A Review on the Roles of Extracellular Polymeric Substances (EPSs) in Wastewater Treatment: Source, Mechanism Study, Bioproducts, Limitations, and Future Challenges

EPSs are high-molecular-weight polymers secreted by microorganisms and occur as soluble/loosely bound and tightly bound fractions. They comprise polysaccharides, proteins, lipids, uronic acids, DNA, humic substances, and other polymers, with composition varying by species, growth conditions, extraction method, and environment. Due to unique composition, EPSs exhibit adsorption, biodegradability, and hydrophilic/hydrophobic properties that underpin their use in diverse industries (cosmetics, food, agriculture) and, critically, in wastewater treatment. In wastewater systems, EPSs protect cells from toxins, provide carbon/energy during starvation, and promote microbial aggregation. Applications include removal of heavy metals and toxins, dye decolorization, suspended solids removal, and sludge dewatering. This review addresses a gap in focusing specifically on EPS sources, production factors, mechanistic roles (flocculation, adsorption, decolorization, degradation), derived bioproducts, limitations, and future directions. The authors queried the SCOPUS database using keywords “EPS” and “wastewater treatment” for 2010–2023, refining to 120 relevant articles for synthesis.

The review synthesizes 120 studies (2010–2023, SCOPUS; keywords: EPS and wastewater treatment), highlighting: (i) EPS sources across bacteria, cyanobacteria, microalgae, yeasts, and fungi, with emphasis on bacterial EPS in wastewater applications; (ii) environmental and operational factors governing EPS production (nutrients, pH, temperature, salinity, oxygen, toxins, signaling molecules, shear); (iii) variability in EPS yields and composition by species/strains; (iv) impacts of bioreactor configurations on EPS production; and (v) mechanistic roles of EPSs in flocculation, adsorption of metals/organics, dye decolorization, and enzymatic degradation. Prior reviews often emphasized treatment units rather than EPS mechanisms and applications; this article consolidates mechanistic insights and performance data across applications and reactors.

This is a narrative review. The authors searched the SCOPUS database (2010–2023) using the keywords “EPS” and “wastewater treatment,” screened and refined the results to 120 articles, and synthesized findings thematically: EPS properties and production factors; EPS yields across taxa and conditions; influence of bioreactor design on EPS production; EPS roles and mechanisms in wastewater treatment (flocculation, adsorption, decolorization, degradation); reactor configurations involving EPS; EPS-derived natural bioproducts; limitations and future research directions. Comparative data from selected studies were collated into summary tables, including EPS yields by species/conditions (Table 1), reactor configurations affecting EPS production (Table 2), and performance metrics across treatment mechanisms (Table 3).

- EPS production drivers: Species/strain identity strongly determines EPS yield and composition. Environmental and operational factors (nutrient limitation, pH, temperature, salinity, oxygen, toxins, shear stress, signaling molecules) modulate EPS synthesis. Examples include enhanced EPS under nutrient limitation and slight stress conditions; low-shear reactors can increase yields for some species. - Reported EPS yields: Examples include Bacillus aerophilus rk1: 3.73 g/L (pH 7, 72 h, 30 °C); Bacillus velezensis: 2 g/L (72 h, 150 rpm, 30 °C); Diaphorobacter nitroreducens R9: 4 g/L (pH 9.5, 33.5 °C); Serratia marcescens: 0.377 g/L (pH 7, 25–27 °C, 72 h); Streptomyces platensis: 0.461 g/L; Rhizobium sp.: 1.04–2.47 g/L; Saccharomyces cerevisiae: 0.97 g/L. - Bioreactors and EPS production: Reactor design significantly affects EPS levels. Membrane bioreactor (MBR) produced 61% w/w EPS vs. 50% w/w in aerobic granular sludge with same inoculum; activated sludge under nitrification yielded 396.9 mg/g MLVSS vs. 82.5 mg/g MLVSS under nitritation. - Flocculation: Mixed-culture EPS generally outperforms pure-culture EPS due to higher production, better adhesion, and lower surface charge. Reported flocculating activities include: 81.8–93.5% in river/municipal/brewery wastewater using a 13-species consortium; 89.1–97.4% in saline/freshwater synthetic systems; Bacillus megaterium EPS achieved 81–90% in kaolin suspension; microalgal EPS aggregated nano-/microplastics with high removal. - Adsorption: EPS functional groups (carboxyl, hydroxyl, phosphoryl, etc.) coordinate/attract cations. Capacities reported: P. aeruginosa EPS adsorbed Cu2+ 300±5.3 mg/g, Cd2+ 250±10.4 mg/g, Pb2+ 380±5.1 mg/g, Co2+ 225±5.6 mg/g, Zn2+ 250±10.7 mg/g; Bacillus megaterium EPS removed 83% Pb2+, 52% Zn2%, 33% Ni2%; Parapedobacter sp. EPS removed 91% Cr(VI). - Decolorization: Bacillus megaterium EPS decolorized Congo red 88% (150 mg/L) and methylene blue 29% (300 mg/L); Aliiglaciecola lipolytica achieved 80% CR decolorization in 8 h and 95% in 36 h. - Degradation: EPS-associated communities achieved COD removal 70–85% (aerobic granular sludge treating fish canning effluent) and TOC removal up to 78% (SBR). Enzymes implicated include azoreductases, laccases, peroxidases, hydrolases, and oxidoreductases. - Mechanisms: Flocculation via bridging and patch models; metal adsorption via electrostatic attraction/complexation and additional hydrophobic/covalent interactions; dye reduction-oxidation facilitated by EPS-bound enzymes; degradation via adsorption followed by extracellular electron transfer and enzymatic breakdown. - EPS-derived bioproducts: EPSs function as bioflocculants (e.g., arsenite removal 86.1% with MBF83), and alginate-like exopolymers (ALEs) from aerobic granular sludge form hydrogels suitable for coatings with demonstrated flame-retardant properties. - Constraints: Complex extraction (chemical/energy intensive), limited species cataloged, low yields hindering scale-up (typical mass production 20–100 mg/L; optimized Bacillus velezensis reached 7.6 g/L).

The review consolidates evidence that EPSs are central to the performance of biological wastewater treatment by enabling aggregation, protection against toxins, and direct participation in pollutant removal through adsorption, redox reactions, and enzymatic degradation. Mechanistic clarity (bridging/patch flocculation; coordination/electrostatic metal binding; enzyme-mediated dye and organics transformation) links EPS composition and structure to observed treatment efficiencies, informing selection of species and operating conditions. Reactor configuration and operating regimes (e.g., MBR vs. granular sludge; nitrification vs. nitritation) significantly modulate EPS production, suggesting process design can be tuned to maximize EPS benefits while mitigating drawbacks (e.g., membrane fouling, excessive biofilm thickness). The synthesis highlights that while EPSs can achieve high flocculation efficiencies (>90%), substantial metal adsorption capacities, and meaningful COD/TOC reductions, translation to full-scale applications remains constrained by extraction complexity and supply limitations. Expanding the repertoire of high-yield, application-tailored EPS producers and simplifying extraction will enhance viability and sustainability of EPS-centric treatment strategies.

EPSs exhibit strong potential in wastewater treatment through flocculation, adsorption, decolorization, and degradation. Reported flocculation efficiencies commonly range from 60% to >90%, adsorption efficiencies can exceed 90% for certain metals, and enzymatic activities within EPSs facilitate dye decolorization and organic degradation. Flocculation mechanisms involve bridging and patch flocculation; adsorption relies on matrix entrapment and functional group coordination; decolorization and degradation are driven by EPS-associated enzymes (e.g., reductases, laccases, peroxidases). Key challenges are the lack of simple, rapid, and green EPS extraction methods and the need for scalable, high-yield production for industrial application. Future work should prioritize discovery of new EPS-producing species matched to specific treatment goals, innovation of low-chemical/low-energy extraction, and optimization and scale-up of production using cost-effective media, including waste-derived substrates.

- Limited knowledge of EPS-producing species with proven wastewater treatment performance; many taxa remain unexplored, and production varies widely by species/strain. - Complex, resource-intensive extraction methods (e.g., ethanol precipitation, membrane filtration, electroseparation) that require significant chemicals, energy, time, and specialized equipment, challenging sustainability and reproducibility. - Low yields and limited mass production capability: typical reported mass production yields are 20–100 mg EPS/L substrate; even optimized processes (e.g., Bacillus velezensis) reached 7.6 g/L. Most studies remain lab-scale; real-scale applications are rare and must address continuous supply, process integration, and potential byproduct toxicity from intermediates. - Operational risks: excessive EPS can cause issues such as membrane fouling or overly thick biofilms requiring control in certain reactor configurations.