Performance characterisation of a stormwater treatment bioretention basin

Enhancing nitrogen and phosphorus removal in plant-biochar-pyrite stormwater bioretention systems: Impact of temperature and high-frequency heavy rainfall

Global climate change and rapid urbanization have resulted in more frequent and intense rainfall events in urban areas, raising concerns about the effectiveness of stormwater bioretention systems. In this study, we optimized the design by constructing a multi-layer filler structure, including plant layer, biochar layer, and pyrite layer, and evaluated its performance in nitrogen (N) and phosphorus (P) removal under different temperatures (5-18 °C and 24-43 °C), rainfall intensity (47.06 mm rainfall depth), and frequency (1-5 days rainfall intervals) conditions. The findings indicate that over 775 days, the plant system consistently removed 62.3% of total nitrogen (TN) and 97.0% of total phosphorus (TP) from 103 intense rainfall events. Temperature fluctuations had minimal impact on nitrate nitrogen (NO3--N) and TP removal, with differences in removal rates of only 1.0% and 0.6%, respectively, among plant groups. Across the multi-layer structure, plant roots mitigated the impact of temperature differences on NO3--N removal, while high-frequency rainfall fluctuated the stability of NO3--N removal. Dense plant roots reinforced N and P removal by facilitating denitrification in the vadose zone (biochar) and strengthening denitrification processes. Biochar and pyrite contributed to stable microenvironments and diverse ecological functions, enhancing NO3--N and PO43- removal. In summary, the synergistic effects of the multi-layer filler structure improved and stabilized N and P removal, providing valuable insights for addressing runoff pollution in bioretention systems amidst rapid urbanization and climate change challenges.

Multi-stage planning of LID-GREI urban drainage systems in response to land-use changes

Long-term planning of urban drainage systems aimed at maintaining the sustainability of urban hydrology remains challenging. In this study, an innovative multi-stage planning framework involving two adaptation pathways for optimizing hybrid low impact development and grey infrastructure (LID-GREI) layouts in opposing chronological orders was explored. The Forward Planning and Backward Planning are adaptation pathways to increase LID in chronological order based on the initial development stage of an urban built-up area and reduce LID in reverse chronological order based on the final development stage, respectively. Two resilience indicators, which considered potential risk scenarios of extreme storms and pipeline failures, were used to evaluate the performance of optimized layouts when land-use changed and evolved over time. Compared these two pathways, Forward Planning made the optimized layouts more economical and resilient in most risk scenarios when land-use changed, while the layouts optimized by Backward Planning showed higher resilience only in the initial stage. Furthermore, a decentralized scheme in Forward Planning was chosen as the optimal solution when taking costs, reliability, resilience, and land-use changes into an overall consideration. Nevertheless, this kind of reverse optimization order offers a novel exploration in planning pathways for discovering the alternative optimization schemes. More comprehensive solutions can be provided to decision-makers. The findings will shed a light on the exploration of optimized layouts in terms of spatial configuration and resilience performance in response to land-use changes. This framework can be used to support long-term investment and planning in urban drainage systems for sustainable stormwater management.

Evaluating bioretention scale effect on stormwater retention and pollutant removal

Bioretention column studies are commonly used in laboratory to assess the performance of such structures in removal of pollutants and to investigate different conceptions aiming to increase their efficiency. However, no studies were found recommending suitable diameters or sizes, or about the uncertainties related to the transfer of results among the different scales (i.e., among different experiments or from the laboratory to field scale). This study assessed the effect of the varying diameters in experimental bioretention columns on the retention and removal of pollutants from stormwater runoff. Three sets of columns with diameters of 400 mm, 300 mm, and 200 mm were assessed. The results showed that runoff retention (R) was affected by the time interval between stormwater events, but not by the bioretention diameter, although the diameter influenced the variability of R results. The removal of TSS (95%), nitrite (98%), and phosphate (96%) did present variability among the different bioretention diameters. However, the nitrate removal was statistically different among the bioretention columns, with removal efficiency above 50% in the 300-mm and 200-mm columns, while the 400-mm columns acted as a source of nitrate by increasing its concentration in the outflow stormwater by up to 285%, suggesting that the removal of this pollutant can be influenced by the scale effect of the bioretention columns and the experiments with small bioretention diameters may not provide reliable results.

Supporting evidences for vegetation-enhanced stormwater infiltration in bioretention systems: a comprehensive review

Stormwater mitigation efficiency of bioretention systems relies for a large part on their capacity to infiltrate rapidly received runoff. Within this context, the primary aim of this literature review was to clarify the vegetation influences on bioretention media hydraulic conductivity, with the ultimate goal of improving guidance on plant choice for system durability. A thorough synthesis of studies dealing with the comparison of plant species, functional types, or traits on infiltration-related processes in biofilters was achieved. Overall, results converged to a positive impact of plants on water infiltration and percolation, either under greenhouse or field conditions. In most cases, vegetation selection had a determining role in maintaining initial media infiltration rates, with in terms of improvement: turfgrass < prairie grass < shrubs < trees. Wind-induced movements of rigid foliage or stems are believed to avoid complete surface clogging. Species with thick, rhizomatous or fleshy (with maximum root diameter near the centimeter range), and tap or deep root systems could be preferred to maximize infiltration rates in permeable bioretention media. In fine-textured soils, higher specific root length, root length density, or mass density could also enhance infiltration. Root mass densities (0.1-2.2 kg.m³) were positively linked with infiltration rates in unlined systems while roots around 1 mm diameter would favor macropore-related preferential flows and increased hydraulic conductivity. Finally, implementation of high-diversity plant communities would ensure the presence of a more functionally rich vegetation community with species possessing adequate physiological adaptations (including root system architecture) to local environmental conditions for perennial cover and proper bioretention hydrological functioning.

Investigating relationships between landscape patterns and surface runoff from a spatial distribution and intensity perspective

Rapid urbanization changes landscape patterns and results in frequent urban waterlogging issues, which affect citizens' daily lives and cause economic loss. Understanding the spatial patterns and impact factors associated with urban waterlogging under different rainfall intensities has significant implications for mitigating this hazard. In this study, the runoff depth calculated according to the Storm Water Management Model (SWMM) simulation results was used to investigate the spatial characteristics of urban waterlogging. Multiple scenario-based designs, a correlation analysis, and a stepwise regression model were employed to detect the relationship between surface runoff depth and landscape patterns under different rainfall intensities. The results show that when the rainfall intensity reached 12.5 mm/12 h, the conversion rate of rainfall to runoff increased significantly, indicating an increased waterlogging risk. Areas with impervious surface proportions of 25-50% and 75-100% were shown to require more attention due to the strong sensitivity of the surface runoff depth to an increase in the impervious surface. It is most cost-effective to maintain the original high-density vegetation or increase the vegetation density from 0-25% to 25-50% for urban green space. Additionally, the landscape configuration also affects the surface runoff depth. The fragmented, scattered, or regular shape of impervious surface patches can reduce surface runoff effectively; larger and less fragmented green space was also shown to have a surface runoff controlling. The adjusted R² values were greater than 0.6 for all stepwise regression models, indicating that the landscape variables selected in the study can effectively predict the surface runoff depth. These models also showed that the landscape composition had a more profound contribution than the landscape configuration on runoff depth. These findings provide meaningful insights and perspectives for urban waterlogging hazard mitigation, quantitative landscape planning, and risk management. The method proposed by this study provides a referable framework for future studies on urban waterlogging and its response to the landscape in the context of global climate change.

Accumulation of high-molecular-weight polycyclic aromatic hydrocarbon impacted the performance and microbial ecology of bioretention systems

Bioretention has been considered as an effective management practice for urban stormwater in the removal of pollutants including polycyclic aromatic hydrocarbons (PAHs). However, the accumulation of high-molecular-weight (HMW) PAHs in bioretention systems and their potential impact on the pollutants removal performance and microbial ecology are still not fully understood. In this study, comparisons of treatment effectiveness, enzyme activity and microbial community in bioretention systems with different types of media amendments were carried out at different spiking levels of pyrene (PYR). The results showed that the removal efficiencies of chemical oxygen demand (COD) and total nitrogen in the bioretention systems were negatively impacted by the PYR levels. The relative activities of soil dehydrogenase and urease were increasingly inhibited by the elevated PYR level, indicating the declining microbial activity regarding organic matter decomposition. The spiking of PYR negatively affected microbial diversity, and distinct time- and influent-dependent changes in microbial communities were observed. The relative abundance of PAH-degrading microorganisms increased in PYR-spiked systems, while the abundance of nitrifiers decreased. The addition of media amendments was beneficial for the enrichment of microorganisms that are more resistant to PYR-related stress, therefore elevating the COD concentration removal rate by ∼50%. This study gives new insight into the multifaceted impacts of HMW PAH accumulation on microbial fingerprinting and enzyme activities, which may provide guidance on better stormwater management practices via bioretention in terms of improved system longevity and performance.

Potential exploration of Fe3O4/biochar from sludge as the media of bioretention system and its comparison with conventional media

The selection and configuration of soil media are a core issue of the bioretention system. A porous carbon material of Fe₃O₄/biochar (BSF) was prepared by adding pickling wastewater to modified sludge biochar, which could serve as a good adsorption performance and cheap media for bioretention system. Through the analytic hierarchy process (AHP), different media were evaluated according to their characteristics. By comparing the characteristics of BSF to bio-ceramic (BC), zeolite (ZE), and activated carbon (AC), it was found that BSF has a larger specific surface area and pore volume. The hydrological characteristics of the medium were also tested. The results show that BSF has better water-absorbing quality and hydraulic conductivity than the other three media, but the water-retention property of the medium seems to be inferior. BSF has stable adsorption performance for ammonia nitrogen (NH₄⁺-N) and total phosphorus (TP) in rainwater. Its high adsorption capacity is maintained at 5-35°C, but it is very susceptible to pH factors. The adsorption process by BSF and other media conforms to pseudo-second-order kinetics and the Langmuir model in rainwater. In general, the performance of BSF is shown to be superior to BC, ZE, and AC, making it a potential medium for bioretention system.

Ranking Three Water Sensitive Urban Design (WSUD) Practices Based on Hydraulic and Water Quality Treatment Performance: Implications for Effective Stormwater Treatment Design

Bioretention basins, constructed wetlands and roadside swales are among the most common Water-Sensitive Urban Design (WSUD) or stormwater quality treatment systems. Although these systems can reduce stormwater quantity and improve quality, their hydraulic and water quality treatment performances are different. The aim of this study was to investigate the hydraulic and water quality performance of a bioretention basin, a constructed wetland and a roadside swale by analyzing monitored water quantity and quality data from a range of rainfall events using a ranking approach. The study outcomes showed that a bioretention basin performed better in relation to peak flow and runoff volume reduction while the constructed wetland tended to produce better outflow water quality. The roadside swale had a relatively lower capacity for treating stormwater. These results suggest that a bioretention basin could be the preferred option when the primary requirement is water quantity improvement. However, if water quality improvement is the primary consideration, a constructed wetland could be more efficient. Additionally, when designing a treatment train, it appears to be preferable to place a bioretention basin prior to a constructed wetland. Further, a swale appears to be more appropriate for use as a pretreatment device. The research study outcomes will contribute to effective stormwater treatment design.

Integrating Tank Model and adsorption/desorption characteristics of filter media to simulate outflow water quantity and quality of a bioretention basin: A case study of biochar-based bioretention basin

Reliable approaches for accurately assessing the performance of stormwater treatment systems is essential for their effective design, including filter media selection which can be a significant constituent in stormwater treatment systems. This study presents an innovative modelling approach integrating the Tank Model with the adsorption-desorption characteristics of the filter media. The resulting modelling approach was applied to simulate a field-scale bioretention basin where biochar was used as filter media with over ten years of rainfall records. The resulting outflow and overflow volumes were compared with observed data for calibration. The Stormwater Treatment Tank Model (STTM) was validated using the Leave-One-Out-Cross-Validation (LOOCV) method. The simulation outcomes include water outflow and overflow (quantity) from the bioretention basin as well as outflow water quality represented by three heavy metals (Pb, Cu, and Zn). The modelling approach developed was found to be capable of accurately simulating outflow and overflow volumes, with outlet water quantity being significantly influenced by the total rainfall depth. The modeling results also suggested that a sole treatment system would not be adequate, particularly for large rainfall events (>100 mm) and a treatment train would be more effective. Simulating long-term (over ten years) pollutant removal performance in the bioretention basin indicated that heavy metals outflow event mean concentration (EMCs) values calculated using simulated results of 30% biochar application rate generated the best pollutant removal with consistent values (2.7 μg/L, 3.0 μg/L, 17.2 μg/L for Pb, Cu, and Zn, respectively). These results confirm that the modelling approach is reliable for assessing long-term treatment performance, as well as a robust tool able to contribute to more effective treatment system design, particularly filter media selection and evaluation.

Study on the Influence of Sponge Road Bioretention Facility on the Stability of Subgrade Slope

With the large-scale application of sponge city facilities, the bioretention facility in urban roads will be one of the key factors affecting the safety of construction facilities in areas with abundant rainfall. In this study, by establishing a three-dimensional finite element model for numerical analysis and combining it with geotechnical tests, the effects of bioretention facility on water pressure distribution, seepage path, and slope stability under rainwater seepage conditions are proposed. In addition, this study puts forward the relationship between the parameters of the bioretention facility and the stability of the slope in combination with the effect of runoff pollution control, which provides direction and basis for the planning, design, and construction of sponge cities in road construction.

Assessment on the cumulative effect of pollutants and the evolution of micro-ecosystems in bioretention systems with different media

Bioretention system is one of the most used green stormwater infrastructures (GSI), and its media is a key factor in reducing runoff water volume and purifying water quality. Many studies have investigated media improvement to enhance the pollutant removal capacity. However, the long-term cumulative effect and microbial effect of pollutants in the modified-media bioretention system is less known. This study investigated the cumulative effect of pollutants and their influence on microbial characteristics in conventional and modified media bioretention system. The addition of modifiers increased the background content of pollutants in the media, and the accumulation of pollutants in planting soil (PS) and bioretention soil mixing + water treatment residuals (BSM+WTR) was relatively higher after the simulated rainfall experiment. The accumulation of pollutants led to a decrease in dehydrogenase activity, and an increase in urease and invertase activities. Ten dominant bacterial species at the phylum level were found in all bioretention systems. The relative abundances of the bacteria with good viability under low nutritional conditions decreased, while the species which could live in the pollutant-rich environment increased. The accumulation of pollutants in the bioretention system led to the extinction of some functional microorganisms. The better the effects of modified media on pollutant removal showed, the more obvious effect on the media micro-ecosystem was. To ensure the long-term efficient and stable operation of the modified-media bioretention system, we recommend balancing the pollutant removal efficiency and cumulative effect in modified-media bioretention systems.

Nutrients and solids removal in bioretention columns using recycled materials under intermittent and frequent flow operations

This research investigated the fate and removal of nitrite (NO₂-N), nitrate (NO₃-N), orthophosphate (PO₄-P), and total suspended solids (TSS) in two bioretention columns, which were designed with three recycled materials. The first column was packed with Recycled Concrete Aggregate (RCA). The second column was a Layered Media (LM), which has layers of RCA with crushed glass and rice husks. The columns were tested under intermittent and frequent operations of synthetic runoff with low and high feed concentrations. The effect of inflow concentration, antecedent dry days (ADD), column age, and the anticipated number of events (EN) was also statistically analyzed on the performance of columns. Depending on column types, nutrient removal was significantly (p < 0.05) increased under frequent flow operations by 26-53% over intermittent. However, TSS removal was notably (p < 0.05) increased by 23-35% under intermittent operations over frequent. Overall, LM showed an increased NO₂-N (92 ± 2%) and NO₃-N (88% ± 2%) removal under low feed frequent operations and TSS removal (97% ± 2%) under initial intermittent operations. On the contrary, RCA showed a maximum of 99% PO₄-P removal under high feed frequent operations. Results showed that the nutrient outflow concentration was found to have a negative correlation with EN and column age and a positive correlation with ADDs throughout the experiments.

Assessment of Blue and Green Infrastructure Solutions in Shaping Urban Public Spaces—Spatial and Functional, Environmental, and Social Aspects

Blue and Green Infrastructure (BGI) provide one of the key Nature Based Solution (NBS) approaches for sustainable stormwater management in cities, in conjunction with extending the scope of Ecosystem Services (ES). In both the process of planning and designing highly urbanized areas, the implementation of BGI is important for the improvement of living conditions and counteracting the negative effects of climate change. Based on the literature review, 19 BGI solutions were identified and then valorized in relation to the following three key aspects: spatial and functional, environmental, and social. The results of the assessment were derived using the scoring method and allowed for the identification of BGI solutions with a high, medium or low value for shaping sustainable urban public spaces. Using the potential of analyzed BGI solutions to improve the functioning and attractiveness of urban areas requires a comprehensive approach. Conscious planning and designing should use the knowledge presented to make the implementation of BGI solutions as effective as possible in relation to the above-mentioned aspects of shaping urban public spaces.

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.

Nitrogen process in stormwater bioretention: the impact of alternate drying and rewetting on nitrogen migration and transformation

Nitrogen migration and transformation in the stormwater bioretention system were studied in laboratory experiments, in which the effects of drying-rewetting were particularly investigated. The occurrence and distribution of nitrogen in the plants, the soil, and the pore water were explored under different drying-rewetting cycles. The results clearly showed that bioretention system could remove nitrogen efficiently in all drying-rewetting cycles. The incoming nitrogen could be retained in the topsoil (0-10 cm) and accumulated in the planted layer. However, the overlong dry periods (12 and 22 days) cause an increase in nitrate in the pore water. In addition, nitrogen is mostly stored in the plants' stem tissues. Up to 23.26% of the inflowing nitrogen can be immobilized in plant tissues after a dry period of 22 days. In addition, the relationships between nitrogen reductase activity in the soil and soil nitrogen content were explored. The increase of soil TN content could enhance the activity of nitrate reductase. Meanwhile, the activity of hydroxylamine reductase (HyR) could be enhanced with the increase of soil NO₃^- content. These results provide a reference for the future development of nitrogen transformation mechanism and the construction of stormwater bioretention systems.

Seasonal performance of field bioretention systems in retaining phosphorus in a cold climate: Influence of prolonged road salt application

Bioretention systems are popular low impact development stormwater management features designed to remove pollutants, including phosphorus (P), from urban stormwater runoff. While the performance of bioretention systems in retaining P has been well studied, seasonal variability of P retention in field-scale systems installed in cold climates, including the influence of high road de-icing salt (sodium chloride) inputs, remains unclear. Two large field-scale bioretention systems installed in London, Ontario, Canada were monitored over their initial operational period to evaluate the seasonal trends in the retention of different forms of P in bioretention systems and the impact of high salt loading. Over the 12-month monitoring period, a net retention of total P and dissolved organic P, and a net release of soluble reactive P and total dissolved P mass were observed. Reduced hydrological performance and increased effluent P concentrations resulted in high P release from the bioretention systems in early to mid-spring (March and April), with most release occurring during a few individual large precipitation events. Laboratory-scale column experiments were performed using the engineered soil media installed in the field-scale bioretention systems to isolate the effect of high salt loading on P release. Column experiments combined with field data indicate that prolonged high salt loads through winter and spring may have contributed to elevated spring P release, mostly in the form of soluble reactive P, from the field-scale bioretention systems. Findings from this study are needed to better understand the performance of bioretention systems with respect to P retention as required to improve urban stormwater management in cold climates. Results have implications for further investigations of the impact of road salt on P mobility in bioretention systems and more broadly in roadside soils and groundwater systems.

Pilot and Field Studies of Modular Bioretention Tree System with Talipariti tiliaceum and Engineered Soil Filter Media in the Tropics

Stormwater runoff management is challenging in a highly urbanised tropical environment due to the unique space constraints and tropical climate conditions. A modular bioretention tree (MBT) with a small footprint and a reduced on-site installation time was explored for application in a tropical environment. Tree species used in the pilot studies were Talipariti tiliaceum (TT1) and Sterculia macrophylla (TT2). Both of the MBTs could effectively remove total suspended solids (TSS), total phosphorus (TP), zinc, copper, cadmium, and lead with removal efficiencies of greater than 90%. Total nitrogen (TN) removal was noted to be significantly higher in the wet period compared to the dry period (p < 0.05). Variation in TN removal between TT1 and TT2 were attributed to the nitrogen uptake and the root formation of the trees species. A field study MBT using Talipariti tiliaceum had a very clean effluent quality, with average TSS, TP, and TN effluent EMC of 4.8 mg/L, 0.04 mg/L, and 0.27 mg/L, respectively. Key environmental factors were also investigated to study their impact on the performance of BMT. It was found that the initial pollutant concentration, the dissolved fraction of influent pollutants, and soil moisture affect the performance of the MBT. Based on the results from this study, the MBT demonstrates good capability in the improvement of stormwater runoff quality.

Hydrology and rainfall runoff pollutant removal performance of biochar-amended bioretention facilities based on field-scale experiments in lateritic red soil regions

Bioretention has been found to lower the effluent loads of various pollutants from rainfall runoff. However, it is still a challenge to effectively use bioretention for rainfall runoff control in lateritic red soil regions where have high rainfall intensity and low soil infiltration capacity. Hence, in this study, the hydrologic performance and rainfall runoff pollutant removal capacity of field-scale biochar-amended bioretention facilities were tested with four rainfall recurrence periods under different biochar distributions, internal water storage (IWS) zone heights, and exfiltration conditions. The results confirmed that incorporation of biochar into planting soil would improve its water content raising capacity (WCRC), especially when the biochar was uniformly mixed with the lateritic red soils. Besides, more infiltrating from the planting soil layer and higher IWS zone heights effectively enhanced WCRC of the stone chip packing layer. For runoff volume control, adding biochar and increasing the IWS zone height could effectively improve runoff volume control capacity. Besides, the unlined bioretention had a higher runoff volume control capacity than lined bioretention. Considering runoff pollutant removal performance, biochar could contribute to significantly improving the runoff pollutant event mean concentration removal rate (R_c) of nutrient pollutants (TN, NO₃-N, NH₃-N, and TP). The average runoff pollutant load removal rate (R_l) of different biochar distributions decreased as follows: biochar was uniformly mixed with the lateritic red soils > biochar was stratified with the lateritic red soils > biochar was excluded in the planting soil layer. The average R_c and average R_l of all pollutants except COD under different IWS zone heights decreased as follows: 40 cm > 20 cm > 0 cm. Meanwhile, the average R_l of the lined bioretention with an IWS zone height of 0 cm was lower than that of the unlined bioretention. Overall, higher rainfall recurrence periods would reduce the treatment capacity of bioretention facilities.

The ability of selected filter materials in removing nutrients, metals, and microplastics from stormwater in biofilter structures

Creative solutions to manage stormwater include ecologically based designs, such as biofilter structures. A laboratory experiment was established to study the ability of biofilters to remove nutrients, metals, total suspended solids (TSS), and total organic C originating from roadside stormwater as melted snow. Special attention was paid to the removal of P. In addition, the fate of microplastics (MPs) in the biofilters was followed. The materials selected for biofilters were (a) crushed light-expanded clay aggregates without biochar or amended with biochar, (b) Filtralite P clay aggregates, (c) crushed concrete, or (d) filter sand. A layer to support grass growth was placed above these materials. Stormwater was rich in TSS with associated P and metals, which were substantially retained by all biofilters. Filtralite and concrete had almost 100% P removal, but the high pH had adverse effects on plants. Light-expanded clay aggregates had lower retention of P, and, when mixed with biochar (30% v/v), the leaching of P increased and N retention was improved. None of the materials was ideal for treating both nutrients and metals, but sand was generally best. Vegetation improved N retention and stormwater infiltration. Plant roots formed preferential pathways for water and associated substances, evidenced by the accumulation of MPs along root channels. No MPs were found in discharge. Given the high loading of suspended solids and associated contaminants in snowmelt from traffic areas and their efficient retention in biofiltration, results of this study suggest the implementation of such stormwater management solutions along road verges.

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.

Urban Flooding Mitigation Techniques: A Systematic Review and Future Studies

Urbanization has replaced natural permeable surfaces with roofs, roads, and other sealed surfaces, which convert rainfall into runoff that finally is carried away by the local sewage system. High intensity rainfall can cause flooding when the city sewer system fails to carry the amounts of runoff offsite. Although projects, such as low-impact development and water-sensitive urban design, have been proposed to retain, detain, infiltrate, harvest, evaporate, transpire, or re-use rainwater on-site, urban flooding is still a serious, unresolved problem. This review sequentially discusses runoff reduction facilities installed above the ground, at the ground surface, and underground. Mainstream techniques include green roofs, non-vegetated roofs, permeable pavements, water-retaining pavements, infiltration trenches, trees, rainwater harvest, rain garden, vegetated filter strip, swale, and soakaways. While these techniques function differently, they share a common characteristic; that is, they can effectively reduce runoff for small rainfalls but lead to overflow in the case of heavy rainfalls. In addition, most of these techniques require sizable land areas for construction. The end of this review highlights the necessity of developing novel, discharge-controllable facilities that can attenuate the peak flow of urban runoff by extending the duration of the runoff discharge.

Vegetation Alters Soil Water Drainage and Retention of Replicate Rain Gardens

Rain gardens are residential bioretention practices widely used to manage urban runoff, yet their design as plant-soil systems lacks understanding. We hypothesized that vegetative treatment (turfgrass, prairie, and shrubs, plus a non-vegetated control) would alter the volume and rate of drainage from 12 replicate mesocosms (i.e., rain gardens) through changes to the belowground system. Roof runoff was collected on-site and distributed equally among the mesocosms following natural rain events for two growing seasons. We monitored stormwater input, drainage output, and soil moisture to assess differences in hydrology by treatment, explained by indices of soil structural development (infiltration, saturated hydraulic conductivity, soil water retention). Drainage volume and response dynamics differed as predicted by vegetative treatment in support of our hypothesis. The greatest reductions in drainage volume were observed beneath shrubs and prairie following smaller stormwater inputs, and accelerated drainage responses were observed beneath turfgrass following larger stormwater inputs. Differences in infiltration, saturated hydraulic conductivity, and plant-induced changes in antecedent soil moisture among vegetative treatments help explain these plant-mediated drainage responses. This study shows that plants can alter the hydrologic dynamics of rain gardens and thus are a critical component of the design and intent of these plant-soil systems.

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.

Remediation and accumulation characteristics of dissolved pollutants for stormwater in improved bioretention basins

Dissolved pollutants in stormwater are more mobile/bioavailable, and are captured via different mechanisms than particles. Column-scale bioretention basins are constructed by filling different media, which is used to study the remediation and accumulation characteristics of dissolved pollutants by improved bioretention basins (increased infiltration and adsorption capacity of the media). The media factor (ratio of specific surface area to cubic of porosity) is used to characterize the basic properties of different media, while considering the key factors: infiltration capacity, inflow concentration, recurrence interval, discharge ratio, antecedent dry period, and rainfall duration. The results showed that stormwater pollutants load reduction rate decreased with the increase of recurrence interval and discharge ratio, and increased with the increase of inflow concentration. Based on response surface methodology (RSM), a quantitative relationship model between major pollutants and influencing factors were established (R² > 0.715), which can be used to estimate the design and operation of the media. By detecting changes in media carbon, nitrogen and phosphorus contents, the results showed that partial pollutants leaching were greater than their accumulation in the initial stage of system operation, and their contents in the media reduced during simulated rainfall. After the pollution contents tend to stabilize, the accumulated pollutants were greater than the leaching, and media pollution contents showed the trend of upper > middle > lower (corresponding to the 10, 35, and 60 cm sections of the media from top to bottom). Six enzymes closely related to the accumulation and migration of nitrogen, phosphorus and organic matter in the media were selected. Pearson correlation analysis found that: the significant correlations between the selected enzyme activity and pollutants were not consistent in bioretention system. For example, catalase was significantly correlated with all the pollutants (P < 0.01), whereas acid phosphatase was not significantly correlated with all the pollutants.

A Review of Nitrogen Removal for Urban Stormwater Runoff in Bioretention System

One of the best management practices (BMPs) for stormwater quality and quantity control is a bioretention system. The removal efficiency of different pollutants under this system is generally satisfactory, except for nitrogen which is deficient in certain bioretention systems. Nitrogen has a complex biogeochemical cycle, and thus the removal processes of nitrogen are typically slower than other pollutants. This study summarizes recent studies that have focused on nitrogen removal for urban stormwater runoff and discusses the latest advances in bioretention systems. The performance, influencing factors, and design enhancements are comprehensively reviewed in this paper. The review of current literature reveals that a bioretention system shows great promise due to its ability to remove nitrogen from stormwater runoff. Combining nitrification and denitrification zones with the addition of a carbon source and selecting different plant species promote nitrogen removal. Nevertheless, more studies on nitrogen transformations in a bioretention system and the relationships between different design factors need to be undertaken.

A review of bioretention components and nutrient removal under different climates-future directions for tropics

Bioretention systems have been implemented as stormwater best management practices (BMPs) worldwide to treat non-point sources pollution. Due to insufficient research, the design guidelines for bioretention systems in tropical countries are modeled after those of temperate countries. However, climatic factors and stormwater runoff characteristics are the two key factors affecting the capacity of bioretention system. This paper reviews and compares the stormwater runoff characteristics, bioretention components, pollutant removal requirements, and applications of bioretention systems in temperate and tropical countries. Suggestions are given for bioretention components in the tropics, including elimination of mulch layer and submerged zone. More research is required to identify suitable additives for filter media, study tropical shrubs application while avoiding using grass and sedges, explore function of soil faunas, and adopt final discharged pollutants concentration (mg/L) on top of percentage removal (%) in bioretention design guidelines.

Design and Performance Characterization of Roadside Bioretention Systems

In the current study, three roadside bioretention systems with different configurations were constructed to investigate their pollutant removal efficiency in different rainfall recurrence intervals. The bioretention systems (referred as units) (unit A: 700 mm height material without submerged zone; unit B: 400 mm height material with 300 mm submerged zone; unit C: 400 mm height material without submerged zone) were used to conduct the rainfall events with uniform 120 min rainfall duration for 2-, 5-, 10-, 15-, and 30-year recurrence intervals. Results reveal that the gradual increase of rainfall return period would have negative effects on TN and NH4+-N removal. The higher filler layer may increase pollutant removal efficiency. Setting a submerged zone could improve the CODMn and TN removal compared to TP and NH4+-N removal. The values for comprehensive reduction rate of pollutant load in the three bioretention systems were recorded as follows: 64% in SS, 50%~80% in TP, 69% in NH4+-N, and 28%~53% in NO3-N separately. These results provide greater understanding of the design and treatment performance of bioretention systems.

Implementation of a specific urban water management - Sponge City

Climate change, rapid urbanization and inappropriate urban planning policies in many countries have resulted in urban water-related problems, such as flooding disasters, water pollution and water shortages. To tackle these issues, the specific urban water management strategy known as Sponge City has been implemented in China since 2013. This is a complex method and one involving many challenges. This paper critically assesses the approaches associated with conventional urban water management. The Sponge City concept and its adoption are then scrutinized to comprehensively assess the limitations and opportunities. It emerges that Sponge City has four main principles, these being: urban water resourcing, ecological water management, green infrastructures, and urban permeable pavement. The uncertainties in Sponge City design and planning, and financial insufficiencies are the most serious problems that can risk the failure of the Sponge City concept. While significant barriers exist, the opportunities for implementing a Sponge City are evident. To obtain multi-ecosystem services of Sponge City, it should be implemented at the watershed scales and be flexible, depending on different decision levels or catchment characteristics. It is essential to apply an intelligent decision-making mechanism and consider the need for close cooperation between various agencies with which the central government can work. A suitable sized and harmonious Sponge City, ensuring a good balance between socio-economic development and environmental conservation, is the ideal.

Stormwater volume reduction and water quality improvement by bioretention: Potentials and challenges for water security in a subtropical catchment

Climate change scenarios tend to intensify extreme rainfall events and drought in Brazil threatening urban water security. Low Impact Development (LID) practices are decentralized alternatives for flood mitigation and prevention. Recently, their potential has increasingly been studied in terms of stormwater harvesting. However, there is still a lack of knowledge about their potentialities in subtropical climate regions. Therefore, this study evaluated the behavior of a bioretention cell in a Brazilian city, during the dry period, which is critical in terms of pollutant accumulation and water availability. In addition to the runoff reduction and pollutant removal efficiency, this paper analyzed the potential for water reuse in terms of the stored volume and water quality guidelines. The results obtained show an average runoff retention efficiency of 70%. Considering only the water availability aspects, the potential stored runoff could be reused for non-potable purposes, reducing the water demand in the catchment by at least half during the dry season. On the other hand, the bioretention presented two different conditions for pollutant removal: Condition A - the concentration values are within the recommended limits for water reuse. The parameters found in this condition were NO₃, NO₂, Zn, Mn, Cu, Cr; Condition B - the pollutant concentrations are above the guideline limits for water reuse and cannot be directly used for different purposes. The parameters found in this condition were Fe, Pb, Ni, Cd and color. Considering water reuse, an additional treatment is required for parameters in this second condition. Further studies should evaluate the design aspects that can allow collection of LIDs effluent, additional treatment if necessary, and reuse in the catchment.

Evaluating the Road-Bioretention Strip System from a Hydraulic Perspective—Case Studies

The two-dimensional overland flow simulation program, FullSWOF_2D, was revised to include submodules of determining infiltration by zones (Z) and grate-inlet (G) drainage from a 2D surface to a 1D pipe flow. The updated program, FullSWOF-ZG, was used to evaluate the performance of a road-bioretention strip (RBS) system and explore/understand key parameters of continuous RBS design. The program was validated using eight pervious surfaces under simulated rainfall events and tested with 20 experimental cases of a locally depressed curb inlet. The mean difference of simulated interception efficiencies (36.6%–86.0%) and observed interception efficiencies (34.8%–84.0%) of the curb inlet was 3.5%, which proves the program predicts the curb-inlet interception efficiency accurately. The 20 road-only and 20 RBS modeling cases were designed and modeled using the FullSWOF-ZG program. These case studies have different road lengths, curb inlet lengths, longitudinal slopes, cross slopes, bioretention-overflow inlet heights, and bioretention soil infiltration parameters. Only 34.6%–48.4% of the total runoff volume is intercepted by the RBS’s curb inlet under heavy rainfall (250 mm/h) and the remaining part of the runoff flows downstream along the road, which may cause local inundation and become a safety hazard. The curb inlet becomes the bottleneck of the RBS system that could impede the runoff flowing into the bioretention strip for detention and infiltration to improve the stormwater quality.

Design parameters and treatment efficiency of a retrofit bioretention system on runoff nitrogen removal

Mixed media design is key factor that affects the operation of bioretention systems. In this study, four types of modifiers, namely, water treatment residual (WTR), green zeolite, fly ash, and coconut bran, were mixed with traditional bioretention soil (65% sand + 30% soil + 5% sawdust, by mass). Consequently, four kinds of modified media were obtained. Ten pilot-scale bioretention basins were constructed by setting different configurations. The steady infiltration rates of the modified packing bioretention systems were 3.25~62.78 times that of plant soil, which was 2.88~55.75 m/day. Results showed that the average concentration removal (ACR) of both mixed and layered fly ash and WTR were better than those of the other media, and the effects could reach over 61.92%. In the bioretention basins with WTR as the modifier, the treatment efficiency of nitrogen under the submerged zone height of 150 mm was relatively optimal, and ACR could reach 65.46%. Outflow total nitrogen (TN) load was most influenced by inflow load, and the correlation coefficient was above 0.765. Relative to the change of inflow concentration (IC), the change of recurrence interval (RI) and discharge ratio (DR) was more sensitive to TN load reduction. The reduction rate of TN load decreased by approximately 15% when the recurrence interval increased from 0.5 to 3 years. It decreased by approximately 12% when the discharge ratio increased from 10 to 20. This study will provide additional insights into the treatment performance of retrofit bioretention systems, and thus, can guide media and configuration design, effect evaluation, and related processes.

Hydrologic performance of bioretention in an expressway service area

Bioretention can be an effective measure for stormwater treatment. However, there is a lack of systematic analysis of the impact of bioretention design parameters on hydrologic performance. Herein, SWMM and RECARGA models were applied to generate the typical annual rainfall runoff and simulate the water balance of the bioretention system in an expressway service area. The purpose of the investigation was to identify key design parameters for the bioretention system and delineate the priorities in developing the design. Results showed that the average groundwater recharge ratios for bioretention basins with and without an underdrain were 58.29% and 92.27%, respectively, the average overflow ratios were 4.13% and 4.19%, the average evapotranspiration ratios were 4.48% and 4.47%, and the average outflow ratio for bioretention with an underdrain was 33.94%. The ratio of the bioretention area to drainage area, and the saturated infiltration rates of planting soil and native soil were the main factors influencing water balance, while the underdrain diameter and gravel layer depth exerted little effect. Based on the impact analysis, multivariate nonlinear regression models of runoff reduction rate for two types of bioretention basin were established, which both exhibited high determination coefficients and acceptable Nash-Sutcliffe coefficients.

Investigation into the long-term stormwater pollution removal efficiency of bioretention systems

In recent years, there has been a steady increase in the number of bioretention systems installed worldwide. However, there has only been limited research on the long-term effectiveness of these sustainable urban drainage system devices. This paper presents the results of a series of controlled field experiments investigating the pollutant removal efficiency of three, 10-year-old, bio-filtration systems that have been in service in the Sunshine Coast in Australia. The results of this study suggest that the long-term pollution removal performance of these systems may not be as effective as previously thought and further research is needed.

The pollution removal and stormwater reduction performance of street-side bioretention basins after ten years in operation

This study evaluated the pollution removal and hydrologic performance of five, 10-year old street-side bioretention systems. The bioretention basins were subjected to a series of simulated rainfall events using synthetic stormwater. Four different pollution concentrations were tested on three of the bioretention basins. The four concentrations tested were: A) no pollution; B) typical Australian urban pollutant loads; C) double the typical pollution loads, and; D) five times the typical pollution loads. Tests were also undertaken to determine the levels of contaminant and heavy metals build-up that occurred in the filter media over the 10 year operational life of the bioretention systems. Although highly variable, the overall hydrological performance of the basins was found to be positive, with all basins attenuating flows, reducing both peak flow rates and total outflow volumes. Total suspended solids removal performance was variable for all tests and no correlation was found between performance and dosage. Total nitrogen (TN) removal was positive for Tests B, C and D. However, the TN removal results for Test A were found to be negative. Total phosphorus (TP) was the only pollutant to be effectively removed from all basins for all four synthetic stormwater tests. The study bioretention basins were found to export pollutants during tests where no pollutants were added to the simulated inflow water (Test A). Heavy metal and hydrocarbon testing undertaken on the bioretention systems found that the pollution levels of the filter media were still within acceptable limits after 10 years in operation. This field study has shown bioretention basin pollution removal performance to be highly variable and dependant on a range of factors including inflow pollution concentrations, filter media, construction methods and environmental factors. Further research is required in order to fully understand the potential stormwater management benefits of these systems.

Field monitoring of a LID-BMP treatment train system in China

In order to assess the urban runoff control effectiveness of a low-impact development best management practice (LID-BMP) treatment train system, a field test of selected LID-BMPs was conducted in China. The LID-BMPs selected include three grassed swales, a buffer strip, a bioretention cell, two infiltration pits, and a constructed wetland. The test site is in a campus in southern China. The LID-BMPs, connected in a series, received stormwater runoff from four tennis courts with an area of 2808 m(2) and eight basketball courts with an area of 4864 m(2). Construction of the LID-BMPs was completed in early spring of 2012, and the sampling was conducted during May of 2012 to September of 2013. During the sampling effort, besides the performance evaluations of grassed swales and the bioretention cell in controlling runoff quantity as well as quality, the emphasis was also on determining the performance of the LID-BMP treatment train system. A total of 19 storm events were monitored, with nine producing no runoff and ten producing runoff. Data collected from the ten storm events were analyzed for estimating runoff quantity (peak flow rate and total runoff volume) and quality reduction by the LID-BMPs. The sum of loads (SOL) method was used for calculating the water quality performance of LID-BMPs. Results indicated that, for peak flow rate, a bioretention cell reduction of 50-84 % was obtained, and grassed swale reduction was 17-79 %, with a runoff volume reduction of 47-80 and 9-74 %, respectively. For water quality, the bioretention cell in general showed good removal for zinc (nearly 100 %), copper (69 %), NH3-N (ammonia nitrogen) (51 %), and total nitrogen (TN) (49 %); fair removal for chemical oxygen demand (COD) (18 %); and poor removal for total suspended solids (TSS) (-11 %) and total phosphorus (TP) (-21 %). And its performance effectiveness for pollutant removal increased in the second year after 1 year of stabilizing. When considering the aggregated effect of the LID-BMP treatment train system, it showed excellent removal for NH3-N (73 %), TN (74 %), and TP (95 %) and fair removal for COD (19 %) and TSS (35 %). The assessment results of the LID-BMP treatment train system provide valuable information on how to link the different types of LID-BMP facilities and maximize the integrated effectiveness on urban runoff control.