Natural pyrite to enhance simultaneous long-term nitrogen and phosphorus removal in constructed wetland: Three years of pilot study

Investigation of pollutants accumulation in the submerged zone for pyrite-based bioretention facilities under continuous rainfall events

Submerged zone in bioretention facilities for stormwater treatment has been approved to be an effective structure amendment to improve denitrification capability. However, the role and influence of water quality changes in the submerged zone under natural continuous random rainfall patterns are still not clear, especially when the rainfall is less than the pore water in the submerged zone. In this study, continuous rainfall events with different rainfall volume (light rain-light rain-heavy rain) were designed in a lab-scale woodchip mulched pyrite bioretention facility to test the effects of rainfall pattern. The results exhibited that light rain events significantly affected the pollutant removal performance of bioretention for the next rainfall. Different effects were observed during the long-term operation. In the 5th month, light rain reduced the ammonia removal efficiency of subsequent rainstorm events by 8.70%, while in the 12th month, when nitrate leakage occurred, light rain led to a 40.24% reduction in the next heavy rain event's nitrate removal efficiency. Additionally, light rain would also affect the concentration of by-products in the next rainfall. Following a light rain, the concentration of sulfate in the subsequent light rainfall can increase by 24.4 mg/L, and by 11.92 mg/L in a heavy rain. The water quality in the submerged zone and media characteristics analysis suggested that nitrogen conversion capacity of the substrate and microbes, such as Nitrospira (2.86%) and Thiobacillus (35.71%), as well as the in-situ accumulation of pollutants under light rain played important roles. This study clarifies the relationship between successive rainfall events and provides a more comprehensive understanding of bioretention facilities. This is beneficial for field study of bioretention facilities in the face of complex rainfall events.

Distinct roles of biochar and pyrite substrates in enhancing nutrient and heavy metals removal in intermittent-aerated constructed wetlands: Performances and mechanism

Constructed wetlands have been widely employed as a cost-effective and environmentally friendly alternative for treating primary and secondary sewage effluents. In this study, biochar and pyrite were utilized as electron donor substrates in intermittent-aerated vertical flow constructed wetlands to strengthen the nutrient and heavy metals removal simultaneously, and the response of nutrient reduction and microbial community to heavy metals stress was also explored. The results indicated that biochar addition exhibited a better nitrogen removal, while pyrite addition greatly promoted the phosphorus removal. Moreover, the high removal efficiencies of Cu2+, Pb2+ and Cd2+ (above 90%) except for Zn2+ were obtained in each system. However, the exposure of heavy metals decreased phosphorus removal while had little effect on nitrogen removal. The influent load and intermittent aeration implementation led to a significant shift in microbial community structures, but microbial biodiversity and abundance decreased under the exposure of heavy metals. Particularly, Thiobacillus and Ferritrophicum, associated with sulfur autotrophic denitrification and iron autotrophic denitrification, were more abundant in pyrite-based wetland systems.

Characteristics and mechanisms of phosphine production in sulfur-based constructed wetlands

Phosphine (PH3) is an important contributor to the phosphorus cycle and is widespread in various environments. However, there are few studies on PH3 in constructed wetlands (CWs). In this study, lab-scale CWs and batch experiments were conducted to explore the characteristics and mechanisms of PH3 production in sulfur-based CWs. The results showed that the PH3 release flux of sulfur-based CWs varied from 0.86±0.04 ng·m-2·h-1 to 1.88±0.09 ng·m-2·h-1. The dissolved PH3 was the main PH3 form in CWs and varied from 2.73 μg·L-1 to 4.08 μg·L-1. The matrix-bound PH3 was a staging reservoir for PH3 and increased with substrate depth. In addition, the sulfur-based substrates had a significant improvement on PH3 production. Elemental sulfur is more conducive to PH3 production than pyrite. Moreover, there was a significant positive correlation between PH3 production, the dsrB gene, and nicotinamide adenine dinucleotide (NADH). NADH might catalyze the phosphate reduction process. And the final stage of the dissimilatory sulfate reduction pathway driven by the dsrB gene might also provide energy for phosphate reduction. The migration and transformation of PH3 increased the available P (Resin-P and NaHCO3-P) from 35 % to 56 % in sulfur-based CW, and the P adsorption capacity was improved by 12 %. The higher proportion of available P increased the plant uptake rate of P by 17 %. This study improves the understanding of the phosphorus cycle in sulfur-based CW and provides new insight into the long-term stable operation of CWs.

Iron-enhanced microscale laboratory aerated filters in the treatment of artificial mariculture wastewater: A study on nitrogen removal performance and the impact on microbial community structure

This study investigates the nitrogen removal efficacy and microbial community dynamics in seawater aquaculture effluent treatment using three different substrate combinations of microscale laboratory aerated filters (MFs) - MF1 (LECA), MF2 (LECA/Fe-C), and MF3 (LECA/Pyrite). The findings indicated that the COD removal exceeded 95% across all MFs, with higher removal efficiencies in MF2 and MF3. In terms of nitrogen removal performance, MF2 exhibited the highest average nitrogen removal of 93.17%, achieving a 12.35% and 3.56% increase compared to MF1 (80.82%) and MF3 (89.61%), respectively. High-throughput sequencing analysis revealed that the Fe-C substrate significantly enhanced the diversity of the microbial community. Notably, in MF2, the salinophilic denitrifying bacterium Halomonas was significantly enriched, accounting for 42.6% of the total microbial community, which was beneficial for nitrogen removal. Moreover, an in-depth analysis of nitrogen metabolic pathways and microbial enzymes indicated that MF2 and MF3 possessed a high abundance of nitrification and denitrification enzymes, related to the high removal rates of NH4+-N and NO3--N. Therefore, the combination of LECA with iron-based materials significantly enhances the nitrogen removal efficiency from mariculture wastewater.

Iron-containing nanominerals for sustainable phosphate management: A comprehensive review and future perspectives

Adsorption, which is a quick and effective method for phosphate management, can effectively address the crisis of phosphorus mineral resources and control eutrophication. Phosphate management systems typically use iron-containing nanominerals (ICNs) with large surface areas and high activity, as well as modified ICNs (mICNs). This paper comprehensively reviews phosphate management by ICNs and mICNs in different water environments. mICNs have a higher affinity for phosphates than ICNs. Phosphate adsorption on ICNs and mICNs occurs through mechanisms such as surface complexation, surface precipitation, electrostatic ligand exchange, and electrostatic attraction. Ionic strength influences phosphate adsorption by changing the surface potential and isoelectric point of ICNs and mICNs. Anions exhibit inhibitory effects on ICNs and mICNs in phosphate adsorption, while cations display a promoting effect. More importantly, high concentrations and molecular weights of natural organic matter can inhibit phosphate adsorption by ICNs and mICNs. Sodium hydroxide has high regeneration capability for ICNs and mICNs. Compared to ICNs with high crystallinity, those with low crystallinity are less likely to desorb. ICNs and mICNs can effectively manage municipal wastewater, eutrophic seawater, and eutrophic lakes. Adsorption of ICNs and mICNs saturated with phosphate can be used as fertilizers in agricultural production. Notably, mICNs and ICNs have positive and negative effects on microorganisms and aquatic organisms in soil. Finally, this study introduces the following: trends and prospects of machine learning-guided mICN design, novel methods for modified ICNs, mICN regeneration, development of mICNs with high adsorption capacity and selectivity for phosphate, investigation of competing ions in different water environments by mICNs, and trends and prospects of in-depth research on the adsorption mechanism of phosphate by weakly crystalline ferrihydrite. This comprehensive review can provide novel insights into the research on high-performance mICNs for phosphate management in the future.

Zeolite facilitates sequestration of heavy metals via lagged Fe(II) oxidation during sediment aeration

Aeration of sediments could induce the release of endogenous heavy metals (HMs) into overlying water. In this study, experiments involving FeS oxygenation and contaminated sediment aeration were conducted to explore the sequestering role of zeolite in the released HMs during sediment aeration. The results reveal that the dynamic processes of Fe(II) oxidation play a crucial role in regulating HMs migration during both FeS oxygenation and sediment aeration in the absence of zeolite. Based on the release of HMs, Fe(II) oxidation can be delineated into two stages: stage I, where HMs (Mn2+, Zn2+, Cd2+, Ni2+, Cu2+) are released from minerals or sediments into suspension, and stage II, released HMs are partially re-sequestered back to mineral phases or sediments due to the generation of Fe-(oxyhydr) oxide. In contrast, the addition of zeolite inhibits the increase of HMs concentration in suspension during stage I. Subsequently, the redistribution of HMs between zeolite and the newly formed Fe-(oxyhydr) oxide occurs during stage II. This redistribution of HMs generates new sorption sites in zeolite, making them available for resorbing a new load of HMs. The outcomes of this study provide potential solutions for sequestering HMs during the sediment aeration.

The effect of reflux ratio on sulfur disproportionation tendency in anaerobic baffled reactor with the heterotrophic combining sulfur autotrophic processes under high concentration perchlorate stress

In a laboratory scale, an anaerobic baffled reactor (ABR) consisting of eight compartments, the heterotrophic combining sulfur autotrophic processes under different reflux ratios were constructed to achieve effective perchlorate removal and alleviate sulfur disproportionation reaction. Perchlorate was efficiently removed with effluent perchlorate concentration below 0.5 μg/L when the influent perchlorate concentration was 1030 mg/L during stages I ~ V, indicating that heterotrophic combining sulfur autotrophic perchlorate reduction processes can effectively achieve high concentration perchlorate removal. Furthermore, the 100% reflux ratio could reduce the contact time between sulfur particles and water; thus, the sulfur disproportionation reaction was inhibited. However, the inhibition effect of reflux on sulfur disproportionation was attenuated due to dilute perchlorate concentration when a reflux ratio of 150% and 200% was implemented. Meanwhile, the content of extracellular polymeric substances (EPS) in the heterotrophic unit (36.79 ~ 45.71 mg/g VSS) was higher than that in the sulfur autotrophic unit (22.19 ~ 25.77 mg/g VSS), indicating that high concentration perchlorate stress in the heterotrophic unit promoted EPS secretion. Thereinto, the PN content of sulfur autotrophic unit decreased in stage III and stage V due to decreasing perchlorate concentration in the autotrophic unit. Meanwhile, the PS content increased with increasing reflux in the autotrophic unit, which was conducive to the formation of biofilm. Furthermore, the high-throughput sequencing result showed that Proteobacteria, Chloroflexi, Firmicutes, and Bacteroidetes were the dominant phyla and Longilinea, Diaphorobacter, Acinetobacter, and Nitrobacter were the dominant genus in ABR, which were associated with heterotrophic or autotrophic perchlorate reduction and beneficial for effective perchlorate removal. The study indicated that reflux was a reasonable strategy for alleviating sulfur disproportionation in heterotrophic combining sulfur autotrophic perchlorate removal processes.

Wetlands as a potential multifunctioning tool to mitigate eutrophication and brownification

Eutrophication and brownification are ongoing environmental problems affecting aquatic ecosystems. Due to anthropogenic changes, increasing amounts of organic and inorganic compounds are entering aquatic systems from surrounding catchment areas, increasing both nutrients, total organic carbon (TOC), and water color with societal, as well as ecological consequences. Several studies have focused on the ability of wetlands to reduce nutrients, whereas data on their potential to reduce TOC and water color are scarce. Here we evaluate wetlands as a potential multifunctional tool for mitigating both eutrophication and brownification. Therefore, we performed a study for 18 months in nine wetlands allowing us to estimate the reduction in concentrations of total nitrogen (TN), total phosphorus (TP), TOC and water color. We show that wetland reduction efficiency with respect to these variables was generally higher during summer, but many of the wetlands were also efficient during winter. We also show that some, but not all, wetlands have the potential to reduce TOC, water color and nutrients simultaneously. However, the generalist wetlands that reduced all four parameters were less efficient in reducing each of them than the specialist wetlands that only reduced one or two parameters. In a broader context, generalist wetlands have the potential to function as multifunctional tools to mitigate both eutrophication and brownification of aquatic systems. However, further research is needed to assess the design of the generalist wetlands and to investigate the potential of using several specialist wetlands in the same catchment.

Enhanced adaptability of pyrite-based constructed wetlands for low carbon to nitrogen ratio wastewater treatments: Modulation of nitrogen removal mechanisms and reduction of carbon emissions

Pyrite-based constructed wetlands (CWs) stimulated nitrate removal performance at low carbon to nitrogen (C/N) ratio has been gaining widely attention. However, the combined effects of pyrite and C/N on the nitrate removal mechanisms and greenhouse gases (GHGs) reduction were ignored. This study found that pyrite-based CWs significantly enhanced nitrate removal in C/N of 0, 1.5 and 3 by effectively driving autotrophic denitrification with high abundance of autotrophs denitrifiers (Rhodanobacter) and nitrate reductase (EC 1.7.7.2), while the enhancement was weakened in C/N of 6 by combined effect of mixotrophic denitrification and dissimilatory nitrate reduction to ammonium (DNRA) with high abundance of organic carbon-degrading bacteria (Stenotrophobacter) and DNRA-related nitrite reductase genes (nrf). Moreover, pyrite addition significantly reduced GHGs emissions from CWs in all stages with the occurrence of iron-coupled autotrophic denitrification. The study shed light on the potential mechanism for pyrite-based CWs for treating low C/N ratio wastewater.

Efficient rural sewage treatment with manganese sand-pyrite soil infiltration systems: Performance, mechanisms, and emissions reduction

The application of soil infiltration systems (SISs) in rural domestic sewage (RDS) is limited due to suboptimal denitrification resulting from factors such as low C/N (<5). This study introduced filler-enhanced SISs and investigated parameter impacts on pollutant removal efficiency and greenhouse gas (GHG) emission reduction. The results showed that Mn sand-pyrite SISs, with hydraulic load ratios of 0.003 m3/m2·h and dry-wet ratios of 3:1, achieved excellent removal efficiency of COD (92.7 %), NH4+-N (95.8 %), and TN (76.4 %). Moreover, N2O and CH4 emission flux were 0.046 and 0.019 mg/m2·d, respectively. X-ray photoelectron spectroscopy showed that the relative concentrations of Mn(Ⅱ) in Mn sand and Fe(Ⅲ) and SO42- in pyrite increased after the experiment. High-throughput sequencing indicated that denitrification was mainly performed by Thiobacillus. This study demonstrated that RDS treatment using the enhanced SIS resulted in efficient denitrification and GHG reduction.

Deciphering the influencing mechanism of hydraulic retention time on purification performance of a mixotrophic system from the perspective of reaction kinetics

At present, eutrophication is increasingly serious, so it is necessary to effectively reduce nitrogen and phosphorus in water bodies. In this study, a pyrite/polycaprolactone-based mixotrophic denitrification (PPMD) system using pyrite and polycaprolactone (PCL) as electron donors was developed and compared with pyrite-based autotrophic denitrification (PAD) system and PCL-based heterotrophic denitrification (PHD) system through continuous flow experiment. The removal efficiency of NO3--N (NRE) and PO43--P (PRE) and the contribution proportion of PAD in the PPMD system were significantly increased by prolonging hydraulic retention time (HRT, from 1 to 48 h). When HRT was equal to 24 h, the PPMD system conformed to the zero-order kinetic model, so NRE and PRE were mainly limited by the PAD process. When HRT was equal to 48 h, the PPMD system met the first-order kinetic model with NRE and PRE reaching 98.9 ± 1.1% and 91.8 ± 4.5%, respectively. When HRT = 48 h, the NRE and PRE by PAD system were 82.7 ± 9.1% and 88.5 ± 4.7%, respectively, but the effluent SO42- concentration was as high as 152.1 ± 13.7 mg/L (the influent SO42- concentration was 49.2 ± 3.3 mg/L); the NRE by PHD system was 98.5 ± 1.7%, but the PO43--P could not be removed ideally. The concentrations of NO3--N, total nitrogen, PO43--P, and SO42- in the PPMD system also showed distinct changes along the reactor column. In addition, the microbial diversity analysis showed that prolonging HRT (from 24 to 48 h) increased the abundance of autotrophic denitrifying microorganisms in the PPMD system, ultimately increasing the contribution proportion of PAD.

Opposite response of constructed wetland performance in nitrogen and phosphorus removal to short and long terms of operation

Constructed wetlands (CWs) have been widely used for treating polluted water since the 1950s, with applications in over 50 countries worldwide. Most studies investigating the pollutant removal efficiency of these wetlands have focused on differences among wetland designs, operation strategies, and environmental conditions. However, there still remains a gap in understanding the variation in wetland pollutant removal efficiency over different time scales. Therefore, the main aim of the study is to address this gap by conducting a global meta-analysis to estimate the variation in nitrogen (N) and phosphorus (P) removal by wetland in short- and long-term pollutant treatment. The findings of this study indicated that the total efficiencies of N and P removal increased during short-term wetland operation but decreased during long-term operation. However, for surface flow CWs specifically, the efficiencies of N and P removal increased during short-term operation and remained stable during long-term operation. Moreover, the study discovered that wetland N removal efficiency was influenced by seasons, with an increase in spring and summer and a decrease in autumn and winter. Conversely, there was no significant seasonal effect on P removal efficiency. Additionally, high hydraulic load impaired wetland N and P removal efficiency during long-term operation. This study offers a critical review of the role of wetlands in wastewater treatment and provides valuable reference data for the design and selection of CWs types during wastewater treatment in the aspect of sustainability.

Development and trends of constructed wetland substrates over the past 30 years: a literature visualization analysis based on CiteSpace

Constructed wetland substrates (CWSs) have received considerable attention owing to their importance in adsorbing and degrading pollutants, providing growth attachment points for microorganisms, and supporting wetland plants. There are differences in the configurations and functions of constructed wetlands (CWs) for treating different water bodies and sewage, resulting in a wide variety of substrates. Research on the application and mechanism of CWSs is not sufficiently systematic. Therefore, the current research advancements and hotspots must be identified. Hence, we used CiteSpace to analyze 1955 English publications from the core collection database of the Web of Science to assess the current state of the CWS research field. Based on the cooperative network analysis, the roles of various countries, institutions, and authors in research on CWSs were reviewed. Keyword co-occurrence and cluster analyses were used to discuss the transformation of CWSs from removing traditional pollutants to emerging pollutants and the transition from incorporating natural substrates to artificial substrates. Finally, we underscored the need for more emphasis to be placed on the collocation and application of the CWSs at different latitudes. Furthermore, the substrate micro-interface process and its effects on the interaction patterns of pollutants and microorganisms should be thoroughly investigated to provide theoretical guidance for the development of wetland applications and mechanisms.

Synthesis of amorphous-MnO2/Clinoptilolite and its utilization for NH4+-N oxidation in an anoxic environment

Mediating the anoxic ammonia oxidation with manganese oxide (MnOx) can reduce the requirements of dissolved oxygen (DO) concentrations in constructed wetlands (CWs) and improve the removal of ammonium nitrogen (NH4+-N). Recent studies that employed natural manganese ore and/or mine waste as substrates in CWs may develop potentially negative environmental effects due to leachates. However, removing NH4+-N by anoxic ammonia oxidation is influenced by the crystal form of MnOx. In this study, a novel clinoptilolite-based amorphous-MnO2 (amorphous-MnO2/clinoptilolite) was synthesized by the sol-gel method as an alternative substrate to improve the efficiency of anoxic ammonia oxidation and reduce the impact of Mn ion leaching. According to the anoxic ammonia oxidation experiment of clinoptilolite, amorphous-MnO2/clinoptilolite, and manganese ore on NH4+-N, the amounts of NH4+-N removed were 24.55 mg/L/d, 44.55 mg/L/d, and 11.04 mg/L/d, respectively, and the initial NH4+-N concentration was 49.53 mg/L. These results indicated that the amorphous-MnO2/clinoptilolite had both the adsorption and the anoxic ammonia oxidation performance. The recycling experiment demonstrated that the effect of anoxic ammonia oxygen mediated by amorphous-MnO2 would not diminish with the gradual saturation of clinoptilolite for NH4+-N. Furthermore, the anoxic ammonia oxidation consumed NH4+-N in the clinoptilolite, which restored the adsorption capacity of the clinoptilolite and simultaneously decreased the leakage of manganese ions in the process, making it environmentally friendly. Therefore, the amorphous-MnO2/clinoptilolite provided an excellent substrate material for the constructed wetland under an anoxic environment, which greatly improved the nitrogen removal capacity compared to existing substrate materials.

Estimating ammonium changes in pilot and full-scale constructed wetlands using kinetic model, linear regression, and machine learning

Constructed wetlands (CWs) are a widely utilized nature-based wastewater treatment method for various effluents. However, their application has been more focused on pilot and full-scale CWs with substantial surface areas and extended operation times, which hold greater relevance in practical scenarios. This study used kinetics, linear regression (LR), and machine learning (ML) models to estimate effluent ammonium in pilot and full-scale CWs. From screening 1476 papers, 24 pilot and full-scale CW studies were selected to extract data containing 15 features and 975 data points. Nine models were fit to this data, revealing that linear models were less effective in capturing CW effluent compared to nonlinear ML algorithms. For training data, the Monod kinetic model predicted the poorest performance with an RMSE of 41.84 mg/L and R2 of 0.34, followed by simple LR (RMSE 24.29 mg/L and R2 0.77) and multiple LR (RMSE 22.63 mg/L and R2 0.80). In contrast, Cubist and Random Forest achieved high performances, with an average RMSE of 12.01 ± 5.38 and an average R2 of 0.93 ± 0.07 for Cubist, and an average RMSE of 15.94 ± 10.69 and an average R2 of 0.91 ± 0.08 for RF. The trained Random Forest performed the best for new data, with an R2 of 0.93 and RMSE of 13.48 mg/L. This ML-based model is a valuable tool for efficiently estimating effluent ammonium concentration in pilot and full-scale CWs, thereby facilitating the design of systems.

Highly efficient nitrate removal in sulfur-based constructed wetlands: Microbial mechanisms and environmental risks

Nitrate is widely distributed in groundwater, posing an increasing threat to both water resources and human health. In this study, the treatment performance, removal mechanisms and environmental risks of sulfur-based constructed wetlands (CWs) for purifying nitrate-contaminated groundwater were investigated. Results showed that sulfur-based CWs could achieve the highest nitrate removal (95%). However, sulfate was largely produced as a by-product in sulfur-based CWs, which declined the nitrogen and phosphorus assimilation by plants. Metagenomic analysis indicated that autotrophs denitrifiers (e.g., Thiobacillus) were enriched, and the abundance of nitrate removal genes was enhanced in sulfur-based CWs. Additionally, sulfur cycle was formed in sulfur-based CWs, which explained the highest nitrate removal reasonably. This study provides comprehensive insights into the nitrate removal mechanisms in sulfur-based CWs and the associated environmental risks in purifying the polluted groundwater.

Biodegradation and sorption of nutrients and endocrine disruptors in a novel concrete-based substrate in vertical-flow constructed wetlands

Removing phosphorus and endocrine-disruptors (EDC) is still challenging for low-cost sewage treatment systems. This study investigated the efficiency of three vertical-flow constructed wetlands (VFCW) vegetated with Eichhornia crassipes onto red clay (CW-RC), autoclaved aerated concrete (CW-AC), and composite from the chemical activation of autoclaved aerated concrete with white cement (CW-AAC) in the removal of organic matter, nutrients, and estrone, 17β-estradiol, and 17α-ethinylestradiol. The novelty aspect of this study is related to selecting these clay and cementitious-based materials in removing endocrine disruptors and nutrients in VFCW. The subsurface VFCW were operated in sequencing-batch mode (cycles of 48-48-72 h), treating synthetic wastewater for 308 days. The operation consisted of Stages I and II, different by adding EDC in Stage II. The presence of EDC increased the competition for dissolved oxygen (DO) and reduced the active sites available for adsorption, diminishing the removal efficiencies of TKN and TAN and total phosphorus in the systems. CW-RC showed a significant increase in COD removal from 65% to 91%, while CW-AC and CW-AAC maintained stable COD removal (84%-82% and 78%-81%, respectively). Overall, the substrates proved effective in removing EDC, with CW-AC and CW-AAC achieving >60% of removal. Bacteria Candidatus Brocadia and Candidatus Jettenia, responsible for carrying out the Anammox process, were identified in assessing the microbial community structure. According to the mass balance analysis, adsorption is the main mechanism for removing TP in CW-AC and CW-AAC, while other losses were predominant in CW-RC. Conversely, for TN removal, the adsorption is more representative in CW-RC, and the different metabolic routes of microorganisms, biofilm assimilation, and partial ammonia volatilization in CW-AC and CW-AAC. The results suggest that the composite AAC is the most suitable material for enhancing the simultaneous removal of organic matter, nutrients, and EDC in VFCW under the evaluated operational conditions.

Autotrophic denitrification based on sulfur-iron minerals: advanced wastewater treatment technology with simultaneous nitrogen and phosphorus removal

Autotrophic denitrification technology has many advantages, including no external carbon source addition, low sludge production, high operating cost efficiency, prevention of secondary sewage pollution, and stable treatment efficiency. At present, the main research on autotrophic denitrification electron donors mainly includes sulfur, iron, and hydrogen. In these autotrophic denitrification systems, pyrite has received attention due to its advantages of easy availability of raw materials, low cost, and pH stability. When pyrite is used as a substrate for autotropic denitrification, sulfide (S2-) and ferrous ion (Fe2+) in the substrate will provide electrons to convert nitrate (NO3-) in sewage first to nitrite (NO2-), then to nitrogen (N2), and finally to discharge the system. At the same time, sulfide (S2-) loses electrons to sulfate (SO42-) and ferrous ion (Fe2+) loses electrons to ferric iron (Fe3+). Phosphates (PO43-) in wastewater are chemically combined with ferric iron (Fe3+) to form ferric phosphate (FePO4) precipitate. This paper aims to provide a detailed and comprehensive overview of the dynamic changes of nitrogen (N), phosphorus (P), and other substances in the process of sulfur autotrophic denitrification using iron sulfide, and to summarize the factors that affect wastewater treatment in the system. This work will provide a relevant research direction and theoretical basis for the field of sulfur autotrophic denitrification, especially for the related experiments of the reaction conversion of various substances in the system.

Long-term performance of a deep oxidation pond with horizontal subsurface flow constructed wetland for purification of rural polluted river water

A full-scale deep oxidation pond with horizontal subsurface flow constructed wetland (DOP-HSCWs) was constructed and used to investigate the nutrient removal and establish a practical inversion prediction model. The high long-term performances of nearly 7 years were obtained with the average removal efficiencies of 76.48 ± 10.11% (chemical oxygen demand, COD), 60.61 ± 29.21% (ammonia nitrogen, NH4+-N), 54.04 ± 21.92% (total phosphorus, TP) and 88.44 ± 6.86% (suspended solids, SS), respectively. The removal efficiency actually increased after 2016 with outflow concentrations lower as compared to initial phase of operation. The effluent concentration in autumn were obviously higher than that in other seasons because of high influent loadings. The Flaml model achieved good performance demonstrating the ability to predict water quality of DOP-HSCWs without human intervention. In addition, COD, NH4+-N, TP concentration of effluent can be significantly affected by SS concentration of influent according to the generalized additive model (p < 0.001). Compared with HSCWs, the DOPs was mainly contributed to pollutant removal. In summer, Cyanobacteria, Cyanobacteria and Proteobacteria were dominated in DOPs, while Proteobacteria was dominated in winter. Although the relative abundance of Proteobacteria in anaerobic zone decreased by 14.99%, the relative abundance of Firmicutes and Chloroflexi increased by nearly 10%, which ensured decontamination effect of the DOPs. Proteobacteria was also dominated in HSCWs, but it was lower than that in DOPs. This study indicated that DOP-HSCWs can achieve a sustainably excellent purification of rural polluted river water during the long period of operation.

Effect of ibuprofen (IBU) on the sulfur-based and calcined pyrite-based autotrophic denitrification (SCPAD) systems with two filling modes: Performance and toxic response mechanism

To investigate the effect of ibuprofen (IBU) on the sulfur-based and calcined pyrite-based autotrophic denitrification (SCPAD) systems, two individual reactors with the layered filling (L-SCPAD) and mixed filling (M-SCPAD) systems were established via sulfur and calcined pyrite. Effluent NO3--N concentration of the L-SCPAD and M-SCPAD systems was first increased to 6.44, 0.93 mg/L under 0.5 mg/L IBU exposure and gradually decreased to 1.66 mg/L, 0 mg/L under 4.0 mg/L IBU exposure, indicating that NO3--N removal performance of the M-SCPAD system was better than that of the L-SCPAD system. The variation of extracellular polymeric substances (EPS) characteristics demonstrated that more EPS was secreted in the M-SCPAD system compared to the L-SCPAD system, which contributed to forming a more stable biofilm structure and protecting microorganisms against the toxicity of IBU in the M-SCPAD system. Moreover, the increased electron transfer impedance and decreased cytochrome c implied that IBU inhibited the electron transfer efficiency of the L-SCPAD and M-SCPAD systems. The decreased adenosine triphosphate (ATP) and electron transfer system activity (ETSA) content showed that IBU inhibited metabolic activity, but the M-SCPAD system exhibited higher metabolic activity compared to the L-SCPAD system. In addition, the analysis of the bacterial community indicated a more stable abundance of nitrogen removal function bacteria (Bacillus) in the M-SCPAD system compared to the L-SCPAD system, which was conducive to maintaining a stable denitrification performance. The toxic response mechanism based on the biogeobattery effect was proposed in the SCPAD systems under IBU exposure. This study provided an important reference for the long-term toxic effect of IBU on the SCPAD systems.

Enhanced treatment of low C/N ratio rural sewage by a modified multi-stage tidal flow constructed wetland at low temperature: Quantitative contributions of key functional genera

Rural sewage treatment was traditionally faced contradiction between low-treatment rates and the need for low-cost development. To address this challenge, we explored the coupling of effluent circulation and step-feeding strategies in a multi-stage tidal flow constructed wetland (TFCW) to achieve stable nitrogen (N) removal performance under conditions of low carbon-to-nitrogen (C/N) ratios and low temperatures. The modified multi-stage TFCW demonstrated the ability to significantly reduce the concentrations of effluent NH4+-N and NO3--N by 33.9 % and 54.8 % respectively, resulting in values of 7.47 mg/L and 3.93 mg/L. Additionally, it achieved an average TN removal efficiency of 69.2 %. The improved N removal performance of rural sewage by the modified multi-stage TFCW at low temperatures was primarily attributed to autotrophic nitrification, heterotrophic nitrification, and autotrophic denitrification. Among the identified functional genera, Nitrosomonas and Nitrosospira played key roles as autotrophic nitrification bacteria (ANB), contributing to 28.2 % of NH4+-N removal. The key heterotrophic nitrification bacteria (HNB) Acidovorax and Rudaea were mainly responsible for 71.3 % of NH4+-N removal via the two-step ammonia assimilation through the organic nitrogen pathway. Furthermore, Rhodanobacter and Acinetobacter emerged as key autotrophic denitrification bacteria (ADNB), accounting for 79.9 % of NO3--N conversion and removal. In summary, this study provides valuable theoretical insights and supports ongoing efforts in biological regulation to address the challenges associated with rural sewage treatment.

Highly efficient phosphorous removal in constructed wetland with iron scrap: Insights into the microbial removal mechanism

Excessive phosphorus (P) in surface water can lead to serious eutrophication and economic losses. Iron-based constructed wetland (CW) is considered as a promising solution to eliminate P effectively due to the advantage of low-cost. However, there is limited available information on the microbial removal mechanism of P in iron-based CW up to now. Therefore, CW with iron scrap was constructed to investigate the treatment performance and microbial removal mechanism in this study. Results showed that efficient and stable P removal (97.09 ± 1.90%) was achieved in iron scrap-based CW during the experiment period, which was attributed to the precipitation of iron and P and improved microbially mediated P removal. Metagenomic analysis showed that microbial diversity was enhanced and phosphate accumulating organisms (e.g., Dechloromonas and Tetrasphaera) were enriched in CW with iron scrap, which explained higher P removal reasonably. In addition, the abundance of genes involved in the P starvation (e.g., phoB), uptake and transport (e.g., pstB) were enhanced in iron scrap-based CW. Enrichment analysis demonstrated that phosphotransferase pathway was also significantly up-regulated in CW with iron scraps, indicating that the energy supply of microbial P removal was enhanced. These findings provide a better understanding of the microbial removal mechanism of P in iron-based CW.

Effect mechanism of micron-scale zero-valent iron enhanced pyrite-driven denitrification biofilter for nitrogen and phosphorus removal

This study aims to explore the effect mechanism of micron-scale zero-valent iron (mZVI) to improve nitrogen and phosphorus removal in a pyrite (FeS2)-driven denitrification biofilter (DNBF) for the secondary effluent treatment. Two similar DNBFs (DNBF-A with FeS2 as fillers and DNBF-B with the mixture mZVI and FeS2 as carrier) were developed. The results showed that NO3--N, total nitrogen (TN) and PO43--P removal efficiencies were up to 91.64%, 67.44% and 80.26% in DNBF-B, which were obviously higher than those of DNBF-A (with NO3--N, TN and PO43--P removal efficiencies of 38.39%, 44.89% and 53.02%, respectively). Kinetic analysis of both PO43--P and NO3--N showed an increase in the rate constant (K) for DNBF-B compared to DNBF-A. The addition of mZVI not only improved the electron transport system activity (ETSA), but also achieved system Fe(II)/Fe(III) redox cycle in DNBF-B. In addition, the high-throughput sequencing analysis indicated that the addition of mZVI could obviously stimulate the enrichment of functional bacteria, such as Thiobacillus (11.99%), Mesotoga (7.50%), JGI-0000079D21 (6.37%), norank_f__Bacteroidetes_vadinHA17 (6.19%), Aquimonas (5.93%) and Arenimonas (3.97%). These genus played the important role in nitrogen and phosphorus removal in DNBF-B. Addition mZVI in the FeS2-driven denitrification biofilter is highly promising for TN and TP removal during secondary effluent treatment.

The enrichment of more functional microbes induced by the increasing hydraulic retention time accounts for the increment of autotrophic denitrification performance

With pyrite (FeS2) and polycaprolactone (PCL) as electron donors, three denitrification systems, namely FeS2-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system and split-mixotrophic denitrification (PPMD) system, were constructed and operated under varying hydraulic retention times (HRT, 1-48 h). Compared with PAD or PHD, the PPMD system could achieve higher removals of NO3--N and PO43--P, and the effluent SO42- concentration was greatly reduced to 7.28 mg/L. Similarly, the abundance of the dominant genera involved in the PAD (Thiobacillus, Sulfurimonas, and Ferritrophicum, etc.) or PHD (Syntrophomonas, Desulfomicrobium, and Desulfovibrio, etc.) process all increased in the PPMD system. Gene prediction completed by PICRUSt2 showed that the abundance of the functional genes involved in denitrification and sulfur oxidation all increased with the increase of HRT. This also accounted for the increased contribution of autotrophic denitrification to total nitrogen removal in the PPMD system. In addition, the analysis of metabolic pathways disclosed the specific conversion mechanisms of nitrogen and sulfur inside the reactor.

Highly Efficient Coremoval of Nitrate and Phosphate Driven by a Sulfur-Siderite Composite Reactive Filler toward Secondary Effluent Polishing

Reactive fillers consisting of reduced sulfur and iron species (SFe-ReFs) have received increasing attention in tertiary wastewater treatment for nitrate and phosphate coremoval. However, the existing SFe-ReFs suffer from either low performance (e.g., pyrrhotite and pyrite) or unsatisfactory use in terms of combustible risk and residual nonreactive impurities (e.g., sulfur mixing with natural iron ores). Here, we developed a new type of sulfur-siderite composite ReF (SSCReF) with a structure of natural siderite powders eventually embedded into sulfur. SSCReFs exhibited many excellent properties, including higher mechanical strengths and hardness and especially much poorer ignitability compared to pure sulfur. By using SSCReF to construct packed-bed reactors, the highest denitrification and dephosphorization rates reached 829.70 gN/m3/d (25 wt % siderite) and 36.70 gP/m3/d (75 wt % siderite), respectively. Dephosphorization was demonstrated to be dependent on sulfur-driven denitrification, in which the acid produced from the later process promoted Fe(II) dissolution, which then directly combined with phosphate to form vivianite or further converted into phosphate adsorbents (ferrihydrite, a green rust-like compound). Water flush was an effective way to finally wash out these surface deposited Fe-P compounds, as well as those nonreactive impurities (Si and Al-bearing compounds) detached from SSCReF. Such a highly efficient and safe SSCReF holds considerable application potential in secondary effluent polishing.

Study on the removal effect and mechanism of calcined pyrite powder on Cr(VI)

Pyrite exhibits considerable potential as an adsorbent in wastewater treatment. However, few pyrite adsorbents are directly obtained from natural pyrite, as most are composite materials that require a complex preparation process. To develop a pyrite-based adsorbent with a simple preparation process, pyrite was processed by calcination at 400, 600, and 800 °C for 4 h and ball-milled into a fine powder. The adsorption properties of the pyrite powder were systematically explored. The calcined pyrite powder was characterized by SEM-EDS and XRD. The results revealed that the pyrite calcined at 600 °C exhibited excellent adsorption properties and was primarily composed of Fe7S8. The optimum conditions for Cr(VI) removal were a temperature of 45 °C, an adsorbent dosage of 1 g, an equilibration time of 60 min, and an initial pH of 3. Moreover, the calcined pyrite powder exhibited excellent reusability, and the Cr(VI) removal rate exceeded 65% after three cycles. The Cr(VI) adsorption on pyrite can be well described by the Freundlich model and pseudo-second-order kinetic equation. The calcination temperature is the main factor affecting the adsorption performance of pyrite. Therefore, the calcined pyrite powder is expected to be an excellent adsorbent for Cr(VI) in the wastewater treatment industry.

Treatment of high-phosphorus load wastewater by column packed with non-burning compound filler/gravel/ceramsite: evaluation of performance and microorganism community

Cost-effective and environmental-friendly substrates are essential for the constructed wetlands (CWs). In this study, the column test was used to explore the differences in pollutant purification performance, microbial community structure and abundance between non-burning compound filler and conventional CWs substrates (i.e. gravel and ceramsite) at low temperature (0-15℃). It was found that the maximum phosphorus removal efficiency of compound filler (99%) was better than gravel (18%) and ceramsite (21%). Besides, the proportion of aerobic heterotrophic bacteria capable of ammonium oxidation, nitrification and denitrification (i.e. Pseudomonas, Acinetobacter, and Acetoanaerobium) was enhanced by compound filler, which has an excellent potential for nitrogen removal in the subsequent purification process. These results demonstrated that the self-made non-burning compound filler was a potential substrate for CWs, which was of great significance for the resource utilization of solid wastes such as polyaluminum chloride residue.

Autotrophic denitrification supported by sphalerite and oyster shells: Chemical and microbiome analysis

This research evaluated the metal-sulfide mineral, sphalerite, as an electron donor for autotrophic denitrification, with and without oyster shells (OS). Batch reactors containing sphalerite simultaneously removed NO₃^- and PO₄^3- from groundwater. OS addition minimized NO₂^- accumulation and removed 100% PO₄^3- in approximately half the time compared with sphalerite alone. Further investigation using domestic wastewater revealed that sphalerite and OS removed NO₃^- at a rate of 0.76 ± 0.36 mg NO₃^--N/(L · d), while maintaining consistent PO₄^3- removal (∼97%) over 140 days. Increasing the sphalerite and OS dose did not improve the denitrification rate. 16S rRNA amplicon sequencing indicated that sulfur-oxidizing species of Chromatiales, Burkholderiales, and Thiobacillus played a role in N removal during sphalerite autotrophic denitrification. This study provides a comprehensive understanding of N removal during sphalerite autotrophic denitrification, which was previously unknown. Knowledge from this work could be used to develop novel technologies for addressing nutrient pollution.

Synergetic effects of pyrrhotite and biochar on simultaneous removal of nitrate and phosphate in autotrophic denitrification system

In the trend of upgrading wastewater treatment plants, developing advanced treatment technologies for more efficient nutrient removal is crucial. This study prepared a pyrrhotite-biochar composite (Fe_x S_y @BC) to investigate its potential for simultaneous removal of nitrate and phosphate under autotrophic denitrification conditions. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the novel composite of Fe_x S_y @BC, which exhibited 9.2 mg N/(L·d) NO₃ ^- -N reduction rate, 97.3% N₂ production, and 81.8 mmol N/(kg·d) NO₃ ^- -N material load with small solid/liquid ratio (0.008). The NO₃ ^- -N removal with Fe_x S_y @BC was 1.2-2.2 times higher than that with pure iron sulfides or biochar or their mixtures, whereas the Δn(S)/Δn(N) of Fe_x S_y @BC was the lowest (1.80). Moreover, the PO₄ ^3- -P reduction rate of Fe_x S_y @BC reached 3.23 mg P/(L·d), as high as that of pure pyrite or pyrrhotite. Thiobacillus was the most dominant denitrifying bacterium. Fe_x S_y @BC exhibited great promise for enhancing nutrient removal from secondary effluent without additional carbon source. PRACTITIONER POINTS: FexSy@BC enhanced nitrate and phosphate removal simultaneously. First-order kinetics and Monod model were fitted for denitrification with FexSy@BC. FexSy@BC had smaller molar ratio of sulfate release to nitrate removal. Thiobacillus was the dominant bacterium in FexSy@BC autotrophic denitrification. Synergistic effects on nutrients removal existed between biochar and pyrrhotite.

Reducing the inhibitive effect of fluorine and heavy metals on nitrate reduction by hydroxyapatite substrate in constructed wetlands

Bio-toxic inorganic pollutants, e.g., fluorine (F) and heavy metals (HMs), in wastewaters are the potential threats to nitrate (NO₃^--N) reduction by microorganisms in constructed wetlands (CWs). Selection of suitable substrate with high F and HMs adsorption efficiency and capacity is a potential alternative for simultaneous removal of these pollutants in CWs. Herein, this study investigated the feasibility of applying hydroxyapatite (HA)-gravel media for F and HMs adsorption and its effect on NO₃^--N reduction in CWs (HA CWs) by comparing the CWs filled with gravel substrate (CK CWs). The results indicated that the removal efficiency of F, Cr, As, and NO₃^--N in HA CWs increased by 113.6-, 3.3-, 2.7-, and 0.6-folds, respectively, compared to CK CWs. The NO₃^--N reduction rate decreased by 11-46% in CK CWs after the presence of F and HMs in influent, while for HA CWs, it was only 13-22%. Excellent F and HMs adsorption capacity of HA substrate availed for wetland plants resisting F/HMs toxicity and making catalase activity lower. The HA substrate in CWs resulted in the certain succession of nitrogen-transforming bacteria, e.g., nitrifiers (Nitrospira) and denitrifiers (Thiobacillus and Desulfobacterium). More importantly, key functional genes, including nirK/nirS, korA/korB, ChrA/ChrD, arsA/arsB, catalyzing the processes of nitrogen biotransformation, energy metabolism, NO₃^--N and metal ions reduction were also enriched in HA CWs. This study highlights HA substrate reduce the inhibitive effect of F and HMs on NO₃^--N reduction, and provides new insights into how microbiota structurally and functionally respond to different substrates in CWs.

Characteristic of KMnO4-modified corn straw biochar and its application in constructed wetland to treat city tail water

Biochar prepared from straw as constructed wetland (CW) substrate reduces straw pollution and simultaneously promotes the wastewater treatment efficiency of CW. In order to further analyze the pollutant removal mechanism of KMnO₄-modified biochar substrate, the KMnO₄-modified biochar was characterized. The experiment on city tail water treatment by CW with biochar was analyzed. The research showed that the surface property improvement on KMnO₄ (0.1 mol/L)-modified biochar was the most obvious. The biochar modified by 0.1 mol/L KMnO₄ increased the SSA and the number of oxygen functional groups and alcohol hydroxyl. KMnO₄-modified biochar improved the removal efficiency of NO₃^--N in CW. KMnO₄-modified biochar substrate with plants improved the TP removal efficiency (about 45%). KMnO₄ as modifier reduced the influence of biochar on electrical conductivity tracing experiment. This study will improve the utilization value of straw and the removal efficiency of CW.

Enhancement and mechanisms of micron-pyrite driven autotrophic denitrification with different pretreatments for treating organic-limited waters

Pyrite-driven autotrophic denitrification (PAD) represents a cheap and promising way for nitrogen removal from organic-limited wastewater, which has obtained increasing attention in recent years. However, the limited denitrification rate and unclear mechanism underlying the process have hindered the engineered application of PAD. This study aims to shed light on the impacts of different pretreatments (i.e., ultrasonication, acid-washing and calcination) on micron-pyrite surface characteristics, denitrification performance and biofilm formation during PAD in batch reactors. A series of solid-phase analyses revealed that all pretreatments could significantly promote biofilm attachment on pyrite granules, but impacted the proportion, distribution and chemical oxidation state of sulfur (S) and iron (Fe) at varying degrees. Batch tests showed that ultrasonication and acid-washing could enhance the total nitrogen reduction rate by 14% and 99%, and decrease the sulfate production rate by 51% and 42%, respectively, when compared with untreated pyrite. Microbial community analysis indicated that Thiobacillus and Rhodanobacter dominated in PAD systems. Two types of indirect mechanisms (i.e., contact and non-contact) for pyrite leaching may co-occur in PAD system, resulting in ferrous iron (Fe²⁺), thiosulfate (S₂O₃^2-) and sulfide (S^2-) as the main electron donors for denitrification. A PAD mechanism model was proposed to describe the PAD electron transfer pathway with the aim to optimize the engineered application of PAD for nitrogen removal.

New insights on simultaneous nitrate and phosphorus removal in pyrite-involved mixotrophic denitrification biofilter for a long-term operation: Performance change and its underlying mechanism

Simultaneous nitrate and phosphorus removal can be completed by pyrite- and influent organics-involved mixotrophic denitrification and chemical phosphorus removal via iron precipitation. However, so far, how their removal performances change with iron precipitation accumulation remains unclear. In this study, the differences in nitrate and phosphorus removal from municipal tailwater between volcanic and pyrite supported biofilters (V-BF, P-BF) for a long-term operation were investigated, as well as the underlying mechanism for these differences. The nitrate removal efficiencies (NREs) in P-BF were greater than those in V-BF due to the synergistic effect of influent organic and pyrite, as evidenced by comparable TOC consumption and Fe²⁺/SO₄^2- production. The NREs in P-BF were gradually lower than in V-BF as a result of bacterial cell-iron encrustation observed in TEM images, which would deteriorate microbial activity. However, the phosphorus removal efficiencies (PREs) in P-BF remained consistently higher than in V-BF, resulting from chemical phosphorus removal which was confirmed that P, Fe and O elements dominated on the pyrite surface after use by SEM-EDS. The dominant denitrifying bacteria differed significantly, autotrophic and heterotrophic denitrifying microorganisms coexisted in P-BF. The relative abundances of the narG coding gene in P-BF were higher than that in V-BF, which was consistent with the total relative abundances of identified denitrifying bacteria. Besides, the mechanism of simultaneous nitrogen and phosphorus removal in the pyrite-involved mixotrophic denitrification process has been deduced. This work has significant implications for the practical application of a pyrite-involved mixotrophic denitrification process for low C/N wastewater treatment.

Synchronous N and P Removal in Carbon-Coated Nanoscale Zerovalent Iron Autotrophic Denitrification─The Synergy of the Carbon Shell and P Removal

Fe⁰ is a promising electron donor for autotrophic denitrification in the simultaneous removal of nitrate and phosphorus in low C/N wastewater. However, P removal may inevitably inhibit bio-denitrification. It has not been well recognized and led to an overdose of iron materials. This study employed carbon-coated zerovalent iron (Fe⁰@C) to support autotrophic denitrification to mitigate the inhibition effects of P removal and enhance both N and P removal. The critical role of the carbon shell in Fe⁰@C was to block the direct contact between Fe⁰ and P and NO₃^--N, to maintain the Fe⁰ activity. Besides, P inhibited the chemical reduction of NO₃^--N by competing for Fe⁰ active sites. This indirectly boosted H₂ generation and promoted bio-denitrification. P removal displayed negligible effects on microbial species but indirectly enhanced the nitrogen metabolic activities because of promoted H₂ in Fe⁰@C-based autotrophic denitrification. Bio-denitrification, in turn, strengthened Fe-P co-precipitation by promoting the formation of ferric hydroxide as a secondary adsorbent for P removal. This study demonstrated an efficient method for simultaneous N and P removal in autotrophic denitrification and revealed the synergistic interactions among N and P removal processes.

Removing nutrients from wastewater by constructed wetlands under perfluoroalkyl acids stress

Constructed wetlands (CWs) are often used to treat wastewater discharged from wastewater treatment plants (WWTPs), while emerging contaminants (such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS)) have been commonly discovered in WWTPs. However, no research has examined whether PFOA/OS (i.e. PFOA and PFOS) affects the performance of CW. Therefore, this study compared the nutrient removal efficiencies of four CWs with varied configurations under PFOA/OS and no PFOA/OS stress conditions. We found that CW containing plants or/and iron-carbon had higher removal efficiency for nutrients (except NH₄⁺-N) than conventional CW in stable operation under wastewater without PFOA/OS. Plants or/and iron increased the nutrient removal efficiency by plant uptake, chemical reaction, and co-precipitation of iron hydroxides. In contrast, the iron-carbon inhibited the nitrification of nitrifying bacteria by consuming dissolved oxygen, converting NO₃^--N to NH₄⁺-N. Although the removal efficiencies of nutrients by CWs differed after introducing PFOA/OS, the removal order was consistent with those before adding PFOA/OS. Plants or/and iron-carbon effectively increased CWs' resistance to PFOA/OS loading and toxicity, and the function of iron-carbon was superior to the plants. In addition, PFOA/OS reduced the abundances of microbes Hydrogenophaga, Pseudomonas, Sphingomonas, Nitrospira, and Candidatus_Accumulibacter that contributed to nutrient removal.

The phylogeny, ecology and ecophysiology of the glycogen accumulating organism (GAO) Defluviicoccus in wastewater treatment plants

This comprehensive review looks critically what is known about members of the genus Defluviicoccus, an example of a glycogen accumulating organism (GAO), in wastewater treatment plants, but found also in other habitats. It considers the operating conditions thought to affect its performance in activated sludge plants designed to remove phosphorus microbiologically, including the still controversial view that it competes with the polyphosphate accumulating bacterium Ca. Accumulibacter for readily biodegradable substrates in the anaerobic zone receiving the influent raw sewage. It looks at its present phylogeny and what is known about it's physiology and biochemistry under the highly selective conditions of these plants, where the biomass is recycled continuously through alternative anaerobic (feed); aerobic (famine) conditions encountered there. The impact of whole genome sequence data, which have revealed considerable intra- and interclade genotypic diversity, on our understanding of its in situ behaviour is also addressed. Particular attention is paid to the problems in much of the literature data based on clone library and next generation DNA sequencing data, where Defluviicoccus identification is restricted to genus level only. Equally problematic, in many publications no attempt has been made to distinguish between Defluviicoccus and the other known GAO, especially Ca. Competibacter, which, as shown here, has a very different ecophysiology. The impact this has had and continues to have on our understanding of members of this genus is discussed, as is the present controversy over its taxonomy. It also suggests where research should be directed to answer some of the important research questions raised in this review.

Investigation into pyrite autotrophic denitrification with different mineral properties

Pyrite autotrophic denitrification (PAD) is considered a promising method for nitrate removal from wastewater and groundwater. However, the results of PAD studies have been contradictory for two decades, and the mechanism is unclear. Here, we investigated mineral properties of two kinds of natural pyrite (YP and TP), their PAD performances, and microbial community shift in their column reactors in parallel. Both pyrite are highly pure crystalline pyrite, but their other mineral properties are quite different. Both batch and column experiments found that PAD of YP occurred but that of TP did not. Thus, the contradictory results of PAD were presented for the first time at the same study. The dominant bacteria in YP and TP columns finally were Thiobacillus (24.55±8.67%) and Flavobacterium (21.11±10.59%), respectively, though their initial microbial communities cultured were similar. Reduced sulfur species and oxide impurities on the surface of pyrite, and small DO in water did not change autotrophic denitrification characteristic of the pyrite itself. This research indicates that mineral property of pyrite caused the contradictory result of PAD. Among pyrite properties, the main crystal plane exposed and chemical state of surficial sulfur and iron were considered the decisive parameters for PAD. The study provides guidelines for selection of pyrite minerals for PAD applications.

Study of the effect of pyrite and alkali-modified rice husk substrates on enhancing nitrogen and phosphorus removals in constructed wetlands

The combined effects and respective advantages of using pyrite and alkali-modified rice husk (RH) were studied as substrates for nitrogen and phosphorus removal from constructed wetlands, and the effects of the carbon to nitrogen (C/N) ratio and the tidal flow mode on system performance were explored. The results showed that alkali-modified RH, which enhances heterotrophic denitrification, had far more advantages than pyrite, which enhances autotrophic denitrification, and alkali-modified RH can be used for the treatment of sewage containing low C/N ratios. At a C/N ratio of 1.5, the total nitrogen (TN) removal rates exceeded 95%. However, the removal efficiency of the system with only pyrite only reached 76.90% when the influent C/N ratio was 6. Pyrite achieved a total phosphorus (TP) removal 10-20% higher than that of the control group. The tidal flow CWs showed enhanced nitrification, and the NH₄⁺-N removal rates increased by approximately 10%, but the increase in dissolved oxygen (DO) was still insufficient to meet the needs of the systems, leading to limited TP removal. The combination of pyrite and alkali-modified RH was optimal for improving the ability of constructed wetlands to treat wastewaters, simultaneously removing nitrogen and phosphorus from sewage containing low C/N ratios. Combined with the tidal flow mode strategy, the use of pyrite and alkali-modified RH as substrates showed substantial advantages for improving water quality.

Simultaneous heterotrophic and FeS2-based ferrous autotrophic denitrification process for low-C/N ratio wastewater treatment: Nitrate removal performance and microbial community analysis

Heterotrophic-autotrophic denitrification reduces the cost of wastewater treatment and the risk of excess chemical oxygen demanded (COD) in the effluent. A mixotrophic denitrification system involving mixed heterotrophic and ferrous autotrophic bacteria was investigated to treat low-C/N ratio (C/N, defined as chemical oxygen demand (COD)/total nitrogen (TN)) wastewater with pyrite and organic carbon as electron donors. The system yielded effluent total nitrogen (TN) of 0.38 mg/L in 48 h due to a synergistic effect when the C/N ratio was 0.5 and influent nitrate nitrogen (NO₃^--N) was 20 mg/L; this TN value was significantly lower than those of the heterotrophic system (14.08 mg/L) and ferrous autotrophic system (12.00 mg/L). The highest abundance of the narG gene was observed in the mixotrophic denitrification system, along with more abundant microbial species. The dominant denitrification bacteria in each system included Thaurea, Ferritrophicum, Pseudomonas, and Thiobacillus, which varied with the initial inoculum source and the environment. Nevertheless, the abundance of the heterotrophic bacteria Thaurea decreased with prolonged operation of the systems. Together, these results implied that the simultaneous heterotrophic and FeS₂-based ferrous autotrophic denitrification process can be an alternative approach for the treatment of low-C/N ratio wastewater.

Synergistic ammonia and nitrate removal in a novel pyrite-driven autotrophic denitrification biofilter

Pyrite is one kind of cost-effective electron donors for nitrate denitrification. In this study, a pyrite-driven autotrophic denitrification biofilter was applied for simultaneous removal of NH₄⁺ and NO₃^- over the 150-day. The influent NH₄⁺/NO₃^- ratio (0.3-1.7) had less effect on system performance, while for the hydraulic retention times (HRTs, 24-3 h), the removal percentage of both > 90% and removal loading rates of 52.8 and 59.4 mg N/(L·d) for NH₄⁺ and NO₃^- removal were obtained at the HRT of 6 h. The 16S rRNA genes analysis showed that Ferritrophicum, Thiobacillus, Candidatus_Brocadia, and unidentified_Nitrospiraceae were predominant. Analyses of nitrogen and sulfur metabolism showed that ammonia was removed by complete nitrification, nitrate was reduced to N₂, and sulfide was oxidized to sulfate. Dynamics of pollutants within the reactor and microbial activity showed nitrification/Anammox and pyrite-driven autotrophic denitrification were responsible for the synergistic removal of NH₄⁺/NO₃^- in this system.

Formation of anaerobic granular sludge (AnGS) to treat high-strength perchlorate wastewater via anaerobic baffled reactor (ABR) system: Electron transfer characteristic, bacterial community and positive feedback mechanism

Anaerobic granular sludge (AnGS) was cultured to treat high-strength perchlorate (reaching to 4800 mg/L) wastewater by an anaerobic baffled reactor (ABR) system with five equal-volume compartments (C1-C5 compartments). Inoculated sludge completely granulated on day 104 with granule size of 0.50-0.75 mm and perchlorate removal efficiency reaching to 97% (influent perchlorate of 2000-4800 mg/L). The Cyclic voltammetry (CV) capacitance increased from 487.5, 465.8 and 407.8 μF to 576.5, 552.4, 549.6 μF in C1, C3 and C5 compartments of ABR system, respectively, suggesting the electron transfer capacity was enhanced under high-strength perchlorate stress. Meanwhile, adenosine triphosphate (ATP) value and electron transport system activity (ETSA) increased to 25.05, 22.87, 20.43 and 6.22, 4.87, 3.95 of C1, C3 and C5 compartments, respectively. The results suggested that high-strength perchlorate stress improved the microbial metabolic activity, which promoted secretion of extracellular polymeric substances (EPS). The more EPS could facilitate the formation and stability of AnGS under high-strength perchlorate stress. In addition, more reasonable metabolic division of labor in functional bacterial (Thauera and Comamonas) was beneficial to AnGS formation, which achieved high-strength perchlorate efficient removal. Finally, a positive feedback mechanism between AnGS formation and high-strength perchlorate removal was established through EPS, microbial metabolic activity and electron transfer characteristic in ABR system. However, excessive perchlorate (5800 mg/L) would exceed the treatment capacity of AnGS, which resulted in the deterioration of removal performance. This work provided an effective information for AnGS application to treat high-strength perchlorate wastewater.

Simultaneous biological removal of nitrogen and phosphorus from secondary effluent of wastewater treatment plants by advanced treatment: A review

With the advancement of water ecological protection and water control standard, it is the general trend to upgrade the wastewater treatment plants (WWTPs). The simultaneous removal of nitrogen and phosphorus is the key to improve the water quality of secondary effluent of WWTPs to prevent the eutrophication. Therefore, it is urgent to develop the applicable technologies for simultaneous biological removal of nitrogen and phosphorus from secondary effluent. In this review, the composition of secondary effluent from municipal WWTPs were briefly introduced firstly, then the three main treatment processes for simultaneous nitrogen and phosphorus removal, i.e., the enhanced denitrifying phosphorus removal filter, the pyrite-based autotrophic denitrification and the microalgae biological treatment system were summarized, their performances and mechanisms were analyzed. The influencing factors and microbial community structure were discussed. The advanced removal of nitrogen and phosphorus by different technologies were also compared and summarized in terms of performance, operational characteristics, disadvantage and cost. Finally, the challenges and future prospects of simultaneous removal of nitrogen and phosphorus technologies for secondary effluent were proposed. This review will deepen to understand the principles and applications of the advanced removal of nitrogen and phosphorus and provide some valuable information for upgrading the treatment process of WWTPs.

Nitrogen and Phosphorus Removal Efficiency and Denitrification Kinetics of Different Substrates in Constructed Wetland

Constructed wetlands (CWs) are generally used for wastewater treatment and removing nitrogen and phosphorus. However, the treatment efficiency of CWs is limited due to the poor performance of various substrates. To find appropriate substrates of CWs for micro-polluted water treatment, zeolite, quartz sand, bio-ceramsite, porous filter, and palygorskite self-assembled composite material (PSM) were used as filtering media to treat slightly polluted water with the aid of autotrophic denitrifying bacteria. PSM exhibited the most remarkable nitrogen and phosphorus removal performance among these substrates. The average removal efficiencies of ammonia nitrogen, total nitrogen, and total phosphorus of PSM were 66.4%, 58.1%, and 85%, respectively. First-order continuous stirred-tank reactor (first-order-CSTR) and Monod continuous stirred-tank reactor (Monod-CSTR) models were established to investigate the kinetic behavior of denitrification nitrogen removal processes using different substrates. Monod-CSTR model was proven to be an accurate model that could simulate nitrate nitrogen removal performance in vertical flow constructed wetland (VFCWs). Moreover, PSM demonstrated significant pollutant removal capacity with the kinetics coefficient of 2.0021 g/m2 d. Hence, PSM can be considered as a promising new type of substrate for micro-polluted wastewater treatment, and Monod-CSTR model can be employed to simulate denitrification processes.

Performance of tidal and non-tidal mangrove constructed wetlands in treating maricultural wastewater

This study aims to evaluate the removal efficiency of nitrogen and phosphorus in the tidal and non-tidal constructed wetlands with typical mangrove (Aegiceras corniculatum) as a wetland plant model to treat simulated marine wastewater. The results showed that the average removals of NO₂^--N, NO₃^--N, NH₄⁺-N, TN and TP were 88.4, 80.5, 81.4, 79.7 and 40.8%, respectively, in the non-tidal subsurface flow (HF) mangrove wetland, and 65.3, 61.3, 90.6, 60.1 and 19.2% in the tidal (TF) mangrove wetland, and 11.4, 64.6, 68.7, 56.6 and 16.3% in the non-tidal free water surface (FWS) mangrove wetland, respectively. Moreover, it was observed that the composition of microbial communities in the HF mangrove wetland was beneficial to the nitrogen cycle and has more quantitative associations of N-metabolism genes. The results indicated that non-tidal HF mangrove wetland has a stable and an effective capacity for potential treatment of marine wastewater compared with the non-tidal FWS mangrove wetland and tidal TF mangrove wetland.

Optimize the Preparation of Novel Pyrite Tailings Based Non-sintered Ceramsite by Plackett-Burman Design Combined With Response Surface Method for Phosphorus Removal

The large amount of untreated pyrite tailings has caused serious environmental problems, and the recycling of pyrite tailings is considered as an attractive strategy. Here, we reported a novel non-sintered ceramsite prepared with pyrite tailings (PTNC) as the main active raw material for phosphorus control, and the dosage effect of ingredients on total phosphorus (TP) removal ability was investigated. The results from Plackett-Burman Design (PBD) suggested the dosages of dehydrated sludge, sodium bicarbonate, and cement were the factors which significantly affect the TP removal ability. The Box-Behnken Design (BBD) based response surface methodology was further employed, and it indicated the interactions between different factors, and the optimized recipe for PTNC was 84.5 g (pyrite tailings), 10 g (cement), 1 g (calcined lime), 1 g (anhydrous gypsum), 3 g (dehydrated sludge), and 0.5 g (sodium bicarbonate). The optimized PTNC was characterized and which presented much higher specific area (7.21 m²/g) than the standard limitation (0.5 m²/g), as well as a lower wear rate (2.08%) rather than 6%. Additionally, the leaching metal concentrations of PTNC were far below the limitation of Chinese National Standard. The adsorption behavior of TP on PTNC was subsequently investigated with batch and dynamic experiments. It was found that the calculated max adsorption amount (q_max) was about 7 mg/g, and PTNC was able to offer a stable TP removal ability under different hydraulic retention time (HRT). The adsorption mechanism was discussed by model fitting analysis combined with XRD and SEM characterization, and cobalt phosphide sulfide was observed as the newly formed substance through the adsorption process, which suggested the existing of both physical and chemical adsorption effect. Our research not only offered an economic preparation method of ceramsite, but also broadened the recycling pathway of pyrite tailings.

Effects and mechanisms of constructed wetlands with different substrates on N2O emission in wastewater treatment

Nitrous oxide (N₂O) emissions from constructed wetlands (CWs) are accompanying problems and have attracted much attention in recent years. CWs filled with different substrates (gravel, biochar, zeolite, and pyrite) were constructed to investigate the nitrogen removal performance and N₂O emissions, which named C-CWs, B-CWs, Z-CWs, and P-CWs, respectively. C-CWs showed the poorest nitrogen removal performance in all CWs. Although B-CWs exhibited the highest fluxes of N₂O emissions, the percentage of N₂O emissions in nitrogen removal (0.15%) was smaller than that of C-CWs (0.18%). In addition, microbiological analysis showed that compared with C-CWs, CWs filled with biochar, zeolite, and pyrite had higher abundance of nitrifying and denitrifying microorganisms and lower abundance of N₂O producing bacteria. In conclusion, biochar, zeolite, and pyrite were more favorable kinds of substrate than the conventional substrates of gravel for the nitrogen removal and reduction of N₂O emissions from CWs.

Vertical-flow constructed wetland based on pyrite intensification: Mixotrophic denitrification performance and mechanism

Deep nitrogen removal from low-carbon wastewater is a pressing water treatment challenge as of yet. Eight sets of vertical-flow constructed wetland (VFCW) intensified by pyrite were designed and applied to treat with low C/N ratio wastewater in this research. The results showed that the addition of pyrite (100% added) significantly promoted TN removal with an efficiency higher than 27.05% under low C/N ratio conditions, indicating that mixotrophic denitrification was achieved in VFCW. Microbial analysis showed that the community structure and diversity of microorganisms were changed significantly, and the growth of autotrophic (Thiobacillus) and heterotrophic bacteria (Thauera) concomitantly enhanced. It is recommended that the addition amount of pyrite is 75% of the wetland volume, meantime, mixing evenly with 25% high porosity substrate (such as activated carbon, volcanic stone, etc.), which could enhance the effective adhesion of microorganisms and their contact area with pyrite, ultimately improve the denitrification capacity of the VFCW.

Can we use mine waste as substrate in constructed wetlands to intensify nutrient removal? A critical assessment of key removal mechanisms and long-term environmental risks

The utilization of natural ores and/or mine waste as substrate in constructed wetlands (CWs) to enhance nutrient removal performance has been gaining high popularity recently. However, the knowledge regarding the long-term feasibility and key removal mechanisms, particularly the potential negative environmental effects of contaminants leached from mine waste is far insufficient. This study, for the first time, performed a critical assessment by using different CWs with three mine waste (coal gangue, iron ore and manganese ore) as substrates in a 385-day experiment treating wastewater with varying nutrient loadings. The results showed that the addition of mine waste in CWs increased removal of total phosphorus (TP) by 17-34%, and total nitrogen (TN) by 11-51%. The higher removal of TP is mainly attributed to the strong binding mechanism of phosphate with the oxides and hydroxides of Mn, Fe and/or Al, which are leached out of mine waste. Moreover, integration of mine waste in CWs also significantly stimulated biofilm establishment and enriched the relative abundance of key functional genes related to the nitrogen cycle, supporting the observed high-rate nitrogen removal. However, leaching of heavy metals (Fe, Mn, Cu and Cr) from the beded mine waste in the experimented CWs was monitored, which further influenced cytoplasmic enzymes and created oxidative stress damage to plants, resulting in a decline of nutrient uptake by plants.

A Novel Constructed Wetland Combined with Microbial Desalination Cells and its Application

Wastewater recycling can alleviate the shortage of water resources. Saline water is seldom treated with biological processes, and its recycling rate is low. Constructed wetland (CW) is a safe, economical, and ecological water treatment method. However, the saline water treatment performance of CW is not good. Microbial desalination cells (MDC) utilizing a bioelectrochemical approach achieve functions of desalination and power generation. In this study, MDC was used to strengthen CW to form a composite system, MDC-CW. Through optimization of design parameters, MDC-CW was applied in the treatment of salt-containing water. The average total nitrogen removal rate in MDC-CW-P1 reached 87.33% and the average COD removal rate was 92.79%. The average desalination rate of MDC-CW-P1 was 55.78% and the average voltage of MDC-CW-P1 reached 0.40 mV. Planting Canna indica in the MDC-CW was conducive to the functions of desalination and power generation. The above results were also verified by the microbial analysis results of gravels in the substrate, plant rhizosphere, and electrodes. In addition, the decontamination of the device mainly depended on the function of the bacteria commonly used in water treatment, such as Proteobacteria and Bacteroidetes, whereas the generation of power depended on the function of Geobacter. Salt ions moved spontaneously to the cathode and anode under the influence of current generation so that the desalination function was realized under the selective isolation function of exchange membranes. The device design and laboratory applications of MDC-CW experimentally achieved the electrochemical function and broadened the treatment scale of CW.