Extracellular polymeric substances (EPSs) are high-molecular-weight natural polymers produced by microorganisms. They are classified into soluble/loosely bound EPSs (dissolved macromolecules, slimes, and colloids) and tightly bound EPS structures (capsular polymers, loosely bound polymers, concentrated gel, and attached organic substances). EPS separation involves chemical (e.g., formaldehyde or sodium hydroxide), physical (e.g., sonication, centrifugation), and physicochemical methods. Their main composition includes polysaccharides, proteins, lipids, uronic acid, DNA, humic substances, and other polymeric compounds, giving them excellent adsorption properties, biodegradability, and variable hydrophilicity/hydrophobicity. However, EPS composition varies depending on factors like culture species, growth profile, extraction method, growth medium, environmental conditions, and operational parameters. EPSs have diverse industrial applications, including thickening, flocculation, emulsification, gelling, and as antitumor agents and stabilizers. The cosmetic and food industries utilize EPSs for their antioxidant, hydration, thickening, texturizing, stabilizing, and emulsifying properties. Agricultural applications include improving soil quality and fertility. This review focuses on the use of EPSs in wastewater treatment, summarizing a comparative study of EPS roles based on 120 articles from the SCOPUS database (2010–2023). The review addresses EPS production and properties, their roles in wastewater treatment, the mechanisms involved, natural bioproducts derived from them, current limitations, and future research directions.

Existing literature extensively covers various aspects of wastewater treatment, focusing on different unit processes and technologies. However, reviews specifically focusing on bacterial EPS production, properties, and detailed mechanisms involved in various wastewater treatment applications remain limited. This gap in comprehensive understanding necessitates this review, which synthesizes current knowledge on EPSs' role in wastewater treatment from diverse perspectives.

This review utilized the SCOPUS database to search for relevant articles published between 2010 and 2023. The keywords "EPS" and "wastewater treatment" were employed to identify articles. A total of 120 articles were selected after careful assessment and refinement. The selected articles were analyzed to comprehensively explore EPS sources, production, properties, mechanisms of action in wastewater treatment, derived bioproducts, limitations, and future challenges. The review is structured into sections to systematically address each aspect, incorporating relevant findings from the selected literature.

EPSs, primarily produced by bacteria but also by cyanobacteria, microalgae, yeasts, and fungi, play crucial roles in wastewater treatment. Bacterial EPS production is significantly influenced by bacterial species, nutrient availability (carbon, nitrogen, phosphorus), temperature, pH, oxygen levels, salinity, signaling molecules, and the presence of toxins. Different bioreactor designs (stirred tank reactors, membrane bioreactors, moving bed bioreactors, fluidized bed bioreactors, trickling filters) influence EPS production yield. EPSs enhance wastewater treatment through several mechanisms: * **Flocculation:** EPS's negatively charged particles promote flocculation, aided by the addition of cations for charge neutralization. High-molecular-weight EPSs and mixed-culture EPSs generally show better flocculation activity. * **Adsorption:** EPSs adsorb heavy metals through electrostatic interactions between negatively charged EPS functional groups (phosphates, carboxylates, acetates, amines, sulfates) and positively charged metal ions. Amino groups in polysaccharides also contribute to metal binding. Coordination bonds between carboxyl or hydroxyl groups and metal ions further enhance adsorption. Other mechanisms include hydrophobic, covalent, and polymer-polymer interactions. * **Decolorization:** EPSs facilitate dye decolorization through reduction-oxidation reactions, either intracellularly or extracellularly. Exoenzymes (azoreductases, laccases, peroxidases) embedded within the EPS contribute to dye degradation by breaking azo linkages and naphthalene rings. Protonation and deprotonation of surface functional groups also influence dye decolorization. * **Degradation:** EPS-bound enzymes (hydrolases, oxidoreductases, amylases, cellulases, proteinases, DNases, and polygalacturonases) catalyze the degradation of organic compounds. Extracellular electron transfer plays a critical role in organic pollutant degradation. Complete degradation often requires multiple species and steps, with potential formation of toxic intermediates. EPSs recovered from wastewater can serve as natural bioflocculants, coating materials, and other bioproducts. EPS bioflocculants demonstrate effectiveness in removing various pollutants. EPSs from aerobic granular sludge show gel-forming properties, potentially useful for applications like moisture retention and flame retardancy. Table 1 summarizes EPS production by various bacterial species under different conditions. Table 2 compares EPS production yields in different bioreactor designs. Table 3 details the applications of EPSs in wastewater treatment, specifying the mechanisms, EPS sources, types of wastewater, and performance metrics.

This review highlights the significant potential of EPSs in enhancing wastewater treatment efficiency. The various mechanisms of action—flocculation, adsorption, decolorization, and degradation—demonstrate EPSs' versatility in removing different types of pollutants. However, the complex extraction methods and relatively low yields of EPS production pose challenges to large-scale industrial applications. The findings underscore the need for further research on identifying new potential EPS-producing species with high yields and developing simple, cost-effective, and environmentally friendly EPS extraction techniques. Optimizing bioreactor design and exploring sustainable mass production methods are crucial for the widespread adoption of EPS-based wastewater treatment technologies. The use of waste materials as growth media to enhance sustainability needs further investigation. The utilization of EPS as a source of value-added bioproducts is also a promising direction to maximize resource utilization.

EPSs demonstrate significant potential in wastewater treatment by facilitating flocculation, adsorption, decolorization, and degradation of various pollutants. However, limitations in EPS extraction methods and mass production currently restrict wider industrial application. Future research should focus on identifying novel EPS-producing species, simplifying extraction processes, and optimizing mass production strategies. Exploring sustainable approaches, such as utilizing waste materials as growth media, is critical for achieving environmentally friendly and economically viable EPS-based wastewater treatment technologies.

This review's main limitations include the reliance on a specific database (SCOPUS), potentially overlooking relevant research from other sources. The focus on bacterial EPSs may limit the scope of broader applications of EPSs from other microbial sources. The limited availability of detailed quantitative data on EPS production yields and cost-effectiveness of extraction methods restricts a thorough comparative analysis of different approaches.