Introduction
Anaerobic digestion (AD) is a process in which complex organic substrate
is degraded to yield methane containing biogas (Kleerebezem, Joosse,
Rozendal, & Van Loosdrecht, 2015). A technology used in the AD process
present-day is the high rate granular sludge technology which enables
effective production of biogas from wastewater (Lettinga, van Velsen,
Hobma, de Zeeuw, & Klapwijk, 1980). Key intermediates in the AD process
are volatile fatty acids (VFA). These VFA can serve as a platform
molecule, in the so-called carboxylate platform (Agler, Wrenn, Zinder,
& Angenent, 2011; Holtzapple & Granda, 2009). VFA serves in this
platform as a building block for the production of chemicals and
polymers such as the production of polyhydroxyalkanoates (Marang, Jiang,
van Loosdrecht, & Kleerebezem, 2013; Jelmer Tamis et al., 2018).
Organic waste, of unknown composition and complexity can be degraded to
these platform molecules using a fermentation process. In order to
acquire VFA from an AD process, methanogenesis should be prevented.
Methanogenesis is the conversion of (in)organic C1 and C2 intermediates
to methane and carbon dioxide containing biogas. This process can be
stopped by inhibiting the methanogens by e.g. working at low solid
retention times (SRT), and/or working at a low pH with high VFA
concentrations (Kleerebezem et al., 2015). Combining the granular sludge
technology and the inhibition of methanogens, the principles of the AD
process using granules can be applied to produce VFA from complex
substrates.
Fermentation processes to yield VFA are usually conducted in continuous
stirred tank reactors (CSTR) fed with glucose (Rombouts, Mos,
Weissbrodt, Kleerebezem, & Van Loosdrecht, 2019; Temudo, Muyzer,
Kleerebezem, & Van Loosdrecht, 2008). A drawback of using a chemostat
type process is that low volumetric productivities are achieved and that
all biomass produced is present in the effluent (J. Tamis, Joosse, van
Loosdrecht, & Kleerebezem, 2015). Another type of operation would be
the usage of granular sludge, an established technology in the AD
process through the development of the Upflow Anaerobic Sludge Bed
(UASB) reactor or more recently the ‘Nereda®’ technology applied in
municipal wastewater treatment (Lettinga et al., 1980; Pronk et al.,
2015). Using granular sludge technology the SRT can be uncoupled from
the hydraulic retention time (HRT), resulting in high-rate systems (J.
Tamis et al., 2015). Additionally, a lower solid content can be realized
in the effluent using granular sludge through biomass removal from the
sludge bed. Low solid contents are beneficial in processes converting
the VFA produced into higher-value non soluble products like
bioplastics. However, the application of granular sludge technology for
the production of VFA is just touched upon and granulation and system
control need to be persistent. In general, anaerobic granular sludge
could serve as a platform to produce on an industrial scale VFA from
organic wastewaters.
Even though VFA production from organic waste using anaerobic granular
sludge has been demonstrated, many research questions remain (Jelmer
Tamis et al., 2018). For example, the factors determining the product
spectrum of the fermentation of glucose and other carbohydrates remain
unclear. For non-granular processes it has been demonstrated that at
higher pH values (6-8) the product spectrum will mainly consist out of
acetate and ethanol, whereas at a lower pH (5-6) an acetate and butyrate
mixture will be obtained (De Kok, Meijer, Van Loosdrecht, &
Kleerebezem, 2013; Temudo, Kleerebezem, & van Loosdrecht, 2007). At
lower pH-values, carbohydrate fermentations that produce organic acids
often generate H2-gas as a by-product. Even though in
some studies H2 is considered the main product of the
fermentation process (Das & Veziroglu, 2008; Hallenbeck & Ghosh,
2009), H2 production lowers the overall VFA product
yield. The produced H2 contains electrons originating
from the substrate and the H2 leaves the bioreactor via
the off-gas, resulting in a decrease of the VFA yield on substrate.
Lastly, also biomass production can be considered as an unwanted side
product of the fermentation process. In anaerobic processes where no
(strong) electron acceptor is present the biomass yield is relatively
low compared to aerobic processes. Still, in order to maximize VFA
production the biomass yield should be minimized. Increasing the SRT
(and therewith lowering the biomass specific growth rate) may result in
higher VFA yields because the biomass yield is reduced due to higher
substrate demands for maintenance purposes. Increasing the SRT can lead
to a higher VFA yield as shown in previous studies (Bengtsson,
Hallquist, Werker, & Welander, 2008; Bolaji & Dionisi, 2017). Overall,
the fermentation of organic waste is a complex process and the factors
that determine the product spectrum are largely unknown.
In this study the effect the SRT has on the product yield and product
spectrum of the fermentation of glucose to VFA was investigated. A
sequenting batch reactor (SBR) operated at pH 5.5 was used in which
pulse-wise glucose was fed to anaerobic granular sludge. The main
process performance indicators were; the VFA yield, the product
spectrum; the sludge volume index (SVI) and the microbial community
structure as identified using 16s rRNA sequencing. Three operational SRT
were chosen namely, 1-2 d SRT, 10-20 d SRT and uncontrolled SRT.
Challenges for this procces will be granulation at different SRT,
avoidance of methanogenesis at longer SRT and reduced VFA production
yield due to production of H2.
Materials and methods