6. Current limitations and potential improvements

Notwithstanding the enormous efforts made to achieve a better understanding of the DF/AD process of AB/TV, it is still necessary to improve not only bioH2 or bioCH4 productivities and yields but also the (long-term) stability of processes for commercialization purposes. TV is a highly complex wastewater that besides high COD and negligible alkalinity, harbors recalcitrant compounds such as phenols, which may act as inhibitors in DF/AD. While the main limitation to use AB as the feedstock is its recalcitrant structure. As mentioned earlier, some of the pretreatment/conditioning steps used in AB have been optimized not only in terms of hydrolysis yield, reaction time, the generation/release and effect of putative fermentation inhibitory compounds, cost-effectiveness but also in terms of bioH2/ bioCH4 production efficiency. However, there is still a need to explore other pretreatments that have not been yet embraced in the field of DF/AD of AB but they have been ascertained as potentially useful in releasing sugars for other applications like the production of bioethanol, such as ammonia fiber explosion (AFEX), autohydrolysis, organosolv, high-energy radiation, ozonolysis, alkaline, ionic liquids, or any combination of those pretreatments. It could be also interesting to explore consolidated processes (direct fermentation) which combine into a single operation the enzymatic hydrolysis of (pretreated) biomass and biological conversion to the desired by-product (in this case bioH2/bioCH4) by mixed consortia.

Besides the features described before, from practical purposes, the highly variable composition of AB/TV constitutes another constraint to produce bioH2 since DF systems are commonly unable to overcome perturbations in feedstock composition. One of the most significant challenges is to assure consistency in the prevailing metabolic pathways during the DF process and favor bioH2-producing pathways over other unwanted routes, for example, homoacetogenesis and methanogenesis. Very little is known about the microbial community structure of DF/AD processes treating AB/TV. In this regard, it is not clear the role of microorganisms and their association with operational parameters (e.g. pH, HRT, and OLR) and process indicators (e.g. VHPR, VMPR, and metabolic composition). Also, much less is known about how microbial assemblage may change through time, and what factors (operating parameters) govern its dynamics. It is worth noticing that HLac monitoring has been disregarded limiting the understanding of integrated DF-AD processes since it, as an intermediate, has a vital role in the carbon flux.

Another concern worth to mention is that most of the previous studies were carried out in batch or semi-continuous reactors. Thus, it is vital to transfer the kinetic knowledge gained from such studies to the expansion of continuous systems. In this context, the development of integrated DF-AD schemes for the continuous production of bioH2 and bioCH4 using AB/TV as feed stocks requires intensive research on interlinking side streams for producing high added-value bioproducts in a biorefinery framework (e.g. HLac-bioH2-bioCH4) for better sustainability of the existing tequila industries.

#### 7. Conclusions

Tequila industry generates huge amounts of AB and TV, which could be subjected to integrated DF-AD processes to produce bioH2 and bioCH4 while reducing their pollution potential. This chapter focused on the state-of-the-art of

environmental conditions for the different groups of microorganisms which have differences in terms of physiology, nutrient intake, nutritional requirements, growth rate, optimum growth conditions such as pH, and adaptation to environmental stress conditions [16]. The acidogenesis and methanogenesis separated in space may also produce bioH2 via DF process [17, 24, 35]. However, it is not necessarily desirable to produce bioH2 in all cases [7]. In the latter case, a stream rich in HLac can be obtained through the HLac-type fermentation which can be further fed to the methanogenic stage [36, 37], where hydrogenotrophic may be benefited for the conversion of HLac to HAc by consuming the intermediate bioH2 gas immediately [52]. The possibility of operating at higher organic loading capacity (in the methanogenic stage), reducing alkali addition, and increasing COD removal efficiency are additional advantages of the two-stage AD as compared to singlestage AD [7, 21, 24]. A small number of reactor configurations devoted to bioH2/ bioCH4 production from AB/TV can be found in the literature (Figure 3). Among them, for both AB and TV, the CSTR and UASB configurations have shown the highest performance to date for producing bioH2 and bioCH4, respectively, that is,

New Advances on Fermentation Processes

Types of reactor configurations used for biohydrogen and biomethane production from tequila processing byproducts. (a) Batch reactor, (b) continuously stirred tank reactor (CSTR) with recirculation, (c) CSTR, (d) anaerobic sequencing batch reactor (AnSBR), (e) trickling bed reactor with recirculation, (f) packed bed reactor, (g) up-flow anaerobic sludge blanket (UASB) reactor. AnSBR can integrate mechanical or hydraulic

Figure 3.

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mixing. UASB can operate with effluent recycle.

configurations and process parameters, metabolic pathways, and microbial ecology of bioH2- and bioCH4-producing reactors. The pretreatment/conditioning steps applied to enhance the valorization of AB/TV were also reviewed. It has been suggested that the HLac-type fermentation coupled to DF and AD can boost the development of cascading design in multi-stage AD processes. This multiproduct approach using AB/TV as resources in the biorefinery scheme may facilitate sustainability to the tequila industry.

VFAs volatile fatty acids VS volatile solid

TV tequila vinasse TRS total-reducing solids TBR trickling bed reactor

Author details

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\*Address all correspondence to: eleon@ciatej.mx

provided the original work is properly cited.

VSS volatile suspended solids

DOI: http://dx.doi.org/10.5772/intechopen.88104

VHPR volumetric biohydrogen production rate VMPR volumetric biomethane production rate

A Comprehensive Overview of the Potential of Tequila Industry By-Products for Biohydrogen…

UASB up-flow anaerobic sludge blanket reactor

Octavio García-Depraect, Daryl Rafael Osuna-Laveaga and Elizabeth León-Becerril\* Department of Environmental Technology, Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco, A.C., Guadalajara, Jalisco, México

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
