140
volumetric productivity and conversion yield. The aerobic dynamic feeding method
has been further enhanced through incorporation of nutrient limitation during the
feast phase to promote polyhydroxyalkanoates synthesis over growth (Silva et al.
2016). In a separate modification, the incorporation of an intermediate settling
phase provides a selective pressure based on the higher density of
polyhydroxyalkanoates- rich biomass to washout non-polyhydroxyalkanoates storing biomass and allows the decanting of remaining soluble organics at the end of the
feast phase to prevent non-polyhydroxyalkanoates growth in the famine phase
(Chen et al. 2016). Zeng et al. demonstrated the value of settling selection in the
enrichment phase, reaching steady-state polyhydroxybutyrate content in two thirds
the time of standard aerobic dynamic feeding. Furthermore, they showed that a
feed-on-demand approach using oxygen uptake rate measurements further improved
storage from 70% to 83% cell dry weight, yield from 0.6 to 0.81 Cmolpolyhydroxybutyrate/CmolAc, and specific polyhydroxybutyrate production from
0.43 to 0.85 Cmol-polyhydroxybutyrate/Cmol-X/h (Zeng et al. 2018).
For microbial mixed culture aerobic dynamic feeding processes, the sequencing
batch reactor is a preferred reactor system. Sequencing batch reactors are easy to
control, highly flexible and allow quick modifications of the process conditions.
Moreover, sequencing batch reactors allow control of process parameters with time,
including substrate concentration and duration profiles and, therefore, are suitable
for time-based feast-famine process control, allowing control over structure and
functions of the composed microbial community (Salehizadeh and van Loosdrecht
2004). They can be operated under intermittent substrate feeding in order to favor
the storage of the polymer by the sludge. For instance, fed-batch fermentation was
proven to be more efficient in producing high cell concentration, due to the ability
of controlling substrate inhibition in the medium (Chee et al. 2010).
In some instances, particularly with highly inhibitive substrates, stepwise fedbatch modes are more likely to prevent inhibition and increase polyhydroxyalkanoates accumulation (Kourmentza et al. 2017; Albuquerque et al. 2007). Continuous
flow–based systems may be even more advantageous with inhibitive substrates.
Such systems also reduce the reliance on integrating multiple reactors to manage
continuous flows typical of most wastewater systems. One two-step continuous flow
layout is to combine a plug flow reactor (PFR) with a subsequent continuous flow
stir tank reactor (CSTR) and settler returning concentrated biomass to the plug flow
reactor. The plug flow reactor promotes feast conditions, while the continuous flow
stir tank reactor provides famine-like conditions (Chee et al. 2010; Salehizadeh and
van Loosdrecht 2004). Contact-stabilization configuration with biomass recycle is
another continuous flow setup that promotes continuous polyhydroxyalkanoates
production. Sarioglu et al. used a contact-stabilization configuration with two aerated reactors, contact and stabilization reactors, separated with a sedimentation
tank. The influent wastewater enters the contact reactor and moves to the sedimentation basin. The overflow of the sedimentation basin leaves as treated effluent, while
the underflow moves to the stabilization reactor, which goes back to the contact
aerator (Sarioglu et al. 2003). Similar to the plug flow reactor coupled with continuous flow stir tank reactor, the recirculation of the biomass results in a continuous
S. Sali and H. R. Mackey
volumetric productivity and conversion yield. The aerobic dynamic feeding method
has been further enhanced through incorporation of nutrient limitation during the
feast phase to promote polyhydroxyalkanoates synthesis over growth (Silva et al.
2016). In a separate modification, the incorporation of an intermediate settling
phase provides a selective pressure based on the higher density of
polyhydroxyalkanoates- rich biomass to washout non-polyhydroxyalkanoates storing biomass and allows the decanting of remaining soluble organics at the end of the
feast phase to prevent non-polyhydroxyalkanoates growth in the famine phase
(Chen et al. 2016). Zeng et al. demonstrated the value of settling selection in the
enrichment phase, reaching steady-state polyhydroxybutyrate content in two thirds
the time of standard aerobic dynamic feeding. Furthermore, they showed that a
feed-on-demand approach using oxygen uptake rate measurements further improved
storage from 70% to 83% cell dry weight, yield from 0.6 to 0.81 Cmolpolyhydroxybutyrate/CmolAc, and specific polyhydroxybutyrate production from
0.43 to 0.85 Cmol-polyhydroxybutyrate/Cmol-X/h (Zeng et al. 2018).
For microbial mixed culture aerobic dynamic feeding processes, the sequencing
batch reactor is a preferred reactor system. Sequencing batch reactors are easy to
control, highly flexible and allow quick modifications of the process conditions.
Moreover, sequencing batch reactors allow control of process parameters with time,
including substrate concentration and duration profiles and, therefore, are suitable
for time-based feast-famine process control, allowing control over structure and
functions of the composed microbial community (Salehizadeh and van Loosdrecht
2004). They can be operated under intermittent substrate feeding in order to favor
the storage of the polymer by the sludge. For instance, fed-batch fermentation was
proven to be more efficient in producing high cell concentration, due to the ability
of controlling substrate inhibition in the medium (Chee et al. 2010).
In some instances, particularly with highly inhibitive substrates, stepwise fedbatch modes are more likely to prevent inhibition and increase polyhydroxyalkanoates accumulation (Kourmentza et al. 2017; Albuquerque et al. 2007). Continuous
flow–based systems may be even more advantageous with inhibitive substrates.
Such systems also reduce the reliance on integrating multiple reactors to manage
continuous flows typical of most wastewater systems. One two-step continuous flow
layout is to combine a plug flow reactor (PFR) with a subsequent continuous flow
stir tank reactor (CSTR) and settler returning concentrated biomass to the plug flow
reactor. The plug flow reactor promotes feast conditions, while the continuous flow
stir tank reactor provides famine-like conditions (Chee et al. 2010; Salehizadeh and
van Loosdrecht 2004). Contact-stabilization configuration with biomass recycle is
another continuous flow setup that promotes continuous polyhydroxyalkanoates
production. Sarioglu et al. used a contact-stabilization configuration with two aerated reactors, contact and stabilization reactors, separated with a sedimentation
tank. The influent wastewater enters the contact reactor and moves to the sedimentation basin. The overflow of the sedimentation basin leaves as treated effluent, while
the underflow moves to the stabilization reactor, which goes back to the contact
aerator (Sarioglu et al. 2003). Similar to the plug flow reactor coupled with continuous flow stir tank reactor, the recirculation of the biomass results in a continuous
S. Sali and H. R. Mackey