Semi-continuous solid substrate anaerobic reactors for H2 production from organic waste: Mesophilic versus thermophilic regime

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Abstract

We evaluated the influence of the operation temperature (mesophilic vs. thermophilic regime) of semicontinuous, acidogenic solid substrate anaerobic digestion (A-SSAD) of the organic fraction of municipal solid waste (OFMSW) at lab scale. The H2 percentage was higher in the thermophilic regime than in the mesophilic operation (58% and 42%, respectively). The H2 yield of thermophilic A-SSAD was significantly higher than in our mesophilic reactors (360 vs. 165 NmL H2/g VSrem) and other studies reported in the literature (range of 62–180 mL/g VS). Mesophilic A-SSAD showed a yield of 37% of the maximum yield based on 4 mol H2/mol hexose, while thermophilic A-SSAD exhibited a yield of 80% of the maximum yield. This result is similar to works with pure cultures of extremophile microorganisms where H2 yields of 83% of the maximum were reported. We found higher concentrations of acetic acid in the digestates of thermophilic A-SSAD, while butyrate was in higher proportion in mesophilic A-SSAD spent solids. The moderate-to-high yields obtained with the semicontinuous process used in this work are in disagreement with previous reports claiming that batch and semicontinuous processes are less efficient than continuous ones.

Introduction

During the past 10 years there has been a renewed interest in new technologies that could supply energy in an environmentally friendly, sustainable way [1]. From an environmental viewpoint, there is an urgent need for appropriate management of municipal solid wastes (MSW). Nearly 1600 million tonne/year of MSW are generated worldwide with up to 43% contributed by Asia and Oceania and 28% contributed by North America and the European Union [2]. On average, almost 50% of the MSW of underdeveloped countries consists of a fermentable, biodegradable fraction.

The anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) for generation of methane and a soil amender has received increased interest in the last 15 years [3], [4], [5], [6], [7], [8], [9]. Yet, even the use of methane as a fuel could be debatable, due to the production of CO2 known to contribute to the greenhouse effect [10]. Hydrogen is a clean fuel since its combustion with oxygen does not generate polluting emissions. According to energy experts, hydrogen is safe, versatile and has a high energy content, high utilization efficiency and is the best option for transport applications [11], [12], [13]. Hydrogen can be produced chemically, electrochemically, as a by-product of oil/coal processing or by the use of microorganisms. There are two main approaches to microbial H2 production, namely, photochemical and fermentation. The first uses photosynthetic microorganisms such as algae and photosynthetic bacteria [14], [15]. The second approach is carried out by fermentative H2-producing microorganisms, such as facultative anaerobes and obligate anaerobes [16], [17].

Some advantages of H2 production by anaerobic fermentation (HAF) are that many fermentative bacteria are capable of high hydrogen generation rate and H2 is produced throughout the day and night at a constant rate since it does not depend on energy provided by sunlight. HAF production has been investigated using pure cultures in sterile conditions and undefined mixed cultures in nonsterile conditions [18]. Moreover, some authors have shown that this valuable fuel can be produced from OFMSW and industrial wastes [19], [20], [21], [22], [23].

Some advantages of using mixed cultures over pure cultures in HAF are lower operational costs (savings in asepsis), operational control based on differential kinetics of microbial subgroups is possible, and septic organic wastes can be used as substrate. Yet, some limitations regarding process development still remain to be solved, such as suppression of hydrogen-consuming microbial subgroups, long-term process feasibility of continuous and semicontinuous processes has not been fully demonstrated, there is limited knowledge of community profile-biochemical performance, acclimation is often required to minimize lag times, etc. [1].

Most HAF-related processes rely on the disruption of the hydrogen uptake of methanogenic archaea since the latter are recognized to be the most significant hydrogen-consumer microbial group in anaerobic consortia [24], [25]. Several approaches have been attempted and reported for achieving this goal, e.g., the use of chemical inhibitors such as acetylene and bromoethane sulfonic acid [19], [23]; heat shock pretreatment (HSP) of inocula [20], [21], [22], [26], [27]; and keeping the pH of the cultures in the acidogenic range (5.8–6.5) [28], [29]. HSP relies on the killing or thermal suppression of methanogenic archaea and nonsporulating eubacteria whereas the culture is enriched in sporulating, hydrogen-producing bacteria such as Clostridia.

At lab scale, HSP usually consists of heating the inocula in a boiling water bath for periods from 15 min to 2 h [1]. HSP has been shown to be effective for batch processes, although its feasibility for semicontinuous or continuous processes seems to be questionable [30], [31]. On the other hand, the acidogenic operation of continuous anaerobic reactors has been demonstrated at pilot-plant and commercial levels [32], [33], although the purpose of this early research was not directed to hydrogen production but to sludge and waste treatment. In this approach, methanogenic archaea are inhibited by the low pH prevalent in the reactor [34], [35], [36]. Low pH is the consequence of the overloading of the reactor that leads to an overproduction of organic acids and other metabolites from the incoming substrate. Thus, we can refer to this as “kinetic” control of the pH although some care has to be exercised in order to keep the pH in a range favorable for hydrogen accumulation [28], [29], [37].

Ueno et al. [38] evaluated the effect of hydraulic retention time (HRT) on the production of hydrogen from sugar-industry wastewater by anaerobic microflora in chemostat culture. Anaerobic microflora was cultured at 60C during 212 days of operation; pH of the cultures was kept at about 6.8. A maximum hydrogen yield of 14 mmol H2/g carbohydrate removed (2.6 mol H2/mol hexose) was obtained at an HRT = 0.5 day. Lay [39] found a hydrogen production of 1600L/m3 day at an HRT 17 h and pH 5.2 in his studies of hydrogen production from soluble starch in a complete mixed reactor. The reactor was started with an HSP anaerobic-digested sludge and incubated at 37C for 60 days. Hydrogen from glucose was produced in the acidogenic phase of a complete mixed reactor seeded with acclimated sludge and incubated at 32C almost 300 h [40]. The authors studied the effect of the dilution rate (0.08, 0.10, 0.125, 0.167 and 0.25h-1, the inverse of the HRT) on hydrogen generation. Percentages of hydrogen in biogas were 71%, 66% and 67% for the dilution rates 0.08, 0.167 and 0.25h-1, respectively.

A study on continuous hydrogen production from the anaerobic acidogenesis of a high-strength rice winery wastewater by a mixed bacterial flora was carried out by Yu et al. [41] who found that the maximum specific rate of hydrogen was 9.33 LH2/(gVSS day) in a thermophilic (55C) upflow reactor at an HRT=2h and pH=5.5. In another work using an upflow anaerobic sludge blanket (UASB) reactor, a constant production of hydrogen from sucrose was observed for 8 months [42]. The lab-scale UASB reactor was operated at a temperature of 35C, pH 6.7 and inoculated with sludge heat-treated at 100C for 45 min. The authors reported that the maximum rate of hydrogen production (270.6 mmol H2/L day) occurred at HRT=8h.

Mesophilic batch experiments with sludge from an anaerobic digestor showed that both HSP and low pH (6.2) were required to maximize hydrogen yield from approximately 1 mol H2/mol glucose [43]. A loss of hydrogen was also observed, due to autotrophic acetogenesis from CO2 and H2 [1] that could not be prevented by HSP.

The majority of the reported research on continuous HAF processes is concentrated on soluble substrates and mesophilic regime. Fortunately, it demonstrates that continuous, acidogenic reactors with low pH are feasible for hydrogen production.

Regarding the use of organic solid waste as substrate in HAF, most previous work, although promising, has been focused on batch processes [19], [20], [21], [22], [23]. There is little information on solid substrate, continuous or semicontinuous, acidogenic fermentation of organic solid wastes (A-SSAD) in the refereed literature. Particularly, no information is available on the long-term stability of A-SSAD and the influence of the temperature regime on hydrogen production. The first issue is crucial for any future full-scale application of A-SSAD. It is likely that the accumulation of organic acids could lead to overall process inhibition [44] or a drop of the reactor pH below 5.8 could foster the switch from acidogenesis to solventogenesis (a hydrogen-consuming metabolic pathway) [45], or both. Concerning the effect of the temperature regime, there is some evidence of the advantage of thermophilic hydrogen fermentation in studies on batch liquid culture with glucose as substrate [46] although a similar result has not been demonstrated for A-SSAD. Therefore, the purpose of this work was to evaluate the effect of the operation temperature (mesophilic vs. thermophilic regime) on the hydrogen production of the acidogenic semicontinuous, solid substrate anaerobic digestion of the organic fraction of municipal solid waste MSW (OFMSW) in lab-scale experiments.

Section snippets

Inocula

The A-SSAD reactors were seeded with digestates from methanogenic solid substrate anaerobic digesters (M-SSAD) degrading a mixture of organic solid wastes. Table 1 shows the average performance of the M-SSAD reactors. Purges from M-SSAD were transferred to both mesophilic and thermophilic A-SSAD twice a week until mass reactor contents of 1 kg was reached. Meanwhile, the “incomplete” A-SSAD reactors were fed twice weekly with a mass of feedstock corresponding to an average of 21 MRT and an

Biogas productivity and hydrogen yield

Fig. 2 shows that the biogas productivity of the thermophilic A-SSAD was almost double than that of the mesophilic reactor in the entire period of operation (typical performance of reactors in first run). The biogas of the two reactors consisted of only hydrogen and carbon dioxide; methane was not present. On average, in two runs the thermophilic A-SSADs showed a higher hydrogen percentage in the biogas than the mesophilic counterparts (58±3% and 42±3% v/v, respectively, Table 3). Consequently,

Conclusions

Both the thermophilic and mesophilic A-SSAD using the organic fraction of municipal solid waste seems to be attractive for hydrogen generation. The thermophilic reactor achieved the highest hydrogen yields and percentages. This was also accompanied by higher biogas productivity and a stable hydrogenogenesis operation.

Acknowledgements

A graduate scholarship to IV-V from CONACYT is gratefully acknowledged. The authors wish to thank Mr. Rafael Hernández-Vera, Ms. Guadalupe M. Simuta Morales, Mr. Alessandro A. Carmona and Ms. Karla M. Muñoz (Environmental Biotechnology R&D Group, Dept. Biotechnology and Bioengineering, CINVESTAV-IPN) for their help with the start-up and the operation of the lab scale reactors. The authors appreciate partial financial aid from CINVESTAV-IPN and support from TESE and COSNET.

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