Discrepant effects of metal and metal oxide nanoparticles on anammox sludge properties: A comparison between Cu and CuO nanoparticles

Leveraging big data to elucidate the impact of heavy metal nanoparticles on anammox processes in wastewater treatment

Anammox is a highly efficient nitrogen removal process, yet the effects of metal/metal-oxide nanoparticles (M/MONPs) on these systems remain underexplored. This study investigates the impact of various M/MONPs on the nitrogen removal rate (NRR). Pearson correlation analysis and statistical evaluation indicates that silver and copper oxide nanoparticles exhibit the highest inhibitory effect, with an inhibition rate of 83.4 % and 73.7 %, respectively. Furthermore, Machine learning models, particularly extreme gradient boost (XGBoost), demonstrate superior performance, with R2 values exceeding 0.91. SHapley Additive exPlanations (SHAP) feature importance analysis highlighted nanoparticles concentration, influent ammonia nitrogen concentration as the most influential factors. Additionally, Partial Dependence Plots (PDP) analysis of key features provided further clarity on the optimal ranges for these critical variables. The present study provides a novel predictive methodology and optimization strategies for enhancing the NRR of anammox system under M/MONPs stress, informed by comprehensive big data analysis.

Effects of nanoparticles on anaerobic, anammox, aerobic, and algal-bacterial granular sludge: A comprehensive review

Nanoparticles (NPs) are of significant interest due to their unique properties, such as large surface area and high reactivity, which have facilitated advancements in various fields. However, their increased use raises concerns about environmental impacts, including on wastewater treatment processes. This review examines the effects of different nanoparticles on anaerobic, anammox, aerobic, and algal-bacterial granular sludge used in wastewater treatment. CeO2 and Ag NPs demonstrated adverse effects on aerobic granular sludge (AGS), reducing nutrient removal and cellular function, while anaerobic granular sludge (AnGS) and anammox granular sludge (AxGS) showed greater resilience due to their higher extracellular polymeric substance (EPS) content. TiO2 NPs had fewer negative effects on algal-bacterial granular sludge (ABGS) than on AGS, as algae played a crucial role in enhancing EPS production and stabilizing the granules. The addition of Fe3O4 NPs significantly enhanced both aerobic and anammox granulation by reducing granulation time, promoting microbial interactions, improving granule stability, and increasing nitrogen removal efficiency, primarily through increased EPS production and enzyme activity. However, Cu and CuO NPs exhibited strong inhibitory effects on aerobic, anammox, and anaerobic systems, affecting EPS structure, cellular integrity, and microbial viability. ZnO NPs demonstrated dose-dependent toxicity, with higher concentrations inducing oxidative stress and reducing performance in AGS and AnGS, whereas AxGS and ABGS were more tolerant due to enhanced EPS production and algae-mediated protection. The existing knowledge gaps and directions for future research on NPs are identified and discussed.

A comparative study on the chronic responses of titanium dioxide nanoparticles on aerobic granular sludge and algal-bacterial granular sludge processes

The chronic effects of titanium dioxide nanoparticles (TiO2 NPs) on aerobic granular sludge (AGS) and algal-bacterial granular sludge (ABGS) was examined in this study. Sequencing batch bioreactors (SBRs) and photo sequencing batch bioreactors (PSBRs) were operated with synthetic wastewater containing 0, 1, 5, 10, 20, 30, and 50 mg L-1 TiO2 NPs for 10 days. Nanoparticles at concentrations of 1 and 5 mg L-1 did not impact nutrient removal but led to an increase in extracellular polymeric substances (EPSs), primarily in protein (PN). With increasing nanoparticle concentration, the negative effect became more pronounced, mainly in the AGS SBRs. At 50 mg L-1 TiO2, chemical oxygen demand (COD), ammonia-nitrogen (NH3-N), and phosphorus (PO43-) removal decreased by 20.9%, 12.2%, and 35.1% in AGS, respectively, while in ABGS, they reached only 13.4%, 5.7%, and 14.2%. ABGS exhibited steady-state nutrient removal at 30 and 50 mg L-1 TiO2 NPs after around 5 days. The higher microbial activity and EPS content in the sludge, coupled with the symbiotic relationship between algae and bacteria, contributed to the higher tolerance of ABGS to nanoparticles. Finally, although nanoparticles reduced biomass in both types of bioreactors, the accumulation of TiO2 NPs in the sludge, confirmed by Energy-dispersive X-ray spectroscopy analysis, and the absence of detectable titanium concentrations in the effluent wastewater, measured by Inductively-coupled plasma mass spectrometry, may be attributed to the specific operational conditions of this study, including the relatively short operation period (10 days) and high initial MLSS concentration (6 g L-1).

Environmental proteomics as a useful methodology for early-stage detection of stress in anammox engineered systems

Anammox bacteria are widely applied worldwide for denitrification of urban wastewater. Differently, their application in the case of industrial effluents has been more limited. Those frequently present high loads of contaminants, demanding an individual evaluation of their treatability by anammox technologies. Bioreactors setting up and recovery after contaminants-derived perturbations are slow. Also, toxicity is frequently not acute but cumulative, which causes negative macroscopic effects to appear only after medium or long-term operations. All these particularities lead to relevant economic and time losses. We hypothesized that contaminants cause changes at anammox proteome level before perturbations in the engineered systems are detectable by macroscopic analyses. In this study, we explored the usefulness of short-batch tests combined with environmental proteomics for the early detection of those changes. Copper was used as a model of stressor contaminant, and anammox granules were exposed to increasing copper concentrations including previously reported IC50 values. The proteomic results revealed that specific anammox proteins involved in stress response (bacterioferritin, universal stress protein, or superoxide dismutase) were overexpressed in as short a time as 28 h at the higher copper concentrations. Consequently, EPS production was also increased, as indicated by the alginate export family protein, polysaccharide biosynthesis protein, and sulfotransferase increased expression. The described workflow can be applied to detect early-stage stress biomarkers of the negative effect of other metals, organics, or even changes in physical-chemical parameters such as pH or temperature on anammox-engineered systems. On an industrial level, it can be of great value for decision-making, especially before dealing with new effluents on facilities, deriving important economic and time savings.

Insights into heavy metals shock on anammox systems: Cell structure-based mechanisms and new challenges

Anaerobic ammonium oxidation (anammox) as a low-carbon and energy-saving technology, has shown unique advantages in the treatment of high ammonia wastewater. However, wastewater usually contains complex heavy metals (HMs), which pose a potential risk to the stable operation of the anammox system. This review systematically re-evaluates the HMs toxicity level from the inhibition effects and the inhibition recovery process, which can provide a new reference for engineering. From the perspective of anammox cell structure (extracellular, anammoxosome membrane, anammoxosome), the mechanism of HMs effects on cellular substances and metabolism is expounded. Furthermore, the challenges and research gaps for HMs inhibition in anammox research are also discussed. The clarification of material flow, energy flow and community succession under HMs shock will help further reveal the inhibition mechanism. The development of new recovery strategies such as bio-accelerators and bio-augmentation is conductive to breaking through the engineered limitations of HMs on anammox. This review provides a new perspective on the recognition of toxicity and mechanism of HMs in the anammox process, as well as the promotion of engineering applicability.

Long-term synergic removal performance of N, P, and CuO nanoparticles in constructed wetlands along with temporal record of Cu pollution in substrate-biofilm

With continued exposure to CuO nanoparticles (NPs) which were toxic to organisms, the performance of wastewater treatment facility might be affected. In present study, the feasibility of constructed wetlands (CWs) for wastewater treatment containing CuO NPs and common pollutants was comprehensively explored. It was found that CWs removed 98.80-99.84% CuO NPs and 90.91-91.83% COD within 300 days. However, N and P removals were affected to varying degrees by CuO NPs. N removal was inhibited only by 0.5 mg/L CuO NPs with 19.75% decreases on the mean from day 200-300. P removal was reduced by 3.80-50.75% and 1.92-7.19% under exposure of 0.5 and 5 mg/L CuO NPs throughout the experiment. Moreover, CuO NPs changed the adsorption potential of P and ammonium-N on sand-biofilm. Cu concentrations in spatial distribution decreased, while they in temporal distribution increased from 36.94 to 97.78 μg/g and from 70.92 to 282.66 μg/g at middle sand layer exposed to 0.5 and 5 mg/L CuO NPs. Mass balance model showed that substrate-biofilm was main pollutant sink for CuO NPs, N, and P. The minor Cu was absorbed by plants exposed to 0.5 and 5 mg/L CuO NPs, which decreased N by 53.40% and 18.51%，and P by 52.35% and 21.62%. Sequencing analysis indicated that CuO NPs also altered spatial microbial community. N-degrading bacteria (Rhodanobacter, Thauera, Nitrospira) changed differently, while phosphate accumulation organisms (Acinetobacter, Pseudomonas, Microlunatus) reduced. Overall, the negative effects of CuO NPs on N and P removal should be noted when CWs as ecological technologies are used to treat CuO NPs-containing wastewater.

Particle size effects in microbial characteristics in thermophilic anaerobic digestion of cattle manure containing copper oxide

Roles of bulk-, micron-, and nano-copper oxide (CuO) on methane production, microbial diversity, functions during thermophilic anaerobic digestion (AD) were investigated in this study. Results showed that bulk-, micron-, and nano-CuO promoted methane production by 27.8%, 47.6%. and 83.1% compared to the control group, respectively. Microbial community analysis demonstrated that different particle sizes could cause various shifts on bacteria community, while had little effect on archaeal diversity. Thereinto, bacteria belonging to phylum Firmicutes and Coprothermobacterota dominated in enhanced hydrolysis process in groups with nano-CuO and bulk-CuO, respectively, while micron-CuO had stronger promotion on the abundances of hydrolytic and fermentative bacteria belonging to families Peptostreptococcaceae, Caloramatoraceae, Erysipelotrichaceae, and Clostridiaceae, than other two CuO sizes. Metabolic pathways revealed that energy-related metabolism and material transformation in bacteria were only boosted by micron-CuO, and nano-CuO and bulk-CuO were important to methanogenic activity, stimulating energy consumption and methane metabolism, respectively.

Long-term effects of copper nanoparticles on volatile fatty acids production from sludge fermentation: Roles of copper species and bacterial community structure

The long-term effects of copper nanoparticles (Cu NPs) on volatile fatty acids (VFAs) production during the waste activated sludge (WAS) fermentation, and the underlying mechanisms regarding copper species distribution and bacterial community evolution were explored. The yield of VFAs in the control was 1086 mg COD/L, whereas those were inhibited by 11.1%, 56.0% and 83.1%, with 25, 50, and 100 mg/g-TSS Cu NPs, respectively. Further investigation indicated that Cu NPs severely affected hydrolysis and acidification of WAS in a dose-dependent manner, while had little impact on solubilization. Besides, Cu NPs enriched the acid-consuming anaerobe while reducing the acid-forming bacteria. The metabolic pathways, microbial function, and enzymatic activities involved were inhibited at all tested dosages. Moreover, soluble and acid-extractable fractions dominated the copper speciation, which were also the main factors inhibiting the VFA production. This study provides a new perspective to interpret the long-term impacts of Cu NPs on WAS fermentation.

Effect of metal and metal oxide engineered nano particles on nitrogen bio-conversion and its mechanism: A review

Metal and metal oxide engineered nano particles (MMO-ENPs) are widely applied in various industries due to their unique properties. Thus, many researches focused on the influence on nitrogen transformation processes by MMO-ENPs. This review focuses on the effect of MMO-ENPs on nitrogen fixation, nitrification, denitrification and Anammox. Firstly, based on most of the researches, it can be concluded MMO-ENPs have negative effect on nitrogen fixation, nitrification and denitrification while the MMO-ENPs have no promotion effect on Anammox. Then, the influence factors are discussed in detail, including MMO-ENPs dosage, MMO-ENPs kind and exposure time. Both the microbial morphology and population structure were altered by MMO-ENPs. Also, the mechanisms of MMO-ENPs affecting the nitrogen transformation are reviewed. The inhibition of key enzymes and functional genes, the promotion of reactive oxygen species (ROS) production, MMO-ENPs themselves and the suppression of electron transfer all contribute to the negative effect. Finally, the key points for future investigation are proposed that more attention should be attached to the effect on Anammox and the further mechanism in the future studies.

Recovery profile of anaerobic ammonium oxidation (anammox) bacteria inhibited by ZnO nanoparticles

The potential effects of nanoparticles (NPs) on biological treatment processes have become significant due to their increasing industrial applications. The purpose of this research was to investigate the self-recovery ability of anammox bacteria following acute ZnO NPs toxicity. In this context, a 2-liter lab-scale anammox reactor was operated for 550 days to enrich the biomass required to the batch exposure tests. Anammox culture was firstly exposed to four different doses of ZnO NPs (50, 75, 100 and 200 mg/L) for 24 h. Then, the ZnO NPs were removed and self-recovery performance of the anammox bacteria was assessed by evaluating the nitrogen removal capacities for 72 h. Besides the nitrogen removal performance, extracellular polymeric substances (EPS) production was also detected to deeply understand the response of the enriched anammox culture against ZnO NPs exposure. The results revealed that sudden and high load of ZnO NPs (100 and 200 mg/L) resulted in persistent impairment to the nitrogen removal performance of the enriched anammox culture. However, relatively lower doses (50 and 75 mg/L) caused deceleration of the nitrogen removal performance during the recovery period. In addition, EPS content in the reactor decreased along with escalating load of ZnO NPs.

How anammox responds to the emerging contaminants: Status and mechanisms

Numerous researches have been carried out to study the effects of emerging contaminants in wastewater, such as antibiotics, nanomaterials, heavy metals, and microplastics, on the anammox process. However, they are fragmented and difficult to provide a comprehensive understanding of their effects on reactor performance and the metabolic mechanisms in anammox bacteria. Therefore, this paper overviews the effects on anammox processes by the introduced emerging contaminants in the past years to fulfill such knowledge gaps that affect our perception of the inhibitory mechanisms and limit the optimization of the anammox process. In detail, their effects on anammox processes from the aspects of reactor performance, microbial community, antibiotic resistance genes (ARGs), and functional genes related to anammox and nitrogen transformation in anammox consortia are summarized. Furthermore, the metabolic mechanisms causing the cell death of anammox bacteria, such as induction of reactive oxygen species, limitation of substrates diffusion, and membrane binding are proposed. By offering this review, the remaining research gaps are identified, and the potential metabolic mechanisms in anammox consortia are highlighted.

Comparison of the dynamic responses of different anammox granules to copper nanoparticle stress: Antibiotic exposure history made a difference

Two types of anaerobic ammonium oxidation (anammox) seed sludge were selected to evaluate their responses to copper nanoparticles (CuNPs) exposure. Antibiotic-exposed anammox granules (R₁) were more likely to be inhibited by 5.0 mg L^-1 CuNPs than the normal anammox granules (C₁). The nitrogen removal efficiency (NRE) of C₁ decreased by 9.00% after two weeks of exposure to CuNPs, whereas that of R₁ decreased by 20.32%. Simultaneously, the abundance of Candidatus. Kuenenia decreased by 27.65% and 36.02% in C₁ and R₁ under CuNPs stress conditions, respectively. Generally, R₁ was more susceptible to CuNPs than C₁. The correlation analysis indicated that the horizontal transfer of antibiotic resistance genes and copA triggered by intI1 facilitated the generation of multiresistance in the anammox process. Moreover, the potential multiresistance mechanism of anammox bacteria was hypothesized based on previous results. The results will generate new ideas for the treatment of complex wastewater using the anammox process.

The impacts of metal-based engineered nanomaterial mixtures on microbial systems: A review

The last decade has witnessed tremendous growth in the commercial use of metal-based engineered nanomaterials (ENMs) for a wide range of products and processes. Consequently, direct and indirect release into environmental systems may no longer be considered negligible or insignificant. Yet, there is an active debate as to whether there are real risks to human or ecological health with environmental exposure to ENMs. Previous research has focused primarily on the acute effects of individual ENMs using pure cultures under controlled laboratory environments, which may not accurately reveal the ecological impacts of ENMs under real environmental conditions. The goal of this review is to assess our current understanding of ENM effects as we move from exposure of single to multiple ENMs or microbial species. For instance, are ENMs' impacts on microbial communities predicted by their intrinsic physical or chemical characteristics or their effects on single microbial populations; how do chronic ENM interactions compare to acute toxicity; does behavior under simplified laboratory conditions reflect that in environmental media; finally, is biological stress modified by interactions in ENM mixtures relative to that of individual ENM? This review summarizes key findings and our evolving understanding of the ecological effects of ENMs under complex environmental conditions on microbial systems, identifies the gaps in our current knowledge, and indicates the direction of future research.

Assessment of Anammox process against acute and long-term exposure of ZnO nanoparticles

The impacts of nanoparticles (NPs) on wastewater treatment have become a great concern because of their widespread applications. Although the acute responses of anammox bacteria to NPs have enhanced the knowledge about the potential risks of them, deep understanding of the cumulative impacts of NPs must be assessed. The purpose of this research was therefore to further extend the current knowledge by evaluating both acute and long-term effects of Zinc oxide (ZnO) NPs on Anammox process based on nitrogen removal performance, self-recovery ability and microbial community structure. The acute exposure tests indicated that, the median inhibition concentration (IC50) of ZnO NPs on Anammox process was 84.7 mg/L (54.82 mg ZnO NPs/g VSS). Acute exposure of 200 mg/L ZnO NPs (117.54 mg Zn/g VSS) caused 80% inhibition in batch assays while the long-term inhibition dosage was 100 mg/L ZnO NPs (187.50 mg ZnO NPs/g VSS) corresponding to 1022 mg/L total Zn (1916.27 mg Zn/g VSS) in the reactor due to the accumulation of NPs. Total, soluble and biomass-associated Zn concentrations were measured throughout the long-term exposure to observe the behavior of ZnO NPs in the reactor. Total Zn in the reactor was cumulatively increased and mostly originated from biomass-associated Zn. Following the long-term inhibition tests, self-recovery of Anammox process within 120 days demonstrated that, the ZnO NPs inhibition is reversible for the applied dose. Furthermore, next generation sequencing results indicated a symbiotic relationship between the microbial groups in the anammox bioreactor while relative abundance of Candidatus (Ca.) Brocadiaceae family showed a decrease parallel to the deterioration in nitrogen removal performance of bioreactor. At the end of the long-term exposure studies, 48.76% decline on anammox quantity was detected.

Resistance of anammox granular sludge to copper nanoparticles and oxytetracycline and restoration of performance

Nanoparticles and antibiotics, the two most frequently detected emerging pollutants from different wastewater sources, are eventually discharged into wastewater treatment plants. In this study, the widely used materials CuNPs and oxytetracycline (OTC) were selected as target pollutants to investigate their joint effects on anaerobic ammonium oxidation (anammox). The results indicated that the environmental concentration slightly inhibited the performance of the reactors, while the performance rapidly deteriorated within a week under high-level combined shocks (5.0 mg L^-1 CuNPs and 2.0 mg L^-1 OTC). After the second shock (2.5 mg L^-1 CuNPs and 2.0 mg L^-1 OTC), the resistance of anammox bacteria was enhanced, with an elevated relative abundance of Candidatus Kuenenia and absolute abundance of hzsA, nirS, and hdh. Moreover, the extracellular polymeric substance (EPS) content and specific anammox activity (SAA) showed corresponding changes. Improved sludge resistance was observed with increasing CuNP and OTC doses, which accelerated the recovery of performance.

Long-term effects of copper nanoparticles on granule-based denitrification systems: Performance, microbial communities, functional genes and sludge properties

The widespread use of copper nanoparticles (CuNPs) has attracted increasing concern because of their potential effects on biological wastewater treatment. However, their effect on granule-based denitrification systems is unclear. Hence, the effects of CuNPs on denitrifying granules were investigated during long-term operation. The results showed that 51.9% of nitrogen removal capacity was lost after exposure to 5 mg L^-1 CuNPs, with the amount of Cu(II) gradually increasing with elevating CuNP levels. Moreover, the relative abundance of denitrifying bacteria (Castellaniella) and denitrifying functional genes (nirK, napA, narG and nosZ) obviously decreased. Meanwhile, the specific denitrification activity, the content of extracellular polymeric substances and dehydrogenase activity decreased by 44.0%, 15.2% and 99.9%, respectively, compared to their values in the initial sludge. Considering the downtrend in the abundance of copper resistance genes, it was deduced that the toxicity of CuNPs was mainly or at least partially due to the release of Cu(II).

Evaluating the response of anaerobic ammonium-oxidizing bacteria to heavy metal spill in freshwater sediment

Anaerobic ammonium-oxidizing (anammox) bacteria can play an important role in nitrogen elimination in the environment. However, the effect of heavy metals on anammox bacteria in aquatic ecosystem remains largely unknown. The present study investigated the variability of anammox bacterial community in a freshwater reservoir after a severe heavy metal spill. The richness (Chao1 richness estimator = 2-18), diversity (Shannon index = 0.26-2.04) and community structure of anammox bacteria changed considerably with sampling date, while anammox bacterial abundance (from 1.38 × 10⁵ to 3.09 × 10⁵ anammox bacterial 16S rRNA gene copies per gram dry sediment) was less responsive to metal spill. Anammox bacterial communities were mainly composed of Brocadia- and Anammoxoglobus-like bacteria as well as novel phylotype, however, there relative abundance varied among sampling dates. This work could add the knowledge of the response of anammox bacteria to heavy metal contamination.