Environmental Engineering Reference
In-Depth Information
well suited for treating highly variable wastewater. Cell-immobilization approaches
and granular processes also showed good H
2
production efficiency.
Various modes of reactor operation viz., batch, fed-batch, semi-batch/continuous,
periodic discontinuous batch (sequencing batch operation) and continuous have
been used to produce H
2
. About a 25% improvement in H
2
production and sub-
strate degradation efficiency was reported with batch mode operation compared to
the corresponding continuous mode operation [92]. The efficacy observed in fed-
batch mode operation might be attributed to the reduced accumulation of soluble
metabolic intermediates formed during acidogenic fermentation due to fill-draw
mode operation [24, 26, 31, 38, 92]. A fed-batch mode of operation with acidic pH
showed highest H
2
production [92]. Poor biomass retention/cell washout encoun-
tered during continuous mode operation can be prevented to some extent with a
batch mode operation [92, 96, 97]. Batch mode operation coupled with a biofilm
configuration combines the operational advantages of both systems and helps to
maintain stable and robust cultures suitable for treating highly variable wastewater
[21-25, 98-100].
Morphologically similar bacteria were observed in the scanning electron
microscopy (SEM) image [26] of the biofilm formed on the fixed-bed of bioreac-
tor producing H
2
from the treatment of chemical wastewater (Fig. 4a). The biofilm
reactor was inoculated with selectively enriched H
2
-producing consortia and oper-
ated under an acidic microenvironment for more than 300 days. SEM imaging
visualized slightly bent, scattered and short chain rods (predominant) along with
a relatively low frequency of cocci shaped bacteria of approximate length of 10
m.
SEM images of isolated bacteria strains from a biofilm reactor (acidogenic mixed
culture) (Fig. 4b, c) visualized slightly bent, rod shaped, thick fluorescent capsid
bacteria with (~10
μ
m in length). Images of both the isolated strain and mixed
consortia showed comparatively similar morphology demonstrating the presence of
related groups of bacteria proliferated in the bioreactor producing H
2
. Transmission
electron microscopy (TEM) image showing sub-cellular structures of the isolated
bacteria from an acidogenic mixed culture [26] (Fig. 4d). TEM image showing oval
centralized spore formation with sub-terminal endospore development in rod shaped
bacteria (1-7
μ
μ
m in length). Terminal bulging with granulose accumulation was not
observed. Flagellum attached subapically to the bacterium (two times length of the
cell body) was observed. Vegetative cell surrounded by thick membrane (peptido-
glycan layer) with two layers (inner and outer forming fibrillar capsule structure) on
the cell surface was also visualized (Fig. 4e).
4.6 Substrate Loading Rate
The organic loading rate (OLR) of the wastewater also influences the H
2
production
pattern, apart from other wastewater characteristics. H
2
yields were inversely pro-
portional to the glucose feeding rate, while the highest H
2
yields were observed
at lowest glucose loading rate [78]. Glucose concentrations exceeding 2 g/l (as
co-substrate) showed suppression in H
2
production [21]. A marked reduction
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