To enrich polyhydroxyalkanoate (PHA) producing microbial communities, generally, a feast-famine regime is applied. Here we investigated the impact of continuous substrate feeding on the enrichment of PHA-producing bacteria in two sequencing batch reactors (SBRs). In the first reactor, the substrate (acetate) was dosed continuously and Zoogloea sp. was enriched. The culture accumulated PHA upon exposure to excess carbon, but the PHA production rate and storage capacity (53 wt.%) were one-fifth of that observed for enrichment cultures in a standard, pulse-fed SBR dominated by the PHA producer Plasticicumulans acidivorans. In the second reactor, half the acetate was dosed at the beginning of the cycle and the other half continuously. Having a true feast phase, the enrichment of P. acidivorans was not impeded by the continuous supply of acetate and the culture accumulated 85 wt.% PHA. This shows that for the enrichment of bacteria with a superior PHA-producing capacity periodic substrate excess – a true feast phase – is essential, while periodic substrate absence – a true famine phase – is not.

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Polyhydroxyalkanoates (PHAs) are microbial storage polymers accumulated by many different prokaryotes as an intracellular carbon and energy reserve. The polymer properties make PHA interesting as bioplastic. Moreover, the monomers could serve as chiral building blocks or be used as a biofuel. Currently, PHA is commercially produced using pure cultures and well-defined substrates. To reduce the cost of PHA production and allow broad application, microbial enrichment cultures could be used. This eliminates the need for axenic conditions and allows the use agro-industrial waste streams as substrate, contributing to the development of a circular economy. The aim of this thesis was to investigate scale-up aspects of the PHA production by microbial enrichment cultures. A translation of the previously developed laboratory process to industrial application raises new questions concerning the impact of (variable) wastewater composition and process design, for example. Chapter 2 and 3, therefore, focus on the fate of different constituents of an acidified waste stream, and the second part (Chapter 4-6) of the thesis focusses on alternative process configurations. Chapter 1 provides a general introduction to the topic and describes the research preceding this thesis. Chapter 7 summarizes and integrates the main findings.@en

The process for non-axenic polyhydroxyalkanoate (PHA) production from organic waste generally comprises three steps: acidogenic fermentation of the waste stream, enrichment of a PHA-producing culture, and production of the PHA. This study assesses the feasibility of combining the enrichment and production step. Harvesting PHA-rich biomass directly from the sequencing batch reactor (SBR) used for enrichment of the microbial culture reduces capital cost, but may increase downstream-processing cost if the PHA content is significantly lowered. Operating an acetate-fed SBR at a volume exchange ratio of 0.75 (18 h cycles, 1 d SRT) allowed the production of biomass with 70 wt% poly(3-hydroxybutyrate) (PHB) in a single-step process. By increasing the exchange ratio to 0.83 (20 h cycles) the PHB content of the harvested biomass increased to 75 wt%, but the operational stability decreased. SBR operation at these high exchange ratios makes that bacteria have to increase their growth rate and external substrate is available for relatively long periods. This allows the establishment of larger flanking populations and negatively affected the kinetic properties of Plasticicumulans acidivorans, the predominant organism. Maximizing the volume exchange ratio is, therefore, a suitable strategy to produce large amounts of PHA in the SBR, but does not ensure the enrichment of a culture with superior PHA productivity.

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