The pressure effects on two-phase anaerobic digestion

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<ul><li><p>ob</p><p>, A</p><p>bDVGW Research Center at the Engler-Bunte-Institute, Karlsruhe Institute of Technology (KIT), Engler-Bunte-Ring 1, D-76131 Karlsruhe, Germany</p><p>h i g h l i g h t s</p><p>tion upe in thecontenmethanery sta</p><p>c lter. Four different pressure levels (the absolute pressure of 1 bar, 3 bar, 6 bar</p><p>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</p><p>ce it temutilizatio</p><p>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 [3]. Nevertheless, biogaspurication and upgrading is usually energy demanding [4]. 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.</p><p>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.</p><p>E-mail addresses: (Y. Chen), (B. Rler), (S. Zielonka), (A. Lemmer), (A.-M. Wonneberger), Thomas.Jungbluth@uni-hohen-</p><p>Applied Energy 116 (2014) 409415</p><p>Contents lists availab</p><p>Applied</p><p>journal homepage: (T. Jungbluth).wastes and energy crops can be degraded and converted into bio-gas, mainly containing CH4 and CO2 [1]. The most common practice</p><p>CO2-removal, drying and pressurization [2]. Sinand spatially separates biogas production from0306-2619/$ - see front matter 2013 Elsevier Ltd. All rights reserved., theaddi-re canhas re-Anaerobic digestionPressureTwo-phaseSubstitute natural gasBiogasBiomethane</p><p>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.</p><p> 2013 Elsevier Ltd. All rights reserved.</p><p>1. Introduction</p><p>Anaerobic digestion is a biological process, in which organic</p><p>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</p><p>a r t i c l e i n f o</p><p>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.</p><p>a b s t r a c t</p><p>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</p><p>a State Institute of Agricultural Engineering and Bioenergy, University Hohenheim, Garbenstrae 9, D-70599 Stuttgart, GermanyThe pressure effects on two-phase anaer</p><p>Yuling Chen a,, Benjamin Rler a, Simon Zielonka a</p><p>ic digestion</p><p>ndreas Lemmer a, Anna-Maria Wonneberger b,</p><p>le at ScienceDirect</p><p>Energy</p><p>vier .com/locate /apenergy</p></li><li><p>a variable, in actual application. In order to improve anaerobicdigestion performance, efforts are primarily focused on the optimi-</p><p>nergzation of more adjustable and controllable parameters, and as a re-sult, the investigation of pressure effects on anaerobic digestionhas been overlooked.</p><p>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 [11], in landlls [12] and at the lower partof anaerobic digestion towers or biogas tower reactors [13] 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 [14]. Bothpiezotolerant and piezophilic microbes are bacteria that are ableto grow and proliferate up to a pressure of 1000 bar [15]. The opti-mal growth rate of piezotolerant microorganisms, however, occursat atmospheric pressure [16]. 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 [16]. This offered a theoreticalbasis for the experiment of anaerobic digestion under pressure.</p><p>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.</p><p>2. Materials and methods</p><p>2.1. Reactors</p><p>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. [17]. 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 [58], 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 [9].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) [10]. Compared toother operational parameters, pressure is a constant, rather than</p><p>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.</p><p>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</p><p>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.</p><p>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).</p><p>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.</p><p>2.2. Experiment procedure</p><p>The anaerobic lter was seeded with the efuent from anotherxed-bed anaerobic reactor, which had been fed with leachate ofgrass silage [18]. 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.</p><p>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.</p><p>2.3. Analytical methods and data acquisition</p><p>y 116 (2014) 409415In this study, pH, pressure and temperature of the anaerobic l-ter were monitored in real-time (pH-sensor: Endress + Hauser</p></li><li><p>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).</p><p>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-</p><p>Fig. 1. Schematic diagram of the two-phase</p><p>Table 1Operating parameters of each run of the experiment.</p><p>Run Duration (days) Absolute pressure of anaerobic lter (bar)</p><p>Target value Daily mean Min. Max.</p><p>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).</p><p>3. Results</p><p>3.1. Substrate characteristics</p><p>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...</p></li></ul>