Environmental Engineering Reference
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dilute out slow growing MB [86]. However, in continuous operation mode, H 2
production was observed at long HRTs of 3 days (pH 6.4) without encountering
problems with methanogenesis [87]. Optimal HRT mostly depends on the nature
and composition of the substrate, function of biocatalyst, loading rate and fermen-
tation pH employed. HRT can be considered as a readily manipulated variable for
process control. Optimum HRTs from 8.0 to 14 h were reported for effective H 2
production [57].
4.4 Temperature
Temperature affects H 2 production, metabolite product distribution, substrate degra-
dation and bacterial growth. Most studies on H 2 production have been conducted
under ambient (15-27 C), mesophilic (30-45 C), and moderate thermophilic
(50-60 C) temperatures, with a few studies of mixed cultures under extreme
thermophilic conditions, over 60 C [88, 89]. The optimal temperature for H 2
production via dark fermentation varies widely based on the type of biocatalyst and
the carbon substrate used. For pure cultures, the optimal temperatures are reported
to be in the range of 37-45 C, whereas for mixed microflora diverse optimum tem-
peratures were reported [62]. Both mesophilic and thermophilic temperatures were
observed to be optimal for fermentative H 2 production processes. Thermophilic
conditions were reportedly advantageous due to its thermodynamics [15, 62] which
gives higher reaction rates with better process performance and decreased problems
with contaminating H 2 -consuming microorganisms. Although higher temperatures
allow more favorable reaction kinetics, rapid changes in system pH may inhibit H 2
producing bacteria [90]. The changes in soluble metabolite composition were also
observed with changes in operating temperature, resulting in metabolic pathway
shifts correlated to bacterial functions dominant at that particular temperature [91].
Temperature control might not be a feasible option for process control.
4.5 Reactor Configuration and Operation
Various reactor configurations, viz., suspended growth, biofilm/packed-bed/fixed
bed, fluidized bed, expanded bed, upflow anaerobic sludge blanket (UASB), gran-
ular sludge, membrane based systems, immobilized systems, etc., have been used
successfully to produce H 2 by fermentation processes. Biofilm/attached-growth sys-
tems are generally robust to shock-loads compared to the corresponding suspended
growth systems, with the biofilms acting as a buffer to reduce the effective concen-
tration of toxic chemicals to which the organisms are exposed, protect the culture
from predation, provide improved reaction potential due to the presence of high cell
densities and provide resilience and resistance to change in the process parameters
[26, 92-95]. Generally bacteria achieve maximum growth rates in biofilm resulting
in improved reaction potential finally leading to stable and robust system which are
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