Polyhydroxyalkanoate Production from Municipal Waste Streams

Abstract: Polyhydroxyalkanoates (PHAs) are a group of bioplastics, which are produced through microorganisms. They are accumulated in granules inside bacteria’s cell cytoplasm and serve as an energy reserve. Moreover, PHAs are completely biodegradable and biocompatible biopolyesters, which make them to suitable materials for medical applications and replace conventional petrochemical plastics. However, it is not economically feasible to produce PHAs yet, as it is four to nine times as expensive as to produce fossil fuel-based plastics. In order to reduce the price, it is possible to use waste streams rich in carbon and mixed cultures as microorganisms, which was applied in this thesis work. In this study, PHAs were synthesized from a volatile fatty acid (VFA) mixture rich in hexanoic acid, which was produced by anaerobic digestion of waste streams. To be able to obtain the maximum PHA content, the experimental work was separated into a selection phase and a production phase respectively. During the selection step, enrichment of the mixed culture took place during 50 days altering feast/famine cycles. The production phase was then conducted in a fed-batch cultivation to accumulate as much PHAs as possible, while utilizing the enriched mixed culture.  The selection phase was seen as successful since the quantity of synthesized PHA increased with time. Solely polyhydroxybutyrate (PHB) was formed during this period. The specific consumption rates for the hexanoic acid and acetic acid were almost the same in this phase (0.10 g hexanoic acid/(g volatile suspended soilds (VSS),h) and 0.11 g acetic acid/(g VSS,h)), which suggests that the consumption of these majoritarian fatty acids was simultaneous. However, the determined consumption rate for butyric acid was approximately solely half of the values for hexanoic acid and acetic acid. The highest PHA yield obtained in the enrichment phase was 0.26 g PHB/g VFA.  In the production phase, the highest achieved PHA content was 31.4 % of VSS, which was obtained after five hours. Both PHB and polyhydroxyvalerate (PHV) were formed in this phase, even though the quantity of accumulated PHB dominated with its approximately 97 weight-%.