Environmental Engineering Reference
In-Depth Information
12, 20 and 24 h suggesting the effective H
+
shuttling throughout the cycle oper-
ation. This helps to maintain the system under acidogenic conditions for longer
periods leading to higher H
2
production. At neutral operation, the e
-
discharge var-
ied with time and approached maximum at 12 h prior to decrease suggesting the
neutralization/reduction behaviour of H
+
by MB.
Acidic pH (below 6) showed less substrate degradation efficiency than the corre-
sponding neutral operation due to reduced methanogenic activity [24]. Neutral pH
illustrated effective substrate removal efficiency over the corresponding acidic oper-
ation. Maintenance of acidic conditions in association with pre-treatment has also
been observed to be effective in H
2
production during treatment of various types of
wastewater [30, 31, 38].
VFA (soluble acid metabolites generated from acidogenic fermentation) and pH
are integral expressions of acid-base conditions of anaerobic microenvironments
which provides information pertaining to the balance between two of the most
important microbial groups (AB and MB). Production of acids gradually reduces
the buffering capacity of system, which, in turn, results in a decline in the system
pH due to accumulation of organic acids leading to process inhibition [23, 81]. If
pH is not maintained in the optimum range, cessation of H
2
production will result
along with a marked shift in microbial population [75]. Relatively higher levels of
soluble metabolite production were observed under acidic operation over the cor-
responding neutral microenvironment, which corroborated well with H
2
production
data [26, 31, 38, 82]. Therefore, pH can be considered as a manipulable variable
for process control. Among the two process variables viz., influent pH and reactor
pH, the later is more difficult to control. Bicarbonate-alkalinity is an important pro-
cess parameter which indicates the system buffering capacity in association with pH
microenvironment and VFA concentrations.
Sulfate, if present in wastes will be converted into hydrogen sulfide by sulfate-
reducing bacteria (SRB) in the anaerobic microenvironment, resulting in toxicity to
other anaerobes [83]. SRB are reported to have H
2
utilization hydrogenase and can
readily use H
2
as the electron donor [84]. pH of the system microenvironment has
a direct influence on the sulfate reduction linked to H
2
production. At acid pH, the
SRB activity gets inhibited wherein H
2
production is unaffected. H
2
production has
markedly recovered and increased when pH was reduced to 5.5, even in the presence
of higher sulfate concentration (3 g SO
4
2-
/
l) [85].
4.3 Hydraulic Retention Time (HRT)
Hydraulic retention time (HRT) influences the H
2
generation process significantly.
Reducing HRT from 18 to 12 h has improved H
2
yield without affecting substrate
removal efficiency [57]. Maximum H
2
yield was reported between 0 and 14 h
in all the experimental variations studied in batch mode during dairy, chemical
and distillery wastewater treatment [22, 25, 31, 38]. Longer fermentation periods
induce a metabolic shift from the acidogenic process to the methanogenic pro-
cess which is unfavorable for H
2
production. Shorter HRT's have been shown to
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