Potential role of nanobubbles in dynamically modulating the structure and stability of anammox granular sludge within biological nitrogen removal process

Heterogeneous microstructure induces floatation in high-rate anammox granules

The floatation of anammox granules can be a serious challenge in practical wastewater treatment, as it can deteriorate reactor performance and cause bacterial loss. To deepen the understanding of floatation mechanism, in this study, both the floating (F-AnGS) and settling anammox granules (S-AnGS) from a high-rate anammox reactor were comparatively investigated. F-AnGS demonstrated 1.6 times higher specific anammox activity compared to S-AnGS, but only 65 % of produced gas could be successfully released, as quantified by anaerobic respirometry. In addition to the overall EPS accumulation, F-AnGS exhibited a heterogeneous microstructure distinct from that of S-AnGS, as revealed by 3D X-ray microscopic imaging at the single granule level. The heterogeneous distribution of EPS, which can form a dense surface layer, was the main cause for granule floatation. The heterogeneous microstructure of F-AnGS can reduce the distance between microorganisms and enhance the metabolic interaction between anammox bacteria and heterotrophs. The abundance of community members did not have a significant variation, but the functional genes related to anammox and partial denitrification pathway were significantly increased, indicating the enhanced nitrite loop in F-AnGS. This study proposed new structural insights into mechanism of anammox granule floatation, suggesting the appropriate activity control of granule-based anammox process.

Impact of ibuprofen on nitrogen removal performance and its biotransformation in a coupled sulfur autotrophic denitrification and anammox system

Ibuprofen (IBU), a commonly used non-steroidal anti-inflammatory drug, is frequently detected in wastewater treatment systems, where it can interfere with nitrogen removal. This study investigated the effects of IBU on nitrogen removal performance and its biotransformation in a coupled sulfur autotrophic denitrification and anammox (SAD/A) system. Moreover, key parameters, such as nitrogen removal efficiency, microbial activity, community structure, and IBU degradation products, were carefully monitored. While IBU concentrations of up to 1 mg/L had negligible impacts on nitrogen removal efficiency due to the counteracting effects of slight inhibition on anammox and enhancement of sulfur autotrophic denitrification, a significant inhibition of ammonia removal occurred when the concentration increased to 10 mg/L. Quantum chemical analyses revealed that IBU underwent biotransformation through decarboxylation and hydroxylation pathways, leading to the formation of two biotransformation products with high ecological toxicity. This study is the first to elucidate the mechanisms by which IBU influences microbial communities and metabolic activities in SAD/A systems. In addition, it highlights the resilience of these systems in maintaining nitrogen removal efficiency under varying IBU concentrations, as well as the environmental risks posed by the biotransformation products of IBU.

Enhancing nitrogen removal performance of anammox granular sludge with ascorbic acid at low temperatures

The application of anammox granular sludge in mainstream wastewater treatment under low-temperature conditions faces several challenges. This research investigates the effects of antioxidant ascorbic acid on the properties of anammox granular sludge subjected to low temperature stress. The results demonstrated that the different concentrations of ascorbic acid significantly improved the nitrogen removal efficiency of anammox granular sludge at low temperature (15 °C). In comparison to the control group without ascorbic acid, the average total nitrogen removal efficiency of anammox reactors increased by 14.97%, 26.35% and 41.03% at 15 °C with 5, 10, and 20 mg/L ascorbic acid, respectively. The strength, stability and mass transfer of granular sludge were promoted by ascorbic acid at 15 °C. Ascorbic acid enhanced the energy metabolism of anammox bacteria (AnAOB) by scavenging intracellular reactive oxygen species (ROS), thereby facilitating the adaptation of AnAOB to low-temperature stress. Notably, the abundance of Candidatus Kuenenia in anammox granular sludge at low temperatures increased significantly. Simultaneously, the hydrophobic amino acids synthesis genes of the bacteria in anammox system were upregulated to modulate the composition of extracellular polymeric substances (EPS) of anammox granular sludge, which enhanced the hydrophobic interactions of granular sludge and reduced the electrostatic barrier on the surface of the sludge. These phenomena facilitated the aggregation of smaller particles into larger ones, restoring the stability of the granular sludge and improving its nitrogen removal capacity in the anammox reactor under low-temperature conditions.

Unraveling the resistance mechanism of anammox granular sludge to iron nanoparticles

Iron nanoparticles (FeNPs) generated from industrial activities could end up into the sewer system, and potentially affect wastewater treatment processes. The impact of FeNPs on anammox process is getting increasing attention. However, the resistance mechanism of anammox granular sludge (AnGS) to FeNPs has not been fully elucidated. The current study investigated the metabolic and morphological response of AnGS to acute and chronic FeNPs exposure. Results showed that nitrogen removal efficiencies were elevated at 1-4 mM FeNPs concentrations compared to 0-0.5 mM FeNPs. Extracellular protein and tyrosine-like and tryptophan-like fluorophore secretions of AnGS were stimulated by FeNPs, which largely contributed to the adsorption of FeNPs on AnGS surface. FeNPs exposure triggered higher necrotic fraction of AnGS compared with no FeNPs condition. Highly absorbed particles appeared inside the bacterial cells of AnGS, soft X-ray imaging illustrated that anammox bacteria maintained intact cellular and anammoxosome structures whereas non-anammox bacterial structures were damaged under FeNPs exposure. Anammox bacterial abundance increased from 4.84% to 20.64%, when FeNPs concentrations increased from 0 mM to 4 mM, and anammoxosome membrane ensured anammox bacterial metabolism under FeNPs exposure. This study extended fundamental understanding of AnGS resistance mechanisms to FeNPs.

Monitoring the Biochemical Activity of Single Anammox Granules with Microbarometers

Granule-based anaerobic ammonium oxidation (Anammox) is a promising biotechnology for wastewater treatments with extraordinary performance in nitrogen removal. However, traditional analytical methods often delivered an average activity of a bulk sample consisting of millions and even billions of Anammox granules with distinct sizes and components. Here, we developed a novel technique to monitor the biochemical activity of individual Anammox granules in real-time by recording the production rate of nitrogen gas with a microbarometer in a sealed chamber containing only one granule. It was found that the specific activity of a single Anammox granule not only varied by tens of folds among different individuals with similar sizes (activity heterogeneity) but also revealed significant breath-like dynamics over time (temporal fluctuation). Statistical analysis on tens of individuals further revealed two subpopulations with distinct color and specific activity, which were subsequently attributed to the different expression levels of heme c content and hydrazine dehydrogenase activity. This study not only provides a general methodology for various kinds of gas-producing microbial processes but also establishes a bottom-up strategy for exploring the structural-activity relationship at a single sludge granule level, with implications for developing a better Anammox process.

Hydrogen Nanobubbles Generated In Situ from Nanoscale Zerovalent Iron with Water to Further Enhance Selenite Sequestration

Gas nanobubbles used for water treatment and recovery give rise to great concern for their unique advantages of less byproducts, higher efficiency, and environmental friendliness. Nanoscale zerovalent iron (nZVI), which has also been widely explored in the field of environmental remediation, can generate gas hydrogen by direct reaction with water. Whether nanoscale hydrogen bubbles can be produced to enhance the pollution removal of the nZVI system is one significant concern involved. Herein, we report direct observations of in situ generation of hydrogen nanobubbles (HNBs) from nZVI in water. More importantly, the formed HNBs can enhance indeed the reduction of Se(IV) beyond the chemical reduction ascribed to Fe(0), especially in the anaerobic environment. The possible mechanism is that HNBs enhance the reducibility of the system and promote electron transport in the solution. This study demonstrates a unique function of HNBs combined with nZVI for the pollutant removal and a new approach for in situ HNB generation for potential applications in the fields of in situ remediation agriculture, biotechnology, medical treatment, health, etc.

Fate of iron nanoparticles in anammox system: Dissolution, migration and transformation

Iron nanoparticles (FeNPs) are commonly used in various industrial processes, leading to their release into the environment and eventual entrance into wastewater treatment plants (WWTPs). FeNPs undergo dissolution, migration, and transformation in WWTPs, which can potentially affect the stable operation of anaerobic ammonia oxidation (anammox) systems and may be discharged with wastewater or biomass. To better understand the fate of FeNPs in anammox systems, exposure experiments were conducted using anammox granular sludges (AnGS) and FeNPs. Results demonstrated that FeNPs released Fe2+ upon contact with water, with a portion being bound to functional groups in extracellular polymeric substances (EPS) and the rest entering the bacteria to form highly absorbable substances. A significant amount of FeNPs was observed to cover the surface of AnGS or aggregate and deposit at the bottom of the reactor, eventually converting into Fe3O4 and stably existing within the anammox system. The findings of this study clarify the fate of FeNPs in anammox systems and provide important insights into the stable operation of anammox systems under FeNPs exposure.

A new pattern of the partial nitrification and Anammox immobilized gel beads: core-shell embedded carrier

Single-stage partial nitrification and Anammox (PN/A) is an efficient and energy-saving denitrification process for wastewater. However, its application is limited by the growth conditions of microorganisms. Therefore, we improved the PN/A by developing a novel core-shell embedded carrier. With Anammox gel as the core and Ammonia-oxidizing bacteria gel as the shell, these beads can achieve dissolved oxygen partitioning and provide a suitable environment for the growth of different bacteria. On this basis, the influence of the shape of core-shell embedded gel on nitrogen removal performance was systematically studied, and the internal morphology and pore size of gel beads were characterized. The results showed that the nitrogen removal efficiency of spherical and square gels was increased by 33.70% and 13.47%, respectively, in the batch test. Fluorescence in situ hybridization confirmed the stratified growth of ammonia-oxidizing bacteria and Anammox in carriers, and the relative abundance value of the two bacteria were 1.25:1 and 1.43:1, respectively. Although the mechanical strength of square gel beads is slightly higher than that of spherical, spherical gel is considered the most suitable gel shape due to its small pore size and poor pore connectivity, which ensures the matching of internal Anammox and external PN reaction. In the long-term experiment, the core-shell embedded beads still had the design characteristics, and the TN removal efficiency was increased by 36.25% despite occasional oxygen excess.

Bamboo charcoal addition enhanced the nitrogen removal of anammox granular sludge with COD: Performance, physicochemical characteristics and microbial community

The effects of different chemical oxygen demand (COD) concentrations on the anammox granular sludge with Bamboo Charcoal (BC) addition were evaluated in UASB reactor. The results showed that the average total nitrogen (TN) removal efficiency was reduced from 85.9% to 81.4% when COD concentration was increased from 50 to 150 mg/L. However, the TN removal efficiency of BC addition reactors was dramatically 3.1%-6.4% higher than that without BC under different COD concentrations. The average diameter of granular sludge was 0.13 mm higher than that without BC. The settling velocity was increased by elevated COD concentration, while the EPS and VSS/SS were increased with BC addition. The high-throughput Miseq sequencing analyses revealed that the bacterial diversity and richness were decreased under COD addition, and the Planctomycetes related to anammox bacteria were Candidatus Brocadia and Candidatus Kuenenia. The Metagenomic sequencing indicated that the abundance of denitrification related functional genes all increased with elevated COD, while the abundance of anammox related functional genes of decreased. The functional genes related to anammox was hydrazine synthase encoding genes (hzsA, hzsB and hzsB). The average relative abundance of hzs genes in the reactor with BC addition was higher than the control at COD concentrations of 50 mg/L and 150 mg/L. The functional genes of denitrification mediated by BC were higher than those without BC throughout the operation phase. It is interesting to note that BC addition greatly enriched the related functional genes of denitrification and anammox.

The Role of Extracellular Polymeric Substances in the Toxicity Response of Anaerobic Granule Sludge to Different Metal Oxide Nanoparticles

Wastewater treatment plants (WWTP) are regarded as the last barriers for the release of incompletely separated and recycled nanoparticles (NPs) into the environment. Despite the importance and ubiquity of microbial extracellular polymeric substances (EPSs) in the complex wastewater matrix, the interaction between NPs and EPSs of anaerobic microflora involved in wastewater treatment and the resultant impact on the biomass metabolomics are unclear. Thus, the impacts of different metal oxide (TiO₂, ZnO, and CuO) NPs on functional bacteria in anaerobic granular sludge (AGS) and the possible toxicity mechanisms were investigated. In particular, the binding quality, enhanced resistance mechanism, and chemical fractional contribution of EPSs from AGS against the nanotoxicity of different NPs was assessed. The results showed that CuO NPs caused the most severe inhibition towards acetoclastic and hydrogenotrophic methanogens, followed by ZnO NPs, whereas TiO₂ NPs caused no inhibition to methanogenesis. Excessive EPS production, especially the protein-like substances, was an effective strategy for reducing certain NPs’ toxicity by immobilizing NPs away from AGS cells, whereas the metabolism restriction on inner microorganisms of AGS induced by CuO NPs can deteriorate the protective role of EPS, indicating that the roles of EPS may not be amenable to generalizations. Further investigations with lactate dehydrogenase (LDH) and reactive oxygen species (ROS) assays indicated that there are greatly essential differences between the toxicity mechanisms of metal NPs to AGS, which varied depending on the NPs’ type and dosage. In addition, dynamic changes in the responses of EPS content to different NPs can result in a significant shift in methanogenic and acidogenic microbial communities. Thus, the production and composition of EPSs will be a key factor in determining the fate and potential effect of NPs in the complex biological matrix. In conclusion, this study broadens the understanding of the inhibition mechanisms of metal oxide NPs on the AGS process, and the influence of EPSs on the fate, behavior, and toxicity of NPs.