Page 80 - Biodegradable Polyesters

58 3 Microbial Synthesis of Biodegradable Polyesters: Processes, Products, Applications

The addition of diammonium hydrogen phosphate (DAHP) resulted in an
increase in PHB production to 67% when raw rice grain-based spentwash was
used. The same wastewater after removal of suspended solids by filtration and
with DAHP supplementation resulted in lower PHB production (57.9%). Supple-
menting other wastes with DAHP led to a substantial decrease in PHB content in
comparison to what was observed in the absence of DAHP. Such studies indicate
the feasibility of using wastewater for PHA production.
Mixed culture production of PHAs from wastewater was also financially attrac-
tive when compared to pure culture PHA production. Both PHA production
processes had similar environmental impacts that were significantly lower than
high-density polyethylene (HDPE) production. There is a large potential for
greater optimization of the PHA process as financial and environmental costs
were primarily due to energy use for downstream processing. Thus, mixed culture
biotechnology could potentially become an attractive addition or alternative to
traditional pure culture-based biotechnology for the production of PHAs, other
chemicals, and/or bioenergy; mixed culture biotechnology could result in stable
and continuous production.

3.8
Application Opportunities of Bacterial Polyhydroxyalkanoates
3.8.1
In Energy Industry: Biofuels Based on PHAs

The PHAs poly-(R)-3HB and medium chain-length polyhydroxyalkanoate
(mcl PHA), were respectively converted to (R)-3-hydroxybutyrate methyl ester
(3HBME) and medium chain length hydroxyalkanoate methyl ester (3HAME)
by acid-catalyzed hydrolysis [61]. It was found that 3HBME and 3HAME had
−1
combustion heat values of 20 and 30 kJ g , respectively. Ethanol has a combus-
−1
tion heat of 27 kJ g , while the addition of 10% 3HBME or 3HAME enhanced
−1
the combustion heat of ethanol to 30 and 35 kJ g , respectively. The addition of
3HBME or 3HAME into n-propanol and n-butanol has led to slight reductions
in their combustion heats. Combustion heats of blended fuels 3HBME–diesel
or 3HBME–gasoline and of 3HAME–diesel or 3HAME–gasoline were lower
than that of pure diesel or gasoline but were still reasonable as fuels. At a rough
estimate, the production costs of PHA-based biofuels from waste resources
including wastewater and activated sludges should be around $1200 per
ton [61].
Biofuels including ethanol and biodiesel have always been controversial regard-
ing “food vs fuel” and “fuel vs arable land.” PHA-based biofuel production from
wastewater or from activated sludge enjoys the advantages of wastewater treat-
ment accompanied by energy generation. These results unlock a new area for PHA
application in the energy sector.