Closing the Nutrient Cycle in Two-Stage Anaerobic Digestion ofIndustrial Waste StreamsLydia Rachbauer,*,† Wolfgang Gabauer,‡ Stefanie Scheidl,‡ Markus Ortner,† Werner Fuchs,‡

and Gunther 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 alternativesubstrate for biogas production by anaerobic digestion. High sulfur loads in the feed caused by acidic pretreatment to enhancebioavailability are responsible for H2S formation during anaerobic digestion. Microbiological oxidation of H2S provides an eleganttechnique to remove this toxic gas compound. Moreover, it allows for recovery of sulfuric acid, the final product of aerobic sulfideoxidation, as demonstrated in this study. Two-stage anaerobic digestion of brewer’s spent grains, the major byproduct in thebrewing industry, allows for the release of up to 78% of total H2S formed in the first pre-acidification stage. Desulfurization ofsuch pre-acidification gas in continuous acidic biofiltration with immobilized sulfur-oxidizing bacteria resulted in a maximum H2Selimination capacity of 473 g m−3 h−1 at an empty bed retention time of 91 s. Complete H2S removal was achieved at inletconcentrations of up to 6363 ppm. The process was shown to be very robust, and even after an interruption of H2S feeding for 10days, 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 andchemicals to minimize process expenses. It was proven that up to 50% of mineral medium that is required in large amountsduring microbiological desulfurization can be replaced by the liquid fraction of the digestate. The conducted study demonstratesthe 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 majorbarrier for further production of renewable energies, such asbiogas. Among others, industrial waste streams and byproductsprovide a valuable but largely unused alternative substrate foranaerobic digestion.1 Thus far, the digestion of some industrialwastes shows substrate-specific challenges. Especially thebrewing industry, distilleries and abattoirs could profitenormously from using their waste or side products for energyproduction onsite to cover their in-house demand for processheat and energy.2 Nevertheless, waste as a substrate foranaerobic digestion brings in certain problems, such as nitrogeninhibition, the requirement for substrate pretreatment as aresult of low bioavailability, or the need for hygienization. Forbrewer’s spent grains (BSG), the major side product in thebrewing industry, different pretreatment technologies, such asthermochemical, enzymatic, and mechanical treatment, havealready been evaluated to break down the lignocellulosiccompound layer and increase its digestibility.2,3 High sulfurloads in the feed caused by an acidic pretreatment, pHadjustment with sulfuric acid, or protein-rich substrates, such asslaughterhouse waste, result in biogas containing elevated levelsof hydrogen sulfide (H2S), reaching up to 10 000 ppm.1 Withdecreasing pH, the solubility of H2S decreases. Thus, H2S isdriven out of the liquid and remains in its gaseous form. Thisstudy shows that two-stage anaerobic digestion allows for therelease, depending upon the pH, of up to 78% of total H2Sformation during the first pre-acidification step compared to asingle-stage system. To prevent corrosion of engines and tominimize maintenance and odor, H2S removal is essential.

Biofiltration for the removal of H2S with various biologicalcarrier materials, such as packed compost,4,5 peat,6,7 and mostpopular, activated carbon,4,8 was previously studied. Morerecent work also evaluated microbiological H2S removal usingsulfur-oxidizing bacteria (SOB) in acidic biofiltration,9−11 butthe recovery of sulfur as sulfuric acid, the final product ofaerobic oxidation reaction of sulfide, was not focused on beforethis study.Digestate, as a residue in anaerobic digestion, contains

nutrients, such as nitrogen, phosphorus, and trace elements, ina sufficient amount to support the growth of the SOB used inthe desulfurization process. Digestate from agricultural biogasplants has a well-established use as a fertilizer, but digestatefrom biogas plants using slaughterhouse waste as a substratecan 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 landis limited.13 For the microbiological desulfurization process,synthetic medium is required in large amounts. The demand forfresh water and synthetic medium can be minimized bysubstitution with the liquid fraction of digestate afterseparation, 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, 2014Revised: February 13, 2015Published: March 12, 2015

Article

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with a strong focus on an efficient desulfurization process andnutrient recirculation. The objective was to efficiently removeand 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 syntheticmedium and fresh water during the desulfurization processusing digestate instead. The results indicate the possibility toreplace up to 50% of fresh water with the liquid fraction ofdigestate. In addition to the sulfur recycling, this uniquecombination of processes means that additional nutrientscontained in the digestate (nitrogen, phosphorus, and traceelements) can be brought back into the overall methaneformation process.

2. EXPERIMENTAL SECTION2.1. Anaerobic Digestion of Pretreated BSG. The substrate for

continuous fermentation was obtained from a local brewery located inthe surroundings of Vienna, Austria. BSG were taken from anintermediate storage tank and stored at −20 °C for long-time storageuntil substrate pretreatment. Pretreatment was performed in amicrowave digestion unit (Ultra CLAVE, MLS GmbH, Germany) ata temperature of 140 °C for 15 min with acid addition of H2SO4,resulting in a final acid concentration of 0.5%. Pretreated substrate wasstored at 4 °C until feeding. Thermochemically pretreated BSG wasfed daily, except for weekends. Continuous lab experiments wereconducted for a total test period of 120 days. The setup consisted oftwo stirred tank reactors with 6 L of working volume with amechanical-sealed central blade stirrer and a heat jacket for automatictemperature control until day 22. The system was then switched to 2 Lflask fermenters with a working volume of 1.8 L. For that, thefermenter content was homogenized by stirring for 30 min before1.8 L aliquots were transferred to the new flask setup. Excess sludgewas discarded. The flask fermenters were incubated at 37 °C on amagnetic 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 hydraulicretention 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 wascontinuously 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 (AWITEBioenergie 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 Desulfuriza-

tion Column. A continuous desulfurization column was constructedfor lab-scale experiments (Figure 1). The column materials werechosen to withstand the strongly acidic culture broth expected.Transparent polyvinyl chloride pipes were connected with poly-ethylene sleeve sockets to function as a desulfurization column. Aurethane-based thermoplastic polymer was chosen for gas and liquidtubing with both Teflon and high-alloy stainless-steel valveconnections (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 ofpolyethylene carrier material was mechanically treated prior to theimmobilization procedure. For this treatment, 271 cm3 of the carrier,which is equivalent to an empty bed volume of 790 cm3, was mixedwith 350 mL of quartz sand and rotated at 2200 min−1 for 15 min. Forsafety reasons, a gas detector was installed in the lab and outlet gas waspurged through a 10% solution of sodium hydroxide to removeresidual H2S. A heating coil was installed on the desulfurizationcolumn 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 industrialdesulfurization plant located in Upper Austria, Austria, that was

centrifuged, and 20 g of the pellet was resuspended in 2 L mineralmedium containing the following: 3.00 g/L KH2PO4, 0.50 g/LMgSO4·7H2O, 1.20 g/L (NH4)Cl, and 0.25 g/L CaCl2·2H2O. Thisculture broth was mixed with the pretreated and washed polyethylenecarrier material, transferred into the desulfurization column, andrecirculated at 30 °C at minimum H2S concentration of approximately200 ppm in addition to air supply for 4 days.

2.2.2. Operation Conditions. During the experimental phase, 12 Lof mineral medium as used during inoculation was supplied. Mediumwas replaced when the pH dropped below 1.2. Before replacement ofmedium, the system was flushed with 2 L of distilled water to rinseresidual sulfuric acid and avoid an immediate pH drop within the freshmedium. An online pH measurement was installed to monitor theoperation conditions throughout the operational phase. Flux of air andsynthetic gas with the composition of 4% H2S in N2 were adjustedaccording to the desired inlet concentration of H2S with two separateflow meters, with a minimum air/H2S ratio of 4:1. The inlet gas washumidified by purging through a temperature-controlled water bottlebefore entering the column.

2.3. Application of the Digestate to Substitute Fresh Waterand Mineral Medium. Digestate was centrifuged at 4000 rpm for10 min to separate solid particles from the liquid fraction. The liquidfraction was applied untreated and pretreated at various concentrationsto substitute thiosulfate mineral medium. Thiosulfate mineral mediumcontained 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 at100 °C for 15 min. The untreated fraction was used to replace themedium at a concentration of 5, 12.5, and 25%, and the strippeddigestate was used at a concentration of 10, 25, and 50%. The amountsof stripped digestate that were added were double the amount ofuntreated digestate to compensate for the reduction in buffer capacityof the stripped fraction caused by elimination of CO2 and ammonia.

Figure 1. Schematic overview of experimental setup for micro-biological desulfurization.

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Experiments were performed in Erlenmeyer flasks containing 50 mLof working volume. All trials were inoculated with 2 mL ofrecirculation broth withdrawn from an industrial desulfurizationplant that was stored at 4 °C. Flasks were incubated at 30 °C on arotary 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 part3, and DIN DEV 38409-H41-1, respectively. Samples for TKN analysiswere digested with sulfuric acid, followed by distillation andsubsequent titration of ammonia (VDLUFA EN 13342, AutoKjel-dahl-unit K-370, Buchi Labortechnik AG, Switzerland). VFA weredetermined by high-performance liquid chromatography (HPLC,column COREGEL 87H, ICE Ion, Agilent Technologies, Inc., SantaClara, 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 coupledplasma−optical emission spectroscopy (ICP−OES, ULTIMA, HoribaGmbH, Austria) after microwave digestion (Ultra CLAVE, MLSGmbH, Germany) of the sample at 240 °C for 40 min at 160 bar with200 mL/L of 65% HNO3 addition. The nitrogen concentration wasdetermined as TKN.The gas quality of the inlet and outlet gas of the acidic biofilter was

measured every second day excluding weekends. Samples werewithdrawn from the system in duplicate in 250 mL gas bags andanalyzed using gas chromatography with a thermal conductivitydetector (GC−TCD, HP 5890 Series II Plus, Hewlett-Packard GmbH,Austria). On sampling days, the liquid broth was also analyzed. Besidesonline pH measurement and frequent temperature control, a samplewas also prepared for determination of the sulfate concentration. Forthis, 1 mL of a 1:10 dilution with ultrapure water was stored at −4 °Cuntil subsequent ion chromatography measurement (an IonPac AS14Acolumn, conductivity detector, Dionex).2.5. Calculations. The H2S removal efficiency (RE), empty bed

retention time (EBRT), and sulfate production rate were used asindicators for conversion efficiency. The H2S RE is calculated by eq 1

=−

×c c

cH S RE (%) 1002

in out

in (1)

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

= × ×VQ

EBRT (s) 1000 60(2)

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 perunit time per bed volume (eq 3)

=−− − c c Q

VEC (g m h )

( )3 1 in out(3)

where cin and cout represent the H2S concentrations in the mixed gasentering the biofilter and outlet gas stream at the biofilter exit in g/m3,respectively, Q is the flow rate of influent gas mixture entering thebiofilter 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 AnaerobicDigestion of Pretreated BSG. The H2S content in theproduced biogas was reduced by up to 78% in themethanogenic stage of the two-stage process compared to thegas composition of single-stage anaerobic digestion (Table 1)of acidically pretreated BSG. This indicates that formed H2S ismainly released during the first step, the pre-acidification,resulting in biogas (from the methanogenic step) with reducedH2S concentrations below 250 ppm (Table 2). In addition, theaverage methane yield was slightly increased from 50.3 ± 1.7 to52.5 ± 1.7%. The results demonstrate that, in addition to amore stable operation in a two-stage process,14 sulfideinhibition during the methane-forming stage can be overcome.Sulfide inhibition is a common problem with substrates rich insulfide, such as industrial wastewaters (e.g., from the rubberlatex industry9) and other waste streams (slaughterhouse wasteand thin stillage).15 The sulfur contained in the substrate wasdemonstrated to be released as gaseous H2S during pre-acidification. Microbiological desulfurization using acidophilicSOB can be applied to the pre-acidification gas to avoid odoremissions and recover the contained sulfur.

3.2. H2S Load Limit: Effect of H2S Shock Loading onRemoval Efficiency. As shown in Figure 2, complete H2Sremoval was achieved with the acidic biofiltration unitestablished at inlet concentrations below 6363 ppm of H2S.When the H2S inlet concentration was increased up to9398 ppm (resulting in a reduced EBRT of 91 s), the removalefficiency gradually decreased. A reduced inlet concentration of8841 ppm of H2S was not sufficient to restore stable RE above

Table 1. Single-Stage Anaerobic Digestion of Pretreated BSG: Operation Conditions and Gas Composition

monitoring parameters gas composition

OLR (g of VS L−1 day−1) COD (g/kg) VFA (g/L) TKN (g/kg) biogas production (NL/g of VS) CH4 (%) CO2 (%) H2 (ppm) H2S (ppm)

0.21−1.96 60.05 ± 3.51 1.06 2.77 ± 0.01 0.66 49.3 38.2 58 12781.96−2.48 79.21 ± 5.75 0.02 3.43 ± 0.09 0.65 48.5 39.3 56 19802.48−2.62 101.71 ± 0.90 0.02 3.32 ± 0.05 0.50 51.8 39.5 56 40232.62−2.87 157.98 ± 14.71 1.21 4.56 ± 1.00 0.66 51.6 40.4 56 >398a

aSensor overload, limit at 5000 ppm of H2S.

Table 2. Two-Stage Anaerobic Digestion of Pretreated BSG: Operation Conditions and Gas Compostion of the MethanogenicStage

monitoring parameters gas composition

OLR (g of VS L−1 day−1) COD(g/kg) VFA (g/L) TKN (g/kg) biogas production (NL/g of VS) CH4 (%) CO2 (%) H2 (ppm) H2S (ppm)

0.13−1.23 37.92 ± 2.87 0.25 1.87 ± 0.18 0.51 51.0 33.0 28 21.23−1.53 63.59 ± 2.65 0.05 2.98 ± 0.01 0.53 51.5 34.2 28 11.53−1.64 95.98 ± 3.65 0.09 3.00 ± 0.10 0.39 54.8 50.8 36 1421.64−2.36 116.03 ± 6.56 0.03 3.25 ± 0.02 0.47 52.8 36.7 54 231

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80%. However, after a period of 10 days without any H2Sfeeding, complete removal was restored at an inlet concen-tration of 6363 ppm. Although Omri et al.6 were able to reach99% RE at EBRT of 60 s at H2S loading below 600 ppm, thisstudy demonstrated that stable H2S removal with 100% RE ispossible for H2S loading of up to 6363 ppm at an increasedEBRT of 133 s.3.3. Performance of Biofiltration for H2S Conversion

to Sulfuric Acid. Figure 3 depicts the rapid drop in pH in the

recirculated nutrient broth caused by the formation of sulfuricacid as a consequence of microbiological H2S oxidation duringacidic biofiltration. It was necessary to replace the media whenthe pH of the recirculation liquid dropped below 1.2 within lessthan 12 h, depending upon the H2S inlet concentration andamount of recirculation liquid provided. These results allow forthe conclusion that H2S from inlet gas was convertedmicrobiologically by the immobilized SOB biofilm in thebiofiltration unit to sulfuric acid at a rapid rate. A maximumsulfate production rate of 0.14 g L−1 h−1 was achieved (Figure4). The produced sulfate was then washed from the carriermaterial and accumulated in the liquid recirculate with a peakconcentration of 4.18 g/L sulfate (data not shown). As shownin Figure 4, the highest sulfate production rate was achieved at100% H2S elimination. A maximum H2S EC of 473 g m−3 h−1

with a total H2S removal of 71.4% was reached at 9398 ppm ofH2S inlet concentration. Similar studies10,16 on acidophilicbiofiltration for biogas desulfurization reached eliminationcapacities between 125 and 256.4 g m−3 h−1. On the basis ofthis result, sulfuric acid production can be clearly linked to H2Selimination, meaning that a recovery of sulfur as intended wasestablished with this setup. During the operational period, a pHof 1.2 was reached prior to medium replacements. This valuecorresponds to a sulfuric acid concentration of 0.3%, which iswithin the range that is required for acidic substratepretreatment prior to anaerobic digestion.

3.4. Digestate as a Suitable Substitute for MineralMedium. Digestate contains micro- and macronutrients insufficient amounts to substitute mineral medium duringmicrobiological desulfurization. As shown in Table 3, thenutrient composition of the digestate is comparable to that ofthe mineral media that are used for cultivation of acidophilicSOB or industrial desulfurization plants with respect tomanganese and phosphorus. The recommended amounts fornitrogen and iron are even exceeded by a factor of 10. Althoughnot required for all listed media, a trace element mix iscommonly used for biogas desulfurization using SOB toprovide a balanced nutrient mix for a diverse microbialcommunity.17 Again, concentrations of Zn, Cu, and Ni are10−100 times higher in the digestate compared to mediacomposition applied in a study by Gonzales Sanchez et al.dealing with cultivation of SOB for full-scale biogasdesulfurization.Sulfate production, in correspondence with a decrease in pH,

was achieved in all trials with digestate addition, as shown inFigure 5. Although double the amount of digestate was addedfor the stripped trials (Figure 5b), only half of the amount ofsulfate was produced when compared to trials where untreateddigestate was applied (Figure 5a). This indicates that themicrobial consortium that is still contained within the untreateddigestate might contribute to thiosulfate oxidation and, hence,increase sulfate production. This hypothesis is also supportedby the fact that sulfate production was enhanced with anincreasing amount of digestate, regardless of digestate treat-ment. In addition, it was demonstrated that a start pH of ∼8.5is feasible for the SOB consortium and has no significantinfluence on sulfate production by a diverse bacterialcommunity.

3.5. Theoretical Sulfur Recovery. The amount of sulfuricacid that was applied during substrate pretreatment (20.2 mg of98% H2SO4/g of OTS) corresponds to a daily sulfuric acid

Figure 2. Performance of acidic biofiltration treating a gas mixturecontaining H2S during the operational period of 35 days: RE of H2S(closed triangles), H2S inlet concentrations (gray bars), and EBRT(open circles).

Figure 3. Sulfuric acid production is indicated by the pH drop (opendiamond) during H2S removal using an acidophilc biofilter.Replacement of media at pH below 1.5 is indicated as open triangles.

Figure 4. H2S elimination in percent (closed squares) andcorresponding EC plotted as g m−3 h−1 (open triangles) are shownwith respect to the amount of sulfate produced under these conditions.

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input of 0.89 mM H2SO4. On the basis of the given daily biogasproduction rate and the measured H2S concentration, anaverage production of 0.39 mM H2S was calculated.Consequently, the maximum (if 100% of H2S is metabolizedto sulfuric acid) theoretical sulfur recovery is 44%.

4. CONCLUSIONThese investigations evaluated the removal of H2S in acidicbiofiltration with focus on optimization of conditions for sulfurrecovery and nutrient recirculation. Major H2S release occurredduring the pre-acidification stage. Nevertheless, remainingsulfur loads in the feed caused several problems in themethanogenic stage. Close pH monitoring was absolutelycrucial for the two-stage anaerobic fermentation of BSG. It wasdemonstrated that efficient removal and conversion of H2S,which is released during pre-acidification (∼4000 ppm) of two-

stage anaerobic digestion by microbiological oxidation tosulfuric acid, are feasible. Furthermore, the possibility toreplace up to 50% of fresh water with the liquid fraction of thedigestate after separation was proven. This reduces theextensive amount of fresh water and mineral medium requiredfor microbiological desulfurization. In addition, nutrientscontained in the digestate (nitrogen, phosphorus, and traceelements) can be recycled.During the desulfurization process, complete H2S removal

was reached. The acid concentration produced was within therange required for thermochemical substrate pretreatment and,therefore, enables direct reuse. The combination of two-stageanaerobic digestion at high sulfur loads, microbiologicaldesulfurization using an acidic biofilter, and digestate utilizationenables us to strongly extend overall efficiency and sustain-ability of the process.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +43-0-2272-66280-535. Fax: +43-0-2272-66280-503. E-mail: lydia.rachbauer@bioenergy2020.eu.NotesThe authors declare no competing financial interest.

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

■ NOMENCLATUREBSG = brewer’s spent grainsCOD = chemical oxygen demandEBRT = empty bed retention timeEC = elimination capacityH2S = hydrogen sulfideOLR = organic loading rateRE = removal efficiencySOB = sulfur-oxidizing bacteriaTKN = total Kjeldahl nitrogenTS = total solidsVFA = volatile fatty acidsVS = volatile solids

■ REFERENCES(1) Al Seadi, T.; Rutz, D.; Janssen, R.; Drosg, B. Biomass resourcesfor biogas production. In The Biogas Handbook: Science, Production and

Table 3. Nutrient Composition of the Digestate and Comparable Mineral Mediaa

mg/L Zn Mn Fe Cu Co Ni N P

digestate 6.477 2.440 69.611 1.008 0.425 0.106 8379.5 223.1LSE 318.3 683.2Ttp 6.501 4.132 10.6 810.9IP 0.456 261.8 455.5Gonzalez Sanchez et al.17 0.020 0.060 0.560 0.090 0.010 375.0 150.0

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

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

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Energy & Fuels Article

DOI: 10.1021/ef502809eEnergy Fuels 2015, 29, 4052−4057

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