18.5 Strategies to Enhance Microbial Hydrogen Production

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18.5.1

Integrative Process

The earlier sections have highlighted various processes involved in biohydrogen

production, which varies with the type of microbes involved. Microbial H2 pro-

duction has several challenges over chemical processes, and yield or production

rate is one such major hurdle of the process. An integrative approach to generate

a single-stage hybrid system or two-stage system for H2 production has recently

gained much attention intending to overcome the limitations of single operated

processes shown in Figure 18.1b,c. The dark fermentation process is characterized

by a massive accumulation of organic acids, which leads to an inhibitory effect

on H2-producing enzymes and the growth of microbes. These acid-rich effluents

are high in carbon content and can act as a substrate for further energy recovery

through a two-stage process. The dark fermentation effluents generated in the

first stage is processed by the second stage and can be used for methanogenesis

for methane production or H2 production through photo-fermentation, microbial

electrolysis cells (MECs) for H2, MFCs for bioelectricity, bioplastic production,

and heterotrophic algae cultivation for lipids [40–44]. These integrated processes

are involved in the efficient valorization of waste effluents for additional energy

production or other value-added products. This makes the integrative approach

more economically feasible and practically applicable to industrial scales.

18.5.2

Medium and Process Optimization

Fermentative hydrogen production is influenced by several factors that have been

discussed in detail in Sections 18.4 of this chapter. The composition of the fermen-

tation medium is very crucial for the activity of enzymes catalyzing H2 production,

like pyruvate ferredoxin oxidoreductase, hydrogenase, formate hydrogen lyase, and

pyruvate formate lyase. The fermentation medium is a source of nutrients essential

for the growth and metabolism of the microorganisms, which includes sources

of carbon, nitrogen, metal ions, and other trace elements. However, an optimum

range of these nutrients is necessary for efficient strain cultivation and production

of metabolites because a higher or lower range may lower the fermentation kinetics,

thereby reducing the product yield. Similarly, an optimum range of operating

conditions is also necessary to be maintained throughout the fermentation process,

which includes temperature, pH of the medium, substrate concentration, and

inoculum. Optimization of both the process parameters and medium components

is, therefore, also essential for biological H2 production. Several studies have

investigated the effect of these factors on the yield of H2 production and estimated

the optimum range of factors required for maximum production [16]. The co-factors

and the enzymes of all microbes are mostly active in an optimum pH and tem-

perature range; therefore, parameter optimization is the key technique to obtain

maximum yields. Different experimental design methods like central composite

design (CCD), Box–Behnken (BB) design, full factorial design, Plackett–Burman

design, Taguchi design, and one-variable-at-a time (OVAT) design can be used

to evaluate the optimum range and the effects of various parameters. Response