Integration of enrichment and accumulation process for Polyhydroxybutyrate production by utilizing wastes from sewage works with mixed microbial culture

Abstract

Polyhydroxybutyrate (PHB) is a biodegradable polyester produced by microorganisms as an intracellular carbon and energy reserve. Its biodegradability and biocompatibility make it a promising alternative to conventional petroleum-based plastics. However, the commercialization of PHB is hindered by high production costs, particularly those associated with sterilization and the use of pure carbon sources. This thesis aims to address these financial barriers by utilizing mixed microbial cultures (MMC) and waste materials as carbon sources and simplifying the production process by integrating enrichment and accumulation process.
The thesis comprises three papers that collectively evaluate potential methods for improving PHB production process. The focus is on the use of MMC and sewage waste, as well as on the integration of the PHB enrichment and accumulation processes to streamline production.
Paper I explores how activated sludge from wastewater treatment plants can be enriched to accumulate PHB. The study analyses the impact of operational parameters, such as organic loading rate (OLR) and settling time, on PHB accumulation. The results indicate that shorter settling times lead to the disintegration of granular sludge but enhance PHB accumulation, while higher OLR increases both biomass concentration and PHB content. These findings offer operational guidelines for future research.
Paper II introduces a novel approach to integrating PHB enrichment and accumulation through nutrient decoupling. The study uniquely evaluates the impact of different nutrient decoupling strategies on PHB production while maintaining a constant feast to famine ratio (F/F). The results reveal that both ammonium and phosphate decoupling have similar effects on PHB content at the end of the feast phase. However, PHB productivity and yield can be enhanced through specific decoupling strategies. The paper also delves into the relationship between microbial community structure and PHB production.
Paper III extends the research by examining the effects of various settling strategies on sludge settleability while maintaining PHB accumulation. The study investigates changes in extracellular polymers (EPS) and their correlation with settleability and PHB levels. The results show that introducing a settling phase after the feast phase improves sludge settleability and dewaterability without compromising PHB levels. A shift in the microbial community from Nitrincola to Thauera was also observed, correlating with improved settleability.
In summary, this thesis introduces innovative approaches to PHB production by integrating enrichment and accumulation processes. By employing unique nutrient decoupling and settling strategies with a fixed F/F ratio, it addresses existing knowledge gaps. It is hoped that these efforts will significantly contribute to the advancement of PHB production.

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ORCID for Yongqiang Liu: orcid.org/0000-0001-9688-1786

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