Environmental Engineering Reference
In-Depth Information
additional micronutrients, organic matter and microbial biomass in the direction of
enhance the process efficiency.
4.7 Nitrogen and Phosphrous
Nitrogen is a necessary component in proteins, nucleic acids and enzymes and is
second only to carbon as a requirement for bacterial growth [103, 104]. Nitrogen
in an appropriate concentration range is beneficial to fermentative H
2
production,
while at a much higher concentration can inhibit the process performance by affect-
ing the intracellular pH of bacteria or inhibiting specific enzymes related to H
2
production [105-107]. Optimal nitrogen concentrations of 0.1 g N/l were reported
for effective H
2
production [104]. Substrate degradation efficiency was also found
to increase with increasing nitrogen concentration from 0 to 0.01 g N/l [104].
Appropriate ratios of C/N are fundamentally important, with the optimum being
47 [103]. Phosphate helps to maintain the system's buffering capacity during the
H
2
fermentative process [91]. Using phosphate as an alternative to carbonate as
a buffering supplement should increase the H
2
gas fraction [108]. An increase in
the carbonate concentration increases the CO
2
fraction in the gas phase due to
carbonate dissolution. Adding phosphate at a proper concentration is a useful strat-
egy for optimal H
2
production [108]. Na
2
HPO
4
affected the H
2
production in a
concentration-dependent way with the optimal concentration being 0.6 g/l. Using a
proper carbonate and phosphate concentration formulation, the H
2
production rate
can be enhanced by 1.9 times which might be due to a shortening of the microflora
lag-phase [108].
5 Combined Process Efficiency
When wastewater is used as a fermentative substrate for H
2
production, the extent
of substrate degradation is important when process efficiency is considered [71].
There is a trade-off between technical efficiency based on H
2
production and sub-
strate removal at different feeding pHs. Neutral pH is ideal for wastewater treatment
while acidic pH is useful for effective H
2
production [21, 26]. Balanced conditions
for effective combined performance and process optimization are especially impor-
tant to sustain process economic viability. Process performance was evaluated using
two diverse mathematical approaches [data enveloping analysis (DEA) and design
of experimental (DOE) methodology] [71]. The role of some important factors such
as type and origin of inoculum, pre-treatment procedure, inlet pH, co-substrate addi-
tion and feed composition were evaluated for combined process efficiency by the
DEA methodology. DEA analysis showed that the untreated anaerobic inoculum
under acidic conditions using simple wastewater as fermentative substrate showed
combined process efficiency. Taguchi's DOE methodology was used to enumerate
the role of selected factors on H
2
production and substrate degradation with the final
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