5 Value-Added Products Recovery:
Polyhydroxyalkanoates
Plastics are essential materials that in 2018 reached a demand of over 60 million
tons in Europe [79]. Though a considerable amount of the plastics was used in
long-life applications, 30 million ended up in the waste streams, with 25% being
disposed in landfills [79]. Worldwide, it is estimated that a considerably higher
percentage of plastics is discarded and directly disposed in nature. In this case, since
conventional plastics cannot naturally feed back into the ecosystem through natural
biogeochemical cycles, they will persist in the environment as invasive
microplastics, interfering with our ecosystems and entering food-feed chains [80,
81]. This problematic is increasing the demand for bio-based and biodegradable
plastics, and polyhydroxyalkanoates (PHAs) have been proposed as an alternative
to petrochemical plastics. PHAs are microbially synthetized polyesters, internally
accumulated as carbon and energy reserves, in the presence of excess carbon. They
display thermoplastic properties similar to traditional plastics [82], yet, PHAs can
be produced from renewable resources and because they are completely
biodegradable upon disposal in the environment, they comprise all the ecological
requirements for the production of sustainable biodegradable plastics.
PHAs are already available in the market, but their current industrial production
is based on single strain processes that use expensive refined substrates, leading to
high production costs and making PHA significantly more expensive than conventional plastics [83]. Efforts to create more economic PHA production strategies
and increase its field of applicability have emerged in recent years for both single
strain processes and mixed microbial cultures (MMC) [84]. In the case of MMC,
low-cost PHA producing systems have been developed by operating non-sterile
open systems that use complex, low-grade, inexpensive wastes as feedstock [85–
87]. Unlike single strain systems that typically use carbohydrates as carbon source,
volatile fatty acids (VFAs) (e.g., acetate, propionate and butyrate) are the best
precursors for PHA production in MMCs, and they can be easily obtained by
fermentation of waste streams [88]. But before the actual PHA production takes
place, the MMC commonly undergoes a microbial selection in order to enrich the
culture in PHA accumulating bacteria. The selection is carried out by subjecting the
culture to transient feeding conditions in a strategy designated as feast and famine
(FF). This consists in alternating periods with excess of external carbon (Feast),
where organisms take up the carbon and accumulate it as PHA, with periods with
no substrate addition (famine), where only the organisms that accumulated PHA
will have carbon reserves and will thus be able to grow. Repeated FF cycles will
favor the cell growth on storage products, thus creating a selection pressure for
organisms with high PHA-storing capacity.
The PHA production systems with MMCs can then be described as a process
that commonly comprises three steps (1) wastewater or agro-industrial waste acidogenic fermentation to VFAs, (2) selection of a PHA accumulating culture under
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J. C. Fradinho et al.
Polyhydroxyalkanoates
Plastics are essential materials that in 2018 reached a demand of over 60 million
tons in Europe [79]. Though a considerable amount of the plastics was used in
long-life applications, 30 million ended up in the waste streams, with 25% being
disposed in landfills [79]. Worldwide, it is estimated that a considerably higher
percentage of plastics is discarded and directly disposed in nature. In this case, since
conventional plastics cannot naturally feed back into the ecosystem through natural
biogeochemical cycles, they will persist in the environment as invasive
microplastics, interfering with our ecosystems and entering food-feed chains [80,
81]. This problematic is increasing the demand for bio-based and biodegradable
plastics, and polyhydroxyalkanoates (PHAs) have been proposed as an alternative
to petrochemical plastics. PHAs are microbially synthetized polyesters, internally
accumulated as carbon and energy reserves, in the presence of excess carbon. They
display thermoplastic properties similar to traditional plastics [82], yet, PHAs can
be produced from renewable resources and because they are completely
biodegradable upon disposal in the environment, they comprise all the ecological
requirements for the production of sustainable biodegradable plastics.
PHAs are already available in the market, but their current industrial production
is based on single strain processes that use expensive refined substrates, leading to
high production costs and making PHA significantly more expensive than conventional plastics [83]. Efforts to create more economic PHA production strategies
and increase its field of applicability have emerged in recent years for both single
strain processes and mixed microbial cultures (MMC) [84]. In the case of MMC,
low-cost PHA producing systems have been developed by operating non-sterile
open systems that use complex, low-grade, inexpensive wastes as feedstock [85–
87]. Unlike single strain systems that typically use carbohydrates as carbon source,
volatile fatty acids (VFAs) (e.g., acetate, propionate and butyrate) are the best
precursors for PHA production in MMCs, and they can be easily obtained by
fermentation of waste streams [88]. But before the actual PHA production takes
place, the MMC commonly undergoes a microbial selection in order to enrich the
culture in PHA accumulating bacteria. The selection is carried out by subjecting the
culture to transient feeding conditions in a strategy designated as feast and famine
(FF). This consists in alternating periods with excess of external carbon (Feast),
where organisms take up the carbon and accumulate it as PHA, with periods with
no substrate addition (famine), where only the organisms that accumulated PHA
will have carbon reserves and will thus be able to grow. Repeated FF cycles will
favor the cell growth on storage products, thus creating a selection pressure for
organisms with high PHA-storing capacity.
The PHA production systems with MMCs can then be described as a process
that commonly comprises three steps (1) wastewater or agro-industrial waste acidogenic fermentation to VFAs, (2) selection of a PHA accumulating culture under
428
J. C. Fradinho et al.