problems which plague ethanol production processes. The off-gases can be
passed through a water cooled condenser before being returned to the bed. At
the end of the fermentation ethanol concentrations in the condensed phase can
be as high as 16.7% (v/v) [219]. However, little further attention has been given
over the last decade to ethanol production and recovery by stripping in gassolid fluidized beds.
In contrast to ethanol production, no attention has been paid to the recovery
of aroma compounds. However, the presence of volatile compounds such as
acetaldehyde, ethyl acetate, ethyl propionate, and 3-methyl butanol in the headspace above substrates on which Rhizopus oryzae is growing [221] suggests that
such recovery could be attempted, although the economics of such a recovery
process are unclear, given that the compounds are present at very low concentrations of less than 0.5 mmol l –1 .
In a related system, during growth of baker’s yeast on a semi-solid substrate,
homogenized whole potatoes predigested with a-amylase, within an airfluidized bed bioreactor, proteins were entrained in the exit air from the bioreactor [222]. These proteins could be recovered by sparging the exit air
through a water chamber. Interestingly, the entrainment of proteins was selective, suggesting the possibility of on-line protein fractionation [222]. Of course,
unlike small organic compounds, the proteins are not volatile. The fractionation
occurs as a partitioning between the aqueous phase within the substrate and
microdroplets of water which exit with the exit gas stream [223].
8.7
Waste Treatment and Ecological Aspects
One of the claimed advantages of SSF is that it often uses agricultural wastes
which would otherwise be dumped. However, for those SSF processes in which
the whole product is not used, but rather the product is extracted from the
solids, there is the important question of how the spent solids are disposed of.
Relatively little attention has been given to this question, probably because
there are relatively few large-scale SSF processes generating solid wastes.
However, if the technology does manage to become established on the large
scale, waste disposal will become a key problem, especially since governments
around the world are imposing stricter environmental controls.
Despite the lack of attention to this issue in the literature, some general
principles can be outlined here. First, one of the important considerations is the
impact of the solids on the environment Second, there is the question of
economics. It is better to take some advantage of a waste material rather than
pay to have to dump it, and it is likely that dumping fees will increase with
increasing shortage of conveniently located suitable dump sites.
Spent solids can potentially generate undesirable odors, or release leachates
rich in organic material. Since extraction will often involve wetting, the solids
may have a high water content, especially if the solids are not pressed, which can
allow the growth of bacteria. The interior regions of piles of solids can quickly
become anaerobic, giving the potential for the production of undesirable odors
by anaerobic bacteria.
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passed through a water cooled condenser before being returned to the bed. At
the end of the fermentation ethanol concentrations in the condensed phase can
be as high as 16.7% (v/v) [219]. However, little further attention has been given
over the last decade to ethanol production and recovery by stripping in gassolid fluidized beds.
In contrast to ethanol production, no attention has been paid to the recovery
of aroma compounds. However, the presence of volatile compounds such as
acetaldehyde, ethyl acetate, ethyl propionate, and 3-methyl butanol in the headspace above substrates on which Rhizopus oryzae is growing [221] suggests that
such recovery could be attempted, although the economics of such a recovery
process are unclear, given that the compounds are present at very low concentrations of less than 0.5 mmol l –1 .
In a related system, during growth of baker’s yeast on a semi-solid substrate,
homogenized whole potatoes predigested with a-amylase, within an airfluidized bed bioreactor, proteins were entrained in the exit air from the bioreactor [222]. These proteins could be recovered by sparging the exit air
through a water chamber. Interestingly, the entrainment of proteins was selective, suggesting the possibility of on-line protein fractionation [222]. Of course,
unlike small organic compounds, the proteins are not volatile. The fractionation
occurs as a partitioning between the aqueous phase within the substrate and
microdroplets of water which exit with the exit gas stream [223].
8.7
Waste Treatment and Ecological Aspects
One of the claimed advantages of SSF is that it often uses agricultural wastes
which would otherwise be dumped. However, for those SSF processes in which
the whole product is not used, but rather the product is extracted from the
solids, there is the important question of how the spent solids are disposed of.
Relatively little attention has been given to this question, probably because
there are relatively few large-scale SSF processes generating solid wastes.
However, if the technology does manage to become established on the large
scale, waste disposal will become a key problem, especially since governments
around the world are imposing stricter environmental controls.
Despite the lack of attention to this issue in the literature, some general
principles can be outlined here. First, one of the important considerations is the
impact of the solids on the environment Second, there is the question of
economics. It is better to take some advantage of a waste material rather than
pay to have to dump it, and it is likely that dumping fees will increase with
increasing shortage of conveniently located suitable dump sites.
Spent solids can potentially generate undesirable odors, or release leachates
rich in organic material. Since extraction will often involve wetting, the solids
may have a high water content, especially if the solids are not pressed, which can
allow the growth of bacteria. The interior regions of piles of solids can quickly
become anaerobic, giving the potential for the production of undesirable odors
by anaerobic bacteria.
130
D.A. Mitchell et al.