Biohydrogen generation from anaerobic digestion of food waste

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<ul><li><p>an</p><p>a</p><p>ian</p><p>ong</p><p>Article history:</p><p>Received 25 April 2013</p><p>which accounts for 36% of the total solid wastes, is dumped to</p><p>biohydrogen and biomethane [2]. Therefore, converting food</p><p>waste to energy is a topic pursued by many researchers [3,4].</p><p>Most studies used mixed sludge in anaerobic digestion to</p><p>produce a biogas with 50e60% biomethane but contained little</p><p>ed by the hydrogen</p><p>ydrogen consuming</p><p>e biomethane [5]. For</p><p>erobic digestion of a</p><p>processing plant, a</p><p>cafeteria, a commercial kitchen, a fish farm and a grease trap</p><p>lum from an anaer-</p><p>food to microorgan-</p><p>and thermophilic</p><p>s contained 62% and</p><p>udy had been carried</p><p>out by Dearman and Bentham [7] using a mixed food waste</p><p>collected fromuniversity kitchens, hospitals andmarkets, and</p><p>the biogas generated during the 70 day digestion period con-</p><p>tained a composition of biomethane ranging from 55 to 75%.</p><p>* Corresponding author. Tel.: 852 23588409..Y. Fung).</p><p>Available online at</p><p>w.</p><p>i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 0 7e1 3 9 1 3E-mail addresses:, (Klandfills every day and the amount of food waste disposed</p><p>from restaurants has doubled in the last five years [1]. Second,</p><p>it is a complete waste to dump a food waste with a high vol-</p><p>atile solids (VS) content of 80e97 wt% which is readily biode-</p><p>gradable in an anaerobic process to generate biogas, namely</p><p>collection service using anaerobic inocu</p><p>obic digester. The biogas generated at a</p><p>isms ratio of 0.5 under mesophilic</p><p>conditions for a residence time of 28 day</p><p>64% biomethane, respectively. Similar st1. Introduction</p><p>The disposal of food waste has been receiving a lot of atten-</p><p>tion for two reasons. First, it takes up the limited landfill</p><p>space. For example, in Hong Kong, about 3280 ton food waste,</p><p>biohydrogen. The biohydrogen produc</p><p>producing bacteria was consumed by h</p><p>bacteria such as methanogens to produc</p><p>example, Chen et al. [6] studied the ana</p><p>mixed food waste collected from a soupReceived in revised form</p><p>1 August 2013</p><p>Accepted 15 August 2013</p><p>Available online 14 September 2013</p><p>Keywords:</p><p>Biohydrogen</p><p>Biomethane</p><p>Anaerobic process</p><p>Food waste0360-3199/$ e see front matter Copyright generated from the anaerobic digestion of a synthetic food waste with con-</p><p>stant composition and a real food waste collected in Hong Kong were studied. This study</p><p>aims at using a monoculture to increase biohydrogen production and determining opti-</p><p>mum conditions for maximum biohydrogen production. Among the nine bacteria screened</p><p>for biohydrogen production, Escherichia cloacae and Enterobacter aerogenes produced the</p><p>largest amount of biohydrogen from the anaerobic digestion of synthetic food waste. The</p><p>optimum anaerobic digestion conditions were determined: initial pH of 7, a water to solids</p><p>ratio of 5 (w/w), a mesophilic temperature (37 1 C), and in the presence of 40 mg/LFeSO4$7H2O. Anaerobic digestion at the optimum operating conditions using collected food</p><p>waste with E. cloacae as the bacterial source was also performed. By adjusting the pH in the</p><p>range of 5e6, a specific biohydrogen production of 155.2 mL/g of volatile solids (VS) in food</p><p>waste was obtained.</p><p>Copyright 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.a r t i c l e i n f o a b s t r a c tWater Bay, Kowloon, Hong KongBiohydrogen generation fromof food waste</p><p>Liping Xiao a, Zhiyi Deng a, Ka Y. Fung b,*, Ka Department of Environmental Engineering, Xiangtan University, Xb Department of Chemical and Biomolecular Engineering, The Hong K</p><p>journal homepage: ww2013, Hydrogen Energy P72aerobic digestion</p><p>M. Ng b</p><p>gtan, Hunan 411105, China</p><p>University of Science and Technology, Clear</p><p>elsevier .com/locate/heublications, LLC. Published by Elsevier Ltd. All rights reserved.</p></li><li><p>species of bacteria such as Enterobacter spp., and Clostridium</p><p>i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 0 7e1 3 9 1 313908spp. which are capable of generating biohydrogen from food</p><p>waste through dark fermentation in the first-stage reactor.</p><p>However, most studies focusing on single species of bacteria</p><p>used simple sugars such as glucose, xylose, and sucrose, or</p><p>substrates rich in sugars such as sugarcane wastewater, sugar</p><p>refinery waste, and orange processing effluent as the nutrient</p><p>source [11e13]. A real food waste containing proteins and</p><p>lipids [14] received limited coverage.</p><p>This paper studies the biohydrogen generation by a single</p><p>species of bacteria in anaerobic digestion. As a control, a</p><p>synthetic food waste with constant composition will be used</p><p>in preliminary tests to screen out two bacteria, among the</p><p>nine bacteria tested, that generate the highest amount of</p><p>biohydrogen. The effect of operating parameters such as the</p><p>initial pH of food waste, water to solids ratio, temperature,</p><p>and the addition of inorganic salts on biohydrogen generation</p><p>of the synthetic food waste will be studied using the two top-</p><p>performing bacteria. Finally, anaerobic digestion of a real food</p><p>waste collected locally will be studiedwith the best performed</p><p>bacteria under the same optimum conditions.</p><p>2. Method and material</p><p>2.1. Food waste</p><p>The synthetic food waste was composed of rice 6% (w/w),</p><p>bread 6% (w/w), vegetable 9.6% (w/w), pork 8.4% (w/w), eggs</p><p>2.4% (w/w), corn oil 1.2% (w/w), table salt 0.2% (w/w) andwater</p><p>66.2% (w/w). The synthetic food waste was made up based on</p><p>the typical carbon to nitrogen ratio and the moisture content</p><p>of food wastes reported in the literature [2]. Collected food</p><p>waste was obtained from two Chinese fast food restaurants at</p><p>the Hong Kong University of Science and Technology. After</p><p>removing bones and shells, food wastes were ground and</p><p>mixed thoroughly in a blender (Westinghouse handheld</p><p>blender) and stored in sealed glass bottles in a refrigerator at</p><p>4 C. Before anaerobic digestion, the food waste was restoredto the ambient temperature (22 1 C).Instead of collecting only biomethane from anaerobic</p><p>digestion, it would be advantageous to collect the biohydrogen</p><p>generated during anaerobic digestion before being consumed</p><p>by the hydrogen consuming bacteria. This requires a two-</p><p>stage reactor where biohydrogen is recovered in the first</p><p>stage and the residual food waste is further degraded in the</p><p>second stage of the reactor to produce biomethane.</p><p>Pretreated sludge which suppresses the activity of</p><p>hydrogen consuming bacteria can be used in the first-stage</p><p>reactor. Various pretreatment methods such as heat-shock</p><p>[8], acid and base treatment [9] have been used for such pur-</p><p>pose. Only biohydrogen and no biomethane was detected in</p><p>the biogas generated in these studies. Wang and Wan [10]</p><p>compared these pretreatment methods using a glucose solu-</p><p>tion and found that heat-shock treatment resulted in the</p><p>highest biohydrogen production rate and biohydrogen yield.</p><p>Treating the mixed sludge by heat or the use of chemicals</p><p>is costly in industrial applications. An option is to use a singleThe characteristics of synthetic and collected food wastes</p><p>such as total solids (TS), volatile solids (VS), moisture content,and total phosphorus (TP) concentration were measured ac-</p><p>cording to standardmethods [15]. Elemental concentration (C,</p><p>H, N, S) of the food waste samples were measured by an</p><p>elemental analyzer (Elementar Analysensysteme Gmbh vario</p><p>EL III CHNS-mode) with sulfanilic acid as the standard. They</p><p>were dried at 105 5 C for 24 h and were ground to particlessmaller than 0.5 mm before measurement. pH of the food</p><p>waste was measured by a pH meter (SevenMulti Mettler</p><p>Toledo).</p><p>2.2. Microorganisms</p><p>Nine bacteria purchased from the Guangdong Microbial Cul-</p><p>ture Collection Center in China or the German Collection of</p><p>Microorganisms and Cell Cultures were used in this study.</p><p>These strainswere selected for the screening study as they are</p><p>commonly used for biohydrogen generation by anaerobic</p><p>digestion [11,12]. These strains were grown under anaerobic</p><p>conditions in various culture media (Table 1). In preparing a</p><p>culture medium, sodium bicarbonate was added to a culture</p><p>medium to adjust its pH. Themediumwas first boiled andwas</p><p>then sparged with N2 for 30 min to remove any oxygen before</p><p>autoclave.</p><p>2.3. Anaerobic digestion of food waste for biohydrogenproduction</p><p>Anaerobic digestion of synthetic or collected food waste was</p><p>carried out in culture bottles and the schematic diagramof the</p><p>experimental setup is shown in Fig. 1. The foodwastewas first</p><p>added to a 100 mL culture bottle. Water was then added to</p><p>obtain the desired water to solids ratio. 1 M NaHCO3 was used</p><p>to adjust its initial pH to the target pH. FeSO4$7H2O or</p><p>NiCl2$6H2Owas used to investigate the effect of inorganic ions</p><p>on performance. The culture bottle was then purgedwith pure</p><p>N2 for 5 min to ensure anaerobic conditions. Bacteria were</p><p>then added to the culture bottles such that the ratio of volatile</p><p>solids in food waste to that in bacteria was kept at 6. The</p><p>bottles were finally sealed with butyl rubber stoppers and</p><p>were put in a water bath to keep the culture medium at the</p><p>desired temperature. Mixing was provided by a stirring mag-</p><p>netic bar in the culture bottle. pH and temperature were</p><p>measured by a pH and temperature probe. Liquid samples</p><p>were collected regularly for measuring chemical oxygen de-</p><p>mand (COD) and the concentration of volatile fatty acids</p><p>(VFAs). The volume of biogas was read from the scales on the</p><p>gas collection tube, and the gas was analyzed for its</p><p>composition.</p><p>For the screening study, 6 g synthetic food waste was used.</p><p>The screening experiment was carried out at 37 1 C, aninitial pH of 7, a water to solids ratio of 5 and without adding</p><p>inorganic salts. The experiment was carried out until no</p><p>biogas was produced for 6 h. Two bacteria that generated the</p><p>highest amount of biohydrogen were then selected to study</p><p>the effect of operating parameters on biohydrogen generation.</p><p>Operating conditions including pH of the initial food waste,</p><p>water to solids ratio, temperature and the presence of inor-</p><p>ganic salts were studied (Table 2). Operating conditions sameas the screening study (base case) was first carried out and the</p><p>operating parameters were then adjusted one at a time. As</p></li><li><p>Table 1 e Bacteria used in the screening study and their culture medium.</p><p>No. Bacteriaa Cultivatingtemperature</p><p>Culture medium</p><p>I Enterobacter aerogenes I 37 C PeptoneBeef extract</p><p>Sodium chloride</p><p>Agar</p><p>5 g/L</p><p>3 g/L</p><p>5 g/L</p><p>20 g/L</p><p>II Enterobacter aerogenes II</p><p>III Escherichia aerogenes</p><p>IV Escherichia cloacae</p><p>V Clostridium beijerinckii 37 C</p><p>pH 6.8 0.2</p><p>VI Clostridium tyrobutyricum</p><p>VII Clostridium amylolyticum</p><p>VIII Clostridium pasteurianum</p><p>C These components were</p><p>X ar</p><p>i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 0 7e1 3 9 1 3 13909experimental results of the screening study showed that no</p><p>biohydrogen was produced after 48 h, experimental studies</p><p>conducted for varying the operating parameters was carried</p><p>out for 48 h only. Biohydrogen generation from anaerobic</p><p>digestion of collected food waste was carried out under the</p><p>IX Clostridium butyricum 30 </p><p>a Bacteria IeIV are facultative anaerobic bacteria, while bacteria VeIoptimum operating conditions determined above. A larger</p><p>amount (12 g) of collected food waste was used in the exper-</p><p>iments and the pHwas controlled in the range of 5e6 with 1 M</p><p>NaHCO3 when the pH dropped below 5.</p><p>The composition of the collected biogas was analyzed by</p><p>gas chromatography (Agilent Technologies, 6890N, Network</p><p>GC System) equipped with a thermal conductivity detector</p><p>and a 2m stainless steel column packed with Hayesep Q (60/</p><p>80 mesh). The operating temperatures of the injection port,</p><p>oven, and detector were all set at 70 C. Nitrogen at a flow rateof 30 mL/min served as the carrier gas.</p><p>For the determination of VFAs in the liquid samples, they</p><p>were first centrifuged at 7000 rpm for 10 min, followed by</p><p>Fig. 1 e Schematic diagram of anaerobic digestion</p><p>experimental setup.filtration through a 0.22 mm cellulose acetate membrane</p><p>before being analyzed by gas chromatography (Agilent Tech-</p><p>nologies, 6890N, Network GC System) equipped with a flame</p><p>ionization detector and a 30m 0.25 mm 0.25 mm capillarycolumn (Heliflex AT-AquaWax-DA; Part No. 14537). The tem-</p><p>added to the above</p><p>Clostridium culture medium</p><p>Sodium bicarbonate</p><p>Sodium caproate</p><p>Sodium chloride</p><p>p-Aminobenzoic acid</p><p>5 g/L</p><p>1.4 g/L</p><p>16 g/L</p><p>40 mg/L</p><p>e strictly anaerobic bacteria.pH 7e7.2</p><p>Tryptose peptone</p><p>Beef extract</p><p>Glucose</p><p>Sodium chloride</p><p>Yeast extract</p><p>Sodium acetate</p><p>Starch (soluble)</p><p>L-cysteine hydrochloride</p><p>Agar</p><p>10 g/L</p><p>10 g/L</p><p>5 g/L</p><p>5 g/L</p><p>3 g/L</p><p>3 g/L</p><p>1 g/L</p><p>0.5 g/L</p><p>20 g/Lperatures of the injector and the detector were set at 250 C.The initial temperature of the column ovenwas set at 50 C for2 min, followed by ramping at 15 C/min for 12.6 min and thetemperature was finally kept at 240 C for 1 min. Helium at aflow rate of 66 mL/min served as the carrier gas.</p><p>COD and pH of the liquid samples were measured using</p><p>sealed tube method (HACH, DR870 spectrophotometer) and a</p><p>pH meter (SevenMulti Mettler Toledo), respectively.</p><p>3. Results and discussion</p><p>3.1. Food waste characteristics</p><p>The characteristics of synthetic food waste and collected food</p><p>waste are summarized in Table 3. Both the synthetic food</p><p>waste and collected food waste were acidic and contained</p><p>about 80%moisture. The carbon to nitrogen (C/N) ratio of both</p><p>Table 2 e Operating parameters studied in theoptimization study.</p><p>Initial food waste pH 5, 6, 7, 8</p><p>Water to solids ratio 3, 4, 5</p><p>Inorganic salt concentration 0, 40, 80 mg/L FeSO4$7H2O</p><p>0, 40, 80 mg/L NiCl2$6H2O</p><p>Temperature 21.5 0.5, 37 0.5 C</p></li><li><p>showed that biogas was not produced after 48 h, the optimi-</p><p>zation experiments were carried out for 48 h only.</p><p>3.3. Effect of operating parameters on biohydrogenproduction</p><p>Anaerobic digestion process was first carried out at the base</p><p>case conditions and the specific biogas and biohydrogen pro-</p><p>duction are summarized in Table 5. Specific biogas production</p><p>is defined as the amount of biogas produced per gram of VS in</p><p>food waste. The effect of operating parameters on bio-</p><p>hydrogen production is discussed below.</p><p>3.3.1. Effect of initial pHAmong the various operating parameters, pH has a larger</p><p>Table 3 e Characteristics of synthetic and collected foodwaste.</p><p>Syntheticfood waste</p><p>Collectedfood waste</p><p>pH 5.0 4.1</p><p>Moisture, % 85.1 82.8</p><p>Total solids (TS), g/100 g</p><p>food waste</p><p>14.9 17.2</p><p>Volatile solids (VS), g/100 g</p><p>food waste</p><p>14.4 16.7</p><p>VS/TS, % 96.5 95.6</p><p>Elemental composition</p><p>among TS</p><p>Total phosphorus, % 0.04 e</p><p>Total nitrogen, % 3.8 2.8</p><p>C, % 45.2 50.0</p><p>i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 0 7e1 3 9 1 313910food wastes were within the suggested C/N ratio (10e20) for</p><p>anaerobic digestion.</p><p>3.2. Screening of bacteria for biohydrogen production inanaerobic digestion</p><p>Nine bacteria were used to study the biohydrogen production</p><p>from anaerobic digestion of synthetic food waste. Th...</p></li></ul>