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Projeto de investigação
Integrating resource recovery of value-added biopolymers into biological wastewater treatment systems
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Lowering polyhydroxyalkanoate bioproduction costs in mixed cultures through integrated optimization of organic loading rate, sludge retention time and biomass withdrawal
Publication . Cruz, Rafaela A.P.; Oehmen, Adrian; Reis, Maria A.M.; DQ - Departamento de Química; UCIBIO - Applied Molecular Biosciences Unit; Faculdade de Ciências e Tecnologia (FCT); Elsevier
Polyhydroxyalkanoates (PHA) are a sustainable alternative to conventional plastics, where mixed microbial cultures can be advantageous for PHA production from industrial waste streams, but lower costs are necessary to facilitate industrial application. This study assesses a novel optimisation strategy for the biomass withdrawal, organic loading rate and solids retention time, with the purpose of lowering bioproduction costs. Conventional and feast-phase biomass withdrawal strategies were compared to determine the most cost-effective operational option. Results demonstrated that increasing the organic loading rate led to an improvement of PHA productivity in each selection reactor, due to increased growth. It was observed that at low solids retention time, the incorporation of an accumulation stage did not improve the productivity of the polyhydroxyalkanoate production process using the feast-phase biomass withdrawal strategy, reducing capital costs. Economic assessment of the two different operational modes was performed, where the results demonstrated that the feast-phase biomass withdrawal strategy at an SRT of 0.8 d was the configuration resulting in the lowest total annual costs and lowest break-even PHA price (1.88 €/kg PHA-crude), due to elimination of the accumulation stage. Overall, this work contributes towards sustainable and cost-effective polyhydroxyalkanoate production from wastes in mixed cultures.
The impact of biomass withdrawal strategy on polyhydroxyalkanoate (PHA) productivity in mixed cultures
Publication . Cruz, Rafaela Alexandra Palma; Reis, Maria D’Ascensão; Oehmen, Adrian
Polyhydroxyalkanoates (PHA) have been pointed as an alternative to reduce the use of
conventional plastics. PHA are natural polyesters that are produced by microorganisms, are
biodegradable, biocompatible and have thermoplastics/elastomeric properties similar to those
of petrochemical plastics. PHA are currently produced based on pure cultures, but the costs of
investment (equipment) and production (energy and refined substrates) have discouraged the
widespread utilization of PHA as commodity plastics.
The production of PHA by mixed microbial cultures (MMC) can reduce the costs of
investment and production, and consequently the PHA price, through use of open systems
(less sophisticated equipment) and low-cost substrates, such as agro-industrial wastes/by-
products. The MMC PHA production process consists generally of three stages: acidogenic
fermentation to obtain VFA as PHA precursors, culture selection to enrich a culture in PHA-
storing microorganisms with high capacity to accumulate PHA and a high growth rate, and
PHA accumulation to maximize the PHA content in the biomass. The culture selection stage is
considered the key step in this process since it determines the biomass productivity and its
capacity of storing PHA. This PhD project has focused on the study of the impact of biomass
withdrawal strategy on the performance of the selection reactor and consequently on biomass
productivity and global PHA productivity.
An experimental setup composed by two sequencing batch reactors (SBR) was operated
under similar conditions, at an organic loading rate (OLR) of 100 Cmmol.L-1.d-1 and solids
retention time (SRT) of 4 days, except for the timing of biomass withdrawal. Reactor 1 (R1)
operated under a conventional strategy of biomass withdrawal (BW) at the end of the famine
phase, while Reactor 2 (R2) performed the BW at the end of the feast phase. A synthetic mixture
of acetic and propionic acid was selected as a representative carbon source composition from a fermented feedstock. The first study in this PhD project aimed to elucidate the effect of two
different BW strategies on the performance of the culture selected.
Both selection reactors had similar PHA storing capacity as proven by similar specific PHA
storage rates of 0.44 ± 0.09 Cmol-PHA.Cmol-XA-1.h-1 in R1 versus 0.36 ± 0.09 Cmol-PHA.Cmol-
XA-1.h-1 in R2 and similar PHA content of the biomass during the accumulation assays (51.7 ±
1.5 wt.% in R1-Ac versus 56.1 ± 1.0 wt.% in R2-Ac). However, R1 presented a much higher
biomass productivity (about 4-fold higher than R2), due to higher biomass concentration (2.8
± 0.4 g-XA.L-1 in R1 and 0.83 ± 0.2 g-XA.L-1 in R2), thus resulting in a much higher global PHA
productivity. This study demonstrated that removing the excess biomass at the end of the
famine phase resulted in a much higher global PHA productivity than removing the excess
biomass at the end of the feast phase, providing important insight into how MMC systems can
be best operated to maximize PHA productivity.
In the second study of this PhD project, the same system was used to assess the influence
of the OLR and SRT in tandem with the BW strategy to maximize the PHA productivity. It was
demonstrated that increasing the OLR led to an improvement of biomass productivity and
consequently to an improvement of the PHA productivity in both selection reactors, while the
reduction of SRT to less than 1 d resulted in the reduction of the storage response in both
reactors, compromising the overall PHA productivity. Additionally, at a SRT lower than 1.7 d,
the accumulation stage did not improve the productivity of the PHA producing process in R2,
and so, the biomass removed at the end of the feast phase could be forwarded directly to
downstream processing, reducing investment and operational costs.
A preliminary cost assessment of the two different operational modes was then
performed, comparing: (1) the utilization of both selection and accumulation reactors and the
conventional BW; and (2) a single selection reactor was used with the feast phase BW. The
results demonstrated that the utilization of a culture selection reactor with the feast phase BW
at the SRT of 0.8 d was the configuration resulting in the lowest total annual costs and lowest
break-even price (1.88 €/kg PHA-crude). Also, this configuration met the minimum limit of 40%
wt.% PHA content in the biomass, which has been previously indicated as a viable threshold
for cost-effective downstream processing.
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Fundação para a Ciência e a Tecnologia
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OE
Número da atribuição
SFRH/BD/110673/2015