Polyhydroxyalkanoates (PHA) are a family of biopolymers produced intracellular
by a range of different bacteria.PHA have attracted widespread attention as
an environmental friendly replacement of fossil-based polymers, because they
have thermoplastic and/or elastomeric properties, and are also biobased and
biodegradable.Moreover, the properties of PHA can be adjusted by tuning the
monomeric composition of the polymer.Currently, more than 150 different monomers
have been discovered which can form the building blocks of the PHA polymer.
PHA production can be divided in three parts: biosynthesis, recovery, and application.
The first part is the biotechnological production of bacteria with PHA
inside their cell. First, an organic substrate can be anaerobically converted into
volatile fatty acids (VFA). These VFA form the preferred substrate for PHA production
by bacteria in the next steps. An approach to make PHA biosynthesis
cost-effective is by using organic waste streams as substrate in combination with
mixed microbial communities. This reduces the relatively large costs for raw materials
and for sterilization of the equipment. Thus far, at least 19 pilot projects
have been operated to produce PHA from municipal or industrial organic waste
streams using this approach. In nearly all cases, the random copolymer poly(3-
hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was produced, indicating that the
biosynthesis of this specific polymer is reasonably well-established. Research on
the production of other types of PHA from waste streams is still scarce (Chapter
2 and 3).
The main obstacles that prevent the large-scale industrial implementation of
waste-derived PHA are the recovery and the application step. First of all, the PHA
recovery costs are responsible for a large fraction of the total production cost
due to high energy and chemical demand. Another challenge of the PHA recovery
step is to achieve a high and consistent quality product when waste is used
as substrate. More research is required to predict the relationship between raw
material input, process parameters, and final mechanical properties of the produced
PHA accurately (Chapter 4).
For the application of PHA, it appeared that introducing waste-derived PHA into
the conventional plastic market is a lasting and complicated procedure. This is
mainly caused by a lack of distribution channels, a lack of experience in bioplastic
processing, and by the small scale at which PHA is currently produced compared
to petrochemical plastics. Therefore, the market entry of waste-derived
PHA could have a higher chance of success if the initial aim is not to produce
bioplastics. Instead, the focus should be on new applications where minor fractions
of impurities, and small variations in polymer characteristics are not regarded
as problematic. Such a niche application can stimulate the introduction
of waste-derived PHA into the market, while avoiding the obstacles and the complexity
of the conventional plastic industry. Moreover, these applications can
potentially exploit the unique properties of PHA (e.g., biodegradability) more effectively
(Chapter 5).
The aim of this thesis was to optimize and balance waste-derived PHA biosynthesis
with recovery, and to target for a niche application of PHA in self-healing
concrete. To this end, research was conducted on all parts of the value chain
from waste to self-healing concrete: PHA biosynthesis (Chapter 2 and 3), PHA
recovery (Chapter 4), and the application of PHA (Chapter 5).
Chapter 2 investigates isobutyrate as sole carbon source for a microbial enrichment
culture in comparison to its structural isomer butyrate. Isobutyrate is a
VFA appearing in multiple waste valorization routes, such as anaerobic fermentation,
chain elongation, and microbial electrosynthesis, but has never been assessed
individually on its PHA production potential. The results reveal that the
enrichment of isobutyrate has a very distinct character regarding microbial community
development, PHA productivity, and even PHA composition. Although
butyrate is a superior substrate in almost every aspect, this research shows that
isobutyrate-rich waste streams have a noteworthy PHA producing potential. The
main finding is that the dominant microorganism, a Comamonas sp., is linked
to the production of a unique PHA family member, poly(3-hydroxyisobutyrate)
(PHiB), up to 37% of the cell dry weight. This chapter is the first scientific report
identifying microbial PHiB production, demonstrating that mixed microbial communities
can be a powerful tool for discovery of new metabolic pathways and
new types of polymers.
In Chapter 3, another uncommon VFA was examined for PHA production, octanoate.
Several enrichment strategies were tested to select for a community
with a high medium-chain-length PHA (mcl-PHA) storage capacity when feeding
octanoate. Based on the analysis of the metabolic pathways, the hypothesis was
formulated that mcl-PHA production is more favorable under oxygen limited conditions
than short-chain-length PHA (scl-PHA). This hypothesis was confirmed by
bioreactor experiments showing that oxygen limitation during the PHA accumulation
resulted in a higher fraction of mcl-PHA over scl-PHA (i.e., a PHA content
of 76 wt% with a mcl-fraction of 0.79 with oxygen limitation, compared to a PHA
content of 72 wt% with a mcl-fraction of 0.62 without oxygen limitation). Physicochemical
analysis revealed that the extracted PHA could be separated efficiently
into a hydroxybutyrate-rich fraction and a hydroxyhexanoate/hydroxyoctanoaterich
fraction. The ratio between the two fractions could be adjusted by changing
the environmental conditions. Almost all enrichments were dominated by
Sphaerotilus sp. This chapter is the first scientific report that links this genus to
mcl-PHA production, demonstrating that microbial enrichments can be a powerful
tool to explore mcl-PHA biodiversity and to discover novel industrially relevant
strains. In solvent extraction of PHA, the choice of solvent has a profound influence
on many aspects of the process design.
Chapter 4 provides a framework to perform a systematic solvent screening
for PHBV extraction. First, a database was constructed of 35 solvents that were
assessed according to six different selection criteria. Then, six solvents were
chosen for further experimental analysis, including 1-butanol, 2-butanol, 2-ethyl
hexanol (2-EH), dimethyl carbonate (DMC), methyl isobutyl ketone (MIBK), and
acetone. The main findings are that the extractions with acetone and DMC obtained
the highest yields (91-95%) with reasonably high purities (93-96%), where
acetone had a key advantage of the possibility to use water as anti-solvent. Moreover,
the results provided new insights in the mechanisms behind PHBV extraction
by pointing out that at elevated temperatures the extraction efficiency is less
determined by the solvent’s solubility parameters and more determined by the
solvent size. Although case-specific factors play a role in the final solvent choice,
we believe that this chapter provides a general strategy for the solvent selection
process.
In Chapter 5, a niche application for waste-derived PHA is proposed and
tested, using it as bacterial substrate in self-healing concrete. Self-healing concrete
is an established technology developed to overcome the inevitable problem
of crack formation in concrete structures, by incorporating a so-called bacteriabased
healing agent. Currently, this technology is hampered by the cost involved
in the preparation of this healing agent. This chapter provides a proof-of-concept
for the use of waste-derived PHA as bacterial substrate in healing agent. The
results show that a PHA-based healing agent, produced from PHA unsuitable
for thermoplastic applications, can induce crack healing in concrete specimens,
thereby reducing the water permeability of the cracks significantly compared to
specimens without a healing agent. For the first time these two emerging fields
of engineering, waste-derived PHA and self-healing concrete, both driven by the
need for environmental sustainability, are successfully linked. We foresee that
this new application will facilitate the implementation of waste-derived PHA technology,
while simultaneously supplying circular and potentially more affordable
raw materials for self-healing concrete.
Chapter 6 will provide a general discussion where overarching topics were
selected for a thorough analysis. Finally, recommendation for further research
are proposed and an outlook for the field is given.