Sustainable wastewater treatment system have increasingly focused on resource recovery from wastewater. Excess sludge, primarily composed of a bacterial cell matrix embedded in extracellular polymeric substances (EPS), offers significant potential in this regard. accounts for approximately 10–40% of the total dry weight of sludge and is recognized as a promising bioresource for producing valuable bioproducts. However, despite the widespread use of flocculent sludge treatment plants, the recovery potential and properties of EPS in flocculent sludge have been largely overlooked.

This thesis focuses on the extracted from flocculent sludge, with the aim of exploring their extraction potential, structural characteristics, conformations, and properties. By analyzing EPS from various full-scale and lab-scale flocculent sludge systems, it examines the factors influencing EPS extraction potential and establishes correlations between these factors and EPS formation and properties. Further investigations into EPS composition and conformation provide a deeper understanding of its structure, shedding light on its role in sludge aggregation and potential applications. This thesis bridges engineering and fundamental perspectives to advance EPS research.

Chapter 1 provides a concise introduction to the growing interest in EPS recovery, highlighting its significance and potential. It also raises key questions about EPS derived from flocculent sludge, establishing a clear roadmap for the thesis and serving as a foundation for the experimental setups in this study.
In Chapter 2, the study focuses on evaluating the EPS recovery potential from flocculent sludge. Samples were collected from various full-scale wastewater treatment plants in China, and EPS was extracted for analysis. Influent characteristics, microbial community profiles and chemical characterizations of EPS were examined to assess their correlations. The EPS yield ranged from 9% to 19% of the organic fraction of raw sludge. The findings also revealed that EPS production is highly influenced by external environmental conditions and strongly linked to bacterial diversity and abundance. This chapter highlights the significant potential of flocculent sludge for EPS recovery.

Chapter 3 aims to explore the connections between various external factors and EPS formation. Lab-scale sequencing batch reactors (SBRs) were operated under controlled conditions, with specific operational and influent parameters designed to cultivate flocculent sludge. The results revealed that sludge fed with starch-rich influent showed significantly enhanced EPS formation, while low temperatures also supported EPS synthesis. In contrast, organic loading rates and sludge retention time (SRT) had minimal impact on EPS yield. Furthermore, adaptations in EPS composition and properties indicated that both influent characteristics and operational conditions played a critical role in shaping EPS composition.
Recognizing the importance of understanding EPS structures, Chapter 4 focuses on a detailed investigation of EPS composition and structure. Extracted EPS was fractionated into distinct components for analysis. Comparisons with commercial alginates revealed that typical alginate units—guluronic acid and mannuronic acid—were absent in all EPS fractions, indicating that EPS from flocculent sludge does not contain alginate structures. Further analysis of these fractions suggested the presence of glycolipid structures, specifically highlighting the significance of lipopolysaccharides (LPS), a type of glycolipid, in EPS. This chapter not only confirmed the absence of alginates but also underscored the critical role of glycolipids in EPS composition.

Chapter 5 delves into the structure of lipopolysaccharides (LPS) and their contributions to EPS properties by comparing EPS from flocculent and granular sludge. LPS was isolated from EPS and subsequently characterized. The study found that LPS comprised approximately 25% of the organic fraction of EPS in flocculent sludge, significantly higher than the 15% observed in granular sludge. LPS from flocculent sludge exhibited unique features, including lower glycan content, shorter glycan chains, lower molecular weight, and a higher prevalence of unsaturated lipids. These structural characteristics led to inverted crosslinks in calcium-bound LPS aggregates, contributing to the fluid-like hydrogel morphology of EPS. In contrast, LPS-Ca aggregates from granular sludge exhibited a bilaminar multilayered structure, resulting in the solid, self-standing hydrogel properties of EPS.

Chapter 6 summarizes the key findings of this thesis, highlighting the insights gained into EPS recovery, structure, and properties. Additionally, it proposes ideas for future research, including exploring bacterial activities involved in EPS biosynthesis, further investigation of LPS structures and their functions, and potential applications of EPS. These suggestions aim to advance the understanding and utilization of EPS in sustainable wastewater treatment and beyond.