The pressure effects on two-phase anaerobic digestion
Post on 31-Dec-2016
bDVGW Research Center at the Engler-Bunte-Institute, Karlsruhe Institute of Technology (KIT), Engler-Bunte-Ring 1, D-76131 Karlsruhe, Germany
h i g h l i g h t s
tion upe in thecontenmethanery sta
c lter. Four different pressure levels (the absolute pressure of 1 bar, 3 bar, 6 bar
in Germany is to transfer biogas into a local combined heat andpower plant for electricity and heat production. However, withthe exception of digester thermal control and heating purposes
ce it temutilizatio
overall energy utilization efciency is largely increased. Intion, the existing gas distribution and storage infrastructube used without modication. Therefore, this applicationceived increased attention in recent years . Nevertheless, biogaspurication and upgrading is usually energy demanding . Two-phase pressurized anaerobic digestion is a possible solution. It isintended to directly remove CO2 and H2S in the digester, makinguse of their high gas solubility under pressure. Thus, biogas pro-duction, purication and pressurization are integrated in one sys-tem, and the expenses involved in the subsequent treatment canbe considerably reduced.
Abbreviations: COD, chemical oxygen demand; ODM, organic dry matter; SBP,specic biogas production; SMY, specic methane yield; TAN, total ammonianitrogen; TIC, total inorganic carbon; VFA, volatile fatty acid. Corresponding author. Tel.: +49 711 459 23348; fax: +49 711 22111.
E-mail addresses: Yuling.Chen@uni-hohenheim.de (Y. Chen),Benjamin.Roessler@uni-hohenheim.de (B. Rler), Simon.Zielonka@uni-hohen-heim.de (S. Zielonka), Andreas.Lemmer@uni-hohenheim.de (A. Lemmer), Anna-Maria.Wonneberger@kit.edu (A.-M. Wonneberger), Thomas.Jungbluth@uni-hohen-
Applied Energy 116 (2014) 409415
Contents lists availab
journal homepage: www.elseheim.de (T. Jungbluth).wastes and energy crops can be degraded and converted into bio-gas, mainly containing CH4 and CO2 . The most common practice
CO2-removal, drying and pressurization . Sinand spatially separates biogas production from0306-2619/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.11.012porallyn, theaddi-re canhas re-Anaerobic digestionPressureTwo-phaseSubstitute natural gasBiogasBiomethane
and 9 bar) were applied to the methane reactor in sequence, with the organic loading rate maintained atapproximately 5.1 kgCODm3 d1. Gas production, gas quality, pH value, volatile fatty acids, alcohol, ammo-nium-nitrogen, chemical oxygen demand (COD) and alkaline buffer capacity were analyzed. No additionalcaustic chemicals were added for pH adjustment throughout the experiment. With the pressure increasingfrom 1.07 bar to 8.91 bar, the pH value decreased from 7.2 to 6.5, the methane content increased from 66%to 75%, and the specic methane yield was slightly reduced from 0.33 lN g1COD to 0.31 lN g1COD. Therewas almost no acid-accumulation during the entire experiment. The average COD-degradation grade wasalways more than 93%, and the average alkaline buffering capacity (VFA/TIC ratio) did not exceed 0.2 atany pressure level. The anaerobic lter showed a very stable performance, regardless of the pressure variation.
2013 Elsevier Ltd. All rights reserved.
Anaerobic digestion is a biological process, in which organic
at the biogas plant, a great amount of cogenerated heat is dissi-pated into the air and wasted. Alternatively, biogas can be injectedinto the gas grid as substitute natural gas after desulfurization,Keywords:pressure-resistant anaerobi The pressure effect on anaerobic diges Increasing pressure decreased pH valu Increasing pressure increased methane Increasing pressure decreased specic The pressurized methane reactor was v
a r t i c l e i n f o
Article history:Received 26 February 2013Received in revised form 2 September 2013Accepted 3 November 2013Available online 27 November 2013to 9 bar was examined.anaerobic lter.t.e yield slightly.ble and performed well.
a b s t r a c t
Two-phase pressurized anaerobic digestion is a novel process aimed at facilitating injection of the producedbiogas into the natural gas grid by integrating the fermentative biogas production and upgrading it to substi-tute natural gas. In order to understand themechanisms, knowledge of pressure effects on anaerobic digestionis required. To examine the effects of pressure on the anaerobic digestion process, a two-phase anaerobicdigestion system was built up in laboratory scale, including three acidogenesis-leach-bed-reactors and oneThomas Jungbluth a
a State Institute of Agricultural Engineering and Bioenergy, University Hohenheim, Garbenstrae 9, D-70599 Stuttgart, GermanyThe pressure effects on two-phase anaer
Yuling Chen a,, Benjamin Rler a, Simon Zielonka a
ndreas Lemmer a, Anna-Maria Wonneberger b,
le at ScienceDirect
vier .com/locate /apenergy
a variable, in actual application. In order to improve anaerobicdigestion performance, efforts are primarily focused on the optimi-
nergzation of more adjustable and controllable parameters, and as a re-sult, the investigation of pressure effects on anaerobic digestionhas been overlooked.
As a matter of fact, anaerobic digestion for methane productionunder pressure is not rare in natural ecosystems or in the wastewa-ter treatment industry. It is common in marine sediments, hun-dreds of meters deep , in landlls  and at the lower partof anaerobic digestion towers or biogas tower reactors  thatare used in wastewater treatment to save ground space. Based onthe pressure adaptability, microorganisms can be divided intothree categories: piezosensitive, piezotolerant and piezophilic mi-crobes. Piezosensitive microbes have optimal growth at atmo-spheric pressure and stop reproduction around 500 bar . Bothpiezotolerant and piezophilic microbes are bacteria that are ableto grow and proliferate up to a pressure of 1000 bar . The opti-mal growth rate of piezotolerant microorganisms, however, occursat atmospheric pressure . Since most microbes in anaerobicdigesters are inoculated from sewage slurry, excrement or waste-water treatment sludge under atmospheric pressure, they are nor-mally not piezophilic. That means, their growth rates are hardlyinhibited by pressure up to 10 bar, according to the research sum-mary of Abe and Horikoshi and Abe . This offered a theoreticalbasis for the experiment of anaerobic digestion under pressure.
This study examined the effects of pressure on anaerobic diges-tion by testing four different pressures (the absolute pressure of1 bar, 3 bar, 6 bar and 9 bar) in a two-phase anaerobic digestionsystem. Gas production, gas quality, pH value, volatile fatty acids(VFAs), chemical oxygen demand (COD) degradation grade, buffercapacity and ammonium were analyzed and compared.
2. Materials and methods
The ow diagram of the two-phase anaerobic digestion systemis shown in Fig. 1. The hydrolysis-acidication was performed inthree parallel-operated acidogenesis-leach-bed-reactors at 55 C.Each reactor had 50-liter volume, and was alternately fed with10 kg (fresh mass) maize silage from the Field-test station of theUniversity Hohenheim (Unterer Lindenhof, Eningen, Germany) ata time interval of seven days. In order to avoid the deciency ofthe nutrients necessary for microbial growth and biological processdisturbances, micronutrients were also added once a week. Thedosage and the composition of the micronutrients was based onthe recommendation of Vintiloiu et al. . The maize silage wasTo better understand pressurized anaerobic digestion, it isessential to rst gain knowledge of how pressure affects the pro-cess. Although various operational parameters (e.g. temperature,pH, hydraulic retention time, organic loading rate and mixingmode) have been thoroughly studied and reviewed , therehave been few discussions on the effect of pressure. The pressurevariable is not given enough recognition in anaerobic digestion,mainly due to the limitations in the available techniques and facil-ities suitable for experimental investigation on pressure effects .In addition, pressure change induces complicated interactionsamong operational conditions and microorganism activity in areactor. For the sake of easy management, the total gas pressurein a common anaerobic digester is maintained slightly above atmo-spheric pressure (up to 0.02 bar overpressure) . Compared toother operational parameters, pressure is a constant, rather than
410 Y. Chen et al. / Applied Eloosely stacked on a perforated grate. In the acidogenesis-leach-bed-reactors, the substrate was gradually decomposed and a leach-ate rich in organic acids, as well as alcohols was produced. Everyweek, approximately 20 l of leachate from each acidogenesis-leach-bed-reactor was introduced into a tank (Tank 1 in Fig. 1)for storage and homogenization. Every six hours, a certain amountof the leachate was pumped from the tank into an anaerobic lter,which was operated under pressure for further degradation. Thefeeding amount was only dependent on the inuent COD concen-tration, since the organic loading rate of the methane reactor re-mained unchanged. For the stable working volume, the sameamount of liquid was eluted from the methane reactor to the othertank (Tank 2 in Fig. 1) for storage under atmospheric pressure. Dueto the pressure difference, the dissolved CO2 could be released, andthe liquid in Tank 2 was distributed evenly to the three acidogen-esis-leach-bed-reactors once a week. A liquid level sensor(Endress + Hauser, Liquicap T FMI21) constantly controlled theworking volume of the anaerobic lter.
The upow-operated anaerobic lter was running at 37 C. Thereactor was composed of a xed bed, a three-phase separator and agas chamber at the top. The 20 l xed bed was packed with sin-tered glass (Sera Siporax) as a carrier material. With 270 m2 l1
biologically effective surfaces, the sintered glass helped the micro-organisms immobilization and biolm development. Despite thexed bed, there was still a certain amount of biomass suspendedin the uid. The three-phase separator prevented the suspendedbiomass from leaving the anaerobic lter with the efuent. In addi-tion, the gas bubbles formed in the process could also be collectedwithout signicant foaming by the separator.
The produced biogas did not immediately leave the anaerobiclter, but accumulated therein, till the desired pressure in theanaerobic lter was reached. At that point, the valve of the gas out-let automatically opened, and the produced biogas was injected toa gasbag. As soon as the gas was released, the pressure of theanaerobic lter started to drop, and the gas outlet was closed again,allowing the auto-generated biogas pressure to increase to the de-sired value. By this means, the anaerobic lter could be set under acertain operating pressure. The entire process was controlled by apressure sensor (Endress + Hauser Ceraphant T PTC31) and a con-trol valve (Brkert 2712).
In addition to the anaerobic lter, the acidogenesis-leach-bed-reactors also had gas outlets. The gas outlet of each reactor wasconnected to a gasbag for gas-quality and -quantity measurement.Furthermore, both the anaerobic lter and the acidogensis-leach-bed-reactors were equipped with pumps for internal circulation(about 1.5 l min1), mixing for ve minutes every ten minutes.
2.2. Experiment procedure
The anaerobic lter was seeded with the efuent from anotherxed-bed anaerobic reactor, which had been fed with leachate ofgrass silage . The reactor start-up period and preliminary testslasted approximately four months. After that, the reactors reacheda steady state, and the experiment on the pressure effects on two-phase anaerobic digestion began.
The experiment was divided into four runs. With the exceptionof the working pressure of the anaerobic lter, all the operatingparameters were maintained at a constant. The inuent COD con-centration stayed at 23 0.9 g l1. The organic loading rate of5.1 0.1 kgCODm3 d1 was applied to the anaerobic lter. Fourdifferent working pressures on the anaerobic lter were tested (Ta-ble 1). No additional caustic chemicals were added for pH adjust-ment throughout the experiment, so that the pressure effects onanaerobic digestion could be clearly examined.
2.3. Analytical methods and data acquisition
y 116 (2014) 409415In this study, pH, pressure and temperature of the anaerobic l-ter were monitored in real-time (pH-sensor: Endress + Hauser
nergY. Chen et al. / Applied ECPS11D; pressure sensor: Endress + Hauser Ceraphant T PTC31;temperature sensor: Greisinger GTF 103 Pt100), and the data werelogged in Labview 11.0.1 (National Instruments). The produced gascomposition and volume were measured under atmospheric pres-sure every day (Gas meter: Ritter TG20/5; Gas analyzer: Sick Mai-hak S710). The gas volume was corrected to dry gas at a standardtemperature and pressure (0 C and 1 atm).
The process liquid in the anaerobic lter was sampled onceevery other day shortly before feeding. It was analyzed for COD,VFAs and the content of sugar, alcohol, total inorganic carbon(alkaline buffer capacity) and ammonium nitrogen (NH4 N). Thecollected leachate from the acidognesis-leach-bed-reactors re-ceived the same analyses once a week. The content of organicdry matter (ODM), VFAs, sugar and alcohol in the maize silagewas evaluated on a weekly basis. ODM was assessed by specifyingthe ash content of dry samples in a mufe furnace at 550 C. CODwas determined with a specic COD analysis system (Hach LangeCompany), including pre-dosed reagents (LCK014, 100010,000 mg l1), a high temperature thermostat (HT 200 S) and asensor array photometer (LASA 20). The concentration of acetate,propionate, n- and iso-butyrate, n- and iso-valerate as well as capr-onate was measured with capillary column gas chromatography(Varian CP-3800). DL-lactic acid, formic acid, sucrose, glucose, fruc-
Fig. 1. Schematic diagram of the two-phase
Table 1Operating parameters of each run of the experiment.
Run Duration (days) Absolute pressure of anaerobic lter (bar)
Target value Daily mean Min. Max.
1 21 1 1.07 0.01 1.06 1.102 27 3 2.97 0.003 2.93 3.003 18 6 5.95 0.004 5.86 6.004 55 9 8.91 0.04 8.79 9.00y 116 (2014) 409415 411tose, ethanol and propylene glycol content were analyzed withhigh-performance liquid chromatography (Bischoff Company).Before gas chromatography and high-performance liquid chroma-tography analyses, maize silage was treated by solidliquidshake-ask extraction. Total inorganic carbon was measured witha titrator (785 DMP Titrino Metrohm Filderstadt). Ammoniumnitrogen was determined by distillation (Vapodest 50, GerhardtCompany) and back titration. All the acquired data were statisti-cally analyzed with ANOVA in SAS (v8.0).
3.1. Substrate characteristics
During the experiments, the maize silage was fed into the aci-dogenesis-leach-bed-reactors for leachate production. The col-lected leachate owed through a 100 lm lter and was used asan intermediate substrate for further acetogenesis and methano-genesis. Therefore, the leachate containing little solid was analyzedwith respect to COD concentration, while the fresh maize silagewas determined with its ODM content. The chemical characteris-tics of the maize silage and the leachate from the acidogenesis-leach-bed-reactors are shown in Table 2.
In spite of different analytical parameters (ODM and COD), itwas clearly observed that the leachate had much less organic frac-tion than the fresh maize silage. This means that a large part of or-ganic substance was still hidden in a solid form. Compared to theadded maize silage, the leachate contained a lower concentrationof acetic acid, DL-lactic acid, sugar and alcohol, but a higher con-tent of other VFAs (C3C6). Nevertheless, acetic acid, n-Butyric acid,and DL-lactic acid were dominant in the organic fraction of theleachate, which was ready for further degradation in the pressur-ized anaerobic lter.
pressurized anaerobic digestion system.
3.3. Biogas production of the anaerobic lter
Fig. 4 illustrates gas quantity under different working pressurelevels of the anaerobic lter. In this study, the specic methaneyield (SMY) and specic biogas production (SBP) represent thestandardized methane and biogas produced from one gram fedCOD, respectively. In contrast to the SMY, the SBP was more sensi-
to 0.31 0.02 lN g1CODadded.
Table 2Chemical characteristic of the substrates used for the pressurized two-phaseanaerobic digestion experiments.
Maize silage Leachate
ODM (g kg1) 312.6 12.7 /COD (g l1) / 23.2 0.9Acetic acid (g kg1) 9.1 0.2 2.7 0.4Propionic acid (g kg1) 0.03 0.01 0.4 0.04iso-Butyric acid (g kg1) 0 0.03 0.01n-Butyric acid (g kg1) 0.05 0.01 3.3 0.3iso-Valeric acid (g kg1) 0 0.05 0.01n-Valeric acid (g kg1) 0 0.1 0.02Capronic acid (g kg1) 0 0.4 0.1DL-lactic acid (g kg1) 16.1 0.9 2.7 0.4Fructose (g kg1) 1.3 0.2 0.1 0.01Ethanol (g kg1) 3.5 0.02 0.9 0.07Propylene glycol (g kg1) 4.5 0.2 0.1 0.04
Table 3Acid and ammonium concentration in the efuent of the anaerobic lter operated atdifferent pressure levels.
Pressure (bar) 1.07 0.01 2.97 0.003 5.95 0.004 8.91 0.04Acetic acid (g kg1) 0 0 0.02 0.008 0.06 0.03Propionic acid (g kg1) 0 0.003 0.003 0.01 0.007 0.02 0.01n-Butyric acid (g kg1) 0 0 0 0n-Valeric acid (g kg1) 0 0 0 0Capronic acid (g kg1) 0 0 0 0NH4 N (g kg
1) 0.66 0.01 0.71 0.01 0.73 0.01 0.74 0.02
412 Y. Chen et al. / Applied Energy 116 (2014) 4094153.2. Control parameters
Fig. 2 shows that the pH value of the anaerobic lter wasstrongly affected by the pressure. The daily average pH value fellfrom approximately 7.2 0.04 at 1.07 0.01 bar to 6.5 0.05 at8.91 0.04 bar. However, the pH reduction rate decreased as pres-sure increased.
The VFA concentrations in the anaerobic lter directly beforethe next feeding are summarized in Table 3. Almost no VFAsaccumulated in the anaerobic lter throughout the experiment.The acetic acid and propionic acid concentration were a littlehigher in the case of higher working pressure. A slight increasewas also observed in ammonium nitrogen concentration, from0.66 0.01 g kg-1 to 0.74 0.02 g kg1 when the pressure in-creased from 1.07 0.01 bar to 8.91 0.04 bar. Furthermore,neither ethanol nor propylene glycol was detected in the efuentfrom the anaerobic lter. The COD concentration was alwaysbelow 1.5 g l1. The COD degradation grade of the anaerobic l-ter remained more than 94% until reaching the operational pres-sure of 5.95 0.004 bar (Fig. 3). Although the difference in CODdegradation grade between 8.91 0.04 bar and the previouslytested pressures reached the statistical signicance, the averageCOD degradation grade at 8.91 0.04 bar was still above 93%.As a guide value for assessing the anaerobic digestion stability,the VFA/TIC ratio stayed between 0.1 and 0.2 at all pressurelevels.Fig. 2. pH Value measured in the anaerobic lter at different pressure levels. The boxmaximum value. The signicant differences among the pressure levels are marked withThe biogas composition was also inuenced by pressure varia-tion (Fig. 5). The methane content increased from 65% to 75%,while the carbon dioxide content decreased from 35% to 25%, aspressure rose from 1.07 0.01 bar to 8.91 0.04 bar. The CH4:CO2ratio in the collected biogas changed from 1.9 to 2.9. The methanewas enriched under higher pressure.
4.1. Effect of pressure on pH value
The pH value decreased considerably when pressure increased.Since Henrys Law can be used for modeling the solubility of CO2 inwater for pressure up to 100 bar , it can be explained in thisway: with the working pressure of the anaerobic lter rising, thetive to the variations of pressure. The rising pressure resulted in aremarkable decrease of the specic biogas production. The specicbiogas production at 2.97 0.003 bar, 5.95 0.004 bar and8.91 0.04 bar was reduced by 6%, 12% and 24%, respectively, incomparison with that at the atmospheric pressure. By contrast,the specic methane yield stayed in the same range (the averageSMY was 0.33 0.01 lN g-1CODadded) at a pressure of 1.07 0.01 to5.95 0.004 bar. The increase of the working pressure from5.95 0.004 bar to 8.91 0.04 bar led to a marginal drop of SMYplot is characterized with the minimum, low quartile, median, upper quartile anddifferent letters (p < 0.05, LSD test).
nergY. Chen et al. / Applied Epartial pressure of CO2 also increases. Based on Henrys Law, thesolubility of CO2 is directly proportional to the partial pressure,and thus it is also increased. For example, it was reported thatthe solubility of CO2 in water at 30 C was 13.5 g CO2 kg1 H2O at10 bar partial pressure of CO2, ten times as that at 1 bar at the sametemperature . The more CO2 that is dissolved, the more car-bonic acid (H2CO3) will be formed. Carbonic acid might dissociateto bicarbonate (HCO3 ) and carbonate (CO
23 ). When those protons
are liberated in the water, the pH value will be decreased. Since thepH value is dened as the decimal logarithm of the proton concen-tration, it does not vary linearly with the pressure. As shown in theresults, the pH value had a slower downward trend as the pressureincreased.
4.2. Anaerobic lter process stability under pressure
Throughout the experiment period, there was very little acidaccumulation detected in the reactor. At 8.91 bar, prior to the nextfeeding, there was only 0.06 g kg1 acetic acid and 0.02 g kg1 pro-pionic acid in the anaerobic lter, much lower than the critical
Fig. 3. COD degradation grade at different pressure levels. The box plot is characterizedsignicant differences among the pressure levels are marked with different letters (p < 0
Fig. 4. Specic biogas production and specic methane yield of the anaerobic lter at diffmedian, upper quartile and maximum value. The signicant differences among the presy 116 (2014) 409415 413threshold level (acetic acid: 1 g kg1 and propionic acid:0.25 g kg1) . The VFA/TIC ratio was also well below the processinstability threshold value which is reported to be 0.5 . Thisindicated that the pressurized anaerobic lter was constantly sta-ble. Moreover, it can be speculated that the pressurized anaerobiclter has potential to deal with an even higher organic loading rate(more than 5 kgCODm3 d1) under pressure of 8.91 bar. The COD-degradation grade of higher than 93% further veried that theanaerobic digestion in the pressurized anaerobic lter accom-plished in an efcient way.
It is widely accepted that the optimal pH value for methanogen-esis should be between 6.8 and 7.4 . At a lower pH value, or-ganic acids are prone to be in an undissociated state. Since acidsin uncharged form more easily penetrate the microbial cell wall,too much undissociated acids accumulated in the digester will ex-ert a strong toxic effect [2,23]. Therefore, pH value is directly re-lated to the process stability of the anaerobic digestion underatmospheric pressure. High acid concentration and even fermentersouring generally accompany an anaerobic digester with low pHvalue. However, the results in this study seem contradictory to
with the minimum, low quartile, median, upper quartile and maximum value. The.05, LSD test).
erent pressure levels. The box plot is characterized with the minimum, low quartile,sure levels are marked with different letters (p < 0.05, LSD test).
nergthe common phenomena. Almost all the fatty acids could be de-graded in time and few acids accumulated in the reactor. Thelow pH value of the pressurized anaerobic lter was exclusivelycaused by the dilution of CO2, instead of organic acid accumulation.Even at a pH value of 6.5, the COD-degradation grade remained at ahigh level of 93%. This might imply that the pH value itself seemsto have less effect on the methanogens than high organic acidconcentration.
Moreover, the reduction of pH value induced by pressure in-crease favors the recombination of protons and anions in the solu-tion. At a low-pH value, the ammonia nitrogen appears more indissociated form in the digester. Vavilin, et al.  have reportedthat ammonium nitrogen tended to increase with pressure rising.
Fig. 5. Methane content and carbon dioxide content measured at different pressurequartile and maximum value. The signicant differences among the pressure levels414 Y. Chen et al. / Applied EIn this study, the upward tendency of ammonium nitrogen wasnot signicant, especially at a pressure of more than 2.97 bar. Be-cause the pH value of the reactor fell to 6.56.8, and almost allammonia nitrogen turns into ammonium nitrogen when pH valueis below 7.0 . Therefore, the ammonium nitrogen concentrationin the anaerobic lter at a pressure of 2.978.91 bar was at the samerange (0.710.74 g kg1), and represents the total ammonia nitrogen(TAN) concentration. Compared with the reported inhibitory TANconcentration (1.714 g l1) , the values in this study were quitelow. Since the inhibition concentration of ammonia to methanogensis higher in undissociated form , the pressurized anaerobic ltercan reduce the risk of ammonia inhibition to some extent.
4.3. Effect of pressure on biogas production
Throughout the experiment, the average SMY of the pressurizedanaerobic lter ranged from 0.31 to 0.33 lN g1CODadded, whichcorresponds with other studies [27,28]. Although the SMY at apressure of 8.91 bar was only a little lower than at other pressures,statistically it was a signicant difference. There are two possiblereasons for that. The rst possibility is that there was a smallamount of acids accumulated in the anaerobic lter at the pressureof 8.91 bar, which indicated that the acids were not completelyconverted into methane at this pressure level. The second possibil-ity is that the methane solubility changed with pressure. Since thepressure dependency of Henry coefcient of methane in the lowpressure range (110 bar) is negligible , according to HenrysLaw, with pressure rising from 1.07 bar to 8.91 bar, the concentra-tion of methane increases correspondingly about nine fold, from0.016 g CH4 kg1 H2O to 0.15 g CH4 kg1 H2O. The calculation isbased on the Henry coefcient given by Wilhelm et al. .Although the CH4 concentration is fairly low in terms of the abso-lute value, it can still be inferred that more produced CH4 tends toremain in the solution at higher pressure. As a result, the SMY wasreduced a little at 8.91 bar.
As previously mentioned, compared with CH4, CO2 dissolvesmuch more readily in water. Due to this signicant difference in sol-ubility between the two gases under pressure, the biogas quality isalso affected. During anaerobic digestion, the gases are producedin the liquid. Once the saturation limit is reached, gases escape from
els. The box plot is characterized with the minimum, low quartile, median, uppermarked with different letters (p < 0.05, LSD test).y 116 (2014) 409415the liquid in the form of gas bubbles and rise into the gas phase.With more CO2 remaining in the solution at higher pressure, lessCO2 enters the gas phase. Thus, the specic biogas production is re-duced with increased pressure and CH4 tends to be dominant in thebiogas. As shown in this study, the CH4 content was signicantly in-creased. A similar observation was also documented in other studies[9,30]. Nevertheless, the CH4 content in this study was lower thanthat in Hayes et al.s research, in which a 93% methane gas was pro-duced . The primary cause of the difference lies in the varied pHvalue. In Hayes et al.s experiment, the pH value of the anaerobic l-ter was maintained at 7.5, while the anaerobic lter in this studywas operated with the pH value down to 6.5. Like pressure, thepH value also inuences the amount of dissolved CO2. More CO2tends to be dissolved in water as pH rises. Wonneberger et al. ex-pressed the relation between the Henry coefcient of CO2 and pHvalue in a formula . With the proper dissociation constant ofCO2  and pH value substituted, the Henry coefcient, as wellas the solubility of CO2 can be calculated. It is found that at the samepressure, the solubility of CO2 at a pH value of 7.5 is about seventimes as much as that at a pH value of 6.5. Therefore, the methaneenrichment in that study was more apparent.
The study has examined the pressure effects on the anaerobiclter in a two-phase anaerobic digestion, in terms of pH value, bio-gas production and process stability. The results presented in this
study demonstrate that an increase in pressure leads to a decrease
weakens the methane enrichment. Therefore, an effective and eco-
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Role of the funding source
This study was nanced by the German Ministry of Educationand Research (BMBF). The funding source was not involved instudy design, the collection, analysis and interpretation of data,the writing of the report or the decision to submit the article forpublication.
The authors gratefully acknowledge the nancial support of thework by the German Ministry of Education and Research withinthe joint research project B2G.Submission declaration and verication
We declare that the work described in the article has not beenpreviously published, and it is not under consideration for publica-tion elsewhere. Its publication is approved by all authors and tac-itly or explicitly by the responsible authorities where the work wasConict of interest
We declare that we have no nancial and personal relationshipswith other people or organizations that can inappropriately inu-ence our work; there is no professional or other personal interestof any nature or kind in any product, service and/or company thatcould be construed as inuencing the position presented in, or thenomical method for maintaining pH value in the pressurized reac-tor should be developed and further veried.
Policy and ethics
We declare that the work described in the article has been car-ried out in accordance with the Code of Ethics of theWorld MedicalAssociation (Declaration of Helsinki) for experiments involving hu-mans; EU Directive 2010/63/EU for animal experiments; Uniformfor biogas purication and upgrading. In addition, the compressedbiogas can be injected into the public gas grid without additionalpressurization. In light of the good process stability, the two-phasepressurized anaerobic digestion might allow operation under evenhigher pressure (more than 9 bar), as well as higher organic load-ing rate (more than 5 kgCODm3 d1). Although low pH value in-duced by rising pressure might not cease the anaerobic digestion, itcounteracts the pressure effect on CO2-solubility, and thus partlyin pH value in the reactor and methane enrichment in the biogas.Based on the COD-degradation grade, the concentration of VFAsand the specic methane yield, the pressurized anaerobic lter inthis study was characterized by a stable performance with workingpressure up to 8.91 bar, despite the low pH value. The increasingmethane content in the raw biogas can largely reduce the costReferences
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The pressure effects on two-phase anaerobic digestion1 Introduction2 Materials and methods2.1 Reactors2.2 Experiment procedure2.3 Analytical methods and data acquisition
3 Results3.1 Substrate characteristics3.2 Control parameters3.3 Biogas production of the anaerobic filter
4 Discussion4.1 Effect of pressure on pH value4.2 Anaerobic filter process stability under pressure4.3 Effect of pressure on biogas production
5 ConclusionPolicy and ethicsConflict of interestSubmission declaration and verificationRole of the funding sourceAcknowledgementReferences