Anaerobic digestion of glycerol derived from biodiesel manufacturing

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<ul><li><p>ro</p><p>S</p><p>rsida</p><p>Kinetic constants</p><p>glycbatco thiculf glsinwas</p><p>phenomenon was observed at the highest load. 2009 Elsevier Ltd. All rights reserved.</p><p>e useand thl., 200comp</p><p>ble ene</p><p>matic growth of the biodiesel industry has created a surplus of glyc-erol that has resulted in a dramatic 10-fold decrease in crudeglycerol prices over the last few years and generated environmentalconcerns associated with contaminated glycerol disposal (Yazdaniand Gonzalez, 2007; Sandun et al., 2007).</p><p>At present, due to its properties, pure glycerol has more than2000 different applications (Elvers et al., 1990). However, the pro-</p><p>properties for use in fuel or solvents (Karinen and Krause, 2006). The microbial conversion (fermentation) of glycerol to 1,3-pro-</p><p>panediol, which can be used as a basic ingredient of polyesters(Barbirato et al., 1998; Ito et al., 2005).</p><p> Other products such as butanol (Biebl, 2001), propionic acid(Bories et al., 2004), ethanol and formate (Jarvis et al., 1997),succinic acid (Lee et al., 2001), dihydroxyacetone (Bories et al.,1991; Claret et al., 1994), polyhydroxyalkanoates (Koller et al.,2005), or hydrogen and ethanol (Ito et al., 2005) were alsoobtained using glycerol as a carbon source.</p><p>* Corresponding author. Tel.: +34 957 218586; fax: +34 957 218625.</p><p>Bioresource Technology 100 (2009) 56095615</p><p>Contents lists availab</p><p>T</p><p>elsE-mail address: a92siloj@uco.es (J.. Siles Lpez).ful emissions and non-toxic fuel, these have drawn much attentionrecently (Ito et al., 2005).</p><p>During the biodiesel production process, oils/fats (triglycerides)aremixedwithmethyl alcohol and alkaline catalysts to produce freefatty-acid esters, with glycerol as a primary by-product (Chi et al.,2007). The production of 100 kg of biodiesel yields approximately10 kg of impure glycerol, with 5590% glycerol (Hazimah et al.,2003). Glycerol is generated as a by-product not only when biodie-sel fuels are produced chemically, but also when they are manufac-tured enzymatically (Du et al., 2003; Vicente et al., 2004) and duringthe productionof bioethanol (Yazdani andGonzalez, 2007). The dra-</p><p>posed as a solution for the economic viability of this product. Severalstrategies based on chemical and biological transformations arebeing pursued to convert glycerol intomore valuable products (Yaz-dani and Gonzalez, 2007). An example of some of these includes:</p><p> The conversion of glycerol into propylene glycol and acetone,through thermo-chemical processes (Chiu et al., 2006; Dasariet al., 2005).</p><p> The etherication of glycerol with either alcohols (e.g. methanolor ethanol) or alkenes (e.g. isobutene) and the production ofoxygen-containing components, which could have suitableBiodegradability</p><p>1. Introduction</p><p>Concern has recently risen over thcost, their sustained availabilitywarming and pollution (Hansen et ahave various advantages such as abased fuel, renewable fuel, a favoura0960-8524/$ - see front matter 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.06.017of fossil resources, theireir impact on global5). Since biodiesel fuelslement to petroleum-rgy balance, less harm-</p><p>duction of crude glycerol exceeds the present commercial demandfor puried glycerol. Furthermore, the purication of glycerol (witha view to being sold) that is generated during biodieselmanufactur-ing is not a viable option for the biodiesel industry (Chi et al., 2007).Although crude glycerol can be burnt, with the consequent ener-getic advantages, the setting up of bioreneries that co-produceproducts of higher economic value alongwith biofuels has beenpro-Glycerol-containing wasteAnaerobic digestion</p><p>option for revalorising this available, impure and low priced by-product derived from the surplus of bio-diesel companies. The organic loading rate studied was 0.210.38 g COD/g VSS d, although an inhibitionAnaerobic digestion of glycerol derived f</p><p>Jos ngel Siles Lpez *, Mara de los ngeles MartnAntonio Martn MartnDepartamento de Qumica Inorgnica e Ingeniera Qumica, Facultad de Ciencias, Univekm 396, 14071 Crdoba, Spain</p><p>a r t i c l e i n f o</p><p>Article history:Received 6 March 2009Received in revised form 1 June 2009Accepted 6 June 2009Available online 9 July 2009</p><p>Keywords:Biodiesel manufacturing</p><p>a b s t r a c t</p><p>The anaerobic digestion of1010 g/kg, was studied innon-granular sludge. Due tthis alkaline catalyst as agrically viable, a volume orevalorisation of glycerol umethane yield coefcient</p><p>Bioresource</p><p>journal homepage: www.ll rights reserved.m biodiesel manufacturing</p><p>antos, Arturo Francisco Chica Prez,</p><p>d de Crdoba, Campus Universitario de Rabanales, Edicio C-3, Ctra. Madrid-Cdiz,</p><p>erol derived from biodiesel manufacturing, in which COD was found to beh laboratory-scale reactors at mesophilic temperature using granular ande high KOH concentration of this by-product, H3PO4 was added to recovertural fertilizer (potassium phosphates). Although it would not be econom-ycerol was distilled and utilised as reference substrate. The anaerobicg granular sludge achieved a biodegradability of around 100%, while the0.306 m3 CH4/kg acidied glycerol. Anaerobic digestion could be a good</p><p>le at ScienceDirect</p><p>echnology</p><p>evier .com/locate /bior tech</p></li><li><p>TecAnaerobic digestion is another possible way to revalorise abun-dant and low-priced glycerol streams. This process may be denedas the biological conversion of organic material to a variety of endproducts including biogas whose main constituents are methaneand carbon dioxide (Gujer and Zehnder, 1983; Olthof andOleszkiewick, 1982; Speece, 1983; Wheatley, 1990). The advanta-ges of anaerobic digestion include low levels of biological sludge,low nutrient requirements, high efciency and the production ofmethane, which can be used as an energy source. The stoichiome-try of the anaerobic digestion of glycerol can be summarised as fol-lows (Christensen and McCarty, 1975; McCarty, 1975):</p><p>C3H8O3 aNH3 ! bCH4 cCO2 dC5H7NO2 eNH4HCO3The products of the reaction are methane, carbon dioxide, bio-</p><p>mass and ammonic bicarbonate, where a, b, c, d and e are stoichi-ometric coefcients. In order to obtain the value of thesecoefcients and determine the methane yield of the anaerobicdigestion of glycerol, a mass balance was made taking into consid-eration a biomass yield of anaerobic bacteria of 0.05 (w/w). Thevalues of a, b, c, d and e were found to be 0.663, 1.648, 0.526,0.041 and 0.622 mol, respectively. In contrast, when an electronbalance was carried out assuming that every electron was usedin methane generation, b reached a value of 1.750. A simple calcu-lation showed that the theoretical methane yield is 94.2%, which isa useful compound due to its caloric power (Lower Caloric Power):35,793 kJ/m3, equivalent to 9.96 kW h/m3. Revalorising glycerol isof special interest as it gives the highest reduced carbon with thecost advantage of anaerobic processes. The aim of this work was</p><p>Nomenclature</p><p>Alk alkalinity (mg CaCO3/L)CODremoved removed chemical oxygen demand (mg/L)CODsoluble soluble chemical oxygen demand (mg/L)COD STO added chemical oxygen demand (mg/L)CODtotal total chemical oxygen demand (mg/L)G cumulative methane volume (mL)Gm cumulative methane volume at innite time (mL)GT experimental maximum methane volume (mL)KG specic methane production kinetic constant (L/g VSS h)K 0G apparent kinetic constant (1/h)MS total mineral solids (mg/L)MSS mineral suspended solids (mg/L)OLR organic loading rate (g COD/g VSS d; kg COD/m3 d)</p><p>5610 J.. Siles Lpez et al. / Bioresourceto evaluate the performance and the stability of anaerobic diges-tion process of glycerol-containing waste derived from biodieselmanufacturing. The study was carried out in six batch labora-tory-scale reactors at mesophilic temperature (35 C).</p><p>2. Methods</p><p>2.1. Experimental set-up</p><p>The experimental set-up used for the anaerobic digestion of thebiodiesel-derived glycerol consisted of six 1-L Pyrex reactors withfour connections to allow for the loading of feedstock, the ventila-tion of the biogas, the injection of inert gas (nitrogen) to maintainthe anaerobic conditions and the removal of efuent. The contentof the reactors was magnetically stirred and temperature wasmaintained by means of a thermostatic jacket containing waterat 37 C. The volume of methane produced during the processwas measured by using 1-L BoyleMariotte reservoirs connectedto each reactor. To remove the CO2 produced during the process,tightly closed bubblers containing a NaOH solution (6 N) were con-nected between the two elements. The methane volume displacedan equal measurable volume of water from the reservoir.</p><p>The reactors were inoculated with methanogenically-activegranular biomass obtained from a full-scale anaerobic reactor usedto treat brewery wastewater from the Heineken S.A. Factory (Jaen,Spain) and non-granular sludge from a full-scale anaerobic reactorused to treat urban wastewater in Jerez de la Frontera (Cadiz,Spain). The granular sludge contained 37,500 mg VSS/L and31,875 mg MSS/L, while the non-granular sludge contained28,400 mg VSS/L and 20,330 MSS/L. The inocula were selected onthe basis of their high methanogenic activity (Field et al., 1988)with values ranging from 0.87 to 0.99 g COD/g VSS d.</p><p>2.2. Glycerol</p><p>The raw material used as substrate was the glycerol-containingwaste discharged after the biodiesel manufacturing process at theBIDA S.A. Factory in Fuentes de Andalucia (Seville, Spain). In general,thiswaste contained glycerol, water,methanol, salts and fatty acids.</p><p>2.3. Raw material pre-treatment</p><p>The substrate was previously treated in two different ways: (a)acidication with phosphoric acid and centrifugation in order to re-cover the catalyst used in the transesterication reaction (KOH) asagricultural fertilizer (potassium phosphates). Additionally, metha-nol and water were removed by vacuum distillation. We call thissubstrate acidied glycerol. (b) Acidication followed by distilla-</p><p>r0 specic rate of methane production (mL CH4/g VSS h)Sremoved removed substrate (g COD)STO added substrate (g COD)t time (h)TS total solids (mg/L)TSS total suspended solids (mg/L)VA volatile acidity (mg acetic/L)VS total volatile solids (mg/L)VSS volatile suspended solids (mg/L)X biomass concentration (mg VSS/L)YCH4=Sremoved methane yield coefcient (mL CH4/g COD removed)</p><p>hnology 100 (2009) 56095615tion (rectication). After the same acidication process, a rectica-tion at laboratory-scale (135140 C; 1.62.0 103 atm) wascarried out. Subsequently, the organic impurities in the distillatewere removed by liquid/liquid extraction with hexane, which waseliminated by vacuum distillation. We call this substrate distilledglycerol. Table 1 shows the characteristics and features of acidied,distilled and commercially available pure glycerol (Elvers et al.,1990). Due to its high COD, the acidied and distilled glycerol werediluted using distilled water until reaching 81.6 and 85.7 g COD/L,respectively, and neutralized by adding sodium hydroxide. Finally,several nutrients and alkalinity (NaHCO3) were added to the dis-tilled glycerol to provide the necessary nutrients for the appropriatemetabolism of the anaerobic microorganisms (DiStefano andAmbulkar, 2006). Table 2 shows the composition of the nutrientand trace element solutions added to the distilled glycerol.</p><p>2.4. Anaerobic digesters. Experimental procedure</p><p>The anaerobic reactors were initially loaded with 12 g VSS ofgranular sludge as inoculum, and the anaerobic digestion of</p></li><li><p>2.6. Software</p><p>Sigma-Plot software (version 9.0) was used to create graphs,perform the statistical analysis and t the experimental data pre-sented in this work.</p><p>Technology 100 (2009) 56095615 5611acidied and distilled glycerol was studied. Another set of reactorswere then loaded with non-granular sludge and the anaerobicdigestion of acidied glycerol was studied. In all cases, the nutrientand trace element solutions described by Fannin (1987) and Fieldet al. (1988) were added when the sludge was loaded. Bothsolutions are very important for activating bacterial growth andmetabolism at the beginning of the process.</p><p>In order to activate the biomass prior to the experiments, thereactors were rst fed with a synthetic solution composed of glu-</p><p>Table 1Composition and features of the acidied, distilled and pure glycerol.</p><p>Parameter Acidiedglycerol</p><p>Distilledglycerol</p><p>Pureglycerol</p><p>Density at 20 C (g/mL) 1.044 1.260 1.261Refraction index at 20 C 1.4440 1.4728 1.4746COD (g/kg) 1010 1155 1217Dynamic viscosity at 50 C (mPa s) 57 150 152Colour Brown Colourless Colourless</p><p>Table 2Composition of the nutrient and trace element solutions added to distilled glycerol.</p><p>Nutrient solution Trace element solution</p><p>Compound g/La Compound g/L</p><p>NH4CI 0.200 MnCl24H2O 0.100K2HPO43H2O 0.100 CoCl26H2O 0.170KH2PO4 0.055 ZnCl2 0.100MgCl26H2O 0.200 CaCl2 0.200Resazurine 0.001 H3BO4 0.019FeCl24H2O 0.100 NiCl26H2O 0.050NaS9H2O 0.500 Na2MoO4H2O 0.100NaHCO3 5.000 Ad 10 mL of trace element solution per litre of diluted</p><p>glycerol</p><p>a Concentration of each compound per litre of diluted glycerol.</p><p>J.. Siles Lpez et al. / Bioresourcecose, sodium acetate and lactic acid (GAL solution) at concentra-tions of 50 g/L, 25 g/L and 20.8 mL/L, respectively. During thisinitial period, the organic load added to the reactors was graduallyincreased from 0.25 to 1.00 g COD over a 16-day period. After thisprevious stage, biomass acclimatization was carried out. The reac-tors were fed with 1 g COD, in which the percentage of glycerolused in the COD was increased from 25% to 100% after four loads.During this acclimatization period, the volume of methane wasmeasured as a function of time. The maximum duration of each as-say was 48 h; the time interval required for the complete biometh-anization of each load. Once this preliminary acclimatization stepwas nished, a series of batch experiments were carried out usingboth sludge types in addition to the acidied and distilled glycerolas substrates. During each set of experiments, the organic loadadded to the reactors was gradually increased from 1.0 to 1.5and 2.0 g COD with distilled glycerol and from 1.0 to 1.5, 2.0 and3.0 g COD with acidied glycerol. In all cases, the volume of meth-ane was measured as a function of time and samples were takenand analysed before and after feeding. The duration of each exper-iment was equal to the time interval required for maximum gasproduction and COD removal. Each glycerol solution load was car-ried out at least in duplicate and the results expressed as means.</p><p>2.5. Chemical analyses</p><p>The following parameters were determined in the efuents ofeach load: pH, COD total, COD soluble, TS, MS, VS, TSS, MSS, VSS,volatile acidity (VA) and alkalinity (Alk). All analyses were carriedout in accordance with the Standard Methods of the APHA (APHA,1989).3. Results and discussion</p><p>3.1. Stability</p><p>The stability of the process is evaluated based on the evolutionof the pH, alkalinity, volatile acidity and volatile acidity/alkalinityratio (VA/Alk) during the anaerobic digestion process of the differ-ent substrates. Table 3 shows the mean value and standard devia-tion of the pH and vol...</p></li></ul>