Next generation swale design for stormwater runoff treatment: A comprehensive approach

Stormwater treatment in infiltration swales with an engineered and planted soil media - Results of a pilot-scale approach

Infiltration swales are well-established stormwater control measures. To meet the future multi-coded requirements, i.e. drainage safety, pollutant retention, and the promotion of biodiversity, we identified a research need regarding the soil layer and its simultaneous adaption of technical and soil functions. In particular, we aimed for higher water storage capacity and increased pollutant retention by developing engineered soil media based on natural topsoil or subsoil with amendments such as compost and brick sand. The media was flushed with real road runoff and monitored over more than two years, focusing on external influencing factors (e.g., antecedent dry-weather period, inflow rate, and de-icing salts) on pollutant retention. We found comparable water retention rates between both engineered soil media and the control but concluded that the amendment of brick sand had a beneficial impact given that the control had a considerably higher fine soil and humus content. An improvement of the retention efficiency was not achieved through the amendments compared to the control and retention efficiencies were >89 %, >97 %, and >94 % for Cu, Zn, and fine suspended solids. External factors had no dominant effect on the retention efficiency, a significant influence could only be demonstrated for organic matter. Additionally, we evaluated a minor but constant leaching for V and DOC. We concluded that engineered media may be suitable in swales through its potential higher water retention capacity with a simultaneous good retention of pollutants. However, attention must be paid to the leaching behavior of the soil amendments.

Nitrate pollution in urban runoff: A comprehensive risk assessment for human and ecological health

Nitrate pollution in urban runoff poses significant environmental and public health risks, with its impact varying across different land use types. This study investigates nitrate concentrations in runoff from residential, commercial, industrial, and traffic zones in Tehran, Iran, using Event Mean Concentration (EMC) analysis and Monte Carlo simulations to assess both ecological and human health risks. The results indicate that industrial and traffic zones exhibit the highest nitrate concentrations, reaching 58.13 mg/L, significantly exceeding regulatory thresholds. Ecological risk assessments highlight the potential for aquatic system degradation, while health risk evaluations reveal hazard index (HI) values surpassing the safe limit (HI > 4), particularly in industrial and high-traffic areas. These findings underscore the need for targeted mitigation strategies, including the implementation of green infrastructure and stricter pollution control measures, to improve urban water quality and reduce associated risks.

Assessing spatial and temporal variability of grass swale infiltration in shallow groundwater conditions

Implementing stormwater green infrastructure in shallow groundwater areas presents major challenges that could restrict widespread adoption of swales in such areas. These limitations are driven by concerns about reduced swale infiltration capacity, which negatively impacts the effectiveness of green measures in managing runoff volumes. This study evaluates the spatial and temporal distribution of infiltration rates in a 30-m grass swale section using a Modified Philip-Dune infiltrometer and full-scale infiltration testing. Groundwater levels were continuously monitored by three piezometers adjacent to the grass swale to assess the impact of unsaturated zone depth on the swale infiltration capacity. Results showed that infiltration rates varied widely from 13 mm/h at the swale bottom to 98 mm/h on the right slope and highlighted the potential overestimation of swale capacity when relying only on point measurements of infiltration. Results from a full-scale infiltration test revealed an overall swale infiltration rate of only 4 mm/h, which is below the values recommended in the literature for swale applicability. A 52 % decrease in infiltration rates was observed between 2022 and 2024. Experimental results indicated that the grass swale had the capacity to recover its storage and managed a subsequent rainfall event within 15 h of the full draw-down. While the findings did not show a strong correlation between swale infiltration rates and the depth of the unsaturated zone, the results underscore the need to balance the soil permeability and groundwater protection for effective stormwater management.

Characterizing nitrogen deposited on urban road surfaces: Implication for stormwater runoff pollution control

Nitrogen (N) is one of the most important pollutants on urban road surfaces. Understanding the N deposition forms, load characteristics, and influential factors can help to provide management and control strategies for road stormwater runoff pollution. This study focuses on a highly urbanized area in Guangzhou, China, and presents the characteristics of both dissolved and particulate N deposition forms as well as their correlations with land-use types and traffic factors. In addition, an artificial neural network (ANN) based classification model is utilized to estimate N pollution hotspot area and total nitrogen (TN) flux from road to receiving water bodies. The results showed that N on urban road surfaces mainly existed in the form of particulate organic nitrogen. Land use types dominated by residential area (RA) and urban village (UV) have higher TN build-up loads. Geodetector analysis indicated that land use has a greater impact on nitrogen build-up loads than traffic factors. Through classification and estimation using the ANN model, RA, and UV were classified as hotspot areas, and the TN flux from roads in the study area was calculated to be 3.35 × 105 g. Furthermore, it was estimated that the annual TN flux from roads in Guangzhou accounts for 19 % of the city's total urban domestic discharge. These findings are expected to contribute to the pollution control of stormwater runoff from urban road surfaces and provide valuable guidance for enhancing the ecological health of urban water environments.

Systematic evaluation of swale length, shape, and longitudinal slope with simulated highway runoff for better swale design

Swales are a low-cost, conveyance and treatment system to manage roadway runoff, but available design guidance is limited. Eight grass swales were constructed in Raleigh, North Carolina, USA, to systematically evaluate the effects of design factors: length, shape, and longitudinal slope under two different storm sizes. Water from an onsite reservoir was used to generate synthetic runoff and simulate flow through the swales. Inflow volume, total suspended sediment (TSS), nitrogen, phosphorus, and four total metals (copper, lead, zinc, and cadmium) were tested with simulated levels representing highway runoff. Efficiency ratios were used to estimate the reductions in inflow volume, pollutant concentrations, and mass loads. Swale length, shape, longitudinal slope, and storm size significantly influenced runoff volume reduction. The longer (30 m) trapezoidal swale constructed on the flatter (1%) longitudinal slope provided maximum reductions in sediment and heavy metal concentrations during small-medium storms. Larger storms had modestly reduced pollutant and volume mitigation. Effluent nutrient concentrations generally exceeded the influent exporting nitrogen and phosphorus from all swale configurations. Significantly better pollutant load reductions were provided by the longer swales for all pollutants, except dissolved phosphorus. Therefore, to optimize swale function, designers could maximize the swale length to the greatest extent practicable, particularly when swales receive inflow from end-of-pipe systems draining roadway surfaces. The trapezoidal cross-section was superior to the triangular cross-section for stormwater treatment. Proper vegetation establishment and maintaining optimal grass height are key to proper swale functioning.

A deep dive into green infrastructure failures using fault tree analysis

Green Infrastructure has transformed traditional urban stormwater management systems by fostering a wide range of service functions. Despite their popularity, green infrastructure's performance can deteriorate over their lifecycle, leading to operational failures. The operation of green infrastructure has predominantly relied on reactive maintenance strategies. To anticipate malfunctions and enhance the performance of green infrastructure in the long run, failure data needs to be recorded so that deterioration processes and component vulnerabilities can be recognized, modelled and included in predictive maintenance schemes. This study investigates possible failures in representative GIs and provides insights into the most important events that should be prioritized in the data collection process. A method for qualitative Fault Tree Analysis using minimal cut sets are introduced, aiming to identify potential failures with the minimum number of events. To identify events of interest fault trees were constructed for bioswales, rain gardens and green roofs, for three groups of service function failures, namely runoff quantity control, runoff quality control and additional service functions. The resulting fault trees consisted of 45 intermediate and 54 basic events. The minimal cut set analysis identified recurring basic events that could affect operation among all three green infrastructure instances. These events are 'trash accumulation', 'clogging due to sediment accumulation', and 'overly dense vegetation'. Among all the possible cut sets, events such as 'plants not thriving', 'invasive plants taking over', and 'deterioration caused by external influences' could potentially disrupt most of the service functions green infrastructure provides. Furthermore, the analysis of interactions between component failures shows vegetation and filter media layer failures have the highest influence over other components. The constructed fault trees and identified basic events could be potentially employed for additional research on data collection processes and calculating the failure rates of green infrastructure and as a result, contribute to a shift toward their proactive operation and maintenance.

Harnessing the runoff reduction potential of urban bioswales as an adaptation response to climate change

Nature-based solutions (NbS), including China's Sponge City Program (SCP), can address the challenges urban communities face due to surface runoff and flooding. The current capacity of SCP facilities in urban environments falls short of meeting the demands placed on communities by climate change. Bioswales are a form of SCP facility that plays an important role in reducing surface runoff by promoting infiltration. This study assesses the potential of SCP facilities to reduce runoff in urban communities under climate change using the storm water management model. The study site in Ningbo, China, was used to evaluate the potential role of bioswales in reducing runoff risks from climate change. We found that bioswales were most effective in scenarios when rainfall peaks occurred early and were less effective in right-skewed rainfall events. The overall performance of SCP facilities was similar across all climate scenarios. To maintain the current protection level of SCP facilities, bioswales would need to cover at least 4% of the catchment area. These findings from Ningbo provide a useful method for assessing NbS in other regions and indicative values for the increase in the bioswale coverage needed to adapt to climate change.

Environmental drivers impact the accumulation and diversity of antibiotic resistance in green stormwater infrastructure

Antibiotic resistance poses an urgent public health concern, with the environment playing a crucial role in the development and dissemination of resistant bacteria. There is a growing body of research indicating that stormwater is a significant source and transport vector of resistance elements. This research sought to characterize the role of green stormwater infrastructure (GSI), designed for stormwater infiltration, in accumulating and propagating antibiotic resistance in the urban water cycle. Sampling included 24 full-scale GSI systems representing three distinct types of GSI - bioswales, bioretention cells, and constructed wetlands. The results indicated that GSI soils accumulate antibiotic resistance genes (ARGs) at elevated concentrations compared to nonengineered soils. Bioretention cells specifically harbored higher abundances of ARGs, suggesting that the type of GSI influences ARG accumulation. Interestingly, ARG diversity in GSI soils was not impacted by the type of GSI design or the diversity of the microbial community and mobile genetic elements. Instead, environmental factors (catchment imperviousness, metals, nutrients, and salts) were identified as significant drivers of ARG diversity. These findings highlight how environmental selective pressures in GSI promote ARG persistence and proliferation independently of the microbial community. Therefore, GSI systems have the potential to be a substantial contributor of abundant and diverse ARGs to the urban water cycle.

Performance of wood waste biochar and food waste compost in a pilot-scale sustainable drainage system for stormwater treatment

Sustainable drainage system (SuDS) for stormwater reclamation has the potential to alleviate the water scarcity and environmental pollution issues. Laboratory studies have demonstrated that the capacity of SuDS to treat stormwater can be improved by integrating biochar and compost in the filter media, whereas their performance in scaled-up applications is less reported. This study examines the effectiveness of a pilot-scale SuDS, bioswale followed by bioretention, amended with wood waste biochar (1, 2, and 4 wt.%) and food waste compost (2 and 4 wt.%) to simultaneously remove multiple pollutants including nutrients, heavy metals, and trace organics from the simulated stormwater. Our results confirmed that SuDS modified with both biochar (2 wt.%) and compost (2 wt.%) displayed superior water quality improvement. The system exhibited high removal efficiency (> 70%) for total phosphorus and major metal species including Ni, Pb, Cd, Cr, Cu, and Zn. Total suspended solids concentration was approaching the detection limit in the effluent, thereby confirming its capability to reduce turbidity and particle-associated pollutants from stormwater. Co-application of biochar and compost also moderately immobilized trace organic contaminants such as 2,4-dichlorophenoxyacetic acid, diuron, and atrazine at field-relevant concentrations. Moreover, the soil amendments amplified the activities of enzymes including β-D-cellobiosidase and urease, suggesting that the improved soil conditions and health of microbial communities could possibly increase phyto and bioremediation of contaminants accumulated in the filter media. Overall, our pilot-scale demonstration confirmed that the co-application of biochar and compost in SuDS can provide a variety of benefits for soil/plant health and water quality.

Predicting stormwater nitrogen loads from a cold-region urban catchment in year 2050 under the impacts of climate change and urban densification

Urban stormwater is a primary source of pollution for receiving water, but there is a shortage of studies on pollutant loads from urban catchments in cold regions. In this study, we coupled a build-up and wash-off model (in Mike Urban) with a climate change model to assess the impacts of climate change and urban densification on stormwater nitrogen loads (TN, TKN, NOx-N, and TAN) in an urban catchment in Canada. We calibrated and validated the Mike Urban model against observed event mean concentrations and nitrogen loads from 2010 to 2016. Results show that the nitrogen loads were mainly governed by rainfall intensity, rainfall duration, and antecedent dry days. Future precipitation data were downscaled using the Global Climate Models (GCMs), and three different Representative Concentration Pathways (RCP 2.5, RCP 4.5, and RCP 8.5) were used. Modeling results show that the TN, TKN, NOx-N, and TAN loads in 2050 will increase by 28.5 - 45.2% from May to September under RCP 2.5 compared to those from 2010 to 2016, by 34.6 - 49.9% under RCP 4.5, and by 39.4 - 53.5% under RCP 8.5. The increase of our projected TN load (from 1.33 to 2.93 kg·N/ha) is similar or slightly higher than the limited studies in other urban catchments. This study provides a reference for predicting stormwater nitrogen loads in urban catchments in cold regions.

Enhanced Escherichia coli removal from stormwater with bermudagrass-derived activated biochar filtration systems

Stormwater treatment and reuse can alleviate water pollution and scarcity while current sand filtration systems showed low treatment performance for stormwater. For enhancing E. coli removal in stormwater, this study applied the bermudagrass-derived activated biochars (BCs) in the BC-sand filtration systems for E. coli removal. Compared with the pristine BC (without activation), the FeCl3 and NaOH activations increased the BC carbon content from 68.02% to 71.60% and 81.22% while E. coli removal efficiency increased from 77.60% to 81.16% and 98.68%, respectively. In all BCs, the BC carbon content showed a highly positive correlation with E. coli removal efficiency. The FeCl3 and NaOH activations also led to the enhancement of roughness of BC surface for enhancing E. coli removal by straining (physical entrapment). The main mechanisms for E. coli removal by BC-amended sand column were found to be hydrophobic attraction and straining. Additionally, under 105-107 CFU/mL of E. coli, final E. coli concentration in NaOH activated BC (NaOH-BC) column was one order of magnitude lower than those in pristine BC and FeCl3 activated BC (Fe-BC) columns. The presence of humic acid remarkably lowered the E. coli removal efficiency from 77.60% to 45.38% in pristine BC-amended sand column while slightly lowering the E. coli removal efficiencies from 81.16% and 98.68% to 68.65% and 92.57% in Fe-BC and NaOH-BC-amended sand columns, respectively. Moreover, compared to pristine BC, the activated BCs (Fe-BC and NaOH-BC) also resulted in the lower antibiotics (tetracycline and sulfamethoxazole) concentrations in the effluents from the BC-amended sand columns. Therefore, for the first time, this study indicated NaOH-BC showed high potential for effective treatment of E. coli from stormwater by the BC-amended sand filtration system compared with pristine BC and Fe-BC.

Implementation of P-Reactive Layer for Improving Urban Water Quality: Kinetic Studies, Dimensioning and Economic Analysis

Urbanization and climate change affecting water quality are the most critical problems that humanity has to encounter globally. Undoubtedly, urban water bodies are heavily contaminated by phosphorus (P). This study aims to identify the mechanisms and efficiency of the P sorption process for selected reactive materials (Autoclaved Aerated Concrete (AAC), Filtralite® Nature P, lightweight expanded clay aggregate (Leca®), limestone, opoka, and zeolite) with surface water as adsorbate and dimension of P-reactive reactive layer supported with economic analysis. Four kinetic models were used to know the sorption mechanism: pseudo-first order, pseudo-second order, Elovich, and intra-particle diffusion model. Calculating the P-reactive layer was based on dimensioning rain retention spaces standards. The pseudo-second model provided the best description of the adsorption kinetics of most materials. The sorption properties obtained after 72 h showed the reduction of 83, 81, 59, 53, 37, and 36% for AAC, opoka, Filtralite® Nature P; limestone, Leca®, and zeolite, respectively. Depending on the volume, the P-reactive layer can remove 29–77 or 61–163 g of P-PO4. The unit cost of removing P-PO4 by the P-reactive layer range from 49.57 to 85.53 €/P-PO4 g. For these reasons, reactive materials seem to be an effective way of removing P from the urban water environment worldwide from both environmental and economic points of view.

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.

Improvement of heavy metal removal from urban runoff using modified pervious concrete

Heavy metals are one of the major chemical pollutant groups in urban runoff. The application of porous concrete is a potential alternative to conventional runoff management systems with the ability to remove heavy metals. Hence, a thorough understanding of the heavy metal removal mechanisms and constraints of conventional porous concrete opens a path for the development of effective modifications. This review critically discusses the major contributors in ordinary porous concrete which supports heavy metal removal. The effects of initial concentration, contact time and competing ions on heavy metal removal using porous concrete are also discussed. Additionally, the effect of decalcification, atmospheric carbonation, acid influent on heavy metal removal is reviewed. The major drawback of porous concrete is the high pH (>8.5) of the effluent water, decalcification of the porous concrete and leaching of adsorbed pollutants. Overall, the addition of adsorbent materials to the porous concrete increases removal efficiencies (7% - 65% increase) without neutralizing the effluent pH. Meanwhile, the addition of Reduced Graphene Oxide is successful in reducing the leachability of the removed heavy metals. The addition of pozzolanic materials can lower the effluent pH while maintaining similar removal efficiencies to unmodified porous concrete. Therefore, developing a novel method of neutralizing the effluent pH must be prioritized in future studies. Additionally, the toxicity that can occur due to the abrasion of modified porous concrete requires study in future research. Further, advanced characterization methods should be used in future studies to understand the mechanisms of removal via the modified porous concrete materials.

Hydraulic and nutrient removal performance of vegetated filter strips with engineered infiltration media for treatment of roadway runoff

As a new strategy for treating excess nutrients in roadway runoff, a self-filtering roadway could be accomplished by including engineered infiltration media within a vegetated filter strip (VFS) located in the roadway shoulder. However, nutrient removal performance will depend on the design to effectively infiltrate roadway runoff and the capacity of subsurface media to sequester or remove nutrients from infiltrated runoff. The objective of this study is to test hydraulic and nutrient removal performance of a roadside VFS over varied rainfall-runoff event sizes and filter widths. Two identical 1:1 scale physical models of roadway shoulders and embankments, one containing engineered media (Treatment model) and the other without (Control model), were tested with simulated rainfall and runoff from 1- and 2-lane roadways. Overall, 32 paired hydraulic experiments and 28 paired nutrient removal experiments were completed to assess performance across frequent and extreme rainfall-runoff events. The results indicate that scalability of performance with filter width varied by parameter. Runoff generation scaled predictably with filter width, as runoff generated close to the pavement and total infiltration increased with filter length. A 6 m-wide VFS containing the engineered media infiltrated all rainfall-runoff except during the most extreme storm events (1-h storms of 76.2 mm and 50.8 mm), where respectively 35% and 22% of rainfall-runoff did not infiltrate and left the system as surface runoff. A majority of phosphorus was retained within a 1.5 m filter while nitrate removal was not observed until 6 m. The Treatment model strongly outperformed the Control model with respect to nitrate (arithmetic mean ± standard deviation of 94 ± 6% reduction vs. 23 ± 64% increase, p < .001) and total nitrogen removal (80 ± 5% vs. 38 ± 23% reduction, p < .001) due to higher rates of microbially-mediated denitrification in the Treatment model. The two models performed comparably with regard to phosphorus reduction (84 ± 9% vs. 82 ± 12% reduction). A minimum 6 m filter width is recommended to ensure sufficient infiltration of runoff and nitrogen removal. Results of this study address uncertainty regarding nutrient removal performance of VFS in urban runoff applications and highlight a potential strategy for standardizing VFS performance across varied soil properties by including engineered media within the filter.

Transport process and source contribution of nitrogen in stormwater runoff from urban catchments

Nitrogen in urban stormwater has been widely studied, and effective management of nitrogen pollution is critical for improving urban stormwater and receiving water quality. This requires an in-depth understanding of the transport process and source contribution to both dissolved and particulate nitrogen in stormwater from urban catchments. In this study, 123 stormwater runoff samples were collected from an urban catchment during different rainfall events. Dissolved and particulate nitrogen concentrations in roof runoff, road runoff, and sewer flow were analyzed. The concentration of dissolved nitrogen was higher in roof runoff than in road runoff and sewer flow. However, the concentration of particulate nitrogen was lower in roof runoff than in road runoff and sewer flow. Isotopic analysis and Bayesian mixing models showed that road runoff was the largest source contributor of both nitrate and particulate organic nitrogen (PON) in sewer flow discharged from the study catchment. In addition, road runoff contributed the majority of PON associated with coarse particles (>105 μm), whereas PON associated with fine particles (<105 μm) was primarily washed-off of sewer sediments. The results provided several suggestions for the management of nitrogen pollution in urban catchments. This study could help to fully understand the transport and sources of nitrogen pollution in urban stormwater and provide recommendations to the government for implementing appropriate stormwater management strategies to minimize stormwater pollution.

Survey of the operational status of twenty-six urban stormwater biofilter facilities in Sweden

This study evaluates the operational status of twenty-six biofilter facilities across nine cities in Sweden, with respect to their functional design criteria, engineered design features (filter media composition, hydraulic conductivity, and drawdown time), and includes a visual inspection of the biofilter components (pre-treatment, in/outlet structures, filter media, and vegetation). These indicators were used to examine the performance level of each biofilter in achieving their design objectives set by the operators. Furthermore, it was investigated whether the biofilter facilities had been properly maintained to meet the objectives. Results indicate that the soil media used was consistent with respect to percentage sand, fines, and organic matter and comparable to design recommendations used by municipalities in other countries. The field-tested hydraulic conductivity for the biofilters ranged from 30 to 962 mm/h. This range of values, along with noticeable sediment accumulation within the biofilter indicate that not all the sites were operating optimally. Pre-treatment stages in poor condition with high volumes of sediment and litter accumulation were the primary causes for, and indicators of, low hydraulic conductivity rates. The ponding volume calculations revealed that at least 40 % of facilities did not have enough capacity to retain every-day and/or design rainfall due to design and/or construction flaws. These analyses raise concerns that, for a considerable number of the biofilters surveyed, water retention and flood protection identified by operators as prioritised objectives are not being met. This raises significant concerns about the functionality of biofilter in practice. Finally, some suggestions are given for tackling the design and maintenance problems discovered.