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more notable at longer sludge retention time and lower organic loading rates (Ciggin
et al. 2012). In the study of Ciggin et al., notable differences in polyhydroxyalkanoates production were observed at organic loading rates of roughly 1000 mg- COD/
L/d and a sludge retention time of 8 days. These differences were reduced when the
sludge age was reduced to 2 days and were negligible when the organic loading was
increased to roughly 3000 mg-COD/L/d.
Organic loading rate has also been demonstrated to impact the polyhydroxyalkanoates monomer composition. Chen et al. showed that, under continuous feeding,
the rate of biomass loading had a significant impact on the
hydroxybutyrate:hydroxyvalerate ratio, such that at around 24 C-mol volatile fatty
acids/C-mol X/d, the hydroxybutyrate:hydroxyvalerate ratio was minimal and was
around half that observed at the lowest loading rate of 4 mol volatile fatty acids/Cmol X/d. The hydroxybutyrate:hydroxyvalerate ratio increased slightly from the
minimum ratio at higher biomass feeding rates (Chen et al. 2015).
A significant issue with volatile fatty acid–rich wastewater under pulse feed
(feast) type operation is the high transient concentrations of volatile fatty acids present in the reactor. Under such cases if the pH is low, unionized volatile fatty acids
may inhibit the microbial culture. Chen et al. (Chen et al. 2015) found that when
operating a system at relatively high volatile fatty acid concentration (115–120 mM),
continuous feeding provided improved accumulation and yields compared with
batch feed at all pH tested, but particularly at the acidic pH of 5 where continuous
feed provided the highest value while for batch feed accumulation and yields were
lowest. Moreover, at lower biomass loading rates, polyhydroxyalkanoates accumulation was found to be less sensitive to all influent pH values tested, all likely as a
result of reduced inhibition.
5.4.5 Nutrient Limitation
Nutrient limitation is when sources of nitrogen or phosphate are depleted in the
environment. Under nitrogen-limiting conditions, protein synthesis is halted, which
in turn stops the progress of the Krebs cycle enzymes such as citrate synthase,
responsible for incorporating acetyl-CoA into the energy generating metabolic
cycle. As a result, the excess acetyl-CoA, a metabolic precursor to polyhydroxybutyrate, is instead directed toward this intracellular storage product (Muralidharan
et al. 2013). It has been observed under nitrogen-limiting conditions, that the bacterial cells do not increase in number, but rather increase in weight and size (Chee
et al. 2010), which is explained by the fact that the polyhydroxyalkanoates production increases during this stage. Additionally, when nutrient-limiting conditions are
coupled with the feast-famine strategy, they enable the polyhydroxyalkanoatesproducing bacteria to maximize nutrient resources as they adjust to the new environmental conditions (Tan et al. 2014).
The carbon to nutrient ratio is an important variable that affects the production of
polyhydroxyalkanoates. Studies concluded that polyhydroxyalkanoates production
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…
more notable at longer sludge retention time and lower organic loading rates (Ciggin
et al. 2012). In the study of Ciggin et al., notable differences in polyhydroxyalkanoates production were observed at organic loading rates of roughly 1000 mg- COD/
L/d and a sludge retention time of 8 days. These differences were reduced when the
sludge age was reduced to 2 days and were negligible when the organic loading was
increased to roughly 3000 mg-COD/L/d.
Organic loading rate has also been demonstrated to impact the polyhydroxyalkanoates monomer composition. Chen et al. showed that, under continuous feeding,
the rate of biomass loading had a significant impact on the
hydroxybutyrate:hydroxyvalerate ratio, such that at around 24 C-mol volatile fatty
acids/C-mol X/d, the hydroxybutyrate:hydroxyvalerate ratio was minimal and was
around half that observed at the lowest loading rate of 4 mol volatile fatty acids/Cmol X/d. The hydroxybutyrate:hydroxyvalerate ratio increased slightly from the
minimum ratio at higher biomass feeding rates (Chen et al. 2015).
A significant issue with volatile fatty acid–rich wastewater under pulse feed
(feast) type operation is the high transient concentrations of volatile fatty acids present in the reactor. Under such cases if the pH is low, unionized volatile fatty acids
may inhibit the microbial culture. Chen et al. (Chen et al. 2015) found that when
operating a system at relatively high volatile fatty acid concentration (115–120 mM),
continuous feeding provided improved accumulation and yields compared with
batch feed at all pH tested, but particularly at the acidic pH of 5 where continuous
feed provided the highest value while for batch feed accumulation and yields were
lowest. Moreover, at lower biomass loading rates, polyhydroxyalkanoates accumulation was found to be less sensitive to all influent pH values tested, all likely as a
result of reduced inhibition.
5.4.5 Nutrient Limitation
Nutrient limitation is when sources of nitrogen or phosphate are depleted in the
environment. Under nitrogen-limiting conditions, protein synthesis is halted, which
in turn stops the progress of the Krebs cycle enzymes such as citrate synthase,
responsible for incorporating acetyl-CoA into the energy generating metabolic
cycle. As a result, the excess acetyl-CoA, a metabolic precursor to polyhydroxybutyrate, is instead directed toward this intracellular storage product (Muralidharan
et al. 2013). It has been observed under nitrogen-limiting conditions, that the bacterial cells do not increase in number, but rather increase in weight and size (Chee
et al. 2010), which is explained by the fact that the polyhydroxyalkanoates production increases during this stage. Additionally, when nutrient-limiting conditions are
coupled with the feast-famine strategy, they enable the polyhydroxyalkanoatesproducing bacteria to maximize nutrient resources as they adjust to the new environmental conditions (Tan et al. 2014).
The carbon to nutrient ratio is an important variable that affects the production of
polyhydroxyalkanoates. Studies concluded that polyhydroxyalkanoates production
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…