Engineered and Environmental Controls of Microbial Denitrification in Established Bioretention Cells

Urban stormwater green infrastructure: Evaluating the public health service role of bioretention using microbial source tracking and bacterial community analyses

Bioretention cells (BRCs) control stormwater flow on-site during precipitation, reducing runoff and improving water quality through chemical, physical, and biological processes. While BRCs are effective in these aspects, they provide habitats for wildlife and may face microbial hazards from fecal shedding, posing a potential threat to human health and the nearby environment. However, limited knowledge exists regarding the ability to control microbial hazards (e.g., beyond using typical indicator bacteria) through stormwater biofiltration. Therefore, the purpose of this study is to characterize changes in the bacterial community of urban stormwater undergoing bioretention treatment, with the goal of assessing the public health implications of these green infrastructure solutions. Samples from BRC inflow and outflow in Columbus, Ohio, were collected post-heavy storms from October 2021 to March 2022. Conventional culture-based E. coli monitoring and microbial source tracking (MST) were conducted to identify major fecal contamination extent and its sources (i.e., human, canine, avian, and ruminant). Droplet digital polymerase chain reaction (ddPCR) was utilized to quantify the level of host-associated fecal contamination in addition to three antibiotic resistant genes (ARGs): tetracycline resistance gene (tetQ), sulfonamide resistance gene (sul1), and Klebsiella pneumoniae carbapenemase resistance gene (blaKPC). Subsequently, 16S rRNA gene sequencing was conducted to characterize bacterial community differences between stormwater BRC inflow and outflow. Untreated urban stormwater reflects anthropogenic contamination, suggesting it as a potential source of contamination to waterbodies and urban environments. When comparing inlet and outlet BRC samples, urban stormwater treated via biofiltration did not increase microbial hazards, and changes in bacterial taxa and alpha diversity were negligible. Beta diversity results reveal a significant shift in bacterial community structure, while simultaneously enhancing the water quality (i.e., reduction of metals, total suspended solids, total nitrogen) of urban stormwater. Significant correlations were found between the bacterial community diversity of urban stormwater with fecal contamination (e.g. dog) and ARG (sul1), rainfall intensity, and water quality (hardness, total phosphorous). The study concludes that bioretention technology can sustainably maintain urban microbial water quality without posing additional public health risks, making it a viable green infrastructure solution for heavy rainfall events exacerbated by climate change.

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.

Soil properties and functional genes in nitrogen removal process of bioretention

The effects of different soil properties on hydrology and nitrogen removal were studied in a simulated bioretention system. Soil capacity of permeability and water retention, changes in the soil environment, leachate concentrations at the surface and bottom layers, quantification of N removal from soil, microorganism and plant by 15N isotope tracer technique, and functional genes abundance at different depths were evaluated. The results showed that shallow root plants, soil compaction, and low organic matter content were not conducive to the infiltration of bioretention systems. In the 72 h experiment, compaction (especially surface compaction) and planting of herbaceous plants (Ophiopogon japonicus) were not beneficial to the removal of TN, TP, and COD. Adding an appropriate amount of organic matter also affects nitrogen and phosphorus removal. In the process of denitrification, the order of the ability to remove nitrogen is soil adsorption > microbial assimilation > plant uptake. The contribution of soil denitrification is affected by soil compaction, compaction location, plant species and organic matter content. The abundance of 16S rRNA, nitrifying, denitrifying and nrfA genes decreased with soil depth. More copies of genes in topsoil were thought to be due to sufficient nutrients, aerobic condition, anaerobic microsites and submerged state. Soil compaction, organic matter content and plant species affected nitrification, denitrification and DNRA gene characteristics.

Multi-media interaction improves the efficiency and stability of the bioretention system for stormwater runoff treatment

Bioretention systems are one of the most widely used stormwater control measures for urban runoff treatment. However, stable and effective dissolved nutrient treatment by bioretention systems is often challenged by complicated stormwater conditions. In this study, pyrite-only (PO), pyrite-biochar (PB), pyrite-woodchip (PW), and pyrite-woodchip-biochar mixed (M) bioretention systems were established to study the feasibility of improving both stability and efficiency in bioretention system via multi-media interaction. PB, PW, and M all showed enhanced dissolved nitrogen and/or phosphorus removal compared to PO, with M demonstrating the highest efficiency and stability under different antecedent drying durations (ADD), pollutant levels, and prolonged precipitation depth. The total dissolved nitrogen and dissolved phosphorus removal in M ranged between 64%-86% and 80%-95%, respectively, with limited organic matter and iron leaching. Pore water, microbial community, and material analysis collectively indicate that pyrite, woodchip, and biochar synergistically facilitated multiple nutrient treatment processes and protected each other against by-product leaching. Pyrite-woodchip interaction greatly increased nitrate removal by facilitating mixotrophic denitrification, while biochar further enhanced ammonium adsorption and expanded the denitrification area. The Fe3+ generated by pyrite aerobic oxidation was adsorbed on the biochar surface and potentially formed a Fe-biochar composite layer, which not only reduced Fe3+-induced pyrite excessive oxidation but also potentially increased organic matter adsorption. Fe (oxyhydr)oxides intermediate product formed by pyrite oxidation, in return, controlled the phosphorus and organic matter leaching from biochar and woodchip. Overall, this study demonstrates that multi-media interaction may enable bioretention systems to achieve stable and effective urban runoff treatment.

2-Hydroxy-1,4-Naphthoquinone: A Promising Redox Mediator for Minimizing Dissolved Organic Nitrogen and Eutrophication Effects of Wastewater Effluent

Researchers and engineers are committed to finding effective approaches to reduce dissolved organic nitrogen (DON) to meet more stringent effluent total nitrogen limits and minimize effluent eutrophication potential. Here, we provided a promising approach by adding specific doses of 2-hydroxy-1,4-naphthoquinone (HNQ) to postdenitrification bioreactors. This approach of adding a small dosage of 0.03-0.1 mM HNQ effectively reduced the concentrations of DON in the effluent (ANOVA, p < 0.05) by up to 63% reduction of effluent DON with a dosing of 0.1 mM HNQ when compared to the control bioreactors. Notably, an algal bioassay indicated that DON played a dominant role in stimulating phytoplankton growth, thus effluent eutrophication potential in bioreactors using 0.1 mM HNQ dramatically decreased compared to that in control bioreactors. The microbe-DON correlation analysis showed that HNQ dosing modified the microbial community composition to both weaken the production and promote the uptake of labile DON, thus minimizing the effluent DON concentration. The toxic assessment demonstrated the ecological safety of the effluent from the bioreactors using the strategy of HNQ addition. Overall, HNQ is a promising redox mediator to reduce the effluent DON concentration with the purpose of meeting low effluent total nitrogen levels and remarkably minimizing effluent eutrophication effects.

Efficient nitrogen removal pathways and corresponding microbial evidence in tidal flow constructed wetlands for saline water treatment

The artificial tidal wetlands ecosystem was believed to be a useful device in treating saline water, and it played a significant part in global nitrogen cycles. However, limited information is available on nitrogen-cycling pathways and related contributions to nitrogen loss in tidal flow constructed wetlands (TF-CWs) for saline water treatment. This study operated seven experimental tidal flow constructed wetlands to remove nitrogen from saline water at salinities of 0-30‰. Stable and high NH4+-N removal efficiency (∼90.3%) was achieved, compared to 4.8-93.4% and 23.5-88.4% for nitrate and total nitrogen (TN), respectively. Microbial analyses revealed the simultaneous occurrence of anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA), nitrification and denitrification, contributing to nitrogen (N) loss from the mesocosms. The absolute abundances were 5.54 × 103-8.35 × 107 (nitrogen functional genes) and 5.21 × 107-7.99 × 109 copies/g (16S rRNA), while the related genera abundances ranged from 1.81% to 10.47% (nitrate reduction) and from 0.29% to 0.97% (nitrification), respectively. Quantitative response relationships showed ammonium transformation were controlled by nxrA, hzsB and amoA, and nitrate removal by nxrA, nosZ and narG. Collectively, TN transformation were determined by narG, nosZ, qnorB, nirS and hzsB through denitrification and anammox pathways. The proportion of nitrogen assimilation by plants was 6.9-23.4%. In summary, these findings would advance our understanding of quantitative molecular mechanisms in TF-CW mesocosms for treating nitrogen pollution that caused algal blooms in estuarine/coastal ecosystems worldwide.

Synthetic phylogenetically diverse communities promote denitrification and stability

Denitrification is an important process of the global nitrogen cycle as some of its intermediates are environmentally important or related to global warming. However, how the phylogenetic diversity of denitrifying communities affects their denitrification rates and temporal stability remains unclear. Here we selected denitrifiers based on their phylogenetic distance to construct two groups of synthetic denitrifying communities: one closely related (CR) group with all strains from the genus Shewanella and the other distantly related (DR) group with all constituents from different genera. All synthetic denitrifying communities (SDCs) were experimentally evolved for 200 generations. The results showed that high phylogenetic diversity followed by experimental evolution promoted the function and stability of synthetic denitrifying communities. Specifically, the productivity and denitrification rates were significantly (P < 0.05) higher with Paracocus denitrificans as the dominant species (since the 50th generation) in the DR community than those in the CR community. The DR community also showed significantly (t = 7.119, df = 10, P < 0.001) higher stability through overyielding and asynchrony of species fluctuations, and showed more complementarity than the CR group during the experimental evolution. This study has important implications for applying synthetic communities to remediate environmental problems and mitigate greenhouse gas emissions.

Influence of double-layer filling structure on nitrogen removal and internal microbial distribution in bioretention cells

The nitrogen removal effect of traditional bioretention cells on runoff rainwater is not stable. The nitrogen removal effect of bioretention cells can be improved by setting up a layered filling structure, but the effect of changes in filling structure on the nitrogen removal process and microbial community characteristics is still unclear. Two types of porosity fillers were set up in the experiment, and a homogeneous bioretention cell and three bioretention cells with layered fillers were constructed by changing the depth range of the upper and lower layers to analyze the influence of the pore variation of different depth fillers on the nitrogen removal process and microbial community characteristics. The experimental results showed that, compared with the homogeneous filing structure, the layered filling structure can strengthen the adsorption of NH₄⁺-N and the conversion of NO₃^--N, so as to increase the removal rates of NH₄⁺-N and NO₃^--N by 20.71-81.56% and 9.25%-78.19%, respectively. Although the low porosity filler structure will reduce the nitrification activity and urease activity by 48.63%-66.68% and 8.00%-20.64% respectively, it can increase the denitrification activity by 19.14%-31.92%, thus significantly reducing the nitrate content in the filler. The low porosity filler structure can affect the growth and reproduction of various phylum bacteria such as Proteobacteria, Chloroflexi, Acidobacteria, and genus bacteria such as Nitrospira, Ellin6067, Rhizobacter, Pseudomonas, which can improve the diversity and richness of microorganisms.

Nitrogen transfer and transformation in bioretention cells under low temperature conditions

The nitrogen removal effect of traditional bioretention cells is generally poor under low temperature conditions, with significant levels of fluctuation and leaching often reported. Therefore, the migration characteristics of nitrogen were explored in bioretention cells under low temperature conditions, with the aim of improving the nitrogen removal effect. Four groups of modified collapsible loess bioretention cells were constructed and operated at 1, 5, 10 and 25 °C. The nitrogen removal effect of the cells was determined at different temperatures and the nitrogen migration and transformation characteristics under low temperature conditions were discussed. Experimental results showed that during the rainfall period, the ammonia nitrogen removal efficiency remained similar at different temperatures (above 97 %), while the nitrate nitrogen removal efficiency varied significantly at 1, 5, 10 and 25 °C, from 28.15 %-65.22 %, 96.68 %-98.8 %, 96.75 %-98.88 % and 80.14 %-96.72 %, respectively. In addition, nitrate nitrogen accumulation occurred in the filler during rainfall events, with lower temperature conditions increasing the final concentration of nitrate nitrogen accumulated. Following a rainfall event, the content of nitrate nitrogen in the filler decreased significantly over a 60 h dry period. However, the nitrate nitrogen reduction rate was significantly lower under low temperature conditions, than at 25 °C. Overall, low temperature conditions had a negative effect on the accumulation of nitrate nitrogen in the filler during rainfall events, as well as the transformation and migration of nitrate nitrogen within the filler during drought periods, with the adverse effects most significant at temperatures lower than 5 °C.

Stormwater biofilter response to high nitrogen loading under transient flow conditions: Ammonium and nitrate fates, and nitrous oxide emissions

Nitrogen (N) in urban runoff is often treated with green infrastructure including biofilters. However, N fates across biofilters are insufficiently understood because prior studies emphasize low N loading under laboratory conditions, or use "steady-state" flow regimes over short time scales. Here, we tested field scale biofilter N fates during simulated storms delivering realistic transient flows with high N loading. Biofilter outflow ammonium (NH₄⁺-N) was 60.7 to 92.3% lower than that of the inflow. Yet the characteristic times for nitrification (days to weeks) and denitrification (days) relative to N residence times (7 to 30 h) suggested low N transformation across the biofilters. Still, across 7 successive storms, total outflow nitrate (NO₃^--N) greatly exceeded (3100 to 3900%) inflow nitrate, a result only explainable by biofilter soil N nitrification occurring between storms. Archaeal, and bacterial amoA gene copies (2.1 × 10⁵ to 1.2 × 10⁶ gc g soil^-1), nitrifier presence by16S rRNA gene sequencing, and outflow δ¹⁸O-NO₃^- values (-3.0 to 17.1 ‰) reinforced that nitrification was occurring. A ratio of δ¹⁸O-NO^3- to δ¹⁵N-NO₃^- of 1.83 for soil eluates indicated additional processes: N assimilation, and N mineralization. Denitrification potential was suggested by enzyme activities and soil denitrifying gene copies (nirK + nirS: 3.0 × 10⁶ to 1.8 × 10⁷; nosZ: 5.0 × 10⁵ to 2.2 × 10⁶ gc g soil^-1). However, nitrous oxide (N₂O-N) emissions (13.5 to 84.3 μg N m ^- 2 h ^- 1) and N₂O export (0.014 g N) were low, and soil nitrification enzyme activities (0.45 to 1.63 mg N kg soil^-1day^-1) exceeded those for denitrification (0.17 to 0.49 mg N kg soil^-1 day^-1). Taken together, chemical, bacterial, and isotopic metrics evidenced that storm inflow NH₄⁺sorbs and, along with mineralized soil N, nitrifies during biofilter dry-down; little denitrification and associated N₂O emissions ensue, and thus subsequent storms export copious NO₃^--N. As such, pulsed pass-through biofilters require redesign to promote plant assimilation and/or denitrification of mineralized and nitrified N, to minimize NO₃^--N generation and export.

Evaluation of nitrogen removal, functional gene abundance and microbial community structure in a stormwater detention basin

Stormwater control measures such as detention basins are used to mitigate the negative effects of urban stormwater resulting from watershed development. In this study, the performance of a detention basin in mitigating nitrogen pollution was examined and the abundance of N-cycling genes (amoA, nirK, nosZ, hzsB and Ntsp-amoA) present in the soil media of the basin was measured using quantitative PCR. Results showed a net export of nitrogen from the basin, however, differences between in- and outflow concentrations were not significant. Furthermore, the quantitative PCR showed that nirK (denitrification gene) was more abundant in the winter season, whereas amoA (nitrification gene) was more abundant in the summer season. The abundance of nirK, Ntsp-amoA and hzsB genes also varied with the sampling depth of soil and based on 16S rRNA gene sequencing of soil samples, Actinobacteria and Proteobacteria were the most dominant phyla. Species diversity appeared higher in summer, while the top and bottom layer of soil clustered separately based on the bacterial community structure. These results underline the importance of understanding nitrogen dynamics and microbial processes within stormwater control measures to enhance their design and performance.

A long term study elucidates the relationship between media amendment and pollutant treatment in the stormwater bioretention system: Stability or efficiency?

Media amendment has been more and more frequently tested in stormwater bioretention systems for enhanced runoff pollutant treatment. However, few studies systematically evaluated the amended system over a long time span, which hindered the further optimization of the proposed amended media. In this study, biochar-pyrite system (PB), conventional sand system (SB), and biochar-woodchip system (WB) were established and operated for 26 months. Media amendment greatly enhanced the dissolved nutrient removal, the highest total dissolved nitrogen removal in PB and WB were 65.6±3.6% and 68.2±2.5%, respectively. Compared with PB, WB could maintain excellent nitrogen removal under long-term operation. In contrast, PB demonstrated stable and more effective total dissolved phosphorus removal during all stages (73.1±3.1%-80.3±4.1%). A high content of phosphorus and organic matter was leached in WB especially at initial operation, while the initial pollutant leaching in PB and SB is much lower, about one-third of WB. Microbial and metabolic function analysis indicated that the microbial community in the bioretention system is complicated and stable. Media amendment enhanced microbial diversity and the relative abundance of functional genera related to nitrogen (Nitrospira, Thauera, Denitratisoma, etc.), sulfur (Thiobacillus, Geobacter, Desulfovibrio, etc.), and carbon cycles (cellulomonas, saccharimonadales, and SBR1031, etc.), which well explained the enhanced pollutant removal and by-product leaching in different systems. Overall, the current study indicates that although media amendment is conducive to enhanced dissolved nutrient removal in bioretention systems, it can hardly maintain both stability and efficiency from initial set-up to long-term operation. In practical application, catchment characteristics, prioritized pollutants, meteorological factors, etc. should all be considered before choosing suitable amended media and its design factors, thereby maximising the stability and efficiency of the bioretention system.

Nitrogen process in stormwater bioretention: effect of the antecedent dry days on the relative abundance of nitrogen functional genes

In this study, we evaluated the relative abundance of nitrogen functional genes (amoA, nirK and nirS) involved in ammonia oxidation and denitrification bacteria in laboratory-scale bioretention columns in response to environmental factors (e.g., moisture content, pH, soil organic matter, soil nitrogen) under different antecedent dry days (ADDs). We observed a decrease tendency of the relative abundance of ammonia-oxidizing bacteria at first and then increased when increasing ADDs from 1 to 22 day, while the relative abundance of denitrifying bacteria showed a downward trend. The abundance of bacteria gene amoA was positively associated with soil ammonia nitrogen concentration (r² = 0.389, p < 0.05) and soil organic matter concentration (r² = 0.334, p < 0.05), while the abundance of bacteria gene nirS was positively correlated with soil ammonia nitrogen (r² = 0.730, p < 0.01), soil organic matter (r² = 0.901, p < 0.01) and soil total nitrogen (r² = 0.779, p < 0.01). Furthermore, gene counts for bacteria gene nirS were correlated negatively with plant root length (r² = 0.364, p < 0.05) and plant biomass (r² = 0.381, p < 0.05). Taken together, these results suggest that both nitrification and denitrification can occur in bioretention systems, which can be affected by environmental factors.

Interactive effect of temperature and plant species on nitrogen cycling and treatment in stormwater biofiltration systems

Stormwater biofiltration systems (also known as biofilters, bioretention, rain gardens) are engineered nature-based solutions, which help mitigate aquatic nitrogen pollution arising from storm runoff. These systems are being increasingly used in a range of climates across the world. A decline in treatment performance is frequently observed in cold weather conditions. While plant species comprise an important design factor influencing system performance, the effect of temperature on the fate of dissolved nitrogen forms, namely ammonium (NH₄⁺) and nitrate (NO₃^-), in the presence of different plant species in these systems remains unclear. A large scale laboratory experiment was undertaken that measured potential rates of nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) as well as the microbial community structure to investigate nitrogen fate and hence removal under two different temperature conditions (2 °C and 15 °C) in the presence of four distinct plant species. The results indicate that lower nitrification rates (reduced by a factor of 4) coupled with potential media NH₄⁺ desorption could be contributing to reduced NH₄⁺ removal during cold conditions. Planting with species exhibiting good nutrient uptake capacity can reduce the extent of this performance decline. While NO₃^- reduction generally remains problematic during cold weather (<0 to 55% reduction), which may not be significantly different from warmer periods, the study demonstrated that the denitrification potential and gene abundance (nap, nar, NirS, norB, nosZ) to be higher than those of nitrification (amoA). Denitrification may not proceeding at optimal rates due to lack of conducive environmental conditions. Nitrogen transformation via DNRA was found to be relatively insignificant. Future studies should investigate the potential of employing cold-resilient plant species to maintain both NH₄⁺ and NO₃^- removal in cold weather conditions.

Application of Reeds as Carbon Source for Enhancing Denitrification of Low C/N Micro-Polluted Water in Vertical-Flow Constructed Wetland

Constructed wetlands have been applied to micro-polluted rivers and lakes. However, they often show poor nitrogen removal efficiency due to insufficient carbon sources for complete denitrification in the waters. In this study, a vertical-flow wetland system was built, in which reeds as a carbon source were added in the middle layer of the substrate. Thereby, the effect of the reed carbon source on denitrification of micro-polluted rivers and lakes with a low C/N ratio in the wetland and the denitrification mechanism were studied. The results showed that the concentrations of NH4+-N, NO3−-N and NO2−-N in the effluent of the constructed wetland were reduced to 0.17–0.35, 0.20–0.49 and 0.01–0.02 mg/L after adding the reed carbon source, and the removal efficiencies of the system for NH4+-N and NO3−-N reached 93.84% and 84.69%, respectively. The abundances of nirK, nirS, hzo and nrfA genes in the wetland substrate increased by 95.51%, 54.96%, 52.89% and 731.95%, respectively, which was considered to be related to the enhanced denitrification, anammox and dissimilatory nitrate reduction to ammonium of the wetland system. Reed planting promoted the increased abundances of amoA and nxrB genes, which might play a positive role in enhancing nitrification in wetland systems. The result of this study may provide a theoretical basis for the ecological restoration of low C/N micro-polluted water bodies.

The use of recycled materials in a biofilter to polish anammox wastewater treatment plant effluent

Anammox is gaining popularity for treating wastewater containing high-strength ammonia due to lower energy demand compared to conventional nitrification-denitrification processes; however, anammox is reported to increase nitrate loads in the effluent. The objective of this study was to assess the applicability of recycled materials [recycled concrete aggregate (RCA) and rice husks (RH)] as a polishing step to improve anammox reactor effluent quality. Anammox effluents were separately passed through two single-stage columns containing RCA and RH, and one two-stage column (50% RCA, 50% RH) to quantify total N, ammonia, nitrate, nitrite, and phosphate removal efficiencies. Langmuir isotherm experiments were conducted to quantify nitrate, nitrite, and phosphate sorption capacities in the columns. The RCA column exhibited the highest phosphate sorption capacity (0.074 mg/g), while the RH column exhibited higher nitrite and nitrate adsorption (0.063 mg/g and 0.023 mg/g respectively). We created a Hydrus-1D model to estimate pseudo-first-order reaction rates in the columns. Because RCA media can form metal-phosphate precipitates, the fastest phosphate reaction rate (1.58 min-1) occurred in the RCA column. The two-stage column demonstrated the greatest overall removals for all nutrients, and removal rates were consistent throughout the experimental period. The two-stage column achieved 15% total N, 94% ammonia-N, 38% nitrate-N, 75% nitrite-N, and 27% phosphate removal. The maximum nitrite, nitrate, and phosphate adsorption capacities in the two-stage column were 0.030 mg/g, 0.017 mg/g, and 0.014 mg/g respectively. This is the first study to demonstrate that recycled materials can successfully be integrated into a biofilter as an effluent polishing step to remove nutrients from anammox wastewater.

Variability of Potential Soil Nitrogen Cycling Rates in Stormwater Bioretention Facilities

Low-impact development (LID) is a common management practice used to infiltrate and filter stormwater through vegetated soil systems. The pollutant reduction potential of these systems is often characterized by a single pollutant removal rate; however, the biophysical properties of soils that regulate the removal of pollutants can be highly variable depending on environmental conditions. The goal of this study was to characterize the variability of soil properties and nitrogen (N) cycling rates in bioretention facilities (BRFs). Soil properties and potential N cycling processes were measured in nine curbside bioretention facilities (BRFs) in Portland, OR during summer and winter seasons, and a subset of six sites was sampled seasonally for two consecutive years to further assess temporal variability in soil N cycling. Potential N cycling rates varied markedly across sites, seasons, and years, and higher variability in N cycling rates was observed among sites with high infiltration rates. The observed seasonal and annual changes in soil parameters suggest that nutrient removal processes in BRFs may be highly variable across sites in an urban landscape. This variability has important implications for predicting the impacts of LID on water quality through time, particularly when estimated removal rates are used as a metric to assess compliance with water quality standards that are implemented to protect downstream ecosystems.

Unconventional microbial mechanisms for the key factors influencing inorganic nitrogen removal in stormwater bioretention columns

Bioretention systems are environmentally friendly measures to control the amount of water and pollutants in urban stormwater runoff, and their treatment performance for inorganic N strongly depends on various microbial processes. However, microbial responses to variations of N mass reduction in bioretention systems are complex and poorly understood, which is not conducive to management designs. In the present study, a series of bioretention columns were established to monitor their fate performance for inorganic N (NH4+and NO3-) by using different configurations and by dosing with simulated stormwater events. The results showed that NH4+ was efficiently oxidized to NO3-, mainly by ammonia- and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO3- removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NO3-mass reduction through root assimilation and enhancement of microbial NO3- reduction in the rhizosphere. The construction of an organic-rich saturation zone can make the redox potential too low for heterotrophic denitrification to occur, so as to ensure high NO3- mass reduction mainly via stimulating chemolithotrophic NO3- reduction coupled with oxidation of reductive sulfur compounds derived from the bio-reduction of sulfate. In contrast, in the organic-poor saturation zone, multiple oligotrophic NO3- reduction pathways may be responsible for the high NO3- mass reduction. These findings highlight the necessity of considering the variation of N bio-transformation pathways for inorganic N removal in the configuration of bioretention systems.

Field Study of the Road Stormwater Runoff Bioretention System with Combined Soil Filter Media and Soil Moisture Conservation Ropes in North China

Growing concerns about urban runoff pollution and water scarcity caused by urbanization have prompted the application of bioretention facilities to manage urban stormwater. The purpose of this study was to evaluate the performance of proposed bioretention facilities regarding road runoff pollutant removal and the variation characteristics of the media physicochemical properties and microbial diversity in dry-cold regions. Two types of bioretention facilities were designed and then constructed in Tianjin Eco-city, China, on the basis of combined soil filter media screened by a laboratory-scale test with a modified bioretention facility (MBF) containing soil moisture conservation ropes. Redundancy analysis was performed to evaluate the relationships between the variation in media physicochemical properties and microbial communities. An increase in media moisture could promote an increase in the relative abundance of several dominant microbial communities. In the MBF, the relatively low nitrate-nitrogen (NO3-N) (0.75 mg/L) and total nitrogen (TN) (4.71 mg/L) effluent concentrations, as well as better removal efficiencies for TN and NO3-N in challenge tests, were mainly attributed to the greater relative abundance of Proteobacteria (25.2%) that are involved in the microbial nitrogen transformation process. The MBF also had greater media microbial richness (5253 operational taxonomic units) compared to the conventional bioretention facility and in situ saline soils. The results indicate that stormwater runoff treated by both bioretention facilities has potential use for daily greening and road spraying. The proposed design approach for bioretention facilities is applicable to LID practices and sustainable stormwater management in other urban regions.

Presence of bacteria capable of PCB biotransformation in stormwater bioretention cells

Core samples from bioretention cell media as well as surface stormwater sediment samples from seven urban areas were collected to assess the potential for biotransformation activity of polychlorinated biphenyls (PCBs). The presence of putative organohalide-respiring bacteria in these samples was studied. Based on extracted DNA, Dehalobacter, Dehalogenimonas and Dehalococcoides were detected. Other organohalide-respiring bacteria like Desulfitobacterium and Sulfurospirillum were not studied. Bacteria containing the genes encoding for biphenyl 2,3-dioxygenase (bphA) or 2,3-dihydroxybiphenyl 1,2-dioxygenase (bphC) were detected in 29 of the 32 samples. These genes are key factors in PCB aerobic degradation. Transcribed bacterial genes from putative organohalide-respiring bacteria as well as genes encoding for bphA and bphC were obtained from the microbial community, thus showing the potential of organohalide respiration of PCBs and aerobic PCB degradation under both aerobic and anaerobic conditions in the surface samples collected at the bioretention site. Presence and concentrations of 209 PCB congeners in the bioretention media were also assessed. The total PCB concentration ranged from 38.4 ± 2.3 ng/g at the top layer of the inlet to 11.6 ± 1.2 ng/g at 20-30 cm at 3 m from the inlet. These results provide documentation that bacteria capable of PCB transformation, including both anaerobic dechlorination and aerobic degradation, were present and active in the bioretention.

Biochar-pyrite bi-layer bioretention system for dissolved nutrient treatment and by-product generation control under various stormwater conditions

Bioretention system with modified media has been increasingly used to control dissolved nutrients in stormwater runoff. However, complicated removal processes and improper design have made most of them hardly achieve comprehensive dissolved nutrient removal and even show by-product generation problem, especially during extreme stormwater events. Here, a modified biochar-pyrite (FeS₂) bi-layer bioretention system was developed and tested under various stormwater conditions with conventional sand-based and woodchip-based bioretention systems as controls. The modified system showed high stability and efficiency for dissolved nutrient treatment. The removal of dissolved organic nitrogen, ammonium, total dissolved nitrogen, and total dissolved phosphorus were 86.3-93.0%, 95.3-98.1%, 41.4-76.5%, and 69.7-88.2%, respectively. Stormwater conditions only influence nitrate removal which decreased with the increase of total received volume and increased with the extension of antecedent drying duration. Net sulfate and total iron generation were very low, less than 8 mg/L and 0.15 mg/L, respectively. Several microbiology, spectroscopy, and media related tests further demonstrated that the vadose zone and submerged zone showed synergy effects during operation. Biochar addition facilitated ammonium adsorption, nitrification, and in situ denitrification in the vadose zone. It also intercepted dissolved oxygen, which alleviated aerobic pyrite oxidation and created an anoxic condition for the submerged zone. Meanwhile, the pyrite-modified submerged zone achieved stable mixotrophic denitrification. The generated iron intermediate products further controlled phosphorus from both influent and vadose zone leaching into stable forms. Mixotrophic denitrification and potential sulfate reduction processes also reduce sulfate generation. Overall, the biochar-pyrite bi-layer bioretention is a highly promising technology for stormwater runoff treatment, with effective dissolved nutrient removal and minimal by-product generation in various stormwater conditions.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.

Biological nitrogen removal from stormwater in bioretention cells: a critical review

Excess nitrogen in stormwater degrades surface water quality via eutrophication and related processes. Bioretention has been recognized as a highly effective low-impact development (LID) technology for the management of high runoff volumes and reduction of nitrogen (N) pollutants through various mechanisms. This paper provides a comprehensive and critical review of recent developments on the biological N removal processes occurring in bioretention systems. The key plant- and microbe-mediated N transformation processes include assimilation (N uptake by plants and microbes), nitrification, denitrification, and anammox (anaerobic ammonia oxidation), but denitrification is the major pathway of permanent N removal. Overall, both laboratory- and field-scale bioretention systems have demonstrated promising N removal performance (TN: >70%). The phyla Bacteroidetes and Proteobacteria are the most abundant microbial communities found to be enriched in biofilter media. Furthermore, the denitrifying communities contain several functional genes (e.g., nirK/nirS, and nosZ), and their concentrations increase near the surface of media depth. The N removal effectiveness of bioretention systems is largely impacted by the hydraulics and environmental factors. When a bioretention system operates at: low hydraulic/N loading rate, containing a saturation zone, vegetated with native plants, having deeper and multilayer biofilter media with warm climate temperature and wet storm events periods, the N removal efficiency can be high. This review highlights shortcomings and current knowledge gaps in the area of total nitrogen removal using bioretention systems, as well as identifies future research directions on this topic.

Estimating nitrogen fates and gross transformations in bioretention systems with applications of 15N labeling methods

Two batches of ¹⁵NH₄⁺ and ¹⁵NO₃^- labeling experiments were conducted to understand the complex nitrogen (N) fates and transformations in bioretention systems, respectively. The fates of ¹⁵NH₄⁺ were first traced in six bioretention systems with different wet-dry regimes and submerged zone settings during four months, indicating: (1) ¹⁵N was mainly leached during the second storm events following the ¹⁵NH₄⁺ addition during the first storm events, suggesting nitrification during the dry period; (2) the main ¹⁵NH₄⁺ fates after four-month exposure were: soil media 59.6%-80.0%, outflow 5.3%-16.4%, plants 2.3%-8.9%, denitrification losses 0-28.4%; (3) longer antecedent dry weather period and submerged zone could help alleviate outflow NO₃^- leaching. The occurrence time, positions and rates of major N transformation processes were later examined by the ¹⁵NO₃^- labeling experiment in a bioretention system over an 8 d wet-dry cycle, indicating: (1) during the brief wet period, hydraulic mixing of "old" water and "new" inflow mainly occurred; (2) during the subsequent dry period, gross rates of nitrification, denitrification and mineralization showed "pulse effects", i.e. peaking at 24-48 h and decreasing significantly within 72 h; (3) denitrification became more dynamic with soil media depth, especially in submerged zone. This study evidenced the feasibility of ¹⁵N labelling method in studying N dynamics in bioretention systems and would inform future engineering and stormwater management practices.

Bioretention for removal of nitrogen: processes, operational conditions, and strategies for improvement

As one of the low-impact development measures, bioretention plays an important role in reducing the runoff peak flow and minimizing runoff pollutants, such as heavy metals, suspended solids, and nutrients. However, the efficiency of nitrogen removal in the bioretention system is unstable, owing to the different chemical properties of various forms of nitrogen and the limitations of current bioretention system for nitrogen transformation. This review article summarizes the recent advances in bioretention system in treatment of urban stormwater and agricultural runoff for nitrogen removal. The microbial characteristics and main processes of nitrogen transformation in bioretention are reviewed. The operational conditions affecting nitrogen removal, including climatic conditions, pH, wet-dry alternation, influent loads and nitrogen concentration, and hydraulic residence time are discussed. Finally, measures or strategies for increasing nitrogen removal efficiency are proposed from the perspectives of structural improvement of the bioretention system, optimization of medium composition, and enhancement of the nitrogen removal reaction processes.

The impact of media, plants and their interactions on bioretention performance: A review

Bioretention systems have gained considerable popularity as a more natural approach to stormwater management in urban environments. The choice of bioretention media is frequently cited as one of the critical design parameters with the ultimate impact on the performance of the system. The goal of this review is to highlight data that challenge the importance of media as being the dominant design parameter and argue that the long-term performance is shaped by the interactions between media and the living components of a bioretention system, especially vegetation. Some of the key interactions are related to the impact of plant roots on media pore structure, which has implications on infiltration, storage capacity, and treatment. Another relevant interaction pertains to evapotranspiration and the associated impacts on the water balance and the water quality performance of bioretention systems. The impacts of vegetation on the media are highlighted and actual, as well as potential, impacts of plant-media interactions on bioretention performance are presented.

Microbial diversity for the improvement of nitrogen removal in stormwater bioretention cells with three aquatic plants

The aquatic plants Iris pseudacorus L., Canna indica L. and Lythrum salicaria L. have been proved to be potential choices for nitrogen removal. However, little is known about microbial diversity for the improvement of nitrogen removal (nitrification and denitrification) in stormwater bioretention cells with the above plants. In this study, batch experiments were conducted to investigate nitrogen removal, substrate layer status, and bacterial community structure to understand microbial diversity and evaluate its effects on performances of nitrogen removal. Ammonia nitrogen removal in the bioretention cell with Lythrum salicaria L. was the highest (88.1%), which was consistent with oxidation reduction potential (ORP) in the bioretention cells. Whilst, removals for both total nitrogen and nitrate were the highest in the bioretention cell with Canna indica L., which was in line with urease activity in the mentioned cells. The used plants had different impact on top 11 dominant microflora at phylum level in the used bioretention cells. Ramlibacter and Nitrosomonadaceaea were both responsible for the difference of nitrogen removal in the bioretention cells with three aquatic plants, suggesting the enhancement of the above dominant microflora could strengthen nitrogen removal in the used bioretention cells.

Plant rhizosphere, soil microenvironment, and functional genes in the nitrogen removal process of bioretention

The coupling effect of plant, soil, and microbes in bioretention provides conditions conductive for nitrogen (N) removal via nitrification, accretion, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA). To assess this process, the effects of plant rhizosphere, isotope labelled inorganic N distribution, soil microenvironment variation, and functional genes' abundance at different pollutant loads in simulated bioretention were evaluated. The results show that both nitrification and denitrification rates decreased with soil depths, and the maximum overall nitrification efficiency was about 3.4-4.4 times higher than that of denitrification. The overall nitrification rates decreased but the overall denitrification rates increased with pollutant loads, showing the limitation of bioretention for N removal at high loads within limited retention time. The radial oxygen loss (ROL) from the roots increased with the pollutant load. DO was higher while pH was lower in the rhizosphere soil compared with those in the soil of the same depth; thus, the nitrification rates was relative higher while the denitrification rates were lower in the rhizosphere soil. The sequence of nitrification rates was O > I > H at all soil depths, which is consistent with the DO and pH profiles of different plants at different loads. The abundance of 16S rRNA genes, nitrifying genes (amoA, nxrB), and denitrifying genes (narG, nirK, qnorB and nosZ) in the soil of plant group were higher than those of the control group without plant, suggesting that plants help increasing soil microbial abundance. The abundance of 16S rRNA genes, nitrifying genes, and denitrifying genes decreased with soil depth. More copies of denitrifying genes in the topsoil, which was thought as aerobic conditions, suggests that denitrification may occur in anoxic microenvironment of the topsoil or during stormwater retention period. Moreover, the root exudates and other organic matter accumulated in the topsoil may provide carbon sources for local denitrification in bioretention.

Evaluation of an enhanced treatment media and permeable pavement base to remove stormwater nitrogen, phosphorus, and metals under simulated rainfall

An enhanced stormwater treatment media, developed previously by the authors, was shown to effectively retain dissolved phosphorus (DP) and total Cu and Zn under simulated rainfall. The media comprises expanded shale aggregate, aluminum-based water treatment residual (WTR), and a psyllium-based binder. A 5-cm layer of media was installed as a permeable pavement base layer in a laboratory mesocosm and subjected to rainfall simulations using synthetic stormwater. At rainfall intensity of 0.66 cm/h, effluent DP event mean concentration (EMC) fell from an average of 0.22 mg/L P to below the EPA water quality criterion of 0.037 mg/L in 8 of 9 storms. DP retention increased at lower rainfall intensity and lower pH. Effluent EMC was lowered to less than 30 μg/L for total Cu and less than 92 μg/L for total Zn, on average, relative to average influent EMCs above 61 and 255 μg/L for total Cu and Zn, respectively. Effluent total Al EMCs were below the 25 μg/L detection limit for all storm simulations, indicating Al leaching from the WTR-containing media not to be an issue. Inclusion of an internal water storage (IWS) zone resulted in a 33% total nitrogen (TN) load reduction when adequate carbon was present to advance denitrification. This study provides an evaluation and demonstrates expected treatment performance of a novel stormwater treatment media under conditions representative of urban stormwater.

Urban stormwater characterization, control, and treatment

This review summarizes over 250 studies published in 2018 related to the characterization, control, and management of urban stormwater runoff. The review covers three broad themes: (a) quantity and quality characterization of stormwater, (b) control and treatment of stormwater runoff, and (c) implementation and assessment of watershed-scale green stormwater infrastructure (GSI). Each section provides an overview of the 2018 literature, common themes, and future work. Several themes emerged from the 2018 literature including exploration of contaminants of emerging concern within stormwater systems, characterization and incorporation of vegetation-driven dynamics in stormwater control measures, and the need for interdisciplinary perspectives on the implementation and assessment of GSI. PRACTITIONER POINTS: Over 250 studies were published in 2018 related to the characterization, control, and treatment of stormwater. Studies cover general stormwater characteristics, control and treatment systems, and watershed-scale assessments. Trends in 2018 include treatment trains, vegetation dynamics, and interdisciplinary perspectives.

Diversity and metabolism effects of microorganisms in bioretention systems with sand, soil and fly ash

Recently, both sand and fly ash have been used for nutrient removal in bioretention systems. However, the improvement in nutrient removal was hampered by a lack of data about of microbial diversity and metabolism effects in the mentioned materials based bioretention systems. Therefore, a mixture with sand, soil and fly ash (1:1:1) was selected as the base in bioretention systems. The investigation of microbial diversity implied that 11 dominant microflora were found, which changed weakly at phylum level but significantly at genus level. The analysis for both urease and extracellular polymer (EPS) showed that urease levels increased with the increase of submerged zone height, which was in line with nitrogen removal, while EPS had the opposite situation. Overall evaluation of microbial role suggested that the enhancement of dominant microflora in the used bioretention systems, like Chloroflexi and Nitrospirae, could strengthen nitrogen removal.

Nitrogen cycling processes within stormwater control measures: A review and call for research

Stormwater control measures (SCMs) have the potential to mitigate negative effects of watershed development on hydrology and water quality. Stormwater regulations and scientific literature have assumed that SCMs are important sites for denitrification, the permanent removal of nitrogen, but this assumption has been informed mainly by short-term loading studies and measurements of potential rates of nitrogen cycling. Recent research concluded that SCM nitrogen removal can be dominated by plant and soil assimilation rather than by denitrification, and rates of nitrogen fixation can exceed rates of denitrification in SCM sediments, resulting in a net addition of nitrogen. Nitrogen cycling measurements from other human-impacted aquatic habitats have presented similar results, additionally suggesting that dissimilatory nitrate reduction to ammonium (DNRA) and algal uptake could be important processes for recycling nitrogen in SCMs. Future research should directly measure a suite of nitrogen cycling processes in SCMs and reveal controlling mechanisms of individual rate processes. There is ample opportunity for research on SCM nitrogen cycling, including investigations of seasonal variation, differences between climatic regions, and trade-offs between nitrogen removal and phosphorus removal. Understanding nitrogen dynamics within SCMs will inform more efficient SCM design and management that promotes denitrification to help mitigate negative effects of urban stormwater on downstream ecosystems.

Bacteria-supported iron scraps for the removal of nitrate from low carbon-to-nitrogen ratio wastewater

A novel bacteria-supported iron scraps (BSIS) system was developed for nitrate removal from low carbon-to-nitrogen ratio (C/N) wastewater. The system consisted of low-cost iron scraps and the accumulated denitrifying-related bacteria enriched from an Fe-wastewater environment when the system was operating. After operating for 39 d, the nitrate removal rate of the system increased to 73.55% within 24 h. The extraction of bacteria from the system revealed that iron scraps and bacteria had a synergistic effect on nitrate removal and bacteria only took effect when cooperating with iron. Microbial analysis using high-throughput sequencing showed that Hydrogenophaga, which is closely related to hydrogenotrophic denitrification, became the dominant genus in the system. The system provides a promising approach to the treatment of nitrate in low C/N wastewater and it has the potential for large-scale application due to the low cost, simple operation and relatively high removal rate.

A cost-effective system for nitrate removal was developed with the key role of iron and genus Hydrogenophaga.

Dynamics and Functional Potential of Stormwater Microorganisms Colonizing Sand Filters

Stormwater management is increasingly relying on engineered infiltration systems (EIS) to reduce the volume and improve the quality of managed stormwater. Yet, EIS in the field will be colonized by a diverse array of environmental microorganisms that change the physiochemical properties of the EIS and provide a habitat for microorganisms with harmful or beneficial qualities. Understanding factors influencing the composition and stability of microbial communities could open up strategies for more efficient management of stormwater. Here, we analyzed the potential pathogenic and metabolic capabilities of stormwater microorganisms colonizing idealized EIS (i.e., sand columns) under laboratory conditions over time. The diversity of microbial communities was analyzed using 16S rRNA gene sequencing, and potential pathogens and denitrifying microbes were identified from taxonomic match to known species. Denitrification potential as determined by nosZ abundance was also assessed with quantitative polymerase chain reaction PCR. Our findings demonstrate that replicate microbial communities colonizing sand columns change in a similar way over time, distinct from control columns and the source community. Potential pathogens were initially more abundant on the columns than in the stormwater but returned to background levels by 24 days after inoculation. The conditions within sand columns select for potential denitrifying microorganisms, some of which were also potential pathogens. These results demonstrate that a diverse suite of stormwater microorganisms colonize sand filters, including a transient population of potential pathogens and denitrifiers. Manipulating the inoculating microbial community of EIS could prove an effective mechanism for changing both potential pathogens and denitrifying bacteria.