4.1.5 Soil
Soil, particularly agriculture soil, has been used to prepare anaerobic mixed culture
inocula for hydrogen production studies from food processing and domestic wastewater [126] and molasses and potato waste [217].
4.2 Culture Pretreatment
To be economically profitable and sustainable, dark fermentation of industrial waste
to recover H 2 should yield a positive net energy gain. Typically, anaerobic mixed
culture produces CH 4 rather than H 2 as the end product. H 2 is an intermediate
metabolite which is produced by acidogenic and acetogenic bacteria and is later
consumed by hydrogenotrophic methanogens, homoacetogens, and sulfate-reducing
bacteria. Therefore, H 2 -consuming microorganisms have to be eliminated or at least
inhibited during dark fermentation so that H 2 accumulates and is recovered before
they consume it while producing CH 4 , acetate, or other by-products. Several
pretreatment methods can achieve the elimination of H 2 -consuming microorganisms. These methods include heat shock [96, 97], acid-base treatment, irradiation,
toxic methanogens-inhibiting chemicals (e.g., 2-bromoethanesulfonic acid (BESA),
chloroform, long-chain fatty acids (LCFAs), etc.) [153, 191, 210, 218–220], ultrasound sonication [221], acid/base [203, 222], and BSAE [203]. Generally, there is a
consensus in the scientific literature on the importance of culture pretreatment but
not on which method is the optimum relative to the others.
4.2.1 Heat Treatment
Heat shock treatment is the most widely used pretreatment, particularly for cultures
used to ferment food waste [67, 93–96, 101, 104, 111, 119, 159, 178]. Heat
treatment selects spore-forming microorganisms such as Clostridia and eliminates
non-spore-forming microorganisms such as methanogens [220]. Unfortunately, heat
pretreatment is not a well-controlled pretreatment; it does not exclusively select
H 2 -producing or kill H 2 -consuming microorganisms [223]. Many H 2 -consuming
bacteria form spores and can survive heat pretreatment, whereas H 2 -producing
bacteria which cannot form spores such as Enterobacter sp. and Citrobacter spp.
[223] are killed. Heat pretreatment does not eradicate methanogens; they regrow
when the conditions are favorable [224]. Repeated heat treatments were necessary
because the H 2 production declined after 1 month of continuous operation [80]. It
must be noted that heat treatment decreases microbial diversity, particularly when
the heat treatment temperature is increased from 65 to 90
C [225]. The duration of
the optimum heat treatment increases with the percent of the total solids
[84]. Reungsang and Sreela-or [158] found that heat treatment of anaerobic sludge
produced a good inoculum for pineapple waste extract fermentation.
350
A. Hajizadeh et al.
Soil, particularly agriculture soil, has been used to prepare anaerobic mixed culture
inocula for hydrogen production studies from food processing and domestic wastewater [126] and molasses and potato waste [217].
4.2 Culture Pretreatment
To be economically profitable and sustainable, dark fermentation of industrial waste
to recover H 2 should yield a positive net energy gain. Typically, anaerobic mixed
culture produces CH 4 rather than H 2 as the end product. H 2 is an intermediate
metabolite which is produced by acidogenic and acetogenic bacteria and is later
consumed by hydrogenotrophic methanogens, homoacetogens, and sulfate-reducing
bacteria. Therefore, H 2 -consuming microorganisms have to be eliminated or at least
inhibited during dark fermentation so that H 2 accumulates and is recovered before
they consume it while producing CH 4 , acetate, or other by-products. Several
pretreatment methods can achieve the elimination of H 2 -consuming microorganisms. These methods include heat shock [96, 97], acid-base treatment, irradiation,
toxic methanogens-inhibiting chemicals (e.g., 2-bromoethanesulfonic acid (BESA),
chloroform, long-chain fatty acids (LCFAs), etc.) [153, 191, 210, 218–220], ultrasound sonication [221], acid/base [203, 222], and BSAE [203]. Generally, there is a
consensus in the scientific literature on the importance of culture pretreatment but
not on which method is the optimum relative to the others.
4.2.1 Heat Treatment
Heat shock treatment is the most widely used pretreatment, particularly for cultures
used to ferment food waste [67, 93–96, 101, 104, 111, 119, 159, 178]. Heat
treatment selects spore-forming microorganisms such as Clostridia and eliminates
non-spore-forming microorganisms such as methanogens [220]. Unfortunately, heat
pretreatment is not a well-controlled pretreatment; it does not exclusively select
H 2 -producing or kill H 2 -consuming microorganisms [223]. Many H 2 -consuming
bacteria form spores and can survive heat pretreatment, whereas H 2 -producing
bacteria which cannot form spores such as Enterobacter sp. and Citrobacter spp.
[223] are killed. Heat pretreatment does not eradicate methanogens; they regrow
when the conditions are favorable [224]. Repeated heat treatments were necessary
because the H 2 production declined after 1 month of continuous operation [80]. It
must be noted that heat treatment decreases microbial diversity, particularly when
the heat treatment temperature is increased from 65 to 90
C [225]. The duration of
the optimum heat treatment increases with the percent of the total solids
[84]. Reungsang and Sreela-or [158] found that heat treatment of anaerobic sludge
produced a good inoculum for pineapple waste extract fermentation.
350
A. Hajizadeh et al.