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was higher under nitrogen-limiting conditions rather than excess of nitrogen or no
nitrogen availability (Saharan et al. 2014; Montiel-Jarillo et al. 2017). Optimal values vary depending on the culture, but are typically in the range of 12 to 25 g-C/g-N
(Silva et al. 2016). Silva et al. found that the timing of nutrient addition to the system played an important role in both the accumulation and the composition of stored
polyhydroxyalkanoates. When nitrogen was added during the famine phase (uncoupled strategy) rather than during the feast phase (coupled strategy), polyhydroxyalkanoates production increased more than twofold reaching 1300 mg-COD/L, while
the hydroxyvalerate monomer content increased by 82% reaching a content of up to
20% wt/wt. Content of hydroxyvalerate monomer showed a decrease, however,
with increasing C/N ratio, while stored polyhydroxyalkanoates also showed a significant deterioration when the C/N ratio increased from 14.3 mol-C/mol-N to
22.3 mol-C/mol-N. Yield and cell storage as cell dry weight both roughly doubled
under the uncoupled nutrient addition strategy. Under this strategy ammonia was
uptaken during polyhydroxyalkanoates degradation for cell biomass growth, promoting a stronger selection pressure for polyhydroxyalkanoates accumulation
(Silva et al. 2016). In another experiment conducted by Chua et al. treating a xenobiotic wastewater in an activated sludge system, different types of polyhydroxyalkanoates monomers were produced from wastewater under nitrogen-limiting
conditions. With an increasing C/N ratio from 20 to 140, the cell storage of polymer
increased from 11% to 39% of cell dry weight (Chua and Yu 1999). Chua and Yu
also reported changes in the copolymer ratios based on the C/N ratio in the microbial mixed culture.
Phosphorus limitation can also be used to control the behavior of polyhydroxyalkanoates production. In chemostat-based systems, phosphorus limitation acts in a
similar manner to nitrogen limitation, preventing cell biomass growth and forcing
carbon toward intracellular storage products (Cavaillé et al. 2016). Careful control
of phosphorus concentration and addition can therefore direct carbon toward cell
growth or polyhydroxyalkanoates. Moreover, the authors found that at lower dilution rates (i.e., lower critical growth rates), the cultures were more adaptive to higher
phosphorus limitation. In contrast, in a sequencing batch reactor, it was found that
phosphorus limitation led to reduced substrate conversion (Korkakaki et al. 2017).
At a C/P ratio of 150 C-mol/P-mol, no significant impact on accumulation was
noticed, but at 300 C-mol/P-mol, polyhydroxyalkanoates accumulation was severely
affected. The relation of substrate conversion vs P limitation allows control over
oxygen uptake rate in the system and therefore peak air demands required by the
aeration system.
5.4.6 pH Impact
According to many studies, pH plays an important role in the production of polyhydroxyalkanoates inside the cells (Montiel-Jarillo et al. 2017), affecting the accumulation of polyhydroxyalkanoates as well as the monomer composition when the pH
S. Sali and H. R. Mackey
was higher under nitrogen-limiting conditions rather than excess of nitrogen or no
nitrogen availability (Saharan et al. 2014; Montiel-Jarillo et al. 2017). Optimal values vary depending on the culture, but are typically in the range of 12 to 25 g-C/g-N
(Silva et al. 2016). Silva et al. found that the timing of nutrient addition to the system played an important role in both the accumulation and the composition of stored
polyhydroxyalkanoates. When nitrogen was added during the famine phase (uncoupled strategy) rather than during the feast phase (coupled strategy), polyhydroxyalkanoates production increased more than twofold reaching 1300 mg-COD/L, while
the hydroxyvalerate monomer content increased by 82% reaching a content of up to
20% wt/wt. Content of hydroxyvalerate monomer showed a decrease, however,
with increasing C/N ratio, while stored polyhydroxyalkanoates also showed a significant deterioration when the C/N ratio increased from 14.3 mol-C/mol-N to
22.3 mol-C/mol-N. Yield and cell storage as cell dry weight both roughly doubled
under the uncoupled nutrient addition strategy. Under this strategy ammonia was
uptaken during polyhydroxyalkanoates degradation for cell biomass growth, promoting a stronger selection pressure for polyhydroxyalkanoates accumulation
(Silva et al. 2016). In another experiment conducted by Chua et al. treating a xenobiotic wastewater in an activated sludge system, different types of polyhydroxyalkanoates monomers were produced from wastewater under nitrogen-limiting
conditions. With an increasing C/N ratio from 20 to 140, the cell storage of polymer
increased from 11% to 39% of cell dry weight (Chua and Yu 1999). Chua and Yu
also reported changes in the copolymer ratios based on the C/N ratio in the microbial mixed culture.
Phosphorus limitation can also be used to control the behavior of polyhydroxyalkanoates production. In chemostat-based systems, phosphorus limitation acts in a
similar manner to nitrogen limitation, preventing cell biomass growth and forcing
carbon toward intracellular storage products (Cavaillé et al. 2016). Careful control
of phosphorus concentration and addition can therefore direct carbon toward cell
growth or polyhydroxyalkanoates. Moreover, the authors found that at lower dilution rates (i.e., lower critical growth rates), the cultures were more adaptive to higher
phosphorus limitation. In contrast, in a sequencing batch reactor, it was found that
phosphorus limitation led to reduced substrate conversion (Korkakaki et al. 2017).
At a C/P ratio of 150 C-mol/P-mol, no significant impact on accumulation was
noticed, but at 300 C-mol/P-mol, polyhydroxyalkanoates accumulation was severely
affected. The relation of substrate conversion vs P limitation allows control over
oxygen uptake rate in the system and therefore peak air demands required by the
aeration system.
5.4.6 pH Impact
According to many studies, pH plays an important role in the production of polyhydroxyalkanoates inside the cells (Montiel-Jarillo et al. 2017), affecting the accumulation of polyhydroxyalkanoates as well as the monomer composition when the pH
S. Sali and H. R. Mackey