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Reorienting Waste Remediation Towards Harnessing Bioenergy 247

in the smaller-scale demonstration phase while supercritical water gasifica-
tion is in its early stage of development (Agarwal et al., 2013). Hydrogen
production by reformation produces large amounts of CO 2 , which contrib-
utes to global warming (Navarro et al., 2009).

6.3.4 Process Limitations

Major limitations observed in the dark-fermentative H 2 production process
are low substrate conversion efficiency, a drop in system redox conditions,
and residual substrate originating from the process as acid-rich wastewater
(Venkata Mohan, 2010). About 40–70% residual organic carbon remains
in the effluent after dark-fermentation even under optimal operating con-
ditions. The persistent accumulation of VFA causes a sharp drop in the
pH, resulting in inhibition of the process (Venkata Mohan et al., 2011a;
Wang and Wan, 2009). Biological limitations such as H 2 -end-product inhi-
bition, acid or solvent accumulation, and H 2 partial pressure limits process
efficiency. Environmental and economic concerns suggest that it is advisable
to use the residual carbon fraction of the acidogenic outlet for additional
energy generation in the process of its treatment (Mohanakrishna
et al., 2010b).
Various integrated approaches were studied to overcome the persis-
tent limitation of the acidogenic process to a certain extent in the context
of biorefinery. Various secondary processes, such as methanogenesis (AF)
for methane production (Mohanakrishna et al., 2010c), acidogenic fer-
mentation for additional H 2 production (Mohanakrishna et al., 2012),
photo-biological process for additional H 2 production (Chandra and
Venkata Mohan, 2011; Srikanth et al., 2009), microbial electrolysis cell
(MEC) for additional H 2 production (LeninBabuetal.,2013a,b), anoxy-
genic nutrient-limiting process for bioplastics production (Venkata
Mohan et al., 2010c), hetrotrophic-algae cultivation for lipid accumula-
tion (Venkata Mohan and Devi, 2012), and MFC for bioelectricity gen-
eration (Chandra et al., 2012; Subhash et al., 2013), were studied by
integration with the primary dark-fermentative H 2 production process
with relatively good degrees of success in the case of product recovery
and wastewater treatment. Integration approaches facilitate a reduction
in wastewater load along with the advantage of value addition to the
existing process in the form of product recovery, making the whole
process economically and environmentally viable (Mohanakrishna and
Venkata Mohan, 2013).