH4+, PO43‑, total organic carbon (TOC), inorganic carbon (IC), NO3− and SO42‑ in the centrates were 515 ± 200 mgN/L, 55 ± 9 mgP/L, 111 ± 108 mgC/L, 817 ± 134 mgC/L, 0 mg/L and 0 mg/L, respectively. A synthetic biogas composed of CO2 (30%vol), H2S (500, 1000, and 5000 ppmv) and balanced with N2 (instead of CH4 due to potential explosion hazards) was used throughout this experimentation (Abello Linde, Spain). Optimization of pH and Liquid Recirculation in CO2 Mass Transfer in Absorption Columns. The performance of a packed bed column and a bubble column (50 cm height × 4.5 cm internal diameter) to remove CO2 from a synthetic gas mixture composed of CO2 (30%vol) and N2 (70%vol) was comparatively assessed. The packed bed absorption column was filled with 4 mm glass Raschig rings and operated with 50 mL/ min of synthetic biogas flowing upward and MSM at different pH values (7, 8, 9, 10) trickling from the top of the column at velocities of 0.027, 0.053, 0.081, and 0.106 cm/s (corresponding to liquid flow rates of 20, 40, 60 80 mL/min). The bubble absorption column consisted of a column supplied with 50 mL/ min of biogas through a ceramic sparger located at the bottom of the system and with MSM at the same pH values and liquid

Figure 1. Schematic diagram of the continuous biogas upgrading experimental plant.

6-blade paddle wheel at a cultivation broth internal circulation velocity of ≈20 cm/s, and illuminated at 80 μE/m2s for 24 h/d using a bench of 15 Gro-Lux fluorescent lamps (Sylvania, Germany). The HRAP was initially filled with 180 L of MSM, inoculated with 1 L of S. platensis and operated in batch mode for the first 70 days (start-up period). Five liters of an alkaliphilic H2S-oxidizing bacterial consortium (0.8 g/L), previously grown on a mineral salt medium reported by Sorokin et al.12 using Na2S2O3 as energy source, were inoculated into the HRAP at day 138. The HRAP was fed from day 70 to 251 with MSM as a nutrient source at a hydraulic retention time (HRT) of 23 days, and with a 7 times diluted centrate from day 251 onward, concomitantly with the inoculation of a nitrifying activated sludge in order to avoid microalgal inhibition due to NH3 accumulation. Synthetic biogas, flowing cocurrently with a recycling microalgal broth stream (20 mL/min) drawn from the HRAP, was supplied to the bubble column from day 57 onward at 20 mL/min through a ceramic sparger located at the bottom of the column (Figure 1). The synthetic biogas contained CO2 (30%), N2 (69.5− 70%) and increasing amounts of H2S depending on the experimental period: 0 ppmv (days 57−137), 500 ppmv (days 138−154), 1000 ppmv (days 155−196) and 5000 ppmv (days 197−518). The height of the bubble column was increased from 0.6 to 1.9 m at day 341, while the pH was automatically controlled at 8.5 from day 433 onward in order to evaluate, respectively, the influence of the bubble column height and cultivation broth pH on H2S-CO2 removal. Table 1 summarizes the main operational conditions tested. CO2, O2 and H2S concentrations at the inlet and outlet of the bubble column were periodically monitored by GC-TCD. Liquid samples were also drawn twice a week from the MSM/ centrate influent and HRAP to monitor the concentrations of TOC, IC, SO42‑, NO3−, NO2−, NH4+, and total suspended solids (TSS). The temperature, pH and dissolved O 2 concentration (DOC) in the HRAP were recorded online. Prior to analysis (except for TSS determination), liquid samples 574

dx.doi.org/10.1021/es403596m | Environ. Sci. Technol. 2014, 48, 573−581


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Table 1. Operational Conditions Evaluated during This Experimentation

L/m2d (http://algae.massey.ac.nz). A microalgae chemical energy content of 21 kJ/g was here used for the estimation of the solar energy assimilated.15,16 The energy required for biogas bubbling and external cultivation broth recirculation in the absorption column was estimated according to Estrada et al.17 assuming a compressor efficiency of 0.7 and a column liquid height of 1.8 m, while an empirical value of 2.5 W/m3 was used for the calculation of the energy used for the internal recirculation of the microalgal cultivation broth at 20 cm/s in the 30 cm deep HRAP.18 The energy consumption of the centrifuge employed for biomass harvesting (0.35 kwh/kg microalgae) was estimated based on a recent economic evaluation published for a real 4 t/y microalgae production plant.19 Analytical Procedures. The gas concentrations of CO2, H2S, CH4, N2 and O2 were analyzed using a Varian CP-3800 gas chromatograph (Palo Alto, CA) coupled with a thermal conductivity detector according to Alzate et al.13 TOC and IC concentrations were determined using a Shimadzu TOC-VCSH analyzer (Japan). N-NH4+ concentration was determined using an Orion Dual Star ammonia electrode (Thermo Scientific, The Netherlands). N-NO3−, N-NO2− and P-PO43‑ concentrations were analyzed via HPLC-IC (Waters 432, conductivity detector). All these analyses, including TSS, were carried out according to Standard Methods.20 The dissolved oxygen concentration (DOC) and temperature in the HRAP were determined online using an O2 transmitter 4100 e (Meter Toledo GmbH, Urdolf, Germany). The pH was automatically controlled using a R305 Consort system (Belgium). Microalgae identification and quantification was carried out by microscopic examination (Olympus IX70) of culture broth samples fixed with lugol acid at 0.5% and stored at 4 °C prior to analysis. Quantification was performed according to Phytoplankton Manual (Sournia, 1978).21

were centrifuged for 10 min at 5000 rpm (Kubota 5000, Kubota, Japan) and filtered through 0.20 μm nylon filters. Biomethane Potential of the Algal-Bacterial Biomass Produced. The mesophilic biomethane potential (BMP) of the residual algal-bacterial biomass generated in the biogas upgrading process was assessed according to Alzate et al.13 The BMP tests were performed in 160 mL glass serum bottles filled with 90 mL of a mixture of algal-bacterial biomass (20 gVS/L) and anaerobic inoculum (10 gVS/L) at different substrate to inoculum ratios (1, 0.5, and 0.25 gVS/gVS). The anaerobic inoculum, previously acclimated to activated sludge, was supplemented with 5 g NaHCO3/L to provide enough buffer capacity for anaerobic digestion. The bottles were closed with butyl septa, sealed with aluminum caps, purged with helium for 15 min and incubated at 35 °C in a rotary shaker at 120 rpm. Control tests containing 90 mL of anaerobic inoculum were carried out in order to determine the CH4 production potential of the inoculum. The production of methane from the inoculum was subtracted from the total methane production to obtain the net methane production from the algal-bacterial biomass generated in the biogas upgrading process. Digestion was monitored by periodic measurements of the pressure of the headspace and biogas composition by GC-TCD. All tests were carried out in duplicate. Mass and Energy Balance of an Integrated HRAPBubble Column System. A mass and energy balance was carried out for the upgrading of an arbitrary biogas flow rate of 38 m3/d containing CH4:CO2:H2S at 64.5:35:0.5 in a largescale integrated HRAP-absorption unit system. Based on the empirical data obtained in this study (CO2 and H2S removals of 90% and 100%, respectively, and a complete depletion of the IC present in the centrate) and assuming a conservative microalgae (C106H181O45N16P) productivity of 15 g/m2d, the biogas upgrading system was composed of a 1000 m2 HRAP (0.3 m deep) interconnected to a 3 m3 bubble column with a microalgae cultivation broth recirculation velocity of 82 m3/d.14 The process was fed with anaerobic effluents from an urban solid waste digester (3800 mgN-NH4+/L, 600 mgP-PO43‑/L, 5000 mgIC/L) and subjected to water evaporation losses of 6

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RESULTS AND DISCUSSION Optimization of pH and Liquid Recirculation in CO2 Mass Transfer in Absorption Columns. The bubble and packed bed columns exhibited a comparable CO2 mass transfer 575



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Figure 2. Influence of the liquid flow rate on CO2 removal efficiency in the bubble (a) and packed bed (b) columns, and O2 concentration in the treated biogas in the bubble (c) and packed bed (d) columns during the absorption experiments conducted at pH 7 (■), 8 (⧫), 9 (▲), and 10 (●). The linear MSM velocities ranged from 0.027 to 0.106 cm/s.

Simultaneous CO2 and H2S Removal in an Integrated HRAP-Bubble Column System. During the start-up period (in the absence of synthetic biogas supply), the pH and T remained approximately constant at 10.1 ± 0.1 and 22 °C, respectively. S. platensis exhibited a specific growth rate of 0.18 d−1 and reached a steady state biomass concentration of 1.04 ± 0.07 g/L. This low specific growth rate recorded was likely due to the low impinging irradiance in our indoors HRAP (80 μE/ m2s), which also limited the CO2 assimilation capacity of the system. Due to the absence of biogas supply during this initial period, cyanobacterial growth caused a gradual decrease in the inorganic carbon concentration from 1200 mg/L to 600 mg/L (Figure 3). From day 57 to 137, the integrated HRAPabsorption column system was provided with synthetic biogas without H2S, which resulted in steady CO2 removal efficiencies of 95 ± 3% and a gradual increase in the IC concentration from 600 mg/L to steady state values of 1291 ± 36 mgC/L (Figure 3). These removal efficiencies were comparable to those recently reported by Gonzalez-Lopez et al.22 (≈ 80%) in a similar experimental two-stage system treating a synthetic flue gas (9% CO2) using a highly carbonated absorption medium. The stripping in the external bubble column of the O2 photosynthetically produced in the HRAP (whose DOC remained constant at 9.05 ± 0.18 mg O2/L) resulted in O2 concentrations in the treated biogas of 0.96 ± 0.16%, which are higher than the maximum O2 concentration of 0.5% required for injection of the upgraded biogas in natural gas networks of European countries such as Germany, The Netherlands or Austria. The addition of CO2 in this operational period did not result in a significant increase in the biomass concentration (1.06 ± 0.07 g/L) but caused a decrease in pH from 10.1 ± 0.1 to steady state values of 9.5 ± 0.1. At day 137, an alkaliphilic H2S oxidizing bacterial consortium23 was inoculated into the HRAP along with the supplementation of the synthetic biogas with H2S at 500 ppmv, which resulted in a complete removal of H2S, a slight decrease in the CO2 removal efficiency to 87 ± 5% and O2 concentrations in the treated biogas of 0.7 ± 0.3%. In this short operational stage at 500 ppmv of H2S, the

potential (Figure 2a, b). At a loading rate of 2.2 gCO2/Lh, the CO2 mass transfer efficiency increased at increasing liquid flow rates as a result of the significant enhancement of the individual liquid-side film kla values (e.g., 3.3, 6.0, 7.6, 9.0, 9.9 h−1 in the bubble absorption column at liquid flow rates of 20, 40, 60, 70, and 80 mL/min, respectively). The pH exerted the largest influence on CO2 absorption due to the strong stepwise increase in the total solubility of CO2 at increasing pH values (0.219; 0.018; 4.242 × 10−4 and 4.913 × 10−6 modified dimensionless Henrýs constants for pH values of 7, 8, 9, and 10, respectively) despite the gradual shift in the mass transfer resistance control from the liquid side to the gas side. In this context, it is worth noting that despite the increase in pH resulted in a decrease in the global volumetric liquid mass transfer coefficient (KLa) in both columns (see S1 in Supporting Information (SI) for a detailed calculation), the pH-mediated enhancement in CO2 solubility finally resulted in the observed increase in CO2 mass transfer from the biogas. Thus, while a pH of 7 resulted in CO2 removals lower than 20% regardless of the type of absorption unit and liquid flow rate, almost a complete CO2 removal was achieved at pH 10 in both systems due to the enhanced gas−liquid concentration gradients for this acid gas at high pH (S1, SI). Besides, since H2S is also an acid gas, its absorption from biogas into aqueous solutions would be also enhanced at increasing pHs. Conversely, the O2 concentration in the outlet gas stream, which must be carefully monitored to ensure an optimum biomethane quality, increased linearly with the liquid flow rate but regardless of the pH and column configuration (Figure 2c, d), with O2 concentrations ranging from 0.3% at 0.027 cm/s to 1.2% at 0.106 cm/s. Therefore, process operation at a pH of 9− 10 and a biogas/liquid flow rate ratio of 1 were chosen to ensure a satisfactory CO2 removal and a complete H2S removal (H2S has three times the aqueous solubility of CO2). Based on their similar transfer performance, the bubble column was selected for further experimentation due to its ease of operation (no clogging problems expected). 576



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Figure 3. Time course of (a) the removal efficiencies of CO2 (◊) and H2S (Δ), and O2 (□) concentrations in the upgraded biogas, (b) concentration of biomass (▲) and inorganic carbon (■) in the HRAP, and (c) concentration of sulfate ( × ), nitrate (•), and ammonium (+) in the HRAP during the continuous biogas upgrading carried out in the combined system HRAP + absorption unit.

temperature of the cultivation broth increased to 26 ± 1 °C, which resulted in a slight increase in biomass, IC and SO4− concentrations to 1.2 ± 0.1 g/L, 1553 ± 23 mgC/L and 1094 ± 32 mg SO4−/L likely due to the higher water evaporation losses (which unfortunately were not accurately recorded). The DOC in the HRAP remained constant at 9.9 ± 0.4 mg O2/L, which suggest that O2 stripping in the absorption column was limited

since otherwise the O2 concentration in the treated biogas would have achieved values of 23% (corresponding to the O2 gas phase concentration in equilibrium with an aqueous cultivation broth at ≈10 mg O2/L). A further step increase in the biogas H2S concentration to 1000 ppmv from day 155 to 196 brought about similar CO2 and H2S removals (89 ± 4% and 100%, respectively), but a significant decrease in the O2 577



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concentration of the treated biogas down to 0.5 ± 0.3% (Figure 3a). This decrease in the upgraded biogas O2 concentration, which was further confirmed in the next operational stage at 5000 ppmv of H2S, was likely due to the increased oxygen demand in the absorption column at increasing H 2 S concentrations mediated by the rapid biological oxidation of H2S to sulfate (requiring 2 mol of O2 per mol of H2S). While biomass concentration and pH remained similar to the steady values recorded in the previous stage (1.2 ± 0.1 g/L and ≈9.5, respectively), the IC concentration gradually increased up to 2000 mgC/L concomitantly with an increase in SO42‑ and NO3− concentrations up to 1160 mg/L and 2656 mg/L, respectively (Figure 3b, c). Surprisingly, the accumulation of NO3− occurred at higher rates than that of SO42‑, which a priori was the only extracellular metabolite produced from the complete oxidation of H2S under the fully oxidizing conditions present in the HRAP. When the experimental setup was challenged with biogas containing 5000 ppmv of H2S during operation with MSM, CO2 and H2S removals remained stable at 86 ± 5% and 100%, respectively, while the higher oxygen demand caused by the higher H2S loading rate decreased the oxygen content in the upgraded biogas below 0.2%, which clearly shows that this innovative biogas upgrading technology can satisfactorily cope with H2S concentrations of at least 0.5% (Figure 3a). During this operational stage, the pH and the concentration of biomass and IC also remained constant at 9.4, 1.2 ± 0.1 g/L and 2020 ± 80 mg/L, respectively. Nitrate and sulfate concentrations increased to steady state values of 3090 ± 200 mg/L and 1516 ± 144 mg/L as a result of the increasing water evaporation losses, and in the particular case of sulfate to its accumulation from H2S oxidation. Process operation with diluted centrate started on day 252 and was preceded by the inoculation of the HRAP with a fresh nitrifying activated sludge in order to prevent any potential inhibition of S. platensis (a cyanobacterium highly sensitive to NH3) by the high NH4+ concentrations typically present in centrates.24 The use of diluted centrate as a free nutrient source instead of MSM to reduce process operating costs caused a gradual decrease in the CO2 removal efficiency from 90% to approximately 40 ± 6% (even when the column height was increased to 1.9 m by day 341), in the biomass concentration from 1.2 g/L to 0.6 ± 0.2 g/L and in the pH of the microalgal cultivation broth from 9.4 to 7 (Figure 3). This decrease in CO2 removal, together with the complete depletion of IC in the HRAP by day 410, suggested that biogas upgrading was limited by the mass transfer of CO2 to microalgae cultivation broth in the external bubble column, which itself was likely hindered by the severe decrease in the ionic strength of the cultivation broth as a result of the replacement of the MSM by the diluted centrate. This hypothesis was confirmed by the visual observation of the larger size of the biogas bubbles flowing upward in the bubble column, since a reduced salinity in the medium is known to promote bubble coalescence. On the other hand, the fact that total nitrogen and phosphate concentrations in the HRAP remained always above 100 mgN/L and 3 mgP/L, respectively, ruled out any deterioration of the biogas upgrading performance due to nutrient limitation. Likewise, the sustained 100% removals achieved for H2S throughout the entire experimentation, together with the efficient microalgae inorganic carbon uptake, also ruled out the possibility of a microbial-mediated deterioration of the biogas upgrading performance. In addition, the low pH values prevailing in the cultivation broth during this experimental period (compared to

process operation with MSM) likely promoted a partial CO2 stripping from the HRAP since significantly higher biomass concentrations (∼0.9 g/L) would be supported by the amount of CO2 absorbed. Process operation with centrates did not result in a decrease of the H2S removal efficiency (100%), except on day 430 where the pH unexpectedly decreased to 5.4 (due to the absence of buffer capacity of the cultivation broth) and H 2 S removal temporarily dropped to 80%. The implementation of a pH control strategy (pH to 8.5 by addition of NaOH) on day 433 restored the complete removal of H2S, which supported steady sulfate concentrations of 183 ± 68 mg/L in the HRAP by the end of the experimentation. The nitrifying community inoculated efficiently oxidized NH4+ to nitrate since NH4+ remained always below 1.5 mgN/L. The DOC remained above 8 ± 1 mg O2/L, while steady NO3− concentrations of 474 ± 30 mg/L were detected at the end of the experimental period (Figure 3c). The high pH and the highly carbonated MSM used during the first 251 days of operation maintained the population of S. platensis free from contamination from other microalgae or cyanobacteria species, but the use of diluted centrate resulted in the gradual disappearance of S. platensis and the establishment of a stable microalgae population composed of Phormidium (71%), Oocystis (20%), and Microspora (9%) (morphological characterization) by the end of the cultivation phase. Spirulina platensis is a cyanobacterium species from the genus Arthrospira (strictly speaking, Spirulina is a cyanobacterium composed of two species of Arthrospira, maxima and platensis, but the older term Spirulina remains in use for historical reasons) that preferentially grows in highly carbonated media at high pH and is highly sensitive to NH3 inhibition. Therefore, the decrease in inorganic carbon concentration from 1000 to 2000 mg C/L to concentrations lower than 50 mg C/L (Figure 3b) concomitant with the decrease in the cultivation broth pH from 9.4 to 7, as a result of HRAP feeding with diluted centrate, likely promoted the observed loss of Spirulina from the system and the establishment of a mixed microalgal/cyanobacterial population able to grow more efficiently under these new environmental conditions. This tailored alkaliphilic algal-bacterial process implemented in this versatile two-stage configuration supported a satisfactory CO2 and H2S removal and was capable of overcoming the typical operational problems of conventional biological desulfurization technologies such as elemental sulfur accumulation in the packed bed of aerobic or anoxic biotrickling filters25,26 or at the top of microaerophilic anaerobic digesters.27 However, this platform technology must be further optimized to reduce both the empty bed residence time in the column from 30 min down to the typical values of 3−6 min of aerobic or anoxic biotrickling filters, and the large footprint of the photobioreactor here employed for CO2 assimilation. In this context, the lower aqueous solubility of CO2 compared to H2S, and the low light utilization efficiencies of HRAPs, entailed both high biogas residence times in the column and the need for extensive HRAPs. However, these technical limitations are counterbalanced by the generation of an O2-free biomethane of sufficient quality to be injected in natural gas networks, the production of a residual biomass that can be used as a feedstock for renewable energy production, and the mitigation of the eutrophication potential of anaerobic effluents. In our particular experimental setup, the low irradiations provided by the fluorescent lamps used (80 μE/m2s) limited the growth rates of microalgae, and therefore, the efficiency of biogas upgrading 578



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These yields were in agreement with those reported by Mussgnug et al.31 for the anaerobic digestion of 6 different microalgae (0.218−0.387 L CH4/gVS). Likewise, the highest initial CH4 production rate and the shortest lag phase were recorded at the lowest substrate to inoculum ratio tested, as expected from the presence of a larger concentration of anaerobic microorganisms. The results here obtained were in agreement with those recently reported by Alzate et al.13 in the evaluation of the BMP of 3 different microalgae consortia at three different substrate to inoculum ratios, who attributed the lowest CH4 yields and the occurrence of longer lag phases recorded at the highest ratio to a process imbalance mediated by the accumulation of volatile fatty acids. In addition, the composition of CH4 in the biogas at the end of the BMP tests decreased when increasing the substrate to inoculum ratio, with values of 66%, 70%, and 74% at 1, 0.5, and 0.25, respectively. Mass and Energy Balance of an Integrated HRAPBubble Column System. The upgrading of an arbitrary biogas flow rate of 38 m3/d to a final biogas composition of CH4:CO2:O2:H2S of 95:5:0:0 (which would allow its direct injection into natural gas networks in most EU countries) would require 364 L/d of anaerobic effluents from urban solid waste digesters and the supply of 6 m3/d of makeup water to compensate for water evaporation losses (S2, SI). The calculation of the anaerobic effluent requirements was based on both its composition and the biomass (C106H181O45N16P) formed (15 kg/d) from the assimilation of 90% of the CO2 present in the biogas and the complete assimilation of the inorganic carbon present in the effluent. The transfer of 90% of the CO2 present in the biogas (and also of 100% of the H2S) occurred in a 3 m3 bubble column (1.9 m height) assuming a similar mass transfer efficiency than that achieved in the pilot experimental setup. At this point it must be noticed that CO2 removal efficiencies of up to 99% (under similar biogas loading rates) have been achieved in our experimental setup by increasing the microalgae cultivation broth recycling by a factor of 3 using diluted centrates (data not shown). In order to maintain a microalgae concentration of 1.5 g/L, 10 m3/d of water from the microalgae centrifugation stage must be recycled back to the HRAP in order to dilute the 15 kg of microalgal biomass daily generated. The absence of water recirculation would entail microalgae concentrations of 15 (kg/d)/0.364 (m3/d) = 41 kg/m3, which would hinder light penetration and ultimately inhibit CO2 assimilation. This water recycling, together with the production of sulfate as a result of H2S oxidation and the incomplete assimilation of all PO43‑ present in the anaerobic effluent, would cause a build-up in phosphate and sulfate concentrations up to 75 mgP-PO43‑/L and 1950 mg SO42‑/L. Assuming a microalgae productivity of 15 g/m2d, the surface of the HRAP needed to produce 15 kg/d of biomass would account for 1000 m2 and the net production of oxygen released to the atmosphere for 21.5 kg O2/d. This scaled-up system supports the production of 5500 kg microalgae per year, which would represent a net solar energy fixation into chemical energy of 31878 kWh/y, based on a typical photosynthetic efficiency of 2.1% (S2, SI). Mixing of the microalgae cultivation broth by recirculation in the HRAP represents the most energy demanding process (≈6600 kWh/ y), followed by centrifugation (1750 kWh/y). Biogas pumping and liquid media recycling in the absorption bubble column accounts for less than 3% of the total energy requirements. From an environmental impact viewpoint, the indirect CO2 released from the production of the electricity required in the

in terms of CO2 and H2S removal. The H2S loading rate was ultimately limited by light supply during process operation with MSM but process performance is expected to be boosted when using solar irradiation (1000−2000 μE/m2s). On the other hand, the results obtained during process operation with diluted centrates were not influenced by the low impinging irradiation since under these particular operating conditions the process was limited by CO2 mass transport in the absorption column rather than light supply for CO2 assimilation in the HRAP. However, there is a clear need for further process optimization in terms of photosynthetic CO2 assimilation (involving a detailed characterization of microalgae growth -Fv/ Fm, Iav, biomass extinction coefficients, etc.) in order to increase the biogas loading rates. The potential of algal-bacterial photobioreactors for biogas upgrading was preliminarily evaluated in the past, but the performance data available is often incomplete and the configurations tested presented severe technological limitations.28,29 Thus, Conde et al.28 achieved an incomplete biogas upgrading (76−94% CO2 removal and 60−67% H2S removal) in a HRAP containing a short biogas absorption column inside the pond (namely BIOLIFT). Mandeno et al.29 achieved a 87% CO2 removal efficiency in a HRAP constructed with a countercurrent pit for biogas absorption, but unfortunately the performance of the system for H2S abatement was not evaluated. This pit-based upgrading system, apart from the significantly higher energy requirements for the recirculation of the microalgae broth through the baffled pit,18 entailed a technological challenge to efficiently collect into the gas collection hood all biogas bubbled. Finally, despite CO2 and H2S removal efficiencies of 97−100% were recorded in 1 L enclosed tubular photobioreactors supplied with biogas containing H2S at 440 ppmv, the upgraded biogas in this system contained oxygen concentrations ranging from 18 to 23%,30 which entails a potential explosion hazard and the impossibility to inject this upgraded biogas in natural gas networks. Biomethane Potential of the Algal-Bacterial Biomass Produced. The anaerobic degradation of the algal-bacterial biomass produced from biogas upgrading resulted in final CH4 yields of 0.27 L CH4/gVS at a substrate to inoculum ratio of 0.25, and 0.21 L CH4/gVS at a ratio of 0.5 and 1 (Figure 4).

Figure 4. Time course of methane productivity for the algal-bacterial biomass produced in the HRAP at a substrate to inoculum ratio of 0.25 (Δ), 0.5 (□), and 1 (◊). Vertical bars represent the standard deviation. 579



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process would account for 3771 kgCO2/y (based on the 0.44 kg CO2/kWh standard CO2 emission factor in Spain), which positively compares with the 7560 kg CO2/y removed from the biogas during the upgrading process. Furthermore, the net electricity gain derived from the anaerobic digestion of the 5500 kg microalgae/y (assuming a VS/TS factor of 0.85, a CH4 productivity yield of 0.27 gCH4/gVS and a biogas to electricity conversion yield of 0.4) would account for approximately 5000 kWh/y. In brief, the technology here evaluated was capable of providing a satisfactory integral biogas upgrading while partially mitigating the eutrophication impact of anaerobic effluents. While process operation at a pH of 9.5 supported an efficient CO2 and H2S transfer in the external absorption column, the rapid H2S oxidation kinetics of the alkaliphilic bacterial community resulted in an O2-free biogas. The use of an external bubble column (as efficient as packed bed columns but with less operational problems) allowed for the complete capture of the upgraded biogas and prevented its contamination with air. The need for a further process optimization in terms of photosynthetic CO2 assimilation was identified in order to increase the biogas loading rate to the absorption column. Process operation with diluted centrates resulted in a process limited by CO2 mass transport and the incomplete depletion of the nutrients present in this residual wastewater, which also constitutes a niche for further research. The mass and energy balance conducted showed the negative CO2 footprint of the process but its significant water footprint, while the anaerobic degradation of the algal-bacterial biomass produced could eventually cover 60% of the total energy demand of the upgrading process.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

S1. Influence of the cultivation pH on the global KLa for CO2 and gas−liquid CO2 concentration gradient in the packed bed and bubble absorption columns at the different external recirculation rates tested. S2. Mass (a) and energy (b) balance calculation for the upgrading of 38 m3/d of biogas in a 1000 m2 HRAP (30 cm deep) interconnected with a 3 m3 bubble absorption column assuming a photosynthetic efficiency of 2.1%. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0034983186424; fax: 0034983423013; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Spanish Ministry of Economy and Competitiveness-CDTI (Projects CTQ201234949, CONSOLIDER-CSD 2007-00055 and SOST-CO2) and Biogas Fuel Cell S.A. Christoph Taetz is gratefully acknowledged for his technical assistance in the BMP tests, Saul Blanco for the identification of the final microalgae population established and Araceli Crespo for her practical assistance. The financial support of the Engineering Institute of Universidad Nacional Autónoma de México, (Project 3319) is also gratefully acknowledged. 580



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Microalgal-Biotechnology As a Platform for an Integral Biogas

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