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Reorienting Waste Remediation Towards Harnessing Bioenergy 243

deactivates AB (Venkata Mohan, 2010; Zhu and Beland, 2006). Hydroge-
nase enzyme activity is inhibited at a low or high pH beyond the optimum
range. The fate of pyruvate depends on the operating pH. Under acidic con-
ditions pyruvate is converted into VFA along with H 2 by AB. Neutral pH
operation leads to the formation of CH 4 and CO 2 by MB. Under basic pH,
anaerobic digestion leads to solventogenesis. Hydrogenase activity is higher
at an acidic pH, but with an increase in pH, the metabolic pathway proceeds
+
to the next step of anaerobic digestion where H get reduced to CH 4
(methanogenesis) or ethanol (solventogenesis). Accumulation of acid
metabolites (VFA) during dark-fermentation causes a marked drop in the
system’s pH, which reduces the buffering capacity, thereby inhibiting the
H 2 production (Devi et al., 2010; Lin and Lay, 2004b). The pH range of
5.5–6.0 is optimum to avoid both methanogenesis and solventogenesis
and can be considered as a manipulated variable for the process control,
especially for dark-fermentation process (Venkata Mohan et al., 2007d,
2010b). A longer fermentation period induces a metabolic shift from acid-

ogenesis to methanogenesis, which is considered to be unfavorable for H 2
production. Maintaining a shorter retention time, between 8.0 and 14 h,
therefore helps to restrict the MB growth as well as activity (Hawkes
et al., 2007; Venkata Mohan, 2010, Venkata Mohan et al., 2007a–d,
2008d,f, 2011a; Vijaya Bhaskar et al., 2008). Methanogens can be suppressed
by maintaining short retention time (2–10 h) as AB grow faster (Fang and
Liu, 2002; Nakamura et al., 1993; Ren et al., 2005; Venkata Mohan,
2009; Zhu and Beland, 2006).
Nitrogen at optimal concentration is beneficial for H 2 production,
while at higher concentrations it can inhibit the process performance by
affecting the intracellular pH of the bacteria or by inhibiting specific
enzymes related to H 2 production (Bisaillon et al., 2006; Chen et al.,
2008; Salerno et al., 2006). The optimal nitrogen concentration of
0.1 g N/L was found to have a positive effect on H 2 production and sub-
strate degradation (Wang et al., 2009). An ideal carbon-to-nitrogen (C/N)
ratio helps in bacterial growth and affects H 2 production by both mixed
and pure cultures. A C/N ratio of 47 has been shown to affect fermentative
H 2 production by mixed microorganisms (Lin and Lay, 2004a). Wastewa-
ter that is low in carbon content can be combined with materials high in N
to attain the desired C:N ratio of 30:1 (Yadvikaetal.,2004). Wastewater
with excess nitrogenous compounds and ammonia inhibits nitrogenase
2+
activity (Redwood and Macaskie, 2006). Iron (Fe )isveryimportant
for the function of hydrogenase and also acts as an active site component