Iron scraps packing rapidly enhances nitrogen removal in an aerobic sludge system and the mechanism

Enhanced nitrogen removal from low C/N ratio wastewater by coordination of ternary electron donors of Fe0, carbon source and sulfur: Focus on oxic/anoxic/oxic process

Insufficient organics was the major obstacle for total nitrogen (TN) removal in conventional pre-anoxic denitrification when treating low carbon to nitrogen (C/N) ratio wastewater. This study constructed a novel ternary-electron donors (Fe0, organics and S0) enhanced oxic/anoxic/oxic (O/A/O) process, integrating simultaneous nitrification and denitrification and autotrophic denitrification (ADN), and evaluated its feasibility to achieve efficient nutrient removal under organics-deficient condition. Long-term operation results showed that TN removal was lower (9.9 %) when Fe0 added individually, then raised to 27.3 %∼46.0 % in simultaneous presence of Fe0 and organics. And the highest TN removal (82.0 %) was obtained by coordination of ternary-electron donors, with 8.46 ± 0.43 mg/L TN in effluent. Meanwhile, the O/A/O process exhibited excellent total phosphorous (TP) removal (84.8 %∼98.4 %) derived from chemical precipitation by Fe0, of which the effluent was <0.76 ± 0.04 mg/L TP. Metabolic characteristics indicated that the coordination of multi-electron donors improved microbial metabolism and denitrifying enzymatic activities, thereby promoting ammonia assimilation and enhancing TN removal. And the secretion of EPS was also stimulated, which favored the bio-utilization of Fe0 and S0 and alleviated organics dependence. Besides, the notable increase in abundances of aerobic denitrifiers (23.95 %∼27.37 %), autotrophic denitrifiers (9.31 %) and denitrifying genes further verified the synergy effect of multi-electron donors on TN removal. This study revealed the enhancement mechanism of O/A/O process by coordination of ternary-electron donors, verified its cost-effectiveness and provided innovative insights on low C/N ratio wastewater remediation.

Iron-carbon micro-electrolysis coupled to heterotrophic nitrification aerobic denitrification treating low carbon/nitrogen mariculture wastewater

Considering the unsatisfied nitrogen (N) and phosphorus (P) treatment performance of mariculture wastewater caused by low carbon/nitrogen (C/N), a novel iron-carbon (Fe-C) micro-electrolysis coupled to heterotrophic nitrification aerobic denitrification (HNAD) process was proposed to enhance the N and P elimination. Results revealed that total nitrogen (TN) removal and total phosphorus (TP) removal efficiencies in Fe-C filter with HNAD (R-Fe) increased by 76.1% and 113.3% compared to filter packed with ceramsite (R-C). Fe-C micro-electrolysis reaction led to the decrease of microbial diversity and richness, the enrichment of heterotrophic nitrification aerobic denitrification bacteria (HNADB) and HNAD genes (napA and napB) by 7.3 times and 56.3%. Besides, a synergistic effect existed that Fe-C substances not only further accumulated main functional genes associated with the transformation of N, carbon (C) and iron (Fe), but also indirectly enhanced electron transport system activity and ATP generation, thus resulting in elevating TN removal.

Insight into enhanced adaptability of iron-carbon biofilter in treating low-carbon nitrogen mariculture wastewater for nitrogen removal and carbon reduction

Iron-carbon (Fe-C) based biofilters have shown significant advantages in treating mariculture wastewater by facilitating the mixotrophic heterotrophic nitrification-aerobic denitrification (HNAD) process. However, the effects of Fe-C materials and varying carbon-to-nitrogen (C/N) ratios on N removal and C reduction performance remain insufficiently explored. This study demonstrated that the Fe-C biofilter (R-Fe) achieved significantly higher NO3--N removal efficiency (65.1-96.0 %) compared to the control filter (-12.1-76.9 %) across all tested C/N ratios. Furthermore, the N2O emission proportion in R-Fe was reduced by 37.4-42.4 % compared to the control. Increasing the influent C/N ratio enhanced N removal efficiency while reducing the proportion of N2O emissions. This improvement correlated with enhanced electron transfer activity and an increased abundance of heterotrophic nitrifying-aerobic denitrifying bacteria (HNADB) and heterotrophic denitrifying bacteria (DNB), while the abundance of autotrophic denitrifying bacteria declined. Strong correlations were observed among microbial electron transfer activity, denitrifying microbial communities, Fe transport genes, denitrification-related functional genes, N removal efficiency, and N2O emission proportion, highlighting the critical role of electron transfer activity in microbial N removal processes.

Differences in the efficiency and mechanisms of different iron-based materials driving synchronous nitrogen and phosphorus removal

Iron-dependent denitrification has been substantially investigated worldwide due to the advantages of low cost, high efficiency, and synchronized phosphorous removal. However, differences in nitrogen metabolism processes with different iron-based materials as electron donors have not been systematically studied. This study investigated the efficacy of nitrogen and phosphate removal using various iron-based materials as electron donors. Substantial nitrogen removal was demonstrated, with complete TN removal with Fe/C powder as electron donor, while relatively lower TN removal was achieved in iron scrap, Fe/C granular, and pyrite systems with removal efficiency of 86 ± 3.5%, 78 ± 5.7%, and 61 ± 3.1%, respectively. However, the high efficiency could only be sustained for a short time and further microbial metabolisms were significantly suppressed by microbial encapsulation caused by iron-bearing minerals. The introduction of fresh electron donors revitalized denitrification activity, with TN removal improved back to more than 80%. Similar trends were observed in phosphate removal, with increased efficiencies corresponding to ferrous ion release, reaching the highest level of 94 ± 0.02% with iron scrap as electron donor. Microbial community analysis revealed distinct compositions, with sulfur autotrophic denitrifying bacteria prevailing in pyrite systems, hydrogenotrophic denitrifying bacteria dominating with Fe/C powder, and Fe(II) oxidizing bacteria (FeOB) governing in Fe scrap and Fe/C granular systems for denitrification. Predicted genomic analysis elucidated mechanisms underlying nitrogen and phosphorus removal, emphasizing the importance of direct electron transfer via cytochrome C and ferrous ion transportation. Furthermore, differences in nitrogen metabolism among systems were highlighted, with Fe/C powder facilitating hydrogen oxidation proteins expression, while pyrite systems favored sulfur oxidation process, and iron scrap selected Fe(II) transportation and oxidation. This study provided valuable insights into revealing mechanism for efficient nitrogen and phosphorus removal with iron-based materials as electron donors.

The variation of resistome, mobilome and pathogen in domestic and industrial wastewater treatment systems

Wastewater treatment plants (WWTPs), including both domestic and industrial facilities, are key contributors to antibiotic resistance genes (ARGs) and human pathogens in the environment. However, the characteristics and dissemination mechanisms of ARGs in domestic (SD) and industrial (SI) wastewater treatment systems remain unclear, leading to uncertainties in risk assessment. Based on metagenomic analysis, we observed significant differences in the compositions of resistome (ARGs and metal resistance genes, MRGs), mobilome (mobile genetic elements, MGEs), and bacterial community between SD and SI. SI exhibited lower diversity of ARGs but higher abundance of MRGs compared to SD. The removal efficiency of resistome was lower in the SI than that in the SD. MGEs emerged as the primary driver of ARG dissemination in the WWTPs, followed by the bacterial community. Environmental conditions (physicochemical parameters, heavy metals, and antibiotics) indirectly influenced the variation of resistome. Significantly, environmental conditions and MGEs highly influenced the composition of resistome in the SI, while bacterial community more associated with resistome in the SD. Additionally, we identified 36 human bacterial pathogens as potential hosts of ARGs, MRGs, and MGEs in wastewater samples. This study provides new insights on the dissemination mechanisms and risk assessment of antimicrobial resistance in the different types of WWTPs.

Enhancing anammox process with granular activated carbon: A study on Microbial Extracellular Secretions (MESs)

Granular activated carbon (GAC), a porous carbon-based material, provides increased attachment space for functional microorganisms and enhances nitrogen removal by facilitating extracellular electron transfer in the anammox process. This study investigates the effects of GAC on the biosynthesis of microbial extracellular secretions (MESs) and explores the roles of these secretions in anammox activities. Four lab-scale reactors were operated: two downstream UASB reactors (D1 and D2) receiving effluents from the upstream UASB reactors (U1: no-GAC, U2: yes-GAC). Our results indicate that MESs were enhanced with the addition of GAC. The effluent from U2 exhibited a 59.62 % higher amino acid content than that from U1. These secretions contributed to an increase in the nitrogen loading rate (NLR) in the downstream reactors. Specifically, NLR in D1 increased from 130.5 to 142.7 g N/m3/day, and in D2, it escalated from 137.5 to 202.8 g N/m3/day, likely through acting as cross-feeding substrates or vital nutrients. D2 also showed increased anammox bacterial activity, enriched Ca. Brocadia population and hao gene abundance. Furthermore, this study revealed that D2 sludge has significantly higher extracellular polymeric substances (EPS) (48.71 mg/g VSS) and a larger average granule size (1.201 ± 0.119 mm) compared to D1 sludge. Overall, GAC-stimulated MESs may have contributed to the enhanced performance of the anammox process.

In-situ generation of iron activated percarbonate for sustainable sludge dewatering

Effective dewatering of sewage sludge could potentially address the issues of high energy consumption and large carbon footprint inherent in the sludge treatment process, advancing toward carbon neutrality in environmental remediation. Yet, the surface hydrophilic characteristics and water-holding interfacial affinity in sludge led to dwindled sludge-water separation performance. Here, the integration of in-situ generation of iron from zero-valent scrap iron (ZVSI) and sodium percarbonate (SPC) was attempted to attenuate the water-retaining interfacial affinity within sludge, thus achieving superior sludge dewatering performance. Results showed that under the optimal conditions, the ZVSI + SPC system led to a remarkable decline of 76.09 % in the specific resistance to filtration of the sludge, accompanied by a notable decline of 34.96 % in the water content. Moreover, the utilization of ZVSI + SPC system could be a viable alternative to the traditional strategies in terms of enhanced sludge dewaterability, offering application potential with stable operating performance, economic feasibility, and reduced carbon emissions. Investigation into dewatering mechanism revealed that ZVSI could maintain the Fe3+/Fe2+ in a stable dynamic cycle and continuously in-situ generate Fe2+, thereby efficaciously fostering the SPC activation for the ceaseless yield of reactive oxygen species. The predominant •OH and 1O2 efficiently decomposed the hydrophilic biopolymers, therefore minimizing the hydrophilic protein secondary structures, along with the hydrogen and disulfide bonds within proteins. Subsequently, the water-holding interfacial affinity was profoundly diminished, leading to intensified hydrophobicity, self-flocculation, and dewaterability. These findings have important implications for the advancement of efficacious ZVSI + SPC conditioning techniques toward sustainable energy and low-carbon prospects.

Spatial distribution characteristics of denitrification functional genes and the environmental drivers in Liaohe estuary wetland

Genes nirS, nirK, and nosZ are specific for the denitrification process, which is associated with greenhouse gas N2O emission. The abundances and diversities of community containing these three genes are usually used as a common index to reflect the denitrification process, and they would be affected by differences in environmental factors caused by changes from warm to cold conditions. The quantification of denitrification in natural wetlands is complex, and straightforward identification of spatial distribution and drivers affecting the process is still developing. In this study, the bacterial communities, gene diversities, and relative abundances involved in denitrification were investigated in Liaohe Estuary Wetland. We analyzed the relative abundances, diversities, and communities of bacteria containing the three genes at warm and cold conditions using Illumina MiSeq sequencing and detected the potential environmental factors influencing their distribution by using a random forest algorithm. There are great differences in the community composition of the bacteria containing genes nirS, nirK, and nosZ. All the abundant taxa of nirS and nirK communities belonged to phylum Proteobacteria. Compared with the community composition of bacteria containing nirS and nirK, the community of bacteria containing nosZ is more diverse, and the subdivision taxa of phylum Euryarchaeota was also abundant in the community containing nosZ. The distribution characteristics of the relative abundance of nirS and nirK showed obvious differences both at warm and cold climate conditions. The oxidation-reduction potential, nitrite nitrogen, and salinity were detected as potential variables that might explain the diversity of nirS. The total nitrogen and nitrite nitrogen were the important variables for predicting the relative abundance of nirS at warm climate condition, while oxidation-reduction potential and pH contributed to the diversity of nirS at cold condition. The bulk density of sediment was detected as a potential variable affecting the relative abundance of nirK at warm and cold conditions, and diversity of nirK at warm condition, while nitrite nitrogen was detected as an important environmental factor for predicting the diversity of nirK at cold condition. Overall, our results show that the key environmental factors, which affect the relative abundance, diversity, and community of bacteria containing the functional denitrification genes, are not exactly the same, and the diversities of nirS, nirK, and nosZ have a higher environmental sensitivity than their relative abundances.

Novel insights into carbon nanomaterials enhancing anammox for nitrogen removal: Effects and mechanisms

Carbon nanomaterials (CNMs) possess the properties including large specific surface area, high porosity, and stable chemical structures, presenting significant application advantages in wastewater treatment. Indeed, CNMs are considered to be added to anammox systems to strengthen anammox function, especially to resolve the challenge of anammox technology, i.e., the slow growth rate of anammox bacteria, as well as its high environmental sensitivity. This paper systematically reviews the promotion effects and mechanisms of CNMs on the nitrogen removal performance of anammox system. Among the zero-, one-, and two-dimensional CNMs, two-dimensional CNMs have best promoting effect on the nitrogen removal performance of anammox system due to its excellent conductivity and abundant functional groups. Then, the promotion effects of CNMs on anammox process are summarized from the perspective of anammox activity and bacteria abundance. Furthermore, CNMs not only enhance the anammox process, but also stimulate the coupling of denitrification pathways with anammox, as well as the improvement of system operational stability (alleviating the inhibitions of low temperature and pH fluctuation), thus contributing to the promoted nitrogen removal performance. Essentially, CNMs are capable of facilitating microbial immobilization and electron transfer, which favor to improve the efficiency and stability of anammox process. Finally, this review highlights the gap in knowledge and future work, aiming to provide a deeper understanding of how CNMs can strengthen the anammox system and provide a novel perspective for the engineering of the anammox process.