Temporary storage or permanent removal? The division of nitrogen between biotic assimilation and denitrification in stormwater biofiltration systems

The impact of deicer and anti-icer use on plant communities in stormwater detention basins: Characterizing salt stress and phytoremediation potential

We present the results of a 1-year study that quantified salt levels in stormwater, soils, and plant tissues from 14 stormwater detention basins across Northern VA in an above-average snow year. We characterize (1) the level of salt stress plants experience, (2) the extent to which current plant communities feature salt tolerant species, and (3) the capacity of these species to phytoremediate soils and reduce the impacts of deicer and anti-icer use. Our results suggest that detention basin vegetation experience a range of salt stress levels that depend on drainage area type (roads: moderate to high > parking lots: low to moderate > pervious areas: none). Established thresholds for salt sensitive vegetation (Na+, Cl+, electrical conductivity, sodium adsorption ratio, exchangeable sodium percentage) were exceeded at least twice in stormwater or soils from all systems draining roads and half of systems draining parking lots. Winter exceedances were most common, but saline conditions did persist into the growing season, particularly at sites draining roads. Two hundred fifty-five plant species were identified across all detention basins, including 48 natives capable of tolerating elevated salt levels (electrical conductivity ≥2 dS/m). Within-tissue concentrations of sodium and chloride ions were highest in Typha (latifolia and angustifolia) (11.1 mg Na+/g; 30 mg Cl-/g), making it our top phytoremediation candidate. Scaling these concentrations up, we estimate that a standard-size highway detention basin (2000-3000 m2) with 100 % cattail cover can phytoremediate up to 100 kg of Na+ and 200 kg of Cl- per year. Uptake at this level is not sufficient to offset winter salt application, constituting only 5-6 % of basin inputs. This suggests that phytoremediation should not be considered a standalone solution to basin salinization, although it could be one approach of many in a broader salt management strategy.

Soil-derived dissolved organic matter and nutrient sources from urban stormwater control measures

There has been significant investment in stormwater control measures (SCM) to reduce erosion, filter pollutants, and mitigate peak storm flows within urban watersheds. SCMs have variable hydrologic connectivity to downstream waterbodies where SCMs may rapidly export water during storm events but dry up in-between events and these alternating wet/dry cycles influence the biogeochemical processes that occur in SCM soils. While the performance of SCMs has been evaluated for nutrient removal, less is known about the potential for mobilization of nutrients and dissolved organic matter (DOM) that accumulate in SCM soils. Because of the significant aquatic ecosystem and human health implications related to excess DOM and nutrients, further work is needed to understand how urban SCMs and wetland soils may be impacting downstream water quality. Water-soluble organic matter (WSOM) was extracted from SCM and urban wetland soils located in the Rappahannock River watershed, part of the larger Chesapeake Bay watershed in the Mid-Atlantic region, to assess the potential mobilization of DOM and nutrients from SCM soils. We found low quantities of readily released organic matter and nutrients regardless of SCM type. WSOM concentration and composition did not vary spatially within SCMs. However, SCM surface water had plant-like signatures and was more aromatic compared to WSOM, indicating that organic matter processing occurs as water moves through SCM soil media. Monthly sampling of SCM soils suggests WSOM and nutrient quantities peak when SCMs are dry. When accounting for spatial extent, SCMs likely serve as smaller potential sources of carbon to downstream aquatic ecosystems relative to larger naturally occurring urban wetlands. Exploring the potential soil-derived DOM in SCMs and urban wetlands furthers our knowledge of how urban systems influence DOM concentration and composition, while examining the performance of SCMs relative to natural soil systems.

Modeling the influences of rainfall conditions on nitrogen removal effects of a permeable pavement system

Permeable pavement systems are important facilities for alleviating runoff pollution. However, few studies have focused on outflow nitrogen processes under varying rainfall conditions. In this study, we proposed a permeable pavement system model that can simulate the nitrogen transformation processes. The model was verified with data on outflow rates and outflow nitrogen concentrations (ammonia nitrogen (NH4-N), nitrate nitrogen (NO3-N), and total nitrogen (TN)) processes for six consecutive rainfall events of a permeable pavement system in Shenzhen, China. Based on the validated model, scenarios were designed to simulate the influences of rainfall conditions on nitrogen removal effects at the single rainfall event and annual scale, respectively. The results indicate that: (i) the model can explicitly describe hydrological and nitrogen processes of the system, and the Nash Sutcliffe Efficiency and percent bias of outflow rates and outflow nitrogen concentrations range from 0.5 to 0.9 and from -22.4% to 24.9%, respectively; (ii) the sensitivity analysis reveals that the empirical coefficient (C) related to evaporation in the gravel layer is sensitive at the annual scale but not at rainfall events scale. Parameters related to mineralization and microbial assimilation (i.e., mineralization rate constant in the gravel layer (k3N,mine), microbial assimilation rate constant of NH4-N, NO3-N, and ON (k1N,bio, k2N,bio, k3N,bio)) are sensitive annually but not at rainfall events scale; (iii) in the single rainfall event simulations, nitrogen removal efficiencies decrease with increasing rainfall return periods but is not significantly affected by rainfall peak coefficients. Nitrogen removal efficiencies improve notably with antecedent dry period extension for single rainfall event with antecedent dry period less than 2 days. In addition, we found that the outflow process significantly affects the outflow nitrogen concentrations process. In annual-scale simulations, extending the antecedent dry period only significantly boosts runoff reduction efficiency and nitrogen removal efficiencies for rainfall events with less than 10 mm.

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.

Plant species contribution to bioretention performance under a temperate climate

Bioretention systems are green infrastructures increasingly used to manage urban stormwater runoff. Plants are an essential component of bioretention, improving water quality and reducing runoff volume and peak flows. However, there is little evidence on how this contribution varies between species, especially in temperate climates with seasonal variations and plant dormancy. The aim of our study was to compare the performance of four plant species for bioretention effectiveness during the growing and dormant periods in a mesocosm study. The species selected (Cornus sericea, Juncus effusus, Iris versicolor, Sesleria autumnalis) are commonly used in bioretention and cover a wide range of biological forms and functional traits.All bioretention mesocosms were effective in reducing water volume, flow and pollutant levels in both of the studied periods. Plants decreased runoff volume and increased contaminant retention by reducing water flow (up to 2.7 times compared to unplanted systems) and increasing water loss through evapotranspiration during the growing period (up to 2.5 times). Plants improved removal of macronutrients, with an average mass removal of 55 % for TN, 81 % for TP and 61 % for K compared to -6 % (release), 61 % and 22 % respectively for the unplanted systems. Except for Sesleria, mass removal of trace elements in planted mesocosms was generally higher than in unplanted ones (up to 8.7 %), regardless of season. Between-species differences in exfiltration rate and improved water quality followed the same order as their evapotranspiration rate and overall size, measured in terms of plant volume, leaf biomass, total leaf area and maximum average root density (Cornus > Juncus > Iris > Sesleria). By increasing evapotranspiration, plants decreased runoff volume and increased contaminant retention. Nutrient removal was partly explained by plant assimilation. Our study confirms the importance of plant species selection for improving water quality and reducing runoff volume during bioretention under a temperate climate.

A review of compaction effect on subsurface processes in soil: Implications on stormwater treatment in roadside compacted soil

Sustainable cities require spacious infrastructures such as roadways to serve multiple functions, including transportation and water treatment. This can be achieved by installing stormwater control measures (SCM) such as biofilters and swales on the roadside compacted soil, but compacted soil limits infiltration and other functions of SCM. Understanding the effect of compaction on subsurface processes could help design SCM that could alleviate the negative impacts of compaction. Therefore, we synthesize reported data on compaction effects on subsurface processes, including infiltration rate, plant health, root microbiome, and biochemical processes. The results show that compaction could reduce runoff infiltration rate, but adding sand to roadside soil could alleviate the negative impact of compaction. Compaction could decrease the oxygen diffusion rate in the root zone, thereby affecting plant root activities, vegetation establishment, and microbial functions in SCM. The impacts of compaction on carbon mineralization rate and root biomass vary widely based on soil type, aeration status, plant species, and inherent soil compaction level. As these processes are critical in maintaining the long-term functions of SCM, the analysis would help develop strategies to alleviate the negative impacts of compaction and turn road infrastructure into a water solution in sustainable cities.

Corncob-pyrite bioretention system for enhanced dissolved nutrient treatment: Carbon source release and mixotrophic denitrification

Solid biomass waste amendment and substrates modification in bioretention systems have been increasingly used to achieve effective dissolved nutrients pollution control in stormwater runoff. However, the risk of excess chemical oxygen demand (COD) leaching from organic carbon sources is often overlooked on most occasions. Pyrite is an efficient electron donor for autotrophic denitrification, but little is known about the efficacy of autotrophic-heterotrophic synergistic effect between additional carbon source and pyrite in bioretention. Here, four bioretention columns (i.e., corncob column (C), pyrite column (P), the corncob-pyrite layered column (L-CP), and the corncob-pyrite mixed column (M-CP)) were designed and filled with soil, quartz sand, and modified media to reveal the synergistic effects. The results showed that the corncob-pyrite layered bioretention could maintain low COD effluent concentration with high stability and efficiency in treating dissolved nutrients. When the influent nitrogen and phosphorus concentrations were 8.46 mg/L and 0.94 mg/L, the average removal rates of ammonia nitrogen, total inorganic nitrogen, and phosphate were 83.6%, 70.52%, and 76.35%, respectively. The scouring experiment showed that placing the corncob in the mulch layer was beneficial to the sustained release of dissolved organic carbon (DOC). Erosion pits were found in the SEM images of used pyrite, indicating that autotrophic denitrifying bacteria in the bioretention could react with pyrite as an electron donor. The relative abundance of Thiobacillus in the submerged zone of the corncob-pyrite layered bioretention reached 38.39%, indicating that the carbon source in the mulch layer increased the relative abundance of Thiobacillus. Coexisting heterotrophic and autotrophic denitrification in this bioretention created a more abundant microbial community structure in the submerged zone. Overall, the corncob-pyrite layered bioretention is highly promising for stormwater runoff treatment, with effective pollution removal and minimal COD emission.

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.

Bioretention system mediated by different dry-wet alterations on nitrogen removal: Performance, fate, and microbial community

In laboratory experiments, the nitrogen migration and transformation in the stormwater bioretention system under different dry-wet alterations were studied. The removal efficiency showed that nitrogen could be removed efficiently in bioretention system under all dry-wet alterations, and the shorter antecedent dry days (ADDs) (1-5 days) were beneficial to the removal of nitrogen before plants decay, compared to the longer ADDs (7-22 days). Using a new method combined with Hydrus-1D model, water transport was simulated and nitrogen migration in bioretention system was quantified, indicating that NH₄⁺-N was mainly removed in the planting layer, and the removal of NO₃^--N was occurred in the submerged layer. Fate experiment showed the main fate of the nitrogen was microorganisms (1-5 ADDs) and soil immobilization (7-22 ADDs). Microbial analysis showed that shorter ADDs (1-5 days) were suitable for Firmicutes growth, while Proteobacteria and Actinobacteria accounted for greater abundance under longer ADDs (7-22 days). Canonical correlation analysis (CCA) revealed the relationships between microbial community and environmental factors. Soil moisture content, soil organic matter (SOM), TN (water), root length, and NO₃^--N (water) were significantly correlated with bacterial community. This work may give new insights into nitrogen migration and transformation, and can provide a reference for the further mechanism study and construction of stormwater bioretention systems.

Spatial performance assessment of reed bed filtration in a constructed wetland

Constructed wetlands (CW) are implemented to improve water quality through filtration by plants (macrophytes), which sequester nutrients and contaminants. Macrophyte beds in CWs reduce the speed of water flow, aiming to improve the water quality by sedimentation and filtration with increasing distance from the inflow. Few studies have assessed spatial distribution and accumulation concentrations of nutrients and contaminants in CW macrophytes as a performance indicator for wetland functionality and management. Macrophytes and water were analysed for nutrient and contaminant accumulation in-situ at a stormwater-fed CW and water remediation site in South Australia. During the austral summer, macrophytes were sampled at 36 sites and water at 46 sites selected by a systematic GIS produced grid covering the entire wetland, which determined distance from the inflow for each site. A total of 144 Schoenoplectus validus (stems and roots) macrophyte samples (i.e. carbon-C, nitrogen-N, Trace elements) and 183 water samples (i.e. total suspended solids-TSS, total nitrogen-TN, total carbon-TC, nitrate-NO₃^-/ nitrite-NO₂^- and ammonia-NH₄⁺) were analysed. Concentrations of water chemistry parameters that significantly increased with distance away from inflow included; TC (P = 0.0008), TN (P = 0.0001), and NH₄⁺ (P = 0.0001), while there was significant decrease in TSS (P = 0.0001). The macrophyte S. validus significantly decreased in height (P = 0.0001) and biomass (P = 0.03) with distance from the inflow. Spatial mapping of nutrients and contaminants with distance from inflow identified increasing TC and C characteristics from inflow to outflow and identified where TSS were removed from the water column. Through this spatial assessment approach of the Oaklands CW, management has identified problem areas with flow regimes that require further investigation to enhance macrophyte water filtration performance which can be used in CWs elsewhere in the world.

A comprehensive review on the long-term performance of stormwater biofiltration systems (SBS): Operational challenges and future directions

Stormwater biofiltration systems (SBS) are a popular technology for mitigating the negative effects of urbanization on the hydrological processes and water quality in urban areas. However, little is known about SBS's long-term performance in actual field conditions. The findings of a review of the scientific literature on the long-term performance of SBS are presented in this paper. The findings show that only a few studies have investigated the performance of SBS and its change over time, and that the results of laboratory and field experiments differed due to the presence of plants, regular maintenance, and some uncertain environmental factors. Based on the existing knowledge gaps in this field, the main challenges observed was the lack of long-term field data series, and the existing mathematical models are not able to accurately forecast the long-term performance of SBS. This could be owing to the difficulties in monitoring activities, the high costs involved and the unpredictability around the operational timeframe. Future study should concentrate on the implementation of simulation and modeling-based research in pilot and full-scale SBS, and the inclusion of new performance indicators should be considered as a priority.

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.

Evaluating low impact development practices potentials for increasing flood resilience and stormwater reuse through lab-controlled bioretention systems

Low impact development practices (LID) as alternative measures of urban drainage can be used within the approach of resources recycling and co-management. This study evaluates the potential contribution of a bioretention system to flood control, non-potable water demands (NPD) and resources co-management. Bioretention setups were tested experimentally under variable conditions to identify operational key-factors to multiple purposes. Additionally, the efficiencies obtained for laboratory scale were extrapolated for household and watershed scale, quantifying the indicators of water demand reduction (WDR), energy demand reduction (EDR) and carbon emission reduction (CER) for hybrid systems with LID. The laboratory results indicated that the use of a bioretention with a submerged zone can improve the quality of the water recovered for reuse, while maintaining the efficiency of runoff retention and peak flow attenuation. Comparing the bioretention effluent quality with the Brazilian standards for stormwater reuse, the parameters color, turbidity, E. coli and metals were above the limits, indicating the necessity of a better treatment for solids particles and disinfection. Expanding the analysis to watershed scale, the bioretention helped to reduce NPD demands up to 45%, leading to a reduction in energy demand and carbon emission from the centralized water supply system.

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.

The effect of intermittent drying and wetting stormwater cycles on the nutrient removal performances of two vegetated biofiltration designs

Vegetated biofiltration systems (biofilters) are now a well-established technology for treatment of urban stormwater, typically showing high nutrient uptake. However, the impact of high temporal variability of rainfall events (further exacerbated by climate change) on nitrogen and phosphorus removal processes, within different biofiltration designs, is still unknown. Hence, a laboratory-based study was conducted to uncover mechanisms behind nutrient removal in biofilters across different drying and wetting regimes. Two sets of experimental columns were based on (1) the standard biofiltration design (unsaturated zone only), and (2) combination of unsaturated and saturated (submerged) zone (SZ) with additional carbon source. Columns were watered with synthetic stormwater according to three drying and wetting schemes, exploring 1, 2, 3, 4 and 7-week drying. Hydraulic performance, soil moisture and pollutant removal were monitored. The results show that hydraulic conductivity of SZ design experiences less change over time compared to standard design, due to slower media drying, crack formation and lower plant die-off. Varied drying lengths challenged both designs differently, with 2-week drying resulting in significant drop of performance across most pollutants in standard design (except ammonia), while SZ design was able to retain high performance for up to four weeks of drying, sustaining microbial and plant uptake. Increased oxygenation of SZ columns during short-term drying was beneficial for ammonia and phosphorus removal. While SZ design showed better performance and quicker recovery for nitrogen removal, in regions with inter-rain event shorter than two weeks, the standard design (no saturated zone, no carbon source) can achieve similar if not better results.

Nitrate removal uncertainty in stormwater control measures: Is the design or climate a culprit?

Eutrophication is caused by excess nitrate and other nutrient exported via stormwater runoff to surface waters, which is projected to increase as a result of climate change. Despite recent increases in the implementation of stormwater control measures (SCM), nutrient export has not abated, indicating poor or inconsistent removal capacities of SCM for nitrate. However, the cause of the variability is unclear. We show that both design and local climate can explain nitrate removal variability by critically analyzing data reported on the international BMP database for nitrate removal by four common types of SCM: bioretention cells, grass swales, media filters, and retention ponds. The relative importance of climate or design on nitrate removal depends on the SCM type. Nitrate removal in grass swales and bioretention systems is more sensitive to local climate than design specifications, whereas nitrate removal in the retention ponds is less sensitive to climate and more sensitive to design features such as vegetation and pond volume. Media filters without amendment have the least capacity compared to other SCM types surveyed, and their removal capacity was independent of the local climate. Adding amendments made up of carbon biomass, iron-based media, or a mixture of these amendments can significantly improve nitrate removal. The type of carbon biomass is also a factor since biochar does not appear to affect nitrate removal. This analysis can help inform the selection of SCM and modification of their design based on local and projected climate to maximize nitrate removal and minimize eutrophication.

Soil nitrogen accumulation, denitrification potential, and carbon source tracing in bioretention basins

Bioretention basins are one of the most commonly used green stormwater features, with the potential to accumulate significant levels of nitrogen (N) in their soil and to permanently remove it through denitrification. Many studies have investigated the N removal potential of bioretention basins through the assessment of inflow and outflow concentrations. However, their long-term N removal through soil accumulation and denitrification potential is less known. This study investigated the temporal variation of total N (TN) accumulation and denitrification potential in soils of 25 bioretention basins within a 13-year soil chronosequence, in a subtropical climate in Australia. The denitrification potential of a subset of seven bioretention basins was investigated in accompaniment with nutrient and soil characteristics. Additionally, stable isotopes (δ¹³C and δ¹⁵N) were used to assess temporal changes in the soil composition as well as to identify the sources of carbon (C) into these basins. Over 13 years of operation, TN accumulated faster in the top 5 cm of soil than deeper soils. Soil TN density was highest in the top 5 cm with an average of 1.4 kg N m^-3, which was about two times higher than deeper soils. Site age and soil texture were the best predictors of soil TN density and denitrification (1 to 9.7 mg N m^-2 h^-1). The isotope values were variable among basins. Low δ¹⁵N values in young basins (-1.02‰) suggested fixation as the main source of N, while older basins had higher δ¹⁵N, indicating higher denitrification. Bioretention plants were the primary source of soil C; although the occurrence of soil amendment also contributed to the C pool. To improve the performance of these bioretention basins, we recommend increasing vegetation at initial years after construction, and enhancing more frequent anaerobic conditions in the high soil profile. These two conditions can improve denitrification potential, and thus the performance of these basins for improving water quality.

The impact of stormwater biofilter design and operational variables on nutrient removal - a statistical modelling approach

Biofiltration systems can help mitigate the impact of urban runoff as they can treat, retain and attenuate stormwater. It is important to select the optimal design characteristics of biofilters (e.g., vegetation, filter media depth) to ensure high treatment performance. Operational conditions (e.g., infiltration rate) can also lead to significant changes in biofilter treatment performance over time. The impact of specific operational conditions on water quality treatment performance of stormwater biofilters is still not well understood. Furthermore, despite the importance of design characteristics and operational conditions on biofilter treatment performance, there is a lack of models that can be used to determine the optimal design and operation. In this paper, we developed a series of statistical models to predict the Total Phosphorus (TP) and Total Nitrogen (TN) removal performance of stormwater biofilters using various numbers of design characteristics and operational conditions. These statistical models were tested using data collected from four extensive laboratory-scale biofilter column studies. It was found that all models performed relatively well with a Nash-Sutcliffe Efficiency (NSE) of 0.42 - 0.61 for TP and 0.37 - 0.63 for TN. The most important design characteristics were filter media type and depth for TP treatment, and vegetation type and submerged zone depth for TN treatment. In addition, infiltration rate and inflow concentrations were the operational conditions that greatly influence outflow TP and TN concentrations from stormwater biofilters. As such, these variables need to be carefully considered when designing and operating stormwater biofilters. Sensitivity analysis results indicate that the model was quite sensitive to all regression coefficients and intercepts. Additional modelling exercises show that the model could be further simplified by reducing the number of cross-correlated parameters. These models can be used by practitioners for not just optimising the design, but also operating biofilters using real-time monitoring and control to achieve optimum performance.

Tracking denitrification in green stormwater infrastructure with dual nitrate stable isotopes

Strategies to mitigate watershed nitrogen export are critical in managing water resources. Green infrastructure (GI) has shown the ability to remove nitrogen from stormwater, but the removal mechanism is unclear. Denitrification removes nitrate from water permanently, making it the most desirable removal mechanism. The year-round field performance of a roadside infiltration GI practice (bioretention) in Northern Virginia was monitored to investigate the transport of nitrogen and the occurrence and contribution of denitrification. Stormwater runoff volumes, nitrogen concentrations, and nitrate isotope ratios (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were measured at the inlet and outlet of the bioretention during 24 storm events over 14 months. Nitrate concentration reductions (inlet vs. outlet) displayed seasonal trends, with higher reductions happening during warmer events and lower reductions or increases occurring during colder events. Cumulative bioretention nitrate and total dissolved nitrogen load reductions were 73% and 70%, respectively. Two out of 24 monitored events displayed denitrification isotope trends, indicating that although bioretention has denitrification potential, it is infrequent and other nitrogen removal mechanisms (i.e. infiltration and plant uptake) are primarily responsible for nitrogen surface effluent reductions. Only approximately 1.4% of the total reduced nitrate surface effluent load over the monitoring period was attributable to denitrification. Denitrification occurred during two of the largest monitored events, suggesting increased hydraulic retention time (HRT) promotes denitrification. Future GI designs should consider increasing HRT to encourage the important ecosystem service denitrification provides.

Best Management Practices for Diffuse Nutrient Pollution: Wicked Problems Across Urban and Agricultural Watersheds

Extensive time and financial resources have been dedicated to address nonpoint sources of nitrogen and phosphorus in watersheds. Despite these efforts, many watersheds have not seen substantial improvement in water quality. The objective of this study is to review the literature and investigate key factors affecting the lack of improvement in nutrient levels in waterways in urban and agricultural regions. From 94 studies identified in the academic literature, we found that, although 60% of studies found improvements in water quality after implementation of Best Management Practices (BMPs) within the watershed, these studies were mostly modeling studies rather than field monitoring studies. For studies that were unable to find improvements in water quality after the implementation of BMPs, the lack of improvement was attributed to lack of knowledge about BMP functioning, lag times, nonoptimal placement and distribution of BMPs in the watershed, postimplementation BMP failure, and socio-political and economic challenges. We refer to these limiting factors as known unknowns. We also acknowledge the existence of unknown unknowns that hinder further improvement in BMP effectiveness and suggest that machine learning, approaches from the field of business and operations management, and long-term convergent studies could be used to resolve these unknown unknowns.

Carbon-to-nitrogen ratio influence on the performance of bioretention for wastewater treatment

Bioretention cell (BRC), bioretention cell with microbial fuel cell (BRC-MFC), and an enhanced combined BRC-MFC system with bimetallic zero-valent iron (BRC-MFC-BZVI) were implemented in current study to treat the domestic wastewater. Nitrogen removal characteristics of three systems were investigated by adjusting influent carbon/nitrogen ratio (C/N ratio of 2.54-19.36). Results revealed that the nitrification and denitrification performances were mainly influenced by organic matter and system combination, which further affected nitrogen removal. When the influent C/N ratio was between 2 to 3, compared with BRC system, in BRC-MFC and BRC-MFC-BZVI system, chemical oxygen demand (COD), total nitrogen (TN), and ammonical nitrogen (NH₄⁺-N) removal efficiencies were still reached to 83.04%, 61.06%, and 42.26% and 86.53%, 43.61%, and 50.99% respectively, which simultaneously achieved high-efficiency of organic matter and nitrogen removal. The efficient supply of electrons in the BRC-MFC and BRC-MFC-BZVI processes was the main reason to achieve profound denitrification removal under the condition of low C/N. Removal rates of nitrate (NO₃^--N) and nitrite (NO₂^--N) were relatively higher due to microbial-driven redox reactions caused by driving electrons to flow in the closed circuit of metal wire connection. Moreover, phylogenetic diversity of bacterial communities mainly induced the catalytic iron, which further enhanced biological nitrogen reduction. The maximum efficient removal of organic matter (OM), TN, and NH4 + -N were obtained in the BRC-MFC-BZVI system, which were 98.42% (C/N = 10.42), 55.61% (C/N = 4.16), and 61.13% (C/N = 4.16), respectively.

Seasonal operation of dual-mode biofilters: The influence of plant species on stormwater and greywater treatment

The use of stormwater biofilters (also known as bioretention systems and raingardens), in tropical and semi-arid areas is hindered by seasonal rainfall patterns which cause extended dry periods. These periods can result in plant die-off, long-term damage to system health and leaching of pollutants when stormwater inflows resume. Using an additional polluted water source during dry periods could minimise system stress and eliminate the need to irrigate biofilters with potable water during dry spells. As such, the presented laboratory study tested the seasonal operation of biofilters, by switching from stormwater treatment in wet months to greywater treatment in dry months. Forty-five single planted biofilter columns, incorporating sedges, grasses, understory ornamentals and vines, were subjected to four months of stormwater inflows, followed by three months of greywater inflows. We also investigated the impact of including a carbon source in the saturated zone on treatment performance. The results showed plant species selection to be critical for nitrogen and phosphorus removal, with consistently effective species such as Carex appressa and Canna x generalis able to maintain low outflow concentrations (e.g. total nitrogen of 0.2-0.3 mg/L and 0.3-0.6 mg/L, respectively) across both water sources. Low outflow phosphorus concentrations were associated with plant species that had high filterable reactive phosphorus removal across both water sources. Similarly, higher removal of ammonia and oxidised nitrogen was associated with lower overall nitrogen concentrations. In contrast, high removal of total suspended sediment (>94%), biochemical oxygen demand (>98%) and some heavy metals (e.g. lead >98% and copper >93%) was reported across all designs. The results suggest that with the careful selection of plant species, biofilters can be operated seasonally as a feasible and practical solution to maintaining system health during extended dry periods.

Nitrate availability affects denitrification in Phragmites australis sediments

Understanding relationships between an increase in nitrate (NO₃ ^- ) loading and the corresponding effects of wetland vegetation on denitrification is essential to designing, restoring, and managing wetlands and canals to maximize their effectiveness as buffers against eutrophication. Although Phragmites australis (Cav.) Trin. ex Steud. is frequently used to remediate nitrogen (N) pollution, no information is available on how NO₃ ^- concentration may affect plant-mediated denitrification. In the present study, denitrification was measured in outdoor vegetated and unvegetated mesocosms incubated in both summer and winter. After spiking the mesocosms with NO₃ ^- concentrations typical of agricultural drainage water (0.7-11.2 mg N L^-1 ), denitrification was quantified by the simultaneous measurement of NO₃ ^- consumption and dinitrogen gas (N₂ ) production. Although denitrification rates varied with vegetation presence and season, NO₃ ^- availability exerted a significant positive effect on the process. Vegetated sediments were more efficient than bare sediments in adapting their mitigation potential to an increase in NO₃ ^- , by yielding a one-order-of-magnitude increase in NO₃ ^- removal rates, under both summer (743-6007 mg N m^-2 d^-1 ) and winter (43-302 mg N m^-2 d^-1 ) conditions along the NO₃ ^- gradient. Denitrification was the dominant sink for water NO₃ ^- in winter and only for vegetated sediments in summer. Nitrification likely contributed to fuel denitrification in summer unvegetated sediments. Since denitrification rates followed Michaelis-Menten kinetics, P. australis-mediated depuration may be considered optimal up to 5.0 mg N L^-1 . The present outcomes provide experimentally supported evidence that restoration with P. australis can work as a cost-effective means of improving water quality in agricultural watersheds.

Dry Wetlands: Nutrient Dynamics in Ephemeral Constructed Stormwater Wetlands

Constructed stormwater wetlands (CSWs) are used to address contaminants in urban stormwater such as nitrogen (N) and phosphorus (P), but their performance is variable. Ephemeral CSWs tend to be less effective than perennial CSWs at removing N and P. We asked: How does wetland vegetation and sediment affect nutrient cycling/release from sediment and vegetation in ephemeral CSWs? We focused on two ephemeral urban CSWs in Pocatello, ID, USA, one densely vegetated and the other nearly bare. We rewetted intact cores of dry wetland sediments and, separately, senesced vegetation for 1 week at the end of the summer dry period to assess whether wetland sediments and vegetation acted as sources or sinks of N and P. For both CSWs, there was a pulse of nutrients immediately following rewetting, but the magnitude of that pulse and subsequent changes in nutrient concentrations suggest different processes dominate at each wetland, driven by differences in wetland vegetation and associated sediment characteristics. There was evidence of denitrification between and during events at the vegetated wetland, but larger fluxes of P at this site suggests a tradeoff between denitrification and P release. While the experimental results suggested specific biogeochemical controls, CSW nutrient concentrations during three events were more dynamic and suggested more biogeochemical complexity than that represented in our experiment, both within events and seasonally. Ephemeral CSWs may create unique biogeochemical conditions and require careful design to ensure N and P retention. Managers will also need to consider whether perennial water sources would improve CSW function.

Migration and transformation of nitrogen in bioretention system during rainfall runoff

Bioretention systems have been extensively studied as a highly efficient technical measure to tackle the global threat of nitrogen pollution during global rainfall runoff. However, the migration and transformation of various forms nitrogen in bioretention system is unclear. So, in this paper, the bioretention systems with different flow regimes and planted configurations were designed to study the nitrogen removal performance and migration and transformation mechanism. The dynamic changes of NH₄⁺-N and NO₃^--N were continuously monitored within 60 h after rainfall, and the abundance of ¹⁵N isotopes in soil layer NH₄⁺-N was simultaneously measured. The results indicated that NH₄⁺-N was mainly intercepted in soil layer in four constructed bioretention systems with similar removal efficiencies (95.42-97.69%). However, NO₃^--N was retained in submerged layer with significant different removal efficiencies (43.03-83.00%). After fitting calculation, the nitrification rate of NH₄⁺-N (0.0626 mg kg^-1 h^-1) in soil was 5.31 times higher than that of the accumulation rate of NO₃^--N (0.0118 mg kg^-1 h^-1). During the elimination process of residual NH₄⁺-N in soil, 41.46% removed by denitrification and plant absorption assimilation, another 57.28% stored in the form of organic nitrogen or inorganic nitrogen, only 1.26% leaked out. Based on this, the content variation of TN, NH₄⁺-N and NO₃^--N could be analyzed by a system-wide and established the nitrogen balance model, which provides a new insight into the enhancement of nitrogen removal in the bioretention system.

Dual-mode stormwater-greywater biofilters: The impact of alternating water sources on treatment performance

The intermittent nature of stormwater runoff impacts the treatment performance of biofilters, also known as stormwater biofiltration or bioretention systems and raingardens. During extended dry periods, which are common even in temperate climates, plants can perish, creating unattractive and non-functional systems that might leach pollutants during the next rainfall event. The current solution is to irrigate during long dry spells, which is costly and unsustainable as biofilters become more widespread. This paper presents the development of dual-mode biofilters, where stormwater and greywater are treated within the same system. Fifty columns, utilising eight plant species, including understory and climbing ornamentals, and designs with and without a carbon source in the submerged zone, were subjected to alternating greywater and stormwater inflows over five months. Six sampling events investigated treatment performance across these switching inflows and an extended dry period (atypical event). Good removal of total suspended solids (>83%), biochemical oxygen demand (>86%) and some heavy metals (e.g. lead >96%) were reported irrespective of design. Plant species selection was critical for the removal of nitrogen (2 to 79%) and phosphorus (12 to 75%) under dual-mode operation. However, following the extended dry period, plants with the lowest nutrient outflow concentrations also experienced some of the highest sediment and carbon concentrations, suggesting that a mixture of plant species may be beneficial for withstanding abnormal conditions. Differences between the treatment performance of designs with and without a carbon source were negligible, with potential benefits possibly negated due to the increased root mass that comes with age (systems were approximately two years old) and the release of carbon from root exudates. The results demonstrate the potential for dual-mode stormwater-greywater biofilters as an alternative to single-mode systems as they can provide effective treatment, along with greater volumes of treated water, while maintaining system performance throughout the year.

Studying the effect of bioswales on nutrient pollution in urban combined sewer systems

The objective of this study was to evaluate the impact of bioswales on nutrient pollution in an urban combined sewershed. This evaluation was based on two criteria: the ability of bioswales to (1) remove nutrient pollution from stormwater runoff directly and (2) decrease sewer overflow volumes, which indirectly reduces total sewershed nutrient pollution during a storm event. Bioswales' direct nutrient removal was determined by analyzing nitrogen and phosphorus levels in water samples at seven bioswales located in the Bronx, New York City (NYC) over 42 storm events, while a bioswale's indirect nutrient removal through combined sewer overflow reduction was estimated by quantifying water retention at one of the bioswales. The study results indicated that: 1) the bioswale retained about 40% of stormwater conveyed to it from a drainage area 231 times its size, 2) bioswales leach nutrients into the subsurface, and 3) nitrogen leaching from bioswales varied seasonally, while phosphorus leaching decreased steadily over the study period. Although the studied bioswales leached a median 1.3 kg nitrogen per year into the subsurface, they provided an aggregate decrease in watershed nutrient pollution, from 7.7 to 6 kg nitrogen per year, due to their reduction of combined sewer overflow via stormwater retention.

A Retrospective Comparison of Water Quality Treatment in a Bioretention Cell 16 Years Following Initial Analysis

One of the most popular stormwater practices in (sub-)urban North Carolina is bioretention. While bioretention has been researched intensively to determine the most efficient designs, few long-term studies have attempted to assess the performance of older bioretention. However, previous research and design guidance for bioretention has predicted long-term water quality treatment. This study compared discharged concentrations and loads of nitrogen and phosphorus from a bioretention cell (1) post-construction and (2) following 17 years of treatment. A conventionally-drained bioretention cell with lateral underdrains in Chapel Hill, North Carolina, USA, was first monitored post-construction for 10-months from 2002–2003 and, again following continuous use, for 14 months from 2017–2018. Estimated mass load reductions during the initial monitoring period were 40% for total nitrogen (TN) and 65% for total phosphorus (TP). Mass load reductions were increased 17 years after construction, with reductions of 72% and 79% for TN and TP, respectively. Plant growth, death, and decay over the 17-year life of the bioretention cell are hypothesized to have contributed additional nitrogen assimilation and carbon to the fill media, serving as a catalyst for nitrogen treatment. Phosphorus removal remained relatively unchanged between the two monitoring periods. Filter media samples indicated the top 20 cm of filter media were nearing phosphorus saturation, but with 1.2 m of filter media, lower depths would most likely continue to provide treatment. If designed, built, and maintained correctly, bioretention appears to provide sustained treatment of stormwater runoff for nitrogen and phosphorus for nearly two decades, and likely longer.

Nutrient, Metal, and Organics Removal from Stormwater Using a Range of Bioretention Soil Mixtures

A column study was conducted to test the ability of bioretention soil mixtures (BSMs) to remove nutrients, metals, and polyaromatic hydrocarbons (PAHs) from stormwater collected from an urban highway. Infiltration rate, plant growth response, and turbidity of the effluent were also measured. The BSMs were made from a range of types and rates of composts and additional materials such as water treatment residuals, sawdust, and oyster shells. Sand was used as a control. Total N and P in stormwater measured 1.8 ± 1 and 0.08 ± 0.03 mg L. All treatments were a source of these nutrients. Metal concentrations in the stormwater were low, with mean Cu and Zn concentrations of 39.8 ± 19.1 and 173 ± 113 μg L, and Cd and Pb close to detection limits. All treatments absorbed Cu and Zn from stormwater with varying levels of removal efficiency. The three treatments tested removed 84 to 100% of the PAHs from the stormwater. In general, contaminant removal (N, P, and Zn) efficiency was not related to infiltration rate, with a slight decrease in Cu removal efficiency observed with increased infiltration rate ( = 0.32). These results indicate that the BSMs tested were a source of nutrients but were generally effective at removing metals and PAHs from stormwater.

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.

Estimate of gas transfer velocity in the presence of emergent vegetation using argon as a tracer: Implications for whole-system denitrification measurements

Denitrification associated with emergent macrophytes is a pivotal process underlying the treatment performance of wetlands and slow-flow waterways. Laboratory scale experiments targeting N losses via denitrification in sediments colonized by emergent macrophytes require the use of mesocosms that are necessarily open to the atmosphere. Thus, the proper quantification of N₂ effluxes relies on the accurate characterization of the air-water gas exchanges. In this study, we present a simple approach for direct measurements of the gas transfer velocity, in open-top mesocosms with Phragmites australis, by using argon as a tracer. Different conditions of water velocity (0, 1.5, 3, and 6 cm s^-1) and temperature (8.5, 16, and 28 °C), were tested, along with, for the first time, the presence of emergent vegetation. The outcomes demonstrated that water velocity and temperature are not the only factors regulating aeration at the mesocosm scale. Indeed, the gas transfer velocity was systematically higher, in the range of 42-53%, in vegetated compared to unvegetated sediments. The increase of small-local turbulence patterns created within water parcels moving around plant stems translated into significant modifications of the reaeration process. The adopted approach may be used to improve the accuracy of denitrification measurements by N₂ efflux-based methods in wetland and slow-flow waterway sediments colonized by emergent macrophytes. Moreover, the present outcomes may have multiple implications for whole-system metabolism estimations from which largely depend our understanding of biogeochemical dynamics in inland waters that have strong connections to worldwide issues, such as nitrate contamination and greenhouse gas emissions.

Quantifying Urban Bioswale Nitrogen Cycling in the Soil, Gas, and Plant Phases

Bioswales are a common feature of urban green infrastructure plans for stormwater management. Despite this fact, the nitrogen (N) cycle in bioswales remains poorly quantified, especially during dry weather in the soil, gas, and plant phases. To quantify the nitrogen cycle across seven bioswale sites located in the Bronx, New York City, we measured rates of ammonium and nitrate production in bioswale soils. We also measured soil nitrous oxide gas emissions and plant foliar nitrogen. We found that all mineralized nitrogen underwent nitrification, indicating that the soils were nitrogen-rich, particularly during summer months when nitrogen cycling rates increase, as indicated by higher levels of ammonium in the soil. In comparison to mineralization (0 to 110 g N m−2 y−1), the amounts of nitrogen uptake by the plants (0 to 5 g N m−2 y−1) and of nitrogen in gas emissions from the soils (1 to 10 g N m−2 y−1) were low, although nitrous oxide gas emissions increased in the summer. The bioswales’ greatest influx of nitrogen was via stormwater (84 to 591 g N m−2 y−1). These findings indicate that bioswale plants receive overabundant nitrogen from stormwater runoff. However, soils currently used for bioswales contain organic matter contributing to the urban nitrogen load. Thus, bioswale designs should use less nitrogen rich soils and minimize fertilization for lower nitrogen runoff.

Plant-Microbe Interactions Drive Denitrification Rates, Dissolved Nitrogen Removal, and the Abundance of Denitrification Genes in Stormwater Control Measures

The microbial community and function along with nitrate/nitrite (NOx) removal rates, and nitrogen (N) partitioning into "uptake", "denitrification", and "remaining" via isotope tracers, were studied in soil bioretention mesocolumns (8 unique plant species). Total denitrification gene reads per million (rpm) were positively correlated with % denitrified ( r = 0.69) but negatively correlated with total NOx removal following simulated rain events ( r = -0.79). This is likely due to plant-microbe interactions. Plant species with greater root volume, plant and microbial assimilation %, and NOx removal % had lower denitrification genes and rates. This implies that although microorganisms have access to N, advantageous functions, like denitrification, may not increase. At the conclusion of the 1.5-year experiment, the microbial community was strongly influenced by plant species within the Top zone dominated by plant roots, and the presence or absence of a saturated zone influenced the microbial community within the Bottom zone. Leptospermum continentale was an outlier from the other plants and had much lower denitrification gene rpm (average 228) compared to the other species (range: 277 to 413). The antimicrobial properties and large root volume of Leptospermum continentale likely caused this denitrification gene depression.

Plenty of room for carbon on the ground: Potential applications of biochar for stormwater treatment

Low impact development (LID) systems are increasingly used to manage stormwater, but they have limited capacity to treat stormwater-a resource to supplement existing water supply in water-stressed urban areas. To enhance their pollutant removal capacity, infiltration-based LID systems can be augmented with natural or engineered geomedia that meet the following criteria: they should be economical, readily available, and have capacity to remove a wide range of stormwater pollutants in conditions expected during intermittent infiltration of stormwater. Biochar, a carbonaceous porous co-product of waste biomass pyrolysis/gasification, meets all these criteria. Biochar can adsorb pollutants, improve water-retention capacity of soil, retain and slowly release nutrients for plant uptake, and help sustain microbiota in soil and plants atop; all these attributes could help improve removal of contaminants in stormwater treatment systems. This article discusses contaminant removal mechanisms by biochar, summarizes specific biochar properties that enhance targeted contaminants removal from stormwater, and identifies challenges and opportunities to retrofit biochar in LID to optimize stormwater treatment.

Engineered and Environmental Controls of Microbial Denitrification in Established Bioretention Cells

Bioretention cells (BRCs) are effective tools for treating urban stormwater, but nitrogen removal by these systems is highly variable. Improvements in nitrogen removal are hampered by a lack of data directly quantifying the abundance or activity of denitrifying microorganisms in BRCs and how they are controlled by original BRC design characteristics. We analyzed denitrifiers in twenty-three BRCs of different designs across three mid-Atlantic states (MD, VA, and NC) by quantifying two bacterial denitrification genes ( nirK and nosZ) and potential enzymatic denitrification rates within the soil medium. Overall, we found that BRC design factors, rather than local environmental variables, had the greatest effects on variation in denitrifier abundance and activity. Specifically, denitrifying populations and denitrification potential increased with organic carbon and inorganic nitrogen concentrations in the soil media and decreased in BRCs planted with grass compared to other types of vegetation. Furthermore, the top layers of BRCs consistently contained greater concentrations and activity of denitrifying bacteria than bottom layers, despite longer periods of saturation and the presence of permanently saturated zones designed to promote denitrification at lower depths. These findings suggest that there is still considerable potential for design improvements when constructing BRCs that could increase denitrification and mitigate nitrogen export to receiving waters.

Inside Story of Gas Processes within Stormwater Biofilters: Does Greenhouse Gas Production Tarnish the Benefits of Nitrogen Removal?

Stormwater biofilters are dynamic environments, supporting diverse processes that act to capture and transform incoming pollutants. However, beneficial water treatment processes can be accompanied by undesirable greenhouse gas production. This study investigated the potential for nitrous oxide (N₂O) and methane (CH₄) generation in dissolved form at the base of laboratory-scale stormwater biofilter columns. The influence of plant presence, species, inflow frequency, and inclusion of a saturated zone and carbon source were studied. Free-draining biofilters remained aerobic with negligible greenhouse gas production during storm events. Designs with a saturated zone were oxygenated at their base by incoming stormwater before anaerobic conditions rapidly re-established, although extended dry periods allowed the reintroduction of oxygen by evapotranspiration. Production of CH₄ and N₂O in the saturated zone varied significantly in response to plant presence, species, and wetting and drying. Concentrations of N₂O typically peaked rapidly following stormwater inundation, associated with limited plant root systems and poorer nitrogen removal from biofilter effluent. Production of CH₄ also commenced quickly but continued throughout the anaerobic interevent period and lacked clear relationships with plant characteristics or nitrogen removal performance. Dissolved greenhouse gas concentrations were highly variable, but peak concentrations of N₂O accounted for <1.5% of the incoming total nitrogen load. While further work is required to measure surface emissions, the potential for substantial release of N₂O or CH₄ in biofilter effluent appears relatively low.

Efficiency promotion and its mechanisms of simultaneous nitrogen and phosphorus removal in stormwater biofilters

Stromwater biofilter technology was greatly improved through adding iron-rich soil, plant detritus and eutrophic lake sediment. Significant ammonium and phosphate removal efficiencies (over 95%) in treatments with iron-rich soil were attributed to strong adsorption capability resulting in high available phosphorus (P) in media, supporting the abundance and activity of nitrifiers and denitrifiers as well as shaping compositions, which facilitated nitrogen (N) removal. Aquatic and terrestrial plant detritus was more beneficial to nitrification and denitrification by stimulating the abundance and activity of nitrifiers and denitrifiers respectively, which increased total nitrogen (TN) removal efficiencies by 17.6% and 22.5%. In addition, bioaugmentation of nitrifiers and denitrifiers from eutrophic sediment was helpful to nutrient removal. Above all, combined application of these materials could reach simultaneously maximum effects (removal efficiencies of P, ammonium and TN were 97-99%, 95-97% and 60-63% respectively), suggesting reasonable selection of materials has important contribution and application prospect in stormwater biofilters.

Temporal variation of nitrogen balance within constructed wetlands treating slightly polluted water using a stable nitrogen isotope experiment

Slightly polluted water has become one of the main sources of nitrogen contaminants in recent years, for which constructed wetlands (CW) is a typical and efficient treatment. However, the knowledge about contribution of individual nitrogen removal pathways and nitrogen balance in constructed wetlands is still limited. In this study, a stable-isotope-addition experiment was performed in laboratory-scale constructed wetlands treating slightly polluted water to determine quantitative contribution of different pathways and temporal variation of nitrogen balance using Na(15)NO3 as tracer. Microbial conversion and substrate retention were found to be the dominant pathways in nitrogen removal contributing 24.4-79.9 and 8.9-70.7 %, respectively, while plant contributed only 4.6-11.1 % through direct assimilation but promoted the efficiency of other pathways. In addition, microbial conversion became the major way to remove N whereas nitrogen retained in substrate at first was gradually released to be utilized by microbes and plants over time. The findings indicated that N2 emission representing microbial conversion was not only the major but also permanent nitrogen removal process, thus keeping a high efficiency of microbial conversion is important for stable and efficient nitrogen removal in constructed wetlands.

Stormwater Bioretention Systems: Testing the Phosphorus Saturation Index and Compost Feedstocks as Predictive Tools for System Performance

A replicated column trial was conducted to evaluate the potential for the phosphorus saturation index (PSI) to predict P movement in bioretention soil mixtures (BSMs). The impact of compost feedstock on BSM performance was also evaluated. Three composts (biosolids/yard, yard/food waste, and manure/sawdust) were each brought to PSI values of 0.1, 0.5, and 1.0 through the addition of Fe-based water treatment residuals (WTRs) to lower the PSI and P salts to increase the PSI. A synthetic stormwater solution was used for 12 leaching events. The PSI predicted total and dissolved P concentrations in column leachate. All composts removed P at PSI 0.1. All composts were a source of P for the higher PSI values tested, with P concentrations in the leachate decreasing over time. Ammonia and nitrate from all treatments decreased over time, with all treatments showing effective N removal. Copper removal (total and dissolved) was >90% for all treatments, with the highest removal observed at PSI 0.1 for all composts. Zinc removal (total) was also greatest in the 0.1 PSI for all composts. At PSI 0.5 and 1.0, the biosolids/yard compost was less effective than the other materials at removing Zn, with a removal efficiency of approximately 50%. Infiltration rates were similar across all treatments and ranged from 0.44 ± 0.1 cm min in the manure/sawdust at PSI 0.1 to 3.8 ± 2.8 cm min in the food/yard at PSI 1.0. Plant growth in the manure/sawdust compost was reduced in comparison to the other composts tested across all PSI levels. The results of this study indicate that the PSI may be an effective tool for predicting P movement in bioretention systems. Compost feedstock does not indicate the ability of composts to filter contaminants filtration, with all composts tested showing high contaminant removal.

Evaluation of sustainable electron donors for nitrate removal in different water media

An external electron donor is usually included in wastewater and groundwater treatment systems to enhance nitrate removal through denitrification. The choice of electron donor is critical for both satisfactory denitrification rates and sustainable long-term performance. Electron donors that are waste products are preferred to pure organic chemicals. Different electron donors have been used to treat different water types and little is known as to whether there are any electron donors that are suitable for multiple applications. Seven different carbon rich waste products, including liquid and solid electron donors, were studied in comparison to pure acetate. Batch-scale tests were used to measure their ability to reduce nitrate concentrations in a pure nutrient solution, light greywater, secondary-treated wastewater and tertiary-treated wastewater. The tested electron donors removed oxidised nitrogen (NOx) at varying rates, ranging from 48 mg N/L/d (acetate) to 0.3 mg N/L/d (hardwood). The concentrations of transient nitrite accumulation also varied across the electron donors. The different water types had an influence on NOx removal rates, the extent of which was dependent on the type of electron donor. Overall, the highest rates were recorded in light greywater, followed by the pure nutrient solution and the two partially treated wastewaters. Cotton wool and rice hulls were found to be promising electron donors with good NOx removal rates, lower leachable nutrients and had the least variation in performance across water types.

Salt tolerant plants increase nitrogen removal from biofiltration systems affected by saline stormwater

Biofiltration systems are used in urban areas to reduce the concentration and load of nutrient pollutants and heavy metals entering waterways through stormwater runoff. Biofilters can, however be exposed to salt water, through intrusion of seawater in coastal areas which could decrease their ability to intercept and retain pollutants. We measured the effect of adding saline stormwater on pollutant removal by six monocotyledonous species with different levels of salt-tolerance. Carex appressa, Carex bichenoviana, Ficinia nodosa, Gahnia filum, Juncus kraussii and Juncus usitatus were exposed to six concentrations of saline stormwater, equivalent to electrical conductivity readings of: 0.09, 2.3, 5.5, 10.4, 20.0 and 37.6 mS cm(-1). Salt-sensitive species: C. appressa, C. bichenoviana and J. usitatus did not survive ≥10.4 mS cm(-1), removing their ability to take up nitrogen (N). Salt-tolerant species, such as F. nodosa and J. kraussii, maintained N-removal even at the highest salt concentration. However, their levels of water stress and stomatal conductance suggest that N-removal would not be sustained at concentrations ≥10.4 mS cm(-1). Increasing salt concentration indirectly increased phosphorus (P) removal, by converting dissolved forms of P to particulate forms which were retained by filter media. Salt concentrations ≥10 mS cm(-1) also reduced removal efficiency of zinc, manganese and cadmium, but increased removal of iron and lead, regardless of plant species. Our results suggest that biofiltration systems exposed to saline stormwater ≤10 mS cm(-1) can only maintain N-removal when planted with salt-tolerant species, while P removal and immobilisation of heavy metals is less affected by species selection.

Genetic associations as indices of nitrogen cycling rates in an aerobic denitrification biofilter used for groundwater remediation

An aerobic denitrification biofilter (ADB) for groundwater remediation was developed with high removal efficiencies (total nitrogen (TN): 82.3-95.8%; NO3(-)-N: 93.2-98.2%). Nitrate (NO3(-)-N) transformation rates stabilized between 21.0 and 23.4 g/(m(3) h), whereas nitrite (NO2(-)-N) and ammonium (NH4(+)-N) transformation rates remained less than 6.0 g/(m(3) h) as the dissolved oxygen (DO) level increased from 1.0 mg/L to 6.0 mg/L. Nitric oxide (NO) and nitrous oxide (N2O) accumulated with great fluctuations (NO: 0-1.6×10(-3) g/(m(3) h); N2O: 0.1-1.1g/(m(3)h)) throughout the experiment. This study suggested that gene associations reflect quantitative relationships with aerobic denitrification rates and can provide useful information regarding aerobic denitrification processes in groundwater. Especially, the qnorB/nosZ ratio acts as the main driver for NO3(-)-N and NH4(+)-N transformation, while the qnorB/nosZ ratio followed by the (nirS+nirK)/nosZ ratio serve a dominant role in the accumulation of N2O and NO.

Methodologies for pre-validation of biofilters and wetlands for stormwater treatment

BACKGROUND

Water Sensitive Urban Design (WSUD) systems are frequently used as part of a stormwater harvesting treatment trains (e.g. biofilters (bio-retentions and rain-gardens) and wetlands). However, validation frameworks for such systems do not exist, limiting their adoption for end-uses such as drinking water. The first stage in the validation framework is pre-validation, which prepares information for further validation monitoring.

OBJECTIVES

A pre-validation roadmap, consisting of five steps, is suggested in this paper. Detailed methods for investigating target micropollutants in stormwater, and determining challenge conditions for biofilters and wetlands, are provided.

METHODS

A literature review was undertaken to identify and quantify micropollutants in stormwater. MUSIC V5.1 was utilized to simulate the behaviour of the systems based on 30-year rainfall data in three distinct climate zones; outputs were evaluated to identify the threshold of operational variables, including length of dry periods (LDPs) and volume of water treated per event.

RESULTS

The paper highlights that a number of micropollutants were found in stormwater at levels above various worldwide drinking water guidelines (eight pesticides, benzene, benzo(a)pyrene, pentachlorophenol, di-(2-ethylhexyl)-phthalate and a total of polychlorinated biphenyls). The 95th percentile LDPs was exponentially related to system design area while the 5th percentile length of dry periods remained within short durations (i.e. 2-8 hours). 95th percentile volume of water treated per event was exponentially related to system design area as a percentage of an impervious catchment area.

CONCLUSIONS

The out-comings of this study show that pre-validation could be completed through a roadmap consisting of a series of steps; this will help in the validation of stormwater treatment systems.