This study investigated a novel thermal-hydrolysis combined with a vacuum fermentation system for high-grade volatile fatty acids (VFA) recovery, and the corresponding changes in the microbial community. Four systems with and without hydrothermal pre-treatment (HTP) and vacuum were mobilized; results revealed that integration of HTP with vacuum has the highest potential in terms of VFA recovery, sludge disintegration, and solid reduction. HTP and vacuum fermentation systems were associated with the highest COD solubilization (45 %), and VFA yield (0.32 g COD/g VSS added). Vacuum fermenters with and without pre-treatment have the highest specific denitrification rates of 7.6 and 7.2 mg NO₃-N/g VSS.h, respectively, compared to all other samples and control (acetate). Changes brought about by vacuum fermentation included a shift in the microbial community toward enriching fermenters, mainly Caprothermobacteria and Thermotagea, responsible for VFA production.

The characteristics of biomass and microbial community dynamics, in relation to autotrophic nitrification, were studied in two 20 L stirred tank reactors (STR) with oxic/hypoxic/oxic zones. The bioreactors were fed with synthetic wastewater with stepwise increasing ammonia concentrations (50–200 N mg/L) without organic substrate in the first phase (autotrophic phase) for 35 days (R1) and 15 days (R2), followed by a heterotrophic phase (with supplementation of organic substrate). The settling properties of the biomass, represented by pin-point flocs, gradually improved in both reactors during the autotrophic phase. The pin-point flocs of R1 exhibited granule-like settling properties. The SVI₃₀ in RI gradually improved to 29 mL/g MLSS, and the corresponding SVI₃₀/ SVI₁₀ gradually improved to 0.88 during the autotrophic phase. The settling properties of the biomass deteriorated in both bioreactors during the heterotrophic phase. The protein to polysaccharide ratio (PN:PS ratio) gradually increased in the extracted EPS (in both, loosely bound (LB) and tightly bound (TB) EPS) during the autotrophic phase, in both bioreactors. The TB:LB EPS ratio was higher when the pin-point flocs of R1 showed granule-like settling properties, followed by a decline in TB:LB EPS ratio during the heterotrophic phase. A combination of molecular approaches (droplet digital-PCR (dd-PCR) and 16S rRNA gene sequencing) revealed that Nitrospira were the predominant nitrifying bacteria in the pin-point flocs that show granular sludge-like settling properties during autotrophic phase in R1. Comammox Nitrospira was the dominant ammonia oxidizer in seed biomass and at low ammonia concentrations in both bioreactors. The relative abundance of canonical ammonia-oxidizing bacteria increased with an increase in influent-ammonia concentrations.

This study examines the effects of polyester and denim microfibers (MFs) on aerobic granular sludge (AGS) over a 42-day period. Treatment performance, granulation, and microbial community changes were assessed at 0, 10, 70, 210, and 1500 MFs/L. Reactors with 70 MFs/L achieved rapid granulation and showed improved settling by day 9, while 0 and 10 MFs/L reactors showed delayed granule formation, which was likely due to limited nucleation and weaker shear conditions. Severe clogging and frequent maintenance occurred at 1500 MFs/L. Despite > 98% MF removal in all reactors, treatment performance declined at higher MF loads. Nitrogen removal dropped from 93% to 68%. Phosphate removal slightly increased in reactors with no or low microfiber loads (96–99%), declined in reactors with 70 or 210 MFs/L (92–91%, 89–88%), and dropped significantly in the reactor with1500 MFs/L (86–70%, p < 0.05). COD removal declined with increasing MF load. Paracoccus (denitrifiers) dominated low-MF reactors; Acinetobacter (associated with complex organic degradation) and Nitrospira (nitrite-oxidizing genus) were enriched at 1500 MFs/L. Performance decline likely stemmed from nutrient transport blockage and toxic leachates, highlighting the potential threat of MFs to wastewater treatment and the need for upstream MF control.

Engineered water systems operate under diverse chemical and physical conditions that influence disinfection efficiency. Hot water networks (HWNs) exemplify this issue, as thermal control is costly and ineffective in cooled sections, and chlorine degrades rapidly at elevated temperatures. Consequently, HWNs remain challenging for controlling pathogens such as Legionella pneumophila. Sustained HWN protection requires a thermally resilient, persistent disinfectant. This study evaluated the stability and efficacy of silver-stabilized hydrogen peroxide (SSHP) in ultrapure, synthetic, and chloraminated municipal tap waters incubated at elevated temperatures for up to 72 h. Culture-based enumeration showed that SSHP antimicrobial efficacy increased with temperature, achieving >2.4-log reductions of L. pneumophila within 5 min at 55 °C. SSHP retained full biocidal activity after 72 h at 60 °C in synthetic tap water, whereas free chlorine lost antimicrobial activity after 24 h. In municipal waters, total and free chlorine decayed below recommended thresholds within 24–40 h at 60 °C, whereas SSHP retained effective residuals after 72 h. Although these controlled experiments cannot capture real-system complexity, SSHP persisted under thermal conditions that accelerated chlorine decay, supporting its use as a resilient HWN disinfectant. This work demonstrates how system-specific evaluation of disinfectants can inform improved strategies for managing microbial risks in water systems.

Efficient simultaneous nitrification, denitrification, and phosphorus removal typically requires either separate tanks with oxic and anoxic conditions or aerobic granular sludge (AGS) with granules larger than 0.2 mm. In this study, small granular flocs (50–100 µm) were cultivated with improved settleability, high biomass retention, strong ammonia and COD removal, late-stage phosphate removal, and enrichment of Paracoccus. The sequencing batch reactor was seeded with biomass from a municipal wastewater treatment plant and fed acetate-based synthetic wastewater for 114 days. The small granular flocs formed densified activated sludge (DAS), with biomass accumulating to approximately 12 g/L mixed liquor suspended solids. DAS developed under selection conditions commonly applied in aerobic granular sludge systems, including unaerated feeding and mixing, elevated hydrodynamic shear, and progressive reduction in settling time. The sludge volume index (SVI) improved to 30 mL/g, comparable to that of granular sludge, with a moderate SVI₃₀/SVI₅ ratio of 72%. The enrichment of Paracoccus is highly desirable in wastewater treatment. Importantly, these findings show that efficient simultaneous nutrient removal does not strictly depend on large granular structures.

Efficient simultaneous nitrification, denitrification, and phosphorus removal typically requires either separate tanks with oxic and anoxic conditions or aerobic granular sludge (AGS) with granules larger than 0.2 mm. In this study, we cultivated small granular flocs (50-100 µm) capable of simultaneous heterotrophic nitrification and aerobic denitrification (HN-AD) in a sequencing batch reactor. The bioreactor was seeded with biomass from a municipal wastewater treatment plant and fed acetate-based synthetic wastewater for 114 days. The small granular flocs formed densified activated sludge (DAS), with biomass accumulating to approximately 12 g/L mixed liquor suspended solids. DAS was cultivated under selection pressures commonly used for AGS formation, including anoxic feeding, extended anoxic mixing, elevated hydrodynamic shear, and a gradual reduction in settling time. The sludge volume index (SVI) improved to 30 mL/g, comparable to that of granular sludge, with a moderate SVI₃₀/SVI₅ ratio of 72%. The relative abundance of Paracoccus reached 72.9%, indicating the metabolic capability of these small granular flocs to carry out HN-AD. Overall, these findings show that efficient simultaneous nutrient removal does not strictly depend on large granular structures. Achieving DAS and HN-AD offers a scalable approach to intensify biological nutrient removal while reducing reliance on distinct anoxic zones.