3.5 Comparison to Other Studies
Table 2 summarizes and compare the solvent titer, productivity,
and yield of ABE fermentation obtained in this study and other notable
continuous fermentation studies. In general, continuous fermentation
systems can dramatically enhance productivity and lower capital costs by
reducing the size of the fermentation system compared to batch
fermentation (Vees et al., 2020). Continuous systems can experience
little to no downtime between batches and have other processing
advantages leading to increased productivity. However, continuous
fermentation has rarely been used in industry because of increased risks
in culture degeneration, cell washout and contamination during operation
for an extended period that may result in catastrophic production lost.
Also, the high productivity obtained at a high dilution rate is usually
at the expense of incomplete or low substrate conversion. To overcome
these problems, continuous fermentation with cell recycling and/or
retention via immobilization in the bioreactor has been used to attain a
high cell density of ~100 g DCW/L reactor volume with
greatly increased reactor productivity of >2 g/L∙h (Huang
et al., 2004; Jang et al., 2013). Various solid support materials have
been applied for cell immobilization via adsorption and entrapment (Badr
et al., 2001; Bankar et al., 2012; Chang et al., 2016; Davison and
Thompson 1993; Frick and Schugerl, 1986; Gallazzi et al., 2015; Huang et
al., 2004; Kong et al., 2015; Qureshi and Maddox, 1988; Qureshi et al.,
2000; Survase et al., 2012; Zhang et al. 2009). However, immobilized
cell bioreactors such as packed-bed and membrane bioreactors often
suffer from lost/declined productivity during prolonged operation due to
the accumulation of dead, aging or non-viable cells, which also causes
clogging/fouling and limits reactor’s operating life to less than a few
weeks (Qureshi and Maddox, 1988; Qureshi et al., 2000; Zhang et al.,
2009). For continuous fermentation with cell recycling through
microfiltration, cell bleeding is necessary to avoid over accumulation
of dead and inactive cells and to prolong the reactor life (Tashiro et
al., 2005).
In the present study, the ABE productivity of 24.2 g/L∙h obtained in the
single-pass FBB with ATCC55025 at the dilution rate of 1.88
h-1 was the highest ever reported to date. In general,
a higher productivity can be obtained with a higher dilution rate in a
continuous fermentation process. However, a higher dilution rate usually
results in lower substrate conversion and butanol production, which will
increase production cost (Huang et al., 2019). Furthermore, a high
dilution rate usually favors cell growth and acid production in ABE
fermentation. To maintain a high productivity while also achieve a high
conversion with high butanol yield, continuous fermentation with stirred
tank reactors (STR) or recirculating packed bed reactors (PBR) may have
to be operated with multiple stages (Badr et al., 2001; Chang et al.,
2016; Frick and Schugerl, 1986), which increases the capital cost. In
this study, butyric acid was used in the feed medium as a co-substrate
with glucose to keep cells in the single-pass FBB in the solventogenesis
phase. It has also been reported that adding butyric acid in the feed
medium could improve butanol production in a continuous STR (Lee SM et
al., 2008).
A PBR with solid support particles like brick (Qureshi et al., 2000) and
ceramic beads (Badr et al., 2001) suffered from low void volume, high
pressure drop, and clogging and channeling due to the accumulation of
cell biomass, which impeded the reactor performance and operating life.
In this study, cells were immobilized in the highly porous fibrous
matrix spirally wound with gaps between the matrix layers as flow
channels to allow for free flow of fermentation broth, suspended solids
(cells), and gases (CO2 and H2) through
the fibrous bed with a low pressure drop without clogging occurring to
conventional packed-bed bioreactors (Zhu et al., 2002). Consequently,
the FBB could have stable performance throughout the entire operation
period of over several months as demonstrated in our previous studies
(Lewis and Yang, 1992). Moreover, the FBB with greatly increased cell
density also facilitated in-process adaptation or evolutionary
engineering of cells to attain higher tolerance to toxic chemicals
(e.g., organic acids and butanol) and increase product titer, yield, and
productivity as demonstrated in previous studies (Huang et al., 1998; Li
et al., 2019; Suwannakham and Yang, 2005; Wei et al., 2013; Yang et al.,
1994; Zhu and Yang, 2003). Since butyric acid and butanol are highly
inhibitory to most microorganisms, no contamination was found throughout
the continuous fermentation study. The continuous FBB was operated for
over 30 days without encountering any performance issues.
The continuous fermentation process can be operated with in situproduct separation to alleviate butanol toxicity and increase final
product titer and reactor productivity (Veza et al., 2021; Yang and Lu,
2013). Gas stripping (Lu et al., 2012; Xue et al., 2016b), adsorption
(Xue et al., 2016a), extraction (Bankar et al., 2012; Davison and
Thompson 1993), and pervaporation (Cai et al., 2016; Zhu et al., 2018)
are the most studied in situ butanol separation methods. More
recently, vacuum distillation was applied to continuously recover ABE
from the fermentation broth in a separate tank, achieving a high final
butanol titer of 550 g/L and productivity of 14 g/L∙h in a continuous
ABE fermentation with cell recycling operated at a dilute rate of 0.076
h-1 (Nguyen et al., 2018). The process maintained a
steady state for ~170 h. The continuous single-pass FBB
can be integrated with gas stripping and pervaporation (or vapor
stripping-vapor permeation, VSVP) to further increase productivity and
product titer to higher than 600 g/L (Du et al., 2021; Lu et al., 2012).