Cold Temperature Effects on Long-Term Nitrogen Transformation Pathway in a Tidal Flow Constructed Wetland

Elevation-associated pathways mediate aquatic biodiversity at multi-trophic levels along a plateau inland river

Aquatic biodiversity plays a significant role in maintaining the ecological balance and the overall health of riverine ecosystems. Elevation is an important factor influencing biodiversity patterns. However, it is still unclear through which pathway elevation influences riverine biodiversity at different trophic levels. In this study, the elevation-associated pathways affecting aquatic biodiversity at different trophic levels were explored using structural equation modeling (SEM) and taking the Bayin River, China as the case. The results showed that the elevational patterns were different among aquatic organisms at different trophic levels. For macroinvertebrates and bacteria, the pattern was hump-shaped; while for phytoplankton and zooplankton, it was U-shaped. Building upon these observed elevational patterns, our investigation delved into the direct and indirect pathways through which elevation influences aquatic biodiversity. We found that elevation exerts an impact on aquatic biodiversity via indirect pathways. For all aquatic organisms investigated, the major pathway through which elevation influences biodiversity is mediated by water temperature and water quality. For aquatic organisms at higher trophic levels, like macroinvertebrates and zooplankton, the crucial pathway is also mediated by the landscape. The results of this study contributed to understanding the effects of elevation on aquatic organisms at different trophic levels and provided an important basis for the assessment of riverine biodiversity at large scales.

Impacts of vegetable processing and cheese making effluent on soil microbial functional diversity, community structure, and denitrification potential of land treatment systems

The cheese making and vegetable processing industries generate immense volumes of high-nitrogen wastewater that is often treated at rural facilities using land applications. Laboratory incubation results showed denitrification decreased with temperature in industry facility soils but remained high in soils from agricultural sites (75% at 2.1°C). 16S rRNA, phospholipid fatty acid (PLFA), and soil respiration analyses were conducted to investigate potential soil microbiome impacts. Biotic and abiotic system factor correlations showed no clear patterns explaining the divergent denitrification rates. In all three soil types at the phylum level, Actinobacteria, Proteobacteria, and Acidobacteria dominated, whereas at the class level, Nitrososphaeria and Alphaproteobacteria dominated, similar to denitrifying systems such as wetlands, wastewater resource recovery facilities, and wastewater-irrigated agricultural systems. Results show that potential denitrification drivers vary but lay the foundation to develop a better understanding of the key factors regulating denitrification in land application systems and protect local groundwater supplies. PRACTITIONER POINTS: Incubation study denitrification rates decreased as temperatures decreased, potentially leading to groundwater contamination issues during colder months. The three most dominant phyla for all systems are Actinobacteria, Proteobacteria, and Acidobacteria. The dominant class for all systems is Nitrosphaeria (phyla Crenarchaeota). No correlation patterns between denitrification rates and system biotic and abiotic factors were observed that explained system efficiency differences.

Microbial denitrification characteristics of typical decentralized wastewater treatment processes based on 16S rRNA sequencing

Despite the widespread application of decentralized wastewater treatment (WWT) facilities in China, relatively few research has used the multi-media biological filter (MMBF) facilities to investigate the microorganism characteristics. This study utilizes 16S rRNA high-throughput sequencing (HTS) technology to examine the microbial biodiversity of a representative wastewater treatment (WWT) system in an expressway service area. The pathways of nitrogen removal along the treatment route were analyzed in conjunction with water quality monitoring. The distribution and composition of microbial flora in the samples were examined, and the dominant flora were identified using LEfSe analysis. The FAPROTAX methodology was employed to investigate the relative abundance of genes associated with the nitrogen cycle and to discern the presence of functional genes involved in nitrogen metabolism. On average, the method has a high level of efficiency in removing COD, TN, NH3-N, and TP from the effluent. The analysis of the microbial community identified a total of 40 phyla, 111 classes, 143 orders, 263 families, and 419 genera. The phyla that were predominantly observed include Proteobacteria, Acidobacteria, Chloroflexi, Actinobacteria, Nitrospirae, Bacteroidetes. The results show that the system has achieved high performance in nitrogen removal, the abundance of nitrification genes is significantly higher than that of other nitrogen cycle genes such as denitrification, and there are six nitrogen metabolism pathways, primarily nitrification, among which Nitrospirae and Nitrospira are the core differentiated flora that can adapt to low temperature conditions and participate in nitrification, and are the dominant nitrogen removal flora in cold regions. This work aims to comprehensively investigate the diversity and functional properties of the bacterial community in decentralized WWT processes.

Microbial community structure analyses and cultivable denitrifier isolation of Myriophyllum aquaticum constructed wetland under low C/N ratio

With the rapid expansion of livestock production, the amount of livestock wastewater accumulated rapidly. Lack of biodegradable organic matter makes denitrification of livestock wastewater after anaerobic digestion more difficult. In this study, Myriophyllum aquaticum constructed wetlands (CWs) with efficient nitrogen removal performance were established under different carbon/nitrogen (C/N) ratios. Analysis of community composition reveals the change of M. aquaticum CWs in microbial community structure with C/N ratios. The proportion of Proteobacteria which is one of the dominant phyla among denitrifier communities increased significantly under low C/N ratio conditions. Besides, to obtain cultivable denitrifier that could be added into CWs in situ, 33 strains belonging to phylum Proteobacteria were isolated from efficient M. aquaticum CWs, while the best-performing denitrification strain M3-1 was identified as Bacillus velezensis JT3-1 (GenBank No. CP032506.1). Redundancy analysis and quadratic models showed that C/N ratio had significant effects on disposal of nitrate (NO₃^--N) and the strains isolated could perform well in denitrification when C/N ratio is relatively low. In addition, they have relatively wide ranges of carbon sources, temperature and a high NO₃^- removal rate of 9.12 mg/(L·hr) at elevated concentrations of 800 mg/L nitrate. Thus, strains isolated from M. aquaticum CWs with low C/N ratio have a practical application value in the treatment of nitrate-containing wastewater. These denitrifying bacteria could be added to CWs to enhance nitrogen removal efficiency of CWs for livestock wastewater with low C/N ratio in the future.

Exploring and comparing the impacts of low temperature to endogenous and exogenous partial denitrification: The nitrite supply, transcription mechanism, and microbial dynamics

Nitrite supply was pretty significant to exogenous or endogenous partial denitrification (ExPD or EdPD) for their combination with anammox in removing nitrogen. This study investigated how temperature impacted the nitrite supply of ExPD and EdPD, through long-term experiments in two 10 L sequencing batch reactors (SBRs) and 12 batch temperature tests, with sodium acetate as organic. It was demonstrated that low temperature (5-15 °C) favored higher nitrite transformation rate (NTR) for two systems (1.1-1.3 and 1.1-1.2 times higher separately), and ExPD owned higher nitrite-supply ability than EdPD (32.8 % higher NTR). Moreover, quantitative reverse transcription PCR and 16srDNA sequencing were conducted, exploring the inherent mechanism and microbial dynamics. Results presented that more inhibition to transcription and translation of nirSK genes than narG in low temperature induced higher NTR. Besides, compared with ExPD, less microbial dynamics and granule size reduction occurred to EdPD, which was more capable of adapting to low temperature.

Effect of ferric iron (Fe(Ш)) on heterotrophic solid-phase denitrification: Denitrification performance and metabolic pathway

The effect of ferric iron (Fe(Ш)) on the performance of heterotrophic solid-phase denitrification (SPD) using biodegradable polymer composite as the electron donor was investigated. The results of continuous batch experiments showed that the addition of over 10 mg/L Fe(Ш) significantly inhibited nitrate removal and led to the accumulation of nitrite. The addition of Fe(Ш) reduced the microbial community diversity and shifted the community dominated by complete denitrifiers (e.g. Thauera) to that dominated by incomplete denitrifiers (e.g. Thermomonas, Stenotrophomonas and Sphingomonas). The predicted analysis of microbial function by PICRUSt2 indicated that the relative abundance of denitrifying genes, including napA/B, nirS and nosZ, were remarkably reduced in the Fe(Ш) groups comparing with the control group. In addition, Fe(Ш) inhibited the genes related to the generation of electron carriers, NADH and FADH₂, in TCA cycle and glycolysis processes, which could result in a lower carbon utilization efficiency for microbial denitrification.

Effect of low temperature on contributions of ammonia oxidizing archaea and bacteria to nitrous oxide in constructed wetlands

Constructed wetlands (CWs) have been widely used for ecological remediation of micro-polluted source water. Nitrous oxide (N₂O) from CWs has caused great concern as a greenhouse gas. However, the contribution of ammonia oxidation driven by ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) to N₂O emission, especially at low temperature, was unknown. This study aimed to quantify the contributions of AOA and AOB to N₂O through lab-scale subsurface CWs. The N₂O emission flux of CW at 8 °C was 1.23 mg m^-2·h^-1, significantly lower than that at 25 °C (1.92 mg m^-2·h^-1). The contribution of ammonia oxidation to N₂O at 8 °C (33.04%) was significantly higher than that at 25 °C (24.17%). The N₂O production from AOA increased from 1.91 ng N·g^-1 at 25 °C to 4.11 ng N·g^-1 soil at 8 °C and its contribution increased from 23.38% to 30.18% (P < 0.05). Low temperature impaired functional gene groups and inhibited the activity of AOB, resulting in its declined contribution. Based on the transcriptional analysis, AOA was less affected by low temperature, thus stably contributing to N₂O. Moreover, community diversity and relationships of AOA were enhanced at 8 °C, while AOB declined. The results confirmed the significant contribution of AOA and demonstrated molecular mechanisms (higher activity and community stability) of the increased contribution of AOA to N₂O at low temperature.

Fate and toxicity of triclosan in tidal flow constructed wetlands amended with cow dung biochar

Triclosan (TC) is one of the threats to the environment due to its bioaccumulative nature, persistency, combined toxicity in aquatic biota, and endocrine-disrupting nature. This study revealed the removal of TC via three distinct setups of vertical flow constructed wetlands (VFCW: B-VFCW (with biochar); PB-VFCW (with plant Colocasia and biochar); C-VFCW (without biochar but with plant)) operated with normal flow and tidal-flow (flooding/drying cycles of 72 h/24 h: B-TFCW; PB-TFCW; C-TFCW) mode for 216 h of the operation cycle. The effluent was analyzed for changes in TC load and wastewater parameters (COD, NO₃-N, NH₄⁺-N, and DO). TC reduction efficiency (%) was found to be higher in PB-TFCW (98.41) followed by, C-TFCW (82.41), B-TFCW (77.51), PB-VFCW (71.83), C-VFCW (64.25), and B-VFCW (52.19) (p < 0.001). Reduction efficiency for COD (29-75 - 53.10%), and NH₄⁺-N (86.5-97.9%) was better in TFCWs than that of setups with a normal mode of operation. TFCWs showed higher DO (3.87-4.89 mg L^-1) during the operation period than that of VFCWs. The toxic impact of TC in plant stand was also assessed and results suggested low phototoxic and oxidative enzyme activities (catalase, CAT; superoxide dismutase, SOD; hydrogen peroxide, H₂O₂; malondialdehyde, MDA) in TFCWs. In summary, biochar addition and tidal flow operation played a significant role in oxidative- and microbial-mediated removals of TC in wastewater. This study provides an alternative strategy for the efficient removals of TC in constructed wetland systems and new insights into the toxic impact of pharmaceuticals on wetland plants.

New insight for purifying polluted river water using the combination of large-scale rotating biological contactors and integrated constructed wetlands in the cold season

Ecological treatment technologies, applied to deal with polluted river water in the low temperature season, remain limited. In this study, a new insight was put forward for purifying polluted river water using a combination system (CS) of large-scale rotating biological contactors (RBCs) and integrated constructed wetlands in autumn and winter. The treatment performance, average removal contribution (RC), nitrification and denitrification rates, microbial community structure, and ecosystem service value were considered to estimate the combination system. Results revealed that the average removal efficiencies of ammonium (NH₄⁺-N), total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) reached 93.9%, 20.8%, 36.5%, and 37.1%, respectively. The combination system showed excellent removal efficiency of NH₄⁺-N regardless of the effect of low temperature. The maximum values of nitrification and denitrification rates were 59.57 g N/(m³·d) and 0.78 g N/(m²·d), respectively. Considerable differences in bacterial community diversity, richness and relative abundance of functional microbes were observed in the main treatment units, resulting in different average RC to pollutants. The unit capital cost of CS purifying polluted river water was 260 USD/m³ and the operation and maintenance cost was 0.144 million USD/yr. Meanwhile, the ecosystem service value of the CS was 0.334 million USD in autumn and winter. CS not only possessed excellent pollutant purifying efficiencies, but also achieved high ecological service value in the cold season.

Evidence of temperature-controlled dissolved inorganic nitrogen distribution in a shallow lake

Dissolved inorganic nitrogen (DIN) plays an important role in aquatic ecosystems as an available source of nitrogen (N). Despite recent advances in our understanding of the effects of climate change on DIN in coastal waters, shallow high-latitude lakes exposed to large seasonal temperature differences have received limited research attention. Therefore, in the present study, Baiyangdian Lake (BYDL) was selected as the study area, as a typical high latitude shallow lake in North China. Based on water and sediment samples collected in spring, summer and winter seasons, DIN accumulation in sedimentary pore water and DIN diffusion fluxes at the sediment-water interface were quantified under different temperature conditions. Correlation analysis was used to establish the effects of temperature on DIN concentration and diffusion in different media. Results show that the diffusion of DIN at the lake sediment-water interface exhibited a strongly positive relationship with temperature, suggesting that high temperature conditions lead to greater DIN release from sediments. Cold temperatures cause DIN accumulation in sedimentary pore water, providing sufficient substrate for N-related bacteria in the sediment under cold temperature conditions. Temperature controls the vertical distribution of DIN by affecting its migratory diffusion and transformation at the sediment-water interface. These findings are valuable for understanding the impact of climate change on the distribution of N in inland shallow lakes, especially in high latitude shallow lakes subjected to large seasonal temperature differences throughout the year.

Effect of steel slag on ammonia removal and ammonia-oxidizing microorganisms in zeolite-based tidal flow constructed wetlands

The ammonia removal performance of tidal flow constructed wetlands (TFCWs) requires to be improved under high hydraulic loading rates (HLRs). The pH decrease caused by nitrification may adversely affect the NH₄⁺-N removal and ammonia-oxidizing microorganisms (AOMs) of TFCWs. Herein, TFCWs with zeolite (TFCW_Z) and a mixture of zeolite and steel slag (TFCW_S) were built to investigate the influence of steel slag on NH₄⁺-N removal and AOMs. Both TFCWs were operated under short flooding/drying (F/D) cycles and high HLRs (3.13 and 4.69 m³/(m² d)). The results revealed that a neutral effluent pH (6.98-7.82) was achieved in TFCW_S owing to the CaO dissolution of steel slag. The NH₄⁺-N removal efficiencies in TFCW_S (91.2 ± 5.1%) were much higher than those in TFCW_Z (73.2 ± 7.1%). Total nitrogen (TN) removal was poor in both TFCWs mainly due to the low influent COD/TN. Phosphorus removal in TFCW_S was unsatisfactory because of the short hydraulic retention time. The addition of steel slag stimulated the flourishing AOMs, including Nitrosomonas (ammonia-oxidizing bacteria, AOB), Candidatus_Nitrocosmicus (ammonia-oxidizing archaea, AOA), and comammox Nitrospira, which may be responsible for the better ammonia removal performance in TFCW_S. PICRUSt2 showed that steel slag also enriched the relative abundance of functional genes involved in nitrification (amoCAB, hao, and nxrAB) but inhibited genes related to denitrification (nirK, norB, and nosZ). Quantitative polymerase chain reaction (qPCR) revealed that complete AOB (CAOB) and AOB contributed more to the amoA genes in TFCW_S and TFCW_Z, respectively. Therefore, this study revealed that the dominant AOMs could be significantly changed in zeolite-based TFCW by adding steel slag to regulate the pH in situ, resulting in a more efficient NH₄⁺-N removal performance.

Fertilizing-induced changes in the nitrifying microbiota associated with soil nitrification and crop yield

Ammonia oxidizing archaea (AOA) and bacteria (AOB), nitrite-oxidizing bacteria (NOB), and comammox Nitrospira (CMX) play pivotal roles in global nitrogen-cycling network. Despite its importance, the driving forces for niche specialization of these nitrifiers, as well as their relative contributions to nitrification and crop yield have not been fully understood. Here, we investigated the niche specialization and environmental prevalence of nitrifying communities, and their importance for the nitrification rate and crop yield across a gradient of nitrogen inputs in a two-decade old field experiment. The results of ¹⁵N-tracer and quantitative PCR revealed that AOB and NOB jointly determined the gross nitrification rates across mineral fertilizer treatments, whereas AOA and AOB contributed more than other nitrifiers to nitrification under with organic fertilizer amendments. Linear regression model revealed that crop yield could be linked with AOB and NOB under inorganic farming but closely associated with CMX under organic management. Amplicon sequencing of these functional genes further demonstrated that mineral and organic fertilizers have distinct influences on the β-diversity and niche breadth of these nitrifying communities, indicating that fertilization triggered niche specialization of nitrifying guilds in agricultural soils. Notably, organic fertilization enhanced the network complexity of these nitrifiers by harboring keystone taxa. Random forest analysis provide robustly evidence for the hypothesis that abundance of functional genes contributed more than a- and β-diversity of these nitrifiers for driving nitrification rates and crop yields. Collectively, these findings provide the empirical evidence for the environmental adaptation and niche specialization of nitrifying communities, and their contributions in nitrification and crop yield when confronted with long-term nitrogen inputs.

Application of thifluzamide alters microbial network structure and affects methane cycle genes in rice-paddy soil

Thifluzamide is an effective agent for controlling rice sheath blight and has a long half-life in soil. However, the effects of thifluzamide on the abundance of microbes harboring methane-cycle genes and soil microbial community assembly patterns are not well known. Thus, we conducted a three-month indoor mesocosm experiment to ascertain the effects of thifluzamide (0.05, 0.5, and 5 mg kg^-1 soil; 0.05 mg kg^-1 soil being recommended) on bacterial and archaeal community structure and on the abundance of methanogen genes using two typical paddy soils: sandy soil from Hangzhou (HZ) and loam sandy soil from Jiansanjiang (JSJ). The effects of thifluzamide on soil microorganisms were related to soil type. In JSJ loam sandy soil, thifluzamide significantly increased bacterial α diversity after 7-30 d and archaeal α diversity at 30 and 60 d. In HZ sandy soil, however, α diversity did not change significantly. Network analysis showed that thifluzamide-treated soils possessed more complex networks with more total nodes and links, a higher average degree of connectivity, and more keystone species. Thifluzamide application increased the number of keystone species associated with methane production in both types of paddy soil. A relatively greater number of modules were significantly negatively correlated with mcrA abundance in the HZ T10 network, but more modules were positively correlated with mcrA abundance in the JSJ T100 network. The half-life of thifluzamide varied for the different doses, i.e., from 152.0 to 419.6 d. The results reveal that methane-cycle genes, soil microbiome assembly, and interactions among microbial species all change in response to thifluzamide stress.

Climate Change Impacts on Urban Sanitation: A Systematic Review and Failure Mode Analysis

Climate change will stress urban sanitation systems. Although urban sanitation uses various infrastructure types and service systems, current research appears skewed toward a small subset of cases. We conducted a systematic literature review to critically appraise the evidence for climate change impacts on all urban sanitation system types. We included road-based transport networks, an essential part of fecal sludge management systems. We combined the evidence on climate change impacts with the existing knowledge about modes of urban sanitation failures. We found a predominance of studies that assess climate impacts on centralized sewerage in high-income contexts. The implications of climate change for urban nonsewered and complex, fragmented, and (partially) decentralized sanitation systems remain under-researched. In addition, the understanding of the impacts of climate change on urban sanitation systems fails to take a comprehensive citywide perspective considering interdependencies with other sectors and combinations of climate effects. We conclude that the evidence for climate change impacts on urban sanitation systems is weak. To date, research neither adequately represents the variety of urban sanitation infrastructure and service systems nor reflects the operational and management challenges of already stressed systems.

Cold temperature mediated nitrate removal pathways in electrolysis-assisted constructed wetland systems under different influent C/N ratios and anode materials

Electrolysis had proven to be useful for the enhanced performance in constructed wetlands (CWs). While at cold temperature, the nitrate removal pathways, plant physiological characteristics and microbial community structure in electrolysis-assisted CWs were unclear. Therefore, the purification performance of three electrolysis-assisted horizontal subsurface-flow constructed wetlands (E-HSCWs) with different anodes and a control system in cold seasons were evaluated in this study. E-HSCWs showed a 2.02-83.21% increase of total nitrogen (TN) removal when compared to control, and the gaps were enlarged with increasing C/N (chemical oxygen demand/total nitrogen, COD/TN) ratios. Nitrite accumulation in E-HSCWs presented a first increase then went down trend with increasing C/N ratios, compared to a steady increase in control system. The optimum C/N ratio was 8 in E-HSCWs for both TN and COD removal. Moreover, Ti|IrO₂-Ta₂O₅ (Ti) anode showed the highest potential for TN and COD removal. Less root weight, shorter root length and reduced TN and total phosphorus (TP) contents in roots were observed in wetland plants (Iris sibirica) of E-HSCWs. In E-HSCWs with Fe and C anodes, the nitrate removal was mainly accomplished by autotrophic denitrifier Hydrogenophaga. While in E-HSCWs with Ti anode, the synergistic effect of autotrophic denitrifier Hydrogenophaga and heterotrophic denitrifiers Acidovorax, Simplicispira, Zoogloea accounted for the nitrate removal. These results showed that E-HSCWs at proper C/N ratio of 8 would be promising for nitrate removal at cold temperature.

Functional genera for efficient nitrogen removal under low C/N ratio influent at low temperatures in a two-stage tidal flow constructed wetland

A two-stage tidal flow constructed wetland (referred to as TFCW-A and TFCW-B) was used to treat low chemical oxygen demand/total nitrogen (COD/TN or simply C/N) ratio influent at low temperatures (<15 °C). The influence of the flooding-resting time (A: 8 h-4 h, B: 4 h-8 h) and effluent recirculation on nitrogen removal and microbial community characteristics were explored. TFCW-B achieved optimal average nitrogen removal efficiency with effluent recirculation (96.05% ammonium nitrogen (NH₄⁺-N); 78.43% TN) and led to nitrate nitrogen (NO₃^--N) accumulation due to the lack of a carbon source and longer resting time. Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were inhibited at low temperatures. Except for nrfA, AOA, AOB, narG and nirS were separated by the flooding-resting time rather than by spatial position. Furthermore, the dominant genera in TFCW-A were Arthrobacter, Rhodobacter, Pseudomonas, and Solitalea, whereas prolonging resting time promoted the growth of Thauera and Zoogloea in TFCW-B. Spearman correlation analysis showed that Zoogloea and Rhodobacter had the strongest correlations with other genera. Moreover, the NH₄⁺-N concentration was significantly positively influenced by Arthrobacter, Rhodobacter, Pseudomonas, and Solitalea but negatively influenced by Thauera and Zoogloea. There was no significant correlation between TN and the dominant genera. This study not only provides a practicable system for wastewater treatment with a low C/N ratio but also presents a theoretical basis for the regulation of microbial communities in nitrogen removal systems at low temperatures.

Response of constructed wetland for wastewater treatment to graphene oxide: Perspectives on plant and microbe

The wide application of graphene oxide (GO) increases its release into environment with less known on environmental effects. This work investigated 120-day interaction between GO (500 and 5000 μg/L) and constructed wetlands (CWs) planted with Iris pseudacorus. CWs showed the effective retention for GO via mature biofilm but less biodegradation. GO significantly induced enzyme activities (urease, neutral phosphatase, and catalase), which was attributed to increases in ecological association and enzyme abundance. GO decreased microbial biomass on day 30, but it had no impacts on day 120. The microbial community showed gradual self-adaption with time due to protection of antioxidant defense system (L-ascorbate oxidase, superoxide reductase, and glutathione related enzyme). The antioxidant enzymes (superoxide dismutase and peroxidase) and lipid peroxidation of Iris pseudacorus were increased by GO, accompanied by reduction on chlorophyll biosynthesis. Overall, the separate effects of GO on micro-regions and individual bodies in CWs were obvious, but it was acceptable that variations in pollutant removal were not evident due to synergetic role of plant-substrate-microbe. Organic matter and phosphorus removals reached to above 93%, and ammonia and total nitrogen removals in GO groups were reduced by 7-8% and 9-13%, respectively.

Nitrogen removal from summer to winter in a field pilot-scale multistage constructed wetland-pond system

A pilot-scale multistage constructed wetland-pond (MCWP) system with a "pre-ecological oxidation pond, two-stage horizontal subsurface flow constructed wetland (HSCW) and surface flow constructed wetland (SFCW) as the core and postsubmerged plant pond" as the process was used to treat actual polluted river water in the field, and the variation in nitrogen removal from summer to winter was investigated. The results showed that the average total nitrogen (TN) removal efficiency in the MCWP was approximately 40.74%. The significant positive correlation between the daily highest temperature and the TN removal efficiency of the whole system was fitted with a nonlinear curve (R² = 0.7192). The TN removal load rate in the HSCWs was 2.7-3.7 times that in the SFCW. The SFCW, which had high-density plants (35 plants/m²), increased the proportion of nitrogen removed by plant harvesting and microbial function. The TN transformed by Iris pseudacorus L. accounted for 54.53% in the SFCW. Furthermore, bacteria completed the nitrogen cycle in the SFCW through a variety of nitrogen removal pathways. This research not only investigated the TN removal performance in an MCWP system but also made it possible to predict the TN removal efficiency according to the daily highest temperature from summer to winter in the field.

Inhibition of ferrous iron (Fe2+) to sulfur-driven autotrophic denitrification: Insight into microbial community and functional genes

The effect of Fe²⁺ on the performance of sulfur-driven autotrophic denitrification (SDAD) using S⁰ as electron donor was evaluated. The experimental results showed that as initial Fe²⁺ concentration increased, nitrate (NO₃^-) removal rate significantly decreased. Fe²⁺ ion (0.1 mM and 1 Mm) inhibited SDAD rate (approximately 10% and 50%) and resulted in an accumulation of nitrite (NO₂^-) and nitrous oxide (N₂O). The relative abundance of Thiobacillus was positively correlated with NO₃^- removal rate, whereas negatively correlated with Fe²⁺ concentration, suggesting that Fe²⁺ inhibited the sulfur-oxidizing denitrifying bacteria. Moreover, the abundance of bacterial 16S rRNA, denitrifying genes (narG, nirS, nirK and nosZ) and sulfur-oxidizing genes (soxB and dsrA) decreased with the increase of Fe²⁺ concentration, among them nosZ and soxB were the most sensitive genes to Fe²⁺, and nosZ/narG, soxB/(bacterial 16S rRNA) and soxB/nirK had influence on NO₃^- removal rate, while nosZ/(bacterial 16S rRNA) affected N₂O accumulation rate.

Various electron donors for biological nitrate removal: A review

Nitrate (NO₃^-) pollution in water and wastewater has become a serious global issue. Biological denitrification, which reduces NO₃^- to N₂ (nitrogen gas) by denitrifying microorganisms, is an efficient and economical process for the removal of NO₃^- from water and wastewater. During the denitrification process, electron donor is required to provide electrons for reduction of NO₃^-. A variety of electron donors, including organic and inorganic compounds, can be used for denitrification. This paper reviews the state of the art of various electron donors used for biological denitrification. Depending on the types of electron donors, denitrification can be classified into heterotrophic and autotrophic denitrification. Heterotrophic denitrification utilizes organic compounds as electron donors, including low-molecular-weight organics (e.g. acetate, methanol, glucose, benzene, methane, etc.) and high-molecular-weight organics (e.g. cellulose, polylactic acid, polycaprolactone, etc.); while autotrophic denitrification utilizes inorganic compounds as electron donors, including hydrogen (H₂), reduced sulfur compounds (e.g. sulfide, element sulfur and thiosulfate), ferrous iron (Fe²⁺), iron sulfides (e.g. FeS, Fe_1-xS and FeS₂), arsenite (As(Ш)) and manganese (Mn(II)). The biological denitrification processes and the representative denitrifying microorganisms are summarized based on different electron donors, and their denitrification performance, operating costs and environmental impacts are compared and discussed. The pilot- or full-scale applications were summarized. The concluding remarks and future prospects were provided. The biodegradable polymers mediated heterotrophic denitrification, as well as H₂ and sulfur mediated autotrophic denitrification are promising denitrification processes for NO₃^- removal from various types of water and wastewater.

Creating tidal flow via siphon for better pollutants removal in a microbial fuel cell-constructed wetland

Oxygen is the electron acceptor in cathode chamber of microbial fuel cell-constructed wetland system (MFC-CW). The objective of the study lies in creating a "tidal flow" (TF) in cathode chamber via a siphon to enhance the oxygen diffusion, thus promoting the system performance. A laboratory scale MFC-CW with a siphon driven TF recirculation was proposed and designed. It allows the variable water level being created in four operational modes. The results demonstrated the significance of the siphon which was reflected by the attractive wastewater treatment performance. Compared with the tested four operational modes under the same hydraulic condition, the highest total nitrogen removal efficiency of 96.32% and COD removal efficiency of 92.37% were achieved, respectively, in 1st full siphon recirculation mode (FSR) and 2nd FSR operation mode. Indeed, the water level variation range played an important role in pollutants removal performance. Reduced water level variation of the TF in cathode chamber hindered excessive oxygen diffusion into MFC-CW and thus adversely affected the system performance. It is clear that the siphon is a wiser input to bring about the better treatment performance, but it is believed that the enhanced microbial activities behind the oxygen promotion is the driven force to exhibiting a better performance in the MFC-CW system.

How to enhance the purification performance of traditional floating treatment wetlands (FTWs) at low temperatures: Strengthening strategies

Pollution of freshwaters poses a major threat to water quality and human health and thus, nutrients have been targeted for mitigation. One such control measure is floating treatment wetlands (FTWs), which are designed to employ vigorous macrophytes above the water surface and extensive plant root system below the water surface to increase plant uptake of nutrients. The efficacy of FTWs in purifying different water systems has been widely studied and reviewed, but most studies have been performed in warm periods when FTW macrophytes are actively growing. In low-temperature conditions, the metabolic processes of macrophytes and microbial activity are usually weakened or reduced by the winter months and are not actively assimilating pollutants. These circumstances hamper the purification ability of FTWs to perform as designed. Furthermore, decayed macrophytes could release pollutants into the water column. Hence, this paper aimed to systematically summarize strategies for use of enhanced FTWs in eutrophic water improvement at low temperature and identify future directions to be addressed in intensifying FTW performance in low-temperature conditions. Low-temperature FTW show variable nutrient removal efficiencies ranging from 22% to 98%. Current amendments to enhance FTW purification performance, ranging from direct strategies for internal components to indirect enhancement of external operation environments encourage the FTW efficacy to some extent. However, the sustainability and sufficiency of water purification efficiency remain a great challenge. Keeping in mind the need for optimizing the FTW components and dealing with high organic and inorganic chemicals, future research should be carried out at the large field-scale and focus on macrophyte- benthos- microorganism synergistic enhancement, breeding of cold-tolerant macrophytes, and combination of FTWs with many strategies, as well as rational design and operational approaches under cold conditions.

Influence of the flooded time on the performance of a tidal flow constructed wetland treating urban stream water

A vertical subsuperficial tidal flow constructed wetland (TFCW) operated under flooded time (FT) variation, was evaluated in the removal of carbonaceous, nitrogenous, and phosphorous matter from urban stream water. The TFCW downflow (117 L) was filled with bricks (44% porosity) and vegetated with Althernanthera philoxeroides (32 plants m^-2). The TFCW was operated under different flooded times - Stage A (48 h), B (36 h), C (24 h), and D (12 h), organic loading rates of 19.58-43.83 gCOD m^-2 d^-1, 3.68-6.94 gTN m^-2 d^-1 and 0.93-2.00 gTP m^-2 d^-1 and volumetric load rates of 46.8, 58.5, 78.0 and 11.7 L d^-1. No significant differences were observed in the removal efficiencies to Chemical Oxygen Demand (COD 66 to 94%), Total Ammonia Nitrogen (TAN 58 to 87%), and Total Nitrogen (TN 53 to 78%) among the stages, and nitrate concentrations lower than 6 mg L^-1 in the effluent. High Total Phosphorus removal was obtained in FT of 48 h (TP 79%). Total phosphorus loading rate was a limiting factor in TP removal, which reduced along with the reduction of FT. The nitrifying community was present over time since ammonia-oxidizing bacteria (Nitrosospira) and nitrite-oxidizing bacteria (Nitrobacter and Nitrospira) were identified in operational stages with variation in relative abundance, but TAN removal efficiency did not show significant differences. There was no change in the denitrifying community structure, indicating that FT did not influence the TN removal. A. philoxeroides was responsible for phytoextraction of 2.1% of TN and 2.7% of TP from the total removed by TFCW. TN removal (65%) was attributed to adsorption in the filtering material and microbial metabolism during the rest time. The findings of this study suggest FT of 12 h to remove COD and TN, and equal to or higher than 48 h to remove TP.

Long-term sulfide input enhances chemoautotrophic denitrification rather than DNRA in freshwater lake sediments

Partitioning between nitrate reduction pathways, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) determines the fate of nitrate removal and thus it is of great ecological importance. Sulfide (S^2-) is a potentially important factor that influences the role of denitrification and DNRA. However, information on the impact of microbial mechanisms for S^2- on the partitioning of nitrate reduction pathways in freshwater environments is still lacking. This study investigated the effects of long-term (108 d) S^2- addition on nitrate reduction pathways and microbial communities in the sediments of two different freshwater lakes. The results show that the increasing S^2- addition enhanced the coupling of S^2- oxidation with denitrification instead of DNRA. The sulfide-oxidizing denitrifier, Thiobacillus, was significantly enriched in the incubations of both lake samples with S^2- addition, which indicates that it may be the key genus driving sulfide-oxidizing denitrification in the lake sediments. During S^2- incubation of the Hongze Lake sample, which had lower inherent organic carbon (C) and sulfate (SO₄^2-), Thiobacillus was more enriched and played a dominant role in the microbial community; while during that of the Nansi Lake sample, which had higher inherent organic C and SO₄^2-, Thiobacillus was less enriched, but increasing abundances of sulfate reducing bacteria (Desulfomicrobium, Desulfatitalea and Geothermobacter) were observed. Moreover, sulfide-oxidizing denitrifiers and sulfate reducers were enriched in the Nansi Lake control treatment without external S^2- input, which suggests that internal sulfate release may promote the cooperation between sulfide-oxidizing denitrifiers and sulfate reducers. This study highlights the importance of sulfide-driven denitrification and the close coupling between the N and S cycles in freshwater environments, which are factors that have often been overlooked.

Comparison of performance of two large-scale vertical-flow constructed wetlands treating wastewater treatment plant tail-water: Contaminants removal and associated microbial community

The removal efficiency of contaminants in large-scale integrated vertical-flow constructed wetland (IVCW) and vertical-flow constructed wetland (VCW) for wastewater treatment plant (WWTP) tail-water was evaluated, and the microbial community was also investigated in this study. The results for 14 months study period indicated that 40.05% chemical oxygen demand (COD), 45.47% ammonia nitrogen (NH₄⁺-N), 62.55% total phosphorus (TP), 55.53% total nitrogen (TN) and 57.20% total suspended solids (TSS) average removal efficiencies were achieved in the IVCW. There was a poor performance of TN removal in the VCW, with an average removal efficiency of 38.13%. There was no significant seasonal difference in TP removal, and a strong positive correlation between influent TP load and removed load. The high-throughput sequencing analysis revealed that Proteobacteria, Planctomycetes, Bacteroidetes and Acidobacteria were dominant in nature and wetland systems. The relative abundance of nitrifying bacteria, denitrifying bacteria and anammox bacteria confirmed that nitrification, denitrification and anammox may be the main processes for nitrogen removal in the IVCW.

Recent advances in the enhanced nitrogen removal by oxygen-increasing technology in constructed wetlands

Constructed wetland has attracted more and more attention for wastewater purification due to its low construction cost and convenient operation recently. However, the unique waterflooding structure of constructed wetland makes the low dissolved oxygen level, which limits the effect of nitrogen removal in the system. Therefore, it is necessary to develop the oxygen-increasing technology to overcome the drawback in constructed wetlands. In this review, the mechanism of nitrogen removal in constructed wetland is discussed and oxygen is main influence factor is concluded. In addition, oxygen-increasing technologies in recent advances which improve the nitrogen removal efficiency greatly, are emphatically introduced. Finally, some future perspectives about oxygen-increasing techniques are also put forward in order to provide reference for further research and engineering application.

Combined influences of process parameters on microorganism-derived dissolved organic nitrogen (mDON) formation at low temperatures: Multivariable statistical and systematic analysis

Regulation of process parameters is a cost-effective approach to control microorganism-derived dissolved organic nitrogen (mDON) formation in low-temperature biological wastewater conditions. However, the integrated influence of multiple parameters in this process is poorly defined. In this study, mathematical methodology was used to evaluate the combined effects of hydraulic retention time (HRT), solids retention time (SRT), and mixed liquor suspended solids (MLSS) on mDON formation at 8 °C. This study also systematically explored how multiple combinations of those three parameters affected mDON chemodiversity (fluorescent properties and molecular compositions), microbial compositions, and specific relationships between mDON molecules and microbial species in activated sludge systems. Results showed that combined effects significantly controlled the mDON formation at 8 °C (P < .05). The systematic analysis suggested that the multi-parameter effects modulated the distribution of different mDON compositions and shaped the microbial communities. Most bacterial phyla as the generalist and a few as the specialist were displayed in 2487 pairs of strong microbe-mDON connections (|r| ≥ 0.6, P < .05). Moreover, network analysis on microbe-mDON relationships identified the network centers as crucial media in terms of combined effects of process parameters on mDON formation. Our results provide comprehensive insight on the roles of multi-parameter covariation influences in regulating the high complexity of mDON traits and microbe-mDON linkages, thereby highlighting the necessity to focus on the combined effects of process parameters for effective and correct controlling strategies on mDON concentrations.

Performance and bacterial communities in unsaturated and saturated zones of a vertical-flow constructed wetland with continuous-feed

In this study, a partially-saturated vertical-flow constructed wetland (VFCW) with continuous-feed was operated to investigate nutrients transformation and possible pathways in unsaturated and saturated zones. Effect of temperature on nutrients removal and microbial community was also evaluated. The variation of temperature barely affected removal of NH₄⁺-N and COD, achieving removal efficiencies of 99.5-100.0% and 96.8-100.0% at effluent temperature of 14.9-27.7 °C. The removal of COD, NH₄⁺-N, total inorganic nitrogen (TIN) and total phosphorus mainly occurred in unsaturated zone, achieving much higher removal rates than saturated zone. Nitrification process in the VFCW was associated with autotrophic/heterotrophic ammonia oxidizing bacteria and nitrite oxidizing bacteria. Denitrification process relied on both autotrophic and heterotrophic denitrifiers. Anaerobic ammonium oxidizing bacteria was also detected, contributing to TIN removal. All of the groups for nutrients removal exhibited higher abundance in unsaturated zone. Diverse pathways co-existed for nitrogen removal, while the main metabolic pathways were different along the depth.

Microbial coupling mechanisms of nitrogen removal in constructed wetlands: A review

Nitrogen removal through microorganisms is the most important pathway in constructed wetlands (CWs). In this review, we summarize the microbial coupling mechanisms of nitrogen removal, which are the common methods of nitrogen transformation. The electron pathways are shortened and consumption of oxygen and energy is reduced during the coupling of nitrogen transformation functional microorganisms. The highly efficient nitrogen removal mechanisms are cultivated from the design conditions in CWs, such as intermittent aeration and tidal flow. The coupling of microorganisms and substrates enhances nitrogen removal mainly by supplying electrons, and plants affect nitrogen transformation functional microorganisms by the release of oxygen and exudates from root systems as well as providing carriers for microbial attachment. In addition, inorganic elements such as Fe, S and H act as electron donors to drive the autotrophic denitrification process in CWs.

The effect of re-startup strategies on the recovery of constructed wetlands after long-term resting operation

The objective of this study was to identify the proper re-startup strategies (RSSs) for constructed wetlands (CWs) after long-term resting operation in terms of the recovery of pollutant removal efficiency (RE) and N-cycle gene abundance. The results suggested that backwashing increased the gene abundance without shortening the recovery time of gene abundance. The RSS involving excavation and washing performed better in terms of chemical oxygen demand (COD) RE, especially at the beginning, and performed slightly better or similarly in terms of N-cycle gene abundance and the REs of ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN). The abundance of the Amox gene was 66.1-92.8, 76.3-161.8 and 1550-2492 times larger than that of the napA, narG and amoA genes, respectively, and the anammox process was the dominant N removal pathway. Therefore, excavation and washing is recommended as the RSS for CWs with a long-term rest period.

Micro-aeration with hollow fiber membrane enhanced the nitrogen removal in constructed wetlands

The nitrogen removal efficiency in constructed wetlands (CWs) was largely affected by the dissolved oxygen (DO). In this study, micro-aeration with different numbers of hollow fiber membrane modules (HFMEs) was adopted to increase the oxygen availability and improve the nitrogen removal efficiency in CWs under different air temperatures and different hydraulic retention time (HRT). Compared to the plant oxygen release (ROL) of wetland plants and traditional mechanical aeration, HFME increased the oxygen availability and enhanced the nitrogen removal efficiency in CWs. The COD and NH₄⁺-N removal efficiencies increased with the increase of the HMFE. TN removal efficiency was increased by 8~16% after the application of HFME in CWs in the high-temperature stage. However, less HFME in CW-M1 realized the highest TN removal efficiency in low- and medium-temperature stages. At low temperature after 4-day HRT, the DO concentration respectively reached 6.25 mg L^-1 and 3.25 mg L^-1 in the upper zone and the bottom of CW-M1. The TN removal efficiencies in the upper zone of CW-M1 (60.69%) and the bottom of CW-M1 (64.98%) were all significantly higher than those in the upper zone of CK (35.98%) and the bottom of CK (39.9%). In addition, the microbial biomass and community analyses revealed that CW-M1 showed the most nitrifying bacteria and the best metabolic activity of bacteria. HEMF in CW-M1 also increased the nitrifying capacity from 0.12 to 0.46 mg kg^-1 h^-1. The application of HFME in CWs accelerated the nitrification process by enhancing nitrifying bacteria and less HFME realized the highest TN removal efficiency through nitrification-denitrification processes. Graphical abstract The application of hollow fiber membrane modules in CWs enhanced the pollutants (TN and COD) removal efficiency in the process of biological nitrification-denitrification and increased the number of nitrifying bacteria.

Characterization and co-occurrence of microbial community in epiphytic biofilms and surface sediments of wetlands with submersed macrophytes

Microbes in epiphytic biofilms and surface sediments play crucial roles in the biogeochemical cycles in wetlands. However, little is known about the compositions of microbial community in wetlands dominated with submersed macrophytes. In this study, bacterial and eukaryotic community in epiphytic biofilms and surface sediments were investigated in wetlands with artificial plants and Myriophyllum verticillatum from September (~27 °C) to January (~9 °C). A total of 30 (including 13 bacterial and 17 eukaryotic) and 34 (including 14 bacterial and 20 eukaryotic) phyla were detected in epiphytic biofilms and sediments, respectively. Microbial community in epiphytic biofilms shifted with decreasing temperature, and biofilms on M. verticillatum were generally similar to those on artificial plants. Though the OTUs and Shannon values were significantly higher in sediments than epiphytic biofilms (p < 0.05), numbers of strongly correlated edges detected in biofilms (64 nodes with 182 edges) were at least three times of those in sediments (40 nodes with 57 edges) as revealed by co-occurrence networks analysis (|r| > 0.7, p < 0.05). These data suggest that there were complex interactions among microbes in epiphytic biofilms than sediments. Positive relationships among microbes revealed the predation, symbiosis, parasitism relationships and the collective degradation of organic matter, while negative ones may be ascribed to their different lifestyles. These results highlight that artificial plants play a similar role as submersed macrophytes as microbial carriers and can be potentially used an alternative substitutes to submersed macrophytes in wetlands.

Experimental investigation and simulation of nitrogen transport in a subsurface infiltration system under saturated and unsaturated conditions

The nitrogen dynamics in a subsurface infiltration system (SIS) are affected by many factors, including temperature, system design, and feed water quality, which are not easily quantified. In this study, a column experiment was conducted to simulate an SIS. The HYDRUS-1D software package was used to investigate and quantify the factors that affect nitrate transport in an SIS. Three treatments were carried out based on different hydraulic conditions, including continuous wetting (CW), wetting/drying (WD), and a specific hydraulic loading rate (SH). The effects of hydraulic conditions and temperature on nitrate transformation were investigated. The model was calibrated and validated using two-year experimental data. Simulations of cumulative outflow volume and nitrate concentration fitted well with the observations. Among the three SISs, the denitrification rate was greatest under unsaturated conditions at high water temperature. The denitrification rate constant had an exponential relationship with temperature. An empirical formula describing this relationship was developed and validated in the SIS. The results showed that the SH column attained the greatest nitrate removal efficiency, mainly due to its low hydraulic loading and long retention time. Overall, the results showed that HYDRUS-1D adequately simulated nitrate transport through the soil column under different temperature and hydraulic conditions in an SIS. The fate of nitrate was directly controlled by the water temperature and hydraulic conditions.

Purification Effect of Sequential Constructed Wetland for the Polluted Water in Urban River

Constructed wetlands can play an active role in improving the water quality of urban rivers. In this study, a sequential series system of the floating-bed constructed wetland (FBCW), horizontal subsurface flow constructed wetland (HSFCW), and surface flow constructed wetland (SFCW) were constructed for the urban river treatment in the cold regions of North China, which gave full play to the combined advantages. In the Yitong River, the designed capacity and the hydraulic loading of the system was 100 m3/d and 0.10 m3/m2d, respectively. The hydraulic retention time was approximately 72 h. The monitoring results, from April to October in 2016, showed the multiple wetland ecosystem could effectively remove chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen (TN), total phosphate (TP), and suspended solids (SS) at average removal rates of 74.79%, 80.90%, 71.12%, 78.44%, and 91.90%, respectively. The removal rate of SS in floating-bed wetland was the largest among all the indicators (80.24%), which could prevent the block of sub-surface flow wetland effectively. The sub-surface flow wetland could remove the NH4-N, TN, and TP effectively, and the contribution rates were 79.20%, 64.64%, and 81.71%, respectively. The surface flow wetland could further purify the TN and the removal rate of TN could reach 23%. The total investment of this ecological engineering was $12,000. The construction cost and the operation cost were $120 and $0.02 per ton of polluted water, which was about 1/3 to 1/5 and 1/6 to 1/3 of the conventional sewage treatment, respectively. The results of this study provide a technical demonstration of the restoration of polluted water in urban rivers in northern China.

Enhanced optimal removal of nitrogen and organics from intermittently aerated vertical flow constructed wetlands: Relative COD/N ratios and microbial responses

Carbon source and dissolved oxygen are the critical factors which sustain the stable redox environment for the microbes to implement the removal of nitrogen and organics in vertical flow constructed wetlands (VFCWs). The effect mechanisms of the COD/N ratios in intermittently aerated VFCWs are needed to be investigated in order to increase the synchronous removal efficiency of pollutants. In this study, the combined effects of COD/N ratios (3, 6, 12) and intermittent aeration in VFCWs on pollutant removal, microbial communities and related function genes were studied. The results showed the increase of COD/N ratios from 3 to 12 enhanced the removal efficiency of TN, NO₃^--N and COD. The removals of NH₄⁺-N decreased as the COD/N ratio increased. The optimal removals of TN (87.65%), NH₄⁺-N (93.20%), NO₃^--N (80.80%) and COD (73.93%) were obtained in VFCW2 (COD/N ratios was 6). Illumina Miseq High-throughput sequencing analysis showed that high COD/N ratios increased the richness and diversity of microbial communities. The absolute abundance of nirK, nosZ, nirS, amoA, nxrA, and anammox bacterial 16S rRNA presented various changes under the different ratios of COD/N. The increase of COD/N ratios enhanced the copy numbers of nirS, nirK and nosZ, which participate in denitrification process. High COD/N ratios (6 and 12) were in favor of Actinobacteria, Firmicutes and Chloroflexi, which mainly play important roles in the process of denitrification. This paper implies that the combination of carbon source and aeration is necessary to sustain high microbial activities during pollutant removal in VFCWs.

Nutrient removal by Rotala rotundifolia: a superior candidate for ecosystem remediation at low temperatures

Temperature is an extremely important factor affecting the nutrient (mainly nitrogen and phosphorus) removal of aquatic macrophytes. A novel submersed Rotala rotundifolia was separately cultivated at room and low temperatures to investigate its ability for nutrient removal. The physiological metabolism was analyzed to explore the mechanism of removing nutrients under a wide temperature range. The results showed that the removal efficiency (RE) of nutrients at low temperature was competitive with that obtained at normal temperature, demonstrating that temperature exerted no obvious influence on the nutrient removal by R. rotundifolia. The root vitality at 5 °C rose from the initial 0.26 to 1.5 mg g⁻¹ h⁻¹, whereas it fell by 38.66% at 10 °C, 28.74% at 20 °C and 5.15% at 30 °C. The peroxidase (POD) activity at 5 °C showed the maximum value on day 7 followed by a notable decline on day 21. All the peak values of soluble sugar and protein as well as MDA showed up at 5 °C and they were 5.5, 437.9 and 10.1 mg g⁻¹, respectively. Chlorophyll a and b reached 8.4 and 4.4 mg g⁻¹ on day 28, respectively, with a total chlorophyll content (a plus b) of 12.4 mg g⁻¹ at 5 °C, all of which were higher than that at 30 °C. These results validated that R. rotundifolia could be a superior candidate suitable for in situ application.

Temperature is an extremely important factor affecting the nutrient (mainly nitrogen and phosphorus) removal of aquatic macrophytes.

Nitrogen metabolism in photosynthetic bacteria wastewater treatment: A novel nitrogen transformation pathway

Nitrogen removal from wastewater is a worldwide environmental problem. Photosynthetic bacteria (PSB) were reported to have a great potential in treating varieties of nitrogen-contaminated wastewater. However, their nitrogen metabolic mechanism is still unclear, which was further explored in this work. The results showed that PSB can efficiently utilize NH₄⁺-N, NO₃^--N and NO₂^--N. Over 90% nitrogen removal efficiencies were obtained under suitable condition. 35 ~ 51% of removed nitrogen was transformed to N₂ and N₂O. In addition, basically no mutual transformation occurred between NH₄⁺-N, NO₃^--N and NO₂^--N in PSB, which is different from other biological technologies. Combining with the analysis of functional gene groups, it indicated that there might be a new direct nitrogen transformation pathway, i.e. NH₄⁺ might be directly oxidized to N₂/N₂O in nitrogen metabolism of PSB, which breaks the limitations of existing technologies, and proposed a new understanding of nitrogen metabolism in PSB.

Stimulation of optimized influent C:N ratios on nitrogen removal in surface flow constructed wetlands: Performance and microbial mechanisms

Exploring optimal C:N ratio is necessary to ensure balanced microbial nitrification and denitrification in constructed wetlands (CWs), which has become an important management practice for more efficient nitrogen removal and sustainability of CWs. Surface flow constructed wetlands (SFCWs) vegetated with Myriophyllum aquaticum were designed to investigate the effects of five different influent C:N ratios (0:1, 2.5:1, 5:1, 10:1, and 15:1) on nitrogen removal performance and microbial communities over a 175-day experimental period. Compared to the influent C:N ratios of 0:1, higher NH₄⁺-N, NO₃^--N, and total nitrogen (TN) removal efficiencies and lower NO₃^--N accumulation were observed at influent C:N ratios higher than 5:1. In addition, the highest TN removal efficiency (70.4%) and the lowest nitrous oxide emission flux (4.12 mg m^-2 d^-1) were obtained at the influent C:N ratio of 5:1. High-throughput sequencing revealed that influent C:N ratios altered the distribution and composition of microbial communities in the sediment, which resulted in a dynamic interplay between N-transforming functional microbes and NH₄⁺-N and NO₃^--N removal. In particular, the dominant denitrifiers, including Desulfovibrio, Zoogloea, and Dechloromonas, were more abundant in the sediment with an influent C:N ratio of 5:1, which contributed to the high N removal rate. These findings may be used to screen for an optimum influent C:N ratio to maintain the sustainability of SFCWs with higher N removal efficiency.

Genetic associations as indices for assessing nitrogen transformation processes in co-composting of cattle manure and rice straw

In this study, mechanism of nitrogen transformation was investigated in terms of genetic associations (nitrogen-related gene groups) in co-composting of cattle manure and rice straw. Mutual validation among KEGG, Pearson correlation, stepwise regression, and Path analyses indicated that the functional genes synergistically affected on nitrogen transformation in composting process. NxrA/qnorB (0.9419 ± 0.0334) and (amoA + anammox)/Bacteria (0.7187 ± 0.0334) were the key functional gene groups mediating NH₄⁺-N transformation. AmoA/(narG + napA) (-0.8400 ± 0.0129), amoA/bacteria (0.8692 ± 0.0273), and (nirK + nirS)/nosZ (1.1652 ± 0.0089) determined NO₃^--N, NO₂^--N and N₂O transformation, respectively. AmoA/(napA + narG) mediated both NO₃^--N and NO₂^--N transformation. AmoA/anammox (-0.7172 ± 0.0591) and (nirK + nirS)/nosZ (-0.6626 ± 0.0825) served as predominant factors for total nitrogen removal. These results provided a molecular-level insight that nitrification, anaerobic ammonia oxidation and denitrification (SNAD) might simultaneously contribute to nitrogen transformation during composting, rather than sequentially.

Biotic factors drive distinct DNRA potential rates and contributions in typical Chinese shallow lake sediments

Dissimilatory nitrate reduction to ammonia (DNRA) is an important nitrate reduction pathway in lake sediments; however, little is known about the biotic factors driving the DNRA potential rates and contributions to the fate of nitrate. This study reports the first investigation of DNRA potential rates and contributions in lake sediments linked to DNRA community structures. The results of ¹⁵N isotope-tracing incubation experiments showed that 12 lakes had distinct DNRA potentials, which could be clustered into 2 groups, one with higher DNRA potentials (rates varied from 2.7 to 5.0 nmol N g^-1 h^-1 and contributions varied from 27.5% to 35.4%) and another with lower potentials (rates varied from 0.6 to 2.3 nmol N g^-1 h^-1 and contributions varied from 8.1% to 22.8%). Sediment C/N and the abundance of the nrfA gene were the key abiotic and biotic factors accounting for the distinct DNRA potential rates, respectively. A high-throughput sequencing analysis of the nrfA gene revealed that the sediment C/N could also affect the DNRA potential rates by altering the ecological patterns of the DNRA community composition. In addition, the interactions between the DNRA community and the denitrifying community were found to be obviously different in the two groups. In the higher DNRA potential group, the DNRA community mainly interacted with heterotrophic denitrifiers, while in the lower DNRA potential group, both heterotrophic and sulfur-driven autotrophic denitrifiers might cooperate with the DNRA community. The present study highlighted the role of the sulfur-driven nitrate reduction pathway in C-limited sediments, which has always been overlooked in freshwater environments, and gave new insights into the molecular mechanism influencing the fate of nitrate.

Keystone taxa regulate microbial assemblage patterns and functional traits of different microbial aggregates in simultaneous anammox and denitrification (SAD) systems

Understanding the impacts of the ecological patterns and niche-based selection on microbial community assembly and nitrogen-cycling network is crucial for achieving energy-neutral wastewater treatment. However, little is known about the niche differentiation and microbial nitrogen-cycling traits of keystone taxa in flocs and granules in anammox-based systems. Herein, the aspects of community assemblage patterns, metabolic functions and nitrogen transformation pathways were explored. The findings discovered that the treatment performance and bacterial community assembly were regulated by core taxa and flocs and granules communities harbored core taxa based on their functional traits. Both niche differentiation and environmental filtering have profound influences on functional bacteria. Furthermore, a combined analysis showed that nitrogen removal in flocs and granules was regulated by different nitrogen transformation pathways. These results suggest that core taxa are the key drivers for the microbial nitrogen-cycling network and improve the understanding of cross-feeding and metabolic pathways between anammox and nitrogen-cycling-related microorganisms.

Effect of microbial activity and microbial community structure on the formation of dissolved organic nitrogen (DON) and bioavailable DON driven by low temperatures

Dissolved organic nitrogen (DON) formed by microbial metabolism in wastewater treatment processes adversely impacts wastewater reuse and receiving waters quality, and microbial metabolism is greatly influenced by temperatures. However, little is known about the effect of microorganisms on DON and bioavailable DON (ABDON) formation under low temperatures. In this study, six reactors were operated at low (8 °C and 15 °C) and room (25 °C) temperatures to evaluate the relationship between microbial activity, microbial communities, and DON and ABDON. Results showed that DON and ABDON concentrations significantly increased at low temperatures (p < 0.05, t-test). DON formation was significantly correlated to microbial activity only, with adenosine triphosphate (negative, r = -0.64) and polysaccharide (positive, r = 0.61) as key indicators; however, ABDON formation was influenced by both microbial activity (polysaccharide > triphenyltetrazolium chloride-dehydrogenases > adenosine triphosphate) and microbial community structure. Short-term tests using the biomass from six reactors were performed at room temperature to further validate the relationship. The distinct differences between these results provide a basis for different strategies on reducing effluent DON and ABDON under low temperatures.

Nitrogen variations during the ice-on season in the eutrophic lakes

Nitrogen accumulation in sediments, and the subsequent migration and transformations between sediment and the overlying water, plays an important role in the lake nitrogen cycle. However, knowledge of these processes are largely confined to ice-free seasons. Recent research under ice has mainly focused on the water eco-environmental effects during winter. Sediment N accumulation during the ice-on season and its associated eco-environmental impacts have never been systematically investigated. To address these knowledge gaps, we chose Wuliangsu Lake in China as a case study site, taking advantage of the spatial disparity between the 13 semi-separated sub-lakes. Based on samples of 35 sampling sites collected before, in the middle, and at the end of ice-on season separately, we performed a quantitative analysis of under-ice lake N accumulation and water-sediment N exchange by analyzing N fraction variations. Hierarchical Cluster Analysis and Relevance Analysis were used to help elucidate the main causes and implications of under-ice N variation. Our results clearly show that existing studies have underestimated the impact of under-ice N accumulation on the lake ecology throughout year: 1) Sediment N accumulated 2-3 times more than that before winter; 2) residual nitrogen (Res-N) contributed to the majority of the accumulated sediment N and was mainly induced by the debris of macrophytes; 3) total available nitrogen (TAN) was the most easily exchanged fractions between sediment and water, and it mainly affected the water environment during winter; 4) the Res-N accumulation during the ice-on season may have a strong impact on the eco-environment in the subsequent seasons. Our research is valuable for understanding the mechanism of internal nutrient cycle and controlling the internal nitrogen pollution, especially in shallow seasonally-frozen lakes that have long suffered from macrophyte-phytoplankton co-dominated eutrophication.

Contribution of the Kodama and 4S pathways to the dibenzothiophene biodegradation in different coastal wetlands under different C/N ratios

Dibenzothiophene (DBT) degradation mechanisms and the transformation of pathways during the incubation of three types of coastal sediments with C/N ratios ranging from 1 to 9 were investigated. The DBT degradation efficiencies were clearly improved with increasing C/N ratio in reed wetland sediments, tidal wetlands sediments and estuary wetland sediments. The quantitative response relationships between DBT degradation rates and related functional genes demonstrate that the Kodama pathway-related gene groups were dominant factors at low C/N ratios, while the 4S-related gene groups mainly determined the degradation rate when the C/N ratio was up to 5. Network analysis also shows that the pathway shifts from the Kodama pathway to the 4S pathway occurred through changes in the connections between functional genomes and rates. Furthermore, there were competition and collaboration between the Kodama and 4S pathways. The 4S pathway-related bacteria were more active in estuary wetland sediments compared with reed wetland sediments and tidal wetland sediments. The higher degradation efficiency in estuary wetland sediments may indicate the greater participation of the 4S pathway in the DBT biodegradation reaction. And the effects of ring cleavage of Kodama pathway caused more complete metabolizing of DBT.

Microbial removal and plant uptake of nitrogen in constructed wetlands: mesocosm tests on influencing factors

Macrophytes and bacteria are key drivers of nitrogen removal in constructed wetlands. Through mesocosm experiments with vegetated submerged beds and free water surface wetlands in various operational modes, wetland configurations, and system layouts, this study developed empirical models for non-destructive estimation of plant biomass growth and associated nitrogen assimilation and explored the combined effects of multiple factors that influence microbial nitrogen removal. The above-ground biomass of individual plants was a power function of plant height for both Cyperus alternifolius and Typha angustifolia. Below- to above-ground biomass ratio was 0.38 for C. alternifolius and 2.73 for T. angustifolia. Because of greater tolerance to ammonia stress, C. alternifolius and C. papyrus grew faster than T. angustifolia. There were no significant effects of wetland type, vegetation, and plant species on microbial nitrogen removal. Microbial nitrogen removal was inhibited by free ammonia at 13.3-16.2 mg N/L. Denitrification and anammox were suppressed at dissolved oxygen greater than 1.9 mg/L. Microbial removal of ammonia in vegetated submerged beds was sensitive mainly to dissolved oxygen, pH, and influent ammonia concentration, while in free water surface wetlands, it was sensitive to influent ammonia concentration, pH, and temperature.

Effects of pipe material on nitrogen transformation, microbial communities and functional genes in raw water transportation

Raw water transportation pipelines are vital in an urban water supply system for transporting raw water to drinking water treatment plants. This study investigated the effects of pipe material on nitrogen transformation, microbial communities and characteristics of related function genes in paint-lined steel pipe (PLSP) and cement-lined steel pipe (CLSP) raw water model systems. We established quantitative relationships between specific functional genes and change rates of nitrogen pollutants, which were verified by field investigation on nitrogen pollutant transformations in real raw water transportation systems. The results showed that the CLSP produced higher ammonia nitrogen (NH₄⁺-N) transformation rates and higher effluent concentrations of nitrate nitrogen (NO₃^--N) and dissolved organic nitrogen (DON) than the PLSP. Both pipes achieved high and stable nitrite nitrogen (NO₂^--N) and low total nitrogen (TN) removal efficiency. Nitrification was found to be the dominant process in both model systems, especially in the CLSP. Characteristics of microbial communities and nitrogen functional genes, which were analysed by high-throughput pyrosequencing and quantitative polymerase chain reaction (qPCR), respectively, varied between the two pipe systems. Nitrogen transformation pathways, identified by path analysis, were also different between the PLSP and CLSP due to different microbial community characteristics and synergistic effects of nitrogen functional genes. In the CLSP, (NH₄⁺-N→NO₂^--N) with part denitrification, was the primary transformation pathway of ammonia nitrogen (NH₄⁺-N), while only ammonia oxidization contributed to NH₄⁺-N transformation in the PLSP. (NO₂^--N→NO₃^--N) was the main pathway involved in NO₂^--N transformation and NO₃^--N accumulation. The TN removal showed complex relationships with nitrification, denitrification and nitrogen fixation processes. These findings provided molecular-level insights into nitrogen pollutant transformations during the transportation of raw water through different types of pipes and technical support for the selection of raw water pipe materials. In our study area, the Taihu basin, China, PLSP was better than CLSP for distributing raw water in a short transportation distance, due to the lower effluent concentrations of DON and NO₃^--N and less abundance of microorganisms.

Seasonal function succession and biogeographic zonation of assimilatory and dissimilatory nitrate-reducing bacterioplankton

The dominance of different nitrate-reducing pathways determines nitrogen cycling patterns. Denitrification (DNF) has been widely studied, but assimilatory nitrate reduction (ANR) and dissimilatory nitrate reduction to ammonium (DNRA) have received much less attention. Their ecological patterns and responsible microbes are poorly understood. Here, we studied the structure and function succession of the three functional groups in the middle route of the South-to-North Water Diversion Project, which is a 1230 km canal spanning 8 degrees of latitude. The results reflected a nitrogen-removing pattern dominated by DNF in the summer and a nitrogen-retaining pattern dominated by ANR and DNRA in the winter. Stenotrophomonas, a typical denitrifier, was the keystone species in the summer and contributed to N₂O production. Clostridium, a genus able to conduct ANR and DNRA, was the keystone species in the winter. Notably, a significant zonation pattern was discovered. According to the community structure, the system could be separated into two biogeographic zones, and the Yellow River (about latitude 35°N) is an important cut-off line. This bacterial biogeography followed different water characteristics and ecological processes. ANR was found to be an important process and seasonally transformed its habitat from the northern zone to the southern zone. DNRA bacteria were acclimated to the northern zone and favored at this region in both seasons. The generation of N₂O, a strong greenhouse gas, also exhibited this zonation pattern. This is the first study to consider assimilatory and dissimilatory nitrate reducers together at a molecular level, and provides new insights into the underlying patterns of a nitrate-reducing bacterioplankton community.

Effect of hydraulic retention time on nitrogen removal and functional gene quantity/transcription in biochar packed reactors at 5 °C: A control-strategy study

This study investigated the effect of hydraulic retention time (HRT = 8, 12, 16, and 24 h) on effluent dissolved organic nitrogen (DON), dissolved total nitrogen (DTN), and functional gene quantity/transcription in biochar packed reactors over a 125-day operation at 5 °C. The lowest effluent DON concentration (0.21 ± 0.14 mg/L) and DTN concentration (10.74 ± 0.41 mg/L) were in R_12h and R_24h, respectively. Adequate HRT (>12 h) was a necessary parameter for poly-β-hydroxybutyrate (PHB) accumulation (PHB/MLSS: 0.1181-0.1522), but higher HRT was detrimental to adenosine triphosphate (ATP) accumulation from 62.77-66.31 µg/g SS (R_8h) to 48.21-48.39 µg/g SS (R_24h). Effluent DON had a negative correlation relation with ATP and the relative abundance of amoA (p = 0.02), and effluent DTN had a negative correlation relation with PHB (p < 0.01), the relative abundance of napA (p = 0.01), and the transcriptional quantity of nirS (p = 0.14) and nxrA (p = 0.03).

Nitrogen loading effects on nitrification and denitrification with functional gene quantity/transcription analysis in biochar packed reactors at 5 °C

This study investigated the nitrogen transformation rates of different nitrogen-loading (20, 30, and 50 mg TN/L) biochar packed reactors (C:N:P = 100:5:1) within 125 days at 5 °C. The results showed that high nitrogen loading resulted in an NH4+ (TN) removal efficiency decline from 98% (57%) to 83% (29%), with biochar yielding a higher NH4+, TN and DON removal rate than conventional activated sludge. Moreover, all biochar packed reactors realized a quick start-up by dropping in temperature stage by stage, and the effluent dissolved organic nitrogen (DON) concentrations of R20, R30, and R50 were 0.44 ± 0.18, 0.85 ± 0.35, and 0.66 ± 0.26 mg/L, respectively. The nirS/amoA, nxrA/amoA, and amoA/(narG + napA) were deemed to be the markers of ammonium oxidation rate (SAOR), specific nitrite oxidation rate (SNOR), and specific nitrate reduction rate (SNRR), respectively. Compared with functional gene quantity data, transcription data (mRNA) introduced into stepwise regression analyses agreed well with nitrogen transformation rates. High nitrogen loading also resulted in the cell viability decreased in R50. Nitrogen loadings and operation time both led to a significant variation in cell membrane composition, and unsaturated fatty acids (UFAs) significantly increased in R30 (46.49%) and R50 (36.34%). High-throughput sequencing revealed that nitrogen loadings increased the abundance of nitrifying bacteria (e.g., Nitrospira) and reduced the abundance of denitrifying bacteria (e.g., Nakamurella, Thermomonas, and Zoogloea) through linear discriminant analysis (LDA).

The influence of complex fermentation broth on denitrification of saline sewage in constructed wetlands by heterotrophic nitrifying/aerobic denitrifying bacterial communities

An experimental vertical-flow constructed wetland (CW) was tested to treat salt-containing sewage. CW clogging deposits and withered Pontederia cordata L. were collected into a complex fermentation broth to serve as the carbon source and its effects on the denitrification capacity and microbial composition of the CW were examined. Addition of the complex fermentation broth into the CW influent (1.8% salinity) led to high removal efficiencies of NH₄⁺-N > 99.82 ± 0.00% and TN > 90.39 ± 0.05%. Heterotrophic nitrifiers and aerobic denitrifiers were entirely dominant in the middle and upper layers of the CW, where obligate halophilic, aerobic denitrifiers Zobellella occurred. The CW successfully cultivated and enriched heterotrophic nitrifying-aerobic denitrifying bacteria, overcoming the effects of salinity and insufficient organic carbon sources on the denitrification capacity of CW. This type of complex carbon sources can also facilitate the utilization of waste resources, such as CW clogging deposits and withered wetland plants.