Closing the Nutrient Cycle in Two-Stage Anaerobic Digestion of

Mar 12, 2015 - University of Natural Resources and Life Science, Gregor-Mendel-Straße 33, 1180 Vienna, Austria. ABSTRACT: Industrial waste streams fr...
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Closing the Nutrient Cycle in Two-Stage Anaerobic Digestion of Industrial Waste Streams Lydia Rachbauer,*,† Wolfgang Gabauer,‡ Stefanie Scheidl,‡ Markus Ortner,† Werner Fuchs,‡ and Günther Bochmann‡ †

Bioenergy2020+ GmbH, Konrad-Lorenz Straße 20, 3430 Tulln, Austria University of Natural Resources and Life Science, Gregor-Mendel-Straße 33, 1180 Vienna, Austria

‡

ABSTRACT: Industrial waste streams from brewing industries and distilleries provide a valuable but largely unused alternative substrate for biogas production by anaerobic digestion. High sulfur loads in the feed caused by acidic pretreatment to enhance bioavailability are responsible for H2S formation during anaerobic digestion. Microbiological oxidation of H2S provides an elegant technique to remove this toxic gas compound. Moreover, it allows for recovery of sulfuric acid, the final product of aerobic sulfide oxidation, as demonstrated in this study. Two-stage anaerobic digestion of brewer’s spent grains, the major byproduct in the brewing industry, allows for the release of up to 78% of total H2S formed in the first pre-acidification stage. Desulfurization of such pre-acidification gas in continuous acidic biofiltration with immobilized sulfur-oxidizing bacteria resulted in a maximum H2S elimination capacity of 473 g m−3 h−1 at an empty bed retention time of 91 s. Complete H2S removal was achieved at inlet concentrations of up to 6363 ppm. The process was shown to be very robust, and even after an interruption of H2S feeding for 10 days, excellent removal efficiency was immediately restored. A maximum sulfate production rate of 0.14 g L−1 h−1 was achieved, and a peak concentration of 4.18 g/L sulfuric acid was reached. Further experiments addressed the reduction of fresh water and chemicals to minimize process expenses. It was proven that up to 50% of mineral medium that is required in large amounts during microbiological desulfurization can be replaced by the liquid fraction of the digestate. The conducted study demonstrates the viability of microbial sulfur recovery with theoretical recovery rates of up to 44%.

1. INTRODUCTION Over the past few years, substrate shortage has become a major barrier for further production of renewable energies, such as biogas. Among others, industrial waste streams and byproducts provide a valuable but largely unused alternative substrate for anaerobic digestion.1 Thus far, the digestion of some industrial wastes shows substrate-specific challenges. Especially the brewing industry, distilleries and abattoirs could profit enormously from using their waste or side products for energy production onsite to cover their in-house demand for process heat and energy.2 Nevertheless, waste as a substrate for anaerobic digestion brings in certain problems, such as nitrogen inhibition, the requirement for substrate pretreatment as a result of low bioavailability, or the need for hygienization. For brewer’s spent grains (BSG), the major side product in the brewing industry, different pretreatment technologies, such as thermochemical, enzymatic, and mechanical treatment, have already been evaluated to break down the lignocellulosic compound layer and increase its digestibility.2,3 High sulfur loads in the feed caused by an acidic pretreatment, pH adjustment with sulfuric acid, or protein-rich substrates, such as slaughterhouse waste, result in biogas containing elevated levels of hydrogen sulfide (H2S), reaching up to 10 000 ppm.1 With decreasing pH, the solubility of H2S decreases. Thus, H2S is driven out of the liquid and remains in its gaseous form. This study shows that two-stage anaerobic digestion allows for the release, depending upon the pH, of up to 78% of total H2S formation during the first pre-acidification step compared to a single-stage system. To prevent corrosion of engines and to minimize maintenance and odor, H2S removal is essential. © XXXX American Chemical Society

Biofiltration for the removal of H2S with various biological carrier materials, such as packed compost,4,5 peat,6,7 and most popular, activated carbon,4,8 was previously studied. More recent work also evaluated microbiological H2S removal using sulfur-oxidizing bacteria (SOB) in acidic biofiltration,9−11 but the recovery of sulfur as sulfuric acid, the final product of aerobic oxidation reaction of sulfide, was not focused on before this study. Digestate, as a residue in anaerobic digestion, contains nutrients, such as nitrogen, phosphorus, and trace elements, in a sufficient amount to support the growth of the SOB used in the desulfurization process. Digestate from agricultural biogas plants has a well-established use as a fertilizer, but digestate from biogas plants using slaughterhouse waste as a substrate can cause problems.12 Because of its high nitrogen levels, regional limits for total nitrogen application are easily exceeded, and therefore, application of such digestate on agricultural land is limited.13 For the microbiological desulfurization process, synthetic medium is required in large amounts. The demand for fresh water and synthetic medium can be minimized by substitution with the liquid fraction of digestate after separation, resulting in sustainable nutrient recycling. This study evaluated the removal of H2S in acidic biofiltration under optimized conditions for sulfur recovery Special Issue: 2nd International Scientific Conference Biogas Science Received: December 16, 2014 Revised: February 13, 2015

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Energy & Fuels with a strong focus on an efficient desulfurization process and nutrient recirculation. The objective was to efficiently remove and convert H2S, which is released during pre-acidification (∼4000 ppm) of two-stage anaerobic digestion to sulfuric acid. Another focus was to minimize the demand for synthetic medium and fresh water during the desulfurization process using digestate instead. The results indicate the possibility to replace up to 50% of fresh water with the liquid fraction of digestate. In addition to the sulfur recycling, this unique combination of processes means that additional nutrients contained in the digestate (nitrogen, phosphorus, and trace elements) can be brought back into the overall methane formation process.

2. EXPERIMENTAL SECTION 2.1. Anaerobic Digestion of Pretreated BSG. The substrate for continuous fermentation was obtained from a local brewery located in the surroundings of Vienna, Austria. BSG were taken from an intermediate storage tank and stored at −20 °C for long-time storage until substrate pretreatment. Pretreatment was performed in a microwave digestion unit (Ultra CLAVE, MLS GmbH, Germany) at a temperature of 140 °C for 15 min with acid addition of H2SO4, resulting in a final acid concentration of 0.5%. Pretreated substrate was stored at 4 °C until feeding. Thermochemically pretreated BSG was fed daily, except for weekends. Continuous lab experiments were conducted for a total test period of 120 days. The setup consisted of two stirred tank reactors with 6 L of working volume with a mechanical-sealed central blade stirrer and a heat jacket for automatic temperature control until day 22. The system was then switched to 2 L flask fermenters with a working volume of 1.8 L. For that, the fermenter content was homogenized by stirring for 30 min before 1.8 L aliquots were transferred to the new flask setup. Excess sludge was discarded. The flask fermenters were incubated at 37 °C on a magnetic stirrer in a water bath for temperature control. For two-stage anaerobic digestion, the pre-acidification stage was performed as a separate fermentation at 300 mL scale. The hydraulic retention time was kept at 15 days at an average organic loading rate (OLR) of 8 g of volatile solids (VS) L−1 day−1. Biogas quantity was continuously measured with high-precision gas counters (MGC-1 V3, Ritter Apparatebau GmbH, Germany). The gas composition (CH4, CO2, H2S, and H2) was analyzed using a gas analyzer (AWITE Bioenergie GmbH, Germany) twice a month. For process monitoring, the amount of volatile fatty acids (VFA) was determined twice a week. Chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) were determined twice a month. 2.2. Experimental Setup and Operation of the Desulfurization Column. A continuous desulfurization column was constructed for lab-scale experiments (Figure 1). The column materials were chosen to withstand the strongly acidic culture broth expected. Transparent polyvinyl chloride pipes were connected with polyethylene sleeve sockets to function as a desulfurization column. A urethane-based thermoplastic polymer was chosen for gas and liquid tubing with both Teflon and high-alloy stainless-steel valve connections (FESTO AG, Germany). The desulfurization column was packed with a polyethylene carrier and had an inner diameter of 0.027 m and a packed height of 0.35 m, resulting in an empty bed volume of 790 cm3. The surface of polyethylene carrier material was mechanically treated prior to the immobilization procedure. For this treatment, 271 cm3 of the carrier, which is equivalent to an empty bed volume of 790 cm3, was mixed with 350 mL of quartz sand and rotated at 2200 min−1 for 15 min. For safety reasons, a gas detector was installed in the lab and outlet gas was purged through a 10% solution of sodium hydroxide to remove residual H2S. A heating coil was installed on the desulfurization column to keep a constant temperature of 30 °C. 2.2.1. Inoculation. The desulfurization column was inoculated as a biofiltration unit using a sample withdrawn from an industrial desulfurization plant located in Upper Austria, Austria, that was

Figure 1. Schematic overview of experimental setup for microbiological desulfurization.

centrifuged, and 20 g of the pellet was resuspended in 2 L mineral medium containing the following: 3.00 g/L KH2PO4, 0.50 g/L MgSO4·7H2O, 1.20 g/L (NH4)Cl, and 0.25 g/L CaCl2·2H2O. This culture broth was mixed with the pretreated and washed polyethylene carrier material, transferred into the desulfurization column, and recirculated at 30 °C at minimum H2S concentration of approximately 200 ppm in addition to air supply for 4 days. 2.2.2. Operation Conditions. During the experimental phase, 12 L of mineral medium as used during inoculation was supplied. Medium was replaced when the pH dropped below 1.2. Before replacement of medium, the system was flushed with 2 L of distilled water to rinse residual sulfuric acid and avoid an immediate pH drop within the fresh medium. An online pH measurement was installed to monitor the operation conditions throughout the operational phase. Flux of air and synthetic gas with the composition of 4% H2S in N2 were adjusted according to the desired inlet concentration of H2S with two separate flow meters, with a minimum air/H2S ratio of 4:1. The inlet gas was humidified by purging through a temperature-controlled water bottle before entering the column. 2.3. Application of the Digestate to Substitute Fresh Water and Mineral Medium. Digestate was centrifuged at 4000 rpm for 10 min to separate solid particles from the liquid fraction. The liquid fraction was applied untreated and pretreated at various concentrations to substitute thiosulfate mineral medium. Thiosulfate mineral medium contained the following: 3.00 g/L KH2PO4, 0.50 g/L MgSO4·7H2O, 1.20 g/L (NH4)Cl, 0.25 g/L CaCl2·2H2O, and 5.00 g/L Na2S2O3· 5H2O. The pretreatment consisted of thermal nitrogen stripping at 100 °C for 15 min. The untreated fraction was used to replace the medium at a concentration of 5, 12.5, and 25%, and the stripped digestate was used at a concentration of 10, 25, and 50%. The amounts of stripped digestate that were added were double the amount of untreated digestate to compensate for the reduction in buffer capacity of the stripped fraction caused by elimination of CO2 and ammonia. B

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Energy & Fuels Table 1. Single-Stage Anaerobic Digestion of Pretreated BSG: Operation Conditions and Gas Composition monitoring parameters OLR (g of VS L−1 day−1) 0.21−1.96 1.96−2.48 2.48−2.62 2.62−2.87 a

COD (g/kg)

VFA (g/L)

± ± ± ±

1.06 0.02 0.02 1.21

60.05 79.21 101.71 157.98

3.51 5.75 0.90 14.71

gas composition

TKN (g/kg)

biogas production (NL/g of VS)

CH4 (%)

CO2 (%)

H2 (ppm)

H2S (ppm)

± ± ± ±

0.66 0.65 0.50 0.66

49.3 48.5 51.8 51.6

38.2 39.3 39.5 40.4

58 56 56 56

1278 1980 4023 >398a

2.77 3.43 3.32 4.56

0.01 0.09 0.05 1.00

Sensor overload, limit at 5000 ppm of H2S.

Table 2. Two-Stage Anaerobic Digestion of Pretreated BSG: Operation Conditions and Gas Compostion of the Methanogenic Stage monitoring parameters OLR (g of VS L−1 day−1) 0.13−1.23 1.23−1.53 1.53−1.64 1.64−2.36

COD(g/kg) 37.92 63.59 95.98 116.03

± ± ± ±

2.87 2.65 3.65 6.56

VFA (g/L) 0.25 0.05 0.09 0.03

gas composition

TKN (g/kg)

biogas production (NL/g of VS)

CH4 (%)

CO2 (%)

H2 (ppm)

H2S (ppm)

± ± ± ±

0.51 0.53 0.39 0.47

51.0 51.5 54.8 52.8

33.0 34.2 50.8 36.7

28 28 36 54

2 1 142 231

1.87 2.98 3.00 3.25

0.18 0.01 0.10 0.02

Experiments were performed in Erlenmeyer flasks containing 50 mL of working volume. All trials were inoculated with 2 mL of recirculation broth withdrawn from an industrial desulfurization plant that was stored at 4 °C. Flasks were incubated at 30 °C on a rotary shaker at 125 rpm. 2.4. Analytical Methods. Total solids (TS), VS, and COD were analyzed according to DIN DEV 38 414 part 2, DIN DEV 38 414 part 3, and DIN DEV 38409-H41-1, respectively. Samples for TKN analysis were digested with sulfuric acid, followed by distillation and subsequent titration of ammonia (VDLUFA EN 13342, AutoKjeldahl-unit K-370, Büchi Labortechnik AG, Switzerland). VFA were determined by high-performance liquid chromatography (HPLC, column COREGEL 87H, ICE Ion, Agilent Technologies, Inc., Santa Clara, CA) following DIN 38 414-19. The trace element (Zn, Mn, Fe, Cu, Co, and Ni) and phosphorus content of the digestate was determined using inductively coupled plasma−optical emission spectroscopy (ICP−OES, ULTIMA, Horiba GmbH, Austria) after microwave digestion (Ultra CLAVE, MLS GmbH, Germany) of the sample at 240 °C for 40 min at 160 bar with 200 mL/L of 65% HNO3 addition. The nitrogen concentration was determined as TKN. The gas quality of the inlet and outlet gas of the acidic biofilter was measured every second day excluding weekends. Samples were withdrawn from the system in duplicate in 250 mL gas bags and analyzed using gas chromatography with a thermal conductivity detector (GC−TCD, HP 5890 Series II Plus, Hewlett-Packard GmbH, Austria). On sampling days, the liquid broth was also analyzed. Besides online pH measurement and frequent temperature control, a sample was also prepared for determination of the sulfate concentration. For this, 1 mL of a 1:10 dilution with ultrapure water was stored at −4 °C until subsequent ion chromatography measurement (an IonPac AS14A column, conductivity detector, Dionex). 2.5. Calculations. The H2S removal efficiency (RE), empty bed retention time (EBRT), and sulfate production rate were used as indicators for conversion efficiency. The H2S RE is calculated by eq 1

H 2S RE (%) =

c in − cout × 100 c in

where V is the empty bed volume of the packed biofilter (790 cm3) and Q is the flow rate of influent mixed gas entering the biofilter (L/ min). H2S elimination capacity (EC) defines the mass of H2S removed per unit time per bed volume (eq 3)

EC (g m−3 h−1) =

V × 1000 × 60 Q

(3)

where cin and cout represent the H2S concentrations in the mixed gas entering the biofilter and outlet gas stream at the biofilter exit in g/m3, respectively, Q is the flow rate of influent gas mixture entering the biofilter in m3/h, and V is the empty bed volume of the biofilter in m3.

3. RESULTS AND DISCUSSION 3.1. Gas Composition in Two-Stage Anaerobic Digestion of Pretreated BSG. The H2S content in the produced biogas was reduced by up to 78% in the methanogenic stage of the two-stage process compared to the gas composition of single-stage anaerobic digestion (Table 1) of acidically pretreated BSG. This indicates that formed H2S is mainly released during the first step, the pre-acidification, resulting in biogas (from the methanogenic step) with reduced H2S concentrations below 250 ppm (Table 2). In addition, the average methane yield was slightly increased from 50.3 ± 1.7 to 52.5 ± 1.7%. The results demonstrate that, in addition to a more stable operation in a two-stage process,14 sulfide inhibition during the methane-forming stage can be overcome. Sulfide inhibition is a common problem with substrates rich in sulfide, such as industrial wastewaters (e.g., from the rubber latex industry9) and other waste streams (slaughterhouse waste and thin stillage).15 The sulfur contained in the substrate was demonstrated to be released as gaseous H2S during preacidification. Microbiological desulfurization using acidophilic SOB can be applied to the pre-acidification gas to avoid odor emissions and recover the contained sulfur. 3.2. H2S Load Limit: Effect of H2S Shock Loading on Removal Efficiency. As shown in Figure 2, complete H2S removal was achieved with the acidic biofiltration unit established at inlet concentrations below 6363 ppm of H2S. When the H2S inlet concentration was increased up to 9398 ppm (resulting in a reduced EBRT of 91 s), the removal efficiency gradually decreased. A reduced inlet concentration of 8841 ppm of H2S was not sufficient to restore stable RE above

(1)

where cin and cout represent the H2S concentrations in the mixed gas entering the biofilter and outlet gas stream at the biofilter exit (ppm), respectively. EBRT was calculated according to eq 2 EBRT (s) =

(c in − cout)Q V

(2) C

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Figure 4. H2S elimination in percent (closed squares) and corresponding EC plotted as g m−3 h−1 (open triangles) are shown with respect to the amount of sulfate produced under these conditions. Figure 2. Performance of acidic biofiltration treating a gas mixture containing H2S during the operational period of 35 days: RE of H2S (closed triangles), H2S inlet concentrations (gray bars), and EBRT (open circles).

with a total H2S removal of 71.4% was reached at 9398 ppm of H2S inlet concentration. Similar studies10,16 on acidophilic biofiltration for biogas desulfurization reached elimination capacities between 125 and 256.4 g m−3 h−1. On the basis of this result, sulfuric acid production can be clearly linked to H2S elimination, meaning that a recovery of sulfur as intended was established with this setup. During the operational period, a pH of 1.2 was reached prior to medium replacements. This value corresponds to a sulfuric acid concentration of 0.3%, which is within the range that is required for acidic substrate pretreatment prior to anaerobic digestion. 3.4. Digestate as a Suitable Substitute for Mineral Medium. Digestate contains micro- and macronutrients in sufficient amounts to substitute mineral medium during microbiological desulfurization. As shown in Table 3, the nutrient composition of the digestate is comparable to that of the mineral media that are used for cultivation of acidophilic SOB or industrial desulfurization plants with respect to manganese and phosphorus. The recommended amounts for nitrogen and iron are even exceeded by a factor of 10. Although not required for all listed media, a trace element mix is commonly used for biogas desulfurization using SOB to provide a balanced nutrient mix for a diverse microbial community.17 Again, concentrations of Zn, Cu, and Ni are 10−100 times higher in the digestate compared to media composition applied in a study by Gonzáles Sánchez et al. dealing with cultivation of SOB for full-scale biogas desulfurization. Sulfate production, in correspondence with a decrease in pH, was achieved in all trials with digestate addition, as shown in Figure 5. Although double the amount of digestate was added for the stripped trials (Figure 5b), only half of the amount of sulfate was produced when compared to trials where untreated digestate was applied (Figure 5a). This indicates that the microbial consortium that is still contained within the untreated digestate might contribute to thiosulfate oxidation and, hence, increase sulfate production. This hypothesis is also supported by the fact that sulfate production was enhanced with an increasing amount of digestate, regardless of digestate treatment. In addition, it was demonstrated that a start pH of ∼8.5 is feasible for the SOB consortium and has no significant influence on sulfate production by a diverse bacterial community. 3.5. Theoretical Sulfur Recovery. The amount of sulfuric acid that was applied during substrate pretreatment (20.2 mg of 98% H2SO4/g of OTS) corresponds to a daily sulfuric acid

80%. However, after a period of 10 days without any H2S feeding, complete removal was restored at an inlet concentration of 6363 ppm. Although Omri et al.6 were able to reach 99% RE at EBRT of 60 s at H2S loading below 600 ppm, this study demonstrated that stable H2S removal with 100% RE is possible for H2S loading of up to 6363 ppm at an increased EBRT of 133 s. 3.3. Performance of Biofiltration for H2S Conversion to Sulfuric Acid. Figure 3 depicts the rapid drop in pH in the

Figure 3. Sulfuric acid production is indicated by the pH drop (open diamond) during H 2S removal using an acidophilc biofilter. Replacement of media at pH below 1.5 is indicated as open triangles.

recirculated nutrient broth caused by the formation of sulfuric acid as a consequence of microbiological H2S oxidation during acidic biofiltration. It was necessary to replace the media when the pH of the recirculation liquid dropped below 1.2 within less than 12 h, depending upon the H2S inlet concentration and amount of recirculation liquid provided. These results allow for the conclusion that H2S from inlet gas was converted microbiologically by the immobilized SOB biofilm in the biofiltration unit to sulfuric acid at a rapid rate. A maximum sulfate production rate of 0.14 g L−1 h−1 was achieved (Figure 4). The produced sulfate was then washed from the carrier material and accumulated in the liquid recirculate with a peak concentration of 4.18 g/L sulfate (data not shown). As shown in Figure 4, the highest sulfate production rate was achieved at 100% H2S elimination. A maximum H2S EC of 473 g m−3 h−1 D

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Energy & Fuels Table 3. Nutrient Composition of the Digestate and Comparable Mineral Mediaa mg/L

Zn

Mn

Fe

Cu

Co

Ni

N

P

digestate LSE Ttp IP González Sánchez et al.17

6.477

2.440

69.611

1.008

0.425

0.106

4.132

0.020

6.501 0.456 0.060

8379.5 318.3 10.6 261.8 375.0

223.1 683.2 810.9 455.5 150.0

0.560

0.090

0.010

a

LSE, DSMZ medium 71, cultivation of SOB Acidithiobacillus thiooxidans (used for continuous setup); Ttp, DSMZ medium 36, cultivation of SOB Thiobacillus thioparus; and IP, medium used for an industrial desulfurization column.

stage anaerobic digestion by microbiological oxidation to sulfuric acid, are feasible. Furthermore, the possibility to replace up to 50% of fresh water with the liquid fraction of the digestate after separation was proven. This reduces the extensive amount of fresh water and mineral medium required for microbiological desulfurization. In addition, nutrients contained in the digestate (nitrogen, phosphorus, and trace elements) can be recycled. During the desulfurization process, complete H2S removal was reached. The acid concentration produced was within the range required for thermochemical substrate pretreatment and, therefore, enables direct reuse. The combination of two-stage anaerobic digestion at high sulfur loads, microbiological desulfurization using an acidic biofilter, and digestate utilization enables us to strongly extend overall efficiency and sustainability of the process.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +43-0-2272-66280-535. Fax: +43-0-2272-66280503. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union’s Seventh Framework Programme managed by the Research Executive Agency (REA) http://ec. europa.eu/research/rea ([FP7/2007−2013][FP7/2007− 2011]) under Agreement 315630. The authors kindly thank the funding organization for the financial support. Furthermore, the authors are very grateful for the valuable contribution of Amin Eisa.

Figure 5. pH drop (solid line) with corresponding sulfate production (broken line) for the addition of the (a) untreated liquid fraction of the digestate and (b) stripped liquid fraction of the digestate at amounts of 25 and 50% (open circles), 12.5 and 25% (closed circles), and 5 and 10% (open triangles), respectively.

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input of 0.89 mM H2SO4. On the basis of the given daily biogas production rate and the measured H2S concentration, an average production of 0.39 mM H 2 S was calculated. Consequently, the maximum (if 100% of H2S is metabolized to sulfuric acid) theoretical sulfur recovery is 44%.

4. CONCLUSION These investigations evaluated the removal of H2S in acidic biofiltration with focus on optimization of conditions for sulfur recovery and nutrient recirculation. Major H2S release occurred during the pre-acidification stage. Nevertheless, remaining sulfur loads in the feed caused several problems in the methanogenic stage. Close pH monitoring was absolutely crucial for the two-stage anaerobic fermentation of BSG. It was demonstrated that efficient removal and conversion of H2S, which is released during pre-acidification (∼4000 ppm) of two-

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NOMENCLATURE BSG = brewer’s spent grains COD = chemical oxygen demand EBRT = empty bed retention time EC = elimination capacity H2S = hydrogen sulfide OLR = organic loading rate RE = removal efficiency SOB = sulfur-oxidizing bacteria TKN = total Kjeldahl nitrogen TS = total solids VFA = volatile fatty acids VS = volatile solids REFERENCES

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