International Journal of Hydrogen Energy 32 (2007) 3141–3146www.elsevier.com/locate/ijhydene

Comparison of two anaerobic systems for hydrogen production from theorganic fraction of municipal solid waste and synthetic wastewater

Liliana M. Alzate-Gaviriaa, P.J. Sebastiana,b,∗, Antonino Pérez-Hernándezc, D. Eapenb

aCentro de Investigación en Energía-UNAM, 62580 Temixco, Morelos, MexicobUniversidad Politécnica de Chiapas, 29010 Tuxtla Gutiérrez, Chiapas, Mexico

cCentro de Investigación en Materiales Avanzados, Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua 31109, Mexico

Received 22 November 2005; received in revised form 26 January 2006; accepted 11 February 2006Available online 2 April 2007

Abstract

Two laboratory scale anaerobic digestion systems for hydrogen production from organic fraction of municipal solid waste (OFMSW) andsynthetic wastewater were compared in this study. One of them was formed by a coupled packed bed reactor (PBR) containing 19.4 L ofOFMSW and the other an upflow anaerobic sludge bed (UASB) of 3.85 L. The reactors were inoculated with a mixture of non-anaerobic inocula.In the UASB the percentage of hydrogen yield reached 51% v/v and 127 N mL H2/gvs removed with a hydraulic retention time (HRT) of 24 h.The concentration of synthetic wastewater in the affluent was 7 g COD/L. For the PBR the percentage yield was 47% v/v and 99 N mL H2/gvsremoved with a mass retention time (MRT) of 50 days and the organic load rate of 16 gvs (Grams Volatile Solids)/(kg-day). The UASB andPBR systems presented maximum hydrogen yields of 30% and 23%, respectively, which correspond to 4 mol H2/mol glucose. These valuesare similar to those reported in the literature for the hydrogen yield (37%) in mesophilic range. The acetic and butyric acids were present inthe effluent as by-products in watery phase. In this work we used non-anaerobic inocula made up of microorganism consortium unlike otherworks where pure inocula or that from anaerobic sludge was used.� 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy.

Keywords: Hydrogen production; Anaerobic digestion; Packed bed reactor; Up-flow anaerobic sludge bed reactor.

1. Introduction

Garbage constitutes many problems for humanity, especiallyin the big cities where a lot of population is concentrated. Dueto the modern way of life, increased human activities and con-sumption, the amount of garbage generated increases day byday. The industrialized world depends largely on petroleumand its derivatives for the energy needs. Nevertheless, the con-cerns on diminishing petroleum reserves and the impact of itsincreased use on environment are on the rise. In this contexthydrogen is considered as the energy vector connecting the pri-mary renewable energy sources and the end use.

The anaerobic digestion of organic waste typically producesmethane that can be used as a renewable fuel in power plants,

∗ Corresponding author. Centro de Investigación en Energía-UNAM, 62580Temixco, Morelos, Mexico. Tel.: +52 55 56229706; fax: +52 777 3250018.

E-mail address: sjp@cie.unam.mx (P.J. Sebastian).

0360-3199/$ - see front matter � 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy.doi:10.1016/j.ijhydene.2006.02.034

automobiles, fuel cells, etc. A gaseous by-product formed dur-ing acido-genesis of the anaerobic digestion is hydrogen, whichis considered as the alternative renewable fuel for the applica-tions mentioned above [1]. Hydrogen possesses an energy con-tent of 122 kJ/g, that is to say, 2.75 times more energy contentthan any other hydrocarbon fuel [2]. One of the main applica-tions of hydrogen is in fuel cells to produce electricity throughan electrochemical process. The feasibility of employing acido-genesis of organic waste to produce hydrogen has been demon-strated in various laboratories [2–4].

The advantages of hydrogen production by anaerobic fer-mentation are that many fermentative bacteria are capable ofhigh yield of hydrogen and hydrogen is produced throughoutday and night at a constant rate since it does not depend on en-ergy provided by an external source. Moreover, some authorshave shown that this valuable fuel can be produced from or-ganic fraction of municipal solid waste (OFMSW) and indus-trial wastes [5].

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The studies concerning continuous bio-hydrogen generation,maintaining high levels of fermentative biomass are the onesreferred as UASB (upflow anaerobic sludge bed) reactors [6],processes that deal with high contents of organic waste, obtain-ing high efficiencies in short hydraulic retention time (HRT)[7]. The mantle of the sludge contains a great variety of mi-croflora that favors the bio-hydrogen generation in a suspendedsystem, which can become a source of inocula culture for hy-drogen production [8].

Similarly, the processes of anaerobic stabilization of solidwaste in systems of packed bed reactors (PBRs) have receivedinterest during the last 13 years, being a technical alternativeand economically viable process, considering the fact that ap-preciable reduction of degradable organic matter of the garbageand high methane yield has been obtained. Nevertheless, theuse of organic solid waste as substrate in anaerobic fermen-tative reactors for hydrogen generation has been employed inbatch-type processes.

The aim of the present study is to compare the two anaerobicsystems (PBR and UASB) in terms of their capacity for hy-drogen generation, both working in mesofília range and usingnon-anaerobic inocula, where the carbon source for the acido-genesis phase varied in the UASB synthetic wastewater and inthe system of PBR with OFMSW.

2. Materials and methods

2.1. Inocula

The inoculation of both systems in this study was preparedwith mixtures of non-anaerobic inocula that consisted of 30 g/Ldeep soil, 300 g/L of excretes vaccine, 150 g/L of pig excretes,1.5 g/L of sodium carbonate and 1 L of water [9]. The inoculawas subjected to pre-treatment called heat shocking, boilingat 100 ◦C for 15 min to inhibit the growth of methanogenicbacteria [5]. The ratio of seed (wet basis) to dry OFMSW inthe PBR was 5% w/w; the UASB was filled with the inoculasuspension (2 g/L) and sucrose (3.85 L).

2.2. Substrate

2.2.1. Synthetic wastewaterThe synthetic wastewater presented the following compo-

sition in g/L: glucose 5; NH4Cl 1; NaHCO3 1; Na2CO3 1;K2HPO4 0.2 and 1 mL/L of a trace metal solution [9]. The car-bon source was glucose supplemented with nutrients and pHneutralizing agent (buffer solution); the pH of the liquor mix-ture in the reactor was adjusted using 1 N NaOH and 1 N HCl.

2.2.2. Organic fraction of municipal solid wasteThe characteristics of the OFMSW packed in the PBR are

shown in Table 1and it was obtained from the cafeteria of ourresearch center. The sample was prepared with waste food, re-cycle paper and cardboard. The solid waste was characterizedaccording to the United States norm US EPA [10]. The de-hydrated solid waste was kept in fresh air before starting the

Table 1Characterization of the OFMSW used in the experiments

Materials Wet weight (g)

Plastic 1691.5Bottle (PET) 476.3Tins 543.8Tetra-Pak 55.8Aluminum 85.4Glass 522.2Wood 632.1Paper 3938.3Cardboard 483.7Waste food 9091.7

Total 17 520.8

packing of the PBR. The content of the volatile solid (VS) was78.4% of the total solid (TS).

2.3. Reactors and experimental procedure

The two laboratory scale systems studied and compared inthis work were constructed using transparent acrylic. The de-sign of the prototypes and the experimental setup are based onthe reactors developed by Chynoweth et al. [11]; Alzate-Gaviriaet al. [9] and Poggi-Varaldo et al. [12] and the flow diagrams ofthe process of anaerobic solid digestion in high concentration[13]. The operating temperature was maintained at 38 ± 2 ◦Cmesophilic using a Cole Parmer� thermostatic bath.

Fig. 1 shows the schematic diagram of the laboratory scaleUASB system, with a working volume of 3.85 L, internal di-ameter of 10 cm and height of 45 cm. The pH inside the reactorwas maintained at 5.7 ± 0.2, HRT of 24 h and continuous flowof synthetic wastewater at 160 mL/h.

The second system consisted of two PBRs with freshOFMSW, a mass retention time (MRT) of 50 days, organicload rate of 16 gvs (Grams Volatile Solids)/(kg-day) and con-tinuous recirculation of synthetic wastewater as shown inFig. 2. The laboratory scale prototypes were constructedof acrylic with a height of 90 cm, base dimensions of20 cm×20 cm and working volume of 22.4 L. The rate of irriga-tion on the beds of the PBR system was 34.5 m3/m2-day. ThePBR reactors were filled with OFMSW and PVC rings (2.5 cmdiameter and 1 cm wide) to increase the cavity in the PBR andfacilitate the percolation of leach. Diaphragm type Milton Roy�

pumps with a maximum flow rate of 1500 L/day and maximumtemperature of 107 ◦C were used for recirculation of the leach.

2.4. Analytical methods

The amount of biogas produced in the reactors was measureddaily adopting the method of displacement of acidified salinesolution [14]. The content of biogas was analyzed in a gaschromatograph (GC-ThermoFinnigan) equipped with a thermalconductivity detector (TCD). The column used was a Molsieve5A (0.53 mm; 50 �m; 25 m), the temperatures in the injector,column and detector were 120, 100 and 180 ◦C, respectively.

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The intermediate, partial and total alkalinity and the coeffi-cient alpha were determined according to the procedures pub-lished by Poggi-Varaldo and Oleszkiewicz [14]. The volatileorganic acid (VOA) concentration, hydrogen potential (pH),temperature, TS, VS in the OFMSW, digested materials andchemical demand for oxygen (CDO) soluble in the liquid cur-rents were determined according to the procedures of StandardMethods [15].

3. Results and discussion

3.1. Start-up

In this study the THF (time to reach a full hydrogen-genesisregime) of the UASB system was 7 days (characteristic of therecirculated effluent: pH 6.15; Alpha 0.5; VOA 540 mg/L HAc[expressed in acetic acid]; %H240% in biogas); while for thePBR system the THF was more than 20 days (characteristic ofthe recirculated effluent: pH 5.60; Alpha 0.5; VOA 3150 mg/LHAc; %H240% in biogas). This may be due to the fact that the

Fig. 1. Laboratory scale reactors. (a) System 1: packed bed reactors loaded with OFMSW. (b) System 2: upflow anaerobic sludge bed reactor loaded withmassive inocula.

liquid-phase system has its carbon source in soluble glucosepresent in the synthetic wastewater, while the reactors with solidphase have their carbon source mainly in the form of OFMSW,a compound consisting of carbohydrates of the waste food andcellulose, hemicellulose of the recycle paper and cardboardpacked in the reactors. Later on, the PBR system is fed by itsown OFMSW and the UASB by the synthetic wastewater.

3.2. pH

The hydrogen generation level reached higher values for pHvalues at around 5.6 ± 0.2. The performances of both systemsin this respect are shown in Figs. 2 and 3(a), respectively. Ourresults are similar to those reported by other authors in theliterature where the influence of pH on hydrogen production isdemonstrated as fundamental [4,6,16]. The composition of theeffluent depended on the pH level in the UASB. In general, thebutyric and acetic acid production increased with the increaseof pH; 25% and 19%, respectively, at the pH of 5.0 and 29%and 22%, respectively, at the pH of 5.8. This change of product

3144 L.M. Alzate-Gaviria et al. / International Journal of Hydrogen Energy 32 (2007) 3141–3146

Fig. 1. (continued).

distribution is probably due to the change of microbial popula-tion in the reactor [6]. In the PBR system, initially the pH was7.7 and after 8 days the pH changed to 6.3, which is reflectedin the absence of hydrogen generation. The hydrogen evolutionstarted after obtaining the pH level of 5.6 ± 0.2 in 12 days,which was maintained at this level by adding HCl accompaniedby the buffer solution.

The by-product accumulation at the end of the hydrogen-genesis process (molecular hydrogen) as butyric and aceticacids is considered as barriers for the fermentation, which canbe surpassed with an external power [17]. Nevertheless, thesemetabolites can be seen as an additional bonus for the processused as raw material in several industries [5].

3.3. Upflow anaerobic sludge bed

The performance of the UASB reactor is shown in Fig. 2.The UASB sludge did not granulate over the entire operatingperiod. Chang and Lin [8] using affluent synthetic wastewa-ter with a concentration of 20 g DQO/L and Lay et al. [3] em-ploying synthetic residual water with a starch concentration of

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Fig. 2. Performance of the leading upflow anaerobic sludge bed reactor ofsystem 1.

4.25 g DQO/L obtained a start-up of 13 and 10 days, respec-tively, slightly superior to what we got for our UASB of 7 daysand with a concentration of 7 g DQO/L, which was operated inrecirculation after having reached the THF. But, the real com-parison of those systems with our UASB indicate that ours isbetter due to the fact that Chang and Lin inoculated their UASBswith pretreated sludge from anaerobic (heat shocking at 100 ◦Cduring 45 min), while in our case a mixture of non-anaerobic in-ocula with similar pre-treatment was used. Nevertheless, in lit-erature superior values are reported for THF compared to ours.Yu et al. [6] report a THF of 20 days in a mesophilic laboratoryscale UASB, where the carbon source used was the industrialresidual water from a rice winery, which may be attributed tothe change of quality of the affluent residual water.

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Table 2Start-up and hydrogen yield of several upflow anaerobic sludge bed (UASB) reactors

System characteristics Y a (mol H2/mol glucose) HRTb (h) T (◦C) H2 (% v/v) Volume (L) Reference

Non-anaerobic seed, flocculent sludge, C: 7 g COD/L(inocula 100 ◦C for 15 min), 7 days start-up, syntheticwastewater (pH 5.8–6.0)

1.98 ± 0.03c 24 38 ± 2 50.5 ± 0.8 3.85 This study (2005)

Anaerobic sludge inocula, C: 20 g COD/L (inocula100 ◦C for 45 min), 13 days start-up, synthetic waste-water

1.50 8 35 ± 1 42.4 3.0 Chang and Lin (2004)

Anaerobic sludge inocula, C: 14–36 g COD/L, 20 daysstart-up, rice winery wastewater

1.37–2.14 2 20.55 ± 1 53–61 2.9 Yu et al. [6]

Anaerobic seed, C: 10 g COD/L sugar-industry waste-water

2.6 12 60 ± 2 64 Lab. Ueno et al. (1996)

Anaerobic digested sludge, C: 4.25 g COD/L, 10 daysstart-up, starch synthetic substrate

1.29 17 37 ± 1 59 3.0 Lay (1999)

aHydrogen yield.bHydraulic retention time.cEquivalent to 127 ± 10 mL H2/gvsremoved.

Table 2 shows the percentage of hydrogen in biogas ob-tained in our study. We obtained 50 ± 2% v/v with a yield of1.98±0.02 mol H2/mol glucose in mesophilic phase. Compar-ing our work with those reported in the literature suggests thatin thermophilic phase one obtains better efficiency. Ueno et al.[18] and Yu et al. [6] obtained H2 % as 64 and 61 v/v, respec-tively, with yield of 2.14 and 2.6 mol H2/mol glucose, respec-tively. This may be explained due to the fact that the bio-kineticsare faster in mesophilic phase. The maximum yield obtainedfrom glucose by stoichiometry is 4 mol H2/mol glucose.

3.4. Coupled PBR with OFMSW

Fig. 3 shows the behavior of our PBR system. The produc-tivity of bio-hydrogen in mesophilic range is lower than that inthermophilic range, according to the results reported in the lit-erature [5]. Biogas obtained in our PBR system consisted onlyof hydrogen, not of carbon dioxide or methane.

Studies on generation of bio-hydrogen from OFMSW arestill a few; much of these studies are done on reactors in theliquid phase fed by soluble wastes. Table 3 shows a compar-ison of our results with similar kind of works reported in theliterature. Valdez-Vázquez et al. [5] reported a higher value ofhydrogen yield, 360 mL H2/gvs (Grams Volatile Solids), pre-sumably due to the fact that their work is in thermophilic regimewhere the biochemical reactions are faster. Our work shows ahydrogen yield of 99 mL H2/gsvremoved and 47%H2 v/v in thePBR, using previously stabilized and pretreated (heat shocking)non-anaerobic inocula. Our values are located in the interme-diate range in comparison with the reports by Lay et al. [3] andValdez-Vázquez et al. [5] all in mesophilic phase with anaerobicsludge inocula, yielding 180 and 165 mL H2/gsvremoved, respec-tively, and with percentage of 60% and 42% H2 v/v, respec-tively. Okamoto et al. [19] worked with three different types ofsubstrates (cabbage, rice and carrot) and obtained yields of 62,96 and 71 mL H2/gsvremoved and 55%, 46% and 47% H2 v/v,respectively.

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Table 3Yield of hydrogen of several packed bed reactors

System characteristics Y (N mL H2/gvs) pH T (◦C) H2 (% v/v) Volume (kg) Reference

Non-anaerobic seed OFMSW (batch) 99 ± 5 NCa 38 ± 2 47 ± 3 2.5 This study (2005)Anaerobic digested seed OFMSW (semi-continuous) 165 5.5 37 ± 2 42 1 Valdez-Vázquez et al. [5]Anaerobic digested seed OFMSW 360 6.4 55 ± 2 58 1 Valdez-Vázquez et al. [5]Hydrogen-producing microorganism seed, FORSU (batch) 180 5.2 37 ± 1 60 0.1 Lay et al. [3]Sludge digested carrot (batch) 71 NC 37 ± 1 47 3 Okamoto et al. [19]Sludge digested rice 96 NC 37 ± 1 46 3 Okamoto et al. [19]Sludge digested cabbage 62 NC 37 ± 1 55 3 Okamoto et al. [19]

aNot controlled.

Our study demonstrated that it is possible to obtain bio-hydrogen from non-anaerobic microorganism consortium,without using pure cultures, hence reducing the hydrogen pro-duction cost. Finally, biogas produced in the UASB was greaterin amount and percentage of hydrogen than that generated bythe PBR, due to massive liquid-phase inocula in the former.

4. Conclusions

Both UASB and PBR systems compared in this study arealternatives for hydrogen production. The liquid-phase systems(UASB) are faster than the solid-phase systems (PBR). Thebio-hydrogen production from organic waste and residual wateris a better option compared to the waste disposal and residualwater treatment, since the former is part of the renewable energyproduction and environmental protection. This also saves thecost of garbage disposal that is expensive and obligatory. Atthe end of the hydrogen-genesis phase, the by-products such asacetic and butyric acids are obtained.

Acknowledgments

The authors wish to acknowledge the financial support re-ceived from PROMEP through the grant UPCHIS-PTC-007and CONACYT through the project G38618-U. The excellentlaboratory assistance received from Rubén Guevara and AreliParra Chavero is also acknowledged.

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