Anaerobic co-digestion of the organic fraction of municipal solid waste with several pure organic co-substrates

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<ul><li><p>oro</p><p>scola dEnginyeria, Universitat Auto`noma de Barcelona, Bellaterra</p><p>a, Spai</p><p>a r t i c l e i n f o</p><p>Article history:</p><p>Received 15 July 2010</p><p>Received in revised form</p><p>23 December 2010</p><p>Accepted 14 January 2011</p><p>(European Commission, 2001; Lema &amp; Omil, 2001; Lettinga,</p><p>2001; McCarty, 2001; Murto, Bjornsson, &amp; Mattiasson, (2004);</p><p>Neves, Oliveira, &amp; Alves, 2006, 2009; Pain, Phillips, &amp; West,</p><p>1988). In addition, the Landfill Directive 1999/31/EC (article 5)</p><p>together with the Working Document on Biological Treatment</p><p>very challenging targets for the reduction of biodegradable</p><p>(IOW).</p><p>However, there are some poorly biodegradable IOW that</p><p>cannot be digested alone due to their characteristics, for</p><p>example, low solubility or unbalanced carbon to nitrogen (C/N)</p><p>ratio. Nevertheless, when mixed with other complementary</p><p>* Corresponding author. Tel.: 34 93 5811019; fax: 34 93 5812013.</p><p>Avai lab le at www.sc iencedi rect .com</p><p>vi</p><p>b i o s y s t em s e n g i n e e r i n g 1 0 8 ( 2 0 1 1 ) 3 5 2e3 6 0E-mail address: antoni.sanchez@uab.cat (A. Sanchez).Biogas technologies (EuropeanCommission, 2009, 2006; Jansen,</p><p>2003; Swedish Institute of Agricultural and Environmental</p><p>Engineering, 2006) are an attractive and well established alter-</p><p>native that allows the production of energy while processing</p><p>different organic wastes or biomass and obtaining a solid</p><p>product that can be used as an organic fertiliser or conditioner</p><p>waste to landfill some years ago. In consequence, at present</p><p>there are a large number of anaerobic digestion facilities</p><p>treating different kinds of organic wastes such as municipal</p><p>solid waste (MSW), source-separated organic fraction of mun-</p><p>icipal solid waste (OFMSW, mainly food and yard wastes),</p><p>manure, sewage sludge and different industrial organic wastesthe OFMSW digestion.</p><p> 2011 IAgrE. Published by Elsevier Ltd. All rights reserved.</p><p>1. Introduction of Biowaste 2nddraft (European Commission, 2001) introducedPublished online 16 February 20111537-5110/$ e see front matter 2011 IAgrEdoi:10.1016/j.biosystemseng.2011.01.007A strategy to improve the operation of working anaerobic digesters treating the organic</p><p>fraction of municipal solid wastes (OFMSW) to increase the biogas production is studied. It</p><p>consists of increasing the organic loading rate of the digesters by adding extra organic</p><p>matter from some problematic organic wastes. Vegetable oil (VO), animal fats (AF), cellu-</p><p>lose and protein (protein) were used as pure co-substrates and the co-digestion anaerobic</p><p>process was analysed in terms of the ultimate methane production, the methane</p><p>production rate and the hydraulic residence time. The analysis of methane or biogas</p><p>production led to different conclusions when expressing this parameter on a volatile solids</p><p>basis or on a reactor volume basis. The need for a combined analysis is highlighted. In</p><p>addition a new model to predict the biodegradability rate and evaluating the organic</p><p>matter fraction susceptible to biodegradation was developed and proved to be suitable for</p><p>assessing anaerobic digestion processes. All four co-substrates used led to some operative</p><p>improvements. Vegetable oil is the most suitable co-substrate to be anaerobically digested</p><p>with the OFMSW since all the parameters evaluated were greatly improved compared toCerdanyola del Valle`s, 08193-Barcelon nComposting Research Group, Department of Chemical Engineering, ESergio Ponsa, Teresa Gea, Antoni Sanchez*Research Paper</p><p>Anaerobic co-digestion of thesolid waste with several pure</p><p>journa l homepage : www.e lse. Published by Elsevier Ltganic fraction of municipalrganic co-substrates</p><p>er . com/ locate / i ssn /15375110d. All rights reserved.</p></li><li><p>b i o s y s t em s e ng i n e e r i n g 1 0 8 ( 2 0 1 1 ) 3 5 2e3 6 0 353wastes, the mixture becomes suitable for anaerobic co-diges-</p><p>tion (Alatriste-Mondragon et al., 2006). There is abundant</p><p>Nomenclature</p><p>BMP biological methane potential, l [CH4] kg1 [VS]</p><p>(normal conditions)</p><p>BOC biodegradable organic carbon, % db (dry basis) or</p><p>TOC (g) basis</p><p>C carbon</p><p>C/N carbon to nitrogen ratio</p><p>CW cellulose wastes</p><p>VO vegetable oil</p><p>AF animal fat</p><p>CH4 methane</p><p>CI inert fraction, %</p><p>CO2 carbon dioxide</p><p>CR rapidly mineralisable fraction, %</p><p>CS slowly mineralisable fraction, %</p><p>CW remaining carbon in the sample, %</p><p>DM dry matter, %</p><p>GB21 cumulative biogas production in 21 days, l [biogas]</p><p>kg1 [TS] (normal conditions)GBn cumulative biogas production in n days, l [biogas]</p><p>kg1 [TS] (normal conditions)</p><p>HRT hydraulic retention time, day</p><p>kR rapid rate constants, day1</p><p>kS slow rate constants, day1</p><p>LCFA long-chain fatty acidsliterature about co-digestion processes, such as co-digestion</p><p>of the OFMSW and agricultural residues (Converti, Drago,</p><p>Ghiazza, Borghi, &amp; Macchiavello, 1997; Kubler et al., 2000),</p><p>organic wastes and sewage sludge (Fernandez, Sanchez, &amp;</p><p>Font, 2005; Neves et al., 2009; Zhang et al., 2008) or more</p><p>specific wastes (Bouallagui, Lahdheb, Ben Romdan, Rachdi, &amp;</p><p>Hamdi, 2009; Buenda, Fernandez, Villasenor, &amp; Rodrguez,</p><p>2009).</p><p>A new strategy to be implemented in already operating</p><p>plants could be the combined treatment of different kinds of</p><p>IOWwith OFMSW. Thus, on the one hand, a new waste would</p><p>acquire value for use in working digesters, whilst avoiding the</p><p>need for new investment. On the other hand, energy would be</p><p>obtained. The most common problematic organic wastes</p><p>produced by industry or municipalities are those that are rich</p><p>in lipids, cellulose and proteins.</p><p>Lipids are characterised either as fats, oils or greases,</p><p>coming mainly from food wastes and some industrial waste-</p><p>waters, such as those from slaughterhouses, dairies or fat</p><p>refineries (Li, Sasaki, Yamashita, Seki, &amp; Kamigochi, 2002).</p><p>Lipids are attractive for biogas production since they are</p><p>reduced organic materials with a high theoretical methane</p><p>potential. However, they also present several problems such</p><p>as inhibition of methanogenic bacteria and adsorption onto</p><p>biomass that can cause sludge flotation and washout (Neves</p><p>et al., 2009).</p><p>Cellulose wastes (CW) come mainly from paper and card-</p><p>board industries (mostly composed of cellulose, hemicellulose</p><p>and lignin) or from textile industries. CW are also part of the</p><p>bulk MSW that is not source-separated, which could be addedto the organic waste flow again in order to be anaerobically</p><p>treated. CWhave aC/N ratio ranging from173:1 to greater than</p><p>M Cumulative BMP, l [CH4] kg1 [VS] (normal</p><p>conditions)</p><p>MSW municipal solid waste</p><p>N nitrogen</p><p>OFMSW organic fraction of municipal solid waste</p><p>OM organic matter content, %, db</p><p>IOW industrial organic wastes</p><p>P maximum methane potential, l [CH4] kg1 [VS]</p><p>(normal conditions)</p><p>P0 specific maximum methane potential, l [CH4] l1</p><p>[reactor] (normal conditions)</p><p>Rmax maximum methane production rate, l [CH4] kg1</p><p>[VS] day1 (normal conditions)Rmax</p><p>0specific maximum methane production rate, l</p><p>[CH4] l1 day1</p><p>T time, hour or day</p><p>TOC Total organic carbon, %, db</p><p>UMP ultimate methane production, l [CH4] kg1</p><p>[VS]loadedUMP0 Specific methane production, l [CH4] l</p><p>1 [reactor](normal conditions)</p><p>VS volatile solids, %, db</p><p>VSloaded loaded volatile solids (considering only substrates</p><p>but not VS corresponding to inoculum), %, db</p><p>l lag phase, day1000:1, while the suggested optimum C/N ratio for anaerobic</p><p>digestion is in the range of 20:1 to 30:1 (Zhang et al., 2008).</p><p>Therefore, mixing CW with the OFMSW can provide suitable</p><p>nutrients for the combined co-digestion (Zhang et al., 2008).</p><p>Wastes with high protein content and consequent high</p><p>nitrogen content come mainly from meat industries, slaugh-</p><p>terhouses and farms (slurry and manure). Since these wastes</p><p>have a high organic content, high biological oxygen demand</p><p>and low C/N ratio, anaerobic co-digestion with the OFMSW or</p><p>sludge is recommended for their treatment (Buenda et al.,</p><p>2009). In addition, large ammonia concentrations in animal</p><p>wastes are found to inhibit anaerobic treatments (Nielsen &amp;</p><p>Angelidaki, 2008). This problem is further accentuated for</p><p>protein-rich wastes, for which the ammonia concentration</p><p>significantly increases during their fermentation (Chen,</p><p>Cheng, &amp; Creamer, 2008).</p><p>To the authors knowledge, there is scarce information</p><p>about high organic load co-digestion of lipids, cellulose and</p><p>protein as pure co-substrates for the OFMSW and its assess-</p><p>ment and possible implementation in working digesters. In</p><p>fact, most industrial co-digestion plants treat the OFMSW</p><p>together (in a relative small percentage) with sewage sludge</p><p>(Mata-Alvarez,Mace, &amp; Llabres, 2000; Rintala &amp; Jarvinen, 1996).</p><p>In addition, there is a scarce industrial application of co-</p><p>digestion (Mata-Alvarez, Mace, &amp; Llabres, 2000), and most of</p><p>the studies published are difficult to apply, and consequently</p><p>only 7% of the overall anaerobic digestion of the OFMSW</p><p>capacity is at present co-digested. The aim of this work is to</p><p>study the feasibility of the co-digestion of the OFMSW with</p><p>different kinds of pure organic co-substrates such as vegetable</p></li><li><p>oil (VO), animal fat (AF), cellulose and peptone (protein) and to</p><p>assess the possibilities of implementing this co-digestion</p><p>process in already working plants. This can be considered as</p><p>a necessary step prior to the assessment of co-digestion with</p><p>real substrates that may have more complex and diverse</p><p>compositions.</p><p>procedure described by the German Institute for Standardi-</p><p>zation (Federal Government of Germany, 2001). This standard</p><p>procedure provides the parameter GB21 expressed as normal</p><p>litres of biogas (temperature: 273 K and pressure 1.01325 bar)</p><p>produced per kg of total solids (l kg1 [TS]) during 21 days. Inthe developed test, biogas production was monitored at</p><p>different times and the test was finished when no significant</p><p>biogas production was observed (after 135 days). Thus, the</p><p>o-s</p><p>V</p><p>b i o s y s t em s e n g i n e e r i n g 1 0 8 ( 2 0 1 1 ) 3 5 2e3 6 0354Table 1 e Characteristics of initial OFMSW, inoculum and c</p><p>Parameter OFMSW Inoculum</p><p>Dry matter (%) 29.0 7.0</p><p>Organic matter (% dry basis) 77.0 65.4</p><p>Fat content (% dry basis) 11.52 7.86</p><p>N-Kjeldahl (% dry basis) 1.83 2.892. Materials and methods</p><p>2.1. Waste sources and inoculum characteristics</p><p>Main properties of the OFMSW, inoculum and co-substrates</p><p>used are presented in Table 1. The OFMSW samples were</p><p>obtained from aMechanical-Biological Treatment (MBT) plant</p><p>(Barcelona, Spain) that treats mixed MSW and OFMSW in two</p><p>separated lines, with a total capacity of 240,000 tons of waste</p><p>per year. The OFMSW samples usedwere taken from the plant</p><p>after mechanical pre-treatment and prior to the anaerobic</p><p>digestion process. Thismaterial, essentially free of impurities,</p><p>is shredded to 20e30 mm and fed to the digester in the plant.</p><p>A representative sample (approximately 40 kg) was obtained</p><p>by mixing four subsamples of about 10 kg each, taken from</p><p>different points of the bulkmaterial. This sample was used for</p><p>biogas and methane production tests and other analyses.</p><p>The four co-substrates used were: (i) commercial vegetable</p><p>(coconut) oil; (ii) animal fat (Trg Debo Fancy, KAO Corporation</p><p>S.A., Spain); (iii) cellulose (cellulose powder, 20 micron, Sig-</p><p>maeAldrich); and protein (bacteriological peptone, Oxoid,</p><p>total nitrogen 14%). An amount of the corresponding co-</p><p>substrate to 20% (dry basis) of the OFMSWwas added to obtain</p><p>the target mixtures, which were intended to achieve 20%</p><p>increase in the loading rate. Vegetable and animal fats were</p><p>additionally characterised in terms of their long-chain fatty</p><p>acids. VO is mainly composed of lauric acid (45.5%), myristic</p><p>acid (18.5%), palmitic acid (10.4%) and oleic acid (8.7%). AF is</p><p>mainly composed of oleic acid (38.0%), palmitic acid (30.0%)</p><p>and stearic acid (17.0%).</p><p>The inoculum for anaerobic digestion was collected from</p><p>the plant anaerobic digester treating OFMSW (4500 m3</p><p>capacity, working temperature 37 C and hydraulic retentiontime, HRT, 21 days). The reactor was continuously fed with</p><p>a mixture of OFMSW:recirculated sludge in a ratio 1:2 (dry</p><p>basis). The anaerobic inoculum was kept at 37 C during twoweeks to remove any remaining easily biodegradable fraction.</p><p>2.2. Biological methane production</p><p>To analyse the biogas andmethane production of the different</p><p>samples, an analytical method was set up by adapting theC/N ratio 14.09 37.45parameter GBn was the biogas production obtained for n days</p><p>of analysis. Results were expressed both as normal litres of</p><p>biogas produced per kg of dry matter and per kg of volatile</p><p>solids (l kg1 [VS]). The ratio of substrate to inoculum used in</p><p>the industrial anaerobic digester (1:2 OFMSW to inoculum</p><p>d.b.), was used in all the experiments involving the use of co-</p><p>substrates and an extra 20% of dry matter was added through</p><p>co-substrate addition. Accordingly, the resulting ratio in the</p><p>co-digestion experiments was a mixture 1:2:0.2 OFMSW:ino-</p><p>culum:co-substrate (dry basis).</p><p>The mixtures were incubated in a temperature-controlled</p><p>room at 37 C in sealed aluminium bottles with a workingvolume of 1 l. Before each experiment, the bottles were purged</p><p>with nitrogen gas to ensure anaerobic conditions. The bottles</p><p>had a ball valve that was connected to a digital manometer</p><p>(SMC model ZSE30, Japan) to measure biogas pressure. The</p><p>bulk density of the mixture was previously determined (in</p><p>triplicate) to calculate the headspace volume of the bottles,</p><p>which was assumed constant during the experiment. During</p><p>the test, the bottles were shaken once a day. Biogas produc-</p><p>tion was calculated according to the ideal gas law from the</p><p>pressure measured in the bottle and considering the head-</p><p>space volume previously measured. To avoid excessive pres-</p><p>sure in the bottle, the biogas producedwas purged periodically</p><p>(typically 25e30 times during the experiment), so that the</p><p>pressure never exceeded 2 bar. This should minimise the</p><p>possible solubilisation of carbon dioxide since methane is</p><p>hardly soluble in aqueous media. As a result, the final biogas</p><p>production over a long period should hardly be affected.</p><p>All biogas production tests were carried out in triplicate</p><p>(three different bottles for each sample). The results are</p><p>expressed as an average with standard deviation. The typical</p><p>deviation found in triplicate samples was in the range of</p><p>5e15%. If one of the reactors presented a deviation greater</p><p>than 20%, it was discarded from the biogas potential calcula-</p><p>tion, as described in the procedure of the Federal Government</p><p>of Germany (2001). A biogas production test containing only</p><p>inoculum and another containing a mixture of inoculum and</p><p>the OFMSW (substrate:inoculum ratio 1:2 d.b.) were also set up</p><p>in triplicate to be used as a blank and control test respectively.</p><p>Specifically the loading rates used were 57.2 g [TS] l1 (37.4 g[VS] l1) for the blank containing only inoculum, 72.6 g [TS] l1</p><p>ubstrates used in the study.</p><p>egetable oil Animal fat Cellulose Protein</p><p>&gt;99 &gt;98 &gt;99 &gt;99</p><p>&gt;99 &gt;99 &gt;99 &gt;99</p><p>&gt;99 &gt;99 0.0 0.0</p><p>4000 &gt;4000 e</p></li><li><p>b i o s y s t em s e ng i n e e r i n g 1 0 8 ( 2 0 1 1 ) 3 5 2e3 6 0 355(49.5 g [VS] l1) for the control, which is the operating loading</p><p>rate of the industrial digester and close to 87 g [TS] l1 (...</p></li></ul>