Improvement of Fruit and Vegetable Waste Anaerobic Digestion

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<ul><li><p>te</p><p>diand</p><p>Keywords:Anaerobic co-digestion</p><p>), ave</p><p>tablish</p><p>trend of these wastes for anaerobic digestion (Bouallagui et al.,2003). In general, hydrolysis is the rate limiting step if the substrateis in particulate form (Veeken and Hamelers, 1999). However, theanaerobic degradation of cellulose-poor wastes like FVW is limitedby methanogenesis rather than by the hydrolysis. A major</p><p>greatest advantage lies in the buffering of the organic loading rate,and anaerobic ammonia production from organic nitrogen, whichreduce the FVW anaerobic digestion limitations.</p><p>Co-digestion is a technology that is increasingly being appliedfor simultaneous treatment of several solid and liquid organicwastes (Poggi-Varaldo et al., 1997; Callaghan et al., 1999; Alatristeet al., 2006; Perez et al., 2006). It combines different organicsubstrates to generate a homogeneous mixture as input to theanaerobic reactor in order to increase process performance</p><p>* Corresponding author. Tel.: 216 22 524406; fax: 216 71 704329.</p><p>Contents lists availab</p><p>Journal of Environm</p><p>ls</p><p>Journal of Environmental Management 90 (2009) 18441849E-mail address: (H. Bouallagui).many types of organic wastes, both solid and liquid (Pain et al.,1988; Ralph and Keith, 1990; Borzacconi et al., 1995; Murto et al.,2004; Neves et al., 2006; Yen and Brune, 2007). This alternativeallows the recovery of energy and a solid product that can be usedas an amendment of soils (Lawson, 1992; Gomez-Lahoz et al.,2007). The nutrient content of the anaerobic compost is favourableand the content of pollutants is low (Krugel et al., 1998; Kubleret al., 2000; Elango et al., 2007).</p><p>The easy biodegradable organic matter content of FVW withhigh moisture facilitates their biological treatment and shows the</p><p>methanogenic bacteria (Misi and Forster, 2001; Bouallagui et al.,2005).</p><p>The addition of co-substrates with high nitrogen content isa solution to adjust nutrient content of FVW. The unbalancednutrients of sh waste (FW), abattoir wastewater (AW) and wasteactivated sludge (WAS) characterised by a low C/N ratio were alsoregarded as an important limitation factor to anaerobic digestion ofthese organic wastes (Mshandete et al., 2004; Gomez et al., 2006;Gannoun et al., 2007). Adding AW, FW and WAS in FVW feedstockto have a balanced C/N ratio was undertaken in this study. TheirStabilitySequencing batch reactorFruit and vegetable wasteAbattoir wastewaterWaste activated sludgeFish waste</p><p>1. Introduction</p><p>Anaerobic digestion is a well es0301-4797/$ see front matter 2008 Elsevier Ltd.doi:10.1016/j.jenvman.2008.12.002were from FVW, and a hydraulic retention time of 10 days. It was observed that AW and WAS additionswith a ratio of 10% VS enhanced biogas yield by 51.5% and 43.8% and total volatile solids removal by 10%and 11.7%, respectively. However FW addition led to improvement of the process stability, as indicated bythe low VFAs/Alkalinity ratio of 0.28, and permitted anaerobic digestion of FVW without chemical alkaliaddition. Despite a considerable decrease in the C/N ratio from 34.2 to 27.6, the addition of FW slightlyimproved the gas production yield (8.1%) compared to anaerobic digestion of FVW alone. A C/N ratiobetween 22 and 25 seemed to be better for anaerobic co-digestion of FVW with its co-substrates. Themost signicant factor for enhanced FVW digestion performance was the improved organic nitrogencontent provided by the additional wastes. Consequently, the occurrence of an imbalance between thedifferent groups of anaerobic bacteria which may take place in unstable anaerobic digestion of FVWcould be prevented.</p><p> 2008 Elsevier Ltd. All rights reserved.</p><p>ed process for treating</p><p>limitation of anaerobic digestion of FVW is a rapid acidication ofthese wastes decreasing the pH in the reactor, and a larger volatilefatty acids production, which stress and inhibit the activity ofAccepted 1 December 2008Available online 30 December 2008nding the better co-substrate for the enhanced performance of co-digestion. The reactors were oper-ated at an organic loading rate of 2.462.51 g volatile solids (VS) l1 d1, of which approximately 90%Received in revised form29 August 2008</p><p>under mesophilic conditions using four anaerobic sequencing batch reactors (ASBR) with the aim ofImprovement of fruit and vegetable wasand stability with co-substrates addition</p><p>H. Bouallagui*, H. Lahdheb, E. Ben Romdan, B. RachLaboratory of Microbial Ecology and Technology, National Institute of Applied Sciences</p><p>a r t i c l e i n f o</p><p>Article history:Received 6 February 2008</p><p>a b s t r a c t</p><p>The effect of sh waste (FWsubstrates on the fruit and</p><p>journal homepage: www.eAll rights reserved.anaerobic digestion performance</p><p>, M. HamdiTechnology, BP 676, 1080, Tunisia</p><p>battoir wastewater (AW) and waste activated sludge (WAS) addition as co-getable waste (FVW) anaerobic digestion performance was investigated</p><p>le at ScienceDirect</p><p>ental Management</p><p>evier .com/locate/ jenvman</p></li><li><p>(Hamzawi et al., 1998; Viotti et al., 2004; Zhang and Banks, 2008). Itpermits the exploitation of complementarity in waste characteris-tics e.g. avoidance of nutrients (N, P) addition when a co-digestedwaste contains nutrients in excess (Gavala et al., 1996; Pavan et al.,2005; Neves et al., 2008). Several studies have shown that multi-component mixtures of agro-wastes, rural wastes and industrialwastes can be digested successfully, although with some mixturesa degree of both synergism and antagonism occurred (Misi andForster, 2001, 2002; Cavinato et al., 2008).</p><p>The aim of this work was to examine the effect of AW, WAS andFW addition as co-substrates on the FVW anaerobic digestionperformance inmesophilic condition using an ASBR. The criteria forjudging the success of a co-digestion were process stability, VSreduction, biogas production rate, and methane yield.</p><p>2. Material and methods</p><p>2.1. Reactors design and operational conditions</p><p>Four laboratory-scale anaerobic sequencing batch reactors (R1,R2, R3 and R4) of 2 l effective volume were used (Fig. 1). Thetemperaturewas controlled at 35 C by a thermostatically regulated</p><p>particles and homogenizing, they were stored at 4 C. WAS wascollected from the activated sludge plant (Cherguia, Tunis) treatingdomestic and industrial wastewaters. It is composed of settledsuspended biomass. The AWwas collected from an abattoir factory(El Ouardia City, Tunis). Analysis of the raw FVW, FW, AW and WASwere carried out several times and the average compositions areshown in Table 1. The FVW consisted of homogenised courgettes,lettuce, tomatoes, apple, orange, pear, potatoes and carrot to give8.3% TS with VS content of 93%. The feedstock was made up byadding a percentage by volume of water, AW, WAS and FW to FVW.Four feedstocks (W1W4) (Table 2) which were prepared with</p><p>H. Bouallagui et al. / Journal of Environmental Management 90 (2009) 18441849 1845water bath. Peristaltic pumps were used to ll the reactors and todraw off the efuents after settling. Mixing in the reactors was doneby a system of magnetic stirring. Each digester was initially inocu-lated with anaerobic sludge obtained from an active mesophilicdigester of FVWs treatment plant (Bouallagui et al., 2007).</p><p>The ASBR was operated with cycles including the following fourdiscrete steps: (i) ll (15 min): 200 ml of different mixtures ofwastes were added to the reactors at the beginning of a cycle, (ii)react (21 h): during this phase, the reactors were stirred andorganic matter was converted to energy and new cells, (iii) settle(2 h and 30 min): settling started when the react phase wasnished and (iv) draw off (15 min): at the end of the settling period,the volume of liquid added at the beginning of the cycle was drawnoff from the reactors.</p><p>2.2. Wastes sources and characteristics</p><p>The fruit, vegetable and sh wastes used in this study werecollected from the group market of Tunis. After shredding to small</p><p>(4) (3)</p><p>(5)(6)</p><p>(1)</p><p>(7)</p><p>(9)</p><p>(2)</p><p>(8)</p><p>Fig. 1. Schematic of the experimental ASBR system: (1) ASBR, (2) water bath and</p><p>heating recirculation, (3) magnetic stirrer, (4) feedstock, (5) feeding pump, (6)discharge pump, (7) efuent stock, (8) sampling valve and (9) biogas collector.average TS contents of 2.7%, 2.74%, 2.9% and 2.8%, were used to loadR1R4. Approximately, 90% of VS in the different feedstocks weregiven from FVWs.</p><p>2.3. Technical analysis</p><p>The biogas produced was measured daily by gas meter (Ritter Bochum Langendreer, Germany) and its compositionwas estimatedusing an ORSAT apparatus (Bouallagui et al., 2003). Total solids (TS),total volatile solids (TVS), total suspended solids (TSS), pH, alka-linity and total volatile fatty acids (VFAs) were determinedaccording to the APHA Standard Methods (1995). Total organiccarbon (TOC) was measured by catalytic oxidation on a TOC Euro-glace analyser. Total nitrogen (TN) was estimated by the Kjeldahlmethod.</p><p>2.4. Statistical analysis</p><p>In order to determine whether the observed differencesbetween digesters performances were signicantly different, datawere subjected to the ANOVA tests (StatSoft Inc, 1997). Differencesbetween co-substrates addition effects (p and p1) were comparedwith 0.05.</p><p>3. Results and discussion</p><p>3.1. Effect of co-substrates addition on the fermentation efciency</p><p>3.1.1. Biogas productionThe biogas production by the digestions of FVW alone and the</p><p>co-digested wastes is shown in Fig. 2. Analysis of biogas productionproles for the substrates combinations showed that there weresignicant differences among the combinations tested. The resultsfor co-digested substrates are better than those obtained fromdigestion of FVW. The average biogas production rate variedbetween 1.53 l d1 and 2.53 l d1, with value being highest for bothmixtures W2 andW3 and lowest for 100% FVW. The specic biogasproductions for the four digestion processes (R1R4) were 0.403,0.611, 0.580 and 0.436 l g1 removal VS.</p><p>The methane yields from FVW, which have been reportedpreviously, are variable depending on the waste composition and</p><p>Table 1Characteristics of raw wastes.</p><p>FVW AW WAS FW</p><p>TS (%) 8.3 0.35 0.6 19.6VS (% of TS) 93 60.3 81.5 60.2TSS (g/l) 46.3 0.46 4.9 53.7tCOD (g/l) 162 3.8 11.7 188.8tCOD/VS 2.1 1.8 2.4 1.6pH 4.2 7.2 6.93 6.1Total Nitrogen (% of TS) 2.1 12.1 5.4 5.4Total Carbon (% of TS) 72 60.2 52.5 48.2</p><p>C/N 34.2 5.14 10.48 8.8</p></li><li><p>the used reactor design. The reported range was from 0.16 to0.4 m3 kg1 VS added (Bouallagui et al., 2005). The results pre-sented in this paper for FVW digestion are therefore comparablewith these earlier results.</p><p>The data for the co-digestion may be also compared with earlierworks. Callaghan et al. (2002) examined the co-digestion of FVWwith cattle slurry and chicken manure. The methane yields theyobtained of 0.350.4 l g1 VS added were very similar to those inthe current study for FVWAW co-digestion. Gomez et al. (2006)have also reported similar results for the anaerobic co-digestion ofFVW and primary sludge. However the methane yield obtainedfrom FVW and WAS anaerobic co-digestion in two stages tubular</p><p>digesters of 0.25 l g1 VS added (Dinsdale et al., 2000) was lowerthan that is presented in this work.</p><p>The addition of AW, WAS and FW enhanced the biogas yield by51.5%, 43.8% and 8.1%, respectively. Biogas yields for FVW:AW andFVW:WAS co-digestions are much greater thanks to the better C/Nratio of these feedstocks (Fig. 3). The ANOVA of the data indicatedthat digesters (R2 and R3) performance enhancement was statis-tically signicant (p&lt; 0.05) (Table 3). Despite a considerabledecrease of C/N ratio from 34.2 to 27.6, the addition of FW slightlyimproves the gas production rate and biogas yield compared toanaerobic digestion of FVW alone (p1&gt;0.05). The C/N ratios of theco-digested FVW:AWand FVW:WAS which ranged between 22 and25 were within the C/N ratio (2025) required for stable and betterbiological conversions reported by others on anaerobic digestion oforganic wastes (Parkin and Owen, 1986; Mshandete et al., 2004;</p><p>Table 2Characteristics of mixtures wastes.</p><p>W1 W2 W3 W4</p><p>TS (%) 2.7 2.74 2.9 2.8VS (% of TS) 92 90.1 86.5 89.5TSS (g/l) 14.1 14.8 18.7 22.6tCOD (g/l) 52.2 50.6 56.4 48.9tCOD/VS 2.1 2.05 2.25 1.95pH 4.3 5.4 4.94 5.1Total Nitrogen (% of TS) 2.1 3.1 2.6 2.5Total Carbon (% of TS) 72 70.2 64.4 69.1C/N 34.2 22.58 24.76 27.6</p><p>1</p><p>1,5</p><p>2</p><p>2,5</p><p>3</p><p>3,5</p><p>Bio</p><p>gas rate (l d</p><p>-1)</p><p>H. Bouallagui et al. / Journal of Environmental Management 90 (2009) 1844184918460</p><p>0,5</p><p>0 5 10 15 20 25 30 35 40 45Time (d)</p><p>0,3</p><p>0,35</p><p>0,4</p><p>0,45</p><p> V</p><p>S rem</p><p>oved</p><p>)0</p><p>0,05</p><p>0,1</p><p>0,15</p><p>0,2</p><p>0,25</p><p>0 5 10 15 20 25 30 35 40 45Time (day)</p><p>Me</p><p>th</p><p>an</p><p>e y</p><p>ie</p><p>ld</p><p> (l g</p><p>-1</p><p>Fig. 2. Biogas rate and methane yield variation during anaerobic digestion of W1:30%FVW/70%Water (-), W2: 30%FVW/70%AW (:), W3: 30%FVW/70%WAS (,) andW4: 30%FVW/1.4%FW/68.6Water (), under mesophilic condition and an HRT of 10days.Yen and Brune, 2007). Kayhanian and Hardy (1994) reported C/Nratios between 25 and 30 as being optimal. However, Kivaisi andMtila (1998) argue that the C/N of approximately between 16 and19 is optimal for methanogenic performance.</p><p>3.1.2. Organic matter removalThe total volatile solids destruction for the various co-digested</p><p>substrates combinations is given in Table 3. The higher degradationefciencies were obtained for the digesters treating FVW:AW andFVW:WAS and operated at an organic loading rate of2.46 gTVS l1 d1 and 2.51 gTVS l1 d1, respectively. They wereassociated with the higher specic biogas production and a lowercontent of volatile solids in the digested efuent, which representsa lesser amount of output stabilised efuent with a better dew-atering properties. The data of Table 3 showed that about 8485.4%of TVS were degraded to methane and carbon dioxide with the co-digestion of organic wastes. These results are in agreement withthose obtained by Fernandez et al. (2005) and better than thoseobtained by Callaghan et al. (1999) and Dinsdale et al. (2000). It isvery likely that the high degradation efciency in the co-fermen-tation was due to an improved ratio of nutrients and better avail-ability of the organic substances, which facilitate their assimilationby anaerobic ora and increases the degree of degradation (Kruppand Schubert, 2005). Furthermore, the AW addition improves theproteins availability that were used by anaerobic bacteria toproduce new cells and enzymes.</p><p>3.1.3. Alkalinity, total VFAs and pH variationIn a well balanced anaerobic digestion process, total VFAs levels</p><p>are low (Fernandez et al., 2005; Chen et al., 2007). In this study allthe combinations examined, except FVWFW showed lower levelsof total VFAs in their digested efuent at steady-state (Fig. 4).During the period days 125 high levels of total VFAs of up to</p><p>0</p><p>0,1</p><p>0,2</p><p>0,3</p><p>0,4</p><p>0,5</p><p>0,6</p><p>0,7</p><p>20 22 24 26 28 30 32 34C/N ratio</p><p>Bio</p><p>gas yield</p><p> (l g</p><p> -1</p><p> rem</p><p>oval V</p><p>S) </p><p>60</p><p>65</p><p>70</p><p>75</p><p>80</p><p>85</p><p>90</p><p>VS</p><p> rem</p><p>oval (%</p><p>)Fig. 3. Effect of C/N ratio variation on the VS removal efciency (,) and biogasyield (-).</p></li><li><p>7,2</p><p>7,3</p><p>7,4</p><p>7,5</p><p>7,6</p><p>7,7</p><p>7,8</p><p>7,9</p><p>pH</p><p>0</p><p>500</p><p>1000</p><p>0 5 10 15 20 25 30 35 40 45</p><p>0 5 10 15 20 25 30 35 40 45</p><p>Time (day)</p><p>VF</p><p>0</p><p>1000</p><p>2000</p><p>3000</p><p>4000</p><p>5000</p><p>6000</p><p>7000</p><p>8000</p><p>Time (d...</p></li></ul>


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