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).