The application of post-denitrification fixed biofilm reactor for polishing secondary effluent: Nitrate removal, soluble microbial products and micropollutants biotransformation

Impact of polyethylene microplastics on the nitrogen removal and bacterial community in sequencing batch reactor at different hydraulic retention times

Both hydraulic retention time (HRT) and microplastics (MPs) are factors affecting the performance of biological wastewater treatment processes, but how MPs affect the role of HRT in biological wastewater treatment performance has not been investigated. In this study, the effects of polyethylene MPs (PE-MPs) on the nitrogen removal performance in a sequencing batch reactor (SBR) at different HRTs were investigated by analyzing the changes in the content and composition of extracellular polymer substances (EPS) and in the bacterial communities and metabolic pathways. In the PE-MPs absence, the HRT was shortened from 1440 to 720 min, resulting in a decrease in the average elimination of chemical oxygen demand and ammonia nitrogen by 13.21 % and 3.78 %, respectively. Whereas the presence of 0.5 mg/L PE-MPs enhanced the reduction by 73.36 % and 93.85 %, respectively. PE-MPs did not change the promotional impacts of HRT shortening on the levels of protein (PN) and polysaccharide (PS) within EPS, while amplified this effect. Aromatic PN of EPS was more sensitive to PE-MPs and HRT than tryptophan-like PN. PE-MPs resulted in a lower enzyme level in nitrification and denitrification metabolism pathways at different HRTs compared to no PE-MPs. This research offers novel perspectives for understanding how PE-MPs intervene in the influence of HRT on biological wastewater treatment performance.

Adaptations of Rhodococcus rhodochrous Biofilms to Oxidative Stress Induced by Copper(II) Oxide Nanoparticles

Copper(II) oxide nanoparticles (CuO NPs) are used in different industries and agriculture, thus leading to their release to the environment, which raises concerns about their ecotoxicity and biosafety. The main toxicity mechanism of nanometals is oxidative stress as a result of the formation of reactive oxygen species caused by metal ions released from nanoparticles. Bacterial biofilms are more resistant to physical and chemical factors than are planktonic cells due to the extracellular polymeric matrix (EPM), which performs a protective function. Hydrocarbon-oxidizing bacteria of the genus Rhodococcus, well-known biodegraders of toxic organic pollutants and bioremediation agents, are capable of producing biofilms, which, as we proposed, are more resistant to metal nanoparticles, while the particular adaptation mechanisms have not yet been clarified. In this study, we study the adaptation mechanisms of Rhodococcus rhodochrous IEGM 1363 biofilms to CuO NPs in a wide range of concentrations (0.001-0.1 g/L), including morphological and ultrastructural cell alterations. The results obtained on the long-term dynamics (≤72 h) and localization of EPM structural components, in particular, lipids, polysaccharides, and proteins, indicated their important role in the complex adaptive response of alkanotrophic Rhodococcus to oxidative stress caused by copper nanooxide. The observed changes in the ultrastructure and element composition included binding of CuO nanoparticles by the cell wall to prevent their penetration inside cells and intracellular accumulation of potassium, magnesium, phosphorus, and sulfur in electron-dense inclusions, which may be associated with a metabolic stress reaction. Understanding the mechanisms of interaction between nanometals and Rhodococcus biofilms will contribute to the development of biocatalysts based on immobilized bacterial cells and bioremediation methods.

Evolution of microbial community and resistance genes in denitrification system under single and combined exposure to benzethonium chloride and methylparaben

Benzethonium chloride (BZC) and methylparaben (MeP) are commonly added into cosmetics as preservatives, which are frequently detected in wastewater treatment plants. Different response patterns of denitrification system were proposed under single and combined exposure to BZC and MeP (0, 0.5, 5 mg/L) by evaluating system performance, functional genes, extracellular polymeric substance (EPS), cytotoxicity, microbial community structure and resistance genes (RGs). The inhibition effect of BZC on denitrification system was stronger than MeP, and the co-exposure of BZC and MeP showed synergistic effect, enhancing the inhibition effect of BZC single exposure. BZC and/or MeP could promote the diffusion of RGs in sludge, including intracellular RGs (si-RGs) and extracellular RGs (se-RGs). Moreover, the single exposure of BZC and co-exposure of BZC and MeP increased the dissemination risks of RGs in water (w-RGs). IntI1 and tnpA-04, mobile genetic elements (MGEs), correlated positively with diverse RGs from different fractions. Notably, the spread of RGs through horizontal gene transfer mediated by MGEs and the flow of si-RGs into extracellular and water were observed in this study.

Effects of Exposure to Different Types of Metal-Organic Framework Nanoparticles on the Gut Microbiota and Liver Metabolism of Adult Zebrafish

Metal-organic framework nanoparticles (MOF NPs) have received much attention for their potential use in nanopesticides. However, little is known about the potential health and environmental risks associated with these materials. In this study, the toxicological responses of zebrafish exposed to five MOF NPs for short and long periods of time were evaluated. The acute toxicity results showed that the toxicity of the five MOF NPs to zebrafish embryos and adult zebrafish was in the order of Cu-MOF > ZIF-90 > ZIF-8 > Fe-MOF > Zr-MOF. Histopathological analysis revealed that ZIF-8, ZIF-90, and Cu-MOF NPs caused liver swelling and vacuolization in zebrafish. The cellular ultrastructure showed that ZIF-8, ZIF-90, and Cu-MOF NPs severely damaged the mitochondrial structure in intestinal epithelial cells and liver cells. The 16S rDNA sequencing data showed that all five MOF NPs significantly altered the dominant microorganisms in the zebrafish intestine. The microbial markers of intestinal inflammation, Proteobacteria (Aeromonas, Plesiomonas, and Legionella), were significantly increased in the Fe-MOF, ZIF-8, Zr-MOF, and Cu-MOF treatment groups. Metabolomics results indicated that the levels of inflammatory promoting factors (Leukotriene E4, 20-hydroxyeicosatetraenoic acid) in arachidonic acid metabolism were decreased, and the levels of inflammatory suppressing factors (8,9-epoxyeicosatrienoic acid) were increased. Metabolites related to oxidative stress, such as glutamine, pyridoxamine, and l-glutamic acid in vitamin B6 metabolism and other signaling pathways, were significantly reduced. Overall, these results suggest that the different MOF NPs had widely varying toxicity to zebrafish, and further attention should be paid to the toxicity of MOF NPs in the real environment.

Simultaneous efficient removal of tetracycline and mitigation of antibiotic resistance genes enrichment by a modified activated sludge process with static magnetic field

To address the increasing issue of antibiotic wastewater, this study applied a static magnetic field (SMF) to the activated sludge process to increase the efficiency of tetracycline (TC) removal from swine wastewater and to reveal its enhanced mechanisms. The results demonstrated that the SMF-modified activated sludge process could achieve almost complete TC removal at sludge loading rates of 0.3 mg TC/g MLSS/d. Analysis of zeta potential and extracellular polymeric substances composition of the activated sludge revealed that SMF increased electrostatic interactions between TC and activated sludge and made activated sludge has much more binding sites, finally resulting in the increased TC biosorption. Metagenomic analysis showed that SMF promoted the enrichment of ammonia-oxidizing bacteria, TC-degrading bacteria, and aromatic compounds-degrading bacteria; it also enhanced ammonia monooxygenase- and cytochrome P450-mediated TC metabolism while upregulating functional genes associated with oxidase, reductase, and dehydrogenase - all contributing to increased TC biodegradation. Additionally, SMF mitigated the enrichment and spread of antibiotic resistance genes (ARGs) by decreasing the abundance of potential hosts of ARGs and inhibiting the upregulation of genes encoding ABC transporters and putative transposase. Based on these findings, this study demonstrates that magnetic field is an enhancement strategy with great potential to relieve the harmful impacts of the growing antibiotic wastewater problem on human health and the ecosystem.

Iron-based mixotrophic denitrification for enhancing nitrate removal from municipal secondary effluent: Performance, microbial community dynamics, and economic feasibility

High nitrate content limits the recycling of the secondary effluent of wastewater treatment plants. In the research, one biomass-iron mixture (BIM) filter material based on mixotrophic denitrification mode (heterotrophic and iron-driven autotrophic denitrification) was developed and used to construct a novel denitrification biological filter (BIM-DNBF) for the nitrogen removal of secondary effluent. BIM-DNBF had a short start-up time (approximately 9 days), and high total nitrogen removal (81 %-89 %) without external addition of organic carbon sources during the whole operation. The coexistence of dominant heterotrophic-denitrification-like Pseudomonas and Erysipelothrix as well as iron-driven autotrophic-denitrification-like Citrobacter, Acidovorax, etc. were found in the BIM-DNBF. Moreover, biomass was recognized as one key player in promoting the reduction of Fe3+ to Fe2+, thereby facilitating the occurrence of iron-driven autotrophic denitrification. In addition, BIM-DNBF was assessed to be affordable. These findings provide evidence that BIM-DNBF can be an efficient technology for nitrogen removal of secondary effluent.

Comparative Analysis of Bacterial Information of Biofilms and Activated Sludge in Full-Scale MBBR-IFAS Systems

This study extensively analyzed the bacterial information of biofilms and activated sludge in oxic reactors of full-scale moving bed biofilm reactor-integrated fixed-film activated sludge (MBBR-IFAS) systems. The bacterial communities of biofilms and activated sludge differed statistically (R = 0.624, p < 0.01). The denitrifying genera Ignavibacterium, Phaeodactylibacter, Terrimonas, and Arcobacter were more abundant in activated sludge (p < 0.05), while comammox Nitrospira was more abundant in biofilms (p < 0.05), with an average relative abundance of 8.13%. Nitrospira and Nitrosomonas had weak co-occurrence relationships with other genera in the MBBR-IFAS systems. Potential function analysis revealed no differences in pathways at levels 1 and 2 based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) between biofilms and activated sludge. However, in terms of pathways at level 3, biofilms had more potential in 26 pathways, including various organic biodegradation and membrane and signal transportation pathways. In comparison, activated sludge had more potential in only five pathways, including glycan biosynthesis and metabolism. With respect to nitrogen metabolism, biofilms had greater potential for nitrification (ammonia oxidation) (M00528), and complete nitrification (comammox) (M00804) concretely accounted for methane/ammonia monooxygenase (K10944, K10945, and K10946) and hydroxylamine dehydrogenase (K10535). This study provides a theoretical basis for MBBR-IFAS systems from the perspective of microorganisms.

Bio-clogging mitigation in constructed wetland using microbial fuel cells with novel hybrid air-photocathode

Excessive accumulation of extracellular polymeric substances (EPS) in constructed wetland (CW) substrate can lead to bio-clogging and affect the long-term stable operation of CW. In this study, a microbial fuel cell (MFC) was coupled with air-photocathode to mitigate CW bio-clogging by enhancing the micro-electric field environment. Because TiO₂/biochar could catalyze and accelerate oxygen reduction reaction, further promoting the gain of electric energy, the electricity generation of the tandem CW-photocatalytic fuel cell (CW-PFC) reached 90.78 mW m^-3. After bio-clogging was mitigated in situ in tandem CW-PFC, the porosity of CW could be restored to about 62.5 % of the initial porosity, and the zeta potential of EPS showed an obvious increase (-14.98 mV). The removal efficiencies of NH₄⁺-N and chemical oxygen demand (COD) in tandem CW-PFC were respectively 31.8 ± 7.2 % and 86.1 ± 6.8 %, higher than those in control system (21.1 ± 11.0 % and 73.3 ± 5.6 %). Tandem CW-PFC could accelerate the degradation of EPS into small molecules (such as aromatic protein) by enhancing the electron transfer. Furthermore, microbiome structure analysis indicated that the enrichment of characteristic microorganisms (Anaerovorax) for degradation of protein-related pollutants, and electroactive bacteria (Geobacter and Trichococcus) promoted EPS degradation and electron transfer. The degradation of EPS might be attributed to the up-regulation of the abundances of carbohydrate and amino acid metabolism. This study provided a promising new strategy for synergic mitigation and prevention of bio-clogging in CW by coupling with MFC and photocatalysis.

Effect of Fe3+ on the nutrient removal performance and microbial community in a biofilm system

In this study, the influence of Fe3+ on N removal, microbial assembly, and species interactions in a biofilm system was determined. The results showed that maximum efficiencies of ammonia nitrogen (NH4+-N), total nitrogen (TN), phosphorus (P), and chemical oxygen demand (COD) removal were achieved using 10 mg/L Fe3+, reaching values of 100, 78.85, 100, and 95.8%, respectively, whereas at concentrations of 15 and 30 mg/L Fe3+ suppressed the removal of NH4+-N, TN, and COD. In terms of absolute abundance, the expression of bacterial amoA, narG, nirK, and napA was maximal in the presence of 10 mg/L Fe3+ (9.18 × 105, 8.58 × 108, 1.09 × 108, and 1.07 × 109 copies/g dry weight, respectively). Irrespective of Fe3+ concentrations, the P removal efficiency remained at almost 100%. Candidatus_Competibacter (10.26–23.32%) was identified as the most abundant bacterial genus within the system. Determinism (50%) and stochasticity (50%) contributed equally to microbial community assembly. Co-occurrence network analysis revealed that in the presence of Fe3+, 60.94% of OTUs in the biofilm system exhibited positive interactions, whereas 39.06% exhibited negative interactions. Within the OTU-based co-occurrence network, fourteen species were identified as key microbes. The stability of the system was found to be predominantly shaped by microbial cooperation, complemented by competition for resources or niche incompatibility. The results of this study suggested that during chemical P removal in wastewater treatment plants using biofilm methods, the concentration of supplemental Fe3+ should be maintained at 10 mg/L, which would not only contribute to P elimination, but also enhance N and COD removal.