141
accumulation of polyhydroxyalkanoates. Continuous flow (feed) systems have the
benefit that polyhydroxyalkanoates accumulation occurs continuously and can
therefore lead to higher volumetric production rates of polyhydroxyalkanoates
(Kourmentza et al. 2017).
Regardless of the type of system selected, it is important to note that the economics and feasibility of the process are driven by three key factors, namely, a high
carbon to polymer yield (particularly if the substrate is costly), a high volumetric
productivity resulting in small reactors and high cell densities, and high intracellular
storage content leading to reduced residual waste. An example of a microbial mixed
culture system highlighting this point is that of Chen et al. (Chen et al. 2015). In
their study they used a continuous feeding mode and found at low biomass loading
rates polyhydroxyalkanoates production was enhanced to a maximum of 70.4% of
cell dry weight. However, lower loading rates lead to less biomass production,
reducing overall productivity, and dilute suspension cultures, making subsequent
processing of the polymer costly. Chua et al.’s study on using nutrient limitation
also highlights this consideration (Chua and Yu 1999). While high C/N ratios led to
the maximum cell dry weight of 39% at a C/N ratio of 140, increasing nutrient limitation decreased cell growth such that the maximum overall polyhydroxyalkanoates
productivity was achieved at a C/N ratio of 100. From a systems perspective, the
optimum ratio would then lie between 100 and 140, depending on cell harvesting
and polymer extraction costs and their relationship to bioreactor cell concentrations.
Process design is further complicated by the economic value of the polymer based
on its monomer composition, which may change as the process design changes.
Given the limited number of pilot microbial mixed culture systems, this is an area
where significant scope exists for further research and may be aided through
computer- aided process design.
5.7 Polyhydroxyalkanoates Quantification
Determining the presence of polyhydroxyalkanoates and its quantity in
polyhydroxyalkanoates- producing microbial cells can be done through various
methods and is important for monitoring the performance of the polyhydroxyalkanoates production process. The most widely used method is gas chromatography
(GC) with either mass spectrometry (MS) detection or flame ionization detection
(FID) as they are highly sensitive and accurate detection methods. Both methods
require depolymerization and conversion into derivatives such as acids or methyl
ester, but these methods provide detailed analysis of polyhydroxyalkanoates monomers, which is useful in process optimization (Tan et al. 2014; Koller et al. 2011).
On average, one test of GC-MS is completed in 4 hours (Godbole 2016). Nuclear
magnetic resonance (NMR) spectroscopy is another method that enables the understanding of polyhydroxyalkanoates monomeric composition (Tan et al. 2014). It
does not require the hydrolysis of the polymer (Godbole 2016). Additionally, it can
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…
accumulation of polyhydroxyalkanoates. Continuous flow (feed) systems have the
benefit that polyhydroxyalkanoates accumulation occurs continuously and can
therefore lead to higher volumetric production rates of polyhydroxyalkanoates
(Kourmentza et al. 2017).
Regardless of the type of system selected, it is important to note that the economics and feasibility of the process are driven by three key factors, namely, a high
carbon to polymer yield (particularly if the substrate is costly), a high volumetric
productivity resulting in small reactors and high cell densities, and high intracellular
storage content leading to reduced residual waste. An example of a microbial mixed
culture system highlighting this point is that of Chen et al. (Chen et al. 2015). In
their study they used a continuous feeding mode and found at low biomass loading
rates polyhydroxyalkanoates production was enhanced to a maximum of 70.4% of
cell dry weight. However, lower loading rates lead to less biomass production,
reducing overall productivity, and dilute suspension cultures, making subsequent
processing of the polymer costly. Chua et al.’s study on using nutrient limitation
also highlights this consideration (Chua and Yu 1999). While high C/N ratios led to
the maximum cell dry weight of 39% at a C/N ratio of 140, increasing nutrient limitation decreased cell growth such that the maximum overall polyhydroxyalkanoates
productivity was achieved at a C/N ratio of 100. From a systems perspective, the
optimum ratio would then lie between 100 and 140, depending on cell harvesting
and polymer extraction costs and their relationship to bioreactor cell concentrations.
Process design is further complicated by the economic value of the polymer based
on its monomer composition, which may change as the process design changes.
Given the limited number of pilot microbial mixed culture systems, this is an area
where significant scope exists for further research and may be aided through
computer- aided process design.
5.7 Polyhydroxyalkanoates Quantification
Determining the presence of polyhydroxyalkanoates and its quantity in
polyhydroxyalkanoates- producing microbial cells can be done through various
methods and is important for monitoring the performance of the polyhydroxyalkanoates production process. The most widely used method is gas chromatography
(GC) with either mass spectrometry (MS) detection or flame ionization detection
(FID) as they are highly sensitive and accurate detection methods. Both methods
require depolymerization and conversion into derivatives such as acids or methyl
ester, but these methods provide detailed analysis of polyhydroxyalkanoates monomers, which is useful in process optimization (Tan et al. 2014; Koller et al. 2011).
On average, one test of GC-MS is completed in 4 hours (Godbole 2016). Nuclear
magnetic resonance (NMR) spectroscopy is another method that enables the understanding of polyhydroxyalkanoates monomeric composition (Tan et al. 2014). It
does not require the hydrolysis of the polymer (Godbole 2016). Additionally, it can
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…