Methods for enhancing bio-hydrogen production from biological process: A review

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JIEC 2062 1–11 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6

Review

Methods for enhancing bio-hydrogen production from biological process: A review Q1 Lakhveer

Singh *, Zularisam A. Wahid *

Faculty of Technology, Universiti Malaysia Pahang (UMP), Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Pahang, Malaysia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 February 2014 Received in revised form 23 May 2014 Accepted 26 May 2014 Available online xxx

Hydrogen (H2) is considered a clean source of energy and an ideal substitute for fossil fuel owing to its high energy content (122 kJ/g), recyclability and non-polluting nature. The conventional physicochemical methods for hydrogen production are costly owing to high energy input requirements. Hydrogen production by means of biological processes is considered the most environmental friendly and relatively easy to operate, with successful operation under ambient conditions and promising techniques, and having significant advantages compared with conventional chemical processes. The major bottlenecks in biological processes are the low hydrogen yield and production rates at a large scale. The target is improvement of the biological method for hydrogen production and devising path even better in comparison to the conventional methods. The present review article aims to highlight various techniques such as pretreatment, cell immobilization, sequential fermentation, combined fermentation that have been used in biological processes for enhancing hydrogen production. Future developments on these techniques are also outlined. ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Biohydrogen Sequential fermentation Combined fermentation Pretreatment Cell immobilization

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pretreatment process . . . . . . . . . . . . . . . . . . . . . . . Cell immobilization techniques. . . . . . . . . . . . . . . Sequential dark and photo fermentation process. Combined dark and photo fermentation process . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abbreviations: GHG, greenhouse gases; TVFA, total volatile fatty acids; HPB, hydrogen producing bacteria; UASB, up flow anaerobic sludge blanket; HCB, hydrogen-consuming bacteria; OFMSW, organic fraction of municipal solid waste; VFAs, volatile fatty acids; ASBRs, anaerobic sequencing batch reactors; VS, volatile solid; IBR, immobilized bioreactor; POME, palm oil mill effluent; AHB, annular hybrid bioreactor; BESA, bromoethanesulfonate; CSTR, continuous stirred tank reactor; PDMS, polydimethylsiloxane; PVA, polyvinyl alcohol; VSS, volatile suspended solids; GAC, granular activated carbon; HRT, hydraulic retention time; CCD, central composite design; EVA, ethylene vinyl acetate copolymer; TCOD, total chemical oxygen demand; PMMA, polymethyl methaacrylate; COD, chemical oxygen demand; CA, calcium alginate matrix; CH, chitosan; TiO2, titanium oxide. * Corresponding authors. Tel.: +60 9 549 2362; fax: +60 9 549 2362. E-mail addresses: [email protected] (L. Singh), [email protected], [email protected] (Z.A. Wahid). http://dx.doi.org/10.1016/j.jiec.2014.05.035 1226-086X/ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

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Introduction

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Worldwide, the extensive use of fossil fuels for power plants, automobiles, and rapid industrialization is increasing day by day, resulting in not only environmental pollution, but also economic and diplomatic problems owing to their limited reserves and uneven distributions [1]. Furthermore, the burning of fossil fuels (coal, natural gas, oil, petroleum) in different sectors contributes to the emission of greenhouse gases as shown in Fig. 1. These gases, such as CO2, CH4, NO2, SO2 and other toxic pollutants, are the main causes of ozone layer depletion, global warming, climate change and acid rain. These problems are surely the most hazardous for the future of the earth and should be solved without delay. It is an urgent issue to find alternative energy sources that are environmentally benign, cost-competitive, and renewable [2,3]. Hydrogen is a viable alternative source to replace conventional fossil fuels owing to its clean, renewable and high energy yielding (122 kJ/g) nature. This energy yield is 2.75 times higher than those of hydrocarbon fuels. When hydrogen is used as a fuel, its main combustion product is water, which can be recycled again to produce more hydrogen, but unlike fossil fuels, hydrogen gas is not readily available in nature, and the commonly used production methods are quite expensive [4]. In recent years, a significant use of hydrogen has been demonstrated for hydrogen-fueled transit buses, ships and submarines, including chemical and petrochemical applications. Currently, about 98% of hydrogen comes from fossil fuels [5]. Worldwide, 40% of hydrogen is produced from natural gas or steam reforming of hydrocarbons, 30% from oil (mostly consumed within refineries), 18% from coal, and the remaining 4% via water electrolysis [6]. However, these processes and also those involving electricity productions are

energy-exhaustive, expensive and not always environmentally friendly. Biological processes for hydrogen production provide an alternative method of hydrogen production can be operated at ambient temperatures and pressures, are less energy intensive and more eco-friendly compared with conventional chemical methods [7]. This approach is not only eco-friendly but also opens new paths for the exploitation of renewable energy resources that are unlimited [8–10]. In addition, they can also use various waste materials, which facilitate waste recycling. Hydrogen production by biological methods can be divided into two main categories: that involving photo energy dependence fermentation [11,12] and anaerobic fermentation [13–15]. Production of hydrogen via dark fermentation has some advantages over other biological processes, such as the high rate of cell growth, non-requirement of light energy, no oxygen limitation problems and the potential for costeffective hydrogen production [16–18]. However, low hydrogen yields, low production rates and reactor instability for continuous high volume hydrogen production remain the major problems of the anaerobic method for hydrogen production at commercial levels. Current hydrogen productivities reported in the literature are 10–20% molar ratio [19–21], much lower than the theoretical value (33%). The main reasons for low hydrogen yield and unstable hydrogen production rate in biological methods are:

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(1) Consumption of hydrogen by hydrogen-consuming bacteria (HCB). (2) Production of more reduced products (e.g., solvents). (3) For high rate and continuous hydrogen production, there is a need for a correspondingly high feeding rate of medium, which will result in washout of the cells.

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Fig. 1. Annual GHG emissions by different sectors. [Source: Global anthropogenic green house gas emissions in 2004].

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These limitations and low hydrogen production rates greatly restrict the biological methods for hydrogen production at industrial levels. Raising the hydrogen yields and hydrogen conversion efficiencies in biological processes are fundamental but complicated tasks. Higher hydrogen yields would be valuable for practical application of the technology. This review paper discusses the efficiency of four operational techniques to enhance the hydrogen production from biological processes: (1) pretreatment method; (2) cell immobilization; (3) sequential dark and photo fermentation; and (4) combined dark and photo fermentation. Moreover, the review identifies several recommendations for realistic applications and further improvement of these techniques for enhancing fermentation-based hydrogen production.

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Pretreatment process

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During anaerobic fermentation, organic matter is first converted into hydrogen, along with volatile fatty acids (VFAs) and alcohols. The transitional metabolites are further degraded by methanogens and are then converted into methane with the reduction of chemical oxygen demand (COD). An anaerobic digestion process produces hydrogen gas in small amounts because methanogens quickly consume hydrogen to produce methane [22–25]. In dark fermentation, hydrogen is produced as a by-product during the acidogenic phase of anaerobic digestion of organic matter. However, in dark fermentation, the yield of hydrogen is relatively low, since hydrogen produced as an intermediate is reduced to methane, acetate, and propionate by HCB. Production of hydrogen from biomass can be increased by suppressing the methanogenic activity, by restricting HCB and by enhancing hydrogen producing bacteria (HPB). Thus, in order to enhance the hydrogen production from a fermentative hydrogen production process, inoculums can be pretreated by certain method to suppress as much hydrogen-consuming bacterial activity as possible while still preserving the activity of the HPB [26]. Different pretreatment methods (heat shock, base, acid, freezing and thawing, aeration, chloroform, iodopropane and 2-bromoethanesulfonic acid or sodium 2-bromoethanesulfonate), have been employed on variety of mixed cultures for selective enrichment of acidogenic hydrogen producing inoculum. Table 1 summarizes the different pretreatment methods for enriching HPB from various inoculums along with the comparative results. Heat treatment: Wang et al. [27] studied the effect of various pretreatments (alkaline, acid and heat) on the anaerobic mixed microflora for increasing the hydrogen production using glucose as substrate. It was found that experiments 80–15 and 80–30 (conditions with heat pretreatment for 15 and 30 min at 80 8C, respectively) showed superior performance with regards to the hydrogen yield and hydrogen production efficiency of 2095.5 and 2152.0 mL, 436.6 and 448.3 mL H2/g COD, respectively, which was 53.2% and 49.0% higher than that of the control, respectively. When the heat pretreatment time increased from 15 min to 30 min at 90 8C, the cumulative H2 yield and hydrogen production efficiency decreased from 1623.0 mL to 1320.5 mL and 338.1 mL/g COD to 275.1 mL/g COD, respectively. Along with an increase in the pretreatment temperature to 100 8C, the cumulative H2 yield decreased to between 1300.0 and 1319.0 mL. Working with sludges, Kawagoshi et al. [28] found the highest hydrogen production with heat-conditioned anaerobic digested sludge, which was almost the same as that obtained with unconditioned digested sludge using glucose as the substrate. Various pretreatments were studied by Dong et al. [29] in their work on anaerobic sludge, using mixtures of rice powder and lettuce powder as the substrate to enhance hydrogen production. The heated anaerobic sludge achieved the highest hydrogen yield of 119.7 mL/g VS. The

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hydrogen yield were ound to be in the order heated > acidified > freeze/thawed > sonicated > aerated > control anaerobic sludge. The methanogens had been effectively suppress by the heat and acid pretreatment, but sonication, freeze/thawing and aeration could not completely suppressed methanogenic activity. Mu et al. [30] confirmed that the hydrogen production yield from seed sludge, pretreated by heat-shock, was the highest compared with the situation found with acid or alkaline treatments. Rossi et al. [31] were investigated the influence of different pretreatment methods (acid, base, heat shock, dry and desiccation, and freezing and thawing) for selecting and conditioning an environmental microbial consortia of bacteria for their use in producing hydrogen from raw glycerol resulting from biodiesel synthesis. The results showed that heat shock produced the highest hydrogen yield followed by dry heat and desiccation, while base pretreatments produced lower than the control indicating that this treatment inhibited the growth of the microbial consortia contributing thereby to a lower hydrogen production and yields. Current evidence proves that heat treatment increases the hydrogen yield compared to non-heat-treated systems. Although the intent has primarily been prevention of methanogenesis, the use of heat treatment does not select exclusively for hydrogen producing bacteria. This is because the characteristic of hydrogen production is not directly associated with the ability to form endospores. There are many hydrogen consuming groups of bacteria that can form spores and therefore survive heat treatment, including acetogens (Acetobacterium, some Clostridium spp, Sporomusa), certain propionate and lactate producers (Propionibacterium, Sporolactobacillus), and a sulphate-reducer (Desulfotomaculum) [32]. Load-shock: The load-shock treatment method was the found to be the best by O-Thong et al. [33] for enriching thermophilic hydrogen-producing seeds from mixed anaerobic cultures, as it completely suppressed methanogenic activity and enhanced the hydrogen production yield of 1.96 mol H2/mol hexose with an hydrogen production rate of 11.2 mmol H2/L h, compared with the situation found without treatment of the anaerobically digested sludge of 0.3 mol H2/mol hexose hydrogen yield. In addition, the microbial community of load-shock treated sludge consisted of Thermoanaerobacterium sp. and Clostridium sp. This result implies that load-shock treatment made an environment more dominated by hydrogen-producing spore forming bacteria and repressed methanogenic activity in mixed anaerobic fermentation. Acid and base: Cheng and Hansen [34] also used different pretreatment methods for enriching HPB from cattle manure sludge and then found that acid pretreatment method was the best amongst the five methods (Table 1) for enhancing the hydrogen production yield. Kim and Shin [35] studied the effect of different pretreatments (continuous spraying acid-pretreatment of food waste at a pH of 2.0 and base pretreatment of food waste at a pH of 12.5) to achieve a reliable start-up method for continuous enriched culture for hydrogen production from food waste and to suppress unintended microbial reactions. It was concluded that the basepretreatment reduced indigenous anaerobic bacteria and enabled more stable long-term operation over 90 days with the maximum hydrogen yield of 0.87 mol H2/mol hexose added, compared with acid and CO2 spraying pretreatments. Chemical: Six pretreatment methods were discussed by Zhu and Beland [36] for enriching the HPB from digested wastewater sludge, and it was found that hydrogen production yield was maximum with iodopropane pretreatment compared with the control in the first batch, while in the second batch, maximum hydrogen yield was obtained with base treatment compared with the control and other treatments. Mohan et al. [37] applied different pretreatment methods on anaerobic mixed inoculums for enhancing hydrogen production using dairy wastewater as

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JIEC 2062 1–11 L. Singh, Z.A. Wahid / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Table 1 Comparison of different pretreatment methods for enriching hydrogen producing bacteria from various inoculums along with the comparative results. Inoculum

Substrate

Reactor type

Inoculum pretreatment method

Best pretreatment method

Hydrogen yield with control

Hydrogen yield/production with treatment

References

Anaerobic mixed microflora acquired from pig manure treating reactor Environmental-isolated consortium of bacteria

Glucose

Batch

Alkaline, acid and heat

Heat

1405.0 mL

2095 mL

[27]

Glycerol

Batch

Heat shock

1.20% mol/g glycerol

5.27% mol/g glycerol

[31]

Anerobic digested sludge (OFMSW)

Rice and Lettuce powder

Batch

Heat

18.8 mL/g VS

119.7 mL/g VS

[29]

Anaerobic digested sludge from a biogas reactor of palm oil mill plant

Sucrose (20 g/L)

Batch

Load-shock

0.3 mol H2/mol hexose

1.96 mol H2/mol hexose

[33]

Anaerobic mixed microflora from an UASB reactor treating wastewater

Dairy wastewater

Batch

BESA

0.0018 mmol/g COD

0.0317 mmol/g COD

[37]

Sludge from secondary settling tank of wastewater treatment plant

Glucose

Batch

Repeated-aeration

51.9 mL

224.5 mL

[39]

Seed sludge from anaerobic digester at the municipal wastewater treatment plant Anaerobic mixed microflora from an UASB treating soybean-processing wastewater Digested sludge from primary anaerobic digester of wastewater treatment plant

Food waste

ASBRs

Base

0.10 mol H2/mol hexose added

0.87 mol H2/mol hexose added

[35]

Sucrose

Heat

[30]

Iodopropanea Alkalineb

Glucose

Batch

(a) 5.64 mol/mol sucrosea (b) 6.12 mol/mol sucroseb 1.0 mol/mol glucose

[36]

Cattle manure sludge

Aerobic activated sludge from, anaerobic digested sludge, soil from watermelon field, soil from kiwi grove, lake sediment, aerobic refuse compost Sewage sludge from aeration tank of a municipal wastewater treatment plant Pseudomonas sp. GZ1 (EFSS1040) seed bacteria

Glucose

Batch

Heat, acid, alkaline, aeration, BESA, iodopropane and control Freezing, thawing, acid, heat-shock, and sodium 2-bromoethanesulfonate Heat, acid and control

0.48 mol H2/mol-glucose (alkaline treated) (a) 5.17 mol/mol sucrosea (b) 4.56 mol/mol sucroseb –

2.00 mol H2/mol-glucose

Sucrose

5-L fermentor reactor Batch

Acid, base, heat shock, dry, desiccation, freezing and thawing Acid, heat, sonication, aeration freeze and thawing Alkaline, acid, BESA, heat, load-shock and control Acid, heat, BESA, acid + heat, BESA + heat, acid + BESA, acid + heat + BESA and control Heat, acid, alkaline, repeated-aeration and control Continuous CO2 spraying, acid and base Heat, acid and alkaline

Heat

1.35 mol H2/mol glucose

1.38 mol H2/mol glucose

[28]

Sewage sludge

Batch

Alkaline and without alkaline

Alkaline

9.1 mL H2/g of digested sludge

16.6 mL H2/g of digested sludge

[38]

Waste water sludge

Batch

Sterilization, microwave and ultrasonication

Sterilization

4.68 mL/g TCOD with ultrasonication

15.02 mL/g TCOD

[40]

a b

214 215 216 217 218 219 220 221 222 223 224 225 226 227 228

Acid

[34]

First batch. Second batch.

substrate and found a maximum hydrogen yield with 2-bromoethanesulfonate (BESA) pretreatment compared with the other pretreatment methods. However, pretreatment using a chemical inhibitor such as BESA is not straightforward because of its high cost [34]. On the other hand, although these chemical inhibitors of methanogenesis have been studied widely, there are still many questions needed to be identified. Their inhibition concentration, characteristics, adverse effects, and the application fields should also be investigated. For example, the BESA and iodopropane, which have been seen as ‘‘specific’’ methanogenic inhibitors, now were found that they also inhibit and influence the nonmethanogens. This means the conclusion from the applications of these chemical inhibitors should be reconsidered and evaluated carefully. Others: The alkaline pretreatment of sewage sludges was extensively examined by Cai et al. [38] who found that the

maximum hydrogen yield (16.6 mL H2/g digested sludge) occurred at a pH of 11.0 compared with raw sludge (9.1 mL H2/g digested sludge). Ren et al. [39] studied four pretreatment methods including heat-shock, acid, alkaline and repeated-aeration pretreatments applied to seed sludge, in order to enrich HPB and to establish highly efficient communities of the mixed microbial cultures. Between the time periods 35–40 h the highest hydrogen yield of 224.5 mL was observed with the repeated-aeration pretreatment method, while the lowest 51.9 mL was obtained as the seed sludge was acidified. The repeated aeration pretreatment could ensure higher microbial diversity. Guo et al. [40] reported the effects of three different pretreatments, microwave, sterilization and ultrasonication on hydrogen production from waste sludge using a new source of inoculums Pseudomonas sp. GZ1 (EF551040). It was found that pretreated sterilized sludge had the largest yield

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of hydrogen production (15.02 mL/g TCOD), compared with microwave or ultrasonication pretreated waste sludge. As is shown in Table 1, there exists certain disagreement on the optimal pretreatment method for enriching hydrogen producing bacteria from mixed and pure cultures. The possible reason for this disagreement was the difference among these studies in the terms of inoculums, pretreatment method studied, specific condition of each pretreatment method and the kind of substrates. Even though heat-shock was the most widely used pretreatment method for enriching hydrogen-producing bacteria from inoculums, it is not always effective for enriching hydrogen-producing bacteria from mixed culture inoculum compared with other pretreatment methods, for it may inhibit the activity of some hydrogenproducing bacteria [36]. In addition, in the reviewed studies, most of the comparisons of various pretreatment methods for enriching HPB from mixed culture inoculum were conducted in batch mode, conducting these comparison in continuous mode is suggested. The effect of the pretreatments is however very dependent on the biomass composition and operating conditions. All these pre-treatments have their advantages and disadvantages and future research is needed for optimization.

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Cell immobilization techniques

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Hydrogen production via dark fermentation has some advantages over other biological processes owing to its capability of attaining high hydrogen production rates [3]. Fermentative production of hydrogen has also an extra merit, as it reduces the burden of organic wastes from the environment by converting biodegradable fractions into more valuable energy resources. Most studies on fermentative hydrogen production from complex wastewater are focused on suspended-cell systems based either on batch or continuous operation [41]. However, in continuous operation on suspended-cell systems, problems of biomass washout at high dilution rates are often encountered. Recycling the biomass has to be used to maintain sufficient cell density for high hydrogen production. Cell immobilization techniques can also

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be used to increase the biomass concentration for both mixed and pure cultures [2]. Thus, immobilization-cell systems are also adopted with a feature of creating a local anaerobic environment, which is well suited to oxygen perceptive fermentative hydrogen production. In addition, immobilized cells also reduce the possibility of cell contamination. This is particularly important for the long-term H2 production process, when cells are hard to keep axenic. Support materials for cell immobilization can be synthetic polymers, such as alginate and polyvinyl alcohol, or naturally available, such as lignocellulosic materials from agricultural residues [41]. Some researchers have concluded that, the immobilized cells culture systems appears to be more effective way to increase hydrogen production. The number of immobilized hydrogen studies in the literature are quite limited (Table 2). Ismail et al. [42] studied hydrogen production from palm oil mill effluent (POME) using polydimethylsiloxane (PDMS) as the immobilization ion matrix and reported the hydrogen production rate of 2.1 L H2/L-POME with immobilized mixed cultures, and they showed efficient soluble carbohydrate consumption and volatile suspended solids removal of 88 and 62% under continuous operation for hydrogen production. Patel et al. [43] and Kumar and Das [44] used mixed and pure cultures immobilized by a lignocellulosic matrix for enhancement of continuous hydrogen production and obtained a higher hydrogen production rate by immobilized cells than that found with free cells. The hydrogen-producing anaerobe, Clostridium tyrobutyricum JM1, was isolated from a food waste treatment process and immobilized in a packed-bed reactor using polyurethane foam as the support medium to improve the efficiency of substrate utilization and continuous hydrogen production [45]. The maximum hydrogen yield and hydrogen production rate were 223 mL/g hexose and 7.2 L H2/L d with an influent glucose concentration of 5 g/L. Therefore, the immobilized reactor using C. tyrobutyricum JM1 was an effective and stable system for continuous hydrogen-production efficient substrate utilization. Immobilization of cells in polymeric materials e.g. ethylene-vinyl acetate copolymer alcohol (EVA) and polymethyl methacrylate (PMMA) have been used successfully [46,47] for hydrogen

Table 2 Cell immobilized technique for enhancing bio-hydrogen production. Material used as solid support matrices for immobilization

Reactor

Inoculums

Substrate

Hydrogen production with suspended-cell system

Hydrogen production with immobilized-cell system

References

Polydimethyl-siloxane

CSTR

2.1 L H2/L POME

[42]

Batch

Palm oil mill effluent Acetate

–

Agar gel

–

Continuous, conditions

3.15 mol H2/mol acetate 1.80 L H2/L h

[51]

PMMA collagen, and activated carbon

Immobilized PDMS-mixed cultures cubes Rhodopseudomonas faecalis strain RLD-53 Anaerobic sludge

Ligno-cellulosic wastes-banana leaves and coconut coir Calcium alginate matrix (CA + CH + TiO2) Biological mycelia pellets Lignocellulosic agroresidues Polyurethane form

Batch

Mixed microbial cultures (MMCs)

Batch

Granular activated carbon (GAC) EVA (ethylene vinyl actate copolymer) Ager gel

AFBR

Sucrose based synthetic water Glucose

1.21 L H2/L h H2/mol sucrose 40–60 mL H2/day

300–330 mL H2/day

[43]

Anaerobic sludge

Sucrose

6.8 mmol/L h

21.3 mmol/L h

[49]

CSTR

Clostridium sp. T2

Xylose

1.13 mmol H2/L h

2.76 mmol H2/L h

[50]

Bioreactor

Lignocellulosic

37 mmol/L h

62 mmol/L h

[44]

Glucose

–

7.2 L H2/L d

[45]

Glucose synthetic wastewater Sucrose

–

2.36 L H2/L h

[54]

Batch

Enterobacte cloacae IIT-BT 08 Clostridium tyrobutyricum JM1 Activated sludge and digested sludge Sewage sludge

–

488 mL H2/g VSS

[46]

Batch

Rhodobacter sphaeroides

Tofu wastewater

1.9 L/h/m2 gel

2.1 L/h/m2 gel

[52]

IBR

[47]

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production from simple sugars in both batch and continuous modes. These materials have been used widely because of low cost, high mechanical strength and durability with ease of handling [48]. Acclimated sludge entrapped in composite polymer matrix (PMMA/collagen/activated carbon) drastically increased the hydrogen production rate from 1.21 L H2/L h (in suspended system) to 1.80 L H2/L h (in immobilized system). Cells immobilized by a calcium alginate matrix supplemented with chitosan and titanium oxide carriers (CA + CH + TiO2) were found by Wu et al. [49] to be more effective for hydrogen production and provided a maximum production rate of 21.3 mmol/L h (at 35 8C and 20 g COD/L sucrose), which was 3-fold greater than that obtained with suspended cells (6.8 mmol/L h). The results of this work suggested the potential use of immobilized-cell systems for hydrogen production. Zhao et al. [50] were investigate the performance of hydrogen production in continuous processes by Clostridium sp. T2 immobilized on mycelia pellets and compared with free cell or traditional carrier sodium alginate. It was obtained that the maximum hydrogen production immobilized on mycelia pellets reached 2.76 mmol H2/L h at the HRT of 10 h, which was considerably improved compared with carrier-free process of 1.13 mmol H2/L h, but slightly lower than the one immobilized in sodium alginate. Liu et al. [51] reported the use of an immobilization technique for photo fermentation bacteria for hydrogen production. However, they reported that the immobilized photo fermentation bacteria could not only enhance hydrogen production but also increase acid-tolerance capacity. Hydrogen was produced even at a pH of 5.0, and so immobilized photo fermentation bacteria can hopefully be applied in combined of dark and photo fermentation for improving the hydrogen production capacity. Hydrogen production from tofu wastewater by the anoxygenic phototrophic bacterium immobilized in agar gel was discussed by Zhu et al. [52] who demonstrated that this provided an effective method for simultaneous hydrogen production and wastewater treatment. The maximum rate of hydrogen production from the wastewater was 2.1 L/h/m2 gel which was even slightly higher than that from glucose medium (as control). Another study reporting high hydrogen formation rate by continuous dark fermentation from fructose (20 g COD/L, 480 kg COD m3 d1) was reported by Lee et al. [53] where a membrane cell recycle reactor (MCR) along with a CSTR fermenter was used. MCR provided high cell concentrations in the fermentor allowing operation at high dilution rates. The highest yield (1.36 mol H2/mol hexose) and hydrogen formation rate (2750 mL H2/L h) were obtained at HRT = 1 h, pH = 6.7 and T = 35 8C. The immobilization protected the bacterium to resist against ammonium ion inhibition. Thus, hydrogen production from immobilized cells is a more promising system than the production from suspension cultures. Available literature shows the high number of immobilization supports proposed by various researchers for hydrogen production. The advantages associated with the production of potable hydrogen using immobilized cell systems (increased rates of productivity, reduced risk of contamination, biocatalyst recycling and high cell density) are well established. However, solid matrices

used for the immobilization of the whole cells were mostly synthetic polymers or inorganic materials. These materials possessed disposal problems and often toxic to microorganisms. Therefore, efforts should be concentrated on cheap, nontoxic and environment friendly immobilization supports, which will improve hydrogen production.

373 374 375 376 377 378

Sequential dark and photo fermentation process

379

Dark and photo fermentation were found to be important hydrogen production technologies. However, the drawback of the single dark/photo fermentation process is lower hydrogen yield owing to the accumulation of short chain organic acids and the expense of pure organic acids. These problems can be overcome by adopting the sequential dark and photo fermentation technique. The waste matter of dark fermentation is used as the substrate for photosynthetic bacteria of the photo fermentation process [55,56]. Fig. 2 shows typical process scheme for bio-hydrogen production by sequential dark and photo-fermentations is described by Redwood et al. [57]. In the second photo fermentative step, the short-chain organic acids are assimilated to hydrogen in the presence of light, producing a maximum of 4 and 6 mol of H2 per mol of acetate and lactate, respectively. The sequential dark and photo fermentation process has a maximum theoretical hydrogen yield of 12 mol H2/mol hexose sugar, and the process is being developed within the European Union 6th framework project ‘‘HYVOLUTION’’ [58]. A sequential dark and photo fermentation is a promising technique for increasing hydrogen production from various carbohydrate-rich wastes and wastewaters. The technique seems to be the most attractive approach to supply future clean and sustainable energy by providing a higher hydrogen yield compared with the single-stage dark and photo fermentation approaches. Various researchers have reported on the sequential dark and photo fermentation technique for increasing hydrogen production as summarized in Table 3. Improving the hydrogen yield from cassava starch by mixed bacteria by a combination of dark and photo fermentation was reported by Cheng at al. [59] In the dark fermentation stage, the maximum hydrogen yield and production rate were 351 mL H2/g starch (2.53 mol H2/mol hexose) and 334.8 mL H2/L h, respectively, by using mixed anaerobic bacteria (mainly Clostridium species). In photo fermentation, mixed immobilized bacteria were used (Rhodopseudomonas palustris species) to produce the hydrogen from soluble metabolite products of dark fermentation. The maximum hydrogen yield in photo fermentation was 489 mL H2/g starch (3.54 mol H2/mol hexose). The total yield of hydrogen increased significantly from 2.53 to 6.07 mol H2/mol hexose by mixed bacteria and cell immobilization in combined of dark and photo fermentation. Similarly, Su et al. [60] reported that hydrogen yield dramatically increased from cassava starch from 240.4 mL H2/g starch in the dark fermentation to 402.3 mL H2/g starch (2.89 mol H2/mol hexose) in the combination mode. Argun et al. [61] investigated hydrogen production from waste wheat starch by sequential dark and photo fermentation. The effluent was first fermented by dark

380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425

Fig. 2. Scheme for sequential dark and photo fermentation process for hydrogen production from biomass [57].

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Table 3 Sequential dark and photo-fermentation method for enhancement of bio-hydrogen production. Reactor

Inoculums

Substrate

H2 yield/production from dark fermentation

H2 yield/production from photo fermentation

Total H2 yield/production from sequential dark-photo fermentation

References

Batch

Activated sludge and immobilized PSB (Rhodopseudomonas palustris) Pre-heated activated sludge and Rhodopseudomonas palustris Anaerobic sludge and Rhodobacter sp. RV

Cassava starch

2.53 mol H2/mol hexose

3.54 mol H2/mol hexose

6.07 mol H2/mol hexose

[59]

Cassava starch

240.4 mL H2/g starch

131.9 mL H2/g starch,

402.3 mL H2/g starch

[60]

Wheat powder solution Glucose

1.87 mol H2/mol glucose

2.68 mol H2/mol glucose

4.55 mol H2/mol glucose

[61]

1.59 mol H2/mol glucose

4.16 mol H2/mol glucose

5.48 mol H2/mol glucose

[68]

Glucose

1.83 mol H2/mol glucose

3.15 mol H2/mol glucose

6.32 mol H2/mol glucose

[69]

Starch

5.40 mmol H2/g COD

10.72 mmol H2/g COD

16.1 mmol H2/g COD

[63]

Sucrose

6.56 mol H2/mol sucrose

5.06 mol H2/mol sucrose

11.61 mol H2/mol sucrose

[62]

Sugar beet molasses

2.1 mol H2/mol glucose

4.75 mol H2/mol glucose

6.85 mol H2/mol glucose

[64]

Sucrose

3.80 mol H2/mol sucrose

–

14.2 mol H2/mol sucrose

[65]

Ground wheat

–

–

0.45 mol H2/mol acetate

[70]

Sweet potato starch + corn steep liquor Glucose

2.7 mol H2/mol hexose

4.5 mol H2/mol hexose

7.2 mol H2/mol hexose

[71]

1.36 mol H2/mol glucose 0.4 mol H2/mol hexose

3.20 mol H2/mol glucose Acetate and ethanol consumed no H2 1.48 mol H2/mol hexose –

4.56 mol H2/mol glucose

[72]

0.4 mol H2/mol hexose

[73]

3.32 mol H2/mol hexose

[66]

Two fold increase over dark-fermentation alone

[74]

Batch

Batch

Batch

Batch

CSTR and Batch Continuous

Batch

Batch + Photobioreactor Continuous

Sequential batch

Sequential batch transfer Sequential batch transfer Sequential batch Sequential batch

426 427 428 429 430 431 432 433 434 435 436 437 438 439 440

C. butyricum and Rhodopseudomonas palustris Ethanoligenens harbinense B49 and Rhodopseudomonas faecalis RLD-5 C. butyricum and Rhodopseudomonas palustris C. butyricum CGS5 and Rhodopseudomonas palustris WP3-5 Caldicellulosiruptor saccharolyticus DSM 8903 and Rhodobacter capsulatus wild type, R. capsulatus hup_ mutant, and Rhodopseudomonas palustris Clostridium pasteurianum CH4 and Rhodopseudomonas palustris WP3-5 Anaerobic sludge and Rhodobacter sphaeroides (NRRL-B1727) C. butyricum, Enterobacter aerogenes co-culture and Rhodobacter sp. M-19 Anaerobic bacteria and Rhodopseudomonas capsulata Escherichia coli HD701 and Rhodobacter sphaeroides O.U.001 Cattle dung and Rhodobacter sphaeroides SH2C Rhodopseudomonas palustris P4 dark adapted and Rhodopseudomonas palustris P4 light adapted

Glucose

Sucrose Glucose

1.84 mol H2/mol hesose 0.041 mol H2 and 5.7 mol organic carbon/mol hexose

fermentation and a hydrogen yield of 1.871 mol H2/mol glucose was reported in this step. In the second step, the effluent of dark fermentation was subjected to NH4-N removal, centrifugation and dilution in order to obtain suitable initial total volatile fatty acids (TVFA) and NH4-N concentrations for photo fermentation. About 60% of TVFA initially present (2.1 g/L) was fermented by Rhodobacter sphaeroides RV at 5 klux illumination with a halogen lamp, and the hydrogen yield was 2.68 mol H2/mol glucose. The overall hydrogen yield reported in sequential dark and photo fermentation was 4.55 mol H2/mol glucose, which was higher than that found with single-stage dark fermentation. A three-step integrated system comprising dark fermentation, photo fermentation, and autotrophic microalgae cultivation could achieve continuous and CO2-free hydrogen production by using different feedstock’s, as reported by Lo et al. [62]. Sucrose was subjected to

sequential dark and photo fermentation using Clostridium butyricum CGS5 and Rhodopseudomonas palustris WP3-5 bacteria. The sequential dark and photo fermentation was operated in a stable manner for nearly 80 days, giving a maximum hydrogen yield of 11.61 mol H2/mol sucrose, which was higher than those found with single-stage dark and photo fermentation (6.56 and 5.06 mol H2/mol sucrose respectively). In the final stage, gaseous CO2 produced from dark and photo fermentation was directly utilized by microalgae (Chlorella vulgaris C-C) culture. This could lead to a marked enhancement in the economic viability of the potential three-stage CO2-free bio-hydrogen production system. In another study, a three-step integrated system consisting of enzymatic hydrolysis, dark fermentation and photo fermentation was employed to enhance the routine production of hydrogen from starch [63]. The dark fermentation stage resulted in a hydrogen

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yield of 5.40 mmol H2/g COD and a COD removal of about 14.3%. Thus, combined dark and photo fermentation processes allowed a marked enhancement in overall hydrogen production yield (16.1 mmol H2/g COD or 3.09 mol H2/mol glucose) and also enabled a 54.3% COD removal. Ozgur et al. [64] reported the batch study of both sequential thermophilic dark fermentation and photo fermentation for hydrogen production using sugar beet molasses as substrate, with the substrate first being subjected to dark fermentation using thermophilic Caldicellulosiruptor saccharolyticus bacteria as inoculums. In dark fermentation, the hydrogen yield was 2.1 mol H2/mol glucose in the absence of NH4+. In the second step, some pretreatments were applied to the effluent of dark fermentation, prior to photo fermentation. The highest hydrogen yield was 4.75 mol H2/mol glucose by photosynthetic bacteria in photo fermentation. The overall hydrogen yield was much higher, at 6.85 mol H2/mol glucose. Sequential two-stage dark and photo fermentation was successfully employed by Chen et al. [65] not only to increase the hydrogen yield, but also to decrease the COD level from the effluent. First, sucrose was subjected to dark fermentation using a pure strain of Clostridium pasteurianum CH4, operated at 32 8C, pH 7.1 and a sucrose concentration of 20,000 mg COD/L, giving a hydrogen yield of 3.80 mol H2/mol sucrose. In the next stage, the soluble metabolite of dark fermentation was used as a substrate and gave the total hydrogen yield of 3.80 (dark fermentation) and 10.02 mol H2/mol sucrose (dark/photo fermentation) using Rhodopseudomonas palustris WP3-5. The overall hydrogen yield of the two-stage process was further enhanced to 14.2 mol H2/mol sucrose with a nearly 90% of COD removal when the photo bioreactor was illuminated with side-light optical fibers and was supplemented with 2.0% (w/v) of clay carriers. Tao et al. [66] employed photo fermentation with Rhodobacter sphaeroides SH2C to convert the fatty acids produced during dark fermentation into hydrogen and increased the total hydrogen yield from sucrose from 1.84 mol H2/mol hexose in dark fermentation to 3.32 mol H2/mol hexose by using the two-step process of combined dark and photo fermentation. Sequential dark and photo fermentation not only increases the hydrogen yield from sucrose but also consumes most of the fatty acids in the dark fermentation effluent except for a small amount of caproate and two-stage systems have been shown to reduce COD load in fermentor effluent by up to 90% [65]. Thus, sequential dark and photo fermentation techniques have the potential for increasing the bio-hydrogen production and yields from industrial waste or biomass. However, sequential dark and photo fermentation was uneconomical because of more reactors and additional space requirements [67]. In addition, the differences inorganic acid production/consumption rates, and therefore potential accumulation of organic acids in the media, decreases in light penetration because of suspended solids are the

major problems in sequential fermentation processes. Production costs for sequential dark and photo fermentation could be reduced by developing more efficient processing schemes and collocating hydrogen production with industries generating waste.

506 507 508 509

Combined dark and photo fermentation process

510

Hydrogen production by fermentation of carbohydrate-rich raw material offers major benefits over costly chemical processes owing to its operation under mild conditions (30–55 8C, 1 atm). However, major obstacle in bio-hydrogen production is low hydrogen yields and rates requiring large fermentation volumes and high residence times [4,8,75,76]. Combined dark and photo fermentation technique have a potential of reducing the fermentation time and improving the hydrogen productivity. Combined dark and photo fermentations process simultaneously takes place in same reactor for hydrogen fermentation is shown in Fig. 3. In addition combined dark-light fermentation was much faster than dark and photo fermentation alone which requires more time for completion. In combined fermentation, simultaneous fermentation of VFAs to H2 and CO2 prevents accumulation of VFAs in the medium and thereby overcomes inhibitions otherwise caused by the high VFA concentrations. The dark-photo combination process could probably reach close to the maximum theoretical hydrogen yield of 12 mol H2/mol glucose in Eq. (1):

511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528

C6 H12 O6 þ 6H2 O ! 12H2 þ 6CO2

DG0 ¼ 3:2 kJ

(1)

Only a limited number of combined dark-photo fermentation studies have been reported for enhancing hydrogen production from carbohydrate rich material (Table 4). Liu et al. [77] investigated the mixed culture of dark and photo fermentation bacteria (Clostridium butyricum and Rhodopseudomonas faecalis RLD-53) in different ratios and used for hydrogen production from glucose. Maximum hydrogen production yield of 1.98 mol/mol glucose was obtained when the ratio of Clostridium butyricum and Rhodopseudomonas faecalis RLD-53 was 1:600 in a mixed culture. The hydrogen yield (1.98 mol/mol glucose) of the mixed culture was slightly higher than that (1.80 mol/mol glucose) obtained with a pure culture of C. butyricum. In another study, Liu et al. [78] reported the production of hydrogen by combining Clostridium butyricum and immobilized Rhodopseudomonas faecalis RLD-53 in batch cultures using glucose as the substrate. A much higher total hydrogen yield of 5.16 mol H2/mol glucose was obtained, and this was further improved and reached 5.374 mol H2/mol glucose, when the effluent’s diluted ratio, the ratio of dark and photo fermentation bacteria and the light intensity were at 1:0.5, 1:2 and 10.25 W/m2, respectively. Miyake et al. [79] reported hydrogen production from glucose in a batch culture and observed an increase in hydrogen yield from

Fig. 3. Scheme for combined dark and photo fermentation process for hydrogen production from biomass [57].

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Table 4 Combined dark and photo-fermentation technique for enhancement of bio-hydrogen production. Reactor

Inoculums

Substrate

H2 yield/production from dark fermentation

H2 yield/production from photo fermentation

H2 yield/production from combined dark-photo fermentation

References

Batch

Anaerobic sludge and Rhodobacter sphaeroid (RS-NRRL and RS-RV) Clostridium acidisoli and Rhodobacter sphaeroides Ethanoligenens harbinense B49 and Rhodopseudomonas faecalis RLD-53 C. butyricum and Rhodopseudomonas faecalis RLD-53 C. butyricum DSM 10702 and Rhodobacter sphaeroides DSM 158 Lactobacillus delbrueckii and Rhodobacter sphaeroides RV C. butyricum and Rhodopseudomonas faecalis RLD-53 C. butyricum and Rhodopseudomonas faecalis RLD-53 Enterobacter cloacae strain DM 11 and Rhodobacter sphaeroides strain O.U.001 Activated sludge and Rhodobacter sp. RV Anaerobic sludge and mixture of Rhodobacter sp. (NRRL B-1727) Rhodobacter sp. (DSMZ-158) Rhodopseudomonas palustris (DSMZ-127) Rhodobacter sp. RV Anaerobic sludge and mixture of Rhodobacter sp. (NRRL B-1727) Rhodobacter sp. (DSMZ-158) Rhodopseudomonas palustris (DSMZ-127) Rhodobacter sp. RV Clostridium beijerinkii DSM-791 and Rhodobacter sp. RV C. butyricum and Rhodobacter sp. M-19

Ground wheat starch

0.14 mol H2/mol glucose

–

0.36 mol H2/mol glucose

[85]

Sucrose

1.83 mol H2/mol hexose

–

5.08 mol H2/mol hexose

[84]

Glucose

1.95 mol H2/mol glucose

–

3.10 mol H2/mol glucose

[82]

Glucose

1.92 mol H2/mol glucose

–

4.13 mol H2/mol glucose

[83]

Glucose

0.50 mL/mL medium

0.55 mL/mL medium

0.60 mL/mL medium

[80]

Glucose

–

–

7.1 mol H2/mol glucose

[86]

Glucose

1.63 mol H2/molglucose

–

5.37 mol H2/molglucose

[78]

Glucose

1.8 mol H2/molglucose

–

1.98 mol H2/molglucose

[77]

Glucose

1.86 mol H2/mol glucose

1.5–1.72 mol H2/mol acetic acid

NR

[87]

Ground wheat starch Wheat starch

–

–

[88]

1.32 mol H2/mol glucose

1.25 mol H2/mol glucose

1.45 mol H2/mol glucose 1.16 mol H2/mol glucose

Wheat starch

–

–

1.03 mol H2/mol glucose

[90]

Wheat starch

–

–

0.6 mol H2/mol glucose

[91]

Starch + yeast extract + glutamate

1.9 mol H2/mol glucose

1.7 mol H2/mol glucose

6.6 mol H2/mol glucose

[81]

Batch

Batch (immobilized)

Batch (immobilized)

Batch

Batch (co-immobilized culture)

Batch

Batch

Batch

Batch Batch

Batch

AHB

Batch

553 554 555 556 557 558 559 560 561 562 563 564 565

1.1 mol H2/mol glucose to 7 mol H2/mol glucose using a co-culture of Clostridium butyricum and the photosynthetic non-sulfur bacterium Rhodopseudomonas sphaeroides RV with an initial glucose concentration of 28 mM (about 5 g/L). However, when Clostridium butyricum and Rhodopseudomonas sphaeroides co-cultured with 5.04 g/L glucose under similar fermentation conditions, the hydrogen yield was much higher (0.60 mL/mL medium), compared with that found with a pure culture of Rhodopseudomonas sphaeroides (0.56 mL/mL medium) [80]. Yokoi et al. [81] obtained a higher yield of hydrogen (4.5 mol/mol glucose) by using a mixed co-culture of Clostridium butyricum and Rhodobacter sphaeroides than that obtained with single-stage dark fermentation (1.9 mol/mol glucose). Hydrogen gas was produced from glucose by using the

[89]

combined process of co-culture dark and photo fermentation [82], with Ethanoligenens harbinense B49 and immobilized Rhodopseudomonas faecalis RLD-53 as inoculums. The maximum hydrogen yield of 3.10 mol H2/mol glucose was obtained in co-culture, when glucose of 6 g/L, phosphate buffer of 50 mmol/L and initial pH of 7.5 were controlled in the medium. Hydrogen yield in the co-culture E. harbinense B49 and immobilized R. faecalis RLD-53 system was increased by 43.3% compared with that (1.95 mol H2/mol glucose) found with the mono-culture (E. harbinense B49). Hydrogen production from glucose in the combined dark and photo fermentation study of Ding et al. [83] used a co-culture of Clostridium butyricum and immobilized Rhodopseudomonas faecalis RLD-53, and the hydrogen yield was 4.134 mol H2/mol glucose in

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6 g/L glucose, 50 mmol/L phosphate buffer, initial pH of 7.5, bacterial ratio of 1:10 and light intensity of 8000 lux. In this study, Rhodopseudomonas faecalis RLD-53 can utilize acetate produced by Clostridium butyricum to produce more hydrogen yield in a coculture (4.134 mol H2/mol glucose) than in the mono-culture of Clostridium butyricum (1.92 mol H2/mol glucose). A Plackett–Burman design and central composite design method was used for increasing the hydrogen production from sucrose through combined dark and photo fermentation under 4 klux light illumination by Sun et al. [84]. The hydrogen yield was reported to be increased from 1.83 mol H2/mol hexose for the pure culture of C. acidisoli DSM12555 to 4.13 mol H2/mol glucose for the coculture under optimal culture conditions (11.43 g sucrose, pH 7.13, and inoculums ratio of 0.83). Ozmihci and Kergi [85] conducted the batch tests of combined dark and light fermentation in order to improve the hydrogen gas production using wheat powder starch as a substrate. The mixture of inoculums used as heat-treated anaerobic sludge and pure culture of C. beijerinckii (DSMZ 791T) were combined with two different light-fermentation bacteria of Rhodobacter sphaeroides (RS-NRRL and RS-RV). The hydrogen yield (0.36 mol H2/mol glucose) obtained by the combination of heattreated anaerobic sludge and Rhodobacter sphaeroides-NRRL was higher than that (0.14 mol H2/mol glucose) found with the pure culture of C. beijerinckii. A maximum hydrogen yield of 7.1 mol H2/ mol glucose was achieved by Asada et al. [86] among all the reported combined fermentation studies. A co-culture of facultative anaerobes, Lactobacillus delbrueckii NBRC 13953 and Rhodobacter sphaeroides-RV was used as a bacterial strain, and experiments were performed with an initial glucose concentration of 4.5 g/L, pH 6.8 and temperature 30 8C. The highest hydrogen yield was reported at the optimal bacterial concentration ratio of L. delbrueckii to Rhodobacter sphaeroides-RV of 1:5. However, combined dark and photo fermentation in single stage systems have limited success for hydrogen production because of the difficulty of reaching optimal conditions for the different organisms as well as balancing the reaction rate to achieve a stable consortium and decreases in light penetration because of suspended solids in mixed fermentation systems [73]. Different nutritional needs of different bacteria, required higher light intensities (>5 klux) due to light penetration problems to the dark-brown fermentation media and VFA accumulation were also main causes for low hydrogen formation rates and yields in combined fermentation. Low performance of combined dark and light processes for hydrogen formation can be overcome by selecting and using of a more effective single strain to perform both dark and light fermentation, optimizing the environmental conditions, improving the light utilization efficiency and developing more efficient bioreactors.

627

Conclusions

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Fermentative techniques are provided an alternative means of producing hydrogen and have distinct advantages over chemical methods. But the most important problems with fermentation techniques for hydrogen production are the low production rates and hydrogen yield. In this review article four types of methods, namely pretreatment, cell immobilization, sequential dark and photo fermentation, and combined dark and photo fermentation methods, are reviewed; all can help in enhancing hydrogen productivity and yields. In pretreatment, diverse means (acid, base, heat shock, chemical, sterilization, aeration and load shock) have been employed on substrate/inoculums to enhance the hydrogen production from anaerobic cultures by eliminating methanogenic bacteria and enriching the spore-forming hydrogen bacteria. Significant enhancements in hydrogen production were reported

when base, load shock and BSEA pretreatment were used, compared with non-treated situations, and use of pretreatment offers an important approach for increasing the hydrogen production from biomass and waste material. Immobilized whole cell techniques represent a reliable approach to dark and photo fermentation for the enhancement of continuous hydrogen production, compared with the suspended cell system. In the immobilization cell system, a variety of natural and synthetic support mediums are used for immobilization and it has been concluded that the immobilized-cell systems have the stable and efficient potential for batch and continuous hydrogen production. However, the yield of hydrogen production is still lower than that of the suspended cell-systems [13]. The major cause for the lower yields could be due to the presence of unfavorable bacterial community structure, low substrate conversion efficiency or mass transfer limitations arising from immobilized cells. In order to obtain higher yield of hydrogen production in immobilization process, it is necessary to develop new immobilized materials for cell entrapment. Sequential dark and photo fermentation (consecutive operation) is an alternative method that could have the potential of enhancing the yield of hydrogen production to the economical level of 8 mol H2/mol glucose and reducing the environmental contamination compared with single-stage dark fermentation. However, hydrogen yields at this level have not been reported yet. Sequential fermentation scheme needs some pre-treatment of the dark fermentation effluent. Removal of dark fermentation bacteria by filtration or centrifugation, adjustment of VFA and NH4-N concentrations by dilution or removal, and optimization of nutrient media composition and adjustment of pH and oxidation-reduction potentials to desired level may result in improved hydrogen production in sequential fermentation. Therefore, some other membrane processes such as ultrafiltration or reverse osmosis may also be required to remove some undesirable compounds from DFE. Sequential fermentation requires larger fermenter volumes and separation/pre-treatment units in between the two stages. In combined fermentation technique both dark and photo fermentation simultaneously take place in a same reactor. Use of a mixed co-culture would not only relieve feedback inhibition of hydrogen production resulting from the accumulation of VFAs produced by dark fermentation but would also increase the spacetime yield of hydrogen. Combined fermentation may seem to be more advantageous than sequential fermentation due to reduction in reactor volumes, simpler procedure and low cost. However, hydrogen gas production rates and yields in combined fermentation were reported to be lower than that of the sequential fermentation due to long lag times between the dark and light fermentations in combined fermentation scheme. Adverse interactions and different nutritional needs of different bacteria also cause low H2 yields and formation rates in combined fermentations. Low hydrogen formation rates and yields may be overcome by selecting and using more effective organisms or mixed cultures, developing more efficient processing systems, optimizing the environmental conditions, improving the light utilization efficiency and developing more efficient bioreactors. Fermentative route of H2 production from carbohydrate rich renewable sources such as biomass or waste materials is a promising approach provided that the rate and the yields of H2 formation were improved to economically feasible levels and large scale operations were developed.

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Acknowledgement

704

The authors are grateful to the FRGS (Grant No. RDU130123), Q2 Universiti Malaysia Pahang (UMP) for financial support.

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JIEC 2062 1–11 L. Singh, Z.A. Wahid / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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