Feasibility of biohydrogen production by anaerobic co-digestion of food waste and sewage sludge
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International Journal of Hydrogen Energy 29 (2004) 16071616www.elsevier.com/locate/ijhydene
Feasibility of biohydrogen production by anaerobicco-digestion of food waste and sewage sludge
Sang-Hyoun Kim, Sun-Kee Han, Hang-Sik Shin
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology,373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
Accepted 24 February 2004
Anaerobic co-digestion of food waste and sewage sludge for hydrogen production was performed in serum bottles undervarious volatile solids (VS) concentrations (0.55.0%) and mixing ratios of two substrates (0:100100:0, VS basis). Throughresponse surface methodology, empirical equations for hydrogen evolution were obtained. The speci5c hydrogen productionpotential of food waste was higher than that of sewage sludge. However, hydrogen production potential increased as sewagesludge composition increased up to 1319% at all the VS concentrations. The maximum speci5c hydrogen production potentialof 122:9 ml=g carbohydrate-COD was found at the waste composition of 87:13 (food waste:sewage sludge) and the VSconcentration of 3.0%. The relationship between carbohydrate concentration, protein concentration, and hydrogen productionpotential indicated that enriched protein by adding sewage sludge might enhance hydrogen production potential. The maximumspeci5c hydrogen production rate was 111:2 ml H2=g VSS=h. Food waste and sewage sludge were, therefore, considered as asuitable main substrate and a useful auxiliary substrate, respectively, for hydrogen production. The metabolic results indicatedthat the fermentation of organic matters was successfully achieved and the characteristics of the heat-treated seed sludge weresimilar to those of anaerobic spore-forming bacteria, Clostridium sp.? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Keywords: Anaerobic co-digestion; Food waste; Hydrogen; Protein; Sewage sludge; VS concentration
Due to the limited resources and pollutants emission(CO2, CO, CnHm, SOx, NOx, ashes, etc.), fossil fuels shouldbe substituted with renewable and non-polluting energysources . As a sustainable energy source with minimal orzero use of hydrocarbons and high-energy yield (122 kJ=g),hydrogen is a promising alternative to fossil fuels. In addi-tion, hydrogen can be directly used to produce electricitythrough fuel cells . Since conventional physico-chemicalhydrogen production methods (e.g. water electrolysis orchemical cracking of hydrocarbons) require electricity de-rived from fossil fuel combustion, interest in biohydrogenproduction has increased signi5cantly . Between two
Corresponding author. Tel.: +82-42869-3613; fax: +82-42869-3610.
E-mail address: firstname.lastname@example.org (H.-S. Shin).
biological processes, fermentative processes that use widerange of organic substances are technically simpler than pho-tosynthetic processes . Carbohydrates are the preferredsubstrate for fermentative hydrogen-producing bacteria suchas Clostridium species (sp.). Stoichiometrically, Clostrid-ium sp. can produce 2 moles of hydrogen with 1 mole ofn-butyrate or 4 moles of hydrogen with 2 moles of acetatefrom 1 mole of hexose . In most cases, using solublede5ned substrates, hydrogen production yield and a ma-jor byproduct were 0.72:1 mole=hexoseconsumed and a ma-jor by-product was n-butyrate, respectively [6,7]. Hydrogenwas hardly produced from protein and lipid . How-ever, protein was indispensable, sometimes, as a nitrogensource for the hydrogen production in both pure and mixedcultures .Organic wastes may become a plentiful source of
inexpensive organic substrate for fermentative hydrogenproduction ; by which reduction and stabilization of
0360-3199/$ 30.00 ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2004.02.018
1608 S.-H. Kim et al. / International Journal of Hydrogen Energy 29 (2004) 16071616
organic wastes also can be accomplished. In recent years,some experimental results using municipal solid waste[810,14], food manufacturing waste [8,13,15,16], wasteactivated sludge [17,18] were reported. The maximum hy-drogen production potential and hydrogen production ratewere in the range of 49298 ml H2=g carbohydrate-CODand 17142 ml H2=g VSS=h, respectively. However, gen-eral guideline on optimum conditions such as water content,carbohydrate concentration, carbohydrate/protein balancewas not suFciently provided yet. Besides, there was nosystematic attempt at co-digestion of diGerent substrates,which might enhance hydrogen production.Food waste and sewage sludge are the most abundant and
problematic organic solid wastes in Korea. The generationof food waste reaches about 11; 000 tons per day, accountingfor 23% of municipal solid wastes . It is the major sourceof odor emanation, vermin attraction, toxic gas emissionand groundwater contamination in collection, transportationand land5ll of solid wastes due to the high organic con-centration (volatile solids/total solids: 0.80.9) and mois-ture content (7585%). However, food waste might be suit-able for fermentative hydrogen production, because it is thecarbohydrate-rich and easily hydrolysable waste . Onthe other hand, approximately 5700 tons of sewage sludgecakes are generated daily , of which 72% is disposed byocean dumping. However, it would be prohibited accordingto London Convention in recent future . The enhance-ment of anaerobic digester is, therefore, urgent to reducethe amount of the sludge cakes and to improve the qualityfor reuse. Co-digestion of sludge with carbon-rich wastes isknown as an economic and feasible approach to retro5t con-ventional digesters [22,23]. If hydrogen can be produced byanaerobic fermentation of food waste with sewage sludge,they would be the important source for hydrogen productiondue to the amount.In this work, the feasibility of anaerobic co-digestion of
food waste and sewage sludge for hydrogen production was,therefore, carried out under various VS concentrations andmixing ratios. Response surface methodology employing afull quadratic model was then conducted to 5nd out the re-lationship between VS concentration, mixing ratio and hy-drogen production.
2. Materials and methods
2.1. Seed sludge
The seed sludge was taken from an anaerobic digesterin a local wastewater treatment plant. The pH, alkalinity,and volatile suspended solids (VSS) concentration of thesludge were 7.6, 2:83 g CaCO3=l, and 5:5 g=l, respectively.It was heat-treated at 90C for 10 min to inactivate hy-drogen consumers and to harvest spore-forming anaerobicbacteria .
Table 1Characteristics of substrates
Parameter Unit Food Sewagewaste sludge
Total solids % 15.9 5.0Volatile solids % 15.2 2.5Total COD g/l 158.4 31.9Soluble COD g/l 50.3 0.14Total carbohydrate g COD/l 84.9 5.0Total protein g COD/l 37.7 18.4Total Kjeldahl nitrogen g N/l 4.4 2.3Total VFApH 4.6 7.5Alkalinity g CaCO3=l 0.4 4.7
The substrate was a mixture of food waste and sewagesludge. Food waste, sampled from a dining hall, was crushedby an electrical blender under anaerobic condition. Sewagesludge was taken from a gravity sludge thickener into whichprimary and secondary sludges were added at the sameamount of VS. All the substrates were 5ltered through astainless steel sieve (US Mesh No. 10 with correspondingsieve opening of 2:00 mm), of which the characteristics aresummarized in Table 1.
2.3. Operating procedure
The experiments were conducted using 415 ml Wheatonmedia lab bottles. A total of 32 bottles with diGerent volatilesolids (VS) concentrations and mixing ratios of food wasteand sewage sludge were simultaneously operated. Total VSconcentrations were controlled to be 0.5%, 1.0%, 1.5%,2.0%, 3.0%, and 5.0%. The mixing ratios of food wasteto sewage sludge were designed to be 100:0, 80:20, 60:40,40:60, 20:80, and 0:100 on VS basis; however, the experi-ments at 20:80, and 0:100 for VS 3.0%, and 40:60, 20:80,and 0:100 for VS 5.0% could not be conducted due to lowVS concentration of sewage sludge. Total carbohydrate andprotein concentrations, therefore, ranged 1.028:0 g COD=land 1.322:1 g COD=l, respectively. Seed sludge of 40 ml,appropriate amounts of food waste, and sewage sludge wereadded to individual bottles, while seed sludge and distilledwater were added to the blank reactor. Each bottle was sup-plemented with 200 mg of KH2PO4, 14 mg ofMgCl24H2O,2 mg of Na2MoO4 4H2O, 2 mg of CaCl2 2H2O, 2:5 mg ofMnCl2 6H2O, and 10 mg of FeCl2 4H2O, which was mod-i5ed from Lay et al. . NaHCO3 was also added to adjusttotal carbohydrate/alkalinity ratio to 1:0 0:1. Each bottlewas then 5lled to 200 ml with distilled water and pH wasadjusted to 6.0 using either 1 M HCl or 1 M KOH. Sub-sequently, the headspaces of the bottles were Lushed withN2 gas for 1 min and the bottles were tightly sealed using
S.-H. Kim et al. / International Journal of Hydrogen Energy 29 (2004) 16071616 1609
open-top screw caps with rubber septa. The bottles werethen placed in a reciprocating shaker at 35C and 100 rpm.The biogas production was determined using a glass syringeof 20200 ml . At the same time, gas composition wasmeasured and the sample from the supernatant was taken toanalyze pH and organic concentrations. During cultivation,if the pH value was out of 5.06.0, it was adjusted usinginjection of either 1 M HCl or 1 M KOH by syringes.
2.4. Analytical methods
Hydrogen content in biogas was measured by a gas chro-matography (GC, Gow Mac series 580) using a thermalconductivity detector and a 1:8 m 3:2 mm stainless-steelcolumn packed with molecular sieve 5A with N2 as a car-rier gas. The contents of CH4, N2, and CO2 were measuredusing a GC of the same model noted previously with a1:8 m 3:2 mm stainless-steel column packed with pora-pak Q (80/100 mesh) using helium as a carrier gas. Thetemperatures of injector, detector, and column were keptat 80C, 90C, and 50C, respectively, in both GCs. VFA(C2C6), and lactate were analyzed by a high performanceliquid chromatograph (Spectrasystem P2000) with anultraviolet (210 nm) detector and an 300 mm 7:8 mmAminex HPX-97H column using H2SO4 of 0:005 M asmobile phase. Aliphatic alcohol was determined using an-other high performance liquid chromatograph (DX-600,Dionex) with an electrochemical detector (ED50A) and an250 mm 4 mm Dionex CarboPac PA10 column usingNaOH of 0:01 M as mobile phase. The liquid samples werepretreated with 0:45 m membrane 5lter before injectionto both HPLCs. Chemical oxygen demands (COD), Sus-pended solids (SS), VSS, TKN, ammonia, and pH weredetermined according to Standard Methods . Carbohy-drate was determined by the colorimetric method of Duboiset al.  with wavelength at 480, 484 and 490 nm us-ing glucose as standard. Total protein was calculated fromorganic nitrogen (9:375 g COD=g organic nitrogen) .
2.5. Assay methods
The hydrogen production curve was 5tted to a modi-5ed Gompertz equation (1), which was used as a suitablemodel for describing the hydrogen production in batch tests.
H = P exp[ exp
( t)e + 1}]
where H was cumulative hydrogen production (ml), P wasultimate hydrogen production (ml), Rm was hydrogen pro-duction rate (ml/day), was lag-phase time (days), and ewas exponential 1.In many biological 5elds, the basic knowledge of phe-
nomena is insuFcient to build a mechanistic model. In thiscase, response surface methodology, a collection of empir-ical models and statistical analyses, can play an extremely
important role in elucidating basic mechanisms in complexsituations and thus providing better process design and con-trol . In this study, the eGects of VS concentrations andmixing ratios of food waste and sewage sludge on biohy-drogen production were analyzed using the full quadraticmodel as shown below .
Y = 0 + 1x1 + 2x2 + 11x21 + 22x
22 + 12x1x2; (2)
where Y was the predicted response, x1 and x2 were indepen-dent variables, 0 was the oGset term, 1 and 2 were linearcoeFcients, 11 and 22 were squared coeFcients, and 12was the interaction coeFcient.All the parameters in Eqs. (1) and (2) were evaluated
using the Fit curve function with a Newtonian algorithm inSigmaplot 2001. In order to minimize the sum of the squareerrors (SSE) between the experiment and the estimation 100iterations were made. The parameters were diagnosed bySSE, correlation coeFcient (R2), standard errors (SE), 95%con5dence limits, t-values of the parameters, and F-test. Theresponse surface contour plots were also constructed usingSigmaplot 2001.
3. Results and discussion
3.1. E6ects of VS concentrations and mixing ratioson fermentative hydrogen production
Hydrogen was not produced in seven reactors includ-ing the blank in which no food waste was added orcarbohydrate concentration was lower than 2:0 g COD=l.The cumulative hydrogen production curves from the 25hydrogen-producing reactors were well described by Eq.(1). All the correlation coeFcients, R2, were larger than0.984 as shown in Table 2. Additionally, all the t-valuesfor parameters were larger than t0:025;5 = 2:571 (table value)(data not shown).The speci5c hydrogen production potential (ml/g
carbohydrate-COD) was obtained from P and the carbohy-drate added. The obtained values were, then, subjected tothe response surface analysis to evaluate the relationshipbetween food waste composition (x1), VS concentration(x2), and the speci5c hydrogen production potential, andthey generated the following:
(Speci5c H2 production potential)
= 39:793 + 2:069x1 + 48:431x20:011x21 8:103x22 0:015x1x2(R2 = 0:898; F = 31:52): (3)
The calculated values of F were greater than F0:05;5;25 =2:60 (table value), which meant that statistically signi5-cant regression models were obtained . Fig. 1 illus-trates that the speci5c hydrogen production potential wasfound to be higher than 75:2 ml=g carbohydrate-COD at
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Table 2Kinetic parameters on hydrogen production calculated from Eq. (1)
VS Food waste Carbohydrate Protein P Rm R2
(%) (%, VS basis) (g COD/l) (g COD/l) (ml H2) (ml/day) (days)
5.0 100 28.0 12.5 339.7 596.6 0.23 0.9935.0 80 24.4 17.3 308.0 367.0 0.23 0.9935.0 60 20.8 22.1 146.3 139.9 0.09 0.9943.0 100 16.8 7.5 355.1 504.2 0.28 0.9993.0 80 14.6 10.4 347.2 419.8 0.26 0.9933.0 60 12.5 13.3 212.5 290.4 0.15 0.9993.0 40 10.3 16.1 48.6 78.9 0.12 0.9992.0 100 11.2 5.0 208.9 359.0 0.78 0.9922.0 80 9.8 6.9 186.5 369.6 0.49 0.9932.0 60 8.3 8.8 120.0 248.2 0.34 0.9992.0 40 6.9 10.8 38.4 71.3 0.12 0.9992.0 20 5.4 12.7 8.8 29.0 0.16 0.9992.0 0 4.0 14.6 1.5 100 8.4 3.8 119.5 79.2 0.24 0.9881.5 80 7.3 5.2 105.7 150.5 0.35 0.9991.5 60 6.2 6.6 55.2 110.9 0.25 0.9941.5 40 5.2 8.1 28.8 29.0 0.23 0.9901.5 20 4.1 9.5 4.7 7.9 0.24 0.9921.5 0 3.0 11.0 1.0 100 5.6 2.5 74.4 81.8 0.44 0.9981.0 80 4.9 3.5 69.4 84.5 0.36 0.9991.0 60...