Environmental Engineering Reference
In-Depth Information
[70]. Many agricultural and food industry wastes contain starch or cellulose, which
are rich in terms of carbohydrate content and can also be used for H
2
production.
The sludge generated in wastewater treatment plants contains large quantities of
carbohydrates and proteins which can also be used for energy production. Table 2
shows data on fermentative H
2
production using various types of waste.
Unlike wastewater, cellulosic material or solid wastes typically require pretreat-
ment to make the organic fraction soluble and bio-available to microorganisms for
conversion to H
2
. Due to its tightly packed, highly crystalline and water-insoluble
nature, cellulose is recalcitrant to hydrolysis into its individual glucose subunits
[70]. In the pretreatment step, a combination of chemical, mechanical, and enzy-
matic processes is typically used. Techniques viz., high temperature, high or low pH,
hydrolytic enzymes, microwaves, ultrasound, radiation, and pulsed electric fields are
being used for this purpose [19]. Some microorganisms can degrade cellulose effec-
tively by using their cellulase enzymes resulting in monosaccharide products that
can be converted into H
2
with dark fermentation [70].
4 Factors Influencing the Fermentative H
2
Production Process
4.1 Biocatalyst
Biocatalyst (inoculum) selection and its pre-treatment plays a vital role in select-
ing requisite microflora for efficient H
2
production [4, 7, 15, 30, 71, 72, 73].
Inoculum preparation affects both start up and the overall efficiency of H
2
pro-
duction. Typical anaerobic mixed cultures cannot produce H
2
as it is rapidly
consumed by H
2
-consuming or CH
4
-producing bacteria (MB) [74]. The most effec-
tive way to enhance H
2
production from anaerobic culture is to restrict or terminate
methanogenesis by allowing H
2
to become a metabolic end product. Physiological
differences between H
2
-producing bacteria (AB) and H
2
-consuming bacteria (MB)
forms the main basis for the preparation of the inoculum to start up the acido-
genic H
2
-producing process [72]. Spore-forming H
2
-producing bacteria can form
spores which protect them when they are in an adverse environment (high tempera-
ture, extreme acidity and alkalinity), but methanogens have no such capability [72].
Some of the pretreatment methods normally used for selective enrichment of an
H
2
-producing inoculum are listed in Table 3. Methanogenesis could also be elim-
inated by maintaining short retention times (2-10 h) during reactor operation [75,
76] as H
2
-producing bacteria grow faster than the methanogens [72]. Combining
different pre-treatment methods also showed a positive effect on the H
2
production
process [4, 21, 30, 38, 71]. In spite of good enhancement in H
2
production, marked
reduction in substrate degradation efficiency was observed after applying pretreat-
ment methods [4, 30, 71], which can be attributed to the inhibition of MB. The
methanogenesis function is required to metabolize intermediates generated from
the acidogenic process. Untreated anaerobic inocula showed low H
2
yield in spite
of effective substrate removal leading to CH
4
formation due to the presence of MB.
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